Molecular characterization of anti-apoptotic activity of Akt

Abstract

The serine-threonine kinase Akt was originally identified as a transforming oncogene. Later, it was found that Akt is phosphorylated and activated in response to mitogens and survival factors in a phosphatidylinositol 3-kinase(PI3K)-dependent manner. After it is activated, Akt regulates important cellular pathways such as cell metabolism, cell proliferation and cell survival by phosphorylating various target proteins. Through its cellular roles, Akt determines the important phenotypes of an organism such as body size and survival. In spite of intensive studies on the PI3K/Akt pathway, we have a limited understanding about the functions and regulations of Akt. Therefore, I tried to screen for novel signaling molecules in the PI3K/Akt pathway by two genetic approaches, yeast two-hybrid assay and fly functional genomics screening. From yeast two-hybrid experiments, I found that the C-terminal region of protein kinase C-related kinase 2 (PRK2) specifically bound to Akt. Cell biological studies revealed that during early stages of apoptosis, this C-terminal fragment of PRK2 was cleaved from the inhibitory N-terminal region and blocked the anti-apoptotic activities of Akt by directly binding in vivo. Moreover, in fly functional genomics assays, the Drosophila ortholog of FKHR (dFKHR) was identified as a regulator of organ size. Additional fly genetics and histology experiments showed that dFKHR is strictly regulated by the PI3K/Akt pathway and induces apoptosis in the Drosophila tissue overexpressing its activated mutants. Strikingly, a removal of dFKHR signaling significantly rescued the size reduction and lethality caused by a loss of function mutant of Dakt1 in a dose-dependent manner. These results, taken together, clearly demonstrated that Akt is the important signaling molecule in regulating fate and size of an organism through regulation of FKHR and is regulated by PRK2 during apoptosis.