Equine granulosa-theca cell tumors express inhibin alpha- and beta A-subunit messenger ribonucleic acids and proteins

The association of equine granulosa-theca cell tumors with atrophied contralateral ovaries and abnormal estrous cycles suggests that these tumors produce hormones that affect pituitary gonadotropin production. Because inhibin, a heterodimer protein secreted by granulosa cells, decreases FSH production, we examined the presence of inhibin alpha- and beta A-subunits and their mRNAs in ovarian tumors obtained from three mares. These tumors contained neoplastic cords and nodules, multiple fluid-filled cysts, and a predominance of neoplastic granulosa cells. Reduced proteins from tumor-conditioned media were analyzed by electrophoresis and immunoblotting using antibodies directed against peptide fragments of the alpha- and beta A-chains of porcine inhibin. Specific bands at 50-kDa and 36-kDa for the inhibin alpha-subunit and at 44 kDa and 13 kDa for the inhibin beta A-subunit were observed in these tumors. Northern blot hybridization of 32P-labeled rat inhibin alpha- and beta A-subunit complementary RNAs to total RNA from each tumor revealed predominant bands of activity in all three tumors at 1.5 and 7 kb for the alpha- and beta A-subunit mRNAs, respectively. These results demonstrate that equine granulosa-theca cell tumors express the mRNAs for inhibin alpha- and beta A-subunits and also secrete inhibin subunits that could potentially affect gonadotropin production in afflicted mares. Furthermore, cells derived from these tumors may provide a useful model for understanding inhibin gene regulation and ovarian tumorigenesis.