PHAPI/pp32 Suppresses Tumorigenesis by Stimulating Apoptosis

Abstract

PHAPI/pp32 is a tumor suppressor whose expression is altered in various human cancers. Although PHAPI possesses multiple biochemical activities, the molecular basis for its tumor-suppressive function has remained obscure. Recently we identified PHAPI as an apoptotic enhancer that stimulates apoptosome-mediated caspase activation. In this study, we defined the structural requirement for its activity to stimulate caspase activation using a series of truncation mutants of PHAPI. Further, utilizing these mutants, we provide evidence to support the model that the apoptotic activity of PHAPI is required for its tumor-suppressive capability. Consistently, pp32R1, a close homolog of PHAPI and yet an oncoprotein, is not able to stimulate caspase activation. A highly discrete region between these two proteins localizes to an essential caspase activation motif of PHAPI. Additionally, PHAPI is predominantly a nuclear protein, and it can translocate to the cytoplasm during apoptosis. Disruption of the nuclear localization signal of PHAPI caused a modest decrease of its tumor-suppressive function, indicating that nuclear localization of PHAPI contributes to, but is not essential for, tumor suppression. PHAPI/pp32 is a tumor suppressor whose expression is altered in various human cancers. Although PHAPI possesses multiple biochemical activities, the molecular basis for its tumor-suppressive function has remained obscure. Recently we identified PHAPI as an apoptotic enhancer that stimulates apoptosome-mediated caspase activation. In this study, we defined the structural requirement for its activity to stimulate caspase activation using a series of truncation mutants of PHAPI. Further, utilizing these mutants, we provide evidence to support the model that the apoptotic activity of PHAPI is required for its tumor-suppressive capability. Consistently, pp32R1, a close homolog of PHAPI and yet an oncoprotein, is not able to stimulate caspase activation. A highly discrete region between these two proteins localizes to an essential caspase activation motif of PHAPI. Additionally, PHAPI is predominantly a nuclear protein, and it can translocate to the cytoplasm during apoptosis. Disruption of the nuclear localization signal of PHAPI caused a modest decrease of its tumor-suppressive function, indicating that nuclear localization of PHAPI contributes to, but is not essential for, tumor suppression. Apoptosis plays a crucial role in multiple physiological events, and its deregulation can lead to various human diseases, including cancer, neurodegeneration, and autoimmune disorders (1Kerr J.F. Wyllie A.H. Currie A.R. Br. J. Cancer. 1972; 26: 239-257Crossref PubMed Scopus (12874) Google Scholar, 2Horvitz H.R. Cancer Res. 1999; 59: 1701s-1706sPubMed Google Scholar). Apoptosis is executed by a family of proteases known as caspases (3Thornberry N.A. Lazebnik Y. Science. 1998; 281: 1312-1316Crossref PubMed Scopus (6159) Google Scholar, 4Budihardjo I. Oliver H. Lutter M. Luo X. Wang X. Annu. Rev. Cell Dev. Biol. 1999; 15: 269-290Crossref PubMed Scopus (2269) Google Scholar). In mammals, a major caspase activation pathway is the mitochondrial cytochrome c-mediated caspase activation pathway, also known as the intrinsic apoptotic pathway (5Jiang X. Wang X. Annu. Rev. Biochem. 2004; 73: 87-106Crossref PubMed Scopus (1128) Google Scholar). In this pathway, a variety of apoptotic stimuli induce release of cytochrome c from mitochondria to the cytoplasm, a process controlled by the Bcl-2 family members (6Chao D.T. Korsmeyer S.J. Annu. Rev. Immunol. 1998; 16: 395-419Crossref PubMed Scopus (1517) Google Scholar, 7Harris M.H. Thompson C.B. Cell Death Differ. 2000; 7: 1182-1191Crossref PubMed Scopus (438) Google Scholar, 8Adams J.M. Cory S. Trends Biochem. Sci. 2001; 26: 61-66Abstract Full Text Full Text PDF PubMed Scopus (811) Google Scholar). The released cytochrome c binds to Apaf-1, the essential mediator of the pathway, and triggers its nucleotide binding/exchanging activity (9Jiang X. Wang X. J. Biol. Chem. 2000; 275: 31199-31203Abstract Full Text Full Text PDF PubMed Scopus (416) Google Scholar, 10Kim H.E. Du F. Fang M. Wang X. Proc. Natl. Acad. Sci. U. S. A. 2005; 102: 17545-17550Crossref PubMed Scopus (238) Google Scholar). Subsequently, the activated Apaf-1 protein forms a multimeric protein complex called the apoptosome, which in turn recruits and activates the initiator caspase, caspase-9. The active apoptosome-caspase-9 holoenzyme (9Jiang X. Wang X. J. Biol. Chem. 2000; 275: 31199-31203Abstract Full Text Full Text PDF PubMed Scopus (416) Google Scholar, 11Rodriguez J. Lazebnik Y. Genes Dev. 1999; 13: 3179-3184Crossref PubMed Scopus (461) Google Scholar) then cleaves and activates the effector caspases, including caspase-3 and caspase-7, and therefore causes apoptotic cell death. The cytochrome c-mediated caspase activation pathway is under precise regulation. For example, in the upstream of mitochondria, there are both pro-apoptotic and anti-apoptotic Bcl-2 family proteins that dictate mitochondrial cytochrome c release (6Chao D.T. Korsmeyer S.J. Annu. Rev. Immunol. 1998; 16: 395-419Crossref PubMed Scopus (1517) Google Scholar, 7Harris M.H. Thompson C.B. Cell Death Differ. 2000; 7: 1182-1191Crossref PubMed Scopus (438) Google Scholar, 8Adams J.M. Cory S. Trends Biochem. Sci. 2001; 26: 61-66Abstract Full Text Full Text PDF PubMed Scopus (811) Google Scholar). In the downstream of mitochondria, inhibitor of apoptosis proteins negatively regulate apoptosis by inhibiting caspase activity (12Duckett C.S. Nava V.E. Gedrich R.W. Clem R.J. Van Dongen J.L. Gilfillan M.C. Shiels H. Hardwick J.M. Thompson C.B. EMBO J. 1996; 15: 2685-2694Crossref PubMed Scopus (523) Google Scholar, 13Uren A.G. Pakusch M. Hawkins C.J. Puls K.L. Vaux D.L. Proc. Natl. Acad. Sci. U. S. 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Moritz R.L. Simpson R.J. Vaux D.L. J. Biol. Chem. 2002; 277: 445-454Abstract Full Text Full Text PDF PubMed Scopus (478) Google Scholar), which are also released from mitochondria during apoptosis. Further, nucleotide binding/exchanging of Apaf-1, a key biochemical event of the pathway, is also closely regulated. Recently, we found that the Apaf-1-dependent caspase activation can be enhanced by PHAPI (putative HLA-associated protein-I, also known as pp32) (20Jiang X. Kim H.E. Shu H. Zhao Y. Zhang H. Kofron J. Donnelly J. Burns D. Ng S.C. Rosenberg S. Wang X. Science. 2003; 299: 223-226Crossref PubMed Scopus (353) Google Scholar). Biochemically, PHAPI along with cellular apoptosis susceptibility and the heat shock protein HSP70, function as the nucleotide exchange factor for Apaf-1, a mechanism reminiscent of G protein regulation (21Kim H.E. Jiang X. Du F. Wang X. Mol. Cell. 2008; 30: 239-247Abstract Full Text Full Text PDF PubMed Scopus (91) Google Scholar). Interestingly, PHAPI is predominantly a nuclear protein (22Chen T.H. Brody J.R. Romantsev F.E. Yu J.G. Kayler A.E. Voneiff E. Kuhajda F.P. Pasternack G.R. Mol. Biol. Cell. 1996; 7: 2045-2056Crossref PubMed Scopus (78) Google Scholar) and can translocate to the cytoplasm during apoptosis, a process involving the RNA-binding protein HuR (23Mazroui R. Di Marco S. Clair E. von Roretz C. Tenenbaum S.A. Keene J.D. Saleh M. Gallouzi I.E. J. Cell Biol. 2008; 180: 113-127Crossref PubMed Scopus (98) Google Scholar). Therefore, subcellular translocation of PHAPI provides an additional regulatory mechanism for PHAPI-stimulated caspase activation. In addition to its activity to promote apoptosis, PHAPI has been shown to possess other biochemical activities, including inhibition of protein phosphatase 2A (PP2A) 3The abbreviations used are: PP2A, protein phosphatase 2A; HAT, histone acetyltransferase; pp32R1, pp32-related-1; NLDM, nuclear localization defective mutant of PHAPI; CMV, cytomegalovirus; GFP, green fluorescent protein; WT, wild type; PRP, PHAPI recombinant protein with residues 140–163 swapped with the correspondent residues of pp32R1. and histone acetyltransferase (HAT), both crucial regulators for various physiological processes (24Li M. Makkinje A. Damuni Z. Biochemistry. 1996; 35: 6998-7002Crossref PubMed Scopus (171) Google Scholar, 25Seo S.B. Macfarlan T. McNamara P. Hong R. Mukai Y. Heo S. Chakravarti D. J. Biol. Chem. 2002; 277: 14005-14010Abstract Full Text Full Text PDF PubMed Scopus (114) Google Scholar). PHAPI can also interact with Ataxin-1, a protein mutated in the neurodegenerative disease spinocerebellar ataxia type I (26Matilla A. Koshy B.T. Cummings C.J. Isobe T. Orr H.T. Zoghbi H.Y. Nature. 1997; 389: 974-978Crossref PubMed Scopus (231) Google Scholar), suggesting a possible role of PHAPI in this type of neurodegeneration. The multiple biochemical activities of PHAPI suggest that this protein is involved in diverse biological events. To date, the most defined biological function of PHAPI is its role as a tumor suppressor. PHAPI expression appears to be greatly decreased during pancreatic cancer progression (27Brody J.R. Witkiewicz A. Williams T.K. Kadkol S.S. Cozzitorto J. Durkan B. Pasternack G.R. Yeo C.J. Mod. Pathol. 2007; 20: 1238-1244Crossref PubMed Scopus (19) Google Scholar). In a study of human non-small cell lung cancer, PHAPI expression level correlated with improved outcome following chemotherapy (28Hoffarth S. Zitzer A. Wiewrodt R. Hahnel P.S. Beyer V. Kreft A. Biesterfeld S. Schuler M. Cell Death Differ. 2008; 15: 161-170Crossref PubMed Scopus (39) Google Scholar). Functionally, overexpression of PHAPI can inhibit oncogene-induced tumorigenesis (22Chen T.H. Brody J.R. Romantsev F.E. Yu J.G. Kayler A.E. Voneiff E. Kuhajda F.P. Pasternack G.R. Mol. Biol. Cell. 1996; 7: 2045-2056Crossref PubMed Scopus (78) Google Scholar, 29Brody J.R. Kadkol S.S. Mahmoud M.A. J.M. Pasternack G.R. J. Biol. Chem. 1999; Full Text Full Text PDF PubMed Scopus Google Scholar, J. Brody J.R. Kadkol S.S. Pasternack G.R. 2001; 20: PubMed Scopus Google Scholar). PHAPI has a close homolog known as which is an of a tumor suppressor J.R. Kadkol S.S. Mahmoud M.A. J.M. Pasternack G.R. J. Biol. Chem. 1999; Full Text Full Text PDF PubMed Scopus Google Scholar, S.S. Brody J.R. J. J. Pasternack G.R. 1999; PubMed Scopus Google Scholar). The expression of is in human with PHAPI expression S.S. Brody J.R. J. J. Pasternack G.R. 1999; PubMed Scopus Google Scholar, S.S. Brody J.R. J. Y. Pasternack G.R. Cancer Res. 2001; PubMed Scopus Google Scholar). The biochemical activity of PHAPI for its tumor-suppressive function has not been Although both and been in cancer PHAPI is able to tumorigenesis inhibiting these two is not the other the activity of PHAPI provides a for its tumor-suppressive which is with that in human cancer and non-small cell lung cancer, PHAPI expression level with the of cancer to apoptosis (28Hoffarth S. Zitzer A. Wiewrodt R. Hahnel P.S. Beyer V. Kreft A. Biesterfeld S. Schuler M. Cell Death Differ. 2008; 15: 161-170Crossref PubMed Scopus (39) Google Scholar, S.S. J.R. M. S. Cancer Res. PubMed Scopus Google Scholar). To this we the structural requirement for the apoptotic activity of PHAPI and evidence to support the model that the apoptotic activity of PHAPI is required for its tumor-suppressive and PHAPI as (20Jiang X. Kim H.E. Shu H. Zhao Y. Zhang H. Kofron J. Donnelly J. Burns D. Ng S.C. Rosenberg S. Wang X. Science. 2003; 299: 223-226Crossref PubMed Scopus (353) Google Scholar). The for mutants of PHAPI by and of for expression of recombinant The nuclear localization defective mutant of PHAPI by mutated to by PHAPI and mutants as proteins in and using by The proteins A and in to for expression of including and To and its mutants for the by from the and then the by for PHAPI-stimulated activity of PHAPI and its mutants to stimulate caspase activation as (20Jiang X. Kim H.E. Shu H. Zhao Y. Zhang H. Kofron J. Donnelly J. Burns D. Ng S.C. Rosenberg S. Wang X. Science. 2003; 299: 223-226Crossref PubMed Scopus (353) Google Scholar). a of cell that we To of of a with A. the with of A. Subsequently, the with A and the protein The in The in for the apoptotic function of PHAPI and its truncation mutants in a A of cytochrome and recombinant PHAPI its truncation mutants as The using a and of fluorescent signal fluorescent as a of of by of caspase-3 activity can also be using the in caspase-3 as the in a of PHAPI by a in A. The to a and The a of using the the and of the in and in of with The by of in and inhibitor the cell to using with for The and by and histone as PHAPI in and in of of PHAPI PHAPI with of the cellular localization of the proteins by fluorescent To PHAPI translocation the with of using a the using and its mutants as for from in by of for with to for in and B. from for in and for For with to in The for and for using a to a to and from of Cell and of of as used in the in of of and of and then the and in a with this are under and to cell of and A additional and activity in the cell to apoptosis in the PHAPI its The using of cell and in A additional The using a and of fluorescent signal fluorescent as a of of by of of identified PHAPI/pp32 as a regulatory of the caspase activation pathway (20Jiang X. Kim H.E. Shu H. Zhao Y. Zhang H. Kofron J. Donnelly J. Burns D. Ng S.C. Rosenberg S. Wang X. Science. 2003; 299: 223-226Crossref PubMed Scopus (353) Google Scholar). PHAPI to the activity of the complex to caspase-9. PHAPI not motif that can to apoptosis. Therefore, to the role of PHAPI in apoptosis, we to the of PHAPI required for its activity to stimulate caspase activation. PHAPI a by to and a region to the highly negatively and recombinant PHAPI protein and a series of mutants with of the protein A and Subsequently, we the activity of these recombinant proteins in c-mediated caspase activation. To we used a from cell and other defined to the caspase activation process (20Jiang X. Kim H.E. Shu H. Zhao Y. Zhang H. Kofron J. Donnelly J. Burns D. Ng S.C. Rosenberg S. Wang X. Science. 2003; 299: 223-226Crossref PubMed Scopus (353) Google Scholar). The Apaf-1, and other required protein for the apoptotic activity of PHAPI (20Jiang X. Kim H.E. Shu H. Zhao Y. Zhang H. Kofron J. Donnelly J. Burns D. Ng S.C. Rosenberg S. Wang X. Science. 2003; 299: 223-226Crossref PubMed Scopus (353) Google Scholar, H.E. Jiang X. Du F. Wang X. Mol. Cell. 2008; 30: 239-247Abstract Full Text Full Text PDF PubMed Scopus (91) Google Scholar). activation by addition of cytochrome c and nucleotide and by using of a a of caspase-3 as the both and we found that there are two of PHAPI protein essential for its apoptotic is the and the other is the region the and also to to the apoptotic activity of of these two caused a modest but decrease of caspase activation by both and a protein the region and the of the two essential for the apoptotic activity of is to support the in caspase activity of PHAPI PHAPI but has a close protein, of a tumor suppressor is an J.R. Kadkol S.S. Mahmoud M.A. J.M. Pasternack G.R. J. Biol. Chem. 1999; Full Text Full Text PDF PubMed Scopus Google Scholar, S.S. Brody J.R. J. J. Pasternack G.R. 1999; PubMed Scopus Google Scholar). recombinant and its caspase activation to not possess apoptotic with the that PHAPI is a tumor is an oncoprotein, that the apoptotic activity and the tumor-suppressive of PHAPI a is not able to stimulate caspase activation highly to that be to the most diverse region of these two to residues 140–163 of PHAPI residues are the region of which is essential for its apoptotic To this we a mutant of PHAPI recombinant protein in which residues 140–163 swapped with the correspondent residues of this mutant as PRP, the mutant of PHAPI a decreased activity to caspase activation is to PHAPI but possesses function, is it possible that tumorigenesis by for the of PHAPI and inhibiting its apoptotic to not be the in in caspase the PHAPI mutants, and can inhibit the activity of the PHAPI to stimulate caspase activation is mechanism that the function of pp32R1. that PHAPI is a protein its in C. J. Biol. Chem. 1997; Full Text Full Text PDF PubMed Scopus Google Scholar). also that both PHAPI from cell and recombinant PHAPI in of suggesting a complex by not PHAPI is a in and the is required for its apoptotic then for to this activity is to with PHAPI in activity by in we used recombinant which a with pp32R1. is not to PHAPI is an in addition to molecular other as the of a protein also its Therefore, we additional to this we to PHAPI can interact with other shown in PHAPI and PHAPI to interact with PHAPI a of PHAPI S.B. Macfarlan T. McNamara P. Hong R. Mukai Y. Heo S. Chakravarti D. J. Biol. Chem. 2002; 277: 14005-14010Abstract Full Text Full Text PDF PubMed Scopus (114) Google Scholar). the that PHAPI can a in we the molecular of recombinant PHAPI by with the that active PHAPI is a also that is a protein not Therefore, suggest that its function a mechanism not to the apoptotic function of PHAPI. The of PHAPI to has been shown to be a tumor but the molecular basis not that PHAPI can caspase activation a (20Jiang X. Kim H.E. Shu H. Zhao Y. Zhang H. Kofron J. Donnelly J. Burns D. Ng S.C. Rosenberg S. Wang X. Science. 2003; 299: 223-226Crossref PubMed Scopus (353) Google Scholar). The that the closely is an and has caspase activation this In it in human expression of PHAPI correlated with of cancer to apoptosis (28Hoffarth S. Zitzer A. Wiewrodt R. Hahnel P.S. Beyer V. Kreft A. Biesterfeld S. Schuler M. Cell Death Differ. 2008; 15: 161-170Crossref PubMed Scopus (39) Google Scholar, S.S. J.R. M. S. Cancer Res. PubMed Scopus Google Scholar). these are with the that the apoptotic activity of PHAPI is required for its tumor-suppressive To this we a to the tumor-suppressive function of a of PHAPI mutants with apoptotic activity defined in an we can the apoptotic activity of PHAPI is correlated with its tumor-suppressive In this we and tumorigenesis by and the The of the to cell in PHAPI the truncation mutants are in these the tumor-suppressive function of these proteins to decrease can be shown in and cell and activity by of the PHAPI. the truncation mutants of PHAPI that possess apoptotic activity as in and not also able to the two other mutants that not the apoptotic activity and the tumor-suppressive Therefore, the apoptotic activity of PHAPI with its tumor-suppressive To the apoptotic activity of PHAPI contributes to its tumor-suppressive function under this we the of PHAPI and its mutants apoptosis by of in these To under the but cell and apoptosis which is for shown in apoptosis by caspase-3 activity in cell of PHAPI enhanced apoptosis by and two mutants, to apoptosis. Further, which a tumor the PHAPI cellular apoptosis activity as this we that the apoptotic activity of PHAPI contributes to its tumor-suppressive function under of the between the tumor-suppressive function and the cellular apoptotic activity of PHAPI and its truncation mutants, it is highly possible that the apoptotic activity of PHAPI is the major for its tumor-suppressive is that the cellular apoptotic activity of is that of the PHAPI in the in activity and we that in there be additional regulation of the apoptotic activity of PHAPI its The of PHAPI in Apoptosis and with (22Chen T.H. Brody J.R. Romantsev F.E. Yu J.G. Kayler A.E. Voneiff E. Kuhajda F.P. Pasternack G.R. Mol. Biol. Cell. 1996; 7: 2045-2056Crossref PubMed Scopus (78) Google Scholar, R. Di Marco S. Clair E. von Roretz C. Tenenbaum S.A. Keene J.D. Saleh M. Gallouzi I.E. J. Cell Biol. 2008; 180: 113-127Crossref PubMed Scopus (98) Google Scholar), we also found that PHAPI is predominantly a nuclear protein in of PHAPI can PHAPI translocate to the cytoplasm to regulate apoptosome-mediated caspase a Additionally, is the nuclear localization of PHAPI required for its tumor-suppressive To nuclear of PHAPI can be in of apoptosis, we in and then with multiple apoptotic found that both and nuclear of with is with the of PHAPI nuclear by (23Mazroui R. Di Marco S. Clair E. von Roretz C. Tenenbaum S.A. Keene J.D. Saleh M. Gallouzi I.E. J. Cell Biol. 2008; 180: 113-127Crossref PubMed Scopus (98) Google Scholar). be that the nuclear in of the be to of apoptosis under these not apoptotic and for the cytoplasm, and it is to nuclear in these PHAPI is a regulatory but not of the mitochondrial caspase pathway, it is highly that its nuclear is by which is an to for the role of PHAPI in apoptosis and tumor suppression. To the role of the nuclear localization of PHAPI in its tumor-suppressive function, we a mutant of PHAPI in which its nuclear localization signal its mutated mutated to possesses activity to stimulate caspase activation in but its nuclear localization we the we found that tumorigenesis indicating nuclear localization of PHAPI is not essential for its tumor-suppressive the other we found that a tumor-suppressive function that of PHAPI and it also cellular apoptosis to a PHAPI suggesting that nuclear localization also contributes to the cellular apoptotic activity and tumor-suppressive function of PHAPI. the mutant and the PHAPI the level of activity in caspase activation in it is the nuclear localization of PHAPI additional to cellular apoptosis. PHAPI/pp32 has been shown to be a tumor suppressor both and human cancer PHAPI tumorigenesis molecular level not Biochemically, PHAPI is able to regulate various including inhibition of and HAT, and of caspase of which cancer biological support the that the apoptotic activity of PHAPI is required for its tumor-suppressive function, PHAPI expression in human correlated with the of cancer to apoptosis (28Hoffarth S. Zitzer A. Wiewrodt R. Hahnel P.S. Beyer V. Kreft A. Biesterfeld S. Schuler M. Cell Death Differ. 2008; 15: 161-170Crossref PubMed Scopus (39) Google Scholar, S.S. J.R. M. S. Cancer Res. PubMed Scopus Google Scholar). In this study, by utilizing multiple truncation mutants of we evidence to support the model that the apoptotic activity of PHAPI is required for its tumor-suppressive In and cellular apoptotic activity of PHAPI and its mutants both with tumor-suppressive PHAPI cellular apoptosis by under which we its tumor-suppressive function, indicating under this the apoptotic activity of PHAPI contributes to tumor suppression. of the highly and the region this to the are essential for both the apoptotic activity and tumor-suppressive function of PHAPI and of the region is it is the major region that PHAPI from its homolog pp32R1, and of the region of PHAPI with the correspondent of the apoptotic activity of PHAPI Consistently, is not able to caspase it also and a and and the of support that the apoptotic activity of PHAPI for its tumor capability. To this the molecular basis the function of is not the apoptotic activity of PHAPI by mechanism by with the of which is a protein Further, to is a protein not it is that tumorigenesis in the The region of PHAPI also its apoptotic activity from other biochemical activities of this tumor suppressor. appears that PHAPI both and its highly negatively protein that both M. S. H. T. B. Biol. Chem. PubMed Scopus Google Scholar, M. Makkinje A. Damuni Z. J. Biol. Chem. 1996; Full Text Full Text PDF PubMed Scopus Google Scholar, S.B. McNamara P. Heo S. A. Chakravarti D. Cell. 2001; Full Text Full Text PDF PubMed Scopus Google Scholar), has a that it also a and highly the highly of these two proteins can inhibit a with which are highly and can from with S.B. Macfarlan T. McNamara P. Hong R. Mukai Y. Heo S. Chakravarti D. J. Biol. Chem. 2002; 277: 14005-14010Abstract Full Text Full Text PDF PubMed Scopus (114) Google Scholar). Additionally, can inhibit and HAT, it is not able to stimulate Apaf-1-dependent caspase activation not Further, has been to be of tumor-suppressive H.T. J. Biol. Chem. 1997; Full Text Full Text PDF PubMed Scopus Google Scholar, C. H. T. Y. K. J. 2005; 26: Google Scholar). In study not that the apoptotic activity of PHAPI is for tumor but also that the biochemical activities of PHAPI to inhibit and role in tumor suppression. PHAPI is predominantly a nuclear protein, and apoptotic can induce translocation of PHAPI to the cytoplasm, of regulatory mechanism for apoptosis (23Mazroui R. Di Marco S. Clair E. von Roretz C. Tenenbaum S.A. Keene J.D. Saleh M. Gallouzi I.E. J. Cell Biol. 2008; 180: 113-127Crossref PubMed Scopus (98) Google Scholar). a PHAPI mutant that nuclear localization but activity to stimulate Apaf-1-dependent caspase activation is to tumorigenesis with that the apoptotic activity of PHAPI for its tumor-suppressive a tumor-suppressive the PHAPI indicating that the nuclear localization of PHAPI also to tumor suppression. nuclear tumor-suppressive function of PHAPI be of it be to the of PHAPI to other nuclear apoptotic proteins as HuR to the cytoplasm, which an additional mechanism for PHAPI to apoptosis (23Mazroui R. Di Marco S. Clair E. von Roretz C. Tenenbaum S.A. Keene J.D. Saleh M. Gallouzi I.E. J. Cell Biol. 2008; 180: 113-127Crossref PubMed Scopus (98) Google Scholar). In addition to the nuclear study also that the of PHAPI also provide basis for additional regulation of the cellular apoptotic activity of PHAPI and and other members of the X. Jiang for and