GnRH agonist treatment decreases progesterone synthesis, luteal peripheral benzodiazepine receptor mRNA, ligand binding and steroidogenic acute regulatory protein expression during pregnancy.

R Sridaran,M Culty,H Li,V Papadopoulos,Gh Philip,Dm Stocco,Z Liu

doi:10.1677/jme.0.0220045

Abstract

We have demonstrated that continuous administration of a gonadotropin-releasing hormone agonist (GnRH-Ag) suppresses luteal steroidogenesis in the pregnant rat. We further demonstrated that the peripheral-type benzodiazepine receptor (PBR) and the steroidogenic acute regulatory protein (StAR) play key roles in cholesterol transport leading to steroidogenesis. The purpose of this study was to understand the cellular and molecular mechanisms involved in the suppression of luteal steroidogenesis leading to a fall in serum progesterone levels in GnRH-Ag-treated rats during early pregnancy. Pregnant rats were treated individually starting on day 8 of pregnancy with 5 microgram/day GnRH-Ag using an osmotic minipump. Sham-operated control rats received no treatment. At 0, 4, 8 and 24 h after initiation of the treatment, rats were killed and corpora lutea (CL) were removed for PBR mRNA, protein and radioligand binding analyses, immunoblot 1-D gel analysis of StAR, P450 scc and 3beta-hydroxysteroid dehydrogenase as well as 2-D gel analysis of StAR. The treatment decreased the luteal PBR mRNA expression at all time periods starting at 4 h compared with that in corresponding sham controls. GnRH-Ag also reduced, in the CL, the PBR protein/ligand binding, the StAR protein and P450 scc protein and its activity as early as 8 h after the treatment and they remained low compared with those in corresponding sham controls. The data from 2-D gel studies suggest that the majority of the decrease in StAR protein appears to be in the phosphorylated forms of StAR. Thus, we have demonstrated, for the first time, the presence of PBR and StAR in the pregnant rat CL and that the coordinated suppression of these proteins involved in the mitochondrial cholesterol transport along with P450 scc by GnRH-Ag leads to reduced ovarian steroidogenesis.

Highlights

We have previously demonstrated that continuous administration of a gonadotropin-releasing hormone agonist (GnRH-Ag) in a pregnant rat suppresses serum levels of pregnenolone and progesterone; the treatment had no effect on the content of free cholesterol in the corpus luteum (CL)(Sridaran et al 1995)
We have demonstrated, for the first time, the presence of peripheral-type benzodiazepine receptor (PBR) and steroidogenic acute regulatory protein (StAR) in the pregnant rat CL and that the coordinated suppression of these proteins involved in the mitochondrial cholesterol transport along with P450 scc by GnRH-Ag leads to reduced ovarian steroidogenesis
We present evidence that PBR and StAR proteins are present in the CL of the rat during early pregnancy and that inhibition of the expression of these two proteins involved in cholesterol transport, by the in vivo administration of GnRHAg, may lead to suppressed luteal steroidogenesis

Summary

Introduction

We have previously demonstrated that continuous administration of a gonadotropin-releasing hormone agonist (GnRH-Ag) in a pregnant rat suppresses serum levels of pregnenolone and progesterone; the treatment had no effect on the content of free cholesterol in the corpus luteum (CL)(Sridaran et al 1995). 46 and others · GnRH decreases progesterone, PBR and StAR during pregnancy receptor (PBR) (Papadopoulos et al 1997a) and the steroidogenic acute regulatory protein (StAR) (Stocco & Clark 1997). PBR is an 18 kDa protein that is present in all steroidogenic tissues (Papadopoulos et al 1997a) including the rat ovarian granulosa cells (Fares et al 1988, Amsterdam & Suh 1991), where it is primarily located on the outer mitochondrial membrane (Anholt et al 1986). PBR was shown to participate in the hormone-induced cholesterol transport in Leydig and adrenocortical cells (Papadopoulos et al 1991). Inhibition of PBR expression both in vivo (Amri et al 1996) and in vitro (Papadopoulos et al 1997b) resulted in decreased steroid production

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