Nicholls, H. T. | Search bioRxiv

bioRxiv2021

Suppression of Fatty Acid Oxidation by Thioesterase Superfamily Member 2 in Skeletal Muscle Promotes Hepatic Steatosis and Insulin Resistance

Thioesterase superfamily member 2 (Them2) is highly expressed in oxidative tissues where it hydrolyzes long chain fatty acyl-CoA esters to free fatty acids and CoA. Although mice globally lacking Them2 (Them2-/-) are protected against diet-induced obesity, insulin resistance and hepatic steatosis, liver-specific Them2-/- mice remain susceptible. To explore the contribution of Them2 in extrahepatic tissues, we created mice with Them2 deleted in skeletal muscle (S-Them2-/-), cardiac muscle (C-Them2-/-) or adipose tissue (A-Them2-/-). When fed a high-fat diet, S-Them2-/- but not C-Them2-/- or A-Them2-/- mice exhibited reduced weight gain. Only S-Them2-/- mice exhibited improved glucose homeostasis together with improved insulin sensitivity in skeletal muscle. Increased rates of fatty acid oxidation in skeletal muscle of S-Them2-/- mice were reflected in alterations in skeletal muscle metabolites, including short chain fatty acids, branched chain amino acids and the pentose phosphate pathway. Protection from diet-induced hepatic steatosis in S-Them2-/- mice was attributable to increased VLDL triglyceride secretion rates in support of demands of increased muscle fatty acid utilization. These results reveal a key role for skeletal muscle Them2 in the pathogenesis of diet-induced obesity, insulin resistance and hepatic steatosis.

molecular biology↗

bioRxiv2020

Thioesterase Superfamily Member 1 Undergoes Stimulus-coupled Reorganization to Regulate Metabolism

In brown adipose tissue, cold exposure promotes thermogenesis, in large part by increasing mitochondrial {beta}-oxidation of lipid droplet-derived fatty acids. This process is suppressed by thioesterase superfamily member 1 (Them1), a long chain fatty acyl-CoA thioesterase that is highly upregulated by cold ambient temperatures. Them1 reduces fatty acid availability for {beta}-oxidation in mitochondria and limits thermogenesis by cellular mechanisms that are not well defined. We show that Them1 regulates metabolism by undergoing marked intracellular conformational changes that occur in response to {beta}-adrenergic stimulation. Mechanistically, Them1 formed puncta that were localized near LD and mitochondria in an immortalized brown adipose cell line. In response to stimulation by norepinephrine, Them1 was phosphorylated by PKC{beta} at S15, which specifically inhibited puncta formation and resulted in a diffuse intracellular localization. This change in Them1 localization also occurred after stimulation in vivo. Puncta formation activated Them1 metabolic activity in vitro, as evidenced by suppression of oxygen consumption following {beta}-adrenergic stimulation. We show by correlative light and electron microscopy that puncta are biomolecular condensates (also known as membraneless organelles) that typically form by phase separation. Them1 contains one intrinsically disordered region at the N-terminus with multiple interacting motifs that is frequently observed in phase-separating proteins. Phosphorylation, which is known to disrupt phase separation and aggregation, results in a diffuse Them1 localization. Our data thus establish that Them1 forms intracellular biomolecular condensates that limit fatty acid oxidation and suppress thermogenesis. During a period of energy demand, the condensates are disrupted by phosphorylation to allow for maximal thermogenesis. The stimulus-coupled reorganization of Them1 thus provides fine-tuning of thermogenesis and energy expenditure.

cell biology↗