INCREASE IN MESSENGER-RIBONUCLEIC-ACID ENCODING THE MYOMETRIAL GAP JUNCTION PROTEIN, CONNEXIN-43, REQUIRES PROTEIN-SYNTHESIS AND IS ASSOCIATED WITH INCREASED EXPRESSION OF THE ACTIVATOR PROTEIN-1, C-FOS

Expression of the myometrial gap junction protein connexin-43 (Cx-43) increases dramatically with the onset of labor in association with an increase in the plasma estrogen to progesterone ratio. Moreover, we re ported that the levels of messenger RNA (mRNA) and protein encoded by the Cx-43 gene are regulated positively by estrogen and negatively by progesterone in both pregnant and nonpregnant rats. However, although the Cx-43 gene has been reported to be responsive to estrogen, it does not contain a palindromic estrogen response element. It does, however , contain a consensus activator protein-1 (AP-1)-binding site that may be of significance, as estrogen has been reported to increase express ion of the AP-1 proteins, Fos and Jun, in the uterus. Therefore, we te sted the hypothesis that the increase in Cx-43 mRNA levels in the myom etrium during labor and after estrogen administration is mediated indi rectly through induction of trans-activating factors, of which Fos and Jun are putative candidates. Treatment of nonpregnant ovariectomized rats with cyclohexamide blocked the estrogen-induced increase in Cx-43 mRNA in the myometrium, suggesting that this action of estrogen requi res newly synthesized protein. Estrogen treatment alone induced an inc rease in mRNA encoding c-fos and c-jun in nonpregnant rats, which prec eded by 2-3 h an increase in Cx-43 mRNA. Labor was also associated wit h a coincident expression of c-fos and Cx-43 in the myometrium, althou gh there was no change in c-jun mRNA levels during this period. These associative data were strengthened by our observation that in paradigm s in which labor was induced prematurely by ovariectomy or blocked by treatment with progesterone, changes in c-fos expression were closely matched by changes in Cx-43 mRNA. These data support our hypothesis th at estrogen (and, by extension, labor-)-induced increases in Cx-43 mRN A are mediated indirectly through newly synthesized trans-activating f actors. Moreover, the temporal correlation between the expression of c -fos and Cx-43 and the presence of AP-1 cis-acting elements within the Cx-43 promoter suggest that c-fos may be one of these trans-activatin g proteins.