Expression of Bcl-2 and Bax in the human corpus luteum during the menstrual cycle and in early pregnancy: Regulation by human chorionic gonadotropin

To investigate the relationship between apoptosis and the Bcl-2/Bax system in the human corpus luteum (CL), the frequency of apoptosis and expression of Bcl-2 and Bax were examined in the CL during the menstrual cycle and in early pregnancy. In situ analysis of DNA fragmentation showed that the numb er of apoptotic cells was much greater in the regressing CL than that in th e midluteal phase CL, whereas there were almost no apoptotic cells in the C L of early pregnancy. Immunohistochemistry revealed that Bcl-2 expression w as observed in the luteal cells in the midluteal phase and early pregnancy, but not in the regressing CL. In contrast, Bax immunostaining was observed in the regressing CL, but not in the midluteal phase and early pregnancy. bcl-2 messenger ribonucleic acid (mRNA) levels in the CL during the menstru al cycle were highest in the midluteal phase and lowest in the regressing C L. In the CL of early pregnancy, bcl-2 mRNA levels were significantly highe r than those in the midluteal phase. In contrast, bax mRNA levels were high est in the regressing CL and remarkably low in the CL of early pregnancy. W estern blot analyses revealed that Bcl-2 expression was significantly lower in the regressing CL than in the midluteal phase and early pregnancy, and that Bax expression was, in contrast, significantly higher in the regressin g CL than in the midluteal phase and was remarkably low in the CL of early pregnancy. When corpora lutea of the midluteal phase were incubated with hC G, hCG significantly increased the mRNA and protein levels of Bcl-2 and sig nificantly decreased those of Bax. In conclusion, Bcl-2 and Bax may play im portant roles in the regulation of the life span of the human CL by control ling the rate of apoptosis. hCG may act to prolong the life span of the CL by increasing Bcl-2 expression and decreasing Bax expression when pregnancy occurs.