DIRECT INHIBITION OF THE SIGNALING FUNCTIONS OF THE MAMMALIAN TARGET OF RAPAMYCIN BY THE PHOSPHOINOSITIDE 3-KINASE INHIBITORS, WORTMANNIN AND LY294002

The immunosuppressant, rapamycin, inhibits cell growth by interfering with the function of a novel kinase. termed mammalian target of rapamy cin (mTOR). The putative catalytic domain of mTOR is similar to those of mammalian and yeast phosphatidyl-inositol (PI) 3-kinases. This stud y demonstrates that mTOR is a component of a cytokine-triggered protei n kinase cascade leading to the phosphorylation of the eukaryotic init iation factor-4E (elF-4E) binding protein, PHAS-1, in activated T lymp hocytes. This event promotes G(1) phase progression by stimulating eIF -4E-dependent translation initiation, A mutant YAC-1 T lymphoma cell l ine, which was selected for resistance to the growth-inhibitory action of rapamycin, was correspondingly resistant to the suppressive effect of this drug on PHAS-1 phosphorylation, In contrast, the PI 3-kinase inhibitor, wortmannin, reduced the phosphorylation of PHAS-1 in both r apamycin-sensitive and -resistant T cells, At similar drug concentrati ons (0.1-1 mu M), wortmannin irreversibly inhibited the serine-specifi c autokinase activity of mTOR, The autokinase activity of mTOR was als o sensitive to the structurally distinct PI 3-kinase inhibitor, LY2940 02, at concentrations (1-30 mu M) nearly identical to those required f or inhibition of the lipid kinase activity of the mammalian p85-p110 h eterodimer. These studies indicate that the signaling functions of mTO R, and potentially those of other high molecular weight PI 3-kinase ho mologs, are directly affected by cellular treatment with wortmannin or LY294002.