TRANSFORMING GROWTH FACTOR-ALPHA AND FACTOR-BETA DIFFERENTIALLY REGULATE GROWTH AND STEROIDOGENESIS OF BOVINE THECAL CELLS DURING ANTRAL FOLLICLE DEVELOPMENT

The actions and interactions of transforming growth factor-alpha (TGF-alpha) and TGF-beta on growth and differentiation of bovine thecal cells were investigated. Bovine thecal interna cells were isolated from small (< 5 mm), medium (5-10 mm), and large (> 10 mm) antral follicles and cultured in the presence or absence of TGF-alpha and/or TGF-beta. Both [H-3]thymidine incorporation and changes in cell number (i.e. DNA levels) were evaluated to determine effects on thecal cell growth. Short term treatment of cells with TGF-alpha (18-24 h) stimulated thymidine incorporation, and longer term treatments (4 days) increased cell number. TGF-beta suppressed thymidine incorporation below that observed in untreated cultures, but had no effect on cell number. When combined with TGF-alpha, TGF-beta suppressed the ability of TGF-alpha to stimulate thymidine incorporation and increase cell number. The response to these growth factors was similar for cells isolated from the different stages of antral follicle development. The effects of TGF-alpha and TGF-beta on thecal cell differentiation were evaluated by quantitating changes in androstenedione and progesterone accumulation in cultures treated with TGFs in the absence (basal) or presence of hCG, estradiol (E2), or a combination of hCG and E2. E2 and hCG were included in this study because previous research has demonstrated that these hormones alter thecal cell steroidogenesis. Treatment with TGF-alpha resulted in a suppression of basal and hormonally stimulated accumulation of androstenedione during days 0-3 of culture, whereas TGF-beta did not significantly alter androstenedione accumulation. TGF-alpha also suppressed progesterone accumulation during days 0-3 of culture in the absence or presence of hormones. In contrast, TGF-beta stimulated accumulation of progesterone in cultures that did not contain E2, which suppressed progesterone during this period. Therefore, during days 0-3 of culture, TGF-alpha appears to have suppressive effects on androstenedione and progesterone production, whereas TGF-beta can stimulate progesterone production in the absence of E2. During days 3-6 of culture, thecal cell differentiation changes, and the capacity to produce androstenedione dramatically declines, while the capacity to produce progesterone increases. During this period, either TGF-alpha or TGF-beta slightly increased basal progesterone accumulation and partially suppressed the ability of hCG to stimulate progesterone. The effects of TGFs on thecal cell steroidogenesis were similar with cells isolated from the different stages of antral follicle development. Results from these studies provide evidence that TGF-alpha and TGF-beta can modulate thecal cell growth and differentiation (i.e. steroidogenesis). The inverse actions of these growth factors on thecal cell proliferation provide an efficient mechanism by which the growth of follicles may be regulated. The effects of TGF-alpha and TGF-beta on steroidogenesis may be associated with these growth effects. The ability of TGF-alpha to stimulate cell growth may decrease cell differentiation, which correlates with the generally suppressive effects of TGF-alpha on steroidogenesis observed. In contrast, TGF-beta inhibits growth, which may promote cell differentiation, and this correlates with the stimulatory effects of TGF-beta on steroidogenesis observed. The ability of thecal cells to produce and respond to these growth factors provides evidence that TGF-alpha and TGF-beta may act as important autocrine factors to influence the growth and differentiation of thecal cells during follicle development.