Estrogen regulation of hepatic 3-hydroxy-3-methylglutaryl coenzyme A reductase and acetyl-CoA carboxylase in xenopus laevis

Mitogen-Activated Protein Kinase 8 (MAP3K8) Mediates the Signaling Pathway of Estradiol Stimulating Progesterone Production Through G Protein-Coupled Receptor 30 (GPR30) in Mouse Corpus Luteum

The corpus luteum (CL) is a transient endocrine gland developed from the ovulated follicles, and the most important function is to synthesize and secrete progesterone (P(4)), a key hormone to maintain normal pregnancy and estrous cycle in most mammals. It is known that estrogen has a vital role in stimulating P(4) synthesis in CL, but it still remains unclear about the mechanism of estradiol (E(2)) regulating P(4) production in CL. Our results here first show that all of the CL cells express MAPK 8 (MAP3K8), and the MAP3K8 level is much higher at the midstage than at the early and late stages during CL development. The further functional studies show that the forced inhibition of endogenous MAP3K8 by using MAP3K8 small interfering RNA and MAP3K8 signaling inhibitor (MAP3K8i) in the luteal cells significantly block the P(4) synthesis and neutralize the enhancing effect of E(2) on P(4) production in the CL. In addition, our results here demonstrate that the stimulating effect of E(2) on P(4) synthesis relies on the estrogen no-classical protein-coupled receptor 30, and MAP3K8 is involved in mediating the protein-coupled receptor 30signaling of E(2) affecting P(4) synthesis via stimulating ERK phosphorylation. These novel findings are critical for our understanding the ovary physiology and pathological mechanism.

Estrogen replacement reverses the hepatic steatosis phenotype in the male aromatase knockout mouse

The aromatase knockout (ArKO) mouse cannot synthesize endogenous estrogens due to a disruption to the Cyp19 gene. Previously we have shown both male and female ArKO mice have an age progressive obese phenotype and a sexually dimorphic disruption to hepatic cholesterol and triglyceride homeostasis. Only ArKO males have elevated hepatic triglyceride levels leading to hepatic steatosis partly due to an increase in expression of enzymes involved in de novo lipogenesis and transporters involved in fatty acid uptake. In this study ArKO males were treated with 17beta-estradiol (3 microg/ kg x d) at 18 wk old for 6 wk. Wild-type controls were not treated, and ArKO controls received vehicle oil injections. Estrogen replacement reverses the previously reported obese and fatty liver phenotypes; this was achieved by reductions in gonadal, visceral, and brown adipose tissue weights and significantly decreased hepatic triglyceride levels. Estrogen deficiency led to a significant up-regulation of hepatic fatty acid synthase expression, which was reduced with 17beta-estradiol replacement, although not quite reaching significance. Acetyl Coenzyme A carboxylase alpha mRNA expression showed no significant changes. Expression of transcripts encoding adipocyte differentiated regulatory protein, a fatty acid transporter, was significantly elevated in estrogen-deficient males, and 17beta-estradiol replacement significantly reduced these levels. Scavenger receptor class b type 1 showed no significantly changes. This study reveals that the previously reported disruption to triglyceride homeostasis in estrogen-deficient males can be reversed with 17beta-estradiol treatment, indicating an important role for estrogen in maintaining triglyceride and fatty acid homeostasis in males.

Feedback and hormonal regulation of hepatic 3-hydroxy-3-methylglutaryl coenzyme A reductase: the concept of cholesterol buffering capacity

Regulation of the expression of hepatic 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase by the major end product of the biosynthetic pathway, cholesterol, and by various hormones is critical to maintaining constant serum and tissue cholesterol levels in the face of an ever-changing external environment. The ability to downregulate this enzyme provides a means to buffer the body against the serum cholesterol-raising action of dietary cholesterol. The higher the basal expression of hepatic HMG-CoA reductase, the greater the "cholesterol buffering capacity" and the greater the resistance to dietary cholesterol. This review focuses on the mechanisms of feedback and hormonal regulation of HMG-CoA reductase in intact animals rather than in cultured cells and presents the evidence that leads to the proposal that regulation of hepatic HMG-CoA reductase acts as a cholesterol buffer. Recent studies with animals have shown that feedback regulation of hepatic HMG-CoA reductase occurs at the level of translation in addition to transcription. The translational efficiency of HMG-CoA reductase mRNA is diminished through the action of dietary cholesterol. Oxylanosterols appear to be involved in this translational regulation. Feedback regulation by dietary cholesterol does not appear to involve changes in the state of phosphorylation of hepatic HMG-CoA reductase or in the rate of degradation of this enzyme. Several hormones act to alter the expression of hepatic HMG-CoA reductase in animals. These include insulin, glucagon, glucocorticoids, thyroid hormone and estrogen. Insulin stimulates HMG-CoA reductase activity likely by increasing the rate of transcription, whereas glucagon acts by opposing this effect. Hepatic HMG-CoA reductase activity undergoes a significant diurnal variation due to changes in the level of immunoreactive protein primarily mediated by changes in insulin and glucagon levels. Thyroid hormone increases hepatic HMG-CoA reductase levels by acting to increase both transcription and stability of the mRNA. Glucocorticoids act to decrease hepatic HMG-CoA reductase expression by destabilizing reductase mRNA. Estrogen acts to increase hepatic HMG-CoA reductase activity primarily by stabilizing the mRNA. Deficiencies in those hormones that act to increase hepatic HMG-CoA reductase gene expression lead to elevations in serum cholesterol levels. High basal expression of hepatic HMG-CoA reductase, whether due to genetic or hormonal factors, appears to result in greater cholesterol buffering capacity and thus increased resistance to dietary cholesterol.

Seasonal commitment of HMGCoA reductase activity to vitellogenin production

In female frogs (Rana Esculenta) during gametogenesis the cholesterol synthesized in the liver by 3-hydroxy-3-methylglutaryl coenzyme A reductase is mostly exported into the blood and taken up by the oocytes. In order to understand the fate of the neosynthesized cholesterol, female and male frogs and estrogenized male controls were injected with the labelled precursor 14C mevalonate. In females and in estrogenized controls, mevalonate-derived radioactivity is found in a plasmatic lipoprotein that has been identified as vitellogenin by immunological detection. The increased 3-hydroxy-3-methylglutaryl coenzyme A reductase activity present in females in Fall is likely to be committed to provide cholesterol for the lipidation of this cholesterol-rich protein.

Circannual variations in mevalonate utilization in frog (Rana esculenta)

1. The fate of mevalonate, the product of HMGCoA reductase, was studied in male and female frogs (Rana esculenta) in order to explain the circannual variations of enzyme activity. 2. The incorporation of 2-14C MVA into unsaponifiable lipids, cholesterol and dolichol in liver, plasma and eggs was followed. 3. Labeled MVA shows a different utilization depending on season and sex. In spring and summer cholesterol synthesis is related to hepatic reserve storage in both sexes, while the peak of enzyme activity, present only in females in fall, seems committed to cholesterol export into the blood and uptake by the oocytes. 4. The presence of a MVA-derived protein identifiable with vitellogenin and labeled on the lipid moiety, suggests that HMGCoA reductase activity in fall is committed to the lipidation of this protein essential for oocyte maturation.

Cholesterol metabolism in frog (Rana esculenta) liver: seasonal and sex-related variations

Many aspects of lipid metabolism have been studied in amphibians, but seasonal lipid modulation in male and female frogs has not been investigated. We describe here the yearlong patterns of hepatic lipid content and enzyme activities related to cholesterol homeostasis, 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase activity and acyl coenzyme A:cholesterol acyltransferase (ACAT) activity in liver of the male and female frog, Rana esculenta. Lipid storage follows distinct seasonal patterns, with an increase in June that is more pronounced in the female than in the male frog. Cholesterol content and cholesterol storage as cholesteryl ester in male liver are consistent with the activity of HMG-CoA reductase and of ACAT enzymes. HMG-CoA reductase activity of the female frog shows an extra peak in fall unrelated to cholesterol storage and probably related to the production of essential compound for oogenesis.