Béatrice Eymin

Camille Leduc

Jean-Luc Coll

Elisabeth Brambilla

Sylvie Gazzeri
email: sylvie.gazzeri@ujf-grenoble.fr

p14 ARF induces G 2 arrest and apoptosis independently of p53 leading to regression of tumours established into nude mice

Keywords: apoptosis, G 2 arrest, tumour regression, ARF, p53

Until recently, the ability of ARF (human p14 ARF , murine p19 ARF ) tumour suppressor protein, encoded by the INK4A/ARF locus, to inhibit cell growth in response to various stimuli was related to its ability to stabilize p53 through the so-called ARF/MDM2/p53 pathway. However, recent data have demonstrated that ARF is not implicated in this unique p53-dependent pathway. By use of transient and stable expression, we show here that human p14 ARF inhibits the growth of human tumoral cells lacking functional p53 by inducing a transient G 2 arrest and subsequently apoptosis. This p14 ARF -induced G 2 arrest was correlated with inhibition of CDC2 activity, inactivation of CDC25C phosphatase and induction of the CDK inhibitor p21 WAF1 . Apoptosis was demonstrated using Hoechst 33352 staining, proteolytic activation of caspase-3 and PARP cleavage. Similar results were obtained in experiments with cells synchronized by hydroxyurea block. Importantly, we were able to reproduce these effects "in vivo" by showing that p14 ARF inhibits the growth of p53 nullizygous human tumours in nude mice and induces the regression of p53 -/-established tumours. In these experiments, tumoral regression was associated with inhibition of cell proliferation as well as induction of apoptosis confirming the data obtained in cell lines.

Introduction

The INK4A/ARF locus encodes two structurally distinct proteins, p16 INK4a and ARF, translated from alternatively spliced mRNAs (Quelle et al., 1995;Sherr, 1998). Both products are expressed in response to inappropriate proliferative signals and activate pivotal pathways implicated in cell-cycle control and tumour suppression (Chin et al., 1998;Sherr, 1998;Sherr, 2001). The cyclin-dependent kinase inhibitor (CDK-I) p16 INK4a prevents phosphorylation and functional inactivation of the retinoblastoma (RB) protein by cyclin-dependent kinases (Serrano et al., 1993). On the other hand, ARF protects against cellular transformation and immortalization by activating the p53 tumour suppressive protein (Sherr, 1998). Expression of ARF is induced in response to activated oncogenes such as Ras (Palmero et al., 1998;Serrano et al., 1997), c-myc (Zindy et al., 1998), E1A (de Stanchina et al., 1998), Abl (Cong et al., 1999;Radfar et al., 1998), and E2F-1 (Bates et al., 1998;Dimri et al., 2000) as well as during replicative senescence (Sherr, 1998). Because ARF-null mice are highly tumour-prone (Kamijo et al., 1997) and die early in life (Kamijo et al., 1999), ARF has been proposed to be most important in tumour surveillance.

Consistent with its role as a tumour suppressor, ARF can induce cell cycle arrest in both G 1 and G 2 phases and/or cell death (Kamijo et al., 1997;Quelle et al., 1995;Stott et al., 1998). Until recently, the growth suppressive function of ARF was related to its ability to stabilize p53 leading to the transcriptional activation of well-known p53-target genes such as p21 WAF1 or 14.3.3σ (Weber et al., 2002). However, recent data have pointed to additional p53independent ARF functions on cell growth but the mechanisms involved remain unclear. In murine systems, p19 ARF can suppress colony formation of p53 -/-cells by affecting the RB pathway (Carnero et al., 2000). Furthermore, mice lacking both p53 and p19 ARF develop a wilder spectrum of tumours than animals lacking either gene alone, and reintroduction of p19 ARF in mouse embryo fibroblasts (MEF) lacking p53, MDM2 and p19 ARF induces cell growth inhibition (Weber et al., 2000). In human, p53-independent ARF activity has been recently ascribed to its ability to impede S-phase progression (Yarbrough et al., 2002) and to induce p53 and Bax-independent apoptosis (Hemmati et al., 2002).

Because we previously showed that human p14 ARF can induce growth inhibition of p53-null cells (Eymin et al., 2001), we were interested in better characterizing the pathway(s) involved in the p53-independent functions of p14 ARF . To this aim, we transiently or stably expressed p14 ARF in human carcinoma cell lines lacking functional p53. Our data show that p14 ARF induces a transient G 2 arrest followed by apoptosis in these p53-deprived cells. This G 2 arrest correlates with inhibition of CDC2 activity, inactivation of CDC25C phosphatase, a major upstream regulator of CDC2, and induction of the CDK-I p21 WAF1 . On the other hand, apoptosis is associated with activation of procaspase-3 and PARP cleavage. Importantly, we also demonstrate that p14 ARF inhibits the growth and induces the regression of p53 nullizygous human tumours established in nude mice through inhibition of cell proliferation and induction of apoptosis, demonstrating the "in vivo" relevance of our data.

Results p14 ARF induces cell cycle arrest in transiently transfected p53-deficient cells

In order to investigate the p53-independent functions of p14 ARF , we transiently transfected two p53-deficient human cell lines derived from bronchioloalveolar lung adenocarcinoma using a full lenght p14 ARF expression vector. In these cell lines, p53 was inactivated either by homozygous deletion (H358 cells) or single point mutation (H322 cells).

In both cell lines, p14 ARF strongly inhibited cell growth after 7 days of G418 selection, as a 50% decrease in cell number was observed upon p14 ARF transfection as compared to cells transfected with the empty vector (Figure 1A). In these cells, p14 ARF was expressed in the nucleus where it resided predominantly in the nucleoli (Figure 1B). In addition, dual-color flow cytometry revealed that both H358 and H322 cells accumulated with a G 2 /M DNA content when transfected with p14 ARF . Since on visual observation there was no evidence of increased mitotic cells content in this population (data not shown), this pattern is likely to represent a G 2 arrest. Altogether, these results demonstrate the ability of human p14 ARF to inhibit cell growth by inducing a G 2 arrest in cells deprived of functional p53.

Inducible p14 ARF expression leads to G 2 arrest in stable transfectants clones

In order to confirm the results of transient transfections, we then established stable, p14 ARF -inducible clones in the H358 cell line using the doxycyclin-inducible expression system (Tet-On). Of note, p14 ARF is expressed at a low level in H358 cells (Figure 1A).

Several stable transfectant clones were obtained and further analyses were performed on four of them. Since all clones were found to give similar results, representative data of only clones 19 and 38 are presented here. As shown in Figure 2A, a strong expression of p14 ARF was detected after 24 hours of doxycyclin induction, which persisted even after 9 days of continuous induction. In contrast, little or no expression of the exogenous p14 ARF was observed in the uninduced state (Figure 2A). In these stable inducible clones, expression of p14 ARF correlated with its nucleolar accumulation (Figure 2B). As shown in Figure 2C (upper panels), a strong cell growth inhibition was observed upon 9 days of p14 ARF induction in both clones. This growth inhibition was significant as early as 72 hours after doxycyclin induction in clone 19 while 6 days were necessary for the clone 38 (Figure 2C, lower panels). Growth inhibition was not detected in an empty tet-on control clone, even in the presence of doxycyclin. Since on the basis of western blot and immunofluorescence analyses, expression of p14 ARF was lower in clone 38 than in clone 19 (data not shown), the differences of inhibition kinetics between clones 19 and 38 was likely related to the different p14 ARF expression levels between the two clones. Consistent with these time course studies of cell growth inhibition, a G 2 /M arrest was observed following 72 hours of doxycyclin induction in clone 19, and 6 days in clone 38 (Figure 2D). Again, no increase in mitotic cells content was detected in these populations based on visual observation, in agreement with G 2 -arrested cells.

Interestingly, we noticed the presence of cells with a DNA content greater than 4n upon p14 ARF induction (data not shown). Taken together, these results confirm, in stable transfectants clones, the ability of p14 ARF to induce a G 2 arrest in a p53-independent manner.

Modulation of cell-cycle protein expression associated with p14 ARF -induced G 2 arrest

As entry in mitosis is timely controlled by the cyclin B 1 /CDC2 complex (Figure 3A; (O'Connell et al., 2000;Takizawa & Morgan, 2000;Weinert, 1997), we analyzed the ability of p14 ARF to modulate its activity. Cyclin B 1 /CDC2 complexes were recovered by immunoprecipitation using anti-CDC2 antibody and their ability to phosphorylate histone H1 "in vitro" was tested in p14 ARF -induced cells. A strong inhibition of cyclin B 1 /CDC2 activity was observed in p14 ARF -expressing cells as compared to uninduced cells (Figure 3B).

Consistent with our cell cycle data (Figure 2D), this inhibition was observed after 72h or 6 days of p14 ARF induction, and persisted during 6 or 9 days in clones 19 and 38 respectively (Figure 3B). Under the same conditions, we did not detect any significant change in cyclin B 1 and CDC2 expression levels in p14 ARF -induced cells immunoblotting when compared to uninduced cells (Figure 3C) and to H358/Tet-On control cells (data not shown).

Before mitosis, cyclin B 1 /CDC2 complexes are maintained in an inactive state by phosphorylation of CDC2 on its Thr14 and Tyr15 residues [Figure 3A; (Blasina et al., 1997;Coleman & Dunphy, 1994;Norbury et al., 1991;Ye et al., 1996)]. Dephosphorylation of both residues causes activation of CDC2 and pushes the cells toward mitosis [ (Takizawa & Morgan, 2000), for review]. In addition, full activation of CDC2 also requires the phosphorylation of a conserved threonine residue (Thr161) (Draetta, 1997;Russo et al., 1996). CDC2 phosphorylation status was then examined using specific antibodies recognizing either Tyr15 or Thr161 phosphorylated residues. As a specific phospho-Tyr14 antibody was not available, CDC2-P Thr14 status could not be studied. As compared to uninduced cells, a marked decrease of CDC2-P Thr161 level was observed in p14 ARF -expressing cells suggesting that CDC2 was hypophosphorylated on this residue (Figure 3C). In contrast, the same apparent level of CDC2-P Tyr15 was detected in both cases (with or without doxycyclin induction ; Figure 3C). As dephosphorylation of CDC2 at tyrosine 15 occurs only in mitotic cells, the level of CDC2-P Tyr15 in a population of asynchronous cells, which contains few mitotic cells, is expected to be high. Therefore, we cannot exclude that this could mask an increased level of phosphorylated CDC2 at Tyr15 residue in the population of p14 ARFexpressing cells principally arrested in G 2 . Taken together, these results suggest that p14 ARF maintains CDC2 into an inactive state, dephosphorylated at Thr161 residue.

Then, we investigated the status of some upper regulators of the cyclin B 1 /CDC2 complexes. At the onset of mitosis, dephosphorylation of both Thr14 and Tyr15 residues on CDC2 is regulated by the CDC25C phosphatase [Figure 3A; (Kumagai & Dunphy, 1991;Millar et al., 1991;Strausfeld et al., 1991)]. CDC25C activity is itself controlled by a complex set of activatory/inhibitory phosphorylations [ (Takizawa & Morgan, 2000), for review].

Throughout cell cycle, except in mitosis, CDC25C is maintained inactivated by phosphorylation on its Ser216 residue that creates a binding site for small phosphoserinebinding proteins called 14-3-3 which inactivate CDC25C by sequestration into the cytoplasm (Peng et al., 1997;Takizawa & Morgan, 2000). In p14 ARF -expressing cells, we detected a slight decrease in CDC25C protein level which was not observed in the H358/Tet-On control cells in the same conditions (data not shown), as well as an increase of its inactive phosphorylated form at Ser216 (Figure 3C andD). This effect was specific for CDC25C as the level of CDC25B and 14.3.3σ proteins did not appear to vary (data not shown). These data strongly suggest that p14 ARF inactivates CDC25C by reducing its total expression level and increasing its inactive phosphorylated form. Furthermore and consistent with a biological inactivation, a cytoplasmic accumulation of CDC2, CDC2-P Tyr15 and CDC25C-P Ser216 was observed in p14 ARF -expressing cells (Figure 3E). In contrast, such accumulation never occured in uninduced cells or in H358/Tet-On control cells in agreement with the property of these proteins to shuttle continuously from the cytoplasm to the nucleus of proliferating cells.

Finally, as CDC2 activity and phosphorylation status can also be controlled by the p21 WAF1 CDK-inhibitors family (Dulic et al., 1998;Harper et al., 1995;Niculescu et al., 1998;Smits et al., 2000), expression of some of them was analyzed. An increase of p21 WAF1 expression level was detected in p14 ARF -expressing cells (Figure 3C) whereas p27 KIP1 level remained unchanged (data not shown).

Overall, these results suggest that p14 ARF induces a G 2 arrest independently of p53 by preventing the activation of cyclin B 1 /CDC2 complexes, at least in part through inactivation of CDC25C phosphatase and induction of p21 WAF1 .

Inhibition of G 2 /M phase progression in synchronized p14 ARF -expressing cells

To confirm the molecular events implicated in the p14 ARF -induced G 2 arrest, synchronized cells were used. Hydroxyurea (HU) treatment was carried out to block the cells in late G 1 and S phases of the cell cycle, allowing a more precise analysis of cellular DNA content in late stages of the cell cycle after block release, particularly between S and mitosis.

In these experiments, p14 ARF was induced 24 hours before HU block. As shown in figures 4A and 4B, uninduced control cells accumulated with a G 2 /M DNA content until 9 hours after block release and started to enter mitosis about 13 hours after replating in fresh medium.

Twenty four hours after the release, they were predominantly in G 0 /G 1 (Figures 4A &4B). In contrast, and consistent with a G 2 /M arrest, a continuous accumulation of p14 ARF -induced cells in G 2 /M phases of the cell cycle was observed between 9 and 13 hours, and persisted even 24 hours (Figures 4A &4B) and 48 hours (data not shown) after the release. As expected, accumulation of control uninduced cells in G 2 /M 13 hours after the block release correlated with a slight upregulation of cyclin B 1 and CDC2 whereas CDC25C protein level did not change (Figure 4C). At this time, and consistent with passage through mitosis, CDC25C was dephosphorylated at Ser216 and CDC2 was activated by dephosphorylation at Tyr15 and phosphorylation at Thr161 residues (Figure 4C). In these control cells, p21 WAF1 was never induced. In contrast, although p14 ARF -expressing cells also accumulated with a G 2 /M DNA content 13 hours after the block release, CDC2 remained phosphorylated on Tyr15 and hypophosphorylated on Thr161 residues (Figure 4C). These data suggest that CDC2 was in an inactive state in these cells. In addition, concomittant downregulation of CDC25C (Figure 4C and D) and upregulation of inactive CDC25C-P Ser216 and of p21 WAF1 protein expression levels (Figure 4C) were also observed. As this specific pattern of protein expression persisted 24 hours (Figure 4C) and even 48 hours (data not shown) after the block release, these data are in agreement with the absence of passage through mitosis of most of the p14 ARF -expressing cells.

We noticed that a small proportion of cells was still able to overpass this G 2 arrest. This might be linked to the shorter time (24 hours) of p14 ARF induction in synchronization experiments compared to those performed on asynchronous cells.

Taken together, these data with synchronized cells confirmed the results obtained in asynchronous p14 ARF -expressing cells and demonstrate the ability of p14 ARF to induce a G 2 arrest independently of p53 by preventing cyclin B 1 /CDC2 activation, at least in part through inactivation of CDC25C phosphatase and induction of p21 WAF1 .

G 2 arrest precedes apoptosis in p14 ARF -expressing cells

Since DNA damage checkpoints may delay cell cycle progression and lead to apoptosis if the cells are unable to undertake repair (Elledge, 1996;Hartwell & Weinert, 1989;Walworth, 2000), we asked whether the p14 ARF -induced G 2 arrest might ultimately lead to apoptosis in our cells. Using Hoechst 33352 staining, we observed an apoptotic pattern in 20% of the p14 ARF -expressing cells after 6 or 9 days of doxycyclin induction in clones 19 and 38 respectively. In contrast, this was never detected in uninduced control cells (Figures 5A &5B), as well as in an inducible control clone, even after 15 days of continuous doxycyclin treatment (data not shown). Under the same conditions, the proteolytic activation of procaspase-3 with appearance of the active p12 and p17 fragments as well as PARP (poly-ADP ribose polymerase) cleavage were specifically observed in p14 ARF -induced cells (Figure 5C). Apoptosis was confirmed in HU-synchronization experiments, since sub-G 1 DNA content, caspase-3 activation and PARP cleavage were specifically detected in p14 ARFexpressing cells 48 hours after the block release (Figure 5D). Overall, these results demonstrate that p14 ARF is also able to induce apoptosis independently of p53. Moreover, as G 2 arrest was clearly observed before apoptosis could take place (Figures 5A &5D), our data suggest that G 2 arrest precedes apoptosis in p14 ARF -expressing cells.

Antitumoral activity of p14 ARF in nude mice is associated with cell cycle arrest and apoptosis

In order to determine whether the activity associated with p14 ARF expression in cells grown in culture could be reproduced in tumours, H358/Tet-On/p14 ARF clones 19 and 38 or H358/Tet-On cells were injected subcutaneously into immunodeficient nude mice. Of note, the growth kinetics of all cells was very similar (data not shown). As identical results were obtained using both clones, representative data of clone 19 are presented here.

We first tested whether the ability of p14 ARF to induce G 2 arrest and apoptosis in cell lines might influence the tumoral progression. Whereas the uninduced H358/Tet-On/p14 ARF cells (mice drinking water) gave rise to rapid tumour growth, a marked inhibition was observed in mice that received doxycyclin (p=0,0043) (Figure 6A). This effect was specific of p14 ARF induction since growth inhibition was not detected in mice injected with H358/Tet-On cells and that received doxycyclin (Figure 6A).

We then asked whether p14 ARF was able to prevent the growth of pre-existing tumours lacking p53. H358/Tet-On/p14 ARF cells were injected into nude mice and tumours were allowed to grow until they reach 50 to 70 mm 3 volume. At this time, doxycyclin was added or not in their drinking water. A doxycyclin treatment of mice with established H358/Tet-On/p14 ARF tumours caused a highly significant tumoral regression after 80 days of treatment (p=0.0043) when compared to mice with same tumours but drinking water only or to mice with established H358/Tet-On tumours and drinking doxycyclin (Figure 6B). This antitumoral activity became statistically significant after 35 days of doxycyclin treatment (p=0.0043).

Immunostaining revealed a clear accumulation of p14 ARF in H358/Tet-On/p14 ARF tumours arising from doxycyclin-treated mice as compared to untreated mice, as well as an inhibition of tumor cell proliferation as determined by KI-67 staining and increased apoptosis as detected by active caspase-3 (Figure 7A) or TUNEL staining (data not shown). When the proliferative index to the apoptotic index was calculated, a lower figure was observed (1.66 versus 4.66) in tumours overexpressing p14 ARF than in those with a physiological level of p14 ARF , in agreement with cell growth inhibition and apoptosis induction. Furthermore, by using western blot analyses on whole tumour extracts, we demonstrated a marked downregulation of total CDC25C which was not detected in the control H358/Tet-On tumours (data not shown) and an apparent stable level of CDC2-P Tyr15 (Figure 7B). However, we could not demonstrate any change in CDC25C-P Ser216 or p21 WAF1 protein levels and were unable to detect CDC2-P Thr161 in treated or untreated tumours (data not shown). On the other hand, clear activation of procaspase-3 and PARP cleavage were specifically observed in treated tumours confirming the existence of apoptosis (Figure 7B). Overall, these results demonstrate the ability of p14 ARF to inhibit cell growth and to induce apoptosis "in vivo".

Discussion

Until recently, the ARF tumour suppressive protein was only ascribed to a welldefined ARF/MDM2/p53 linear pathway. In this model, ARF induces the stabilization and activation of p53 leading to a dual block in G 1 and G 2 phases of the cell cycle and/or cell death (Kamijo et al., 1997;Quelle et al., 1995;Tao & Levine, 1999;Weber et al., 2002;Weber et al., 1999;Zhang & Xiong, 1999). In contrast and despite the existence of many contradictory data, it is now well admitted that ARF also acts onto p53-independent pathways to inhibit cell growth. However, the exact nature of the molecular mechanisms involved in these processes is still unclear and the targets remain unknown. In this study, we demonstrate that p14 ARFexpressing cells undergo a G 2 arrest in the absence of functional p53 suggesting that p53 might be necessary for G 1 checkpoint but dispensable for G 2 arrest in response to p14 ARF induction. Consistently, it was previously reported that p53 -/-cells can accumulate in G 2 following DNA damage (Fan et al., 1995;Li et al., 1995;Lowe et al., 1993;Powell et al., 1995). However, as a prolonged G 2 -M arrest requires p53 (Bunz et al., 1998), this could explain why p14 ARF induces only a transient G 2 arrest that precedes apoptosis in the absence of active p53. According to our data, the ability of p14 ARF to induce p53-independent apoptosis was recently described in p53-deleted SAOS-2 cells and HCT116 p53-/-colon cancer cells (Hemmati et al., 2002). However, cell cycle analysis was not performed in these cells. On the other hand, Weber et al. recently reported a strict dependency of an intact p53 pathway for p14 ARF -mediated cell cycle arrest following adenovirus infection (Weber et al., 2002).

However, in this last paper, all experiments were performed in a 24-48 hours time frame and no long-term effects were analyzed in contrast to our study. Therefore, as mentionned by the authors, p14 ARF might be able to induce arrest or apoptosis in a p53-negative background once activated over a longer time period.

A key step in regulating the entry of eukaryotic cells into mitosis is the activation of the cyclin B 1 -CDC2 complex [ (Takizawa & Morgan, 2000), for review]. Consistently, a strong inhibition of CDC2 activity was associated with G 2 arrest in p14 ARF -expressing cells.

When we investigated the mechanisms involved in this inhibition, we observed that CDC2 was markedly hypophosphorylated on Thr161 and apparently hyperphosphorylated on Tyr15.

The inhibitory phosphorylations on Tyr15 and Thr14 of CDC2 are counteracted by the CDC25C phosphatase (Kumagai & Dunphy, 1991;Millar et al., 1991;Strausfeld et al., 1991).

In our p14 ARF -expressing cells, the level of CDC25C protein was reduced. Total CDC25C protein level is not significantly affected during cell cycle progression (Goldstone et al., 2001) in contrast to its phosphorylation status. Therefore, it is unlikely that the specific decrease of CDC25C level following p14 ARF induction could represent the sole consequence of G 2 arrest. Interestingly, as for CDC25A which is downregulated during G 1 checkpoint in response to DNA damage (Mailand et al., 2000), downregulation of CDC25C has been already reported in response to UVB radiations (Athar et al., 2000), γ radiations (Wang et al., 2000) or infection by adenovirus (Raj et al., 2001) and related to a decrease of transcription (Wang et al., 2000) or a degradation by the proteasome complex (Raj et al., 2001). Whether these mechanisms could be implicated in the downregulation of CDC25C protein following p14 ARF induction remains to be determined. On the other hand, overexpression and cytoplasmic accumulation of the CDC25C-P Ser216 inactive form was observed upon p14 ARF induction.

Maintenance of the inhibitory phosphorylation of CDC2 on Thr14/Tyr15 mediated by a CHK-CDC25C pathway has been previously implicated in a G 2 arrest following DNA damage (Peng et al., 1997;Poon et al., 1997;Rhind et al., 1997;Sanchez et al., 1997). In such conditions, inactive phosphorylation of CDC25C on Ser216 by CHK1 and CHK2 kinases was observed, allowing binding to the proteins of the 14.3.3 family and preventing CDC2 activation (Furnari et al., 1997;Graves et al., 2000;Peng et al., 1997;Sanchez et al., 1997). In this way, it would be interesting to study the implication of CHK1 and CHK2 kinases in the p14 ARF -mediated phosphorylation of CDC25C on Ser216. Overall, although further studies are required to assess the role of CDC25C in p14 ARF -mediated G 2 arrest, we propose that p14 ARF might inactivate CDC25C by both down-regulating its global expression and increasing its inhibitory phosphorylation on Ser216, both mechanisms cooperating to reinforce the G 2 cell cycle checkpoint. Therefore, although we cannot exclude that Tyr15 (and Thr14) phosphorylations of CDC2 could simply be a result rather than a cause of cyclin B 1 -CDC2 inactivation linked to multiple feedback loops controlling the activity of p34 CDC2 complexes (Poon et al., 1997), it is reasonnable to assume that inactivation of CDC25C is implicated in the apparent accumulation of inactive CDC2-P Tyr15 upon p14 ARF expression.

Concomitantly with CDC25C inactivation, a strong induction of the cyclin-CDK inhibitor p21 WAF1 was observed in p14 ARF -expressing cells. Under normal growth conditions, p21 WAF1 protein accumulates in G 2 (Dulic et al., 1998;Li et al., 1994) and promotes a pause in late G 2 (Dulic et al., 1998). However, several studies have also demonstrated the efficient role of p21 WAF1 in G 2 checkpoint (Agarwal et al., 1995;Niculescu et al., 1998). Interestingly, Bunz et al. have reported that p21 WAF1 is essential to sustain rather than to initiate the G 2 checkpoint after DNA damage in human cells (Bunz et al., 1998). Despite its low affinity for the cyclin B 1 -CDC2 complexes (Dulic et al., 1998;Harper et al., 1995;Winters et al., 1998), p21 WAF1 can inhibit CDC2 activity (Yu et al., 1998) and has been recently involved in the inhibition of its activating phosphorylation on Thr161 (Smits et al., 2000). Whether p21 WAF1 affects directly or not the phosphorylation status of CDC2 and mediates some of the p53-independent effects of p14 ARF remains to be determined. Overall, although we are aware that further studies are required to extend the mechanisms by which p14 ARF inhibits cell growth and to clearly identify direct targets, our data suggest that p14 ARF induces a G 2 arrest independently of p53 by preventing CDC2 activity through both induction of p21 WAF1 and inactivation of CDC25C.

Finally and perhaps the most significant aspect of this work was the finding that p14 ARF conditional expression inhibits tumour growth and induces the regression of p53-/human tumours established in nude mice. Importantly, these biological events were associated with an inhibition of cell proliferation as well as an increased apoptosis. Furthermore, although we were unable to reproducibly detect variations in the level of p21 WAF1 and CDC25C-P Ser216 , CDC2, CDC2-P Tyr15 , CDC25C, caspase-3 and PARP protein levels varied similarly "in vitro" and "in vivo" strengthening our data on cell lines. Taken together, our results reinforce the notion that p14 ARF can affect pathways and targets other than p53 and demonstrate for the first time that these p53-independent effects can contribute to the tumour suppressive functions of p14 ARF "in vivo". They also predicts that loss of p14 ARF in tumours that have already inactivated p53, might confer an additive deleterious effect on cell growth. Consistently, Weber et al recently reported that tumours arising in mice lacking ARF and p53 are more aggressive than those in mice lacking either gene alone (Weber et al., 2000). Furthermore, we previously reported the double inactivation of p14 ARF and p53 in a large proportion of high grade NE lung tumours (Gazzeri et al., 1998), the most aggressive lung tumours, a concept that received now full agreement in numerous other cancer types (Esteller et al., 2000;Lindstrom et al., 2000;Sanchez-Cespedes et al., 1999). Therefore, reintroduction of p14 ARF in these tumours could restore some of the normal checkpoint pathways and thus constitute a major goal of future therapy.

Materials and methods

Cell lines, transfection, proliferation, apoptosis and synchronization assays

H358 and H322 human bronchioloalveolar lung carcinoma cell lines were cultured in RPMI-1640 medium (GIBCO, Cergy Pontoise, France) supplemented with 10% (v/v) heatinactivated FCS, 2 mM L-glutamine and 100 units/ml penicillin/streptomycin in 5% CO 2 at 37°C. To ensure exponential growth, cells were resuspended in fresh medium 24 hours before transfection. Transient transfections were carried out using FUGENE 6 (Roche Diagnostic) according to the manufacturer's protocol. The total amount of DNA used for transfection was held constant by adding empty control vector. In the growth suppression assay, geneticin (G418; 800 µg/ml) was added to the media 48h post transfection and trypan blue excluding cells were counted one week later. Cell cycle analysis of transient transfection experiments was performed 4 days after same geneticin treatment. For clonogenic assays using p14 ARF inducible clones, cells were fixed in 100% ethanol at the indicated times, stained with 1% methylene blue in 0.01 M Borate buffer, pH 8.5 and optical density was measured at 630 nm after elution in HCl 0.1N. Evaluation of apoptosis in stable transfectants clones was carried out after 48h, 72h, 6 days and 9 days of doxycyclin treatment (1 µg/ml) using Hoechst 33352 staining (5 µg/ml, 30 min at 37°C). The percentage of apoptotic cells, identified by their fragmented chromatin, was scored on at least 300 cells using fluorescence microscopy analysis. In the synchronized assays, exponentially H358/Tet-On/p14 ARF clones were pretreated during 24 hours with or without 1 µg/ml doxycyclin and further cultured in the presence of 1 mM hydroxyurea (HU) (Sigma). 24 hours after HU treatment, cells were washed twice with PBS and cultured in fresh medium in the presence or absence of doxycyclin for additional indicated times. Cells were then collected and further analyzed for DNA content and protein expression. Cell-cycle and western blot analyses were processed as described below.

Generation of stable p14 ARF inducible clones

Obtention of stable p14 ARF inducible clones was performed using a modified tetracyclin-regulated inducible expression system (Tet-On System, Clontech, Ozyme, Saint Quentin en Yvelines, France) according to the manufacturer's protocol. Briefly, H358 cells were transfected in a first round with the pTet-On plasmid which contains the reverse doxycyclin repressor (rtTA) expression cassette, using TFX-50 (Promega, Charbonnières, France) and transferrin (Tfe) (Becton Dickinson, le Pont de Claix, France), and selection of transfectants was performed in the presence of G418 (800 µg/ml). Stable H358/Tet-On clones were screened using the Promega Luciferase Assay System after transient transfections with pTRE-Luc plasmid. A second round of tranfection with pTRE-p14 ARF and pTK-Hyg plasmids was then carried out using FUGENE 6. The pTRE-p14 ARF plasmid was constructed by subcloning wild-type p14 ARF cDNA downstream of the Tet-regulated promoter into pTRE plasmid. Double transfectants were then selected in the presence of hygromycin (200 µg/ml) and geneticin (100 µg/ml), and screened for p14 ARF expression by western blotting following 48 hours of doxycyclin induction (1 µg/ml). Four clones were arbitrarily selected for further experiments.

Antibodies

Monoclonal antibodies (mAbs) used included anti-CDC2/CDK1 (Ab-3, Oncogene Research), anti-CDC2 p34 (sc-54, Santa-Cruz, TEBU, Le Perray-en-Yvelines), anti-CDC25C (67211A, Pharmingen), anti-phospho-CDC25C (Ser216) (9D1, Cell Signaling), anti-cyclin B 1 (14541A, Pharmingen), anti-human KI67 antigen (clone Ki-67, DAKO) and anti-p21 WAF1 (Ab-1, clone EA10, Oncogene Research). Polyclonal antibodies used included anti-actin (20)(21)(22)(23)(24)(25)(26)(27)(28)(29)(30)(31)(32)(33)Sigma,Saint Quentin Fallavier,France), anti-caspase-3 (65906E, Pharmingen), antihuman caspase-3 active (R&D System), anti-phospho-CDC2 (Tyr15) (Cell Signaling), antiphospho-CDC2 (Thr161) (Cell Signaling), anti p14 ARF (Ab-1, Oncogene Research) and anti-PARP (Roche Diagnostics).

Cell-cycle analysis

Cells were washed twice in PBS and fixed in ice-cold acetone for 5 min at -20°C.

After two PBS washes, cells were saturated with 2% BSA, 2% goat serum in PBS for 30 min at room temperature and incubated overnight at +4°C with gentle rotation in presence of anti-p14 ARF (Ab-1, 1/200). After extended washes in PBS, Alexa TM 488 goat anti-rabbit-IgG (H+L) conjugate (2 mg/ml; 1/500, Interchim, Montluçon) was added for 30 min at room temperature.

Cells were then washed twice in PBS, incubated at 37°C for 30 min with 200 U/ml of RNAse A (Sigma) and then with propidium iodide (10 µg/ml in PBS). Cell-cycle distribution of p14 ARF -expressing cells was determined by dual-color flow cytometry using the Cellfit software (Becton Dickinson, Grenoble, France).

Indirect immunofluorescence 50x10

3 cells/well were seeded onto 4-wells LAB-TEK R . At indicated times, cells were fixed in 4% paraformaldehyde for 5 min at room temperature (RT), washed three times, saturated in presence of 2% BSA, 2% goat serum in PBS for 30 min at RT and further incubated with anti-p14 ARF (Ab-1, Oncogene Research; 1/1000) for 2 hours. After three PBS washes, Alexa TM 488 goat anti-rabbit IgG (H+L) conjugate (2 mg/ml; 1/500, Interchim, Montluçon) was added for 30 min at RT and the cells were then whashed twice in PBS.

Normal rabbit IgG at the same concentration as the primary antibody served as a negative control. Cells were counterstained with Hoechst and observed using an Olympus microscope (20x magnification). Images were captured with a Coolview CCD camera (Photonic Science) and digitally saved using Visilog software.

Immunoblotting

Cells or frozen tumour tissues were lysed in RIPA buffer for 30 min on ice, pelleted and the protein concentration was determined using the Biorad Dc protein assay. 20-40 µg protein were then separated by SDS-PAGE in 7-12% gels and electroblotted onto PVDF membranes (Amersham Pharmacia, France). Membranes were incubated overnight at +4°C with primary antibody and proteins were detected using horseradish peroxidase-conjugated goat anti-mouse or anti-rabbit antibodies (Jackson ImmunoResearch Laboratories, West Grove, PA) and enhanced chemiluminescence detection system (Amersham, Les Ulis, france). To confirm the slight variation of CDC25C expression level, immunoblots were analyzed by densitometry using the NIH 1.6 software. Further quantification was performed by normalizing the signals with respect to that of the actin loading control. Results were expressed as a CDC25C/actin ratio which was arbitrarily normalized to a 100% expression level in uninduced cells at each time studied.

For cytoplasmic extracts, cells were incubated for 30 min into an ice-cold buffer containing 250 mM Sucrose, 20 mM HEPES-KOH, pH 7.5, 1.5 mM MgCl 2 , 1 mM EDTA, 1 mM EGTA, 1 mM DTT and protease/phosphatase inhibitors. Permeabilization was carried out by serial passages of the cell suspension through a 25G seringue until the obtention of 70-80% trypan blue positive cells. Centrifugation at 1700 rpm was then performed for 5 min and supernatants were ultracentrifugated for 30 min at 55.000 rpm in order to obtain the cytosolic fraction.

"In vitro" kinase reaction

Cells were washed three times with PBS, resuspended in lysis buffer (150 mM NaCl, 1mM EDTA, 1 mM EGTA, 50 mM Tris-HCl pH 7.4, 1% Triton 100X, 0.2 mM Na 3 VO 4 , 0.5% Nonidet P40, 0.1 mM PMSF, 2.5 µg/ml pepstatin, 10 µg/ml aprotinin, 5 µg/ml leupeptin), incubated on ice for 30 min and centrifugated for 20 min at 10.000 rpm. Protein concentration was adjusted to 200 µg in 200 µl NET buffer (150 mM NaCl, 50 mM Tris-HCl pH 7.5, 0.1% NP40, 1 mM EDTA pH 8, 0.25% gelatin, 0.02% sodium azide) supplemented with protease inhibitors before use and incubated overnight at +4°C with anti-cyclin B 1 or anti-CDC2 antibodies. 1 mg of protein A/protein G agarose beads was added to each reaction and samples were incubated for 1 hour at + 4°C with gentle rotation. After two washes in NET buffer, beads were subjected to a final wash in kinase buffer (50 mM Tris-HCl, pH 7.6, 10 mM MgCl 2 , 1 mM DTT). 10 µl aliquots were added to 40 µl of kinase buffer containing 50 µM ATP, 10 µCi (γ-32 P) ATP (3000 Ci/mmol) and 50 µg of histone H1 and incubated 30 min at 30°C. Samples were boiled in the presence of 50 µl SDS-PAGE loading buffer and subjected to electrophoresis on a 12% polyacrylamide gel. CDC2 activity was visualized by autoradiography.

Tumour treatment

"In vivo" experiments were conducted onto six-weeks-old female Swiss nude mice (Iffa Credo, Marcy l'Etoile, France). In all experiments, 20x10 6 H358/Tet-On/p14 ARF cells were injected subcutaneously into the nude mice and randomized into two groups, one receiving doxycyclin (2 mg/ml) in their drinking water and the second one receiving water

only. An additional group of nude mice injected with 20x10 6 H358/Tet-On cells and treated with doxycyclin was used as control. The tumour volumes were measured once a week in two perpendicular diameters and calculated as follows: a x b 2 x 0.4, where "a" and "b" are the largest and smallest diameters, respectively. All experiments were repeated twice for both 19 and 38 clones. In the tumour uptake assay, doxycyclin was provided immediately after cells injection. Each group was composed of 8 animals and difference in tumour volume was assessed using a non parametric Mann-Whitney t test. Study of tumour regression was performed on 10 animals per group that received doxycyclin or water when tumour volumes ranged from 50 to 70 mm 3 (about two or three weeks later). Mice were sacrified at differents times and tumours were divided into two parts before frozing in order to perform western blotting or immunohistochemistry. Statistical analysis between the tumour volumes of the different groups was performed using a non parametric Mann-Whitney t test on 6 animals per group.

Immunohistochemical staining

Immunostaining of p14 ARF , active caspase-3 and KI67 was performed on frozen sections, as previously described for p14 ARF and KI67 (Gazzeri et al., 1998;Labat-Moleur et al., 1998) and according to the manufacturer's protocol for caspase-3. Briefly, three step immunohistochemical method was applied on 5 µM thick frozen sections. After fixation, non specific binding sites were blocked by incubating the sections with 0.1% (w/v) BSA in phosphate-buffered saline (PBS) for 30 minutes at room temperature. The sections were subsequently incubated with primary antibodies overnight at 4°C, washed in PBS several times and exposed to biotinylated secondary antibodies consisting of either anti-rabbit biotinylated donkey F(ab')2 (1/1000; The Jackson Laboratory; West Grove; PA) or antimouse biotinylated donkey F(ab')2 (1/500; The Jackson Laboratory) for 1 hour at room temperature. Slides were then washed in PBS and incubated with the streptavidin-biotinperoxydase complex (1/200; Strept-AB complex; DAKO) for 1 hour at room temperature.

The slides were counterstained with Harris' hematoxylin. Normal rabbit or mouse IgG at the same concentration as the primary antibodies served as negative controls. H358/Tet-On/p14 ARF clone 19 was pretreated during 24 hours with (Dox +) or without (Dox -)

Legends to figures

1 µg/ml doxycyclin and further cultured during 24 hours in the presence of 1 mM hydroxyurea (HU). HU block was then released and cells were cultured in fresh medium with (Dox +) or without (Dox -) doxycyclin. Six hours (T6), 9 hours (T9), 13 hours (T13) and 24 hours (T24) after the block release, the cells were collected and processed for dual-color flow cytometry using the Cellfit software (Becton Dickinson) (A, B) and western-blot analysis (C).

Representative results of three independent experiments are presented. (D) Quantification of CDC25C expression level by densitometry in the presence (DOX+) or absence (DOX-) of doxycycline. Analysis was performed as explained in Figure 3D.

Figure

  Figure 1: p14 ARF inhibits cell growth and induces a G 2 arrest in transiently transfected p53-null cell lines.

Figure 2 :

 2 Figure 2: Inducible p14 ARF expression leads to G 2 arrest in stable transfectants clones. In all experiments, expression of p14 ARF was either uninduced (Dox -) or induced (Dox +) in H358/Tet-On/p14 ARF clones 19 and 38 in presence of 1 µg/ml of doxycyclin and experiments were performed at the indicated times. (A) Western blotting of p14 ARF protein expression in H358/Tet-On/p14 ARF clones 19 and 38. (B) Nucleolar accumulation of p14 ARF in both clones

Figure 4 :

 4 Figure 4: p14 ARF inhibits the G 2 /M phase transition in HU-synchronized cells.

Figure 5 :

 5 Figure 5: p14 ARF induces apoptosis independently of p53.In all experiments, H358/Tet-On/p14 ARF clones 19 and 38 were cultured with (Dox +) or

Figure 6 :

 6 Figure 6: p14 ARF inhibits tumour progression and induces tumour regression in nude

Figure 7 :

 7 Figure 7: p14 ARF inhibits cell proliferation and induces apoptosis in nude mice.H358/Tet-On/p14 ARF cells (clone 19) were sub-cutaneously injected into nude mice. After 3

G0

  + Dox -Dox + Dox -Dox + Dox -Dox + Dox -Dox +

Acknowledgements

This work was supported by a grant to E.B. from La Ligue Nationale Contre le Cancer as an « équipe labellisée » and by the Association pour la Recherche sur le Cancer. B.E. was supported by INSERM (Poste Accueil). We thank Christiane Oddou, Pascal Perron, Christine Claraz and Sylvie Veyrenc for technical assistance.

SD: standard deviation. (D) Cell-cycle analysis of the p14 ARF positive cells was performed using dual-color flow cytometry. Representative data of at least three independent experiments are shown. Similar results were obtained on the whole cell population after propidium iodide staining (data not shown).


The yin and the yang of p27 Kip1 as a target for cancer therapy B.Eymin and E. Brambilla Schematically, human tumors arise because tumor cells escape from extracellular signals that physiologically monitor cellular proliferation as well as cell death and acquire the property to grow continuously. During tumorigenesis, many proteins that normally control the proper timing of cell growth such as the cyclins or prevent the occurrence of unappropriate apoptosis are abnormally expressed or activated and act as typical oncogenes leading to hyperproliferation and/or protecting tumor cells from death. In contrast, proteins that normally prevent for example damaged cells to progress into the next phase of the cell cycle or act as apoptosis inducers if the damage can not be repaired are frequently inactivated in tumor cells and act as tumor suppressor genes. Therefore, loss of tumor suppressor genes associated with abnormal hyperactivity of oncogenes classically contribute to human carcinogenesis. However, affecting a clear-cut function of tumor suppressor or oncogene remains a subject of debate for certain proteins. p27 Kip1 is a member of the cyclin dependent kinase inhibitory proteins which also include p21 WAF1 , the target gene of p53, p16 INK4a , a member of the retinoblastoma (Rb) pathway and p57 Kip2 . With p16 INK4a , it plays a major role in counteracting the activity of cyclins D 1 and E on Rb phosphorylation and subsequent G 1 -S transition. Thus, it coordinates the activation of cyclin E-cdk2 with accumulation of cyclin D-cdk4 and initiates the timely exit of cells from the cell cycle in response to antimitogenic signals. Beside its classical anti-proliferative function, p27 Kip1 can also promote or inhibit apoptosis depending on the context (1,2). Based on these data, p27 Kip1 presents most of the classical features of a tumor suppressive protein that might be loss in proliferative tumor cells. Unlike most of the tumor suppressor genes which loss of function responds to the classical "two-hit hypothesis" of tumor suppression (Knudson law), p27 Kip1 gene is not deleted (neither loss of heterozygozity nor homozygous deletion) not mutant, and does not suffer aberrant methylation of its 5'-end in tumors. Rather, in mouse models, p27 Kip1 was shown to be a dosage dependent tumor suppressor since loss of a single allele of p27 Kip1 increases the frequency and decreases the latency of tumors to a level intermediate to that seen in wild-type and null counterparts (3). In human tumors, a direct correlation between p27 Kip1 protein levels and survival chances was first noted in colon cancer and later in cancers of the breast, prostate, bladder, ovary, lung and other tissues (for review, see 4). The evidence that reduced p27 Kip1 may contribute to tumor development by either increasing the proliferation of cells or decreasing their apoptosis might explain why the loss of p27 Kip1 is a common marker among many different tumor types and suggest that p27 Kip1 might be a valuable target for both stratifying patient risk and, perhaps, selecting treatment. However, observation that some human tumors exhibit high levels of p27 Kip1 makes this scheme more complex. For example, in aggressive small cell lung cancer, p27 Kip1 overexpression has been suggested to protect cells from apoptosis in unfavorable microenvironments [START_REF] Masuda | Protective function of p27(KIP1) against apoptosis in small cell lung cancer cells in unfavorable microenvironments[END_REF]. In addition, p27 Kip1 might affect the efficacy of chemotherapies as these treatments works better in proliferating rather than in growth-arrested cells. Therefore, we need to keep in mind that modulating p27 Kip1 levels can have unforeseen consequences on therapy depending on the cell type and the contextual clues. In this issue, Ishii and colleagues (6) have studied the consequences of p27 Kip1 overexpression or downregulation in human lung adenocarcinoma. They first show that overexpression of p27 Kip1 inhibits cell growth and rescues cells from death instead of inducing apoptosis. More interestingly, using siRNA technology, they demonstrate that neutralization of endogenous p27 Kip1 induces cell death of several non small cell lung cancer cell lines without affecting their cell cycle status. Finally, after analysis of p27 Kip1 subcellular localization and phosphorylation, they describe the predominant cytoplasmic accumulation of endogenous p27 Kip1 in these tumor cells which might preclude it from regulating the G 1 /S transition. p27 Kip1 activity appears to be directly targeted by its mislocalization to the cytoplasm in colon or ovarian tumors [START_REF] Ciaprrone | Localization and expression of p27Kip1 in multistage colorectal carcinogenesis[END_REF][START_REF] Hurteau | Expression and subcellular localization of the cyclin-dependent kinase inhibitor p27(KIP1) in epithelial ovarian cancer[END_REF] and cytoplasmic p27 Kip1 seems to correlate with poor-long term survival and tumor grades in breast carcinoma [START_REF] Viglietto | Cytoplasmic relocalization and inhibition of the cyclin-dependent kinase inhibitor p27Kip1 by PKB/Akt-mediated phosphorylation in breast cancer[END_REF]. Mislocalization effectively inactivates p27 Kip1 inhibitory activity, as cytoplasmic p27 Kip1 is partitioned from its nuclear cyclin-cdk targets. However, as suggested by Ishii and colleagues, such cytoplasmic localization of p27 Kip1 could also contribute to its anti-apoptotic functions. Therefore, before considering therapies, we need to predict how cells in a particular tumor will respond to accumulation or neutralization of p27 Kip1 . In this context, the study of Ishii and co-workers suggests that targeting p27 Kip1 in non small cell lung cancer could inhibit their growth by reducing cell viability. In contrast, in other tumor types, it will be the restoration of p27 Kip1 that will prevent tumor growth, either by inhibiting cellular proliferation or inducing apoptosis. So, depending on the cancer type, the goal of therapy targeting p27 Kip1 will tend to either neutralize or reintroduce it. It is the reason why comprehensive understanding of p27 Kip1 biology would facilitate development of such therapeutic responses in human cancer. So, it would be too simplistic to ascribe p27 Kip1 only as a tumor suppressive protein since its anti-apoptotic function might also predict an "oncogenic" role. As a typical example of this "gain of function" are the highly agressive small cell lung cancer which carry elevated levels of nuclear p27 Kip1 . Obviously, more need to be done to decipher the real contribution of p27 Kip1 in human lung tumor growth. In conclusion, it is now well established that the biological effects of numerous proteins might be quite different, sometimes even opposite, depending on the cellular model, the context as well as their subcellular localization. Therefore, before designing general therapies against cancer, we have to study very carefully the specific proteins involved in each carcinogenetic process as well as to investigate the molecular mechanisms by which the loss or hyperactivity of such proteins contribute to the carcinogenesis. Finally, we have to think about the fact that modulating the level of proteins that controls essential processes such as cellular proliferation and apoptosis might interfere with the response of tumor cells to chemotherapeutic treatments as well as to have unpredicted consequences.