Targeting redox alterations and sensors in colorectal cancer

Biguanides are a class of antidiabetic drugs known for their anticancer properties, but their mechanism of action is still poorly understood. The most widely accepted model links biguanides antitumoral activity to the inhibition of Complex I of mitochondrial respiratory chain and consequent activation of the AMP Kinase, a sensor of energy stress. However, this model has raised several concerns since inhibition of Complex I requires millimolar concentrations of the drugs, which cannot be reached in patients or animal models, where the maximum tolerated doses are in the range of low micromolar range. This observation suggests that at physiologic doses alternate mechanisms of action may contribute to the antitumor effects of biguanides. In this study, we have investigated the mechanism of action of biguanides, using as a model colorectal cancer (CRC), a disease where biguanides have shown promising therapeutic properties. Oral administration of biguanides to APC-mutated mice revealed that phenformin, but not metformin, causes a significant reduction of intestinal tumorigenesis and its maximum concentration in the gut is within the low micromolar range. Treatment of CRC cell lines with low micromolar concentrations result in a significant inhibitory effect of cell proliferation, mediated by an increase of cytoplasmic NADH/NAD+ levels. This redox imbalance is due to inhibition of mitochondrial Glycerol-3-Phosphate Dehydrogenase (mGPD), a component of the glycerophosphate shuttle (GPS), that is inhibited by biguanides. The anti-proliferative effect of redox imbalance is reproduced by alterations of lactate/pyruvate ratio and can be rescued by specific NADH oxidases. We show that the redox dependent inhibitory effect is mediated by the corepressor C-terminal Binding Protein 2 (CtBP2), that senses redox alterations through its NADH-binding domain. Relevantly, we demonstrate that dietary limitation through short-term starvation (STS) significantly enhances the redox-dependent inhibition of cancer cells growth after phenformin treatment, both in in vitro and in vivo CRC models. Collectively we demonstrate that therapeutic concentrations of phenformin exert a robust anticancer activity in CRC cells by a coupled metabolic-corepressor mechanism and that a combination of calorie restricted diet and phenformin may represent a promising novel therapeutic opportunity to treat colorectal cancer.