Dottorato in MEDICINA MOLECOLARE

Titolo della tesi: Phenformin-mediated anti-tumor properties rely on a novel redox/corepressor interplay

The biguanides metformin and phenformin are antidiabetic drugs associated to well-established antitumor properties in preclinical and clinical settings. Despite intensive efforts and studies, the mechanism of action of biguanides in cancer is still unclear. The most accepted model links the effect of these drugs to their ability to inhibit Complex I of the mitochondrial electron transport chain, thereby decreasing respiration and causing energy stress and elevation of the redox state. However, this model has been questioned since inhibition of Complex I requires millimolar concentrations of biguanides and these values cannot be reached in patients and animal models, where the maximum tolerated doses are within the low micromolar range (i.e. 1-5 μM). Using Hedgehog-dependent Medulloblastoma (Shh MB) as a study model, we have demonstrated that therapeutic doses of phenformin do not inhibit Complex I but rather mitochondrial Glycerol-3-Phosphate Dehydrogenase (mGPD), a component of the glycerophosphate shuttle (GPS). This inhibition causes the increase of cytoplasmic redox state and NADH. We show that C-terminal Binding Protein 2 (CtBP2) senses the elevations of NADH and binds Gli1, the transcription factor and downstream effector of Hedgehog signaling, causing its functional inhibition and suppression of tumor growth. To understand whether this newly discovered mechanism is also effective in other types of malignancies, we studied the effect of biguanides on colorectal cancer (CRC) growth. As shown by our preliminary data, phenformin seems to inhibit cancer cell proliferation through a redox mechanism, mediated by the corepressor CtBP2 also in CRC. Notably, we observed that Free Fatty Acids (FFAs) act as redox-inducing nutrients as they significantly enhance the phenformin-induced NADH content and inhibition of cancer growth. Collectively, with our work we have discovered a previously uncharacterized mechanism of action of biguanides in cancer, whereby the mGPD-mediated elevation of redox state represents a key event, leading the subsequent corepressor-mediated inhibition of tumor growth. These results pave the way to new studies focused on the use of biguanides and/or other redox enhancing agents in cancer.