A Cytoplasmic PML Mutant Inhibits p53 Function

[Cell Cycle 5:22, 2688-2692, 15 November 2006]; ©2006 Landes Bioscience

Report

A Cytoplasmic PML Mutant Inhibits p53 Function Cristian Bellodi1,4 Karin Kindle2 Francesca Bernassola3 Andrea Cossarizza4 David Dinsdale1 Gerry Melino1 David Heery2 Paolo Salomoni1,*

Abstract

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3IDI-IRCCS Biochemistry Lab; c/o Department of Experimental Medicine; University

of Rome; Tor Vergata, Rome Italy

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2School of Pharmacy; University of Nottingham; Nottingham UK

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1MRC Toxicology Unit; Leicester UK

4Department

The promyelocytic leukaemia gene (Pml) is a tumor suppressor identified in acute promyelocytic leukaemia (APL), where it is fused to RARa gene as a result of the chromo‑ somal translocation t(15;17). Pml encodes both nuclear and cytoplasmic isoforms. While nuclear PML has been intensively investigated, cytoplasmic PML proteins are less characterized. PML nuclear isoforms (nPML) are the essential components of sub‑nuclear structures referred to as PML nuclear bodies (PML‑NB). In response to cellular insults such as DNA damage and oncogenic activation, nPML modulates p53 activity through CBP‑mediated acetylation and activates its pro‑apoptotic and growth suppressive functions. Two missense mutations resulting in truncated PML cytoplasmic proteins (Mut PML) have been identified in aggressive APL cases. Here we report that cytoplasmic PML is able to induce the relocation of nPML to the cytoplasm, thus reducing the number of PML‑NBs. Remarkably, Mut PML inhibits p53 transcriptional, growth suppressive, and apoptotic functions, thus suggesting that cytoplasmic expression of PML has an impact on survival through inhibition of nuclear PML. Overall our findings shed new light on the role of PML cytoplasmic proteins in the regulation of p53.

of Biomedical Sciences; University of Modena and Reggio Emilia;

Modena Italy

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Key words

ACKNOWLEDGEMENTS

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p53, cell death, leukaemia, PML, cytoplasm

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We thank Vincenzo de Laurenzi, Martina Stagno D’Alcontres (MRC, Leicester), Keith Leppard (Warwick University, UK), Bruno Calabretta (Thomas Jefferson University, Kimmel Cancer Center, Philadelphia, USA) for reagents, protocols and useful discussion. We also thank Kulvinder Sikand for confocal microscopy analysis. This work is supported by the MRC.

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Previously published online as a Cell Cycle E-publication: http://www.landesbioscience.com/journals/cc/abstract.php?id=3504

The promyelocytic leukaemia gene PmL encodes a tumor suppressor protein that was initially identified in acute promyelocytic leukaemia (APL).1,2 The vast majority of APL cases are characterized by the reciprocal and balanced translocation t(15;17) that leads to the generation of the PML‑RARa fusion protein, which has a critical role in leukemo‑ genesis.1 PML‑RARa negatively affects the functions of both PML and RARa rendering leukemic blasts resistant to apoptosis and insensitive to differentiation mediated by the retinoic acid (RA).1,3,4 In line with this, inactivation of PmL in an animal model of APL results in increased incidence and early onset of the disease, thus indicating that PML could act as tumor suppressor.5 The Pml gene consists of nine exons that generate a number of spliced transcripts encoding nuclear and cytoplasmic isoforms.6 PML nuclear isoforms (nPML) are the essential components of sub‑nuclear complexes referred to as PML nuclear bodies (PML‑NB).6,7 An increasing number of proteins are found transiently or constitu‑ tively associated with PML‑NB; however, only a few directly interact with PML including p53, pRB, Daxx, CBP and eIF4E.1,8‑10 The PML‑NB component CREB‑binding protein (CBP) is an acetyltransferase acting as a nuclear receptor coactivator, and nuclear PML acts as a CBP cofactor.11,12 Upon oncogenic transformation, PML and CBP promote p53 tran‑ scriptional activation, a function exerted through the recruitment of p53 into PML‑NB and its sequential CBP‑mediated acetylation.1,9,10 Consequently, Pml‑/‑ cells are resistant to g‑radiation‑induced apoptosis and oncogene‑induced cellular senescence.8‑10 In APL, PML‑RARa inhibits p53 activity by promoting its deacetylation and degradation.13 Although the majority of studies focused on PML nuclear functions, accumulating evidence implies a potential role of PML in the cytosol. Firstly, a specific cytoplasmic PML isoform has been found associated with TGF‑b signalling pathway.14 In a murine plasmacytoma cell line a mutation of PML was identified, which results in the generation of a truncated, cytoplasmic PML protein (Mut ex3) displaying dominant negative proper‑ ties.15 One of PML nuclear isoforms, PML1, has been recently found to localize in the cytoplasm due to the presence of a nuclear export sequence.16 In APL cells, proteolytic cleavage of PML‑RARa by neutrophyl elastase causes cytoplasmic accumulation of the PML portion.17 Importantly, in a study conducted on a cohort of seventeen RA‑resistant APL cases two missense mutations 1272delAG and IVS3‑1 G → A were identified in

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Original manuscript submitted: 09/18/06 Manuscript accepted: 10/05/06

Introduction

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*Correspondence to: Paolo Salomoni; MRC Toxicology Unit; Lancaster Road; Box138; Leicester, Leicestershire LE1 9HN UK; Tel.: +44.116.2525568; Fax: +44.116.2525616; Email: [email protected]

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A Cytoplasmic PML Mutant Inhibits p53 Function

the remaining Pml allele, which are associated with a very aggres‑ sive course of the disease.18 Both mutations introduce a stop codon upstream the nuclear localization signal (NLS) of PML, which results in truncated PML proteins (Mut PML) that localize to the cyto‑ plasm.18 Finally, we have recently shown that Mut PML accumulates in cytoplasmic bodies and interacts with PML‑RARa. This results in potentiation of PML‑RARa‑mediated inhibition of RA‑dependent transcription and differentiation.19 In this study, we employed Mut PML as a tool to investigate the relationship between cytoplasmic PML and p53. Remarkably, we found that cytoplasmic PML mutants are able to inhibit p53 transcriptional, growth suppressive, and apoptotic functions possibly by sequestering nuclear PML and therefore inducing the disruption of the PML‑NB. Taken together, our observation shed new light on the functions of cytoplasmic PML proteins in the regulation of survival.

Results and Discussion p53 activation is in part dependent on nuclear PML (nPML) coactivating function,9 while the role of cytoplasmic isoforms is currently unknown. p53 is recruited into PML‑NB, where nPML promotes its activation through CBP‑mediated acetylation. We have recently demonstrated that an APL‑associated PML mutant (Mut PML) forms cytoplasmic bodies where PML‑RARa accumulates, and exacerbates the maturation block observed in APL cells.19 We employed this cytoplasmic mutant as a tool to investigate the relationship between cytoplasmic PML and p53. We set out to determine the effect of Mut PML expression on the localization pattern of nPML and on p53 function. We started by investigating the localization of nuclear PML in the presence of Mut PML. Remarkably, we found that in Mut PML‑transfected SAOS‑2 cells, both exogenous and endogenous nPML partially colocalized with Mut PML in PML‑CB (Fig. 1A upper and bottom panel, respectively). Moreover, transfected GFP‑PML accumulated in the cytoplasm of cells expressing Mut PML in live microscopy experi‑ ments (Fig. 1B). In haematopoietic cells, Mut PML is able to cause a dramatic reduction in the number of PML‑NBs and to delocalize nPML to the cytoplasm (Fig. 1C; middle panels). Furthermore, in a small number of Mut PML‑expressing cells, nPML was completely delocalized to the cytoplasm (Fig. 1C; bottom panels). Therefore, as p53 function is in part regulated by its association with nPML and the PML‑NB, it is conceivable that Mut PML could inhibit p53 by affecting the localization of nPML and other PML‑NB components, such as CBP.19 Since nPML is a strong p53 transcriptional activator,9 we set out to determine whether Mut PML negatively affected p53 transcriptional activity. To this purpose, we over‑expressed Mut PML along with p53 in SAOS‑2 cells. Interestingly, we found that the activity of a GADD45 reporter was clearly inhibited in the presence of Mut PML (Fig. 2A). In contrast, Mut PML did not affect the activity of an unrelated Myb reporter (Fig. 2D). In order to assess whether this effect was also evident in haematopoietic cells, we tested whether Mut PML could affect p53‑dependent transcription in haematopoietic cells. To this end, we infected HL60 cells with Mut PML or vector (pBABE) retroviruses. Similarly to the results obtained using SAOS‑2 cells, p53‑dependent activation of the Gadd45 reporter was impaired in Mut PML‑infected but not in control cells (Fig. 2B). Interestingly, over‑expression of nPML in SAOS‑2 cells completely rescued the inhibitory effects of Mut PML on p53‑de‑ pendent transcription, thus suggesting that Mut PML and nPML www.landesbioscience.com

Figure 1. Mut PML colocalizes with nuclear PML. (A) Mut PML colocalizes with nPML in SAOS2. Upper panel: SAOS‑2 cells were transduced with with FLAG‑tagged nPML and Mut PML and stained with anti‑FLAG (Sigma) (red) and anti‑HA (green) antibodies. Bottom panel: cells were transfected with control or Mut PML vectors and stained with anti‑PML antibody (Santa Cruz) (red). (B) Mut PML induces the delocalization of nPML in live cells. SAOS2 cells were transfected with nPML alone or with nPML and Mut PML. Cells were analyzed by live microscopy. (C) Mut PML colocalizes with nPML in hematopoietic cells. HL60 cells were infected with HA‑tagged Mut PML and control (pBABE) retroviruses. Control (upper panel) and Mut PML infected cells were cytospun and stained with anti‑PML (Chemicon), which does not crossreact with Mut PML, and anti‑HA antibodies. Nuclei were counterstained using DAPI and slides were analyzed by confocal microscopy.

could counteract each other (Fig. 2C). We next studied the effect of Mut PML over‑expression on p53 cellular localization. Coexpressed p53 and nPML partially colocalized in PML‑NB (not shown), as previously reported.9 Conversely, expression of Mut PML did not

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Figure 2. p53 transcriptional activaty is inhibited by Mut PML. (A) Mut PML inhibits p53 in SAOS‑2 cells. Cells were transiently transfected with the following plasmids (100 ng unless otherwise stated): pCH110 (b‑galactosidase repoter), p53, 500 ng pGADD45‑Luc, 1000 ng of HA‑MutPML expression vectors. Luciferase activity was measured 36 hrs after transfection by using Dual Light System kit (Applied Biosystem). Transcriptional assays values are means ± standard deviations of three separate experi‑ ments performed in triplicate. (B) Mut PML inhibits p53 in hematopoietic cells. HL60 cells were infected with MutPML or control (pBABE) viral particles. After selection with puro‑ mycin pBABE and MutPML expressing clones were trans‑ fected with 500 ng p53, 2500 ng pGADD45‑Luc along with 100 ng of the TK‑Renilla plasmid (Promega) by means of nucleofection (AMAXA) Amaxa Nucleofector system (amaxa GmbH) according to the manufacturer’s instruc‑ tions. Luciferase activity was assayed 8 hrs after transfec‑ tion using the dual luciferase assay system (Promega). (C) Over‑expression of nPML (PML isoform IV) rescues MutPML inhibition of p53 trascriptional activity. Different combina‑ tions of p53, pCH110, 500 ng pGADD45‑Luc, 500 ng of nPML and MutPML were cotransfected in SAOS‑2 cells, and luciferase activity was evaluated. (D) The activity of a Myb reporter (MIM1) construct is not regulated by Mut PML in SAOS2 cells. Different combinations of p53, pCH110, 500 ng MIM1 and 500 ng of Mut PML were introduced in SAOS2 cells, and luciferase was measured.

Figure 3. Mut PML inhibits p53 biological functions. (A) Mut PML inhibits p53 growth suppressive function in colony forming assays. H1299 cells were transfected with different combination of p53, Mut PML, HDM2 expressing vectors, and colonies were stained with crystal violet and counted 15 days after transfection. Right panel shows mean of three independent experiments (expressed as percentage of increase in colony formation) ± standard deviations. (B) Mut PML inhibits p53‑dependent cell death. H1299 cells were transfected with different combinations of p53, Mut PML and HDM2 expressing vectors, and cell death was evaluated 24 hours after by trypan blue exclusion assay.

result in p53 relocation to the cytoplasm (not shown). This was not unexpected, because Mut PML lacks the carboxy‑terminal portion responsible for the interaction with p53. In order to test whether Mut PML could affect p53 growth suppressive functions, we performed colony‑forming assays in p53‑deficient H1299 cells transfected with p53 alone or in combination with Mut PML, nPML and, as a control, HDM2, which is one of the major negative regulators of p53.20 Cells transfected with p53 and Mut PML formed a significantly higher number of colonies compared to p53 only‑transduced cells (Fig. 3A, left panel). Importantly, Mut PML inhibited p53 growth suppressive activity to an extent similar to HDM2 (Fig. 3A). Mut PML is also able to inhibit p53‑dependent cell death, as in cells cotransfected with 2690

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(Fig. 4A). The observed effects were not due to differences in protein levels as the expression of H‑RasV12 was comparable in the two cell populations analyzed (Fig. 4B). Taken together our findings clearly demonstrate that cytoplasmic delocalization of PML results in p53 inhibition in different experi‑ mental settings. We propose that Mut PML exerts its inhibitory functions by relocating nPML and other important p53 coactivators to the cytoplasm. It is therefore conceivable that the balance between nuclear and cytoplasmic PML isoforms could represent an important regulatory switch for p53 activation. In this respect, a recent paper demonstrated that PML is found in cytoplasmic bodies in early G1 before entering the nucleus to reform PML‑NBs.22 This suggests that high levels of cytoplasmic PML could block nuclear PML isoforms from entering the nucleus after mitosis. It would also be interesting to test whether cytoplasmic forms of PML‑RARa19 are also able to delocalize nuclear PML and to affect p53 function. It has to be said that we cannot exclude the possibility that cytoplasmic PML could also affect transcription‑independent functions of p53. In this regard, a recent study revealed the impor‑ tance of cytoplasmic p53,23 thus suggesting that Mut PML could also affect this pathway. Remarkably, the analysis of PML localiza‑ tion in human cancer revealed that PML is found in cytoplasmic granules/bodies in a high percentage of hepatocellular carcinomas and in skin carcinomas, suggesting a role for cytoplasmic PML in cancer.16,24 Nevertheless, it is unclear what are the mechanisms responsible for PML delocalization in cancer and what is the relation‑ ship between PML delocalization and p53 mutation status. This and other outstanding questions remain to be addressed in order to gain a better understanding of the biological and pathological implications of PML cytoplasmic localization. References Figure 4. Mut PML inhibits H‑RasV12‑induced cellular senescence. (A) MEFs were infected with different combinations of H‑RasV12 �� and Mut PML retrovi‑ ruses, and cellular senescence was evaluated by measuring SA‑bGal activity by using the senescence b‑galactosidase staining kit (Cell Signaling) at day 1 (d1) and 2 (d2; no further increase in staining was detected at later days). Upper panel shows a representative image of infected cells; lower panel shows means of fold induction over empty vector infected cells ± standard deviations. (B) Levels of Ras are not influences by Mut PML expression. Western blot analysis showing the levels of H‑RasV12 and Mut PML in the cells.

p53 and Mut PML the percentage of cell death was significantly reduced (Fig. 3B). Finally, we set out to determine whether Mut PML inhibits p53 function in more physiological settings. To do this, we assayed the effects of Mut PML expression on oncogene‑induced senescence in primary mouse embryo fibroblasts (MEF), where this phenomenon is entirely p53‑dependent.21 In particular, it has been shown that nPML promotes CBP‑mediated activation of p53 in response to oncogenic transformation, and this results in induction of premature cellular senescence.10 We infected MEF using an oncogenic form of H‑Ras (H‑RasV12) in combination with Mut PML viral particles. After puromycin selection, cellular senescence was evaluated at different days by measuring acidic b‑galactosidase (SA‑bGal) activity. Remarkably, we found that the number of SA‑bgal positive cells was significantly lower in cells cotransduced with H‑RasV12 and Mut PML compare to H‑RasV12 infected cells at both day one and two, thus indicating that Mut PML could phenocopy PML inactivation www.landesbioscience.com

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A Cytoplasmic PML Mutant Inhibits p53 Function 16. Condemine W, Takahashi Y, Zhu J, Puvion‑Dutilleul F, Guegan S, Janin A, de The H. Characterization of endogenous human promyelocytic leukemia isoforms. Cancer Res 2006; 66:6192‑8. 17. Lane AA, Ley TJ. Neutrophil elastase cleaves PML‑RARalpha and is important for the development of acute promyelocytic leukemia in mice. Cell 2003; 115:305‑18. 18. Gurrieri C, Nafa K, Merghoub T, Bernardi R, Capodieci P, Biondi A, Nimer S, Douer D, Cordon‑Cardo C, Gallagher R, Pandolfi PP. Mutations of the PML tumor suppressor gene in acute promyelocytic leukemia. Blood 2004; 103:2358‑62. 19. Bellodi C, Kindle K, Bernassola F, Dinsdale D, Cossarizza A, Melino G, Heery D, Salomoni P. Cytoplasmic function of mutant PML and PML‑RARalpha. J Biol Chem 2006. 20. Oren M. Regulation of the p53 tumor suppressor protein. J Biol Chem 1999; 274:36031‑4. 21. Serrano M, Lin AW, McCurrach ME, Beach D, Lowe SW. Oncogenic ras provokes pre‑ mature cell senescence associated with accumulation of p53 and p16INK4a. Cell 1997; 88:593‑602. 22. Dellaire G, Eskiw CH, Dehghani H, Ching RW, Bazett‑Jones DP. Mitotic accumulations of PML protein contribute to the reestablishment of PML nuclear bodies in G1. J Cell Sci 2006; 119:1034‑42. 23. Chipuk JE, Bouchier‑Hayes L, Kuwana T, Newmeyer DD, Green DR. PUMA couples the nuclear and cytoplasmic proapoptotic function of p53. Science 2005; 309:1732‑5. 24. Terris B, Baldin V, Dubois S, Degott C, Flejou JF, Henin D, Dejean A. PML nuclear bodies are general targets for inflammation and cell proliferation. Cancer Res 1995; 55:1590‑7.

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