We have tested the hypothesis that the
antidiabetic biguanide metformin can be used to manipulate the threshold for stress-induced senescence (SIS), thus accelerating the onset of
cancer-protective cellular senescence in response to oncogenic stimuli. Using senescence-prone murine embryonic fibroblasts (MEFs), we assessed whether
metformin treatment modified the senescence phenotype that is activated in response to
DNA damaging inducers.
Metformin significantly enhanced the number of MEFs entering a senescent stage in response to
doxorubicin, an
anthracycline that induces cell senescence by activating DNA damage signaling pathways (e.g., ATM/ATR) in a
reactive oxygen species (ROS)-dependent manner. Using WI-38 and BJ-1 human diploid fibroblasts (HDFs), we explored whether
metformin supplementation throughout their entire replicative lifespan may promote the early appearance of the
biomarkers of replicative senescence. Chronic
metformin significantly reduced HDFs' lifespan by accelerating both the loss of replicative potential and the acquisition of replicative senescence-related
biomarkers (e.g., enlarged and flattened cell shapes, loss of arrayed arrangement, accumulation of intracellular and extracellular debris and SA-β-gal-positive staining).
Metformin functioned as a bona fide stressful agent, inducing monotonic, dose-dependent, SIS-like responses in BJ-1 HDFs, which are highly resistant to ROS-induced premature senescence.
Metformin-induced SIS in BJ-1 fibroblasts was accompanied by the striking activation of several
microRNAs belonging to the miR-200s family (miR-200a, miR-141 and miR429) and miR-205, thus mimicking a recently described ability of ROS to chemosensitize
cancer cells by specifically upregulating anti-EMT (epithelial-to-mesenchymal transition) miR-200s. Because the unlimited proliferative potential of stem cells results from their metabolic refractoriness to SIS, we finally tested if
metformin treatment could circumvent the stress (e.g., ROS)-resistant phenotype of induced pluripotent stem cells (iPSCs).
Metformin treatment drastically reduced both the number and the size of iPSC colonies and notably diminished the staining of the pluripotency marker
alkaline phosphatase. Our current findings, altogether, reveal for the first time that
metformin can efficiently lower the threshold for SIS to generate an "stressed" cell phenotype that becomes pre-sensitized to oncogenic-like stimuli, including
DNA damaging, proliferative and/or stemness inducers.