Estrogen-related receptor α1 transcriptional activities are regulated in part via the ErbB2/HER2 signalling pathway

We previously showed that (a) estrogen-related receptor alpha 1 (ERR alpha 1) down-modulates estrogen receptor (ER)-stimulated transcription in low ErbB2-expressing MCF-7 mammary carcinoma cells, and (b) ERR alpha, and ErbB2 mRNA levels positively correlate in clinical breast tumors. We show here that ERR alpha 1 represses ER alpha-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, ERR alpha 1 activated estrogen response element- and ERR response element- mediated transcription in ER alpha-positive, high Erb132-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 ERR alpha 1 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 ERR alpha 1. Two-dimensional PAGE of radiophosphate-labeled ERR alpha 1 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 ERR alpha 1 phosphorylation. Additionally, mitogen-activated protein kinases (MAPK) and Akts, components of the ErbB2 pathway, phosphorylated ERR alpha 1 in vitro. ERR alpha 1-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, ERR alpha 1 activities are regulated, in part, via Erb132 signaling, with ERR alpha 1 likely positively feedback-regulating Erb132 expression. Taken together, we conclude that ERR alpha 1 phosphorylation status shows potential as a biomarker of clinical course and antihormonal- and Erb132-based treatment options, with ERR alpha 1 serving as a novel target for drug development.