Different composite regulatory elements direct expression of the human alpha subunit gene to pituitary and placenta

To identify elements of the human alpha subunit gene necessary for cell-specific expression, we generated an array of block mutations spanning approximately 400 base pairs (bp) of promoter proximal region and examined them using transient transfection analysis in pituitary (alpha T3) and placental (BeWo) cell lines. Comparison of promoter activity in the two cell types revealed both common and unique elements required for transcription in pituitary and placenta. Two strong elements, the cyclic AMP response element (CRE) and the upstream regulatory element (URE), regulate expression of the alpha subunit gene in BeWo cells. In contrast, promoter activity in alpha T3 cells requires an array of weaker elements. These include the CREs, the URE, as well as two previously described elements, pituitary glycoprotein hormone basal element (PGBE) and gonadotrope-specific element (GSE), and two new elements we designated as the alpha basal elements 1 and 2 (alpha BE1 and alpha BE2). These new elements reside between -316 and -302 bp (alpha BE1) and -296 and -285 bp (alpha BE2) of the human alpha subunit promoter and bind distinct proteins designated alpha BP1 and alpha BP2, respectively. Southwestern blot analysis revealed that alpha BE1 specifically binds 54- and 56-kDa proteins. Additional studies disclosed several potential interactions between proteins that bind the CRE and proteins that occupy PGBE, alpha BE1, and alpha BE2, suggesting that gonadotrope-specific expression occurs through a unique composite regulatory element that includes components of the placenta-specific enhancer.