The Fn14 Receptor is Up-regulated by HER2 Overexpression and Heregulin1-β1 (HRG) Signaling in Breast Cancer Cells and Contributes to HRG-MMP9-driven Cell Invasion.

Abstract

Human epidermal growth factor receptor (HER)-2 overexpression occurs in ~25% of all breast cancers and is associated with increased metastatic potential and poor patient survival. Abnormal HER2 activation, either through HER2 overexpression or heregulin (HRG):HER3 binding, elicits the formation of potent HER2-HER3 heterodimers and drives breast cancer cell growth and metastasis. Fibroblast growth factor-inducible 14 (Fn14), a member of the TNF receptor superfamily, is overexpressed in human breast tumors, and high expression levels strongly correlate with both the invasive HER2+/ER- intrinsic subtype and indicators of poor prognosis. We report here that HER2 and Fn14 are also co-expressed in a transgenic mouse model of HER2-driven breast tumorigenesis. In consideration of these findings, we investigated whether HER2 activation in breast cancer cells could directly induce Fn14 gene expression. We found that transient or stable transfection of MCF7 cells with HER2 increased Fn14 protein levels and this effect was primarily due to an increase in Fn14 stability. Also, HRG1-beta1 treatment of MCF7 cells transiently induced Fn14 mRNA and protein expression. Both the HER2-and HRG1-beta1-induced increase in Fn14 expression in MCF7 cells as well as increased Fn14 expression in HER2 gene-amplified AU565 cells could be blocked by HER2 kinase inhibition with lapatinib or HER2/HER3 depletion using siRNA. We also report that Fn14-depleted, HER2-overexpressing MCF7 cells have reduced basal cell migration capacity and reduced HRG1-beta1-stimulated cell migration, invasion and matrix metalloproteinase (MMP)-9 expression. Furthermore, expression of Fn14 in Fn14-depleted HER2-overexpressing cells increased HRG1-beta1-stimulated invasion and MMP-9 expression. Preliminary results also suggest that Fn14 depletion in MCF7/HER2-18 and SKBR3 cells might be decreasing anchorage-dependent and -independent growth in vitro and tumor xenograft growth in vivo. Together, these results indicate that the Fn14 receptor may be an important downstream regulator of both HER2 overexpression- and HRG1-beta1-driven HER2/HER3 signaling in breast cancer cells.