HER2/EGFR-AKT Signaling Switches TGFß from Inhibiting Cell Proliferation to Promoting Cell Migration in Breast Cancer.

ABSTRACT

TGFß signaling inhibits cell proliferation to block cancer initiation, yet it also enhances metastasis to promote malignancy during breast cancer development. The mechanisms underlying these differential effects are still unclear. Here, we report that HER2/EGFR signaling switches TGFß function in breast cancer cells from antiproliferation to cancer promotion. Inhibition of HER2/EGFR activity attenuated TGFß-induced epithelial-mesenchymal transition and migration but enhanced the antiproliferative activity of TGFß. Activation of HER2/EGFR induced phosphorylation of Smad3 at Ser208 of the linker region through AKT, which promoted the nuclear accumulation of Smad3 and subsequent expression of the genes related to EMT and cell migration. In contrast, HER2/EGFR signaling had no effects on the nuclear localization of Smad2. Knockdown of Smad3, but not Smad2, blocked TGFß-induced breast cancer cell migration. We observed a positive correlation between the nuclear localization of Smad3 and HER2 activation in advanced human breast cancers. Our results demonstrate a key role for HER2/EGFR in differential regulation of Smad3 activity to shift TGFß function from antitumorigenic to protumorigenic during breast cancer development.Significance: TGFß signaling can shift from inhibiting to promoting breast cancer development via HER2/EGFR AKT-mediated phosphorylation of Smad3 at S208, enhancing its nuclear accumulation and upregulation of EMT-related genes.Graphical Abstract http//cancerres.aacrjournals.org/content/canres/78/21/6073/F1.large.jpg Cancer Res; 78(21); 6073-85. ©2018 AACR.