Estrogen synthesis in gastric parietal cells and secretion into portal vein

Aromatase is the enzyme responsible for conversion of C₁₉ androgenic steroids to the corresponding estrogens: a reaction known as aromatization. In systemic circulation, the main source of 17β-estradiol, an estrogen, is the ovary. However, some reports describe that the gastric parietal cells synthesize large amounts of estrogens into the portal vein in both male and female rats. Although the author found many estrogen-producing cells in the stomach of younger and older groups of men and women, details of gastric estrogens have remained unclear. The author therefore investigated the fundamental kinetics of gastric 17β-estradiol using rats, obtaining important findings. In postnatal development, the gastric aromatase increases gradually from 20 days after birth. Gastric 17β-estradiol might contribute to liver growth. The regulation of gastric 17β-estradiol differs from that of other gastric endocrine systems and gastric acid secretion. Although ovarian estrogen fluctuates, gastric 17β-estradiol synthesis remains stable during the estrus cycle. The synthesis of gastric 17β-estradiol is independent of the regulatory system of the ovary. The circadian rhythm of the arterial 17β-estradiol level depends on the hepatic estrogen receptor α expression, and also on the concentration of gastric 17β-estradiol in the portal vein because portal venous 17β-estradiol level is synchronized with arterial concentration throughout the day. Liver dysfunction associated with experimental and pathological causes such as portal vein ligation, partial hepatectomy, and bile duct ligation evoke an influx of gastric estrogen into systemic circulation. These findings provide new insights into the gastro-hepatic axis and elucidate stomach and liver functions.