Impaired mTOR/SREBP1-Mediated Lipogenesis as a Mechanism of Hepatic Glycogen Accumulation in a Carnivorous Fish Model, Largemouth Bass (Micropterus salmoides).

Hepatic glycogen accumulation is a hallmark of glucose intolerance in carnivorous fish, yet the molecular mechanisms governing the partitioning of surplus carbohydrates remain poorly understood. This study integrated physiology and functional assays to elucidate how insulin-dependent mTOR/SREBP1 signaling governs hepatic glucose partitioning and glycogen accumulation. In the present study, we used largemouth bass (poor glucose utilization) and Nile tilapia (efficient glucose utilization) fed diets containing graded carbohydrate levels for 8 weeks. The results revealed that high-carbohydrate (HC) diets suppressed the PI3K/AKT1/mTOR axis in largemouth bass, reducing nuclear SREBP1 and causing massive hepatic glycogen accumulation. Conversely, tilapia efficiently activated this axis to promote lipid synthesis in response to excessive carbohydrates. Meanwhile, srebp1 knockdown in primary hepatocytes of largemouth bass decreased lipogenic gene expression and triglyceride content while increasing glycogen level. Mechanistic validations in largemouth bass demonstrated that insulin treatment restored AKT1/mTOR pathway activity and SREBP1 nuclear translocation, alleviating glycogen overload while promoting lipogenesis. Knockdown of akt1 or s6k1 prevented SREBP1 activation, whereas tsc2 knockdown rescued mTOR phosphorylation. Furthermore, mTOR inhibition by rapamycin abolished insulin-induced SREBP1 transactivation of lipogenic targets, mimicking the HC-induced glycogen-overload phenotype. In summary, this study identifies the insulin-responsive mTOR/SREBP1 signaling axis as the critical pathway governing lipogenesis in largemouth bass. Functional impairment of the insulin-responsive mTOR/SREBP1 signaling axis is associated with a metabolic shift favoring glycogen storage over lipogenesis, providing mechanistic insights relevant to glucose intolerance across vertebrates.