Metformin against TGFβ-induced epithelial-to-mesenchymal transition (EMT): From cancer stem cells to aging-associated fibrosis

Transforming Growth Factor-b (TGFb) is a major driving force of the Epithelial-to-Mesenchymal (EMT) genetic program, which becomes overactive in the pathophysiology of many age-related human diseases. TGFb-driven EMT is sufficient to generate migrating cancer stem cells by directly linking the acquisition of cellular motility with the maintenance of tumor-initiating (stemness) capacity. Chronic diseases exhibiting excessive fibrosis can be caused by repeated and sustained infliction of TGFb-driven EMT, which increases collagen and extracellular matrix synthesis. Pharmacological prevention and/or reversal of TGFb-induced EMT may therefore have important clinical applications in the management of cancer metastasis as well as in the prevention and/or treatment of end-state organ failures. Earlier studies from our group have revealed that clinically-relevant concentrations of the biguanide derivative metformin, the most widely used oral agent to lower blood glucose concentration in patients with type 2 diabetes and metabolic syndrome, notably decreased both the self-renewal and the proliferation of trastuzumab-refractory breast cancer stem cell populations. Given that: a.) tumor-initiating cancer stem cells display a significant enrichment in the expression of basal/mesenchymal or myoepithelial markers, including an increased secretion of TGFb; b.) metformin treatment impedes the ontogeny of generating the stem cell phenotype by transcriptionally repressing key drivers of the EMT genetic program (e.g. ZEB1, TWIST1, SNAIL2 [Slug], TGFbs), we recently hypothesized that prevention of TGFb-induced EMT might represent a common molecular mechanism underlying the anti-cancer stem cells and anti-fibrotic actions of metformin. Remarkably, metformin exposure not only impedes TGFb-promoted loss of the epithelial marker E-cadherin in MCF-7 breast cancer cells but it prevents further TGF-induced cell scattering and accumulation of the mesenchymal marker vimentin in Madin-Darby canine kidney (MDCK) cells. We now propose that metformin, by weakening the ability of TGFb signaling to fully induce mesenchymal cell states in a variety of pathological processes including fibrosis (e.g. chronic renal disease, non-alcoholic steatohepatitis, heart failure or sclerosis) and malignant progression (and likely by reducing TGFb-regulated inflammation and immune responses -inflamm-aging-), molecularly behaves as a bona fide anti-aging modality.