Role Of PP2A Research Articles

Angiotensin II Type 2 Receptor (AT2R) Modulates CD4+ T Cells into Regulatory T Cells (Tregs) in Kidney Ischemia-Reperfusion Injury in Rats: Role of PP2A

Angiotensin II Type 2 Receptor (AT2R) Modulates CD4+ T Cells into Regulatory T Cells (Tregs) in Kidney Ischemia-Reperfusion Injury in Rats: Role of PP2A

Identification of miRNAs involved in liver injury induced by chronic exposure to cadmium

To elaborate the molecular mechanism underlying the hepatotoxicity induced by chronic exposure to cadmium (Cd), a mouse model with hepatocyte-specific deletion of Ppp2r1a (encoding protein phosphatase 2 A Aα subunit, PP2A Aα) gene was used to investigate the effect of cadmium exposure on liver injury. The wild type littermates (WT) and PP2A Aα-/- mice (KO) were treated with cadmium chloride (CdCl2) at concentrations of 0 mg/L, 10 mg/L, 100 mg/L in drinking water for 3, 6 and 9 months (KO mice only for 9 months), respectively. The pathological findings were characterized by progressive inflammation, steatosis, and liver fibrosis upon treatment of CdCl2 in a dose-response and time-dependent manner. Notably, PP2A Aα depletion leads to a more profound liver injury induced by CdCl2 treatment. The transcriptome analysis in livers of KO mice revealed 20 differentially expressed microRNAs (miRNAs) appeared in both 3- and 9-month. Particularly, the alterations of miR-34a-5p, miR-345-5p, and miR-30e-5p expressions were implicated in the development of liver disease and correlated with the degree of liver injury induced by cadmium treatment. Further analysis indicated that miR-34a-5p, miR-345-5p, and miR-30e-5p might be involved in CdCl2-induced liver injury, in part by dysregulation of lipid metabolism and inflammation. The in vitro studies showed that miR-34a-5p was involved in regulation of CdCl2-induced cytotoxicity through directly targeted adiponectin receptor 2 (AdipoR2) mRNA. Taken together, we identified that specific miRNAs were implicated in hepatotoxicity induced by chronic exposure to CdCl2. These findings also provide new insight into the role of PP2A in regulation of miRNAs-mediated liver injury.

Proteome analysis revealed the essential functions of protein phosphatase PP2A in the induction of Th9 cells

Proteomic analysis identifies post-translational functions of proteins, which remains obscure in transcriptomics. Given the important functions of Th9 cells in anti-tumor immunity, we performed proteome analysis of Th9 cells to understand the involvement of proteins that might be crucial for the anti-tumor functions of Th9 cells. Here we performed a comprehensive proteomic analysis of murine Th0 and Th9 cells, and identified proteins that are enriched in Th9 cells. Pathway analysis identified an abundance of phosphoproteins in the proteome of Th9 cells as compared to Th0 cells. Among upregulated phosphoproteins, Ppp2ca (catalytic subunit of protein phosphatase, PP2A) was found to be highly enriched in Th9 cells. Although the role of PP2A has been shown to regulate the differentiation and functions of Th1, Th2, Th17 and Tregs, its role in the differentiation and functions of Th9 cells is not identified yet. Here we found that PP2A is required for the induction of Th9 cells, as PP2A inhibition leads to the suppression of IL-9 and expression of key transcription factors of Th9 cells. PP2A inhibition abrogates Th9 cell-mediated anti-tumor immune response in B16-OVA melanoma tumor model. Thus, we report that PP2A is essential for the differentiation and anti-tumor functions of Th9 cells.

PP2Ac upregulates PI3K-Akt signaling and induces hepatocyte apoptosis in liver donor after brain death

Multiple research groups have demonstrated that the outcome of patients receiving liver grafts from brain death donors (DBD) is poorer when compared with patients receiving grafts from living donors. This might be due to an increased hepatocyte apoptosis induced after brain death (BD). In this work, we found that the activity of PP2A-Akt pathway is significantly increased in clinical donor ex vivo hepatocytes after BD by iTRAQ protein quantification analysis. The same results were confirmed in animal models. A time-dependent promotion of apoptosis was also found in DBD rabbit liver, as demonstrated by the increased levels of cleaved Caspase 3 and the decreased of Bcl-2. To further investigate the roles of PP2A and Akt in regulating apoptosis of hepatocytes after BD, we cultivated human liver cell line L02 with serum deprivation and hypoxia, to simulate the ischemic and hypoxic conditions of hepatocytes in DBD. Increased apoptosis and decreased viability were observed during the time in this model. Meanwhile PP2A activity and Akt activity were respectively increased and decreased. Notably, the proportion of Akt phosphorylation at Ser473 decreased, while other known targets of PP2A (p38, JNK and ERK) were not affected in terms of protein levels or phosphorylation. These results suggested that PP2A is involved in apoptotic induction of hepatocytes after brain death by specific suppression of Akt. This discovery was further confirmed with pharmaceutical and genetic methods. Our work implied potential targets for reducing liver cell apoptosis and improving organ donor quality after BD.

The phosphatase inhibitor Sds23 regulates cell division symmetry in fission yeast.

Animal and fungal cells divide through the assembly, anchoring, and constriction of a contractile actomyosin ring (CAR) during cytokinesis. The timing and position of the CAR must be tightly controlled to prevent defects in cell division, but many of the underlying signaling events remain unknown. The conserved heterotrimeric protein phosphatase PP2A controls the timing of events in mitosis, and upstream pathways including Greatwall–Ensa regulate PP2A activity. A role for PP2A in CAR regulation has been less clear, although loss of PP2A in yeast causes defects in cytokinesis. Here, we report that Sds23, an inhibitor of PP2A family protein phosphatases, promotes the symmetric division of fission yeast cells through spatial control of cytokinesis. We found that sds23∆ cells divide asymmetrically due to misplaced CAR assembly, followed by sliding of the CAR away from its assembly site. These mutant cells exhibit delayed recruitment of putative CAR anchoring proteins including the glucan synthase Bgs1. Our observations likely reflect a broader role for regulation of PP2A in cell polarity and cytokinesis because sds23∆ phenotypes were exacerbated when combined with mutations in the fission yeast Ensa homologue, Igo1. These results identify the PP2A regulatory network as a critical component in the signaling pathways coordinating cytokinesis.

C. elegans PTEN and AMPK block neuroblast divisions by inhibiting a BMP-insulin-PP2A-MAPK pathway.

Caenorhabditis elegans that hatch in the absence of food stop their postembryonic development in a process called L1 arrest. Intriguingly, we find that the postembryonic Q neuroblasts divide and migrate during L1 arrest in mutants that have lost the energy sensor AMP-activated protein kinase (AMPK) or the insulin/IGF-1 signaling (IIS) negative regulator DAF-18/PTEN. We report that DBL-1/BMP works upstream of IIS to promote agonistic insulin-like peptides during L1 arrest. However, the abnormal Q cell divisions that occur during L1 arrest use a novel branch of the IIS pathway that is independent of the terminal transcription factor DAF-16/FOXO. Using genetic epistasis and drug interactions we show that AMPK functions downstream of, or in parallel with DAF-18/PTEN and IIS to inhibit PP2A function. Further, we show that PP2A regulates the abnormal Q cell divisions by activating the MPK-1/ERK signaling pathway via LIN-45/RAF, independently of LET-60/RAS. PP2A acts as a tumor suppressor in many oncogenic signaling cascades. Our work demonstrates a new role for PP2A that is needed to induce neuroblast divisions during starvation and is regulated by both insulin and AMPK.

The role of PP2A in an inflammatory induced increase in Brain Microvascular Permeability

The role of PP2A in an inflammatory induced increase in Brain Microvascular Permeability

Evidence that PP2A activity is dispensable for spindle assembly checkpoint-dependent control of Cdk1.

Progression through mitosis, the cell cycle phase deputed to segregate replicated chromosomes, is granted by a protein phosphorylation wave that follows an activation-inactivation cycle of cyclin B-dependent kinase (Cdk) 1, the major mitosis-promoting enzyme. To ensure correct chromosome segregation, the safeguard mechanism spindle assembly checkpoint (SAC) delays Cdk1 inactivation by preventing cyclin B degradation until mitotic spindle assembly. At the end of mitosis, reversal of bulk mitotic protein phosphorylation, downstream Cdk1 inactivation, is required to complete mitosis and crucially relies on the activity of major protein phosphatases like PP2A. A role for PP2A, however, has also been suggested in spindle assembly and SAC-dependent control of Cdk1. Indeed, PP2A was found in complex with SAC proteins while small interfering RNAs (siRNAs)-mediated downregulation of PP2A holoenzyme components affected mitosis completion in mammalian cells. However, whether the SAC-dependent control of Cdk1 required the catalytic activity of PP2A has never been directly assessed. Here, using two PP2A inhibitors, okadaic acid and LB-100, we provide evidence that PP2A activity is dispensable for SAC control of Cdk1 in human cells.

Abstract 5762: PR55-alpha associated PP2A promotes pancreatic tumorigenesis

Abstract PP2A holoenzyme consists of a catalytic subunit, a scaffold subunit, and a regulatory subunit. Loss of function analysis using PP2A catalytic inhibitors or inhibition via tumor viral proteins suggest PP2A as a putative tumor suppressor. However, PP2A has also been shown to facilitate the activation of oncogenic signaling pathways when associated with specific regulatory subunits. In this study, we investigated the possible oncogenic role of PP2A in pancreatic cancer. We found a striking increase in the expression of PR55α, a PP2A regulatory subunit, in pancreatic cancer cells compared to normal pancreatic epithelial cells. Consistently, PR55α expression was markedly elevated in pancreatic ductal adenocarcinoma tissues compared to adjacent normal pancreatic tissues (P<0.0001) and correlated with poor survival of pancreatic cancer patients (P<0.0003). RNAi-mediated depletion of PR55α in pancreatic cancer cells resulted in diminished phosphorylation of both AKT and ERK1/2 and decreased protein levels of β-catenin. Accordingly, pancreatic cancer cells with reduced PR55α expression exhibited significantly impaired properties of transformation, including attenuated cell growth, clonogenicity, mobility, and anchorage-independent growth. Moreover, orthotopic implantation of PR55α-depleted pancreatic cancer cells into nude mice showed a marked reduction in tumorigenicity (P<0.001) and distant metastases. These results suggest that PR55α promotes pancreatic cancer development by sustaining hyperactivity of multiple oncogenic signaling pathways, including AKT, ERK and Wnt. Citation Format: Ying Yan, Ashley L. Hein, Parthasarathy Seshacharyulu, Satyanarayana Rachagani, Yuri M. Sheinin, Michel M. Ouellette, Moorthy P. Ponnusamy, Surinder K. Batra. PR55-alpha associated PP2A promotes pancreatic tumorigenesis [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 5762. doi:10.1158/1538-7445.AM2017-5762

1537P - The role of PP2A in innate resistance to HER2-targeted therapy

1537P - The role of PP2A in innate resistance to HER2-targeted therapy

Editorial: The Emerging Role for PP2A in Hematologic Malignancies

Editorial: The Emerging Role for PP2A in Hematologic Malignancies

Protein phosphatase 2A promotes the transition to G0 during terminal differentiation in Drosophila.

Protein phosphatase type 2A complex (PP2A) has been known as a tumor suppressor for over two decades, but it remains unclear exactly how it suppresses tumor growth. Here, we provide data indicating a novel role for PP2A in promoting the transition to quiescence upon terminal differentiation in vivo. Using Drosophila eyes and wings as a model, we find that compromising PP2A activity during the final cell cycle prior to a developmentally controlled cell cycle exit leads to extra cell divisions and delays entry into quiescence. By systematically testing the regulatory subunits of Drosophila PP2A, we find that the B56 family member widerborst (wdb) is required for the role of PP2A in promoting the transition to quiescence. Cells in differentiating tissues with compromised PP2A retain high Cdk2 activity when they should be quiescent, and genetic epistasis tests demonstrate that ectopic Cyclin E/Cdk2 activity is responsible for the extra cell cycles caused by PP2A inhibition. The loss of wdb/PP2A function cooperates with aberrantly high Cyclin E protein levels, allowing cells to bypass a robust G0 late in development. This provides an example of how loss of PP2A can cooperate with oncogenic mutations in cancer. We propose that the PP2A complex plays a novel role in differentiating tissues to promote developmentally controlled quiescence through the regulation of Cyclin E/Cdk2 activity.

The role of PP2A in regulation of cytotoxicity and DNA repair

The role of PP2A in regulation of cytotoxicity and DNA repair

Comprehensive logic based analyses of Toll-like receptor 4 signal transduction pathway.

Among the 13 TLRs in the vertebrate systems, only TLR4 utilizes both Myeloid differentiation factor 88 (MyD88) and Toll/Interleukin-1 receptor (TIR)-domain-containing adapter interferon-β-inducing Factor (TRIF) adaptors to transduce signals triggering host-protective immune responses. Earlier studies on the pathway combined various experimental data in the form of one comprehensive map of TLR signaling. But in the absence of adequate kinetic parameters quantitative mathematical models that reveal emerging systems level properties and dynamic inter-regulation among the kinases/phosphatases of the TLR4 network are not yet available. So, here we used reaction stoichiometry-based and parameter independent logical modeling formalism to build the TLR4 signaling network model that captured the feedback regulations, interdependencies between signaling kinases and phosphatases and the outcome of simulated infections. The analyses of the TLR4 signaling network revealed 360 feedback loops, 157 negative and 203 positive; of which, 334 loops had the phosphatase PP1 as an essential component. The network elements' interdependency (positive or negative dependencies) in perturbation conditions such as the phosphatase knockout conditions revealed interdependencies between the dual-specific phosphatases MKP-1 and MKP-3 and the kinases in MAPK modules and the role of PP2A in the auto-regulation of Calmodulin kinase-II. Our simulations under the specific kinase or phosphatase gene-deficiency or inhibition conditions corroborated with several previously reported experimental data. The simulations to mimic Yersinia pestis and E. coli infections identified the key perturbation in the network and potential drug targets. Thus, our analyses of TLR4 signaling highlights the role of phosphatases as key regulatory factors in determining the global interdependencies among the network elements; uncovers novel signaling connections; identifies potential drug targets for infections.

Abstract 3861: Downregulation of protein phosphatase 2A promotes prostate cancer progression and metastasis.

Abstract Clinical progression of prostate cancer (PCa) is characterized by a transition from castration-sensitive (CS) to castration-resistant (CR) phenotype. The resulting CR tumors are highly aggressive and metastatic, and thus pose increased risk of morbidity and death to PCa patients. Therefore, identification of novel gene targets associated with CR growth and metastatic behavior remain a priority area in PCa research. Previously, we identified that PPP2CA, which encodes for the α–isoform of catalytic subunit of PP2A (a serine/threonine phosphatase), is downregulated in CRPCa. In additional findings, we reported that PP2A downregulation sustained the growth of PCa cells under steroid–deprived conditions. In the present study, we examined the role of PP2A in malignant behavior of PCa cells using in vitro and in vivo functional assays. Furthermore, we also delineated the underlying molecular mechanisms. Our data demonstrated that downregulation of PPP2CA (in CS LNCaP cells) promoted, whereas its overexpression (in CR C4-2 and PC3 cells) decreased the migration and invasion of human PCa cells. Similarly, we observed a loss of homotypic interactions in PPP2CA-silenced LNCaP cells, while it increased in PPP2CA-overexpressing C4-2 and PC3 cells. These changes were associated with epithelial to mesenchymal transition (EMT) or vice versa in PPP2CA-silenced or overexpressing PCa cells, respectively. When examined in vivo in an orthotopic mouse model, PPP2CA-overexpressing PC3 cells exhibited dramatic decrease in tumorigenesis due to diminished proliferation and enhanced apoptosis as compared to the control cells. Moreover, significant reduction in metastatic incidence was also observed. Mechanistic studies revealed that PPP2CA downregulation increases nuclear accumulation of β-catenin and NF-κB and subsequent activation of transcriptional activity of the responsive gene promoters. We also observed cooperative involvement of both β-catenin and NF-κB in the PPP2CA downregulation-induced EMT and invasiveness of PCa cells. Lastly, our data demonstrated a role of PP2A/Akt axis in enhanced nuclear accumulation of β-catenin and NF-κB through phosphorylation-induced inactivation of Gsk3-β and IkB-α, respectively. Altogether, our data suggest that loss of PPP2CA is associated with PCa progression and metastasis, and restoration of PP2A activity could serve as an effective preventive and/or therapeutic approach against the advanced disease. Citation Format: Arun Bhardwaj, Seema Singh, Sanjeev K. Srivastava, Sumit Arora, Stephen J. Hyde, Richard E. Honkanen, William E. Grizzle, Ajay P. Singh. Downregulation of protein phosphatase 2A promotes prostate cancer progression and metastasis. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 3861. doi:10.1158/1538-7445.AM2013-3861

Abstract 293: c-Myc Inactivation by PP2A is Necessary for Dopaminergic Activity in the Proximal Tubule

We have previously shown that c-Myc regulates the expression of G protein-coupled receptor kinase 4 (GRK4). The inhibition of c-Myc can revert a dopamine-1 receptor (D 1 R) uncoupled renal proximal tubule cell (uRPTC) to normal (nRPTC). The c-Myc inhibitor, 10074-G5 (30 μM, 3 hr), restored coupling of D 1 R mediated adenylyl cyclase stimulation in uRPTCs, causing a significant increase in cAMP levels (16.21% + 0.97; p<0.01; n=47) in response to the dopamine D 1 R/D 5 R agonist, fenoldopam (FEN), than when compared to cells pre-treated with DMSO vehicle (VEH). We have also shown that FEN leads to PP2A activation. Therefore we tested the hypothesis that an inhibitory role of PP2A on c-Myc may play a role in regulating dopaminergic function. PMA (100 nM) was added to either nRPTC or uRPTC for 3hrs. In nRPTC, the total c-Myc from immunoprecipitation increased by 138.7% + 4.1%; p<0.01; n=3 with respect to VEH, while in uRPTC the total c-Myc increased by 214.5% + 11.3%; p<0.01; n=3. The PP2A/Myc association decreased by 46.0% + 1.16%; p<0.05; n=3 with respect VEH in nRPTC and decreased in uRPTC by 46.9% + 8.71%; p<0.05; n=3. These data suggest that c-Myc is activated only when PP2A binding is low. Addition of FEN (3 hrs) decreased the total c-Myc in nRPTC by 22.6% + 9.1%; p=0.06; n=3 but not in uRPTC. In nRPTC, the PP2A/Myc association increased by 56.6% + 15.1%; p <0.05; n=3 when compared to VEH, but showed no response in uRPTC. Addition of natriuretic peptide angiotensin-III (Ang III, 10 nM) decreased the expression of c-Myc by 29.4% + 16.8%; p<0.05; n=3, but showed no response in uRPTC. The PP2A/Myc binding increased by 109.3% + 30.1%; p<0.05; n=3 in nRPTC, with no increase in uRPTC. Here we show for the first time in nRPTC that both FEN and Ang III decrease total c-Myc while increasing PP2A/Myc co-immunoprecipitation, leading to inactivation of c-Myc. In uRPTC, no significant change in the total c-Myc or the association of PP2A/Myc was detected in response to FEN or Ang III. This suggests that both a lack of expressional control of c-Myc, in addition to a lack of inhibition of c-Myc by PP2A in uRPTC, may have a role in the etiology of D 1 R uncoupling and human renal proximal tubule-related hypertension. Therefore, c-Myc inhibitors may provide a new approach for treating hypertension.

A phosphatase threshold sets the level of Cdk1 activity in early mitosis in budding yeast.

Entry into mitosis is initiated by synthesis of cyclins, which bind and activate cyclin-dependent kinase 1 (Cdk1). Cyclin synthesis is gradual, yet activation of Cdk1 occurs in a stepwise manner: a low level of Cdk1 activity is initially generated that triggers early mitotic events, which is followed by full activation of Cdk1. Little is known about how stepwise activation of Cdk1 is achieved. A key regulator of Cdk1 is the Wee1 kinase, which phosphorylates and inhibits Cdk1. Wee1 and Cdk1 show mutual regulation: Cdk1 phosphorylates Wee1, which activates Wee1 to inhibit Cdk1. Further phosphorylation events inactivate Wee1. We discovered that a specific form of protein phosphatase 2A (PP2A(Cdc55)) opposes the initial phosphorylation of Wee1 by Cdk1. In vivo analysis, in vitro reconstitution, and mathematical modeling suggest that PP2A(Cdc55) sets a threshold that limits activation of Wee1, thereby allowing a low constant level of Cdk1 activity to escape Wee1 inhibition in early mitosis. These results define a new role for PP2A(Cdc55) and reveal a systems-level mechanism by which dynamically opposed kinase and phosphatase activities can modulate signal strength.

Mitochondrial Protein Phosphatase 2A Regulates Cell Death Induced by Simulated Ischemia in Kidney NRK-52E Cells

Acute renal failure can occur after an ischemic injury and results in significant mortality. The stress-signaling pathways that are activated during renal ischemia are unknown. PP2A has emerged as an important regulator of cell death. To study the role of PP2A in ischemia-induced cell death, we used an in vitro model of simulated ischemia. In the present study, simulated ischemia in rat renal tubule epithelial NRK-52E cells (a) results in cell death that involves both necrosis and apoptosis, (b) activates PP2A, and (c) up-regulates the PP2A B56 α regulatory subunit. Previous data have shown that PKC α negatively regulates B56 α protein expression. Consistent with this finding, simulated ischemia suppressed PKC α and up-regulated B56 α. Treatment of NRK-52E cells with ceramide suppressed PKC α and activated PP2A in a manner that mimicked simulated ischemia. A role for PP2A in simulated ischemia-induced cell death is likely since inhibition of PP2A protected NRK-52E cells. In addition, overexpression of exogenous B56 α but not B55 in NRK-52E cells enhanced simulated ischemia-induced cell death. These findings suggest that activation of a PP2A isoform that contains the B56 α regulatory subunit is required for ischemia-induced cell death in kidney epithelial proximal tubule cells.

Mechanisms of the HRSL3 tumor suppressor function in ovarian carcinoma cells

HRSL3 (also known as H-REV107-1) belongs to a class II tumor suppressor gene family and is downregulated in several human tumors including ovarian carcinomas. To unravel the mechanism of HRSL3 tumor suppressor action, we performed a yeast two-hybrid screen and identified the alpha-isoform of the regulatory subunit A of protein phosphatase 2A (PR65alpha) as a new interaction partner of HRSL3. Interaction between HRSL3 and PR65alpha was confirmed in vitro and by co-immunoprecipitation in mammalian cells. We demonstrate that HRSL3 binds to the endogenous PR65alpha, thereby partially sequestering the catalytic subunit PR36 from the PR65 protein complex, and inhibiting PP2A catalytic activity. Furthermore, binding of HRSL3 to PR65 induces apoptosis in ovarian carcinoma cells in a caspase-dependent manner. Using several mutant HRSL3 constructs, we identified the N-terminal proline-rich region within the HRSL3 protein as the domain that is relevant for both binding of PR65alpha and induction of programmed cell death. This suggests that the negative impact of HRSL3 onto PP2A activity is important for the HRSL3 pro-apoptotic function and indicates a role of PP2A in survival of human ovarian carcinomas. The analysis of distinct PP2A target molecules revealed PKCzeta as being involved in HRSL3 action. These data implicate HRSL3 as a signaling regulatory molecule, which is functionally involved in the oncogenic network mediating growth and survival of ovarian cancer cells.

Polyoma and SV40 proteins differentially regulate PP2A to activate distinct cellular signaling pathways involved in growth control

Binding of Src family kinases to membrane-associated polyoma virus middle T-antigen (PyMT) can result in the phosphorylation of PyMT tyrosine 250, which serves as a docking site for the binding of Shc and subsequent activation of the Raf-MEK-ERK (MAP) kinase cascade. In a screen for PyMT variants that could not activate the ARF tumor suppressor, we isolated a cytoplasmic nontransforming mutant (MTA) that encoded a C-terminal truncated form of the PyMT protein. Surprisingly, MTA was able to strongly activate the MAP kinase pathway in the absence of Src family kinase and Shc binding. Interestingly, the polyoma small T-antigen (PyST), which shares with MTA both partial amino acid sequence homology and cellular location, also activates the MAP kinase cascade. Activation of the MAP kinase cascade by both MTA and PyST has been demonstrated to be PP2A-dependent. Neither MTA nor PyST activate the phosphorylation of AKT. The SV40 small T-antigen, which is similar to PyST in containing a J domain and in binding to the PP2A AC dimer, does not activate the MAP kinase cascade, but does stimulate phosphorylation of AKT in a PP2A-dependent manner. These findings highlight a novel role of PP2A in stimulating the MAP kinase cascade and indicate that the similar polyoma and SV40 small T-antigens influence PP2A to activate discrete cellular signaling pathways involved in growth control.