PP2A是細胞中一種主要的Ser/Thr蛋白磷酸酶，參與在許多細胞活動中。PP2A完全酶複合體包含了三個次單元，包括了36kDa的催化次單元(PP2A/C)、65kDa的結構次單元(PP2A/A)，以及一個相當多樣化的調節性次單元(PP2A/B)。近來的文獻認為，B調節性次單元具有決定PP2A受質特異性(substrate specificity)及次細胞定位(subcellular localization)的功能。而根據我們實驗室之前的結果指出，PP2A AB55alphaC完全酶會與Akt有直接的交互作用，而Akt在許多細胞的活性上又扮演著關鍵的角色。本研究最終的目標是要探討是否PP2A AB55alphaC完全酶與Akt的交互作用會抵制Akt的活性。我們的結果指出，在血清刺激的NIH3T3細胞與IL-3仰賴性的FL5.12細胞中，B55alpha的overexpression會顯著地降低Akt Thr308位點的磷酸化，但是對於Ser473位點的磷酸化僅是些微的減低。另一方面，利用shRNA媒介的RNA干擾(shRNA-mediated RNA interference)，降低NIH3T3細胞及FL5.12細胞中內生性B55alpha的表現，皆會明顯地增加Thr308位點的磷酸化。相似地，在IL-3仰賴型的FL5.12細胞中，以藥理性抑制的方式抑制PP2A的活性，Akt Thr308位點磷酸化的情形也會較Ser473位點來得顯著。與細胞實驗結果相同地，在試管內Akt去磷酸化分析(in vitro Akt dephosphorylation assay)的實驗中，PP2A AB55alphaC完全會選擇性地針對Akt的phospho-Thr308進行去磷酸化。與上述結果ㄧ致地，在NIH3T3細胞及FL5.12細胞中，部份Akt下游受質的磷酸化會受到B55alpha overexpression的影響而減弱，這也暗示著Akt部份的活性會因為B55alpha overexpression而降低。更進一步地，我們也發現，B55alpha的overexpression會遲滯NIH3T3細胞的增生，而B55alpha的knockdown則會增加FL5.12細胞在IL-3移除下的存活率。我們的結果闡明了，B55alpha次單元會引導著PP2A完全酶調控Akt的活性，並且進一步地在細胞生長及細胞存活上扮演著重要的角色。

PP2A is one of the major serine/threonine phosphatases involved in regulation of many cellular processes. A PP2A holoenzyme complex consists of three subunits, including a 36 kDa catalytic subunit (PP2A/C), a 65 kDa scaffold subunit (PP2A/A), and a third variable regulatory subunit (PP2A/B). B regulatory subunit is implicated to control substrate specificity and subcellular localization of PP2A. Our lab has previously found that PP2A AB55alphaC holoenzyme directly interacted with Akt, which plays a crucial role in regulating multiple cellular activities. The ultimate goal of this study is to investigate whether the interaction between PP2A AB55alphaC holoenzyme and Akt counteracts Akt activity. Our results showed that in both serum-stimulated NIH3T3 cells and IL-3-dependent FL5.12 cells, B55alpha overexpression impaired phosphorylation of Thr308 of Akt significantly, but impaired Ser473 modestly. On the other hand, silence of endogenous B55alpha expression by shRNA-mediated RNA interference markedly increased phosphorylation at Thr308 but not at Ser473 in both NIH3T3 cells and FL5.12 cells. Similarly, we found that pharmacological inhibition of PP2A increased Thr308 phosphorylation of Akt more significantly than Ser473 in IL-3-dependent FL5.12 cells. In parallel with the cellular findings, PP2A AB55alphaC holoenzymes were found to preferentially dephosphorylate phospho-Thr308 rather than phospho-Ser473 in in vitro Akt dephosphorylation assays. Consistently, phosphorylation of a subset of Akt substrates was significantly impaired by B55alpha overexpression in both FL5.12 and NIH3T3 cells, suggesting that B55alpha overexpression partially reduced Akt activity. Furthermore, B55alpha overexpression retarded proliferation of NIH3T3 cells, and B55alpha knockdown increased survival of FL5.12 cells upon IL-3 withdrawal. Our data demonstrate that B55alpha subunit targets PP2A holoenzyme to regulate Akt activity and has an important role in cell growth and cell survival.

1. Andjelkovic, M. et al. Activation and phosphorylation of a pleckstrin homology domain containing protein kinase (RAC-PK/PKB) promoted by serum and protein phosphatase inhibitors. Proc Natl Acad Sci U S A 93, 5699-5704 (1996).
2. Coffer, P.J. & Woodgett, J.R. Molecular cloning and characterisation of a novel putative protein-serine kinase related to the cAMP-dependent and protein kinase C families Eur J Biochem 201, 475-81 (1991).
3. Bellacosa, A., Testa, J.R., Staal, S.P. & Tsichlis, P.N. A retroviral oncogene, Akt encoding a Serine-Threonine kinase containing an SH2-like region. Science 254, 274-7 (1991).
4. Murthy, S.S., Tosolini, A., Taguchi, T. & Testa, J.R. Mapping of AKT3, encoding a member of the Akt/protein kinase B family, to human and rodent chromosomes by fluorescence in situ hybridization. Cytogenet Cell Genet 88, 39–40 (2000).
5. Chen, W.S. et al. Growth retardation and increased apoptosis in mice with homozygous disruption of the Akt1 gene. Genes Dev 15, 2203-8 (2001).
6. Cho, H., Thorvaldsen, J.L., Chu, Q., Feng, F. & Birnbaum, M.J. Akt1/pkbalpha is required for normal growth but dispensable for maintenance of glucose homeostasis in mice. J Biol Chem 276, 38349-52 (2001).
7. Cho, H. et al. Insulin resistance and a diabetes mellitus-like syndrome in mice lacking the protein kinase Akt2 (PKB beta). Science 292, 1728-31 (2001).
8. George, S. et al. A family with severe insulin resistance and diabetes due to a mutation in AKT2. Science 304, 1325-8 (2004).
9. Tschopp, O. et al. Essential role of protein kinase B gamma (PKB gamma/Akt3) in postnatal brain development but not in glucose homeostasis. Development 132, 2943-54 (2005).
10. Franke, T.F. et al. The protein kinase encoded by the Akt proto-oncogene is a target of the PDGF-activated phosphatidylinositol 3-kinase. Cell 81, 727-36 (1995).
11. Chan, T.O., Rittenhouse, S.E. & Tsichlis, P.N. AKT/PKB and other D3 phosphoinositide-regulated kinases:kinase activation by phosphoinositide-dependent phosphorylation. Annu Rev Biochem 68, 965-1014 (1999).
12. Vivanco, I. & Sawyers, C.L. The phosphatidylinositol 3-kinase AKT pathway in human cancer. Nat Rev Cancer 2, 489-501 (2002).
13. Luo, J., Manning, B.D. & Cantley, L.C. Targeting the PI3K-Akt pathway in human cancer: rationale and promise. Cancer Cell 4, 257-62 (2003).
14. Cross, D.A., Alessi, D.R., Cohen, P., Andjelkovich, M. & Hemmings, B.A. Inhibition of glycogen synthase kinase-3 by insulin mediated by protein kinase B. Nature 378, 785-9 (1995).
15. Wendel, H.G. et al. Survival signalling. by Akt and eIF4E in oncogenesis and cancer therapy. Nature 428, 332-7 (2004).
16. Harris, T.E. & Lawrence, J.C., Jr. TOR signaling. Sci STKE 2003, re15 (2003).
17. Chen, X. et al. Constitutively active Akt is an important regulator of TRAIL sensitivity in prostate cancer. Oncogene 20, 6073-83 (2001).
18. Wang, Q. et al. Regulation of TRAIL expression by the phosphatidylinositol 3-kinase/Akt/GSK-3 pathway in human colon cancer cells. J Biol Chem 277, 36602-10 (2002).
19. Hennessy, B.T., Smith, D.L., Ram, P.T., Lu, Y. & Mills, G.B. Exploiting the PI3K/AKT pathway for cancer drug discovery. Nat Rev Drug Discov 4, 988-1004 (2005).
20. Nicholson, K.M. & Anderson, N.G. Cell Signal 14, 381-395 (2002).
21. Carroll, P.E. et al. Centrosome hyperamplification in human cancer: chromosome instability induced by p53 mutation and/or Mdm2 overexpression. Oncogene 18, 1935-44 (1999).
22. Toi, M. et al. MDM2 in breast cancer. Breast Cancer 25, 264–68 (1997).
23. Sherr, C.J. & Weber, J.D. The ARF/p53 pathway. Curr Opin Genet Dev 10, 94-9 (2000).
24. Testa, J.R. & Bellacosa, A. AKT plays a central role in tumorigenesis. Proc Natl Acad Sci U S A 98, 10983-5 (2001).
25. Sarbassov, D.D., Guertin, D.A., Ali, S.M. & Sabatini, D.M. Phosphorylation and regulation of Akt/PKB by the rictor-mTOR complex. Science 307, 1098-101 (2005).
26. Jacinto, E. et al. SIN1/MIP1 maintains rictor-mTOR complex integrity and regulates Akt phosphorylation and substrate specificity. Cell 127, 125-37 (2006).
27. Alessi et al. Characterization of a 3-phosphoinositide-dependent protein kinase which phosphorylates and activates protein kinase Bα. Curr. Biol. 7, 261–269 (1997).
28. Stephens, L. et al. Protein kinase B kinases that mediate phosphatidylinositol 3,4,5-trisphosphate-dependent activation of protein kinase B. Science 279, 710–714 (1998).
29. Bayascas, J.R. & Alessi, D.R. Regulation of Akt/PKB Ser473 phosphorylation. Mol. Cell 18, 143–145 (2005).
30. Dong, L.Q. & Liu, F. PDK2: the missing piece in the receptor tyrosine kinase signaling pathway puzzle. Am. J. Physiol. Endocrinol. Metab. 289, E187–E196 (2005).
31. Feng, J., Park, J., Cron, P., Hess, D. & Hemmings, B.A. Identification of a PKB/Akt Hydrophobic Motif Ser-473 Kinase as DNA-dependent Protein Kinase. J. Biol. Chem. 279, 41189–41196 (2004).
32. Kawakami, Y. et al. Protein kinase C II regulates Akt phosphorylation on Ser-473 in a cell type- and stimulus-specific fashion. J. Biol. Chem. 279, 47720–47725 (2004).
33. Troussard, A.A. et al. Conditional knock-out of integrin-linked kinase demonstrates an essential role in protein kinase B/Akt avtivation. J. Biol. Chem. 278, 22374–22378 (2003).
34. Woodgett, J.R. Recent advances in the protein kinase B signaling pathway. Curr. Opin. Cell Biol. 17, 150–157 (2005).
35. Meier, R., Alessi, D.R., Cron, P., Andjelkovic, M. & Hemmings, B.A. Mitogenic activation, phosphorylation, and nuclear translocation of protein kinase B. J Biol Chem 272, 30491-7 (1997).
36. Meier, R. & Hemmings, B.A. Regulation of protein kinase B. J Recept Signal Transduct Res 19, 121-8 (1999).
37. Meier, R., Thelen, M. & Hemmings, B.A. Inactivation and dephosphoryla-. tion of protein kinase Bα (PKBα) promoted by hyperosmotic stress. EMBO J 17, 7294-303 (1998).
38. Gao, T., Furnari, F. & Newton, A.C. PHLPP:a phosphatase that directly dephosphorylates Akt,promotes apoptosis,and suppresses tumor growth. Mol Cell 18, 13-24 (2005).
39. Chen, D., Fucini, R.V., Olson, A.L., Hemmings, B.A. & Pessin, J.E. Osmotic shock in- hibits insulin signaling by maintaining Akt Protein-Kinase-B man inactive dephosphorylated state. Mol Cell Biol 19, 4684-4694 (1999).
40. Sato, S., Fujita, N. & Tsuruo, T. Modulation of Akt kinase activity by binding to Hsp90. Proc Natl Acad Sci U S A 97, 10832-10837 (2000).
41. Ivaska, J. et al. Integrin alpha 2 beta 1 promotes activation of protein phosphatase 2A and dephosphorylation of Akt and glycogen synthase kinase 3 beta. Mol Cell Biol 22, 1352-1359 (2002).
42. Resjo, S. et al. Protein phosphatase 2A is the main phosphatase involved in the regulation of protein kinase B in rat adipocytes. Cell Signal 14, 231-238 (2002).
43. Hunter, T. Protein kinase and phosphatases: the yin and yang of protein phosphorylation and signaling. Cell 80, 225-36 (1995).
44. Shenolikar, S. Protein serine/threonine phosphatase-New avenues for cell regulation. Annu Rev Cell Biol 10, 55-86 (1994).
45. Barford, D. Molecular mechanisms of the. protein serine/threonine phophatases. Trends Biochem Sci 21, 407-12 (1996).
46. Wera, S. & Hemmings, B.A. Serine/threonine protein phosphatases. Biochem J 311 ( Pt 1), 17-29 (1995).
47. Millward, T.A., Zolnierowicz, S. & Hemmings, B.A. Regulation of protein kinase cascades by protein phosphatase 2A. Trends Biochem Sci 24, 186-91 (1999).
48. Stone, S.R., Hofsteenge, J. & Hemmings, B.A. Molecular cloning of cDNAs encoding. two isoforms of the catalytic subunit of protein phosphatase 2A. Biochemistry 26, 7215-20 (1987).
49. Green, D.D., Yang, S.I. & Mumby, M.C. Molecular cloning and sequence. analysis of the catalytic subunit of bovine type 2A protein phosphatase. Proc Natl Acad Sci U S A 84, 4880-4 (1987).
50. Arino, J., Woon, C.W., Brautigan, D.L., Miller, T.B., Jr. & Johnson, G.L. Human liver phosphatase 2A: cDNA and amino acid sequence of two catalytic subunit isotypes. Proc Natl Acad Sci U S A 85, 4252-6 (1988).
51. Khew-Goodall, Y. & Hemmings, B.A. Tissue-specific expression of mRNAs encoding alpha- and beta-catalytic subunits of protein phosphatase 2A. FEBS Lett 238, 265-8 (1988).
52. Khew-Goodall, Y., Mayer, R.E., Maurer, F., Stone, S.R. & Hemmings, B.A. Structure and transcriptional regulation of protein phosphatase 2A RT catalytic subunit genes. Biochemistry 30, 89-97 (1991).
53. Cormier, P. et al. Protein phosphatase 2A from Xenopus oocytes. Characterization during meiotic. cell division. FEBS Lett 295, 185-8 (1991).
54. Van Hoof, C. et al. Molecular cloning and developmental regulation of expression of two isoforms of the catalytic subunit of protein phosphatase 2A from Xenopus laevis. Biochem Biophys Res Commun 215, 666-73 (1995).
55. Orgad, S. et al. The structure of protein phosphatase 2A is as highly conserved as that of protein phosphatase 1. FEBS Lett 275, 44-8 (1990).
56. MacKintosh, R.W., Haycox, G., Hardie, D.G. & Cohen, P.T. Identification by molecular cloning of two cDNA sequences from the plant Brassica napus which are very similar to mammalian protein phosphatases-1 and -2A. FEBS Lett 276, 156-60 (1990).
57. Arino, J. et al. Protein phosphatases in higher plants: multiplicity of type 2A phosphatases in Arabidopsis thaliana. Plant Mol Biol 21, 475-85 (1993).
58. Kinoshita, N., Ohkura, H. & Yanagida, M. Distinct, essential roles of type 1 and 2A protein phosphatases in the control of the fission yeast cell division cycle. Cell 63, 405-15 (1990).
59. Sneddon, A.A., Cohen, P.T. & Stark, M.J. Saccharomyces cerevisiae protein phosphatase 2A performs an essential cellular function and is encoded by two genes. EMBO J 9, 4339-46 (1990).
60. Cohen, P.T., Brewis, N.D., Hughes, V. & Mann, D.J. Protein serine/threonine phosphatases; an expanding family. FEBS Lett 268, 355-9 (1990).
61. Hemmings, B.A. et al. alpha- and beta-forms of the 65-kDa subunit of protein phosphatase 2A have a similar 39 amino acid repeating structure. Biochemistry 29, 3166-73 (1990).
62. Ruediger, R., Hentz, M., Fait, J., Mumby, M. & Walter, G. Molecular model of the A subunit of protein phosphatase 2A: interaction with other subunits. J Virol 68, 123-9 (1994).
63. Ruediger, R. et al. Identification of binding sites on the regulatory A subunit of protein phosphatase 2A for the catalytic C subunit and for tumor antigens of simian virus 40 and polyomavirus. Mol Cell Biol 12, 4872-82 (1992).
64. Andrade, M.A. & Bork, P. HEAT repeats in the Huntington's disease protein. . Nat Genet 11, 115-6 (1995).
65. Groves, M.R., Hanlon, N., Turowski, P., Hemmings, B.A. & Barford, D. The structure of the protein phosphatase 2A PR65/A subunit reveals the conformation of its 15 tandemly repeated HEAT motifs. Cell 96, 99-110 (1999).
66. Mayer, R.E. et al. Structure of the 55-kDa regulatory subunit of protein phosphatase 2A: evidence for a neuronal-specific isoform. Biochemistry 30, 3589-97 (1991).
67. Healy, A.M. et al. CDC55, a Saccharomyces cerevisiae gene involved in cellular morphogenesis: identification, characterization, and homology to the B subunit of mammalian type 2A protein phosphatase. Mol Cell Biol 11, 5767-80 (1991).
68. Zolnierowicz, S. et al. Diversity in the regulatory Bsubunits of protein phosphatase 2A: identification of a novel isoform highly expressed in brain. Biochemistry 33, 11858-67 (1994).
69. Strack, S., Chang, D., Zaucha, J.A., Colbran, R.J. & Wadzinski, B.E. Cloning and characterization of B delta, a novel regulatory subunit of protein phosphatase 2A. FEBS Lett 460, 462-6 (1999).
70. Strack, S., Zaucha, J.A., Ebner, F.F., Colbran, R.J. & Wadzinski, B.E. Brain protein phosphatase 2A: developmental regulation and distinct cellular and subcellular localization by B subunits. J Comp Neurol 392, 515-27 (1998).
71. Mayer-Jaekiel, R.E. & Hemmings, B.A. Protein phosphatase type 2A-menage a trois. Trends Cell Biol. 4, 287–291 (1994).
72. Mayer-Jaekel, R.E. et al. The 55 kd regulatory subunit of Drosophila protein phosphatase 2A is required for anaphase. Cell 72, 621-33 (1993).
73. Turowski, P., Myles, T., Hemmings, B.A., Fernandez, A. & Lamb, N.J. Vimentin dephosphorylation by protein phosphatase 2A is modulated by the targeting subunit B55. Mol Biol Cell 10, 1997-2015 (1999).
74. McCright, B. & Virshup, D.M. Identification of a new family of protein phosphatase 2A regulatory subunits. J Biol Chem 270, 26123-8 (1995).
75. Csortos, C., Zolnierowicz, S., Bako, E., Durbin, S.D. & DePaoli-Roach, A.A. High complexity in the expression of the B subunit of protein phosphatases 2A. Evidence for the existence of at least seven novel iso-forms. J Biol Chem 271, 2578-88 (1996).
76. Tehrani, M.A., Mumby, M.C. & Kamibayashi, C. Identification of a novel protein phosphatase 2A regulatory subunit highly expressed in muscle. J Biol Chem 271, 5164-70 (1996).
77. Tanabe, O. et al. Molecular cloning of a 74-kDa regulatory subunit (B" or delta) of human protein phosphatase 2A. FEBS Lett 379, 107-11 (1996).
78. McCright, B., Rivers, A.M., Audlin, S. & Virshup, D.M. The B56 family of protein phosphatase 2A (PP2A) regulatory subunits encodes differentiation-induced phosphoproteins that target PP2A to both nucleus and cytoplasm. J Biol Chem 271, 22081-9 (1996).
79. Zolnierowicz, S. et al. The variable subunit associated with protein phosphatase 2A0 defines a novel multimember family of regulatory subunits. Biochem J 317 ( Pt 1), 187-94 (1996).
80. Nagase, T. et al. J Biochem (Tokyo) 122, 178-87 (1997).
81. Graeber, T.G. et al. Hypoxia induces accumulation of p53 protein, but activation of a G1-phase checkpoint by low-oxygen conditions is independent of p53 status. Mol Cell Biol 14, 6264-77 (1994).
82. Hendrix, P. et al. Structure and expression of a 72-kDa regulatory subunit of protein phosphatase 2A. Evidence for different size forms produced by alternative splicing. J Biol Chem 268, 15267-76 (1993).
83. Cegielska, A., Shaffer, S., Derua, R., Goris, J. & Virshup, D.M. Different oligomeric forms of protein phosphatase 2A activate and inhibit simian virus 40 DNA replication. . Mol Cell Biol 14, 4616-23 (1994).
84. Yan, Z., Fedorov, S.A., Mumby, M.C. & Williams, R.S. PR48, a Novel Regulatory Subunit of Protein Phosphatase 2A, Interacts with Cdc6 and Modulates DNA Replication in Human Cells. Mol Cell Biol 20, 1021-9 (2000).
85. Voorhoeve, P.M., Watson, R.J., Farlie, P.G., Bernards, R. & Lam, E.W. Oncogene 18, 679-88 (1999).
86. Moreno, C.S. et al. J Biol Chem 275, 5257-63 (2000).
87. Westphal, R.S., Anderson, K.A., Means, A.R. & Wadzinski, B.E. Science 280, 1258-1261 (1998).
88. Westphal, R.S., Coffee, R.L., Jr., M., A. , Pelech, S.L. & Wadzinski, B.E. J Biol Chem 274, 687-692 (1999).
89. Peterson, R.T., Desai, B.N., Hardwick, J.S. & Schreiber, S.L. Proc Natl Acad Sci U S A 96, 4438-4442 (1999).
90. Lee, T., Kim, S.J. & Sumpio, B.E. J Cell Physiol 194, 349-355 (2003).
91. Heriche, J.K. et al. Science 276, 952-955 (1997).
92. Fu, D.X., Kuo, Y.L., Liu, B.Y., Jeang, K.T. & Giam, C.Z. J Biol Chem 278, 1487-1493 (2003).
93. Kray, A.E. et al. J Biol Chem 280, 35974-35982 (2005).
94. Ory, S., Zhou, M., Conrads, T.P., Veenstra, T.D. & Morrison, D.K. Curr Biol 13, 1356-1364 (2003).
95. Liu, Q. & Hofmann, P.A. Am J Physiol Heart Circ Physiol 286, H2204-2212 (2004).
96. Yellaturu, C.R., Bhanoori, M., Neeli, I. & Rao, G.N. J Biol Chem 277, 40148-40155 (2002).
97. Liu, W. et al. Cell Death Differ 10, 772-781 (2003).
98. Borgatti, P. et al. J Cell Physiol 196, 79-88 (2003).
99. Trotman, L.C. et al. Nature 441, 523-527 (2006).
100. Silverstein, A.M., Barrow, C.A., Davis, A.J. & and Mumby, M.C. Proc Natl Acad Sci U S A 99, 4221-4226 (2002).
101. Van Kanegan, M.J. J Biol Chem 280, 36029-36 (2005).
102. Adams, D.G. et al. J Biol Chem 280, 42644-42654 (2005).
103. Letourneux, C., Rocher, G. & Porteu, F. EMBO J 25, 727-738 (2006).
104. Rocher, G., Letourneux, C., Lenormand, P. & Porteu, F. J Biol Chem 282, 5468-5477 (2007).
105. Sontag, E., Fedorov, S., Kamibayashi, C., Robbins, D., Cobb, M. & and Mumby, M. Cell 75, 887-897 (1993).
106. Chen, W. et al. Identification of specific PP2A complexes involved in human cell transformation. Cancer Cell 5, 127-136 (2004).
107. Yuan, H., Veldman, T., Rundell, K. & Schlegel, R. Simian Virus 40 Small Tumor Antigen Activates AKT and Telomerase and Induces Anchorage-Independent Growth of Human Epithelial Cells. J Virol 76, 10685-10691 (2002).
108. Zhao, J.J. et al. Human mammary epithelial cell transformation through the activation of phosphatidylinositol 3-kinase. Cancer Cell 3, 483-495 (2003).
109. Arroyo, J.D. & Hahn, W.C. Involvement of PP2A in viral and cellular transformation. Oncogene 24, 7746-7755 (2005).
110. Chiang, C.W. Protein phosphatase 2A activates the proapoptotic function of BAD in interleukin- 3-dependent lymphoid cells by a mechanism requiring 14-3-3 dissociation. Blood 97, 1289-97 (2001).
111. Brummelkamp, T.R., Bernards, R. & Agami, R. A system for stable expression of short interfering RNAs in mammalian cells. Science 296, 550-553 (2002).
112. Songyang, Z., Baltimore, D., Cantley, L.C., Kaplan, D.R. & and Franke, T.F. Interleukin 3-dependent survival by the Akt protein kinase. Proc Natl Acad Sci U S A 94, 11345-11350 (1997).
113. Yamaguchi, H. & Wang, H.G. Tissue transglutaminase serves as an inhibitor of apoptosis by cross-linking caspase-3 in thapsigargin-treated cells. Oncogene 20, 7779-7786 (2001).
114. Scheid, M.P., Marignani, P.A. & Woodgett, J.R. Multiple Phosphoinositide 3-Kinase-Dependent Steps in Activation of Protein Kinase B. Mol Cell Biol 22, 6247-6260 (2002).
115. Strack, S. Overexpression of the protein phosphatase 2A regulatory subunit B promotes neuronal differentiation by activating the MAP kinase (MAPK) cascade. J Biol Chem 277, 41525-41532 (2002).
116. Dagda, R.K., Zaucha, J.A., Wadzinski, B.E. & Strack, S. A developmentally regulated, neuron-specific splice variant of the variable subunit B{beta} targets protein phosphatase 2A to mitochondria and modulates apoptosis. J Biol Chem 278, 24976-24985 (2003).
117. Yamada, T. et al. 3-phosphoinositide-dependent protein kinase 1, an Akt1 kinase, is involved in dephosphorylation of Thr-308 of Akt1 in Chinese hamster ovary cells. J Biol Chem 276, 5339-5345 (2001).
118. Xu, Y. et al. Structure of the protein phosphatase 2A holoenzyme. Cell 127, 1239-1251 (2006).
119. Cho, U.S. & Xu, W. Crystal structure of a protein phosphatase 2A heterotrimeric holoenzyme. Nature 445, 53-57 (2007).
120. Yang, J. et al. Crystal structure of an activated Akt/protein kinase B ternary complex with GSK3-peptide and AMP-PNP. Nat Struct Biol 9, 940-944 (2002).
121. Shiota, C., Woo, J.T., Lindner, J., Shelton, K.D. & Magnuson, M.A. Multiallelic disruption of the rictor gene in mice reveals that mTOR complex 2 is essential for fetal growth and viability. . Dev Cell 11, 583-589 (2006).
122. Yan, L. et al. PP2A regulates the pro-apoptotic activity of FOXO1. J Biol Chem (2008).
123. Yang, J.Y. et al. ERK promotes tumorigenesis by inhibiting FOXO3a via MDM2-mediated degradation. Nat Cell Biol (2008).
124. Janssens, V., Goris, J. & Van Hoof, C. PP2A: the expected tumor suppressor. Curr Opin Genet Dev 15, 34-41 (2005).