Abstract 4421: GPER-mediated activation of the cAMP response element-binding protein (CREB) and subsequent IGFBP-1 expression is altered during the development of tamoxifen resistance

Abstract

Abstract Tamoxifen is a commonly prescribed anti-estrogen treatment for breast cancer patients; this selective estrogen receptor modulator functions as an ERα-antagonist in breast tissue and inhibits cell proliferation and survival of breast cancer cells. More recently, the active metabolite of tamoxifen, 4-OHT (Tam), has been shown to activate the G protein-coupled estrogen receptor (GPER). Previous studies from our laboratory identified a novel, GPER-dependent mechanism of Tam action in breast cancer cells that requires the extracellular accumulation of the insulin-like growth factor binding protein-1 (IGFBP-1). The molecular mechanisms of Tam-induced IGFBP-1 production mediated by GPER have not been determined. GPER activation has been shown to stimulate cAMP production and PKA activation in several cell types. We hypothesized the cAMP response element-binding protein (CREB) activation is required for Tam-induced IGFBP-1 production in breast caner cells. To determine if CREB activation may be involved in IGFBP-1 transcription, CREB phosphorylation (S133) status and nuclear accumulation were determined in MCF-7 cells after Tam treatment. The phosphorylation and nuclear accumulation of CREB were observed in Tam-treated MCF-7 cells suggesting that CREB activation may be required for Tam-induced IGFBP-1 transcription. Increased CREB phosphorylation and IGFBP-1 transcription were inhibited by pretreatment with the PKA inhibitor H-89 supporting a role for CREB in Tam-induced IGFBP-1 expression. H-89 pretreatment also significantly decreased the previously reported inhibition of IGF-1-stimulated signaling by Tam treatment in breast cancer cells. We have previously reported that Tam-induced IGFBP-1 expression is dysregulated in Tam-resistant MCF-7 cells (MCF-7TamR). To determine if aberrant CREB signaling results in the observed dysregulation, similar experiments were performed in MCF-7TamR cells. Our results show that CREB phosphorylation and nuclear accumulation are decreased in Tam-resistant cells compared to Tam-sensitive cells. Taken together, these results suggest that activation of GPER results in CREB-dependent IGFBP-1 production and subsequent inhibition of IGF-1-stimulated signaling in breast cancer cells treated with Tam. Furthermore, dysregulation of GPER-mediated CREB activation may be involved in the development of Tam resistance in breast cancer. Citation Format: Ali Vaziri-Gohar, Kevin D. Houston. GPER-mediated activation of the cAMP response element-binding protein (CREB) and subsequent IGFBP-1 expression is altered during the development of tamoxifen resistance. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 4421. doi:10.1158/1538-7445.AM2015-4421