Enhancer and promoter element interactions dictate cyclic adenosine monophosphate mediated and cell-specific expression of the glycoprotein hormone α-gene

Abstract

The glycoprotein hormone α-gene is preferentially expressed in placental cell lines, but it is also expressed in several other cell lines indicating that the differential activity of the a-gene regulatory elements in various cell types is more quantitative than qualitative. The 5'-flanking region of the α-gene contains several distinct DNA regulatory sequences including an upstream regulatory element [(URE) -181 to -150 base pairs (bp)] that stimulates basal expression and an 18 bp twice-repeated cAMP-responsive element [(CRE) -146 to -111 bp]. We constructed an array of fusion genes containing the URE and/or the CRE linked to different truncated promoters [α-gene, somatostatin (SRIF), glucagon, Simian Virus 40]. These constructions were transiently expressed in placental, fibroblast, or islet cell lines to identify regulatory sequences involved in cell-specific expression as well as interactions between the URE, the CRE, and different promoter elements. The URE, CRE, and a-promoter elements contribute approximately 3-, 6-, and 5-fold, respectively, to preferential expression in JEG-3 cells. In JEG-3 cells, the URE is strictly dependent on the CRE for activity, but it functions in a promoter-independent manner. In contrast, the CRE is markedly promoter dependent. When linked to heterologous enhancers, the α-promoter is more active in JEG-3 cells than in other cell lines, thereby contributing substantially to preferential expression in placental cells. Although the CREs derived from the α and SRIF genes both activate expression of the α promoter, only the αCRE activates the SRIF promoter in JEG-3 cells. Thus, the α and SRIF CREs are not entirely interchangeable and enhancer-promoter interactions can dramatically influence patterns of cell-specific expression. These studies indicate that enhancer-promoter combinations determine preferential expression of the α-gene in JEG-3 cells. © 1989 by The Endocrine Society.