β ₃ Integrin and Src facilitate transforming growth factor-β mediated induction of epithelial-mesenchymal transition in mammary epithelial cells

Abstract

Introduction

Transforming growth factor (TGF)-β suppresses breast cancer formation by preventing cell cycle progression in mammary epithelial cells (MECs). During the course of mammary tumorigenesis, genetic and epigenetic changes negate the cytostatic actions of TGF-β, thus enabling TGF-β to promote the acquisition and development of metastatic phenotypes. The molecular mechanisms underlying this conversion of TGF-β function remain poorly understood but may involve signaling inputs from integrins.

Methods

β ₃ Integrin expression or function in MECs was manipulated by retroviral transduction of active or inactive β ₃ integrins, or by transient transfection of small interfering RNA (siRNA) against β ₃ integrin. Altered proliferation, invasion, and epithelial-mesenchymal transition (EMT) stimulated by TGF-β in control and β ₃ integrin manipulated MECs was determined. Src involvement in β ₃ integrin mediated alterations in TGF-β signaling was assessed by performing Src protein kinase assays, and by interdicting Src function pharmacologically and genetically.

Results

TGF-β stimulation induced α _vβ ₃ integrin expression in a manner that coincided with EMT in MECs. Introduction of siRNA against β ₃ integrin blocked its induction by TGF-β and prevented TGF-β stimulation of EMT in MECs. β ₃ integrin interacted physically with the TGF-β receptor (TβR) type II, thereby enhancing TGF-β stimulation of mitogen-activated protein kinases (MAPKs), and of Smad2/3-mediated gene transcription in MECs. Formation of β ₃ integrin:TβR-II complexes blocked TGF-β mediated growth arrest and increased TGF-β mediated invasion and EMT. Dual β ₃ integrin:TβR-II activation induced tyrosine phosphorylation of TβR-II, a phosphotransferase reaction mediated by Src in vitro. Inhibiting Src activity in MECs prevented the ability of β ₃ integrin to induce TβR-II tyrosine phosphorylation, MAPK activation, and EMT stimulated by TGF-β. Lastly, wild-type and D119A β ₃ integrin expression enhanced and abolished, respectively, TGF-β stimulation of invasion in human breast cancer cells.

Conclusion

We show that β ₃ integrin alters TGF-β signaling in MECs via Src-mediated TβR-II tyrosine phosphorylation, which significantly enhanced the ability of TGF-β to induce EMT and invasion. Our findings suggest that β ₃ integrin interdiction strategies may represent an innovative approach to re-establishing TGF-β mediated tumor suppression in progressing human breast cancers.