Obesity-dependent changes in interstitial ECM mechanics promote breast tumorigenesis

Obesity leads to fibrotic remodeling of mammary adipose tissue, and the resulting increase in interstitial extracellular matrix stiffness promotes breast tumor malignancy. Fat fibrosis and breast cancer One of the many risk factors for cancer is obesity—but why? Seo et al. examined the cellular, structural, and molecular changes that happen in breast tissue in obese animals and people. They found that obesity induces fibrotic remodeling of the mammary fat pad, leading to changes in extracellular matrix (ECM) mechanical properties, via myofibroblasts and adipose stem cells (ASCs), regardless of ovary function. Through altered mechanotransduction, ECM from obese mice promoted human breast cancer cell growth, as well as the growth of premalignant breast cells (those that have yet to become cancerous). Tissues from obese patients revealed more severe fibrotic remodeling around tumors and higher levels of a key mechanosignaling component, YAP/TAZ, than their lean counterparts. The authors further demonstrated that caloric restriction in obese mice decreased fibrosis in mammary fat, suggesting a therapeutic angle for obesity-related cancers. By linking tumorigenesis to the behavior of fat cells and ECM mechanics, the authors point toward new drug targets for preventing cancer progression. However, a cautionary tale also exists in the use of adipose tissue and cells for patients after mastectomy, as ASCs from obese individuals may have the capacity to promote breast cancer recurrence. Obesity and extracellular matrix (ECM) density are considered independent risk and prognostic factors for breast cancer. Whether they are functionally linked is uncertain. We investigated the hypothesis that obesity enhances local myofibroblast content in mammary adipose tissue and that these stromal changes increase malignant potential by enhancing interstitial ECM stiffness. Indeed, mammary fat of both diet- and genetically induced mouse models of obesity were enriched for myofibroblasts and stiffness-promoting ECM components. These differences were related to varied adipose stromal cell (ASC) characteristics because ASCs isolated from obese mice contained more myofibroblasts and deposited denser and stiffer ECMs relative to ASCs from lean control mice. Accordingly, decellularized matrices from obese ASCs stimulated mechanosignaling and thereby the malignant potential of breast cancer cells. Finally, the clinical relevance and translational potential of our findings were supported by analysis of patient specimens and the observation that caloric restriction in a mouse model reduces myofibroblast content in mammary fat. Collectively, these findings suggest that obesity-induced interstitial fibrosis promotes breast tumorigenesis by altering mammary ECM mechanics with important potential implications for anticancer therapies.