Regulation of whole-body glucose and lipid metabolism by skeletal muscle

Abstract

Obesity and associated diseases like type 2 diabetes are rapidly growing health concerns across the globe. The uptake and expenditure of energy in the body are tightly regulated by a plethora of enzymes and hormones in central and peripheral tissues. Skeletal muscle is an important organ in this regulatory network and exhibits remarkable flexibility with regard to fuel utilization and modulates whole-body glucose and lipid metabolism as underlined by the work presented in this thesis.

The enzyme diacylglycerol kinase (DGK) is involved in lipid signaling and metabolism. Ablating the isoform DGKε allowed us to assess its regulatory role in whole-body energy metabolism. We observed an enrichment of diacylglycerol lipid species in skeletal muscle of high-fat fed DGKε kockout mice which was paradoxically associated with improved glucose tolerance. Nonetheless, the loss of DGKε promoted a greater whole-body reliance on lipids as fuel source. Taken together, this data identifies DGKε as a modulator of skeletal muscle lipid metabolism affecting whole-body energy handling.

Signaling of the heterotrimeric AMP-activated protein kinase (AMPK) stimulates ATPgenerating processes when energy levels are low. We characterized the extent to which activity of the regulatory AMPK subunit γ1 in skeletal muscle modifies whole-body metabolism by expressing the constitutively active transgene AMPKγ1 H151R in skeletal muscle. This led to increased whole-body insulin sensitivity with a greater reliance on glucose as a fuel source. Furthermore, sex-specific effects on adipose tissue were observed. Our findings underline the potential therapeutic value of tissue-specific AMPK activation as it may protect against the development of insulin resistance. Conversely, the activation of AMPKγ3, another regulatory subunit isoform abundant in skeletal muscle, did not affect the whole-body lipid oxidation rate. For this assessment, we established an in vivo assay relying on the intravenous administration of 3H-palmitic acid combined with non-β-oxidizable 14C-2-bromopalmitic acid. Independently of the level of AMPK activation in skeletal muscle, we report an increased whole-body fatty acid oxidation in high-fat fed mice compared to chow fed mice.

Skeletal muscle adapts to obesity and insulin resistance by altering the abundance of certain proteins. With a state-of-the-art mass spectrometry-based workflow, we identified over 6,000 proteins in quadriceps muscle of lean and morbidly obese, insulin resistant mice lacking the satiety hormone leptin (ob/ob mice). Enzymes involved in lipid metabolism and proteins characteristic for slow oxidative type I muscle fibers were among the 118 differentially abundant proteins in skeletal muscle from obese in comparison to lean mice. Together with the increased abundance of proteins associated with mitochondria and peroxisomes, key organelles in the handling of energetic processes and cellular stress, this data indicates that obesity increases fatty acid oxidation in skeletal muscle.

In conclusion, the enzymes DGKε and AMPK, with its regulatory subunits γ1 and γ3, modulate skeletal muscle energy homeostasis and influence whole-body glucose and lipid metabolism. We find that obesity and insulin resistance are associated with the remodeling of the proteome of skeletal muscle suggesting increased lipid oxidation.