Molecular Bases for the Actions of Ovarian Sex Steroids in the Regulation of Proliferation and Apoptosis of Human Uterine Leiomyoma

Abstract

Uterine leiomyomas appear during the reproductive years and regress after menopause, indicating the ovarian steroid-dependent growth potential. In order to characterize the molecular mechanism of sex steroidal regulation of leiomyoma growth, we examined whether sex steroids could influence the proliferation of leiomyoma cells. As epidermal growth factor (EGF) has been shown to mediate estrogen action and play a crucial role in regulating leiomyoma growth, we also investigated the effects of sex steroids on EGF and EGF receptor (EGF-R) expression in leiomyoma cells. In cultures of leiomyoma cells, the addition of either estradiol (E₂; 10 ng/ml) or progesterone (P₄; 100 ng/ml) resulted in an increase in proliferating cell nuclear antigen (PCNA) expression in the cells, whereas in cultures of normal myometrial cells, the addition of E₂ augmented PCNA expression in the cells, but P₄ did not. Immunoblot analysis revealed that leiomyoma cells contained immunoreactive EGF and that P₄ treatment resulted in an increase in EGF expression in the cells, whereas E₂ treatment resulted in a lower EGF expression in the cells. By contrast, E₂ treatment augmented EGF-R expression in cultured leiomyoma cells, but P₄ did not. These results indicate that P₄ upregulates the expression of PCNA and EGF in leiomyoma cells, whereas E₂ upregulates the expression of PCNA and EGF-R in those cells. It is, therefore, conceivable that P₄ and E₂ act in combination to stimulate the proliferative potential of leiomyoma cells through the induction of EGF and EGF-R expression. We also found that Bcl-2 protein, an apoptosis-inhibiting gene product, was abundantly expressed in leiomyoma relative to that in normal myometrium and that Bcl-2 protein expression in leiomyoma cells was upregulated by P₄, but downregulated by E₂. It seems, therefore, likely that P₄ may also participate in leiomyoma growth through the induction of Bcl-2 protein in leiomyoma cells. The abundant expression of Bcl-2 protein in leiomyoma cells may be one of the molecular bases for the enhanced growth of a leiomyoma relative to that of normal myometrium in the uterus.