Drouin Gaetan
email: gaetan.drouin@agrocampus-ouest.fr

Rioux Vincent
email: vincent.rioux@agrocampus-ouest.fr

Legrand Philippe
email: philippe.legrand@agrocampus-ouest.fr

Gaetan Drouin

Philippe Legrand

Vincent Rioux

The n-3 docosapentaenoic acid (DPA): a new player in the n-3 long chain polyunsaturated fatty acid family

Keywords: docosahexaenoic acid DHA, eicosapentaenoic acid EPA, polyunsaturated fatty acid

The n-3 docosapentaenoic acid (n-3 DPA) is less studied n-3 long-chain polyunsaturated fatty acid (LCPUFA), compared to its counterparts eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA). Present in food sources in non-negligible quantities, as well as in human milk, dietary n-3 DPA is of current interest both for its ability to increase EPA and DHA tissue status and for its specific or shared biological effects. Indeed, some evidence showed that dietary n-3 DPA is a source of EPA and slightly DHA in the major metabolic organs. n-3 DPA is also the precursor of a large panel of lipid mediators (protectins, resolvins, maresins, isoprostanes) principally implicated in the pro-resolution of the inflammation with specific effects compared to the other n-3 LCPUFA. Recent results showed that n-3 DPA is implied in the improvement of cardiovascular and metabolic disease risk markers, especially plasma lipid parameters, platelet aggregation, insulin sensitivity and cellular plasticity. Moreover, n-3 DPA is the most abundant n-3 LCPUFA in the brain after DHA and it could be specifically beneficial for elderly neuroprotection, and early-life development. These results led to the development of two drugs specifically containing n-3 DPA. This review summarizes the different knowledge about n-3 DPA direct and indirect sources, availability and purification methods, focusing thereafter on the recent findings showing n-3 DPA relationship with fatty acid metabolism, lipid mediators, Finally, the n-3 DPA biological and pharmacological effects are described.

Highlights

• n-3 DPA could be considered like a dietary source of EPA tissue content • No evidence showed that dietary n-3 DPA increased brain DHA • Hydroxy-metabolites from n-3 DPA are involved in the pro-resolution of inflammation • More and more evidences about the n-3 DPA specific effects to decrease lipid parameters • n-3 DPA purification methods will allow its accessibility for further in vivo studies

Introduction

The n-3 long-chain polyunsaturated fatty acids (LCPUFA) have been widely studied and contribute to numerous beneficial effects, mainly associated with cardiovascular prevention [START_REF] Innes | The differential effects of eicosapentaenoic acid and docosahexaenoic acid on cardiometabolic risk factors: A systematic review[END_REF], neurodevelopment, but also with the reduction of the risk of neurodegenerative diseases. [START_REF] Dyall | Long-chain omega-3 fatty acids and the brain: A review of the independent and shared effects of EPA, DPA and DHA, Front[END_REF]. Indeed, these n-3 LCPUFA are involved in many processes such as the increase of membrane plasticity, the synthesis of oxygenated metabolites and the resolution of inflammation or the regulation of genes [START_REF] Rhee | Fish Consumption, Omega-3 Fatty Acids, and Risk of Cardiovascular Disease[END_REF].

The majority of the n-3 LCPUFA studies were conducted using fish oils, composed of a mixture of three major n-3 LCPUFA: docosahexaenoic acid (DHA), eicosapentaenoic acid (EPA) and n-3 docosapentaenoic acid (n-3 DPA).

Most of the beneficial effects of fish oils have been attributed to DHA and then to EPA, for which there is growing interest in the independent and shared functions. On the other hand, the literature concerning the potential protective effects of n-3 DPA is brief because n-3 DPA represents less than one-third of each EPA and DHA in fish oils. In addition, n-3 DPA is not commercially available in sufficient quantity, with high purity (> 98%) and at an affordable price to set up in vivo nutritional supplementation studies [START_REF] Byelashov | Dietary sources, current intakes, and nutritional role of omega-3 docosapentaenoic acid[END_REF].

Studies about n-3 DPA have however begun to grow in recent years. n-3 DPA is indeed the second n-3 LCPUFA found in the brain (w/w), although its cerebral concentration is about 70 times lower than DHA. Moreover, the level of n-3 DPA in human milk is higher than that of EPA, similar to that of DHA and its level is more stable [START_REF] Delplanque | Lipid Quality in Infant Nutrition: Current Knowledge and Future Opportunities[END_REF], implying a potential impact of n-3 DPA during pregnancy and development, which is the subject of a recent review [6]. While obtaining optimal tissue status in n-3 LCPUFA is one of the current public health challenges, n-3 DPA is also the only intermediate between EPA and DHA in the n-3 LCPUFA conversion pathway from α-linolenic acid (ALA) present in significant quantities in the diet. Therefore, could n-3 DPA serve as a dietary source or biological reservoir of DHA and EPA?

This paper reviews and summarizes the different knowledge about n-3 DPA and focus on the most recent findings synthesized in Table 1. To more thorough review concerning specific knowledges, the reader is encouraged to read another reviews specific about n-3 DPA or including it, as mentioned in adequate sections below [START_REF] Dyall | Long-chain omega-3 fatty acids and the brain: A review of the independent and shared effects of EPA, DPA and DHA, Front[END_REF][START_REF] Byelashov | Dietary sources, current intakes, and nutritional role of omega-3 docosapentaenoic acid[END_REF][6][START_REF] Yazdi | A review of the biologic and pharmacologic role of docosapentaenoic acid n-3[END_REF][START_REF] Kaur | Docosapentaenoic acid (22:5n-3): A review of its biological effects[END_REF][START_REF] Kaur | Short update on docosapentaenoic acid: A bioactive long-chain n-3 fatty acid[END_REF][10][11][12].

Dietary sources and availability of n-3 DPA

Commercial availability

Suppliers who offer n-3 DPA> 98-99% purity cannot provide on demand enough quantities of n-3 DPA for in vivo studies at prices that are affordable for most research laboratories (supplier communications, 2017). Thus, studies focusing on n-3 DPA are often association studies in humans or in vitro studies. Most in vivo studies thus used n-3 DPA with a purity level <98% which therefore also contains mainly DHA, EPA and n-6 DPA, limiting the interpretation of these findings [START_REF] Kaur | Docosapentaenoic acid (22:5n-3): A review of its biological effects[END_REF][START_REF] Kaur | Short update on docosapentaenoic acid: A bioactive long-chain n-3 fatty acid[END_REF]13]. In addition, only one supplier offers radiolabelled 14 C-n-3 DPA (ARC, 10µCi), which is also very expensive and inaccessible for n-3 DPA metabolism monitoring studies, although two studies have reported its use in vivo [START_REF] Kaur | Orally administered [14C]DPA and [14C]DHA are metabolised differently to [14C]EPA in rats[END_REF][START_REF] Alvarez | Docosapentaenoic acid is converted to docosahexaenoic acid in the retinas of normal and prcd-affected miniature poodle dogs[END_REF]. A n-3 DPA-based dietary supplement has also recently appeared on the market and contains 15% n-3 DPA in proportion to n-3 LCPUFAs (Super n-3 DPA Fish Oil®, Swanson, USA, 2018).

Purification and synthesis

To alleviate this problem of commercial availability as well as to study n-3 DPA in various dietary lipid forms, some teams have been interested in the synthesis or in the purification of n-3 DPA from natural sources. Foremost, a five-step synthesis of n-3 DPA from EPA has been described in 30% overall yield for making multi-milligram quantities of n-3 DPA [START_REF] Jakobsen | Concise syntheses of three ω-3 polyunsaturated fatty acids[END_REF]. Concerning n-3 DPA purification, historically, a Japanese study first focused on the industrial scale purification of n-3 DPA in ethyl ester form from Schizochytrium sp. algae oil by industrial highperformance liquid chromatography (HPLC) using two reverse phase columns. They obtained n-3 DPA (and DHA) > 99% purities with a production of 70 g/hour of n-3 DPA [START_REF] Yamamura | Industrial high-performance liquid chromatography purification of docosahexaenoic acid ethyl ester and docosapentaenoic acid ethyl ester from single-cell oil[END_REF]. A Chinese team then focused on purifying n-3 DPA in the laboratory from tuna oil, firstly by crystallization of the fatty acids complexed with urea, then by purification by liquid chromatography (LC) on a silver nitrate silica column. [START_REF] Mu | Combined Urea Complexation and Argentated Silica Gel Column Chromatography for Concentration and Separation of PUFAs from Tuna Oil: Based on Improved DPA Level, JAOCS[END_REF]. They obtained n-3 DPA at 22.3% purity with a purification efficiency of 70.7%. Very recently, the same team managed to purify n-3 DPA from 16.4% to 28.1% of the total fatty acids in the form of diacylglycerol by crystallization (6h, -80 ° C) from the Schizochytrium sp oil [19]. Various patents have also been filed to produce, extract and purify n-3 DPA (and DHA) as fatty acid methyl esters from ulkenia [START_REF] Tanaka | United States Patent[END_REF][START_REF] Ding | Process for preparing and separating methyl docosapentaenoate and methyl docosahexenoate[END_REF][START_REF] Tanaka | Process for preparing docosahexaenoic acid and docosapentaenoic acid[END_REF]. In addition, n-3 DPA monoglycerides have been synthetized through esterification at the sn-1 position of the glycerol backbone by using n-3 DPA ethyl esters as starting materials [START_REF] Forin | Polyunsaturated fatty acid monoglycerides, derivatives, and uses thereof -Patent -PubChem[END_REF]. The self-assembly of this compound has been structurally characterized [24], which could give it a better bioavailability than n-3 DPA ethyl ester and triglyceride forms, as shown previously for EPA monoglycerides [25]. More recently, we have purified tens of grams of n-3 DPA> 99% by flash and preparative LC from commercial fish oils enriched with n-3 LCPUFA [26,27]. The purification involved seven successive purification cycles with a purification efficiency > 70% and allowed to purify 8g of n-3 DPA/week [26] as well as EPA and DHA> 99% [27].

The advance of these methods could quickly allow access to the amounts of n-3 DPA needed for clinical studies in humans, requiring several hundred grams of pure n-3 DPA. n-3 DPA-enriched dietary supplements could be a good source for the purification of n-3 DPA by LC compared to fish or algae oils, as well as the combination of LC purification methods, crystallization and distillation [START_REF] Magallanes | Highly concentrated omega-3 fatty acid ethyl esters by urea complexation and molecular distillation[END_REF] or the use of drug under development enriched with n-3 DPA [25,[START_REF] Maki | Effects of MAT9001 containing eicosapentaenoic acid and docosapentaenoic acid, compared to eicosapentaenoic acid ethyl esters, on triglycerides, lipoprotein cholesterol, and related variables[END_REF].

Dietary sources and consumption

The major source of n-3 DPA is obviously seafood, including fish from the Clupeidae family that gave the n-3 DPA its common name: clupanodonic acid (USDA, 2014). Seal meat and fats appear to be the richest in n-3 DPA, containing 5.6% of n-3 DPA [START_REF] Bang | The composition of the Eskimo food in north Western Greenland[END_REF], which would amount to a daily consumption between 1.7 and 4.0 g of n-3 DPA per day for the Inuit population [START_REF] Bang | Lipid Metabolism and Ischemic Heart Disease in Greenland Eskimos[END_REF]. Among the most common consumer products in Western society, salmon contains 393 mg of n-3 DPA per 100g serving, Atlantic mackerel 200 mg, and other oily fish between 100 and 200mg (USDA, 2014). Beef liver and lamb are the richest land-based sources of n-3 DPA but are also highly variable in terms of provenances [START_REF] Byelashov | Dietary sources, current intakes, and nutritional role of omega-3 docosapentaenoic acid[END_REF]. Thus, they contain about 140 mg of n-3 DPA/100 g in New Zealand, but only 20-30mg in the United Kingdom, while American beef does not contain any, surely associated with differences in pasture production and quality [START_REF] Williams | Composition of Australian red meat 2002 3. Nutrient profile[END_REF][START_REF] Sciences | Australian red meat consumption -predominantly lean in response to public[END_REF][START_REF] Morin | Eicosapentaenoic acid and docosapentaenoic acid monoglycerides are more potent than docosahexaenoic acid monoglyceride to resolve inflammation in a rheumatoid arthritis model[END_REF]. Indeed, the quantities of n-3 DPA in organic meat are around 50% greater than in conventional meat [35]. While the dominant sources of EPA and DHA are seafood products that contain less n-3 DPA, n-3 DPA is the most prevalent n-3 LCPUFA in meat [START_REF] Curtis | Analysis of omega-3 fatty acids in foods and supplements[END_REF], thus varying the sources and amounts consumed of n-3 DPA according to eating habits. In another hand, the ruminal biohydrogenation of n-3 DPA was similar to that of EPA and appears complete without the formation of intermediate derivatives expected after biohydrogenation of DHA [37]. Food supplements made from fish oils enriched with n-3 LCPUFA may also provide a n-3 DPA intake, with mackerel oils containing about 4.9% n-3 DPA, compared with 3.0% for salmon or 2% for sardine.

The main dietary sources of n-3 DPA in pregnant and lactating women are seafood (59%), poultry (14%), meat products (11%) and dairy products (9%) [START_REF] Byelashov | Dietary sources, current intakes, and nutritional role of omega-3 docosapentaenoic acid[END_REF]. About two-thirds of seafood-derived n-3 DPA intake in these women was attributed to salmon consumption [START_REF] Enti | Scientific Opinion on the Tolerable Upper Intake Level of eicosapentaenoic acid (EPA), docosahexaenoic acid (DHA) and docosapentaenoic acid (DPA)[END_REF]. In Australian children, intake of EPA and DHA is strongly correlated with consumption of fish and seafood, while intake of n-3 DPA is moderately correlated with meat consumption [START_REF] Rahmawaty | Dietary intake and food sources of EPA, DPA and DHA in Australian children[END_REF]. The main contributor to n-3 DPA consumption among these children was meat, poultry and wildfowl (56%), fish and seafood (23%), cereal products and dishes (5.7%), dairy products (5.6%) and finally egg products (3.6%) [START_REF] Rahmawaty | Dietary intake and food sources of EPA, DPA and DHA in Australian children[END_REF][START_REF]European Food Safety Authority, Scientific opinion on dietary reference values for fats, including saturated fatty acids, polyunsaturated fatty acids, monounsaturated fatty acids, trans fatty acids, and cholesterol[END_REF]. In Europe, the average daily intake of n-3 DPA in adults is between 25 mg/day (Belgium, women aged 18-39) and 75 mg/day (France, male> 45 years), with quartile intakes of 12 to 80 mg/day. In France, the maximum intake, estimated by the five highest percentiles of the population, is 129 mg / day of n-3 DPA [START_REF] Enti | Scientific Opinion on the Tolerable Upper Intake Level of eicosapentaenoic acid (EPA), docosahexaenoic acid (DHA) and docosapentaenoic acid (DPA)[END_REF]. There is no database of young children for n-3 DPA (0-3 years old) and adolescents (13-19 years old). Swedish, Norwegian and German consumption data indicate that the average daily intake of n-3 DPA in 4-year-old children is 30 mg/day, 40 mg/day for 8-12 years and 120 mg/day for 8-12 years old [START_REF] Enti | Scientific Opinion on the Tolerable Upper Intake Level of eicosapentaenoic acid (EPA), docosahexaenoic acid (DHA) and docosapentaenoic acid (DPA)[END_REF][START_REF]European Food Safety Authority, Scientific opinion on dietary reference values for fats, including saturated fatty acids, polyunsaturated fatty acids, monounsaturated fatty acids, trans fatty acids, and cholesterol[END_REF]. Thus, n-3 DPA could contribute up to 30% of the intake of n-3 LCPUFA in the diet of these populations [START_REF] Howe | Dietary intake of long-chain ω-3 polyunsaturated fatty acids: Contribution of meat sources[END_REF] but there are important disparity of consumption beyond populations and ethnicity [42].

Dietary sources of n-3 DPA can also be indirect, either by providing the precursors of the n-3 LCPUFA conversion pathway, or by providing lipid mixture to increase the conversion from ALA to n-3 DPA. Several studies have shown that n-3 DPA can be increased in the blood compartment in human as well as in tissues in animals after a diet rich in ALA or EPA [START_REF] Arterburn | Distribution, interconversion, and dose response of n-3 fatty acids in humans[END_REF]. More recently, the addition of echium oil rich in stearidonic acid in the diet of rats increased the tissue n-3 DPA status, showing that stearidonic acid could also be a source of n-3 DPA, in addition to a source of EPA for which it is mainly described [START_REF] Kuhnt | Dietary Echium Oil Increases Long-Chain n-3 PUFAs, Including Docosapentaenoic Acid, in Blood Fractions and Alters Biochemical Markers for Cardiovascular Disease Independently of Age, Sex, and Metabolic Syndrome[END_REF]45]. While human milk contains n-3 DPA and cerebral accumulation of n-3 LCPUFA occurs mainly during the first years of life [START_REF] Delplanque | Lipid Quality in Infant Nutrition: Current Knowledge and Future Opportunities[END_REF], some studies have shown that the addition of milk lipids in the diet of young people, whose fatty acid composition are closer to breast milk than a mixture of vegetable oils, also increased tissue n-3 DPA status. First, in a monocentric, double-blind controlled and randomized trial, healthy newborns fed formula containing a mixture of dairy lipids and plant oils from birth to 4 month-old increased their n-3 DPA content in red blood cells (RBC) compared to newborns fed with a formula composed with plant oils only [46]. Moreover, this increase in RBC n-3 DPA was more important than in breastfed newborns. In healthy post-weaning Sprague Dawley rats, a partial incorporation of dairy lipids in the diet with vegetable oils (50% w/w) during 6 weeks increased the n-3 DPA status in the RBC, brain, liver and principally in the heart, compared to 100% vegetable oil diets [47]. Thus, dairy lipids could be a potential indirect help to increase the n-3 DPA status in early life. Surprisingly, it has been shown that n-3 DPA supplementation (0.5% of total fatty acids, 10% lipid w/w) and the partial incorporation of dairy lipids in the diet (50% w/w) had a complementary effect to increase the n-3 LCPUFA status in tissues of Sprague Dawley rats fed from weaning for 6 weeks, especially EPA and n-3 DPA tissue contents [26].

Markers of food consumption

The n-3 DPA composition of the RBC membranes and the different lipid classes of plasma in humans are positively correlated with dietary intake of n-3 LCPUFA, as for EPA and DHA. This increase, however, tends to be relatively limited compared to that of EPA and DHA [START_REF] Stark | Global survey of the omega-3 fatty acids, docosahexaenoic acid and eicosapentaenoic acid in the blood stream of healthy adults[END_REF]. In addition, n-3 DPA is present in whole blood in lower amounts than EPA and DHA and the proportion of n-3 DPA decreases less than EPA or DHA as the proportion of n-6 PUFA increases in the blood [START_REF] Bibus | Balancing proportions of competing omega-3 and omega-6 highly unsaturated fatty acids (HUFA) in tissue lipids[END_REF].This suggests that n-3 DPA present in other blood compartments than RBC (plasma, platelets, peripheral blood mononuclear cells) may have a different metabolism than EPA and DHA and that the n-3 DPA blood level would be preserved [START_REF] Harris | The Omega-3 Index: A new risk factor for death from coronary heart disease?[END_REF]. n-3 DPA is for now not considered in the calculation of the Omega-3 index (EPA + DHA of the RBC membranes), which is used as a marker of consumption but also as a risk marker for total mortality, sudden cardiac death or other cardiovascular risks in epidemiological studies [51]. Although the inclusion of n-3 DPA in the Omega-3 index was more precise to estimate n-3 LCPUFA contents in whole blood [START_REF] Harris | The Omega-3 Index: A new risk factor for death from coronary heart disease?[END_REF], it did not improve the prediction of the risks associated with the existing Omega-3 index [11].

Nutritional recommendations

The different health national agencies agreed that the data about n-3 DPA were insufficient to produce specific recommendations for this n-3 LCPUFA. Nevertheless, England [START_REF] Contreras | Effectiveness of silver diamine fluoride in caries prevention and arrest: A systematic literature review[END_REF], Australia and New Zealand [START_REF] Nhmrc | Nutrient Reference Values for Australia and New Zealand Including Recommended Dietary Intakes[END_REF] as well as the Netherlands [START_REF] Gezondheidsraad | Guidelines for healthy diet 2006[END_REF] included the n-3 DPA in the sum of the recommended n-3 LCPUFA (EPA + n-3 DPA + DHA).

Products derived from n-3 DPA

A reservoir of EPA and DHA

n-3 DPA is the direct intermediate between EPA and DHA, in the conversion pathway from ALA, to be present in significant amounts in the human diet compared to the C24:5 n-3 and C24:6 n-3 derivatives [START_REF] Rahmawaty | Dietary intake and food sources of EPA, DPA and DHA in Australian children[END_REF]. This conversion pathway is well known and involves a sequence of desaturase enzymes adding one double bond to the carbon chain and elongase enzymes extending the carbon-chain of two carbons (Figure 1). The n-3 LCPUFA conversion pathway is parallel to that of n-6 LCPUFA using the same sequence and enzymes. Thus, the two pathways are in competition for substrates with each other. In the n-3 LCPUFA conversion pathway, the n-3 DPA is elongated to the C24:5 n-3 derivative by the elongase-5 and weaker elongase-2 enzymes, then desaturated to the C24:6 n-3 by the action of the ∆6-desaturase, and finally converted to DHA by a peroxisomal β-oxidation step [START_REF] Gregory | Molecular basis for differential elongation of omega-3 docosapentaenoic acid by the rat Elovl5 and Elovl2[END_REF]. The ∆6desaturase and more recently elongase-2 are considered as the limiting enzymes in this conversion pathway to DHA [START_REF] Gregory | Elongase reactions as control points in Long-Chain polyunsaturated fatty acid synthesis[END_REF]. Thus, it has been hypothesized that dietary n-3 DPA could be a better precursor of DHA than dietary EPA, bypassing the conversion of EPA to n-3 DPA using elongase-2 and elongase-5 as well [26,27]. One ∆4desaturase activity has also been described in vitro in humans to convert n-3 DPA directly to DHA but these results need to be confirmed [START_REF] Park | The fatty acid desaturase 2 (FADS2) gene product catalyzes ∆4 desaturation to yield n-3 docosahexaenoic acid and n-6 docosapentaenoic acid in human cells[END_REF]. Some authors have also hypothesized that n-3 DPA could not only serve as a reservoir of DHA but also of EPA in humans, in farm animals and potentially in other mammals [START_REF] Miller | A short-term n-3 DPA supplementation study in humans[END_REF].

The ability of dietary n-3 DPA to increase tissue status in DHA remains however controversial and seems tissuedependent. In humans, a 7-day supplementation with a single daily dose of pure n-3 DPA resulted in an increase in plasma DHA in triglycerides only [START_REF] Miller | A short-term n-3 DPA supplementation study in humans[END_REF]. Gavage of rats for 7 days with 50 mg of n-3 DPA/day in the form of free fatty acid led to an increase in DHA in the liver only compared to ALA control [START_REF] Kaur | Short-term docosapentaenoic acid (22:5n-3) supplementation increases tissue docosapentaenoic acid, DHA and EPA concentrations in rats[END_REF]. In the miniature poodle, intravitreal injection of 1-14 C-n-3 DPA showed an increase in radiolabeled DHA in the retina [START_REF] Alvarez | Docosapentaenoic acid is converted to docosahexaenoic acid in the retinas of normal and prcd-affected miniature poodle dogs[END_REF]. Conversely, in the C57BL/KsJ-db/db obese mouse supplemented for 4 weeks with tri-n-3 DPA, no increase in DHA was found in studied tissues [START_REF] Gotoh | Effects of three different highly purified n-3 series highly unsaturated fatty acids on lipid metabolism in C57BL/KsJ-dbl db mice[END_REF], as in the C57BL/6J mouse fed a high-fat diet and force-fed with 50mg of n-3 DPA daily for 6 weeks [13]. In these mice, n-3 DPA supplementation tended to impact tissue fatty acid composition more like DHA than EPA supplementations [13]. While brain tissue composition is known to remain highly stable, oral administration of n-3 DPA to rats resulted in the increase of cerebral n-3 DPA and DHA, regardless of animal age [START_REF] Kelly | The polyunsaturated fatty acids, EPA and DPA exert a protective effect in the hippocampus of the aged rat[END_REF]. However, these both studies used n-3 DPA nearby 70% purity containing DHA and is not easy to interpret.

The addition of n-3 DPA in endothelial cell cultures of aorta [START_REF] Achard | Interconversions and distinct metabolic fate of eicosapentaenoic, docosapentaenoic and docosahexaenoic acids in bovine aortic endothelial cells[END_REF][START_REF] Kanayasu-Toyoda | Docosapentaenoic acid (22:5, n-3), an elongation metabolite of eicosapentaenoic acid (20:5, n-3), is a potent stimulator of endothelial cell migration on pretreatment in vitro[END_REF], rat hepatocyte line (FaO) [START_REF] Kaur | Docosapentaenoic acid (22:5n-3) down-regulates the expression of genes involved in fat synthesis in liver cells[END_REF], primary rat hepatocytes [START_REF] Pawar | Unsaturated fatty acid regulation of peroxisome proliferator-activated receptor ?? activity in rat primary hepatoctes[END_REF] or human hepatocyte line (HepG2) [66] caused an increase in EPA in cells and media, as well as DHA in media, but DHA was conversely not increased in human intestine (Caco-2) or monocyte (THP-1) cell lines [66].

Dietary n-3 DPA could also be used as a source of EPA. The EPA produced from n-3 DPA certainly comes from the retroconversion of n-3 DPA, implicating, as for DHA, the acyl-CoA oxidase and one peroxisomal β-oxidation step. This pathway was demonstrated in acyl-CoA oxidase deficient fibroblasts which do not led to n-3 LCPUFA [START_REF] Christensen | Peroxisomal beta-oxidation of polyunsaturated long chain fatty acids in human fibroblasts. The polyunsaturated and the saturated long chain fatty acids are retroconverted by the same acyl-CoA oxidase[END_REF]. In Sprague Dawley rats, we showed that the overall tissue fatty acid change following a dietary n-3 DPA supplementation was more similar to EPA supplementation than DHA supplementation [27]. In this study, the apparent retroconversion of n-3 DPA into EPA was particularly important in the kidney (68%), the liver (38%) and the spleen (20%). These results are confirmed by another study showing a higher apparent retroconversion of n-3 DPA in the kidney than in the liver in rats (50 mg/day by oral gavage) [START_REF] Holub | Oral docosapentaenoic acid (22:5n-3) is differentially incorporated into phospholipid pools and differentially metabolized to eicosapentaenoic acid in tissues from young rats[END_REF] (Figure 1). Finally, the apparent retroconversion of n-3 DPA to EPA was shown in humans [START_REF] Miller | A short-term n-3 DPA supplementation study in humans[END_REF], Sprague-Dawley rat [START_REF] Kaur | Short-term docosapentaenoic acid (22:5n-3) supplementation increases tissue docosapentaenoic acid, DHA and EPA concentrations in rats[END_REF], C57BL/6J-db/db mouse [START_REF] Gotoh | Effects of three different highly purified n-3 series highly unsaturated fatty acids on lipid metabolism in C57BL/KsJ-dbl db mice[END_REF], C57BL/6J mouse (high-fat diet) [13] and miniature poodle [START_REF] Alvarez | Docosapentaenoic acid is converted to docosahexaenoic acid in the retinas of normal and prcd-affected miniature poodle dogs[END_REF]. However, this retroconversion is still estimated relative to control without dietary n-3 DPA and there is a lack of labeled n-3 DPA monitoring studies to really quantify the importance of this pathway.

We recently investigated for the first time the tissue distribution of n-3 LCPUFA in 18 tissues, following a 3 week nutritional pure n-3 DPA supplementation (0.5% of the total fatty acids included in the diet) of healthy Sprague Dawley rats in post weaning [26]. The n-3 DPA content increased in half of the studied tissues and mostly in the spleen, lung, heart, liver and bone marrow from +50% to +110% compared to the control group. n-3 DPA was mostly retroconverted into EPA, especially in the liver (35%) and the kidney (46%) and slightly converted into DHA, showing an increased content of these two LCPUFA in affected tissues. Moreover, the n-3 DPA supplementation decreased total n-6 PUFA in affected tissues and especially n-6 DPA and adrenic acid (C22:4 n-6) [26]. In organs most affected by its supplementation, another study showed that dietary n-3 DPA was mainly incorporated into the phospholipid fractions, (phosphatidylethanolamines and phosphatidylcholines) [START_REF] Holub | Oral docosapentaenoic acid (22:5n-3) is differentially incorporated into phospholipid pools and differentially metabolized to eicosapentaenoic acid in tissues from young rats[END_REF].

To conclude, dietary n-3 DPA is mainly incorporated in major metabolic organs and esterified into the same lipid species than EPA and DHA. It is mainly retroconverted into EPA and slightly converted to DHA. It could therefore be considered as a source of EPA but also DHA to a lesser extent, implying potential physiological effects associated within these tissues.

Bioavailability

Only one study investigated the bioavailability (efficiency with which dietary n-3 DPA is used systematically through normal metabolic pathways) of dietary n-3 DPA. After oral administration of 2.5 µCi of 1-14 C-n-3 DPA, 1-14 C-EPA or 1-14 C-DHA in the rat housed in metabolic cage for 6h, 1-14 C-n-3 DPA was catabolized to 14 CO 2 in the same proportion as the 1-14 C-DHA (about 7% of the ingested dose) and less than the 1-14 C-EPA (about 18%). In addition, the percentage of the ingested radioactivity measured in the heart and muscle was similar from 1-14 C-n-3 DPA and 1-14 C-DHA and higher from 1-14 C-EPA. Conversely, the radioactivity found in liver, brain and kidney was similar from 1-14 C-n-3 DPA and 1-14 C-EPA but less than from 1-14 C-DHA [START_REF] Kaur | Orally administered [14C]DPA and [14C]DHA are metabolised differently to [14C]EPA in rats[END_REF].

The digestibility (difference between intake and excretion) of dietary n-3 DPA in ethyl ester form (96.6%) was similar than DHA (96.9%) and lower than EPA (98.3%) in post weaning Sprague Dawley healthy rats [27]. In another study, the excretion of n-3 DPA was 4.6 times greater than that of EPA after ingestion of 250 mg/day of n-3 DPA or EPA as free fatty acids for 3 days in rats [START_REF] Ghasemi Fard | Comparison of the bioavailability of docosapentaenoic acid (DPA, 22:5N-3) and eicosapentaenoic acid (EPA, 20:5n-3) in the rat[END_REF]. n-3 DPA was also preferentially hydrolyzed by porcine pancreatic lipase in vitro compared to EPA and DHA, suggesting a faster absorption [START_REF] Akanbi | Pancreatic lipase selectively hydrolyses DPA over EPA and DHA due to location of double bonds in the fatty acid rather than regioselectivity[END_REF].

Thus, the n-3 DPA seems slightly less absorbed than the other n-3 LCPUFA. Nevertheless, its digestibility remains greater than 95% regardless of its form of intake. More studies are needed to better address the n-3 DPA bioavailability compared to other n-3 LCPUFA.

Precursor of lipid mediators

Like DHA, n-3 DPA is a precursor of docosanoids whose physiological effects are however poorly known. Several metabolites of n-3 DPA are indeed discovered each year in different tissues, [10,[71][START_REF] Tungen | Total Synthesis of the Antiinflammatory and Pro-resolving Lipid Mediator MaR1 n -3 DPA Utilizing an sp 3 -sp 3 Negishi Cross-Coupling Reaction[END_REF][START_REF] Aursnes | Total synthesis of the lipid mediator PD1n-3 DPA: Configurational assignments and anti-inflammatory and proresolving actions[END_REF][74][START_REF] Dalli | Novel n-3 immunoresolvents: Structures and actions[END_REF]. The multiple lipid mediators identified from n-3 DPA and their deduced biosynthesis pathways are summarized in Figure 2. and di-hydroxylated derivatives from n-3 DPA. The 15-LOX pathway is the most efficient, converting 85% of the n-3 DPA to its 17S-hydro(peroxy) n-3 DPA derivative in tubo, compared to only 10% for 12-LOX and 5-LOX [START_REF] Dangi | Biogenic synthesis, purification, and chemical characterization of anti-inflammatory resolvins derived from docosapentaenoic acid (DPAn-6)[END_REF]. Some of these mono-hydroxylated metabolites as well as Resolvin D5 n-3 DPA and Maresin 1 n-3 DPA were detected in human serum after n-3 DPA supplementation for 7 days [START_REF] Markworth | Divergent shifts in lipid mediator profile following supplementation with n-3 docosapentaenoic acid and eicosapentaenoic acid[END_REF]. n-3 DPA can additionally undergo the induced action of cyclooxygenase-2 (COX-2), to form the 13R-hydroxy-7Z,10Z,13R,14E,16Z,19Z-n-3 DPA which will be able, as for derivatives from DHA, to be reduced to 13-oxo derivatives (EFOX). The 17-EFOX-D5 was for instance produced when aspirin was added to the culture medium of macrophages, as for EFOX derivatives from DHA [START_REF] Kuda | Bioactive metabolites of docosahexaenoic acid[END_REF]. The 13-serie resolvins from 13R-hydroxy-7Z,10Z,13R,14E,16Z,19Z-n-3 DPA (RvT1: 7,13R,20-tri hydroxy-n-3 DPA, RvT2: 7,12,13R-tri hydroxy-n-3 DPA, RvT3: 7,8,13R-trihydroxy-n-3 DPA, RvT4: 7,13R-dihydroxy-n-3 DPA) have been identified in co-incubations of neutrophils and endothelial cells [START_REF] Serhan | Protectins and maresins: New pro-resolving families of mediators in acute inflammation and resolution bioactive metabolome[END_REF]. These derivatives are formed by COX-2 then by S-nitrosylation. The cytochrome P450 can also metabolize n-3 DPA, but to a lesser extent than other n-3 LCPUFAs, although this result must be confirmed [START_REF] Dupuy | Simultaneous quantitative profiling of 20 isoprostanoids from omega-3 and omega-6 polyunsaturated fatty acids by LC-MS/MS in various biological samples[END_REF][START_REF] Faouder | LC-MS/MS method for rapid and concomitant quantification of pro-inflammatory and pro-resolving polyunsaturated fatty acid metabolites[END_REF]. Isoprostanes resulting from the peroxidation of n-3 DPA are not yet known.

Nevertheless, a series of isoprostanes derived from n-3 DPA have recently been described from the n-6 isomer of n-3 DPA [START_REF] De Las Heras-Gómez | Potential applications of lipid peroxidation products -F4-neuroprostanes, F3-neuroprostanesn-6 DPA, F2-dihomoisoprostanes and F2-isoprostanesin the evaluation of the allograft function in renal transplantation[END_REF]. Very few studies report on the overall distribution and action of oxygenated metabolites from n-3 DPA because of their very recent discovery and the lack of synthetic standards [START_REF] Serhan | Discovery of specialized pro-resolving mediators marks the dawn of resolution physiology and pharmacology[END_REF]. More information on the chemical synthesis, availability and biological effects of n-3 DPA-derived metabolites can be found in these reviews [10].

Biological and pharmacological effects of n-3 DPA

Inflammation and cancer

The decrease in inflammation associated with n-3 DPA seems to come mainly from these lipid mediators, and mainly the specialized pro-resolving mediator (SPM) (maresins, protectins, resolvins). Indeed, incubation of human macrophages with Protectin D1 n-3 DPA increased the monocyte differentiation, the phagocytic activity of macrophages and the apoptosis of neutrophils, which are key factors in the resolution of inflammation [71,76].

Moreover, the incubation of n-3 DPA-derived Maresin1 n-3 DPA also stimulated macrophage phagocytosis and clearance of human apoptotic neutrophils in a similar manner to DHA-derived Maresin1 [START_REF] Tungen | Total Synthesis of the Antiinflammatory and Pro-resolving Lipid Mediator MaR1 n -3 DPA Utilizing an sp 3 -sp 3 Negishi Cross-Coupling Reaction[END_REF]. In human inflammatory bowel disease colon biopsies, the Protectin D1 n-3 DPA and Resolvin D5 n-3 DPA increased [86]. These lipid mediators protected against colitis and intestinal inflammation in mice and decreased the extent of leukocyte adhesion and emigration post-stimulation. Contrary, the inhibition to their metabolic pathway (15-LOX) led to increased intestinal inflammation [86]. In a series of other studies, 13-series resolvins from n-3 DPA accelerated the resolution of inflammation and increased survival by 60% in Escherichia coli-infected mice [START_REF] Tungen | Total Synthesis of the Antiinflammatory and Pro-resolving Lipid Mediator MaR1 n -3 DPA Utilizing an sp 3 -sp 3 Negishi Cross-Coupling Reaction[END_REF][START_REF] Serhan | Protectins and maresins: New pro-resolving families of mediators in acute inflammation and resolution bioactive metabolome[END_REF][START_REF] Dalli | Elucidation of novel 13-series resolvins that increase with atorvastatin and clear infections[END_REF]. The independent effects of each 13-series resolvins from n-3 DPA were well described previously (supp. data) [START_REF] Dalli | Elucidation of novel 13-series resolvins that increase with atorvastatin and clear infections[END_REF]. In LPS-activated murine macrophage like RAW264.7 cells incubated with n-3 DPA, EPA or DHA, n-3 DPA increased EPA and DHA cell contents, down-regulated mRNA expression of pro-inflammatory factors (IL-6, IL-1β, iNOS, COX-2), and especially decreased IL-6 mRNA expression dose-dependently more than EPA and similarly than DHA [88]. Interestingly, the down-regulation of IL-6 and IL-1β mRNAs were similar when cells were incubated with an inhibitor of the delta-6 desaturase, demonstrating that n-3 DPA exhibited anti-inflammatory effects independent of DHA conversion [88].

Several studies from the same research team have shown that n-3 DPA-monoacylglyceride had greater antiinflammatory, anti-proliferative and pro-apoptotic effects than EPA-or DHA-monoacylglycerides [25], in a model of colorectal cancer [START_REF] Morin | Anti-proliferative effects of a new docosapentaenoic acid monoacylglyceride in colorectal carcinoma cells[END_REF], in a model of rheumatoid arthritis [START_REF] Morin | Eicosapentaenoic acid and docosapentaenoic acid monoglycerides are more potent than docosahexaenoic acid monoglyceride to resolve inflammation in a rheumatoid arthritis model[END_REF], and in pulmonary hypertension where n-3 DPA reduced the markers of inflammation and remodeled vascular pattern [START_REF] Morin | Docosapentaenoic acid monoacylglyceride reduces inflammation and vascular remodeling in experimental pulmonary hypertension[END_REF]. As a consequence, a prescription drug containing monoglyceride n-3 DPA is in the final stages of development by SCF Pharma for its anti-inflammatory and anti-proliferative properties and has been patented [START_REF] Vallières | Mitochondrial Ferredoxin Determines Vulnerability of Cells to Copper Excess[END_REF]. In healthy adult men, a high level of n-3 DPA was correlated with a lower inflammation score, mainly associated with decrease in C-reactive protein (CRP) and TNFα scores [START_REF] Labont� | Association of red blood cell n-3 polyunsaturated fatty acids with plasma inflammatory biomarkers among the Quebec Cree population[END_REF]. In another study, n-3 DPA reduced the expression of genes involved in the inflammation of blood vessel membranes [START_REF] Kishida | Docosahexaenoic acid enrichment can reduce L929 cell necrosis induced by tumor necrosis factor[END_REF]. Treatment of aortic endothelial cells with n-3 DPA strongly inhibited angiogenesis, implicated in tumor growth, inflammation, and microangiopathies [START_REF] Tsuji | Docosapentaenoic acid (22: 5, n-3) suppressed tube-forming activity in endothelial cells induced by vascular endothelial growth factor[END_REF]. A prospective study associated dietary n-3 DPA with a reduction of breast cancer risk (like EPA and DHA intake) [95].

On the other hand, n-3 DPA anti-inflammatory properties could benefit adults with comprised pulmonary health. n-3 DPA was positively associated with forced expiratory volume in the first second (FIV 1 ), forced vital capacity (FEV) and FEV 1 /FVC, modified by smoking and sex, in meta-analyses across seven cohorts (n=16,134) [96]. A recent epidemiological study has shown that the consumed n-3 DPA was the most fatty acid associated with a better average FEV 1 and slower FEV 1 decline in the smoking patient [42]. Interestingly, the FEV 1 decline from the adverse effect of continuous current smoking was completely negated with high n-3 DPA intake in the Lovelace Smokers cohort [42]. In human bronchus and guinea pig trachea preparations treated with n-3 DPAmonoacylglyceride, the higher concentration of n-3 DPA reduced the TNF-α/NF κB pathway, suppressed COX-2 expression, decreased the Ca 2+ sensitivity of bronchial explants and reversed the induced contractile reactivity [97].

Cardiovascular and metabolic diseases

The effect of n-3 DPA on the lipid parameters associated with the prevention of cardiovascular diseases is the most documented topic (anti-inflammatory properties, inhibition of cytokine synthesis, decrease in thrombosis, decrease in plasma lipids, inhibition of atherosclerosis...). Studies on the potential effect of n-3 DPA in the prevention of cardiovascular diseases in humans are usually association studies and concern the blood compartment. It has been shown that a high level of n-3 DPA in the red blood cell membranes in men and in women is associated with a lower risk of developing metabolic syndrome in Chinese adults [START_REF] Dai | Association between n-3 polyunsaturated fatty acids in erythrocytes and metabolic syndrome in Chinese men and women[END_REF]. In addition, a high plasma n-3 DPA (and DHA) level would also be the most correlated with a reduction of the risk of cardiovascular disease with plasma DHA level [51,[START_REF] Guo | n-3 Polyunsaturated Fatty Acids and Metabolic Syndrome Risk: A Meta-Analysis[END_REF][START_REF] Oda | Relationships between serum unsaturated fatty acids and coronary risk factors: negative relations between nervonic acid and obesityrelated risk factors[END_REF][START_REF] Rissanen | Fish oil-derived fatty acids, docosahexaenoic acid and docosapentaenoic acid, and the risk of acute coronary events: The Kuopio Ischaemic Heart Disease Risk Factor Study[END_REF][START_REF] Miura | Plasma eicosapentaenoic acid is negatively associated with all-cause mortality among men and women in a population-based prospective study[END_REF]. Moreover, in a pool of 19 cohort studies, n-3 DPA (in plasma and adipose tissue) was the only n-3 LCPUFA associated with a lower risk of total coronary heart disease, while all n-3 LCPUFA were associated with a lower risk of fatal coronary heart disease [START_REF] Del Gobbo | ω-3 Polyunsaturated fatty acid biomarkers and coronary heart disease: Pooling project of 19 cohort studies[END_REF]. In addition, a cross-sectional study found that decrease in RBC n-3 DPA concentrations (3.0% vs. 3.9%) was associated with an increased incidence in cardiovascular diseases [START_REF] Paganelli | Altered erythrocyte n-3 fatty acids in Mediterranean patients with coronary artery disease[END_REF]. Plasma n-3 DPA was also inversely correlated with arterial obstruction and stroke. n-3 DPA was the only fatty acid whose plasma level is inversely correlated with arterial obstruction in smokers [42,[START_REF] Leng | Plasma essential fatty acids, cigarette smoking, and dietary antioxidants in peripheral arterial disease. A population-based case-control study[END_REF]. In addition, a cross-sectional study of carotid ultrasonography showed an association between n-3 DPA consumption and carotid wall thickness reduction [START_REF] Hino | Very long chain N-3 fatty acids intake and carotid atherosclerosis: An epidemiological study evaluated by ultrasonography[END_REF]. Serum n-3 DPA (1 st vs 3 rd tertile) was the only n-3 LCPUFA inversely associated with the risk of orthostatic hypotension [START_REF] Nyantika | Serum long-chain omega-3 polyunsaturated fatty acids and risk of orthostatic hypotension[END_REF]. However, the association of n-3 DPA with the reduction of cardiovascular risk was moderate (but significant) [START_REF] Schumacher | Comparison of fatty acid intakes assessed by a cardiovascular-specific food frequency questionnaire with red blood cell membrane fatty acids in hyperlipidaemic Australian adults: A validation study[END_REF] and these results remain controversial because some studies didn't show any impact of n-3 DPA intake on these factors [11].

In another hand, in children, the highest RBC levels of n-3 DPA have also been associated with a decreased risk of pancreatic islet autoimmunity in children with type I diabetes [START_REF] Norris | Erythrocyte membrane docosapentaenoic acid levels are associated with islet autoimmunity: The Diabetes Autoimmunity Study in the Young[END_REF]. Moreover, plasma n-3 DPA and DHA were strongly and positively associated with insulin sensitivity using a global lipidomic approach in rats fed high-fat or high-fructose diets [START_REF] Huang | Docosapentaenoic acid and docosahexaenoic acid are positively associated with insulin sensitivity in rats fed high-fat and high-fructose diets[END_REF]. Serum n-3 DPA was influenced by lifestyle: in obese adolescent with cardiometabolic syndrome following a 1-year interdisciplinary therapy, changes in n-3 DPA were negatively associated with leptin and leptin/adiponectin ratio and positively with adiponectin [START_REF] Masquio | The role of free fatty acids in the inflammatory and cardiometabolic profile in adolescents with metabolic syndrome engaged in interdisciplinary therapy[END_REF]. Moreover, high-fat diets supplemented with n-3 DPA decreased serum adiponectin level in mice [13].

Some in vitro and in vivo studies, mainly in the rodent model, tried to decrypt the mechanisms by which n-3 DPA could help improving cardiovascular diseases risk markers. Compared with ALA, EPA or DHA contents, the rate of n-3 DPA in RBC was inversely and dose-dependently the most correlated with fasting plasma triglycerides in humans, after a 5 month supplementation with EPA + DHA [START_REF] Skulas-Ray | Red blood cell docosapentaenoic acid (DPA n-3) is inversely associated with triglycerides and C-reactive protein (CRP) in healthy adults and dose-dependently increases following n-3 fatty acid supplementation[END_REF]. Thus, several studies in animals have shown that n-3 DPA improved the lipid profile of plasma like EPA and DHA. In healthy rats fed 0.1% n-3 DPA (in energy) for 6 weeks, plasma total cholesterol, non-HDL cholesterol and cholesterol esters decreased compared to control group [26]. Compared to EPA and DHA supplementations in ethyl ester forms, n-3 DPA-fed rats (7.6 mg/day/kg.bw) were the only one with lower plasma triglycerides, total cholesterol, non-HDL cholesterol, total cholesterol/ HDL cholesterol ratio and cholesterol esters concentrations [27]. The n-3 DPA-supplemented group also increased its plasma total antioxidant status and superoxide dismutase activity like EPA and DHA fed-rats, with no change in complete blood count, white blood cell and splenocytes subpopulations [27]. In hamster fed high-cholesterol diets, supplementation with n-3 DPA (50 mg/day) reduced plasma total cholesterol and non-HDL cholesterol, associated with inhibition of SREBP2 mRNA, resulting in a decrease in the transcription of the HMG-CoA Reductase involved in cholesterol synthesis [START_REF] Chen | DPA n-3, DPA n-6 and DHA improve lipoprotein profiles and aortic function in hamsters fed a high cholesterol diet[END_REF]. Mice supplemented with dietary n-3 DPA showed a decrease in the activity of Fatty Acid Synthase (FAS), plasma total cholesterol and triglyceride concentrations [START_REF] Nagao | Comparison of the lipid-lowering effects of four different n-3 highly unsaturated fatty acids in HepG2 cells[END_REF]. A very interesting recent study in C57BL/6J mice fed high-fat diets (23% w/w) and supplemented with n-3 DPA (72% purity), pure EPA or pure DHA for 6 weeks showed that only n-3 DPA improved insulin resistance (HOMA) [13]. In this study, both n-3 DPA and DHA prevented the increase in serum alanine aminotransferase (ALT) levels, probably associated with inhibition of the TLR-4/NFκB signaling pathway and decreased liver lipogenesis. Another study showed that C57BL/KsJ-db/db mice receiving purified tri-n-3 DPA or DHA decreased their hepatic triglyceride levels significantly more than mice treated with EPA [START_REF] Gotoh | Effects of three different highly purified n-3 series highly unsaturated fatty acids on lipid metabolism in C57BL/KsJ-dbl db mice[END_REF]. In a human liver cell culture model (HepG2), the decrease in triglyceride synthesis after supplementation with n-3 LCPUFA in the medium was associated with a decrease in FAS mRNA expression and was classified as depending on the supplementations as follow: C24:6 n-3> DHA> n-3 DPA> EPA [START_REF] Nagao | Comparison of the lipid-lowering effects of four different n-3 highly unsaturated fatty acids in HepG2 cells[END_REF]. In a rat hepatocyte cell line model (FAO) treated with 50 µM EPA, n-3 DPA or DHA, n-3 DPA decreased most strongly the expression of HMG-CoA reductase, FAS, acetyl-CoA carboxylase 1 (ACC-1), SREBP1-c and ChREBP [START_REF] Kaur | Docosapentaenoic acid (22:5n-3) down-regulates the expression of genes involved in fat synthesis in liver cells[END_REF]. On the other hand, the concentration of postprandial plasma chylomicrons decreased in healthy women after a breakfast supplemented with n-3 DPA, compared to supplementation with EPA or olive oil [START_REF] Miller | A short-term n-3 DPA supplementation study in humans[END_REF]. Finally, it appears that the n-3 DPA is transported mainly in the triglyceride part of the chylomicrons and not in the PL after the evaluation period of 5 hours in humans [START_REF] Linderborg | Postprandial metabolism of docosapentaenoic acid (DPA, 22:5n-3) and eicosapentaenoic acid (EPA, 20:5n-3) in humans[END_REF].

Based on these studies, Matinas BioPharma has patented and placed on the market a prescription-only drug containing a mixture of n-3 DPA and EPA in the form of ethyl ester and containing only traces of DHA for the treatment of severe hypertriglyceridemia [START_REF] Maki | Effects of MAT9001 containing eicosapentaenoic acid and docosapentaenoic acid, compared to eicosapentaenoic acid ethyl esters, on triglycerides, lipoprotein cholesterol, and related variables[END_REF]. Compared to the ingestion of EPA ethyl esters in humans with severe hypertriglyceridemia (200-400 mg of triglycerides/dL plasma), this drug (MAT9001) helped reducing the plasma triglycerides, total cholesterol, non-HDL cholesterol and VLDL concentrations more significantly [START_REF] Maki | Effects of MAT9001 containing eicosapentaenoic acid and docosapentaenoic acid, compared to eicosapentaenoic acid ethyl esters, on triglycerides, lipoprotein cholesterol, and related variables[END_REF]. The lack of comparison with the ethyl ester of DHA nevertheless reduces the scope of these findings. n-3 DPA can also decrease platelet aggregation more significantly than EPA and DHA. n-3 DPA indeed inhibited collagen or ARA-stimulated platelet aggregation in a dose-dependent manner using rabbit platelets [START_REF] Akiba | Involvement of Lipoxygenase Pathway in Docosapentaenoic Acid-Induced Inhibition of Platelet Aggregation[END_REF] or human platelets, but this regulation appears sex-dependent in humans because only platelet aggregation in women were inhibited [START_REF] Phang | Eicosapentaenoic and Docosahexaenoic Acid Supplementations Reduce Platelet Aggregation and Hemostatic Markers Differentially in Men and Women[END_REF][START_REF] Phang | Inhibition of platelet aggregation by omega-3 polyunsaturated fatty acids is gender specific-Redefining platelet response to fish oils[END_REF]. n-3 DPA also increased the LOX pathway and may act as a strong inhibitor of COX-1 and COX-2 activities leading to decreased platelet aggregation and active aortic tension [START_REF] Chen | DPA n-3, DPA n-6 and DHA improve lipoprotein profiles and aortic function in hamsters fed a high cholesterol diet[END_REF][START_REF] Akiba | Involvement of Lipoxygenase Pathway in Docosapentaenoic Acid-Induced Inhibition of Platelet Aggregation[END_REF]. n-3 DPA also stimulated the migration of endothelial cells, whose migration and proliferation are processes involved in the control of the healing response of blood vessels [START_REF] Kanayasu-Toyoda | Docosapentaenoic acid (22:5, n-3), an elongation metabolite of eicosapentaenoic acid (20:5, n-3), is a potent stimulator of endothelial cell migration on pretreatment in vitro[END_REF]. In addition, the treatment of aortic endothelial cells with n-3 DPA inhibited their migratory activity due to the stimulation of vascular endothelial growth factor (VEGF) [START_REF] Kanayasu-Toyoda | Docosapentaenoic acid (22:5, n-3), an elongation metabolite of eicosapentaenoic acid (20:5, n-3), is a potent stimulator of endothelial cell migration on pretreatment in vitro[END_REF].

The supplementation of Sprague Dawley rats with echium oil, rich in stearidonic acid and potential source of n-3 DPA, showed an anti-arrhythmic action comparable to that obtained by supplementation with fish oils rich in DHA and was associated with tissue augmentation. of the n-3 DPA [START_REF] Abeywardena | Rise in DPA following SDA-rich dietary echium oil less effective in affording anti-arrhythmic actions compared to high DHA levels achieved with fish oil in Sprague-Dawley rats[END_REF].

The association between n-3 DPA blood status and prevention of cardiovascular and metabolic risks remains uncertain. Nevertheless, an even greater level of evidence is accumulating in favor of a specific effect of n-3 DPA on the improvement of risk factors associated with metabolic diseases, including improvements in blood lipid parameters, in platelet aggregation, in pro-resolution of inflammation, improvement of insulin sensitivity or modulation of adiponectin. On the other hand, these effects and their mechanisms remain to be elucidated in humans.

Neuroprotection and development

n-3 DPA is the most abundant n-3 LCPUFA in the brain after DHA and it could be specifically beneficial for neuroprotection and for depression prevention [START_REF] Dyall | Long-chain omega-3 fatty acids and the brain: A review of the independent and shared effects of EPA, DPA and DHA, Front[END_REF]. In a mouse model of epilepsy, Frigerio et al. interestingly showed that in the hippocampus of 72 post-status epilepticus mice, IL1β and TNFα transcripts (neuroinflammation markers) as well as 5-LOX and 15-LOX transcripts (key enzymes in pro-resolving mediator biosynthesis) were upregulated supporting the hypothesis that neuroinflammation in epileptogenesis could result from a failure to engage pro-resolving mechanisms. The authors then showed that some lipid mediator production derived from n-3 DPA were downregulated in the hippocampi of epileptogenic mice (Resolvin D2 n-3 DPA , Resolvin D5 n-3 DPA ), while Protectin D1 n-3 DPA was upregulated [START_REF] Frigerio | n-3 Docosapentaenoic acid-derived protectin D1 promotes resolution of neuroinflammation and arrests epileptogenesis[END_REF]. Moreover, the intracerebroventricular administration of Protectin D1 n-3 DPA in methylester form (20-200ng) during epileptogenesis dose-dependently controlled the onset and the propagation of neuroinflammation during epiletpgenesis in the hippocampus. Providing new leads for treatment, the authors also showed that the injection of Protectin D1 n-3 DPA improved weight recovery, decreased cognitive deficit, the frequency (-2-fold) and the average duration (-40%) of spontaneous seizures [START_REF] Frigerio | n-3 Docosapentaenoic acid-derived protectin D1 promotes resolution of neuroinflammation and arrests epileptogenesis[END_REF].

Elderly rats (20-22 months) supplemented with n-3 DPA, EPA, or purified monounsaturated fatty acids for 56 days had neuro-restorative benefits associated with decrease in microglial activation and oxidative stress in the hippocampus, two mechanisms involved in the loss of synaptic functions and therefore related to cognitive decline [START_REF] Kelly | The polyunsaturated fatty acids, EPA and DPA exert a protective effect in the hippocampus of the aged rat[END_REF]. Moreover, n-3 DPA (and n-6 DPA) inhibited sphingosylphosphorylcholine-induced Ca 2+ -sensitization of vascular smooth muscle contraction by inhibiting Rho-kinase activation and translocation to the cell membrane, a major cause of cerebrovascular vasospasm [START_REF] Zhang | Omega-3 and omega-6 DPA equally inhibit the sphingosylphosphorylcholine-induced Ca2+-sensitization of vascular smooth muscle contraction via inhibiting Rho-kinase activation and translocation[END_REF]. Some authors also hypothesized that RBC n-3 DPA content could be one of the diagnostic marker of Alzheimer's disease and one therapeutic target because RBC n-3 DPA decreased in cognitively normal elderly participants with high neocortical β-amyloid load [START_REF] Goozee | Alterations in erythrocyte fatty acid composition in preclinical Alzheimer's disease[END_REF]. Conversely, no association was found between serum n-3 DPA and performance on neuropsychological tests in an older population [START_REF] D'ascoli | Association between serum long-chain omega-3 polyunsaturated fatty acids and cognitive performance in elderly men and women: The Kuopio Ischaemic Heart Disease Risk Factor Study[END_REF].

While n-3 LCPUFA are well-known to decrease the risk of age-related macular degeneration, plasma level of n-3 DPA was the only n-3 LCPUFA associated with higher macular pigment optical density in subjects with family history of age-related macular degeneration [START_REF] Merle | Plasma long-chain omega-3 polyunsaturated fatty acids and macular pigment in subjects with family history of age-related macular degeneration: the Limpia Study[END_REF].

Concerning depression, purified n-3 DPA supplementation at 150 mg/kg/day for 6 days in rats resulted in a reduction of symptoms associated with depression and increased levels of cerebral n-3 DPA [START_REF] Laino | Fluoxetine potentiation of omega-3 fatty acid antidepressant effect: Evaluating pharmacokinetic and brain fatty acid-related aspects in rodents[END_REF]. In a prospective cohort study in aged subjects, the third quartile of n-3 DPA intake was also correlated with a reduced risk of major depressive disorder as well as EPA and fish intake. However, only n-3 DPA and fish intake remained significant when odd ratios were adjusted for cancer, myocardial infarction, stroke and diabetes [START_REF] Matsuoka | Dietary fish, n-3 polyunsaturated fatty acid consumption, and depression risk in Japan: a population-based prospective cohort study[END_REF]. RBC n-3 DPA (and both ALA and DHA) levels were also negatively associated with depression of postmenopausal women only if they used hormone therapy, suggesting an interaction between n-3 DPA and hormones on depression [START_REF] Jin | Association between erythrocyte levels of n-3 polyunsaturated fatty acids and depression in postmenopausal women using or not using hormone therapy[END_REF]. n-3 DPA is also present in non-negligible quantities in human and mammalian milk, so it could be involved in fertility [START_REF] Kokoli | Dietary omega-3 polyunsaturated fatty acids induce plasminogen activator activity and DNA damage in rabbit spermatozoa[END_REF], pregnancy [START_REF] Pinto | Serum n-3 polyunsaturated fatty acids are inversely associated with longitudinal changes in depressive symptoms during pregnancy[END_REF]and early-life development. Indeed, high n-3 DPA intake by lactating mothers was linked to better neuro-development and bone health of children [6]. Moreover, n-3 DPA blood levels in mothers were associated with lower allergic diseases in children and mothers. These findings were recently well reviewed [6] and are not detailed here.

Conclusion and prospects

An increasing number of association studies support the hypothesis that n-3 DPA is a bioactive fatty acid beneficial to human health. The suggested mechanisms involve the importance of n-3 DPA-derived lipid metabolites in the pro-resolution of inflammation in various models and mainly the importance of protectin D1 and resolvin D5 found in humans. Many other n-3 DPA-derived metabolites have also recently been identified and their potential physiological effects are not yet known. Likewise, in vivo and in vitro studies suggest that n-3 DPA is implied in the improvement of cardiovascular and metabolic disease risk markers, especially plasma lipid parameters, platelet aggregation, insulin sensitivity and cellular plasticity. Moreover, n-3 DPA is the most abundant n-3 LCPUFA in the brain after DHA and it could be specifically beneficial for elderly neuroprotection, and early-life development.

Nevertheless, there is still a lack of clinical intervention studies in humans to elucidate the specific biological effects of n-3 DPA and its underlying mechanisms and no studies are currently underway (search on ClinicalTrials.gov, September 21, 2018). The increase in commercial n-3 DPA availability as well as the efficiency and diversity of n-3 DPA purification methods should facilitate the implementation of new studies in the coming years.

While the effects associated with n-3 LCPUFA are studied independently, it remains difficult to differentiate the effects specific to n-3 DPA itself compared to those of EPA and DHA as they are biologically interconverted. For this, labeled-n-3 DPA monitoring studies are necessary to better understand the independent effects of n-3 DPA compared to other n-3 LCPUFA. In contrast, dietary n-3 DPA appears to be a good source of EPA and a low source of DHA in major metabolic organs, in addition to being well assimilated. The n-3 DPA could thus contribute to increasing the omega-3 status. Indeed, n-3 DPA is more present in meat than EPA or DHA and while the sources of fatty fish are limited, its food consumption is not negligible and n-3 DPA should surely be considered as well as DHA and EPA within the next nutritional recommendations. While food sources most often contain a mix of different n-3 LCPUFAs, they are studied independently, and it would be interesting to see if the impact of dietary n-3 DPA alone on omega-3 status is of nutritional interest. compared to a mixture of n-3 LCPUFA representative of human consumption.

  The 17S-hydroperoxy-n-3 DPA is first synthesized by the action of the 15-lipoxygenase. It can thereafter be converted either into 16S,17S-epoxy-7Z, 10Z, 12E, 14E, 19Z-n-3 DPA which will be converted by enzymatic hydrolysis into the family of Protectins n-3 DPA : Protectin 1 n-3 DPA (10R,17S-dihydroxy-7Z,11E,13E,15Z,19Z-n-3 DPA) and Protectin 2 n-3 DPA (16,17R-dihydroxy-7Z,10,13,14,19Z-n-3 DPA)[76,77]; or into 7,17S-dihydroperoxyn-3 DPA by the 5-LOX to give the family of Resolvins n-3 DPA : Resolvin D1 n-3 DPA (7,8,17S-trihydroxy-9,11,13,15E,19Z-n-3 DPA), Resolvin D2 n-3 DPA (7,16,17-trihydroxy-8,10,12,14E,19Z-n-3 DPA) and Resolvin D5 n-3 DPA (7S,17S-dihydroxydocosa-8E,10Z,13Z,15E,19Z-n-3 DPA) [75]. The maresins derived from n-3 DPA come from the action of the 12-LOX followed by the reduction of 14S-hydroperoxy-7Z,10Z,12E,16Z,19Z-n-3 DPA to 13,14S-epoxy n-3 DPA, itself enzymatically hydrolyzed to give the family of Maresins n-3 DPA : Maresin 1 n-3 DPA (7S,14S-dihydroxy-8E,10E,12Z,16Z,19Z-n-3 DPA), Maresin 2 n-3 DPA (13,14-dihydroxy-7Z,9,11, 16Z,19Z-n-3 DPA) and Maresin 3 n-3 DPA (14, 21-dihydroxy-7Z,10Z,12E,16Z,19Z-n-3 DPA). The 5-LOX can also produce mono-

Figure 1 -

 1 Figure 1 -Bioconversion pathways of n-3 and n-6 polyunsaturated fatty acid families∆: desaturase, ε: elongase, ALA: α-linolenic acid, ADA: adrenic acid, ARA: arachidonic acid, EPA: eicosapentaenoic acid, GLA: γ-linolenic acid, DHA: docosahexaenoic acid, n-3 DPA: n-3 docosapentaenoic acid, LA: linoleic acid, STA: stearidonic acid.

Figure 2 -

 2 Figure 2 -Biosynthesis pathway of n-3 docosapentaenoic acid-derived metabolites COX: cyclooxygenase, DPA: docosapentaenoic acid, EFOX: oxo derivative, isoP: isoprostane, LOX: lipoxygenase, HDPA: hydroxy-DPA, H(p)-DPA: hydro(peroxy)-DPA

Acknowledgments

We thank Pr. Thierry Durand (Institute of Biomolecules Max Mousseron, Montpellier, France) for the design of isoprostanes and Daniel Catheline (Laboratory of Biochemistry and Human Nutrition, Agrocampus Ouest, Rennes, France) for his help in the figure realization.

This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.