Unliganded estrogen receptor α stimulates bone sialoprotein gene expression

Estrogen is one of the steroid hormones essential for skeletal development. The estrogen receptor (ER) is a transcription factor and a member of the steroid receptor superfamily. There are two different forms of the ER, usually referred to as alpha and beta, each encoded by a separate gene. Hormone-activated ERs form dimers, since the two forms are coexpressed in many cell types. Bone sialoprotein (BSP) is a tissue-specific acidic glycoprotein that is expressed by differentiated osteoblasts, odontoblasts and cementoblasts during the initial formation of mineralized tissue. To determine the molecular basis of the tissue-specific expression of BSP and its regulation by estrogen and the ER, we have analyzed the effects of beta-estradiol and ER alpha. on BSP gene transcription. ERa protein levels were increased after ER alpha overexpression in ROS17/2.8 cells. While BSP mRNA levels were increased by ERa overexpression, the endogenous and overexpressed BSP mRNA levels were not changed by beta-estradiol (10(-8) M, 24 h). Luciferase activities of different sized BSP promoter constructs (pLUC3 -6) were increased by ERa overexpression, whereas basal and induced luciferase activities by ERa overexpression were not influenced by beta-estradiol. Effects of ERa overexpression Were abrogated by 2 bp mutations in either the cAMP response element (CRE) or activator protein 1 (AP1)/glucocorticoid response element (GRE). Gel shift analyses showed that ERa overexpression increased binding to the CRE and AP1/GRE elements. Notably, the CRE-protein complexes were disrupted by ERa, CREB and phospho-CREB antibodies. The AP1/GRE-protein complexes were supershifted by the c-Fos antibody. These studies demonstrate that ERa. stimulates BSP gene transcription in a ligand-independent manner by targeting the CRE and AP1/GRE elements in the rat BSP gene promoter. (c) 2014 Elsevier B.V. All rights reserved.