Differential regulation of fibronectin expression and fibrillogenesis by autocrine TGF-β1 signaling in malignant and benign mammary epithelial cells

Remodeling of the extracellular matrix (ECM) is a key hallmark of cancer progression. A critical component of ECM remodeling is the assembly of the glycoprotein fibronectin (FN) into insoluble fibrils, which provide a scaffold for invading vascular endothelial cells and escaping cancer cells, as well as a framework for collagen deposition and oncogenic cytokine tethering. FN fibril assembly is induced by Transforming Growth Factor-81 (TGF-81), which was originally identified for its role in malignant transformation. Addition of exogenous TGF-81 drives FN fibril assembly while also upregulating endogenous TGF-81 expression and autocrine signaling. In the current study, we sought to determine if autocrine TGF-81 signaling plays a role in FN fibril formation in either MCF10A mammary epithelial cells, which behave similarly to healthy epithelia, or malignant MDA-MB-231 breast cancer cells. Our results show two interesting findings: first, malignant MDA-MB-231 cells assemble less FN into fibrils, despite expressing and secreting more soluble FN; second, autocrine TGF-81 signaling is required for FN fibril formation in MCF10A epithelial cells, even in the presence of exogenous, active TGF-81. This suggests that autocrine TGF-81 is signaling through distinct pathways from active exogenous TGF-81. We hypothesized that this signaling was mediated by interactions between the TGF-81 latency associated peptide (LAP) and av integrins; indeed, incubating MCF10As with soluble LAP, even in the absence of the active TGF-81 ligand, partially recovered FN fibril assembly. Taken together, these data suggests that autocrine TGF-81 plays a critical role in FN fibril assembly, and this interaction is mediated by LAP-integrin signaling.