Metformin: update on mechanisms of action and repurposing potential

Abstract

Currently, metformin is the first-line medication to treat type 2 diabetes mellitus (T2DM) in most guidelines and is used daily by >200 million patients. Surprisingly, the mechanisms underlying its therapeutic action are complex and are still not fully understood. Early evidence highlighted the liver as the major organ involved in the effect of metformin on reducing blood levels of glucose. However, increasing evidence points towards other sites of action that might also have an important role, including the gastrointestinal tract, the gut microbial communities and the tissue-resident immune cells. At the molecular level, it seems that the mechanisms of action vary depending on the dose of metformin used and duration of treatment. Initial studies have shown that metformin targets hepatic mitochondria; however, the identification of a novel target at low concentrations of metformin at the lysosome surface might reveal a new mechanism of action. Based on the efficacy and safety records in T2DM, attention has been given to the repurposing of metformin as part of adjunct therapy for the treatment of cancer, age-related diseases, inflammatory diseases and COVID-19. In this Review, we highlight the latest advances in our understanding of the mechanisms of action of metformin and discuss potential emerging novel therapeutic uses.

Fig. 1. Proposed mechanisms for metformin-induced reductions in blood levels of glucose.

Left: Complex I inhibition-dependent mechanisms. In the liver, metformin induces a mild inhibition of mitochondrial respiratory chain complex I, leading to a moderate decrease in ATP synthesis and a concomitant increase in cellular levels of AMP. The metformin-induced decrease in hepatic gluconeogenic flux, an ATP-dependent metabolic process, could result from this reduction in ATP levels. In addition, increased AMP levels lead to inhibition of the activity of enzymes that are regulated by AMP and are involved in gluconeogenesis, such as adenylate cyclase and fructose-1-6-bisphosphatase (FBP1), which contributes to decreased glucose output. Of note, the metformin-induced increase in the AMP to ATP ratio also activates AMP-activated protein kinase (AMPK), but this has no direct effect on the regulation of glucose production. The inhibition of complex I by metformin is also accompanied by an increase in cellular redox potential (NADH:NAD⁺). Middle: Mitochondrial glycerol-3-phosphate dehydrogenase (mGPDH)-dependent and complex IV inhibition-dependent mechanisms. Metformin directly inhibits mGPDH, resulting in an increased cytosolic redox state (NADH:NAD⁺), reduced gluconeogenesis from lactate and reduced activity of the glycerol–phosphate shuttle (which transfers NADH from the cytosol to mitochondria). In addition, metformin raises the hepatic redox state through an increase in the glutathione to oxidized glutathione ratio (GSH:GSSG), leading to inhibition of genes encoding enzymes involved in gluconeogenesis through a let-7–TET3–HNF-4α pathway. Finally, metformin inhibits mitochondrial respiratory chain complex IV, which can also result in an indirect inhibition of mGPDH activity. Right: AMPK activation-dependent mechanisms in lysosomes. Metformin at low concentrations binds presenilin enhancer 2 (PEN2), which is recruited to ATPase H⁺ transporting accessory protein 1 (ATP6AP1) independent of changes in AMP levels, leading to inhibition of v-ATPase and phosphorylation and/or activation of AMPK in lysosomes through the formation of a supercomplex containing the v-ATPase, Ragulator, AXIN, liver kinase B1 (LKB1) and AMPK. Thereafter, metformin-activated AMPK from lysosomes reduces lipid accumulation in the liver via acetyl-CoA carboxylase (ACC) inhibition and increases glucagon-like peptide 1 (GLP1) secretion in the gut, inducing reductions in blood levels of glucose. cGPDH, cytosolic glycerol-3-phosphate dehydrogenase; HNF-4α, hepatocyte nuclear factor 4α; LDH, lactate dehydrogenase; OCT1, organic transporter 1; TET3, Tet methylcytosine dioxygenase 3.

Fig. 2. Anti-inflammatory and immunomodulatory effects of metformin.

a, Following putative transporter-mediated internalization in various immune cell subsets, metformin inhibits the mitochondrial respiratory chain complex I and can modulate cell-specific inflammatory processes by both AMP-activated protein kinase (AMPK)-independent and AMPK-dependent mechanisms. b, Metformin can modulate the immune system, which could have beneficial effects in various pathological conditions (such as certain cancers, infections and hyperinflammatory diseases). These effects have been reported to involve various innate and adaptive immune cells, leading to modulation of several cell–cell interactions in local niches, as well as in immunometabolic and cellular processes. ATF3, activating transcription factor 3; CXCL1, chemokine (C-X-C motif) ligand 1; FASN, fatty acid synthase; FOXO3, forkhead box O3; IFNγ, interferon-γ; ISGs, interferon-stimulated genes; mTOR, mammalian target of rapamycin; NAFLD, non-alcoholic fatty liver disease; NASH, non-alcoholic steatohepatitis; NF-κB, nuclear factor-κB; NGAL, neutrophil gelatinase-associated lipocalin; PD1, programmed cell death protein 1; Pi, inorganic phosphate; PRF1, perforin 1; ROS, reactive oxygen species; STAT1 and STAT3, signal transducer and activator of transcription 1 and 3; TGFβ, transforming growth factor-β; TNF, tumour necrosis factor.