INCREASED EXPRESSION OF BCL-2 PROTEIN IN HUMAN UTERINE LEIOMYOMA AND ITS UP-REGULATION BY PROGESTERONE

Uterine leiomyoma is the most common benign smooth muscle cell tumor o f the myometrium. Although Bcl-2 protein is known to be an apoptosis-i nhibiting gene product and to prevent apoptotic cell death in a variet y of cells, there are no published data regarding whether human leiomy omas express Bcl-2 protein. In the present study, we examined the expr ession of Bcl-2 protein in leiomyomas in comparison with that in the n ormal myometrium using an immunohistochemical method and immunoblot an alysis with a monoclonal antibody to human Bcl-2 protein. Furthermore, we investigated whether sex steroid hormones could influence the leve ls of Bcl-2 protein expression in leiomyoma cells cultured in vitro un der serum-free, phenol red-free conditions. Immunohistochemical staini ng for Bcl-2 protein was prominent in leiomyoma cells, but was scarcel y present in normal myometrial smooth muscle cells. The expression of Bcl-2 protein in leiomyoma cells was most abundant in the secretory, p rogesterone-dominated, phase of the menstrual cycle, but was less abun dant in the proliferative phase of the menstrual cycle. Western blot a nalyses of leiomyoma and myometrium tissue extracts revealed that Bcl- 2 protein, with a molecular mass estimated at approximately 26 kDa, wa s abundantly present in leiomyoma tissue extracts, but was undetectabl e in normal myometrial tissue extracts. In monolayer cultures of uteri ne leiomyoma cells under a serum-free condition, the addition of proge sterone (100 ng/mL) resulted in a striking increase in Bcl-2 protein e xpression in the cultured leiomyoma cells relative to that in control cultures, whereas the addition of 17 beta-estradiol (10 ng/mL) resulte d in a reduction in Bcl-2 protein expression in the cells. The concent rations of sex steroids used were within the physiological tissue conc entrations found in leiomyomas and myometrium. The present results sug gest that the abundant expression of Bcl-2 protein may have a molecula r basis characteristic of leiomyomas in the human uterus and that prog esterone may play a vital role in the enhanced expression of Bcl-2 pro tein in human uterine leiomyoma cells.