Rat Thecal/Interstitial Cells Secrete a Transforming Growth Factor-β-Like Factor that Promotes Growth and Differentiation in Rat Granulosa Cells*

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Abstract
Rat granulose cells respond to transforming growth factor-.beta. (TGF-.beta.) with increase in DNA synthesis and FSH-stimulated aromatase activity. To determine if the cells surrouding 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 TSH-induced aromatase activity. Treatment of the conditioned medium with heat, acid, or alakli 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 by 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.