Distinct contributions of mTOR and AMPK/JNK to trans-10,cis-12 conjugated linoleic acid-induced hepatic lipid accumulation in lean and db/db mice

An elevation in circulating free fatty acid (FFA) levels due to dysregulated adipose lipolysis is a major factor in the development of metabolic dysfunction-associated steatotic liver disease (MASLD) as a consequence of obesity. Hepatic lipid content also increases in response to trans -10, cis -12 conjugated linoleic acid (t10,c12 CLA), but this can occur independent of obesity. T10,c12 CLA treatment also stimulates adipose lipolysis, thus increasing FFA levels, but whether FFAs contribute indirectly to CLA-induced hepatic lipid accumulation has not been previously investigated. To address this issue, changes in triglyceride accumulation, lipid droplets and regulatory pathways (mTOR, AMPK, PPARα, JNK) related to hepatic lipid uptake (CD36, FABP1), synthesis (ACC, FAS) and secretion (MTP) were compared in lean and obese db/db mice fed control diet or diet containing t10,c12 CLA (0.4% w/w) for 4 weeks. Dietary t10,c12 CLA increased hepatic lipid accumulation in lean mice but not in db/db mice with established hepatic steatosis. FABP1 was increased in both genotypes, while FAS ( de novo lipogenesis) was higher in lean mice and MTP (triglyceride secretion) was greater in db/db mice. t10,c12 CLA upregulated the mTOR pathway (S6RP) in both mouse models, with PPARα elevated in lean mice and JNK and AMPK higher in db/db mice. Rat H4IIE hepatoma cells were used to examine which metabolic and regulatory processes were responsive to t10,c12 CLA or the FFA oleic acid. Lipid uptake and storage were increased in hepatomas by both treatments, but t10,c12 CLA only activated the mTOR pathway (p-PASK), while oleic acid increased lipogenesis-associated (AMPK, p-ACC) and cell stress (JNK) proteins. These findings indicate hepatic lipid accumulation in response to t10,c12 CLA in vivo involves both direct and indirect (i.e. FFA-mediated) effects on lipid metabolic processes through distinct intracellular signaling pathways.