α-ketoisocaproic acid promotes ER stress through impairment of autophagy, thereby provoking lipid accumulation and insulin resistance in murine preadipocytes

α-ketoisocaproic acid (AKA), a metabolite of l-leucine, is one of the branched-chain amino acids (BCAAs) involved in energy metabolism. However, the effects of AKA on lipid metabolism, insulin signaling, and related mechanisms in preadipocytes have not been reported. Herein, we investigated the impacts of AKA on lipid accumulation in 3T3-L1 murine preadipocytes. Treatment with AKA for 4 days enhanced lipid accumulation and expression of lipogenic proteins, such as cleaved sterol-regulatory element-binding proteins (SREBP1) and stearoyl-CoA desaturase-1 (SCD1) in 3T3-L1 cells. Increased endoplasmic reticulum (ER) stress markers, such as phosphorylation of eukaryotic initiation factor 2 (eIF2) and CHOP, were observed in the presence of AKA. AKA treatment increased mTOR phosphorylation and reducing autophagy markers, such as LC3 conversion and degradation of p62. Treatment with rapamycin, an mTOR inhibitor, reduced the effects of AKA on ER stress and lipogenesis in 3T3-L1 preadipocytes. The present study demonstrates that AKA increases ER stress by impairing mTOR/autophagy signaling, thus promoting lipid accumulation and insulin resistance in preadipocytes. In particular, if AKA is taken together with substances that can suppress ER stress, more effective skeletal muscle gain will be possible.