Metformin Inhibits Hepatic mTORC1 Signaling via Dose-Dependent Mechanisms Involving AMPK and the TSC Complex

Abstract

Metformin is the most widely prescribed drug for the treatment of type-2 diabetes. However, knowledge of the full effects of metformin on biochemical pathways and processes in its primary target tissue, the liver, is limited. One established effect of metformin is to decrease cellular energy levels. The AMP-activated protein kinase (AMPK) and mechanistic target of rapamycin (mTOR) complex 1 (mTORC1) are key regulators of metabolism that are respectively activated and inhibited in acute response to cellular energy depletion. Here we show that metformin robustly inhibits mTORC1 in mouse liver tissue and primary hepatocytes. Using mouse genetics, we find that at the lowest concentrations of metformin that inhibit hepatic mTORC1 signaling, this inhibition is dependent on AMPK and the tuberous sclerosis complex (TSC) protein complex (TSC complex). Finally, we show that metformin profoundly inhibits hepatocyte protein synthesis in a manner that is largely dependent on its ability to suppress mTORC1 signaling.

Regulation of hepatic glucose metabolism in health and disease

“…In addition, both galegine (a related guanide compound) and metformin suppressed hepatic gluconeogenesis within 20 min of intravenous infusion in rats, a time frame that is inconsistent with that of transcriptional mechanisms 83 . In hepatocytes, the activation of AMPK in response to guanide and biguanide compounds is well established 78,80,82,84–86 and occurs rapidly following the intravenous administration of galegine 83 . However, interestingly, pharmacological activation of AMPK with A-769662 (which activates AMPK to a similar extent to galegine) did not suppress HGP in awake hepatic glycogen-depleted rats 83 .…”

AMPK: guardian of metabolism and mitochondrial homeostasis

“…This mouse model revealed that these phosphorylation events mediate the insulin-sensitizing effect of metformin, thus establishing AMPK as a relevant target in the action of metformin. Despite controversy 224 , AMPK is widely viewed as an essential component of the action of metformin at physiological concentrations 219,225 . Given the role of the AMPK–ACC pathway in regulating fatty acid synthesis, AMPK activation is also an attractive treatment option for conditions associated with increased fatty acid production, such as nonalcoholic fatty liver disease (NAFLD) 85 .…”

Cytoprotective activated protein C averts Nlrp3 inflammasome–induced ischemia-reperfusion injury via mTORC1 inhibition

“…We acknowledge that the role of mTOR signaling in myocardial IRI is complex and depends on its temporal activation pattern, the cell type, the extent of mTOR activity, and the involvement of mTORC1 vs mTORC2. 14,41,60 The ability of TSC1 deficiency to abolish aPC-mediated mTORC1 inhibition indicates that aPC regulates the mTORC1 complex. 60,61 However, considering the multifaceted interactions of TSC1/TSC2, mTORC1, and mTORC2, the precise mechanism needs to be evaluated in future studies.…”

“…TSC1 is a pivotal inhibitor of mTORC1, and its deficiency causes constitutive mTORC1 activation. 41 BMDMs were isolated from mice with inducible TSC1 deficiency, and TSC1 expression was inhibited by transient expression with Cre recombinase ex vivo 41 ( Figure 4F; control cells transiently expressed GFP). In TSC1-deficient BMDMs, aPC failed to inhibit expression of Raptor and HK1 and phosphorylation of p70S6K ( Figure 4G-H), and importantly, aPC-mediated Nlrp3 inflammasome restriction was abolished ( Figure 4I-J).…”