DIRECT ESTRADIOL DOWN-REGULATION OF SECRETOGRANIN-II AND CHROMOGRANIN-A MESSENGER-RNA LEVELS IN RAT PITUITARY-CELLS

Estradiol (E2) has been previously shown to negatively regulate, in vivo, the secretogranin (SgII) and chromogranin A (CgA) mRNA levels in the rat pituitary. Using cultured pituitary cell aggregates, experiments were undertaken to discriminate between direct or indirect effects of E2 on SgII and CgA levels. SgII, CgA and gonadotropin alpha- and beta-subunit mRNA levels were determined by Northern blotting. SgII and CgA protein levels were quantitated by Western blotting, and by immunoprecipitation of radioactive SgII after [S-35]methionine labeling. Increasing concentrations of E2 (from 10(-12) M to 10(-8) M) in the culture medium promoted a decrease of SgII and CgA mRNA levels to 30% and 50% of the control, respectively, after 72 h treatment. By contrast, none of the gonadotropin subunit mRNAs exhibited changes in concentration. A 24 h treatment with 10(-8) M E2 was sufficient to promote such a decrease in SgII and CgA mRNAs. Quantitation of the proteins after Western blotting revealed that 10(-8) M E2 lowered by 30% in CgA content of aggregates (P < 0.05 vs. control) while SgII content remained unaffected. Moreover, quantitation of the newly synthesized SgII by immunoprecipitation of the S-35-labeled SgII gave evidence for a lack of E2 effect. These data demonstrate: (1) a direct effect of E2 on the pituitary cells to down-regulate SgII and CgA mRNA steady-state levels; (2) though contained within the same secretory granules, a distinct pathway for negative E2 regulation of the gonadotropins and both granins; and (3) a differential effect of E2 on cell SgII and CgA contents as was previously demonstrated in vivo.