The cellular energy sensor AMP-activated protein kinase (AMPK) is a metabolic regulator that mediates adaptation to nutritional variations to maintain a proper energy balance in cells. We show here... Show moreThe cellular energy sensor AMP-activated protein kinase (AMPK) is a metabolic regulator that mediates adaptation to nutritional variations to maintain a proper energy balance in cells. We show here that suckling-weaning and fasting-refeeding transitions in rodents are associated with changes in AMPK activation and the cellular energy state in the liver. These nutritional transitions were characterized by a metabolic switch from lipid to glucose utilization, orchestrated by modifications in glucose levels and the glucagon/insulin ratio in the bloodstream. We therefore investigated the respective roles of glucose and pancreatic hormones on AMPK activation in mouse primary hepatocytes. We found that glucose starvation transiently activates AMPK, whereas changes in glucagon and insulin levels had no impact on AMPK. Challenge of hepatocytes with metformin-induced metabolic stress strengthened both AMPK activation and cellular energy depletion under limited-glucose conditions, whereas neither glucagon nor insulin altered AMPK activation. Although both insulin and glucagon induced AMPK? phosphorylation at its Ser(485/491) residue, they did not affect its activity. Finally, the decrease in cellular ATP levels in response to an energy stress was additionally exacerbated under fasting conditions and by AMPK deficiency in hepatocytes, revealing metabolic inflexibility and emphasizing the importance of AMPK for maintaining hepatic energy charge. Our results suggest that nutritional changes (i.e. glucose availability), rather than the related hormonal changes (i.e. the glucagon/insulin ratio), sensitize AMPK activation to the energetic stress induced by the dietary transition during fasting. This effect is critical for preserving the cellular energy state in the liver. Show less
Despite its position as the first-line drug for treatment of type 2 diabetes mellitus, the mechanisms underlying the plasma glucose level-lowering effects of metformin (1,1-dimethylbiguanide) still... Show moreDespite its position as the first-line drug for treatment of type 2 diabetes mellitus, the mechanisms underlying the plasma glucose level-lowering effects of metformin (1,1-dimethylbiguanide) still remain incompletely understood. Metformin is thought to exert its primary antidiabetic action through the suppression of hepatic glucose production. In addition, the discovery that metformin inhibits the mitochondrial respiratory chain complex 1 has placed energy metabolism and activation of AMP-activated protein kinase (AMPK) at the centre of its proposed mechanism of action. However, the role of AMPK has been challenged and might only account for indirect changes in hepatic insulin sensitivity. Various mechanisms involving alterations in cellular energy charge, AMP-mediated inhibition of adenylate cyclase or fructose-1,6-bisphosphatase 1 and modulation of the cellular redox state through direct inhibition of mitochondrial glycerol-3-phosphate dehydrogenase have been proposed for the acute inhibition of gluconeogenesis by metformin. Emerging evidence suggests that metformin could improve obesity-induced meta-inflammation via direct and indirect effects on tissue-resident immune cells in metabolic organs (that is, adipose tissue, the gastrointestinal tract and the liver). Furthermore, the gastrointestinal tract also has a major role in metformin action through modulation of glucose-lowering hormone glucagon-like peptide 1 and the intestinal bile acid pool and alterations in gut microbiota composition. Show less
Thomas, A.; Belaidi, E.; Moulin, S.; Horman, S.; Zon, G.C. van der; Viollet, B.; ... ; Guigas, B. 2017
Inhibition or blockade of HSCs (hepatic stellate cells), the main matrix-producing cells involved in the wound-healing response, represents an attractive strategy for the treatment of liver... Show moreInhibition or blockade of HSCs (hepatic stellate cells), the main matrix-producing cells involved in the wound-healing response, represents an attractive strategy for the treatment of liver fibrosis. In vitro studies have shown that activation of AMPK (AMP-activated protein kinase), a key player in the regulation of cellular energy homoeostasis, inhibits proliferation of myofibroblasts derived from HSCs. If AMPK is a true regulator of fibrogenesis then defective AMPK activity would enhance fibrogenesis and hepatic fibrosis. To test this, in the present work, in vitro studies were performed on mouse primary HSCs treated or not with the AMPK activator AICAR (5-amino-4-imidazolecarboxamide ribonucleotide) or isolated from mice lacking the AMPK alpha 1 catalytic subunit (AMPK alpha 1(-/-)) or their littermates (AMPK alpha 1(+/+)). Liver fibrosis was induced in vivo in AMPK alpha 1(-/-) and AMPK alpha 1(+/+) mice by repeated injections of CCl4 (carbon tetrachloride). During culture activation of HSCs, AMPK protein and activity significantly increased and regulatory AMPK gamma 3 mRNA was specifically up-regulated. Stimulation of AMPK activity by AICAR inhibited HSC proliferation, as expected, as well as collagen alpha 1(1) expression. Importantly, AMPK alpha 1 deletion inhibited proliferation of HSCs, but not fibrogenesis, in vivo. Moreover, AMPK alpha 1 deletion was not associated with enhanced CCl4-induced fibrosis in vivo. In conclusion, our present findings demonstrate that HSC transdifferentiation is associated with increased AMPK activity that could relate to the stabilization of AMPK complex by the gamma 3 subunits. Activation of AMPK in HSCs inhibits in vitro fibrogenesis. By contrast, low AMPK activity does not prevent HSC activation in vitro nor in in vivo fibrosis. Show less
