Characterization of the stimulatory actions of thymic factor(s) on basal and gonadotropin-induced steroidogenesis in cultured rat granulosa cells

Abstract

Recently, there have been numerous reports that demonstrate the importance of the thymus gland in reproductive physiology. Previously, we have reported that thymic factors (TFs) which are present in thymic cell culture-conditioned medium (TCM) could stimulate basal progesterone and estradiol production from cultured rat granulosa cells. The current study attempts to characterize the stimulatory actions of TFs on both basal and FSH induced steroidogenesis. Thymic epithelial cells from immature female rats were isolated and used for production of TCM. Granulosa cells were obtained from immature diethylstilbestrol (DES)-treated rats. TFs stimulated both basal and FSH-induced progesterone secretions 80 and 17 times, respectively, as compared to the control media. The effects of TFs on basal and FSH-induced 20α-hydroxyprogesterone secretion were comparable to those on progesterone production (40X and 10X, respectively). In addition, TCM stimulated basal and FSH-induced estradiol secretion approximately 4 and 2.5 times, respectively, compared to control. Stimulation of aromatase enzyme activity followed a similar trend as estradiol secretion, and TCM stimulated basal and FSH-stimulated aromatase enzyme activity approximately 15 and 3 times, respectively compared to control. Thus, these results indicate that the observed increases in progesterone and estradiol secretions in TCM-treated rat granulosa cells are likely to be due to elevated activities of specific steroidogenic enzymes. Measurements of total cell protein and DNA synthesis indicate that enhanced steroidogenesis in TCM-treated cells is not due to increased cell growth and/or proliferation. Rather, the enhanced steroidogenesis is probably due to an increased steroid biosynthetic capability of the cells. In addition, not only can TCM stimulate production of these steroid hormones in the absence of FSH, but also in the presence of FSH, suggesting physiological relevance of its stimulatory actions. Overall, these results show that thymic epithelial cells are a potential source of steroidogenic factors. Moreover, the thymic factor(s) may have a physiologic role in ovarian function.