LKB1 is a tumor suppressor that may also be fundamental to cell metabolism, since LKB1 phosphorylates and activates the energy sensing enzyme AMPK. We generated muscle-specific LKB1 knockout (MLKB1KO) mice, and surprisingly, found that a lack of LKB1 in skeletal muscle enhanced insulin sensitivity, as evidenced by decreased fasting glucose and insulin concentrations, improved glucose tolerance, increased muscle glucose uptake in vivo, and increased glucose utilization during a hyperinsulinemic-euglycemic clamp. MLKB1KO mice had increased insulin-stimulated Akt phosphorylation and a >80% decrease in muscle expression of TRB3, a recently identified Akt inhibitor. Akt/TRB3 binding was present in skeletal muscle, and overexpression of TRB3 in C2C12 myoblasts significantly reduced Akt phosphorylation. These results demonstrate that skeletal muscle LKB1 is a negative regulator of insulin sensitivity and glucose homeostasis. LKB1-mediated TRB3 expression provides a novel link between LKB1 and Akt, critical kinases involved in both tumor genesis and cell metabolism.LKB1 is a serine/threonine kinase that links a diverse array of cellular processes, including cancer, cellular polarity, and metabolism. Originally identified as the tumor suppressor protein mutated in Peutz-Jeghers syndrome (17, 21), LKB1 has since been shown to regulate polarity in a number of systems, including Caenorhabditis elegans (48), Drosophila melanogaster (28), Xenopus (33), and mammalian cells (3). Biochemically, LBK1 can phosphorylate and activate at least 13 members of the AMP-activated protein kinase (AMPK) subfamily of protein kinases (20, 27) when associated with two regulatory proteins essential for catalytic activity, STE20-related adapter protein and mouse protein 25 (MO25) (15). AMPK, the most studied of LKB1's downstream substrates, is a conserved serine/threonine kinase that functions in the regulation of energy metabolism (16,18,29). Studies with cell culture (15,40,51) or conditional knockouts of LKB1 in skeletal muscle (38), cardiac muscle (39), or liver (41) have found that LKB1 regulates AMPK activity both in vitro and in vivo. Other than AMPK and the microtubule affinity-regulating kinase (MARK) proteins, which have been implicated in the control of cellular polarity (9), relatively little is known about the function of the AMPK-related kinases. The well-established role of AMPK in metabolism, however, directly implicates LKB1 in the maintenance of energy balance.The protein kinase Akt functions both in the control of cell proliferation and as a critical node in insulin signaling, appearing to mediate most of the metabolic effects of insulin (44). Akt is activated by phosphorylation of Thr 308 within the T loop of the catalytic domain and Ser 473 , located in a C-terminal, noncatalytic region of the enzyme (1). A mammalian homolog of D. melanogaster tribbles, TRB3, was recently identified as a negative regulator of Akt activity in human embryonic kidney 293 (HEK293) cells and mouse liver (10). In HEK293 cells and liver, TRB3 bind...

Endoplasmic Reticulum (ER) stress has been linked to insulin resistance in multiple tissues but the role of ER stress in skeletal muscle has not been explored. ER stress has also been reported to increase tribbles 3 (TRB3) expression in multiple cell lines. Here, we report that high fat feeding in mice, and obesity and type 2 diabetes in humans significantly increases TRB3 and ER stress markers in skeletal muscle. Overexpression of TRB3 in C2C12 myotubes and mouse tibialis anterior muscles significantly impairs insulin signaling. Incubation of C2C12 cells and mouse skeletal muscle with ER stressors thapsigargin and tunicamycin increases TRB3 and impairs insulin signaling and glucose uptake, effects reversed in cells overexpressing RNAi for TRB3 and in muscles from TRB3 knockout mice. Furthermore, TRB3 knockout mice are protected from high fat diet-induced insulin resistance in skeletal muscle. These data demonstrate that TRB3 mediates ER stress-induced insulin resistance in skeletal muscle.