Stiff collagen matrices increase tumorigenic prolactin signaling in breast cancer cells

Clinically, circulating prolactin levels and density of the extracellular matrix (ECM) are individual risk factors for breast cancer. As tumors develop, the surrounding stroma responds with increased deposition and cross-linking of the collagen matrix (desmoplasia). In mouse models, prolactin promotes mammary carcinomas that resemble luminal breast cancers in women, and increased collagen density promotes tumor metastasis and progression. Although the contributions of the ECM to the physiologic actions of prolactin are increasingly understood, little is known about the functional relationship between the ECM and prolactin signaling in breast cancer. Here, we examined consequences of increased ECM stiffness on prolactin signals to luminal breast cancer cells in three-dimensional collagen I matrices in vitro. We showed that matrix stiffness potently regulates a switch in prolactin signals from physiologic to protumorigenic outcomes. Compliant matrices promoted physiological prolactin actions and activation of STAT5, whereas stiff matrices promoted protumorigenic outcomes, including increased matrix metalloproteinase-dependent invasion and collagen scaffold realignment. In stiff matrices, prolactin increased SRC family kinase-dependent phosphorylation of focal adhesion kinase (FAK) at tyrosine 925, FAK association with the mitogen-activated protein kinase mediator GRB2, and pERK1/2. Stiff matrices also increased co-localization of prolactin receptors and integrin-activated FAK, implicating altered spatial relationships. Together, these results demonstrate that ECM stiffness is a powerful regulator of the spectrum of prolactin signals and that stiff matrices and prolactin interact in a feed-forward loop in breast cancer progression. Our study is the first reported evidence of altered ECM-prolactin interactions in breast cancer, suggesting the potential for new therapeutic approaches.