Estrogen-Related Receptor α1 Transcriptional Activities Are Regulated in Part via the ErbB2/HER2 Signaling Pathway

We previously showed that ( a ) estrogen-related receptor α1 (ERRα1) down-modulates estrogen receptor (ER)–stimulated transcription in low ErbB2–expressing MCF-7 mammary carcinoma cells, and ( b ) ERRα and ErbB2 mRNA levels positively correlate in clinical breast tumors. We show here that ERRα1 represses ERα-mediated activation in MCF-7 cells because it failed to recruit the coactivator glucocorticoid receptor interacting protein 1 (GRIP1) when bound to an estrogen response element. In contrast, ERRα1 activated estrogen response element– and ERR response element–mediated transcription in ERα-positive, high ErbB2–expressing BT-474 mammary carcinoma cells, activation that was enhanced by overexpression of GRIP1. Likewise, regulation of the endogenous genes pS2, progesterone receptor , and ErbB2 by ERRα1 reflected the cell type–specific differences observed with our reporter plasmids. Importantly, overexpression of activated ErbB2 in MCF-7 cells led to transcriptional activation, rather than repression, by ERRα1. Two-dimensional PAGE of radiophosphate-labeled ERRα1 indicated that it was hyperphosphorylated in BT-474 relative to MCF-7 cells; incubation of these cells with anti-ErbB2 antibody led to reduction in the extent of ERRα1 phosphorylation. Additionally, mitogen-activated protein kinases (MAPK) and Akts, components of the ErbB2 pathway, phosphorylated ERRα1 in vitro . ERRα1-activated transcription in BT-474 cells was inhibited by disruption of ErbB2/epidermal growth factor receptor signaling with trastuzumab or gefitinib or inactivation of downstream components of this signaling, MAPK kinase/MAPK, and phosphatidylinositol-3-OH kinase/Akt, with U0126 or LY294002, respectively. Thus, ERRα1 activities are regulated, in part, via ErbB2 signaling, with ERRα1 likely positively feedback-regulating ErbB2 expression. Taken together, we conclude that ERRα1 phosphorylation status shows potential as a biomarker of clinical course and antihormonal- and ErbB2-based treatment options, with ERRα1 serving as a novel target for drug development. (Mol Cancer Res 2007;5(1):71–86)