AMPK activation prevents excess nutrient-induced hepatic lipid accumulation by inhibiting mTORC1 signaling and endoplasmic reticulum stress response

Abstract

Lipid accumulation is a central event in the development of chronic metabolic diseases, including obesity and type 2 diabetes, but the mechanisms responsible for lipid accumulation are incompletely understood. This study was designed to investigate the mechanisms for excess nutrient-induced lipid accumulation and whether activation of AMP-activated protein kinase (AMPK) prevents the hepatic lipid accumulation in excess nutrient-treated HepG2 cells and high fat diet (HFD)-fed mice. Exposure of HepG2 cells to high levels of glucose or palmitate induced the endoplasmic reticulum (ER) stress response, activated sterol regulatory element-binding protein-1 (SREBP-1), and enhanced lipid accumulation, all of which were sensitive to ER stress inhibitor and gene silencing of eukaryotic initiation factor 2α. The increases in ER stress response and lipid accumulation were associated with activation of mammalian target of rapamycin complex 1 (mTORC1) signaling. Inhibition of mTORC1 signaling attenuated the ER stress response and lipid accumulation induced by high glucose or by deletion of tuberous sclerosis 2. In addition, AMPK activation prevented the mTORC1 activation, ER stress response, and lipid accumulation. This effect was mimicked or abrogated, respectively, by overexpression of constitutively active and dominant-negative AMPK mutants. Finally, treatment of HFD-fed mice with 5-aminoimidazole-4-carboxamide-1-β-4-ribofuranoside inhibited the mTORC1 pathway, suppressed the ER stress response, and prevented insulin resistance and hepatic lipid accumulation. We conclude that activation of AMPK prevents excess nutrient-induced hepatic lipid accumulation by inhibiting mTORC1 and ER stress response.