Insulin resistance is a major hallmark in the development of type II diabetes, which is characterized by the failure of insulin to promote glucose uptake in muscle and to suppress glucose production in liver. The serine-threonine kinase Akt (PKB) is a principal target of insulin signaling that inhibits hepatic glucose output when glucose is available from food. Here we show that TRB3, a mammalian homolog of Drosophila tribbles, functions as a negative modulator of Akt. TRB3 expression is induced in liver under fasting conditions, and TRB3 disrupts insulin signaling by binding directly to Akt and blocking activation of the kinase. Amounts of TRB3 RNA and protein were increased in livers of db/db diabetic mice compared with those in wild-type mice. Hepatic overexpression of TRB3 in amounts comparable to those in db/db mice promoted hyperglycemia and glucose intolerance. Our results suggest that, by interfering with Akt activation, TRB3 contributes to insulin resistance in individuals with susceptibility to type II diabetes.

The nuclear factor CREB stimulates the expression of cellular genes following its protein kinase A-mediated phosphorylation at Ser-133. Ser-133 phosphorylation, in turn, activates target gene expression by promoting recruitment of the co-activator CBP. Recent studies showing that CREB and its paralog CREM are required for survival of certain cell types prompted us to examine whether CREB is a nuclear target for activation via the growth factor-dependent Ser/Thr kinase Akt/PKB. When overexpressed in serum-stimulated cells, Akt/PKB potently induced Ser-133 phosphorylation of CREB and promoted recruitment of CBP. Correspondingly, Akt/ PKB stimulated target gene expression via CREB in a phospho(Ser-133)-dependent manner. Akt/PKB induced CREB activity only in response to serum stimulation, and this effect was suppressed by the phosphatidylinositol 3-kinase inhibitor LY 294002. Our results support the notion that Akt/PKB promotes cell survival, at least in part, by stimulating the expression of cellular genes via the CREB/CBP nuclear transduction pathway.Originally characterized on the basis of its sequence homology with the v-Akt oncogene and with protein kinase A (1-3), the Ser/Thr kinase Akt has been shown to block cellular apoptosis and to promote cell survival in response to growth factor induction (reviewed in Ref. 4). Akt/PKB-mediated phosphorylation of BAD, for example, blocks cellular apoptosis by promoting binding of BAD to the 14-3-3 protein and thereby sequestering BAD from Bcl-X L (5-7). Following activation by PI3-K, 1 Akt/PKB translocates to the nucleus where it is thought to regulate specific genetic programs by catalyzing the phosphorylation of specific nuclear factors (8, 9).A number of growth factors and hormones have been shown to stimulate the expression of cellular genes by inducing the phosphorylation of the nuclear factor CREB at . Originally characterized as a target for PKA-mediated phosphorylation (11), CREB is also recognized by other cellular kinases including protein kinase C (12), pp90 RSK (13), calmodulin kinases II and IV (14, 15), and microtubule-activated protein kinase-activated protein 2 (16).Recent studies with transgenic and knockout mice indicate that CREB and its paralog CREM are important for cell survival. CREM-deficient mice, for example, exhibit a spermatogenesis defect secondary to enhanced apoptosis of germ cells (17,18). Overexpression of a dominant negative CREB transgene, moreover, induces apoptosis in T cells, following growth factor stimulation (19). The involvement of CREB family members in cell survival and the resemblance of Akt/PKB to protein kinase A, not only in primary sequence but also in its apparent substrate specificity, prompted us to examine whether CREB is a regulatory target for Akt/PKB. Here we demonstrate that Akt/PKB promotes phosphorylation of CREB, stimulates recruitment of CBP to the promoter, and activates cellular gene expression via a CRE-dependent mechanism. Our results suggest that CREB may contribute importantly to cell survival in response...