Effects of oestrogen on adenylate cyclase system and glucose output in rat liver

Sex Hormone-Dependent Physiology and Diseases of Liver

Sexual dimorphism is associated not only with somatic and behavioral differences between men and women, but also with physiological differences reflected in organ metabolism. Genes regulated by sex hormones differ in expression in various tissues, which is especially important in the case of liver metabolism, with the liver being a target organ for sex hormones as its cells express estrogen receptors (ERs: ERα, also known as ESR1 or NR3A; ERβ; GPER (G protein-coupled ER, also known as GPR 30)) and the androgen receptor (AR) in both men and women. Differences in sex hormone levels and sex hormone-specific gene expression are mentioned as some of the main variations in causes of the incidence of hepatic diseases; for example, hepatocellular carcinoma (HCC) is more common in men, while women have an increased risk of autoimmune liver disease and show more acute liver failure symptoms in alcoholic liver disease. In non-alcoholic fatty liver disease (NAFLD), the distinction is less pronounced, but increased incidences are suggested among men and postmenopausal women, probably due to an increased tendency towards visceral fat accumulation.

Sex Hormones and Their Receptors Regulate Liver Energy Homeostasis

The liver is one of the most essential organs involved in the regulation of energy homeostasis. Hepatic steatosis, a major manifestation of metabolic syndrome, is associated with imbalance between lipid formation and breakdown, glucose production and catabolism, and cholesterol synthesis and secretion. Epidemiological studies show sex difference in the prevalence in fatty liver disease and suggest that sex hormones may play vital roles in regulating hepatic steatosis. In this review, we summarize current literature and discuss the role of estrogens and androgens and the mechanisms through which estrogen receptors and androgen receptors regulate lipid and glucose metabolism in the liver. In females, estradiol regulates liver metabolism via estrogen receptors by decreasing lipogenesis, gluconeogenesis, and fatty acid uptake, while enhancing lipolysis, cholesterol secretion, and glucose catabolism. In males, testosterone works via androgen receptors to increase insulin receptor expression and glycogen synthesis, decrease glucose uptake and lipogenesis, and promote cholesterol storage in the liver. These recent integrated concepts suggest that sex hormone receptors could be potential promising targets for the prevention of hepatic steatosis.

The involvement of the stimulatory G protein in sexual dimorphism of beta-adrenergic receptor-mediated functions in rat liver

In rat hepatocytes, beta-adrenergic receptor (beta-AR)-mediated cAMP generation was found to be higher in the female than in the male. As compared to the male, the number of beta-AR, detected by [125I]iodocyanopindolol, was elevated in the female. In agonist competition experiments, the proportion of beta-AR in the high-affinity state was promoted in the female than in the male. The alpha subunit of the stimulatory G protein (Gs alpha) was quantified using ADP-ribosylation catalyzed by cholera toxin. The amount of Gs alpha, both small, 42 kDa (Gs alpha S), and large, 47 kDa (Gs alpha L), forms increased in parallel with enhancement of catecholamine-sensitive adenylate cyclase activity in the female. The female showed a disproportionate increase in Gs alpha L, which is preferentially coupled to beta-AR, compared with Gs alpha S. In addition, 17 beta-estradiol facilitated isoproterenol-induced cAMP generation in both male and female rats, whereas castration or testosterone had no effect on this response. It is proposed that the cellular sites for sexual dimorphism in hepatic beta-adrenergic functions are the coupling state of beta-AR to Gs and the amount of Gs alpha as well as the level of beta-AR.

Effects of androgen treatment on adenylate cyclase system in rat hepatic membranes

Effects of androgen treatment of young female rats on glucagon- and catecholamine-sensitive adenylate cyclase activity and adrenergic receptors of hepatic membranes have been studied. Injections of testosterone propionate for 7 days showed a significant decrease in the adenylate cyclase activity responding to isoproterenol and glucagon. The decrease in hormonal stimulation of the enzyme was accompanied with the fall in activation by non-hormonal stimuli, such as forskolin, sodium fluoride, Gpp(NH)p and Mn, without any changes in the number and the affinity of beta-adrenergic receptors of the membrane. These results suggest that androgens exert post-receptor effects by inhibiting the activity of the catalytic unit of adenylate cyclase system in rat hepatic membranes.

Sex difference in adrenergic receptor-mediated glycogenolysis in rat livers

Catecholamine-induced stimulation of hepatic glycogenolysis in male and female rats was studied by detecting the cytosolic free Ca2+ concentration ([Ca2+]i), cAMP generation and adrenergic receptor function. Increase in alpha 1-adrenergic receptor-mediated [Ca2+]i and beta-adrenergic receptor-mediated cAMP generation were examined using isolated hepatocytes. No difference was found in the alpha 1-adrenergic receptor-mediated response of [Ca2+]i in fura-2-loaded hepatocytes between males and females, while epinephrine-induced cAMP accumulation in hepatocytes was about 3-fold higher in females. The alpha 1- and beta-adrenergic receptor properties of the plasma membrane were evaluated by ligand binding studies using [3H]prazosin (alpha 1-adrenergic antagonist) and [125I]iodocyanopindolol (beta-adrenergic antagonist); and little sex difference was found in either affinity or the number of binding sites of [3H]prazosin and [125I]iodocyanopindolol. Activation of adenylate cyclase by forskolin and GTP-gamma-S was also similar for both sexes. These results suggest that the sex difference of beta-adrenergic response is due to a difference in the guanine nucleotide regulatory binding proteins (G proteins) and/or beta-adrenergic receptor-Gs protein (the stimulatory G protein of adenylate cyclase) coupling ability.