Metformin targets Stat3 to inhibit cell growth and induce apoptosis in triple-negative breast cancers

A distinct group of breast cancers, called basal or triple-negative (TN) cancers, express both basal cytokeratins and the epidermal growth factor receptor but fail to express estrogen receptors, progesterone receptors or HER 2 and have stem-like or mesenchymal features. They are particularly aggressive, are frequently chemoresistant, with p53 mutation and upregulation of IL-6 and Stat3. Because TN cells are particularly sensitive to the anti-diabetic agent metformin, we hypothesized that it may target JAK2/Stat3 signaling. The effects of metformin upon Stat3 expression and activation were examined in four human TN cell lines. Metformin's effects were also studied in sublines with forced overexpression of constitutively active (CA) Stat3 as well as lines with stable knockdown of Stat3. Metformin inhibited Stat3 activation (P-Stat3) at Tyr705 and Ser727 and downstream signaling in each of the four parental cell lines. CA-Stat3 transfection attenuated, whereas Stat3 knockdown enhanced, the effects of metformin upon growth inhibition and apoptosis induction. A Stat3-specific inhibitor acted synergistically with metformin in reducing cell growth and inducing apoptosis. An mTOR inhibitor showed no significant interaction with metformin. In summary, Stat3 is a critical regulator of metformin action in TN cancer cells, providing the potential for enhancing metformin's efficacy in the clinical setting.