Attenuation of human chorionic gonadotropin release by nitric oxide in choriocarcinoma cell lines

Nitric oxide (NO) is involved in the regulation of endocrine functions, but only a few studies have been reported about its role in placental hormone secretion. We investigated whether NO has any function in the release of human chorionic gonadotropin (hCG) in two different choriocarcinoma cell lines, JEG-3 and BeWo. First, nitric oxide synthase (NOS) was characterized in the choriocarcinoma cells. NOS activity was localized mainly in the particulate fraction and depended on calcium/calmodulin. Activity was inhibited by the presence of the L-arginine analog, NG-monomethyl-L-arginine (L-NMMA; 1 x 10(-4) M). Western blot analysis showed that the choriocarcinoma cells contained an endothelial isoform of NOS. The NO donor, sodium nitroprusside (SNP; 1 x 10(-5) and 1 x 10(-4) M), significantly inhibited hCG secretion in both choriocarcinoma cell lines. The suppression of hCG release by SNP (1 x 10(-5) M) was blocked by the addition of an NO scavenger, hemoglobin (1 x 10(-6) M). L-Arginine (1 x 10(-2) M), a NOS substrate, inhibited basal hCG secretion in JEG-3 cells. Incubation of the cells with L-NMMA (1 x 10(-4) and 1 x 10(-3) M) significantly increased hCG release. Exposure of both cell lines to increasing concentrations of a cyclic GMP analog (8-bromo-cyclic GMP; 1 x 10(-4) to 1 x 10(-2) M) caused a dose-dependent inhibition of hCG release. Cyclic GMP accumulation in response to SNP (1 x 10(-4) M), however, was not detected in either JEG-3 or BeWo cells. These data demonstrated that the endothelial isoform of NOS and a functional L-arginine-NO pathway are present in the choriocarcinoma cell lines. In addition, these findings support the hypothesis that NO produced in these cell lines is involved in the regulation of hCG secretion. We assume that although cyclic GMP is likely to play a role as a second messenger, a cyclic GMP-independent pathway cannot be excluded as a possible physiological mechanism in the attenuation of hCG release by NO.