Novel pathways regulating glucose and lipid metabolism in human skeletal muscle

Abstract

The utilization of glucose and lipids as energy substrates in skeletal muscle is strictly regulated. As skeletal muscle is the body s chief consumer of glucose and lipids, it plays a critical role in the maintenance of whole-body homeostasis. Under normal physiological conditions, skeletal muscle displays a certain metabolic flexibility, allowing the tissue to switch between the utilization of glucose and lipids. Generally, skeletal muscle manifests a preference for lipids as the primary energy substrate, in the fasting state, or during such metabolic challenges as starvation or exercise. In these situations, glucose availability is low. However, in the post-prandial state, glucose availability is high and energy substrate utilization is shifted toward glucose. A compromised or reduced plasticity in this ability to shift substrate utilization results in metabolic inflexibility. Conditions associated with metabolic inflexibility include the metabolic syndrome, insulin resistance and type 2 diabetes mellitus. To address the etiology and pathogenesis of these disorders, investigation of the molecular events governing these processes is warranted. In this thesis, novel pathways regulating glucose and lipid metabolism in human skeletal muscle were investigated with the specific aim of discovering and validating new potential therapeutic targets to treat type 2 diabetes.

Cultured primary human skeletal muscle cells were used to determine the consequence of targeted malonyl CoA decarboxylase (MCD) (Study I), and inhibitor of nuclear factor kappaB kinase (IKKbeta) (Study II), reduction on metabolic and signaling parameters. The effect of exogenous fibroblast growth factor 21 (FGF-21) (Study III) and clenbuterol treatment (Study IV) on metabolic and signaling parameters were also determined. Targeted genetic reduction of either MCD or IKKbeta increased glucose uptake and improved insulin signaling in cultured muscle cells. MCD reduction was sufficient to shift substrate utilization from lipid to glucose oxidation. IKKbeta gene silencing prevented TNF-alpha-mediated insulin resistance in cultured human skeletal muscle. FGF-21 is a novel member of the fibroblast growth factor family of proteins, distinguished by its hormone-like action on metabolism. FGF-21 treatment positively impacted glucose metabolism in human skeletal muscle cells and in incubated intact mouse skeletal muscle. Finally, chronic clenbuterol exposure was sufficient to increase glycogen synthesis, and reduce lipid oxidation in human skeletal muscle myotubes and in cultured rat L6 muscle cells.

In conclusion, the studies presented in this thesis provide insight into molecular pathways governing glucose and lipid metabolism in skeletal muscle. Several new targets have been identified and validated for potential therapeutic intervention to treat insulin resistance. Moreover, these investigations provide insight not only into the signaling paradigm of a particular pathway but rather into the interconnectivity of complex metabolic regulation.