Plasma FGF21 is increased in T2D patients, and positively correlated with fasting insulin and BMI. However, FGF21 has direct effects in enhancing skeletal muscle glucose uptake, providing additional points of regulation that may contribute to the beneficial effects of FGF21 on glucose homeostasis. Whether increased plasma FGF21 in T2D is a compensatory mechanism to increase glucose metabolism remains to be determined.

OBJECTIVE-Malonyl coenzyme A (CoA) decarboxylase (MCD) is a key enzyme responsible for malonyl-CoA turnover and functions in the control of the balance between lipid and glucose metabolism. We utilized RNA interference (siRNA)-based gene silencing to determine the direct role of MCD on metabolic responses in primary human skeletal muscle.RESEARCH DESIGN AND METHODS-We used siRNA to silence MCD gene expression in cultured human myotubes from healthy volunteers (seven male and seven female) with no known metabolic disorders. Thereafter, we determined lipid and glucose metabolism and signal transduction under basal and insulinstimulated conditions. RESULTS-RNA interference-based silencing of MCD expression (75% reduction) increased malonyl-CoA levels twofold and shifted substrate utilization from lipid to glucose oxidation. RNA interference-based depletion of MCD reduced basal palmitate oxidation. In parallel with this reduction, palmitate uptake was decreased under basal (40%) and insulin-stimulated (49%) conditions compared with myotubes transfected with a scrambled sequence. MCD silencing increased basal and insulin-mediated glucose oxidation 1.4-and 2.6-fold, respectively, compared with myotubes transfected with a scrambled sequence. In addition, glucose transport and cell-surface GLUT4 content was increased. In contrast, insulin action on IRS-1 tyrosine phosphorylation, tyrosine-associated phosphatidylinositol (PI) 3-kinase activity, Akt, and glycogen synthase kinase (GSK) phosphorylation was unaltered between myotubes transfected with siRNA against MCD versus a scrambled sequence.CONCLUSIONS-These results provide evidence that MCD silencing suppresses lipid uptake and enhances glucose uptake in primary human myotubes. In conclusion, MCD expression plays a key reciprocal role in the balance between lipid and glucose metabolism. Diabetes 57:1508-1516, 2008

OBJECTIVE-Proinflammatory cytokines contribute to systemic low-grade inflammation and insulin resistance. Tumor necrosis factor (TNF)-␣ impedes insulin signaling in insulin target tissues. We determined the role of inhibitor of nuclear factor-B kinase (IKK)␤ in TNF-␣-induced impairments in insulin signaling and glucose metabolism in skeletal muscle.RESEARCH DESIGN AND METHODS-Small interfering RNA (siRNA) was used to silence IKK␤ gene expression in primary human skeletal muscle myotubes from nondiabetic subjects. siRNA gene silencing reduced IKK␤ protein expression 73% (P Ͻ 0.05). Myotubes were incubated in the absence or presence of insulin and/or TNF-␣, and effects of IKK␤ silencing on insulin signaling and glucose metabolism were determined.RESULTS-Insulin increased glucose uptake 1.7-fold (P Ͻ 0.05) and glucose incorporation into glycogen 3.8-fold (P Ͻ 0.05) in myotubes from nondiabetic subjects. TNF-␣ exposure fully impaired insulin-mediated glucose uptake and metabolism. IKK␤ siRNA protected against TNF-␣-induced impairments in glucose metabolism, since insulin-induced increases in glucose uptake (1.5-fold; P Ͻ 0.05) and glycogen synthesis (3.5-fold; P Ͻ 0.05) were restored. Conversely, TNF-␣-induced increases in insulin receptor substrate-1 serine phosphorylation (Ser 312 ), Jun NH 2 -terminal kinase phosphorylation, and extracellular signal-related kinase-1/2 mitogen-activated protein kinase (MAPK) phosphorylation were unaltered by siRNA-mediated IKK␤ reduction. siRNA-mediated IKK␤ reduction prevented TNF-␣-induced insulin resistance on Akt Ser 473 and Thr 308 phosphorylation and phosphorylation of the 160-kDa Akt substrate AS160. IKK␤ silencing had no effect on cell differentiation. Finally, mRNA expression of GLUT1 or GLUT4 and protein expression of MAPK kinase kinase kinase isoform 4 (MAP4K4) was unaltered by IKK␤ siRNA.CONCLUSIONS-IKK␤ silencing prevents TNF-␣-induced impairments in insulin action on Akt phosphorylation and glucose uptake and metabolism in human skeletal muscle.

Background: Contractions activate the sodium pump, Na ϩ ,K ϩ -ATPase, and the energy sensor, AMP-activated protein