Estrogen-producing steroidogenic pathways in parietal cells of the rat gastric mucosa

Abstract

Recently we demonstrated the presence of aromatase (P450_arom), estrogen synthetase, and the active production of estrogen in parietal cells of the rat stomach. We therefore investigated the steroidogenic pathways of estrogen and also other steroid metabolites in the gastric mucosa of male rats, by showing the mRNA expression of steroidogenic enzymes using RT-PCR and in situ hybridization histochemistry, and by measuring the blood concentration of steroids in the artery and the portal vein. RT-PCR analysis showed the strong mRNA expression of 17α-hydroxylase/17,20-lyase (P450_17α), 17β-hydroxysteroid dehydrogenase (HSD) type III and P450_arom, and the weak mRNA expression of 17β-HSD type II, 5α-reductase type I and 3α-HSD. The other mRNAs of steroidogenic enzymes examined were not detected. In situ hybridization histochemistry demonstrated the localization of mRNAs for P450_17α, 17β-HSD type III and P450_arom in the parietal cells. Higher levels of progesterone and testosterone were found in the artery compared with the portal vein. Higher amounts of estrone and 17β-estradiol, by contrast, were present in the portal vein compared with the artery. These results indicate that parietal cells of rat stomach convert circulating progesterone and/through androstenedione and testosterone to synthesize both estrone and 17β-estradiol, which then enter the liver via the portal vein.

Introduction

Steroid production is well known to occur in classical steroidogenic organs such as adrenal cortex, testis, ovary and placenta. These organs convert cholesterol to pregnenolone by P450 cholesterol side-chain cleavage enzyme (P450_scc), and then transform pregnenolone to other active steroids, for example, aldosterone and cortisol in adrenal cortex, testosterone in testis, and 17β-estradiol and progesterone in ovary and placenta. We have recently demonstrated a potent biosynthesis of estrogen in the parietal cells of rat gastric mucosa (Ueyama et al., 2002), while an earlier report showed that androgen was produced in these cells (Le Goascogne et al., 1995). Both mRNA and enzyme activity of aromatase (P450_arom), estrogen synthetase (Simpson et al., 1994), were found in large quantities, which were equivalent to those levels present in the ovary, in the gastric mucosa of rats (Ueyama et al., 2002), and a high concentration of 17β-estradiol, arising from the stomach, was present in the portal vein (Ueyama et al., 2002). It has not been quite understood which steroidogenic pathway is involved in the production of estrogen in the rat gastric mucosa, and whether other active steroids such as corticoids are also produced. In this study, we investigated both expression and localization of mRNAs encoding steroidogenic enzymes in the stomach by RT-PCR and in situ hybridization histochemistry. We measured also the blood concentrations of steroids in the artery and the portal vein to confirm the in vivo metabolism of steroid compounds in the gastric parietal cells of rats.