Correction to “Modulation of oncogenic potential by alternative gene use in human prostate cancer”

The retinoblastoma gene product (Rb) is a tumor suppressor that affects apoptosis paradoxically. Most sporadic cancers inactivate Rb by preferentially targeting the pathway that regulates Rb phosphorylation, resulting in resistance to apoptosis; this contrasts with Rb inactivation by mutation, which is associated with high rates of apoptosis. How phosphorylated Rb protects cells from apoptosis is not well understood, but there is evidence that Rb may sequester a pro-apoptotic nuclear factor. pp32 (ANP32A) is a pro-apoptotic nuclear phosphoprotein, the expression of which is commonly increased in cancer. We report that hyperphosphorylated Rb interacts with pp32 but not with the closely related proteins pp32r1 and pp32r2. We further demonstrate that pp32-Rb interaction inhibits the apoptotic activity of pp32 and stimulates proliferation. These results suggest a mechanism whereby cancer cells gain both a proliferative and survival advantage when Rb is inactivated by hyperphosphorylation.The retinoblastoma protein (Rb) 1 is a nuclear phosphoprotein that regulates proliferation, differentiation, and apoptosis. As a tumor suppressor, Rb inhibits proliferation by repressing E2F1-mediated transcription when hypophosphorylated. Hyperphosphorylation of Rb relieves E2F1 repression and allows cell cycle progression to occur (1). The importance of Rb is underscored by the fact that Rb function is disrupted in virtually all human cancers (2). Paradoxically and inconsistent with its role as a tumor suppressor, hyperphosphorylated wild-type Rb inhibits apoptosis in both cell culture and animal models (3)(4)(5)(6)(7)(8)(9)(10). Because Rb inactivation is pivotal for carcinogenesis, this poses the problem of how cancer cells escape apoptosis when Rb function is disrupted.Inherited cancers and cells in which Rb is inactivated by mutation have increased rates of both proliferation and apoptosis (11). Most sporadic cancers preferentially inactivate Rb by hyperphosphorylation, which may occur through mutation of cyclin D, cdk4, or p16. Such cancers are generally slow growing and resistant to apoptosis induced by chemotherapy or radiation (12). It is possible that in these cancers the tumor suppressor function of Rb is inhibited, whereas the anti-apoptotic function remains intact (13). Inactivation by hyperphosphorylation might promote proliferation by increasing free E2F1, as well as inhibit apoptosis by retaining the anti-apoptotic function of Rb. This is consistent with evidence suggesting that it is the hyperphosphorylated form of Rb rather than Rb per se that protects cells from apoptosis (14). The induction of apoptosis in various cell lines is accompanied by a shift in Rb from the hyperphosphorylated to the hypophosphorylated form (15,16). Rb dephosphorylation, which has been shown to be required for apoptosis, occurs in the early stage of apoptosis (17,18). Inhibition of Rb dephosphorylation prevents apoptosis, whereas induction of dephosphorylation leads to apoptosis (19). In DBA/2 mice, increased levels of hyperphosphoryla...

Section: Pp32-rb Interaction Inhibits Pp32 Apoptotic Activitymentioning

confidence: 99%

Phosphorylated Retinoblastoma Protein Complexes with pp32 and Inhibits pp32-mediated Apoptosis

Adegbola¹,

Pasternack

2005

“…2A). Intriguingly, instead of being a tumor suppressor like PHAPI, pp32R1 is an oncogene (29,31). We generated purified recombinant pp32R1 and examined its effect on Apaf-1-mediated caspase activation (Fig.…”

Section: Phapi/pp32 Suppresses Tumorigenesis By Stimulating Apoptosismentioning

confidence: 99%

“…Intriguingly, PHAPI has a close homolog known as pp32R1 (pp32-related-1), which is an oncoprotein instead of a tumor suppressor (29,31). The expression of pp32R1 is switched on in certain human cancers with diminished PHAPI expression (31,32).…”

mentioning

confidence: 99%

PHAPI/pp32 Suppresses Tumorigenesis by Stimulating Apoptosis

Pan

Graca

Shao

et al. 2009

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 (1, 2). Apoptosis is executed by a family of proteases known as caspases (3,4). In mammals, a major caspase activation pathway is the mitochondrial cytochrome c-mediated caspase activation pathway, also known as the intrinsic apoptotic pathway (5). 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 (6 -8). The released cytochrome c binds to Apaf-1, the essential mediator of the pathway, and triggers its nucleotide binding/exchanging activity (9, 10). 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 (9, 11) 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 (6 -8). In the downstream of mitochondria, inhibitor of apoptosis proteins negatively regulate apoptosis by inhibiting caspase activity (12-15). The function of inhibitor of apoptosis proteins can be antagonized by Smac/Diablo (16,17) and Omi/ Htr2A (18,19), 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 enha...

“…The principal function of these motifs seems to be the provision of a structural framework that allows protein-protein interactions (11). PHAPI is also known as pp32, a potent tumor suppressor (12)(13)(14)(15) that is down-regulated in cancer with reciprocal increased expression of pp32r1 and pp32r2, other members of the pp32 gene family (12). Although the physiological role of PHAPI/pp32 is not fully understood, it has been shown to be a potent inhibitor of PP2A in vitro (16,17) and also promotes apoptosis by caspase-9 activation (18).…”

mentioning

confidence: 99%

Protein Phosphatase 2A, a Negative Regulator of the ERK Signaling Pathway, Is Activated by Tyrosine Phosphorylation of Putative HLA Class II-associated Protein I (PHAPI)/pp32 in Response to the Antiproliferative Lectin, Jacalin

Yu¹,

Packman

Weldon

et al. 2004

Protein phosphatase 2A (PP2A) is a family of mammalian serine/threonine phosphatases that is involved in the control of many cellular functions including those mediated by extracellular signal-regulated kinase (ERK) signaling. While investigating the reversible antiproliferative effect of the dietary lectin, jacalin, which binds the Thomsen-Friedenreich antigen (galactose ␤1-3 N-acetylgalactosamine ␣-), we have found that this lectin (30 g/ml) induces rapid, transient, tyrosine phosphorylation of putative human HLA-DR-associated protein I (PHAPI, also known as the tumor suppressor pp32) in HT29 human colon cancer cells. This is accompanied by the release of PP2A from association with PHAPI, allowing increased phosphatase activity of PP2A (by 42 ؎ 10% at 10 min) and consequent complete dephosphorylation of the ERK kinase, MEK1/2, by 10 min and of ERK1/2 by 60 min. PHAPI knockdown by RNA interference abolished the effects of jacalin on PP2A activation and MEK inhibition. Thus phosphorylation of PHAPI/pp32 is a critical regulatory step in PP2A activation and ERK signaling.Protein phosphatase 2A (PP2A) 1 is a family of mammalian serine/threonine phosphatases that accounts for the major portion of serine/threonine kinase activity in most cells. PP2A is a trimeric holoenzyme complex and is composed of a catalytic C subunit, a structural A subunit, and a regulatory B-type subunit, each encoded by separate genes (1, 2). Up to 75 different trimeric ABC holoenzymes may exist. The C subunit is the enzymatically active component, A is a scaffolding component, and B acts as a targeting module that directs the enzyme to various intracellular locations and also provides substrate specificity.PP2A was initially thought to act passively and nonspecifically in antagonizing the action of kinases by removal of phosphate groups from their substrates, but it has become clear that it is tightly regulated by mechanisms that include variation in its subunit composition (3), phosphorylation, and/or methylation of its catalytic subunit and signal-regulated targeting to specific subcellular compartments (1-4). PP2A is an important negative regulator of the extracellular signal-regulated kinase (ERK) signaling pathway (5-7) and is involved in the control of many cellular functions including metabolism, transcription, translation, RNA splicing, DNA replication, cell cycle progression, transformation, and apoptosis (1-8).PHAPI, first purified by affinity with an agarose-conjugated synthetic cytoplasmic region of HLA-DR ␣ chain, is a putative HLA class II-associated cytosolic protein (9), later identified as a phosphoprotein (10). It belongs to a group of proteins containing 20 -29-residue leucine-rich repeat motifs. The principal function of these motifs seems to be the provision of a structural framework that allows protein-protein interactions (11). PHAPI is also known as pp32, a potent tumor suppressor (12-15) that is down-regulated in cancer with reciprocal increased expression of pp32r1 and pp32r2, other members of the pp32 gene family (...