Repurposing metformin: an old drug with new tricks in its binding pockets

Abstract

Improvements in healthcare and nutrition have generated remarkable increases in life expectancy worldwide. This is one of the greatest achievements of the modern world yet it also presents a grave challenge: as more people survive into later life, more also experience the diseases of old age, including type 2 diabetes (T2D), cardiovascular disease (CVD) and cancer. Developing new ways to improve health in the elderly is therefore a top priority for biomedical research. Although our understanding of the molecular basis of these morbidities has advanced rapidly, effective novel treatments are still lacking. Alternative drug development strategies are now being explored, such as the repurposing of existing drugs used to treat other diseases. This can save a considerable amount of time and money since the pharmacokinetics, pharmacodynamics and safety profiles of these drugs are already established, effectively enabling preclinical studies to be bypassed. Metformin is one such drug currently being investigated for novel applications. The present review provides a thorough and detailed account of our current understanding of the molecular pharmacology and signalling mechanisms underlying biguanide-protein interactions. It also focuses on the key role of the microbiota in regulating age-associated morbidities and a potential role for metformin to modulate its function. Research in this area holds the key to solving many of the mysteries of our current understanding of drug action and concerted effects to provide sustained and long-life health.

Keywords: aging; biguanides; cancer; cardiovascular disease; metformin; microbiota; phenformin; type 2 diabetes.

Figure 1. Proposed mechanisms of metformin action in T2D

Metformin enters the hepatocyte through OCT1 and accumulates in the mitochondria where it inhibits complex I. This leads to a reduction in ATP and concomitant rise in AMP. Elevated AMP levels lead to activation of AMPK, although metformin may also promote AMPK activation in a direct manner. AMPK inhibits gluconeogenic gene transcription by preventing formation of the CREB–CBP–CRTC2 complex, both directly and via SIRT1. Furthermore, AMPK inhibits lipogenesis through ACC, ChREBP and SREBP phosphorylation, which helps to improve insulin sensitivity. Several AMPK-independent mechanisms of metformin action also exist. The reduction in cellular energy status can directly inhibit gluconeogenic flux. Additionally, increased AMP has an inhibitory effect on adenylate cyclase leading to decreased cAMP production. This in turn reduces the activity of PKA and its downstream targets, which include CREB. Metformin also inhibits mGPD. This prevents glycerol from contributing to gluconeogenesis and increases the cytosolic redox state, which in turn makes the conversion of lactate to pyruvate unfavourable thus limiting the use of lactate as a gluconeogenic substrate.

Figure 2. Proposed mechanisms to explain the anti-cancer effects of metformin

At the systemic level metformin may inhibit tumour development by reducing insulin/IGF-1 signalling, preventing the release of pro-inflammatory cytokines through NF-κB and enhancing the immune response [mediated by natural killer cells (NK cells) and cytolytic T-cells (T-cells)] to cancer cells. Metformin also has direct effects in cancer cells, many of which are mediated by AMPK. When activated, AMPK disrupts cancer cell energy metabolism by inhibiting fatty acid synthesis (inhibition of ACC) and suppressing the Warburg phenotype mediated by the action of the hypoxia-induced factor (HIF-1) on the glycolytic enzymes pyruvate kinase (PDK) and pyruvate dehydrogenase (PDH). AMPK may also block folate and methionine metabolism, which is required for nucleotide synthesis. AMPK down-regulates the oncogene c-MYC by inducing the expression of Dicer and up-regulates the tumour suppressor p53 by inhibiting its negative regulator MDMX. Furthermore, AMPK blocks the mTORC1 signalling pathway by inhibiting Raptor and activating TSC2. AMPK independent mechanisms have also been described. Metformin can protect against DNA damage by inhibiting complex I and suppressing ROS production. Metformin also inhibits hexokinase activity and impairs glucose uptake. Metformin can block mTORC1 signalling in the absence of AMPK via inhibition of Rag GTPases. Additionally, metformin reduces levels of the cell cycle regulator cyclin D1 in a p53- and REDD1-dependent manner. Finally, metformin promotes apoptosis by down-regulating the Stat3/Bcl-2 pathway through release of cytochrome C and also promotes autophagy.

Figure 3. Effects of metformin in gut physiology to regulate T2D and survival in model organisms and humans

Metformin increases C. elegans lifespan when co-cultured with a sensitive but not a tolerant E. coli strain. In Drosophila, high concentrations of metformin lead to intestinal dyshomoeostasis and increased concentration of faecal output that correlates with an observed shortening of lifespan. Metformin normalizes the gut microbiota of rodents on a high-fat diet and improves glucose homoeostasis due to effects on the gut microbiota. In T2D diabetic patients, metformin promotes weight loss and improved glucose tolerance either by reducing gut glucose uptake or by direct alterations of the gut microbiota which can ultimately lead to increased survival.