The biguanides metformin and phenformin are antidiabetic drugs with documented antitumor properties in colorectal cancer (CRC). Despite intensive research, the mechanism of action of biguanides in cancer is still unclear. The most accepted model links their effect to inhibition of mitochondrial complex I, thereby causing energy stress with activation of AMPK and mTOR inhibition. However, this model has been questioned since complex I inhibition requires millimolar concentrations of biguanides but these values cannot be reached in patients and animal models, where the maximum tolerated doses are within the micromolar range. Recent studies in diabetic patients have shown that the weight- and glucose-lowering properties of biguanides are related to their ability to increase secretion of GDF15, a stress induced cytokine, independently of complex I. However, whether GDF15 may also mediate the therapeutic effect of biguanides in cancer is currently unknown.
Our working hypothesis is that biguanides inhibit mitochondrial GPD2, a component of the glycerophosphate shuttle, thereby causing elevations of NADH (reductive stress) with consequent activation of the redox sensor CtBP2. We believe that this mechanism governs the increase of GDF15, which in turn inhibits intestinal tumorigenesis, likely inducing an antitumor immune response. We propose that specific combinations of biguanides, nutrients and/or novel drugs enhance this mechanism by elevating NADH content and GDF15 expression and may be exploited for therapeutic purposes in CRC.
The current project aims at elucidating these issues by 1) investigating the biguanide-GDF15 axis in preclinical tumor models of CRC, 2) defining its mode of action and 3) developing new approaches to exploit this mechanism for therapeutic purposes. Results from this study will provide invaluable information to understand the mechanism of action and conditions of usage of a widely studied class of metabolic drugs in CRC.
Despite metformin is the most prescribed antidiabetic drug worldwide and it is recognized that biguanides may represent valuable tools against cancer, how these drugs exert their antitumor effects is still highly debated. The classical interpretation is that they inhibit cellular respiration by inhibiting complex I, causing a decrease of ATP/ADP ratio (energy stress) and activation of AMPK-mTOR mediated antitumor response. However, the drug concentrations required to activate this mechanism are too elevated compared to the maximum tolerated doses in humans and animal models.
In this project we propose a completely novel idea to explain how biguanide work in colorectal cancer, a disease where these drugs have been shown to exert potential therapeutic and preventive effects. We propose that the antitumor effects may be mediated by increased GDF15, a protein that has been very recently identified as the key mediator of glucose and weight lowering effect of metformin in diabetes.
We propose that GDF15 is a key regulator of the anticancer properties of biguanides, connecting a previously unappreciated metabolic alteration, the reductive stress, (i.e. elevation of NADH/NAD+) to the antitumor response. We suggest that biguanides regulates this peculiar metabolic alteration by inhibiting GPD2, a component of the mitochondrial glycerophosphate shuttle, thereby causing an increase of cytoplasmic NADH. These elevations lead to activation of the redox sensor CtBP2, which functions as transcriptional regulator, leading to activation of GDF15 transcription. We hypothesize that GDF15 acts by recruiting the immune system against tumor cells and, importantly, we believe that this putative new mechanism could be exploited by specific strategies aimed at increasing the intracellular redox state. To our knowledge, this is a completely novel view about the mechanism of action of biguanides and, in general, about the role of intracellular NADH/redox state in cancer.
We believe that the idea that biguanides may act through reductive stress-mediated increase of GDF15 may represent a real breakthrough in Oncology as it introduces the challenging and, to some extent, provocative idea that biguanides may act by connecting a previously unappreciated metabolic alteration (intracellular increase of NADH) with an anticancer response, likely recruiting the immune system. In other words, we suggest that biguanides may act as immune-metabolic modulators against cancer.
It is known that in many tumor-associated metabolic alterations, such as aerobic glycolysis, the levels of NADH increase, to provide reducing equivalents to the mitochondrial electron transport chain to build up ATP. Thus, the idea that the increase of NADH may have deleterious consequences for a cancer cell may sound somehow counterintuitive. However, as for many other alterations, the imbalance of a key metabolite may also represent a condition of stress that is sensed by a dedicated machinery that activates compensatory responses, leading to restoration of proper homeostatic levels, but also associated to inhibition of cell proliferation or cell death. In this context, GDF15 would represent the signaling molecule that mediates this integrated stress response.
Although it is well recognized that biguanides have significant antitumor properties in experimental settings, and some clinical trials have documented their efficacy in the chemoprevention and treatment of some forms of cancer, including CRC, their use in clinical oncology has not been approved yet. One of the reasons is likely the lack of understanding of their pharmacodynamics, which is largely attributable to the discrepancies between the drug concentrations used in experimental settings and the doses routinely administered to patients or animal models. To overcome this limitation, the key point of this project is that the proposed model works at the low (micromolar), therapeutic doses of the drugs and that clinically relevant concentrations and conditions will be used throughout the whole project.
The overall new idea proposed in this project is that, if the model will be experimentally confirmed, it will be possible to study novel strategies aimed at targeting these novel players: the reductive stress or, more specifically, the circulating levels of GDF15. As described in the proposal this could be obtained with specific foods (e.g. oleic acid) that synergize with biguanides, or with novel drugs that selectively target this metabolic alteration (e.g. C17 or additional GPD2 inhibitors), possibly without altering other pathways, or even by direct administration of GDF15.