Metformin: Multi-faceted protection against cancer

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.

The molecular mechanisms used by breast cancer stem cells (BCSCs) to survive and/or maintain their undifferentiated CD44+CD24-/low mesenchymal-like antigenic state remains largely unexplored. Autophagy, a key homeostatic process of cytoplasmic degradation and recycling evolved to respond to stress conditions, might be causally fundamental in the biology of BCSCs. Stable & specific knockdown of autophagy-regulatory genes by lentiviral-delivered small hairpin (sh) RNA drastically decreased the number of JIMT-1 epithelial BC cells bearing CD44+CD24-/low cell-surface antigens from ~75% in parental and control (-) shRNA-transduced cells to 26% and 7% in ATG8/LC3 shRNA- and ATG12 shRNA-transduced cells, respectively. Autophagy inhibition notably enhanced transcriptional activation of CD24 gene, potentiating the epithelial-like phenotype of CD44+CD24+ cells versus the mesenchymal CD44+CD24-/low progeny. EMT-focused Real Time RT-PCR profiling revealed that genetic ablation of autophagy transcriptionally repressed the gene coding for the mesenchymal filament vimentin (VIM). shRNA-driven silencing of the ATG12 gene and disabling the final step in the autophagy pathway by the antimalarial drug chloroquine both prevented TGFb1-induced accumulation of vimentin in JIMT-1 cells. Knockdown of autophagy-specific genes was sufficient also to increase by up to 11-times the number of CD24+ cells in MDA-MB-231 cells, a BC model of mesenchymal origin that is virtually composed of CD44+CD24-/low cells. Chloroquine treatment augmented the number of CD24+ cells and concomitantly reduced constitutive overexpression of vimentin in MDA-MB-231 cells. This is the first report demonstrating that autophagy is mechanistically linked to the maintenance of tumor cells expressing high levels of CD44 and low levels of CD24, which are typical of BCSCs .

In the early 1970s, Professor Vladimir Dilman originally developed the idea that antidiabetic biguanides may be promising as geroprotectors and anticancer drugs ("metabolic rehabilitation"). In the early 2000s, Anisimov´s experiments revealed that chronic treatment of female transgenic HER2-/neu mice with metformin significantly reduced the incidence and size of mammary adenocarcinomas and increased the mean latency of the tumors. Epidemiological studies have confirmed that metformin, but not other anti-diabetic drugs, significantly reduces cancer incidence and improves cancer patients' survival in type 2 diabetics. At present, pioneer work by Dilman & Anisimov at the Petrov Institute of Oncology (St. Petersburg, Russia) is rapidly evolving due to ever-growing preclinical studies using human tumor-derived cultured cancer cells and animal models. We herein critically review how the antidiabetic drug metformin is getting reset to metabolically fight cancer. Our current perception is that metformin may constitute a novel "hybrid anti-cancer pill" physically combining both the long-lasting effects of antibodies -by persistently lowering levels of blood insulin and glucose- and the immediate potency of a cancer cell-targeting molecular agent -by suppressing the pivotal AMPK/mTOR/S6K1 axis and several protein kinases at once, including tyrosine kinase receptors such as HER1 and HER2-. In this scenario, we discuss the relevance of metformin doses in pre-clinical models regarding metformin's mechanisms of action in clinical settings. We examine recent landmark studies demonstrating metformin's ability to specifically target the cancer-initiating stem cells from which tumor cells develop, thereby preventing cancer relapse when used in combination with cytotoxic chemotherapy (dandelion hypothesis). We present the notion that, by acting as an efficient caloric restriction mimetic, metformin enhanced intrinsic capacity of mitotically competent cells to self-maintenance and repair (hormesis) might trigger counterintuitive detrimental effects. Ongoing chemopreventive, neoadjuvant and adjuvant trials should definitely establish whether metformin's ability to kill the "dandelion root" beneath the "cancer soil" likely exceeds metformin-related dangers of hormesis.

Aging can be viewed as a quasi-programmed phenomenon driven by the overactivation of the nutrient-sensing mTOR gerogene. mTOR-driven aging can be triggered or accelerated by a decline or loss of re...