PERMISSIVE EFFECT OF CYCLIC-AMP AND CYCLOHEXIMIDE FOR INDUCTION BY SODIUM-BUTYRATE OF THE GLYCOPROTEIN HORMONE ALPHA-SUBUNIT GENE IN CHORIOCARCINOMA CELLS

Expression of the chorionic gonadotropin alpha-subunit (alphaCG) gene is regulated differently in human tumor cell lines derived from tropho blastic and nontrophoblastic tissues. This is based, at least in part, on the observations that production of alphaCG is increased by cAMP b ut not by sodium butyrate in choriocarcinoma cells (JEG-3), whereas pr oduction of alphaCG is increased by butyrate but not by cAMP in cervic al carcinoma cells (HeLa). Data presented herein confirm that the stea dy-state levels of alphaCG mRNA are increased approximately 10-fold by cAMP in JEG-3 cells, but additionally demonstrate that these levels c an be further increased (to 25-fold) by sodium butyrate in conjunction with the cyclic nucleotide. Similar effects were achieved with the ph orbol ester 12-O-tetradecanoylphorbol-13-acetate, which also acted syn ergistically with cAMP to increase alphaCG mRNA levels (24-fold). Buty rate and 12-O-tetradecanoylphorbol-13-acetate had little or no effect when added alone (1.5-fold and 2.5-fold, respectively) or in combinati on with one another (3.8-fold). Induction of the alphaCG gene by cAMP was independent of protein synthesis, as mRNA levels were comparable w hen JEG-3 cells were treated with cAMP in the absence and presence of cycloheximide (CHX). Unexpectedly, alphaCG mRNA levels were also eleva ted 8- to 10-fold in response to a combination of butyrate and CHX, bu t CHX alone or in combination with 12-O-tetradecanoylphorbol-13-acetat e had no effect. Gel mobility shift assays demonstrated changes in the pattern of proteins binding to a cAMP response element and to a troph oblast-specific element with nuclear extracts from cells treated with CHX but not with cAMP or sodium butyrate. Together, these results sugg est that induction patterns of the alphaCG gene by butyrate and cAMP i n trophoblast- and nontrophoblast-derived tumor cell lines are less di stinct than suggested previously and indicate that CHX, which is permi ssive for induction by butyrate, causes a significant shift in electro phoretic mobility of trans-acting factors involved in alphaCG gene exp ression.