Protein kinase B (c-Akt): a multifunctional mediator of phosphatidylinositol 3-kinase activation

Abstract

While a plethora of extracellular molecules exist that modulate cellular functions via binding to membrane receptors inside the cell, their actions are mediated by relatively few signalling mechanisms. One of these is activation of phosphatidylinositol 3-kinase (PI-3K), which results in the generation of a membranerestricted second messenger, polyphosphatidylinositides containing a 3h-phosphate. How these molecules transduced the effects of agonists of PI-3K was unclear until the recent discovery that several protein kinases become activated upon exposure to 3h-phosphorylated inositol lipids. These enzymes include protein kinase B (PKB)\AKT and PtdIns(3,4,5)P $ -dependent kinases 1 and 2, the first two of which interact with 3h-phosphorylated ORIGINS OF AKTThe AKR strain of mice exhibit a high incidence of leukaemias and lymphomas from spontaneous thymoma [1]. A retrovirus termed AKT8 was isolated from one of these lines derived from a spontaneous thymoma. This virus was demonstrated to uniquely transform only mink lung cells (CCL64) in culture, while virus inoculated into newborn mice was shown to be tumorigenic [2]. The non-viral DNA component transduced from the mouse genome was subsequently identified, and two human homologues, AKT1 and AKT2, cloned [3]. The location of the human AKT locus was mapped to chromosome 14q32, proximal to the immunoglobulin-heavy-chain locus [4], a region frequently affected by translocations and inversions in human Tcell leukaemia\lymphoma, mixed-lineage childhood leukaemia and clonal T-cell proliferations in ataxia telangiectasia, supporting a role for this oncogene in formation of a variety of tumours [5]. Analysis of a panel of human tumours revealed a 20-fold amplification of AKT1 in a primary gastric adenocarcinoma. AKT2, on the other hand, was mapped to chromosome region 19q13.1-q13.2 and shown to be amplified and overexpressed in several ovarian carcinoma and pancreatic cancer cell lines [6,7]. A recent large-scale study of AKT2 alterations in ovarian and breast tumours revealed amplification in 12.1 % ovarian and 2.8 % breast carcinomas [8]. Furthermore, amplification of AKT2 was especially frequent in undifferentiated tumours, suggesting that AKT2 alterations may be associated with tumour aggressiveness.Abbreviations used : PI-3K, phosphatidylinositol 3-kinase ; PKB, protein kinase B ; PKA, protein kinase A ; PKC, protein kinase C ; RAC-PK, related to A-and C-kinase ; PH, pleckstrin homology ; PtdIns, phosphoinositide ; PDGF, platelet-derived growth factor ; EGF, epidermal growth factor ; bFGF, basic fibroblast growth factor ; IL, interleukin ; ERK, extracellular-signal-regulated kinase ; Btk, Bruton tyrosine kinase ; PDK, PtdIns(3,4,5)P 3 -dependent kinase ; MAPKAP, mitogen-activated protein kinase-activated protein ; SH2, Src homology 2 ; gag, group-specific antigen ; GLUT, glucose transporter ; GSK, glycogen synthase kinase ; PFK2, 6-phosphofructose-2-kinase ; IGF, insulin-like growth factor ; 4E-BP1, 4E-binding protein ; PHAS, pH-and acid-stable ; BCR-ABL, breakpoint...

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