S6 kinase (S6K) deletion in metazoans causes small cell size, insulin hypersensitivity, and metabolic adaptations; however, the underlying molecular mechanisms are unclear. Here we show that S6K-deficient skeletal muscle cells have increased AMP and inorganic phosphate levels relative to ATP and phosphocreatine, causing AMP-activated protein kinase (AMPK) upregulation. Energy stress and muscle cell atrophy are specifically triggered by the S6K1 deletion, independent of S6K2 activity. Two known AMPK-dependent functions, mitochondrial biogenesis and fatty acid beta-oxidation, are upregulated in S6K-deficient muscle cells, leading to a sharp depletion of lipid content, while glycogen stores are spared. Strikingly, AMPK inhibition in S6K-deficient cells restores cell growth and sensitivity to nutrient signals. These data indicate that S6K1 controls the energy state of the cell and the AMPK-dependent metabolic program, providing a mechanism for cell mass accumulation under high-calorie diet.

SummaryIn contrast to the class I phosphoinositide 3-kinases (PI3Ks), the organismal roles of the kinase activity of the class II PI3Ks are less clear. Here, we report that class II PI3K-C2β kinase-dead mice are viable and healthy but display an unanticipated enhanced insulin sensitivity and glucose tolerance, as well as protection against high-fat-diet-induced liver steatosis. Despite having a broad tissue distribution, systemic PI3K-C2β inhibition selectively enhances insulin signaling only in metabolic tissues. In a primary hepatocyte model, basal PI3P lipid levels are reduced by 60% upon PI3K-C2β inhibition. This results in an expansion of the very early APPL1-positive endosomal compartment and altered insulin receptor trafficking, correlating with an amplification of insulin-induced, class I PI3K-dependent Akt signaling, without impacting MAPK activity. These data reveal PI3K-C2β as a critical regulator of endosomal trafficking, specifically in insulin signaling, and identify PI3K-C2β as a potential drug target for insulin sensitization.

Vps34 PI3K is thought to be the main producer of phosphatidylinositol-3-monophosphate, a lipid that controls intracellular vesicular trafficking. The organismal impact of systemic inhibition of Vps34 kinase activity is not completely understood. Here we show that heterozygous Vps34 kinase-dead mice are healthy and display a robustly enhanced insulin sensitivity and glucose tolerance, phenotypes mimicked by a selective Vps34 inhibitor in wild-type mice. The underlying mechanism of insulin sensitization is multifactorial and not through the canonical insulin/Akt pathway. Vps34 inhibition alters cellular energy metabolism, activating the AMPK pathway in liver and muscle. In liver, Vps34 inactivation mildly dampens autophagy, limiting substrate availability for mitochondrial respiration and reducing gluconeogenesis. In muscle, Vps34 inactivation triggers a metabolic switch from oxidative phosphorylation towards glycolysis and enhanced glucose uptake. Our study identifies Vps34 as a new drug target for insulin resistance in Type-2 diabetes, in which the unmet therapeutic need remains substantial.