Biosynthesis of the oocyte maturation inducing steroid by granulosa cells of the ovarian follicle in rainbow trout, Oncorhynchus mykiss
Biosynthèse du stéroïde inducteur de la maturation ovocytaire par les cellules du granulosa du follicule ovarien de truite arc-en-ciel, Oncorhynchus mykiss
Résumé
The purpose of this study was to reinvestigate the two-cells type model proposed for the meiotic maturation inducing steroid (MIS= 17αhydroxy.20βdihydroprogesterone : 17α20β0HP) by ovarian follicles of the rainbow trout, Oncorhynchus mykiss. Isolated granulosa cells were cultured in vitro to study steroidogenesis pathways by means of two methods. The first one was based on analysis of tritiated precursors (pregnelone: P5. 17αhydroxyprogesterone: 17α0HP, and androstenedione : λ4) metabolism. The second one used radioimmunoassays to estimate 17α20β0HP production from unlabeled precursors. 3βHSD activity has been identified in granulosa cells after characterization of 17α0HP by means of measure cocristallization until constant isotopic ratio (3H/14C). Besides 22 R-hydroxycholesterol and several λ5 steroids are used for 17α20β0HP synthesis by the granulosa cells. Particularly, 20β0HP5 and 17α20β0HP5 are metabolised actively, and the existence of a λ5 pathway for 17α20β0HP synthesis is suggested. Stimulation of 20βHSD activity, a key enzyme in the MIS synthesis, by salmon maturational gonadotropin (sGtH) has been confirmed. On the other hand, no salmo prolactine action could be detected: while salmon growth hormone showed uncertain synergistic effects with sGtH. Finally, testosterone and even more estradiol inhibite 20βHSD activity at physiological concentrations.These different resuls allow to perfect the theca/granulosa cellular cooperation model, that has been reported by other authors. We have demonstrated that granulosa cells have steroidogenic potentiality for MIS production from other precusors than 17α0HP. The possibility of other sources than thecal cell for these precursors needs to be investigated.
Ce travail avait pour objectif l'identification des potentialites steroidogenes des cellules de granulosa de follicule ovarien de truite arc-en-ciel, oncorhynchus mykiss, et l'etude de leur regulation en rapport avec la synthese du steroide inducteur de la maturation meiotique (17 alpha-hydroxy, 20 beta-dihydroxyprogesterone). La presence d'une activite 3 beta-hydroxysteroide dehydrogenase a ete demontree par identification, jusqu'a cocristallisation a ratio isotopique constante, de la 17 alpha-hydroxyprogesterone parmi les metabolites de la pregnenolone. Par ailleurs le 22(r) hydroxycholesterol et divers delta-5-steroides sont utilises par la granulosa pour synthetiser la 17 alpha, 20 beta hydroxyprogesterone. En particulier la 20 beta-hydroxypregnenolone et la 17 alpha, 20 beta, hydroxypregnenolone sont activement metabolises, et il est suggere l'existence d'une voie delta-5 pour la synthese de la 17 alpha, 20 beta, hydroxyprogesterone. La stimulation par la gonadotropine maturante de saumon (sgth) de l'activite 20 beta-hydroxysteroide dehydrogenase, enzyme clef dans la synthese du mis, est confirmee. Par contre aucune action de la prolactine de saumon n'a pu etre detectee; tandis que l'hormone de croissance de saumon presentait des effets synergiques, positifs avec la sgth, aleatoires. Enfin la testosterone et plus encore l'oestradiol, hormone de la vitellogenese inhibe l'activite 20 beta-hydroxysteroide-dehydrogenase a des concentrations physiologiques. Ces differents resultats permettent de completer le modele de cooperation cellulaire theque/granulosa, qui avait ete propose par d'autres auteurs, en prenant davantage en compte les potentialites propres de la granulosa.