Metformin suppresses HIF-1α expression in cancer-associated fibroblasts to prevent tumor-stromal cross talk in breast cancer

The tumor microenvironment (TME) is a crucial factor in cancer progression. In breast cancer, cancer-associated fibroblasts (CAFs) and the derived stromal components have been recognized as comprising the majority of the pathological structure of the TME. In this study, we show that metformin (Met), a diabetes drug, transforms CAFs in the TME. Met disrupts tumor-stromal cross talk by preventing breast cancer cell transforming growth factor-beta (TGF-beta) signaling and the production of stromal-derived factor-1 (SDF-1) and interleukin-8 (IL-8) by CAFs. The suppression of bidirectional signaling between tumor cells and CAFs by Met is attributed to increased phospho-AMP kinase (p-AMPK) levels. By upregulating p-AMPK in CAFs, Met induces prolyl hydroxylases (PHDs), leading to the degradation of hypoxia-inducible factor-1 alpha (HIF-1 alpha) in CAFs. Moreover, interruption of HIF-1 alpha-driven SDF-1 signaling in CAFs by Met leads to decreased breast cancer cell invasion. These findings suggest that Met may be used to target tumor-promoting signaling between CAFs and breast cancer cells in the TME.