Rat thecal/interstitial cells secrete a transforming growth factor-beta-like factor that promotes growth and differentiation in rat granulosa cells.

Rat granulosa cells respond to transforming growth factor-beta (TGF beta) with increases in DNA synthesis and FSH-stimulated aromatase activity. To determine if the cells surrounding the granulosa cells generate TGF beta-like activity, which influences the growth and differentiation of granulosa cells, we cultured rat ovarian thecal/interstitial cells under serum-free conditions and collected their secretion products. Conditioned medium generated by primary cultures of thecal/interstitial cells over a 6-day collection period stimulated DNA synthesis, suggesting the presence of TGF beta-like bioactivity. At the onset of the collection period extracts of cultured cells contained undetectable levels of TGF beta-like activity, suggesting that the factor was produced and released by thecal/interstitial cells. FSH augmented the actions of both TGF beta and conditioned medium on DNA synthesis. Using an independent assay that relies on the response of the aromatase complex we showed that TGF beta and conditioned medium had parallel effects; TGF beta and conditioned medium alone did not influence the basal levels of aromatase activity, but both treatments augmented FSH-induced aromatase activity. Treatment of the conditioned medium with heat, acid, or alkali resulted in an increase in TGF beta-like bioactivity, suggesting that the TGF beta-like factor could be activated under these conditions. A commercially available antibody known to neutralize the bioactivity of TGF beta blocked the actions of TGF beta on DNA synthesis and blocked the growth-promoting activity of conditioned medium in a dose-dependent manner. Fractionation of the proteins in conditioned medium by elution through a Sephadex G-75 (superfine) column under acidic conditions showed that the growth-promoting activity and the aromatase activator eluted in fractions containing proteins with mol wt 25K. Standard TGF beta eluted in the same fractions. In summary, rat thecal/interstitial cells in culture secrete a TGF beta-like factor. The TGF beta-like factor may act together with FSH to promote growth and activate the aromatase complex of granulosa cells developing in the intrafollicular lumen in close association with the oocyte.