The role of long-chain Acyl-Coenzyme A synthetase 1 (ACSL-1) in lipid metabolism in human skeletal muscle primary myotubes

Obesity is considered a major health threat to the U.S. due to being a strong risk factor for developing type 2 diabetes and other metabolic diseases. The prevalence and severity of obesity is even greater among some subpopulations in the U.S. (African-American Women). In this regard, metabolic dysfunction may be associated with an impairment of mitochondrial fatty acid oxidation (mtFAO) which can lead to over accumulation of bioactive lipids such as fatty acyl-CoA species. While reductions in mitochondrial content may be a precipitating variable, reductions in key enzymes that lead to partitioning fatty acids towards mitochondrial oxidation may also be a contributing factor. Recently, reductions of acyl-CoA synthetase (ACS) activity have been identified in skeletal muscle. Long chain acyl-CoA synthetase (ACSL) exists as five different isoforms, the roles of which are to activate fatty acids to acyl-CoAs in the initial step of fatty acid metabolism (synthesis or oxidation). In liver of rodents, ACSL-1 has been thought to direct fatty acids toward mtFAO, but little data exists in human skeletal muscle. The purpose of this study was to understand the potential role of ACSL-1 activity in lipid metabolism in human skeletal muscle. To address the purpose of the study, we employed a model of underexpression/knockdown (UEX/KD) of ACSL-1 in primary human skeletal muscle cells (HSKM). Based on data from our laboratory, ACSL-1 overexpression significantly increased mtFAO in HSKM cells from obese individuals. Therefore, we hypothesized that ACSL-1 UEX/KD would reduce mtFAO in this tissue. To address our hypothesis, we conducted fatty acid oxidation and lipid synthesis experiments following 48 h of lipid exposure in HSKM primary myotubes obtained from percutaneous biopsies of the vastus lateralis transfected with either shRNA (KD) or scrambled RNA (control) plasmid vectors. Results demonstrated that ACSL-1 was significantly reduced (P<0.05) following KD vs. control. However, following ACSL-1 KD, we observed an absence of change in complete (CO2) and acid soluble metabolites (ASM) incomplete metabolites oxidation palmitate. In addition, we also reported no alterations of total lipid synthesis and esterification of acyl-CoA toward MAG, DAG, and TAG synthesis despite the supply of exogenous lipids in our cell model. This is the first report of successful transfection and ACSL-1 KD in HSKM cells. Given the inconsistent findings with our original hypothesis, we now hypothesize the presence of compensatory mechanisms that exist following UEX/KD of ACSL-1 to offset the negative effects of ACSL-1 KD. Alternatives include upregulation of additional ACSL isoforms (e.g., ACSL-5) and/or elevations in peroxisomal activity.