Kinase cascades in the regulation of glucose homeostasis

Therapeutic strategies to treat Type 2 Diabetes Mellitus (T2DM) aim at improving muscle insulin sensitivity by either directly modulating, or bypassing defective insulin signaling. Insulin induces metabolic and gene regulatory responses via PI 3-Kinase (PI3K) and Mitogen-Activated Protein Kinase (MAPK) signaling pathways, respectively. MAPK signaling pathways can also be activated by insulin-independent stimuli such as exercise/contraction and stress/hypoxia. AMP-activated Protein Kinase (AMPK) also transduces signals to glucose transport and gene regulatory responses by insulin-independent stimuli. Identifying the molecular mechanisms underlying insulin -dependent and -independent signaling pathways will reveal molecular targets for the pharmacological treatment of insulin resistance. Glucose induces insulin resistance in skeletal muscle. Glucose sensitive targets in the PI3K signaling pathway were identified in Wistar and Goto-Kakizaki (GK) rats exposed to a 3hr hyperglycemic infusion. In skeletal muscle, glucose directly activates Phosphoinositide Dependent Kinase 1 (PDK-1) and downstream Protein Kinase C (PKC) isoforms (α/β, δ and ζ) through a mechanism that is independent of changes in PI3K, Protein Kinase B (PKB) and Extracellular signal-Regulated protein Kinase (ERK) phosphorylation. This may account for defects in insulin signaling in non-obese diabetic GK rats, whereby PKCζ was also activated by hyperglycemia. Thus, hyperglycemia acutely modulates signal transduction and this may account for skeletal muscle insulin resistance at the level of the PI3K pathway. MAPK activation in skeletal muscle from insulin resistant ob/ob mice was measured to determine if insulin-dependent and -independent induction of MAPK signaling is intact in insulin resistant tissue. Insulin action on MAPK was impaired in ob/ob mice, whereas contraction-mediated effects were preserved. In addition Phorbol 12-Myristate-13-Acetate (PMA) elicited divergent effects on MAPK signaling in ob/ob mice; c-Jun NH2-terminal Kinase (JNK) and ERK phosphorylation were preserved, whereas p38-MAPK phosphorylation was refractory. Thus appropriate MAPK response can be elicited in insulin-resistant skeletal muscle via insulin-independent mechanisms. Exercise and muscle contraction may circumvent aberrant MAPK signaling in insulin-resistant skeletal muscle. Exerciseand hypoxia-stimulated AMPK activity and glucose transport were measured in skeletal muscle from insulin resistant obese Zucker fa/fa rats. An isoform-specific defect in AMPKα1 activity in response to contraction was observed, which was inconsequential to glucose transport. Thus, AMPK signaling to glucose transport is unaffected by insulin resistance. Constitutive activation of AMPK can be achieved by AMPKγ3 missense mutation (R225Q). Transgenic mice overexpressing the R225Q γ3 mutant form were challenged with a 55% fat diet. Transgenic mice have increased lipid oxidation in the presence of lipid oversupply, which attenuates the expected accumulation of intramuscular triglyceride (TG) content and protects against skeletal muscle insulin resistance. In contrast, ablation of AMPKγ3 resulted in increased TG content and impaired insulin action. Thus, the muscle specific AMPKγ3-subunit is a putative target for pharmacological treatment of diet-induced skeletal muscle insulin resistance. In conclusion insulin signaling defects at the level of PI3K and MAPK pathways appear to contribute to skeletal muscle insulin resistance. Insulin-independent pathways via MAPK and AMPK may prove valuable in the treatment of insulin resistance in T2DM.