A High-Fat Diet Promotes Mammary Gland Myofibroblast Differentiation through MicroRNA 140 Downregulation

Human breast adipose tissue is a heterogeneous cell population consisting of mature white adipocytes, multipotent mesenchymal stem cells, committed progenitor cells, fibroblasts, endothelial cells, and immune cells. Dependent on external stimulation, adipose-derived stem cells differentiate along diverse lineages into adipocytes, chondrocytes, osteoblasts, fibroblasts, and myofibroblasts. It is currently not fully understood how a high-fat diet reprograms adipose-derived stem cells into myofibroblasts. In our study, we used mouse models of a regular diet and of high-fat-diet-induced obesity to investigate the role of dietary fat on myofibroblast differentiation in the mammary stromal microenvironment. We found that a high-fat diet promotes myofibroblast differentiation by decreasing microRNA 140 (miR-140) expression in mammary adipose tissue through a novel negative-feedback loop. Increased transforming growth factor β1 (TGF-β1) in mammary adipose tissue in obese mice activates SMAD3 signaling, causing phospho-SMAD3 to bind to the miR-140 locus and inhibit miR-140 transcription. This prevents miR-140 from targeting SMAD3 for degradation, resulting in amplified TGF-β1/SMAD3 signaling and miR-140 downregulation-dependent myofibroblast differentiation. Using tissue and coculture models, we found that myofibroblasts and the fibrotic microenvironment created by myofibroblasts impact the stemness and proliferation of normal ductal epithelial cells and early-stage breast cancer invasion and stemness.