Methods Measurements of AMPK, ACC, and fatty acid oxidation in primary hepatocytes.Hepatocytes were isolated from male Sprague Dawley (SD) rats by collagenase digestion (18). For the AMPK assay, cells were seeded in six-well plates at 1.5 × 10 6 cells/well in DMEM containing 100 U/ml penicillin, 100 µg/ml streptomycin, 10% FBS, 100 nM insulin, 100 nM dexamethasone, and 5 µg/ml transferrin for 4 hours. Cells were then cultured in serum-free DMEM for 16 hours followed by treatment for 1 hour or 7 hours with control medium, 5-amino-imidazole carboxamide ribo-

5 AMP-activated protein kinase (AMPK) can be activated in response to cellular fuel depletion and leads to switching off ATP-consuming pathways and switching on ATP-regenerating pathways in many cell types. We have hypothesized that AMPK is a central mediator of insulin-independent glucose transport, which enables fuel-depleted muscle cells to take up glucose for AT P regeneration under conditions of metabolic stress. To test this hypothesis, rat epitrochlearis muscles were isolated and incubated in vitro under several conditions that evoke metabolic stress accompanied by intracellular fuel depletion. Rates of glucose transport in the isolated muscles were increased by all of these conditions, including contraction (5-fold above basal), hypoxia (8-fold), 2,4-dinotrophenol (11-fold), rotenone (7-fold), and hyperosmolarity (8-fold). All of these stimuli simultaneously increased both 1 and 2 isoform-specific AMPK activity. There was close correlation between 1 (r 2 = 0.72) and 2 (r 2 = 0.67) AMPK activities and the rate of glucose transport, irrespective of the metabolic stress used, all of which compromised muscle fuel status as judged by AT P, phosphocreatine, and glycogen content. 5-Aminoimidazole-4-carboxamide ribonucleoside, a pharmacological AMPK activator that is metabolized to an AMP-mimetic ZMP, also increased both glucose transport and AMPK activity but did not change fuel status. Insulin stimulated glucose transport by 6.5-fold above basal but did not a ffect AMPK activity. These results suggest that the activation of AMPK may be a common mechanism leading to insulin-independent glucose transport in skeletal muscle under conditions of metabolic stress. D i a b e t e s 49:XXX-XXX, 2000

The Akt substrate of 160 kDa (AS160) is phosphorylated on Akt substrate (PAS) motifs in response to insulin and contraction in skeletal muscle, regulating glucose uptake. Here we discovered a dissociation between AS160 protein expression and apparent AS160 PAS phosphorylation among soleus, tibialis anterior, and extensor digitorum longus muscles. Immunodepletion of AS160 in tibialis anterior muscle lysates resulted in minimal depletion of the PAS band at 160 kDa, suggesting the presence of an additional PAS immunoreactive protein. By immunoprecipitation and mass spectrometry, we identified this protein as the AS160 paralog TBC1D1, an obesity candidate gene regulating GLUT4 translocation in adipocytes. TBC1D1 expression was severalfold higher in skeletal muscles compared with all other tissues and was the dominant protein detected by the anti-PAS antibody at 160 kDa in tibialis anterior and extensor digitorum longus but not soleus muscles. In vivo stimulation by insulin, contraction, and the AMP-activated protein kinase (AMPK) activator AICAR increased TBC1D1 PAS phosphorylation. Using mass spectrometry on TBC1D1 from mouse skeletal muscle, we identified several novel phosphorylation sites on TBC1D1 and found the majority were consensus or near consensus sites for AMPK. Semiquantitative analysis of spectra suggested that AICAR caused greater overall phosphorylation of TBC1D1 sites compared with insulin. Purified Akt and AMPK phosphorylated TBC1D1 in vitro, and AMPK, but not Akt, reduced TBC1D1 electrophoretic mobility. TBC1D1 is a major PAS immunoreactive protein in skeletal muscle that is phosphorylated in vivo by insulin, AICAR, and contraction. Both Akt and AMPK phosphorylate TBC1D1, but AMPK may be the more robust regulator.A defining pathology of type 2 diabetes is impaired insulinstimulated glucose uptake in skeletal muscle. Skeletal muscle is the largest tissue in the human body by mass and is the chief site of insulin-stimulated glucose disposal. Insulin stimulation causes translocation of GLUT4 glucose transporters from intracellular regions to the plasma membrane and t-tubule system where they function to import glucose. In individuals with type 2 diabetes, insulin fails to stimulate adequate GLUT4 translocation, resulting in impaired glucose uptake and poor glucose tolerance.Skeletal muscle is unique as an insulin-sensitive tissue because voluntary contraction during exercise causes GLUT4 translocation completely independent of insulin signaling (1, 2). Contraction-stimulated glucose uptake is preserved in the muscle of individuals with type 2 diabetes, thus demonstrating the existence of signaling pathways that circumvent defective components of the insulin signaling pathway (3). If and where insulin-and contraction-stimulated glucose uptake pathways converge have been topics of considerable interest. Recently, the Akt substrate of 160 kDa (AS160) 2 was identified as a mediator of both insulin-and contraction-stimulated glucose uptake and, therefore, a potential nexus for convergent signaling (4, 5).A...