The Pim and AKT serine/threonine protein kinases are implicated as drivers of cancer. Their regulation of tumor growth is closely tied to the ability of these enzymes to mainly stimulate protein synthesis by activating mTORC1 (mammalian target of rapamycin complex 1) signaling, although the exact mechanism is not completely understood. mTORC1 activity is normally suppressed by amino acid starvation through a cascade of multiple regulatory protein complexes, e.g., GATOR1, GATOR2, and KICSTOR, that reduce the activity of Rag GTPases. Bioinformatic analysis revealed that DEPDC5 (DEP domain containing protein 5), a component of GATOR1 complex, contains Pim and AKT protein kinase phosphorylation consensus sequences. DEPDC5 phosphorylation by Pim and AKT kinases was confirmed in cancer cells through the use of phospho-specific antibodies and transfection of phospho-inactive DEPDC5 mutants. Consistent with these findings, during amino acid starvation the elevated expression of Pim1 overcame the amino acid inhibitory protein cascade and activated mTORC1. In contrast, the knockout of DEPDC5 partially blocked the ability of small molecule inhibitors against Pim and AKT kinases both singly and in combination to suppress tumor growth and mTORC1 activity in vitro and in vivo. In animal experiments knocking in a glutamic acid (S1530E) in DEPDC5, a phospho mimic, in tumor cells induced a significant level of resistance to Pim and the combination of Pim and AKT inhibitors. Our results indicate a phosphorylation-dependent regulatory mechanism targeting DEPDC5 through which Pim1 and AKT act as upstream effectors of mTORC1 to facilitate proliferation and survival of cancer cells.
Keywords: AKT kinase; DEPDC5; GATOR1; Pim kinase; mTORC1.