Endothelin-1 as a luteinization inhibitor: inhibition of rat granulosa cell progesterone accumulation via selective modulation of key steroidogenic steps affecting both progesterone formation and degradation

Abstract

Endothelin (ET)-1, is a 21 amino acid vasoactive peptide subject to regulation by cellular oxygen tension. However, an increasing body of information now suggests that ET-1 is a multifunctional peptidergic regulator the actions of which are not limited to the vascular system. Although ET-1 has been shown to inhibit the gonadotropin-supported accumulation of progesterone by cultured granulosa cells, the precise cellular mechanism(s) involved remain unknown. It was therefore the objective of this study to examine in greater detail the effects of ET-1 on progestin economy in cultured granulosa cells from immature rats. Treatment with ET-1 was inhibitory to the FSH-supported accumulation of progesterone in a dose-dependent manner, an action characterized by a median inhibitory dose of 2 x 10(-11) M and a maximal inhibitory effect of 90%. This inhibitory action of ET-1 was reversible following extensive washing and could not be accounted for by a decrease in the viable cell mass. Evaluation of the activities of progesterone-forming enzymes revealed ET-1 to be a potent (P < 0.01) inhibitor of cholesterol side-chain cleavage and 3 beta-hydroxysteroid dehydrogenase (HSD)/isomerase (76.1 +/- 1.2% and 47.3 +/- 8.6% inhibition, respectively). Cellular radiolabeling with [3H]pregnenolone confirmed an ET-1-induced inhibition of the FSH-supported accumulation of radiolabeled progesterone. However, this effect was concomitant with enhancement of the accumulation of more distal metabolites, i.e. 20 alpha-dihydroprogesterone, 5 alpha-pregnane-3 alpha, 20 alpha-diol, and 5 alpha-pregnane-3 alpha-ol-20-one. Analysis of the FSH-supported activities of the progesterone-degrading enzymes revealed ET-1 as a potent (P < 0.05) stimulator of 20 alpha-HSD and 5 alpha-reductase (3.6 +/- 1.0 and 1.7 +/- 0.3-fold, stimulation respectively). In contrast, no significant changes were observed in 3 alpha-HSD activity. Taken together, our findings demonstrate that the ET-1 induced inhibition of gonadotropin-supported progesterone accumulation constitutes a complex phenomenon wherein ET-1 inhibits the activities of steroidogenic enzymes concerned with progesterone formation while enhancing the activities of enzymes concerned with progesterone degradation. We speculate that ET-1, possibly of intraovarian origin, acts as a luteinization-inhibitor to suppress premature luteinization at a time when continued preovulatory expression of ET-1 (in the intact but not ruptured follicle) may be contingent upon relative intrafollicular hypoxia.