Abstract
Metabolic diseases and their serious sequelae such as non-alcoholic fatty liver disease (NAFLD) pose a substantial clinical burden. It is now well recognized that skeletal muscle is a major site for the metabolism of all major macronutrients, and derangements in these muscle processes significantly contribute to metabolic disease. Studies over the last 15 years have identified the transcription factor Krüppel-like factor 15 (KLF15) as an important regulator and effector of metabolic processes across various tissues, and furthermore, genome-wide studies have identified human KLF15 variants with increased body mass index and diabetes. Given the importance of skeletal muscle in maintaining metabolic homeostasis, we generated a skeletal muscle specific KLF15 knockout (K15-SKO) mouse to study the role of skeletal muscle KLF15 in regulating systemic metabolism. We found that this animal is prone to developing obesity and insulin resistance at baseline, a phenotype that is greatly exacerbated in response to high fat diet (HFD). Strikingly, K15-SKO mice show a propensity toward developing NAFLD, as demonstrated by increased micro- and macrovesicular steatosis, hepatocellular ballooning, increased hepatic fatty acid and triglyceride deposition, and elevated Cd36 expression. A potential cause of NAFLD is the accumulation of excess lipids and lipid intermediates due to defects in the lipid flux pathway in extrahepatic tissues. Indeed, we see defects in the expression of genes involved in the carnitine shuttle and a paucity of long-chain acylcarnitines in K15-SKO skeletal muscle. Furthermore, RNA sequencing of skeletal muscle from K15-SKO mice shows downregulation in a number of pathways involved in lipid handling. This indicates that KLF15 serves as a novel extrahepatic molecular regulator of hepatic health. It has been previously shown that a diet rich in short-chain fatty acids (SCFA) can bypass defects in lipid handling and ultimately improve metabolic health. To explore this therapeutic avenue, we gave K15-SKO mice either normal chow (NC) or a SCFA-rich diet for 7 weeks. We observed decreased weight gain and improved glucose homeostasis in SCFA-rich diet fed mice. In addition to being a preventative strategy, SCFA-rich diets may also serve as a potential therapy to rescue from metabolic disease. To this end, we gave K15-SKO mice HFD for 5 weeks followed by 7 weeks of either NC or SCFA-rich diet. We observed that providing SCFAs can improve metabolic health and ameliorate the phenotype seen due to defects in skeletal muscle lipid handling: mice given SCFA-rich diet following HFD had significantly decreased weight gain and improved insulin sensitivity. These studies demonstrate that skeletal muscle KLF15 serves as an important regulator of lipid flux and hepatic health, and that SCFA-rich diets are a promising candidate for metabolic disease resultant of impaired lipid handling.
Skeletal Muscle Lipid Droplets and the Athlete’s Paradox
