Src phosphorylates Tyr284 in TGF-β type II receptor and regulates TGF-β stimulation of p38 MAPK during breast cancer cell proliferation and invasion

Genetic and epigenetic events often negate the cytostatic function of transforming growth factor-beta (TGF-beta) in mammary epithelial cells (MEC), which ultimately enables malignant MECs to proliferate, invade, and metastasize when stimulated by TGF-beta. The molecular mechanisms underlying this phenotypic conversion of TGF-beta function during mammary tumorigenesis remain poorly defined. We previously established alpha(v)beta(3) integrin and Src as essential mediators of mitogen-activated protein kinase (MAPK) activation, invasion, and epithelial-to-mesenchymal transition stimulated by TGF-beta in normal and malignant MECs. Mechanistically, 3 integrin interacted physically with the TGF-beta type II receptor (T beta R II-II), leading to its tyrosine phosphorylation by Src and the initiation of oncogenic signaling by TGF-beta. We now show herein that Src phosphorylated T beta I-II on Y284 both in vitro and in vivo. Interestingly, although the expression of Y284F-T beta R-II mutants in breast cancer cells had no effect on TGF-beta stimulation of Smad2/3, this T beta R-II mutant completely abrogated p38 MAPK activation by TGF-beta. Accordingly, Src-mediated phosphorylation of Y284 coordinated the docking of the SH2 domains of growth factor receptor binding protein 2 (Grb2) and Src homology domain 2 containing (She) T beta R-II, thereby associating these adapter proteins to MAPK activation by TGF-beta. Importantly, Y284F-T beta R-II mutants also abrogated breast cancer cell invasion induced by alpha(v)beta(3) integrin and TGF-beta as well as partially restored their cytostatic response to TGF-beta. Our findings have identified a novel alpha(v)beta(3) integrin/Src/Y284/T beta R-II signaling axis that promotes oncogenic signaling by TGF-beta in malignant MECs and suggest that antagonizing this signaling axis may one day prove beneficial in treating patients with metastatic breast cancers.