Rat inhibin: Molecular cloning of α- and β-subunit complementary deoxyribonucleic acids and expression in the ovary

Inhibin is a gonadal protein hormone that suppresses the secretion of FSH from pituitary gonad-otrophs. It has previously been characterized as a heterodimer of two dissimilar subunits (α, 18 kilo- daltons and β, 14 kilodaltons) the smaller of which exists in two forms (β_A and β_B) and can form dimers that stimulate the secretion of FSH. In the present work, cDNA clones encoding the inhibin α- and β_A- subunits have been isolated from rat ovary and characterized. The a-inhibin cDNA predicts a precursor protein of 366 amino acids containing the 133 amino acid mature α-subunit at its COOH-terminus. The β_A-inhibin cDNA predicts a precursor protein of 424 amino acids containing the 116 amino acid β_A- subunit at its COOH-terminus. Analysis of rat ovarian RNA indicates that α-inhibin mRNA levels are stimulated by PMSG treatment in vivo. In cultured granulosa cells, FSH also stimulates α-inhibin mRNA, and the FSH effect is suppressed by cotreatment with GnRH. Hybridization in situ to rat ovarian tissue demonstrates that both the α-inhibin and β_A-inhibin mRNAs are specifically expressed in granulosa cells of the developing follicles.