Chi Wu Chiang

Regulation of Phosphorylation of Thr-308 of Akt, Cell Proliferation, and Survival by the B55α Regulatory Subunit Targeting of the Protein Phosphatase 2A Holoenzyme to Akt

Akt is a protein serine/threonine kinase that is involved in the regulation of diverse cellular processes. Phosphorylation of Akt at regulatory residues Thr-308 and Ser-473 leads to its full activation. The protein phosphatase 2A (PP2A) has long been known to negatively regulate Akt activity. The PP2A holoenzyme consists of the structural subunit (A), catalytic subunit (C), and a variable regulatory subunit (B). Here we report the identification of the specific B regulatory subunit that targets the PP2A holoenzyme to Akt. We found endogenous association of PP2A AB55C holoenzymes with Akt by co-immunoprecipitation analyses in pro-lymphoid FL5.12 cells. Akt was shown to associate with ectopically expressed B55α subunit in NIH3T3 cells. The direct interaction between B55α subunit and Akt was confirmed using in vitro pulldown analyses. Intriguingly, we found that overexpression of B55α subunit significantly impaired phosphorylation at Thr-308, but to a lesser extent at Ser-473 of Akt in both FL5.12 and NIH3T3 cells. Concomitantly, phosphorylation of a subset of Akt substrates, including FoxO3a, was substantially decreased by B55α overexpression in these cells. Silencing of B55α expression markedly increased phosphorylation at Thr-308 but not at Ser-473 in both FL5.12 cells and NIH3T3 cells. Consistently, PP2A AB55αC holoenzymes preferentially dephosphorylated phospho-Thr-308 rather than phospho-Ser-473 in in vitro dephosphorylation assays. Furthermore, B55α overexpression retarded proliferation of NIH3T3 cells, and knockdown of B55α expression increased survival of FL5.12 cells upon interleukin-3 deprivation. Together, our data demonstrate that B55α-dependent targeting of the PP2A holoenzyme to Akt selectively regulates Akt phosphorylation at Thr-308 to regulate cell proliferation and survival.

Protein Phosphatase 2A Dephosphorylation of Phosphoserine 112 Plays the Gatekeeper Role for BAD-Mediated Apoptosis

ABSTRACT BAD, a proapoptotic molecule of the BCL2 family, is regulated by reversible phosphorylation. During survival, BAD is sequestered by 14-3-3 through serine 136 phosphorylation and is dissociated from BCL-XL through serine 155 phosphorylation. We report that phosphoserine 112 (pSer112) dephosphorylation functions as a gatekeeper for BAD-mediated apoptosis. During apoptosis, dephosphorylation of pSer112 preceded pSer136 dephosphorylation. Dephosphorylation of pSer112 accelerated dephosphorylation of pSer136, and inhibition of pSer112 dephosphorylation prevented pSer136 dephosphorylation, indicating that dephosphorylation of pSer112 is required for dephosphorylation of pSer136. Protein phosphatase 2A (PP2A) is the major pSer112 phosphatase. PP2A competed with 14-3-3 for BAD binding, and survival factor withdrawal enhanced PP2A association with BAD. Dephosphorylation of the critical residue, pSer136, could only be blocked by inhibition of all known subfamilies of serine/threonine phosphatases, suggesting that multiple phosphatases are involved in pSer136 dephosphorylation. Inhibition of PP2A rescued FL5.12 cells from apoptosis, demonstrating a physiologic role for PP2A-mediated pSer112 dephosphorylation. Thus, PP2A dephosphorylation of pSer112 is the key initiating event regulating the activation of BAD during interleukin-3 withdrawal-induced apoptosis.

GSK-3β acts downstream of PP2A and the PI 3-kinase-Akt pathway, and upstream of caspase-2 in ceramide-induced mitochondrial apoptosis

The signaling of glycogen synthase kinase-3β (GSK-3β) has been implicated in stress-induced apoptosis. However, the pro-apoptotic role of GSK-3β is still unclear. Here, we show the involvement of GSK-3β in ceramide-induced mitochondrial apoptosis. Ceramide induced GSK-3β activation via protein dephosphorylation at serine 9. We previously reported that ceramide induced caspase-2 and caspase-8 activation, Bid cleavage, mitochondrial damage, and apoptosis. In this study, we found that caspase-2 activation and the subsequent apoptotic events were abolished by the GSK-3β inhibitors lithium chloride and SB216763, and by GSK-3β knockdown using short interfering RNA. We also found that ceramide-activated protein phosphatase 2A (PP2A) indirectly caused GSK-3β activation, and that the PP2A-regulated PI 3-kinase-Akt pathway was involved in GSK-3β activation. These results indicate a role for GSK-3β in ceramide-induced apoptosis, in which GSK-3β acts downstream of PP2A and the PI 3-kinase-Akt pathway, and upstream of caspase-2 and caspase-8.

BAD/BCL-XL heterodimerization leads to bypass of G0/G1 arrest

The pro-apoptotic molecule BAD binds BCL-xL or BCL2 and inactivates their survival function. In addition to their anti-apoptotic function, BCL2 and BCL-xL also delay cell cycle entry from quiescence. We found that the BH3-only molecule BAD also exerted a cell cycle effect. BAD expression resulted in failure to cell cycle block in growth arrest conditions. In low serum and in confluence, fibroblasts constitutively or inducibly expressing BAD persisted in S phase, continued to incorporate BrdU, and exhibited sustained cyclin E/cdk2 activity. Mutation analysis indicated that the cell cycle effect of BAD was not dependent on its phosphorylation status or subcellular localization, but strictly co-segregated with BCL-xL binding. bclx−/− MEFs expressing BAD and bad−/− MEFs both arrested in G0/G1 in low serum similar to wild-type controls, suggesting that the ability to overcome the G0/G1 checkpoint resulted from the presence of BAD/BCL-xL heterodimers, rather than the absence of BCL-xL or BAD. These data provide evidence that in addition to regulating apoptosis, the BAD/BCL-xL heterodimer has a novel cell cycle function.

Insertional tagging of at least two loci associated with resistance to adenine arabinoside in Toxoplasma gondii, and cloning of the adenosine kinase locus

A genetic approach has been exploited to investigate adenylate salvage pathways in the protozoan parasite Toxoplasma gondii, a purine auxotroph. Using a new insertional mutagenesis vector designed to facilitate the rescue of tagged loci even when multiple plasmids integrate as a tandem array, 15 independent clonal lines resistant to the toxic nucleoside analog adenine arabinoside (AraA) were generated. Approximately two-thirds of these clones lack adenosine kinase (AK) activity. Parallel studies identified an expressed sequence tag (EST) exhibiting a small region of weak similarity to human AK, and this locus was tagged in several AK-deficient insertional mutants. Library screening yielded full-length cDNA and genomic clones. The T. gondii AK gene contains five exons spanning a approximately 3 kb locus, and the predicted coding sequence was employed to identify additional AK genes and cDNAs in the GenBank and dbEST databases. A genomic construct lacking essential coding sequence was used to create defined genetic knock-outs at the T. gondii AK locus, and AK activity was restored using a cDNA-derived minigene. Hybridization analysis of DNA from 13 AraA-resistant insertional mutants reveals three distinct classes: (i) AK-mutants tagged at the AK locus; (ii) AK- mutants not tagged at the AK locus, suggesting the possibility that another locus may be involved in regulating AK expression; and (iii) mutants with normal AK activity (potential transport mutants).

Lithium inhibits ceramide- and etoposide-induced protein phosphatase 2A methylation, Bcl-2 dephosphorylation, caspase-2 activation, and apoptosis.

Lithium confers cell protection against stress and toxic stimuli. Although lithium inhibits a number of enzymes, the antiapoptotic mechanisms of lithium remain unresolved. Here, we report a novel role of lithium on the blockage of ceramide- and etoposide-induced apoptosis via inhibition of protein phosphatase 2A (PP2A) activity. Overexpression of PP2A resulted in caspase-2 activation, mitochondrial damage, and cell apoptosis that were inhibited by okadaic acid (OA) and lithium. Lithium and OA abrogated ceramide- and etoposide-induced Bcl-2 dephosphorylation at serine 70. Furthermore, ceramide- and etoposide-induced PP2A activation involved methylation of PP2A C subunit, which lithium suppressed. Lithium caused dissociation of PP2A B subunit from the PP2A core enzyme, whereas ceramide caused recruitment of the B subunit. Taken together, lithium exhibited an antiapoptotic effect by inhibiting Bcl-2 dephosphorylation and caspase-2 activation, which involved, at least in part, a mechanism of down-regulating PP2A methylation and PP2A activity.

Hepatitis B Virus X Protein Upregulates mTOR Signaling through IKKβ to Increase Cell Proliferation and VEGF Production in Hepatocellular Carcinoma

Hepatocellular carcinoma (HCC), a major cause of cancer-related death in Southeast Asia, is frequently associated with hepatitis B virus (HBV) infection. HBV X protein (HBx), encoded by a viral non-structural gene, is a multifunctional regulator in HBV-associated tumor development. We investigated novel signaling pathways underlying HBx-induced liver tumorigenesis and found that the signaling pathway involving IκB kinase β (IKKβ), tuberous sclerosis complex 1 (TSC1), and mammalian target of rapamycin (mTOR) downstream effector S6 kinase (S6K1), was upregulated when HBx was overexpressed in hepatoma cells. HBx-induced S6K1 activation was reversed by IKKβ inhibitor Bay 11-7082 or silencing IKKβ expression using siRNA. HBx upregulated cell proliferation and vascular endothelial growth factor (VEGF) production, and these HBx-upregulated phenotypes were abolished by treatment with IKKβ inhibitor Bay 11-7082 or mTOR inhibitor rapamycin. The association of HBx-modulated IKKβ/mTOR/S6K1 signaling with liver tumorigenesis was verified in a HBx transgenic mouse model in which pIKKβ, pS6K1, and VEGF expression was found to be higher in cancerous than non-cancerous liver tissues. Furthermore, we also found that pIKKβ levels were strongly correlated with pTSC1 and pS6K1 levels in HBV-associated hepatoma tissue specimens taken from 95 patients, and that higher pIKKβ, pTSC1, and pS6K1 levels were correlated with a poor prognosis in these patients. Taken together, our findings demonstrate that HBx deregulates TSC1/mTOR signaling through IKKβ, which is crucially linked to HBV-associated HCC development.

Anti-angiogenesis mediated by angiostatin K1-3, K1-4 and K1-4.5. Involvement of p53, FasL, AKT and mRNA deregulation

The molecular mechanism mediated by multiple forms of angiostatin via acting on proliferating vascular endothelium remains elusive. To address whether three forms of angiostatin, K1-3, K1-4 or K1-4.5, utilized similar or distinct pathways to mediate anti-angiogenesis, we adopted an adenoviral expression system to express secretable angiostatin molecules for CM collection. The anti-angiogenic activity of K1-3, K1-4 or K1-4.5 was confirmed by using proliferation, migration, tube formation and apoptotic assays of human endothelial cells. These angiostatin molecules at comparable expression level inhibited various in vitro angiogenesis assays with some variations. Furthermore, K1-3, K1-4 or K1-4.5 increased the expression of p53 protein and its downstream effectors, enhanced FasL-mediated signaling pathways, and decreased activation of AKT. At least three different receptors, Fas, integrin alpha(v)beta3 and ATP synthase, were involved in the anti-angiogenic action of angiostatin molecules. Besides, the expression of 189 genes at mRNA level was significantly altered by K1-3, K1-4 or K1-4.5. More than 70% of these genes participate in growth, inflammation, apoptosis, migration and extracellular matrix. Taken together, K1-3, K1-4 and K1-4.5, regardless of the number of kringles in the angiostatin molecules, mediated anti-angiogenesis via mostly similar pathways. We are the first to demonstrate the involvement of DAPK1 in the mediation of anti-angiogenesis by angiostatin.

Proteolysis of integrin α5 and β1 subunits involved in retinoic acid-induced apoptosis in human hepatoma Hep3B cells

Abstract Our previous report demonstrated that all-trans-retinoic acid (ATRA) induces detachment and death under serum starvation in several human tumor cell lines. In this study, we examined the influence of cell–extracellular matrix interaction on the ability of ATRA to induce apoptosis. Plating of human hepatoma Hep3B cells onto poly-hydroxyethylmethacrylate-coated plates in the absence of serum resulted in the acceleration of ATRA-induced apoptosis. In contrast, ATRA-induced apoptosis was significantly suppressed by plating cells onto Matrigel-coated plates but not suppressed by culturing onto collagen-, laminin-, vitronectin-, or fibronectin-coated plates. Exogenously added soluble collagen, laminin, fibronectin, vitronectin or Matrigel failed to suppress ATRA-induced apoptosis. Results from the adhesion assay indicated that the cell attachment to fibronectin was significantly inhibited by ATRA. Treatment with perturbing antibody against integrin α5 or β1 subunits resulted in promotion of ATRA-induced apoptosis. Moreover, the proteolytic cleavage of α5β1 integrin and focal adhesion kinase (FAK) proteins is linked to the early phase of the ATRA-induced apoptotic process. Furthermore, ATRA-induced detachment, death, and cleavage of α5β1 integrin and FAK were drastically suppressed by plating cells onto Matrigel-coated plates. These findings provide evidence that abrogation of cell adhesion, through proteolysis of α5β1 integrin and FAK, is closely linked to ATRA-induced apoptosis in Hep3B cells.

The B56γ3 Regulatory Subunit of Protein Phosphatase 2A (PP2A) Regulates S Phase-specific Nuclear Accumulation of PP2A and the G1 to S Transition

Protein phosphatase 2A (PP2A) is a heterotrimeric enzyme consisting of a scaffold subunit (A), a catalytic subunit (C), and a variable regulatory subunit (B). The regulatory B subunits determine the substrate specificity and subcellular localization of the PP2A holoenzyme. Here, we demonstrate that the subcellular localization of the B56γ3 regulatory subunit is regulated in a cell cycle-specific manner. Notably, B56γ3 becomes enriched in the nucleus at the G1/S border and in S phase. The S phase-specific nuclear enrichment of B56γ3 is accompanied by increases of nuclear A and C subunits and nuclear PP2A activity. Overexpression of B56γ3 promotes nuclear localization of the A and C subunits, whereas silencing both B56γ2 and B56γ3 blocks the S phase-specific increase in the nuclear localization and activity of PP2A. In NIH3T3 cells, B56γ3 overexpression reduces p27 phosphorylation at Thr-187, concomitantly elevates p27 protein levels, delays the G1 to S transition, and retards cell proliferation. Consistently, knockdown of endogenous B56γ3 expression reduces p27 protein levels and increases cell proliferation in HeLa cells. These findings demonstrate that the dynamic nuclear distribution of the B56γ3 regulatory subunit controls nuclear PP2A activity, which regulates cell cycle controllers, such as p27, to restrain cell cycle progression, and may be responsible for the tumor suppressor function of PP2A.