Avruch, Joseph MD Cell Regulation by small GTPases and Protein kinases
Elucidation of the TOR pathway. TOR is a giant protein kinase that responds to nutrient sufficiency and insulin/IGF-1 (and other mitogens) to control the growth/size of all cells (e.g., pancreatic beta cells), and the proliferation of some, such as T cells and vascular smooth muscle cells. TOR also negatively regulates insulin signaling to PI-3 kinase. TOR functions in two physically separate, independently regulated hetero-oligomeric complexes (mTOR complexes 1 and 2); The major effort is directed at understanding how the small, ras-like GTPase, Rheb, which we showed acts directly on the mTOR polypeptide, controls the kinase activity of mTORC1. The activity of mTORC1 is also controlled by amino acids especially leucine. We showed that leucine controls the interaction of Rheb with mTOR in vivo; the basis for this control appears to be mediated by another set of small GTPases, the Rag polypeptides, and current effort is focused on the regulation of the latter. mTOR interaction with its substrates (e.g., 4E-BP, S6K1) appears to be regulated; the basis for this regulation is under study. Finally, the mechanisms by which mTOR gates the translation of the IGF2 mRNA isoform specifically associated with the RNA binding protein IMP2, as well as the identification of the cohort of mRNAs bound to IMP2 is under investigation; IMP2 is a candidate T2DM locus and fetal IGF2 is relevant to the nutrient control of fetal growth.
The regulation and physiologic functions of the Mst1 and Mst2 protein kinases, which are the mammalian orthologs of the “hippo” kinase, a central element in an anti-proliferative developmental pathway defined in Drosophila. We identified Mst1/2 as constitutive partners of the tumor suppressor proteins RASSF1/Nore1; the latter bind specifically to the active forms of the Ras and Rap1 small GTPases. Transfection and biochemical analyses defined the regulation of the Nore1/Mst1 complex by ras-like GTPases and identified the cyclin-like polypeptide Mob1 as a preferred Mst1/2 substrate. To gain an understanding of the physiologic functions of these kinases, Mst1 and Mst2 KO mice were generated. The Mst1 KO mice exhibit major abnormalities in the function of T cells; elimination of Mst1 greatly enhances the proliferative response of mature, naïve T cells to stimulation of the T cell antigen receptor, whereas the clustering and activation of integrins is severely impaired. Surprisingly, these effects of Mst1 KO are independent of the downstream elements of the canonical “hippo” pathway. In contrast, Mst1 and Mst2 act as redundant tumor suppressors in liver, largely through inhibition of the YAP transcriptional coactivator, paralleling the behavior of “hippo”. The regulation of Mst1/Mst2 in their multiple signaling pathways and their physiologic targets are under investigation.