INCREASED FOLLISTATIN (ACTIVIN-BINDING PROTEIN) GENE-EXPRESSION IN RAT ANTERIOR-PITUITARY TISSUE AFTER OVARIECTOMY MAY BE MEDIATED BY PITUITARY ACTIVIN

For lack of evidence to the contrary, it is now believed that the FSH- suppressing actions of follistatin are due to its ability to bind endo genous pituitary activin. Recent data have demonstrated a role for pit uitary activin-B in mediating FSH hypersecretion after ovariectomy (OV X) and during the secondary FSH surge on estrus. Therefore, given that follistatin is produced within anterior pituitary tissue, and conside ring the potentially important function of follistatin to modulate act ivin bioactivity, we sought to gain insights into the regulation of fo llistatin gene expression in the anterior pituitary gland of adult fem ale rats. At the termination of all in vivo investigations, rats were killed, trunk blood was collected for determination of serum LH and FS H levels by RIA, and pituitary tissue was collected, pooled (two or th ree glands per pool), and processed for determination of follistatin m essenger RNA (mRNA) levels by a solution-hybridization RNase protectio n assay. In the first experiment, pituitary follistatin mRNA levels we re significantly (P < 0.01) increased 3 weeks after OVX. Treatment of long-term ovariectomized rats with a Nal-Glu LHRH antagonist restored serum LH levels to precastration levels and suppressed serum FSH conce ntrations by 70%, but follistatin message levels were not altered. In contrast, treatment of castrated rats with recombinant human follistat in-288 selectively suppressed serum FSH levels (50%) and completely ab olished OVX-induced increases in follistatin mRNA levels. Subsequent e xperiments revealed that OVX-induced increases in follistatin gene exp ression could be observed in pituitary tissue grafted underneath the k idney capsule of hypophysectomized rats. Furthermore, follistatin mRNA levels were significantly (P < 0.05) higher in pituitary glands taken from estrous rats during the secondary FSH surge (0200 h) than in gla nds obtained from rats on proestrous morning when serum FSH levels wer e basal. Because increased steady state follistatin mRNA levels in the latter two instances were associated with selective FSH hypersecretio n, and such hypersecretion was previously shown to be dependent to a s ignificant degree on pituitary activin, we next tested the hypothesis that increased pituitary follistatin gene expression is mediated by ac tivin. Using cultures of dispersed pituitary cells, addition of recomb inant human activin-A for 72 h increased follistatin mRNA levels 3-fol d while enhancing only FSH secretion. Collectively, the present result s demonstrate a coupling of follistatin gene expression in the anterio r pituitary gland with changes in pituitary FSH secretion under condit ions where LH secretion is unaltered. Viewed in the context of previou s work, the data also suggest that changes in follistatin mRNA levels may be linked to activin signaling.