EXPRESSION OF INHIBIN/ACTIVIN SUBUNITS AND FOLLISTATIN MESSENGER RIBONUCLEIC-ACIDS AND PROTEINS IN OVARIAN FOLLICLES AND THE CORPUS-LUTEUM DURING THE HUMAN MENSTRUAL-CYCLE

The physiological role of intraovarian activin (beta/beta) and inhibin (alpha/beta) dimers in humans is unclear. The identification of follistatin as a beta-subunit-specific high affinity binding protein has added complexities for the interpretation of in vitro studies concerning the functionalities of these ovarian peptides. We, therefore, have attempted to define in. vivo compartmental distributions of gene expression and protein localization for inhibin and activin subunits (alpha,beta(A), and beta(B)) concurrent with follistatin in ovarian follicles and corpus lutea obtained from a large number of human ovaries. In situ hybridization and immunohistochemistry were used for detecting the expression of genes encoding inhibin/activin subunits and follistatin and their gene products, the proteins. Granulosa cells of small antral follicles (1-8 mm) were found to express mRNA for alpha-, beta(A) and beta(B)-subunits as well as follistatin, and the strongest signals were localized in the cumulus granulosa cells. In the thecal cell layer, only alpha-subunit mRNA was detected. Proteins were localized in cellular compartments corresponding to their mRNA, but in addition, proteins for beta(A)-subunit and follistatin were detected in the thecal cell layers in the absence of gene expression, an observation compatible with a paracrine action. Thus, granulosa cells of the small antral follicle have the potential to form all dimers of inhibin and activin, and their autocrine and paracrine actions may be modulated by follistatin in both granulosa cell and thecal cell layers. With the development of a dominant follicle, remarkable switches in subunit gene expressions occurred; beta(B)-subunit mRNA was no longer detectable in any cell type, and beta(A)-subunit expression emerged in the thecal cells along with continued abundant expression of beta(A)-subunit and follistatin in the granulosa cells. Proteins were found only in granulosa cells corresponding to their mRNAs. In the corpus luteum, the inhibin/activin alpha- and beta(A)-subunits and follistatin mRNA and proteins were expressed exclusively in the luteinized granulosa cells. Luteinized thecal cells were devoid of detectable mRNA message or proteins. Thus, the inhibin-activin-follistatin system in the corpus luteum appears to function in an autocrine fashion. We conclude that 1) both thecal and granulosa cells are endowed with the ability to synthesize inhibin/activin subunits as well as follistatin with differential expression in small and dominant follicles and corpus luteum; 2) the intensity, compartmentalization, and developmental changes in gene expression and protein localization of the inhibin-activin-follistatin system during the ovarian cycle in the human suggest paracrine and autocrine modes of regulation of follicular maturation and atresia; and 3) the marked gene expression of this system in cumulus granulosa cells suggests a role in the growth and development of oocytes.