Different outcomes of unliganded and liganded estrogen receptor-alpha on neurite outgrowth in PC12 cells

A precise description of the mechanisms by which estrogen receptor-alpha (ERalpha) exerts its influences on cellular growth and differentiation is still pending. Here, we report that the differentiation of PC12 cells is profoundly affected by ERalpha. Importantly, depending upon its binding to 17beta-estradiol (17betaE2), ERalpha is found to exert different effects on pathways involved in nerve growth factor (NGF) signaling. Indeed, upon its stable expression in PC12 cells, unliganded ERalpha is able to partially inhibit the neurite outgrowth induced by NGF. This process involves a repression of MAPK and phosphatidylinositol 3-kinase/Akt signaling pathways, which leads to a negative regulation of markers of neuronal differentiation such as VGF and NFLc. This repressive action of unliganded ERalpha is mediated by its D domain and does not involve its transactivation and DNA-binding domains, thereby suggesting that direct transcriptional activity of ERalpha is not required. In contrast with this repressive action occurring in the absence of 17betaE2, the expression of ERalpha in PC12 cells allows 17betaE2 to potentiate the NGF-induced neurite outgrowth. Importantly, 17betaE2 has no impact on NGF-induced activity of MAPK and Akt signaling pathways. The mechanisms engaged by liganded ERalpha are thus unlikely to rely on an antagonism of the inhibition mediated by the unliganded ERalpha. Furthermore, 17betaE2 enhances NGF-induced response of VGF and NFLc neuronal markers in PC12 clones expressing ERalpha. This stimulatory effect of 17betaE2 requires the transactivation functions of ERalpha and its D domain, suggesting that an estrogen-responsive element-independent transcriptional mechanism is potentially relevant for the neuritogenic properties of 17betaE2 in ERalpha-expressing PC12 cells.