REGULATION OF INHIBIN ACTIVIN SUNBUNIT MESSENGER RIBONUCLEIC-ACIDS (MESSENGER-RNAS) BY ACTIVIN-A AND EXPRESSION OF ACTIVIN RECEPTOR MESSENGER-RNAS IN CULTURED HUMAN GRANULOSA-LUTEAL CELLS

Recent studies have indicated that activin and inhibin may act as loca l regulators of cell growth and steroidogenesis in the human ovary. We studied the effect of recombinant human activin A and purified bovine inhibin A on the steady state messenger RNA (mRNA) levels of the inhi bin/activin alpha-, beta(A)-, and beta(B)-subunits in cultured granulo sa-luteal (GL) cells from preovulatory ovarian follicles of women unde rgoing in vitro fertilization. Activin A induced the expression of a 4 .8-kilobase beta(B)-subunit mRNA transcript without affecting basal ex pression levels of the alpha- and beta(A)-subunit mRNAs. It stimulated beta(B)-subunit mRNA levels in a concentration- and time-dependent ma nner. Maximal stimulation of beta(B)-subunit mRNA levels was obtained with 30-100 ng/ml activin A. The level of beta(B)-subunit mRNAs increa sed significantly 8 h after stimulation, rising gradually thereafter t o a maximum at 48 h. Inhibin A did not affect the mRNA levels of any i nhibin/activin subunits, nor did it inhibit the effect of activin A. R ecombinant human follistatin did not affect basal beta(B)-subunit mRNA levels, but it neutralized the effect of activin A. Although hCG indu ces inhibin/activin alpha- and beta(A)-subunit mRNA levels in human GL cells, it did not increase basal beta(B)-subunit levels. By contrast, it inhibited activin A-induced beta(B)-subunit mRNA levels. On the ot her hand, activin A decreased hCG-induced mRNA levels of the inhibin a lpha-subunit and cytochrome P450 side-chain cleavage (P450scc) enzyme, an important rate-limiting enzyme in human GL cell progestin synthesi s. Moreover, we observed by Northern blot analysis that cultured human GL cells as well as freshly isolated preovulatory granulosa cells exp ress the specific mRNAs for all currently known serine/threonine kinas e activin receptors, i.e. activin receptors I, IB, II, and IIB. Our re sults suggest that in GL cells, activin A may locally stimulate synthe sis of the beta(B)-subunit in an autocrine or paracrine manner, and th at in human ovary, regulation of the beta(B)-subunit differs from that of the alpha- and beta(A)-subunits.