Steroid receptors and regulation of LH and secretogranin II (GP87) secretion in pituitary cell aggregates.

The gonadotrope polypeptide (GP87), a secretogranin II form, has previously been found to be co-released with luteinizing hormone (LH) by rat gonadotrope cells under luteinizing hormone-releasing hormone (LHRH) stimulation. Nothing is hitherto known concerning its function and regulation. In the present paper, the modulatory effects of steroids on GP87 release were investigated using cultures of rat pituitary cell aggregates. As steroid effects are dependent on the presence of receptors, we firstly demonstrated that the steroid receptors are maintained with their typical characteristics in this culture system. In steroid-free medium, the estrogen receptor concentration increased significantly (P < 0.01) within 7 days in culture (646 +/- 122 fmol/mg DNA, n = 3) when compared to the level in freshly dispersed cells (355 +/- 59 fmol/mg DNA, n = 4) suggesting that cells can synthesize the estrogen receptor and that this cell culture system is suitable for studying regulatory effects of steroids. Pituitary cell aggregates from 14 day-old female rats were maintained for 24 h in the absence or presence of 10 nM estradiol (E(2)) or 50 nM 5?-dihydrotestosterone (DHT) and incubated for 2.5 h with [(35)S] methionine. Perifusion was initiated and 4 min pulses of 20 nM LHRH occurred every 1 h. The effects of steroids were investigated on LH, total and labeled GP87. The results indicated that: (1) DHT pretreatment reduced the amplitude of LHRH-stimulated GP87 and LH release responses; (2) by contrast, E(2) pretreatment enhanced the effects of LHRH on total or labeled GP87 release whereas the LH release was not significantly modified; and (3) when E(2) treatment started only 60 min before the first LHRH pulse, no differences between responses of E(2)-treated and control aggregates were observed. The present data show for the first time that sex steroids can modulate the GP87 release through a direct effect on the pituitary. E(2) could either stimulate the GP87 synthesis or increase its intracellular trafficking. That LH and GP87 do not strictly exhibit the same release response further suggests either some heterogeneity of GP87 localization in gonadotrope sub-types or within secretory granules.