Membrane translocation and activation of the Akt kinase in growth factor-stimulated hematopoietic cells

Abstract

In the last two years, there has been considerable excitement regarding the mechanisms of signal propagation from receptor tyrosine kinases to the serinethreonine kinase Akt [l-3]. The study by Zhang and Vik, published in a recent issue of Leukemia Research [4], describes activation of Akt in the GM-CSFdependent megakaryocytic cell line M07e. Following serum and growth factor starvation, Akt kinase activity in this cell line is dowmegulated within 1 h. Stimulation with a variety of factors inducing proliferation or differentiation (GM-CSF, IL11, and phorbol 12-myristate 13-acetate) leads to activation of Akt with rapid kinetics. More importantly, the authors show that the increase in Akt kinase activity is paralleled by translocation of the protein to the cell membrane. The report highlights the importance of membrane translocation in the process of Akt activation. The Akt kinase is encoded by c-akt, the transduced cellular homologue of the viral oncogene v-akt. The vakt oncogene was identified in an acute transforming retrovirus from a T-cell lymphoma of the high leukemia mouse strain AKR [5,6]. Akt is now known to define a family of related genes that are highly conserved in evolution. In man, two homologous cellular sequences have been identified: AKTl and AKT2 [5]. AKTI is the true human homologue of mouse c-akt (98% identity at the amino acid level) [7]. The AKTI gene is identical to MC-PKcl, which was cloned by virtue of its homology to genes encoding protein kinases A and C [8], and the protein kinase Bcr (PKBa) gene [9]. AKT2 is identical to MC-PKB and PKBB [9-l 11. Recently, a gene encoding another Akt family member, rat Rat-PKy, has also been identified [ 121. The amino terminus of each Akt family member is comprised of a pleckstrin homology (PH) domain (amino acids 2-l 12), a region shared by a large family of proteins with diverse functions, especially molecules involved in signal transduction pathways [ 13-151. AKTl and AKT2 each contain a 70-amino acid long carboxy-terminal tail (C-tail) that appears to have a regulatory role [16]. The most divergent regions between AKTl and AKT2 are the amino-terminal PH domain and the C-tail (Fig. l), suggesting that these two regions may confer functional differences between AKTl and AKT2. Both AKTl and AKl2 have been implicated in human cancer. Amplification and/or overexpression of AKT2 have been observed in lO-20% of human ovarian and pancreatic carcinomas [l 1,17-191. We have demonstrated that overexpression of AKT2 protein is sufficient to transform NIH 3T3 cells, and that the oncogenic activity of AKT2 is abolished by truncation of the 70amino acid proline-rich C-tail [16]. Furthermore, AKT2 antisense RNA has been shown to inhibit the tumorigenie phenotype of human pancreatic cancer cells exhibiting amplification and/or overexpression of AKT2 [18]. In contrast to AKT2, AKTI has been shown to be amplified in only a single human gastric carcinoma [5]. Because of its location at chromosome band 14q32, proximal to the ZGH locus [20], AKTI had been proposed as a candidate gene targeted by 14q32 chromosome rearrangements in human T-cell malignancies, prolymphocytic leukemias, and mixed lineage childhood leukemias [2&22]. However, we have observed no alterations of AKTI in an analysis of more than 30 such hematologic specimens or in approximately 100 specimens from various types of solid tumors (J. R. Testa and J. Q. Cheng, unpublished data). The Akt/AKTl kinase can be activated by serum, various growth factors including platelet derived growth factor (PDGF), epidermal growth factor and basic fibroblast growth factor, insulin, and cellular stress such as heat shock and hyperosmolarity. Akt is catalytically inactive in serum-starved cells, but its kinase activity is rapidly induced following treatment with PDGF, and activation of Akt is abrogated by mutations in the PH