Cyclic AMP responsiveness of human gonadotropin-alpha gene transcription is directed by a repeated 18-base pair enhancer. Alpha-promoter receptivity to the enhancer confers cell-preferential expression

Abstract

The human chorionic gonadotropin-alpha (CG-alpha) gene is transcriptionally activated by cAMP. Sequencing the CG-alpha 5'-flanking region identified two copies of a palindrome, 5'-TGACGTCA-3', homologous to sequences in other cAMP-responsive genes. The two palindromes are contained within two identical 18-base pair (bp) sequences arranged as adjacent direct repeats. One or two synthetic copies of the 18-bp sequences were inserted into plasmids containing either the CG-alpha promoter or the SV40 promoter directing transcription of the chloramphenicol acetyltransferase gene. The 36-bp (double) element markedly enhanced chloramphenicol acetyltransferase activity in placental choriocarcinoma (JEG-3) cells when inserted in either orientation both 5' to the cap site or 3' of the coding sequence, thus defining it as an enhancer. Moreover, 8-br-cAMP stimulated the enhancer activity 30-40-fold. A single 18-bp element also stimulated chloramphenicol acetyltransferase activity, although 5-fold less than the double element, but still imparted a 35-fold transcriptional cAMP responsivity. The enhancer activates its homologous promoter much more efficiently than the SV40 promoter in JEG-3 cells. The alpha-promoter is not nearly as receptive to activation by the enhancer in baby hamster kidney fibroblasts, whereas the more modest enhancement of the SV40 promoter is less cell-specific. These studies suggest that the interaction of a 36-bp enhancer-like element with the homologous promoter represents part of the mechanism of cell-specific expression of the CG-alpha gene and that the enhancer is co-localized with a highly effective cAMP-response element.