Regulation of three key enzymes in cholesterol metabolism by phosphorylation/dephosphorylation

In(OTf)3-catalyzed N-α phosphonylation of N,O-acetals with triethyl phosphite

A practical approach to α-aminophosphonates has been developed through an In(OTf)₃-catalyzed N-α phosphonylation of N,O-acetals with triethyl phosphite 7. Indoline and isoindoline N,O-acetals 6a-6j and 9a-9j and chain N,O-acetals 11a-11p were subjected to a Lewis acid catalyzed N-α phosphonylation process. As a result, the desired α-aminophosphonates 8a-8j, 10a-10j and 12a-12p were obtained in moderate to good yields.

Lipidomic Alterations and PPARα Activation Induced by Resveratrol Lead to Reduction in Lesion Size in Endometriosis Models

Endometriosis is an estrogen-dependent chronic inflammatory disease that affects approximately 10% of women of reproductive age and up to 50% of women with infertility. The heterogeneity of the disease makes accurate diagnosis and treatment a clinical challenge. In this study, we generated two models of endometriosis: the first in rats and the second using human ectopic endometrial stromal cells (HEcESCs) derived from the lesion tissues of endometriosis patients. We then applied resveratrol to assess its therapeutic potential. Resveratrol intervention had significant efficacy to attenuate lesion size and to rectify aberrant lipid profiles of model rats. Lipidomic analysis revealed significant lipidomic alterations, including notable increases of sphingolipids and decreases of both glycerolipids and most phospholipids. Upon resveratrol application, both proliferation capacity and invasiveness parameters decreased, and the early apoptosis proportion increased for HEcESCs. The activation of PPARα was also noted as a factor potentially contributing to recovery from endometriosis in both models. Our study provides valuable insight into the mechanisms of resveratrol in endometriosis and therefore strengthens the potential for optimizing resveratrol treatment for this disease.

Lipidomic Alterations and PPARα Activation Induced by Resveratrol Lead to Reduction in Lesion Size in Endometriosis Models

Endometriosis is an estrogen‐dependent chronic inflammatory disease that affects approximately 10% of women of reproductive age and up to 50% of women with infertility. The heterogeneity of the disease makes accurate diagnosis and treatment a clinical challenge. In this study, we generated two models of endometriosis: the first in rats and the second using human ectopic endometrial stromal cells (HEcESCs) derived from the lesion tissues of endometriosis patients. We then applied resveratrol to assess its therapeutic potential. Resveratrol intervention had significant efficacy to attenuate lesion size and to rectify aberrant lipid profiles of model rats. Lipidomic analysis revealed significant lipidomic alterations, including notable increases of sphingolipids and decreases of both glycerolipids and most phospholipids. Upon resveratrol application, both proliferation capacity and invasiveness parameters decreased, and the early apoptosis proportion increased for HEcESCs. The activation of PPARα was also noted as a factor potentially contributing to recovery from endometriosis in both models. Our study provides valuable insight into the mechanisms of resveratrol in endometriosis and therefore strengthens the potential for optimizing resveratrol treatment for this disease.

Interplay between cholesterol and drug metabolism

Cholesterol biosynthetic and metabolic pathways contain several branching points towards physiologically active molecules, such as coenzyme Q, vitamin D, glucocorticoid and steroid hormones, oxysterols, or bile acids. Sophisticated regulatory mechanisms are involved in maintenance of the homeostasis of not only cholesterol but also other cholesterogenic molecules. In addition to endogenous cues, cholesterol homeostasis needs to accommodate also to exogenous cues that are imported into the body, such as chemicals and medications. Steroid and nuclear receptors together with sterol regulatory element-binding protein (SREBP) mediate the fine tuning of biosynthetic and metabolic routes as well as transports of cholesterol and its derivatives. Similarly, drug/xenobiotic metabolism is the subject to the feedback regulation of cytochrome P450 enzymes and transporters. The regulatory mechanisms that maintain the homeostasis of cholesterogenic molecules and are involved in drug metabolism share similarities. Cholesterol and cholesterogenic compounds (bile acids, glucocorticoids, vitamin D, etc.) regulate the xenosensor signaling in drug-mediated induction of the major drug-metabolizing cytochrome P450 enzymes. The key cellular receptors, pregnane X receptor (PXR), constitutive androstane receptor (CAR), vitamin D receptor (VDR), and glucocorticoid receptor (GR) provide a functional cross-talk between the pathways maintaining cholesterol homeostasis and controlling the expression of drug-metabolizing enzymes. These receptors serve as metabolic sensors, resulting in a coordinate regulation of cholesterogenic compounds metabolism and of the defense against xenobiotic and endobiotic toxicity. Herein we present a comprehensive review of functional interactions between cholesterol homeostasis and drug metabolism involving the main nuclear and steroid receptors.

ACAT2 and human hepatic cholesterol metabolism: identification of important gender-related differences in normolipidemic, non-obese Chinese patients

OBJECTIVE

ACAT2 is a major cholesterol esterification enzyme specifically expressed in hepatocytes and may control the amount of hepatic free (unesterified) cholesterol available for secretion into bile or into HDL. This study aims to further elucidate physiologic roles of ACAT2 in human hepatic cholesterol metabolism.

METHODS AND RESULTS

Liver biopsies from 40 normolipidemic, non-obese gallstone patients including some gallstone-free patients (female/male, 18/22) were collected and analyzed for microsomal ACAT2 activity, protein and mRNA expression. Plasma HDL-cholesterol (HDL-C) was significantly higher in females than in males, while triglycerides were significantly lower. ACAT2 activity in females was significantly lower than observed in males, regardless of the presence of gallstone disease. Moreover, the activity of ACAT2 correlated negatively with plasma levels of HDL-C (r=-0.57, P<0.05) and with Apo AI (r=-0.49, P<0.05).

CONCLUSION

This is the first description of a gender-related difference in hepatic ACAT2 activity in normolipidemic non-obese Chinese patients suggesting a possible role for ACAT2 in the regulation of cholesterol metabolism in humans. The negative correlation between ACAT2 activity and HDL-C or Apo AI may reflect this regulation. Since ACAT2 activity generally has been found to be pro-atherogenic in animal models, the observed sex-related difference may contribute to female protection from complications of coronary heart disease (CHD).

Biosynthesis of bile acids in mammalian liver

The biosynthesis of bile acids in mammalian liver and its regulation, together with the physiological role of bile acids, are reviewed in this article. Bile acids are biosynthesized from cholesterol in hepatocytes. Several steps are involved including epimerisation of the 3beta-hydroxyl group, reduction of the delta4 double bond to the 5beta-H structural arrangement, introduction of alpha-hydroxyl groups at C7 or C7 and C12 and, finally, oxidative degradation of the side chain by three carbon atoms. This gives the primary bile acids, cholic and chenodeoxycholic acids. Cholesterol-7alpha-hydroxylation is the rate determining step in the biosynthesis of cholic and chenodeoxycholic acids. Feedback regulation of cholesterol biosynthesis occurs by various mechanisms including termination of the synthesis of specific cytochromes P-450, modulation of specific cytosol proteins, short-term changes in the process of phosphorylation-dephosphorylation and changes in the capacity of the cholesterol pool as a substrate. Prior to being exported from the liver, bile acids are conjugated with glycine and taurine to produce the bile salts. After excretion into the intestinal tract, primary bile acids are partly converted to secondary bile acids, deoxycholic and lithocholic acids, by intestinal microorganisms. The majority of bile acids is absorbed from the intestinal tract and returned to the liver via the portal blood, so that only a small fraction is excreted in the feces. Bile acids returned to the liver can be reconjugated and reexcreted into the bile in the process of enterohepatic recycling. In addition to the physiological function of emulsifying lipids in the intestinal tract, bile acids are particularly important in respect of their ability to dissolve and transport cholesterol in the bile.

Cholesterol homeostasis in development: the role of Xenopus 7-dehydrocholesterol reductase (Xdhcr7) in neural development

7-dehydrocholesterol reductase (7-Dhcr) catalyses the final step in the pathway of cholesterol biosynthesis. Human patients with inborn errors of 7-Dhcr (Smith-Lemli-Opitz-Syndrome) have elevated serum levels of 7-dehydrocholesterol but low levels of cholesterol, which in phenotypical terms can result in growth retardation, craniofacial abnormalities including cleft palate, and reduced metal abilities. This study reports the isolation and molecular characterisation of 7-dehydrocholesterol reductase (Xdhcr7) from Xenopus laevis. During early embryonic development, the expression of Xdhcr7 is first of all spatially restricted to the Spemann's organizer and later to the notochord. In both tissues, Xdhcr7 is coexpressed with Sonic hedgehog (Shh), which itself is cholesterol-modified during autoproteolytic cleavage. Data from Xdhcr7 overexpression and knockdown experiments reveals that a tight control of cholesterol synthesis is particularly important for proper development of the central and peripheral nervous system.

Cholesterol 7alpha-hydroxylase is phosphorylated at multiple amino acids

The activity of cholesterol 7alpha-hydroxylase (gpCYP7A1), the rate limiting enzyme in bile acid synthesis, has been postulated to be regulated by phosphorylation/dephosphorylation. This study has found that several kinase activators rapidly reduce the amount of bile acid produced by the human hepatoma cell line, HepG2, and that gpCYP7A1 from HepG2 cell extracts eluted in the phosphoprotein fraction of FeIII columns. After incubating the HepG2 cells with radioactive orthophosphate, the band identified as gpCYP7Al on immunoblots was strongly labeled. Recombinant gpCYP7A was expressed as 6xHIS fusion polypeptides and subjected to kinase assays. The locations of phosphorylation were mapped further by screening synthetic peptides against AMP-activated protein kinase (AMPK), c-Jun N-terminal kinase, protein kinase A, and a panel of nine protein kinase C isoforms. AMPK, also known as 3-hydroxy-3-methylglutaryl coenzyme A reductase kinase, phosphorylated cholesterol 7alpha-hydroxylase, suggesting a potential mechanism of coordination of cholesterol synthesis and degradation.

Oxysterols: modulators of cholesterol metabolism and other processes

Oxygenated derivatives of cholesterol (oxysterols) present a remarkably diverse profile of biological activities, including effects on sphingolipid metabolism, platelet aggregation, apoptosis, and protein prenylation. The most notable oxysterol activities center around the regulation of cholesterol homeostasis, which appears to be controlled in part by a complex series of interactions of oxysterol ligands with various receptors, such as the oxysterol binding protein, the cellular nucleic acid binding protein, the sterol regulatory element binding protein, the LXR nuclear orphan receptors, and the low-density lipoprotein receptor. Identification of the endogenous oxysterol ligands and elucidation of their enzymatic origins are topics of active investigation. Except for 24, 25-epoxysterols, most oxysterols arise from cholesterol by autoxidation or by specific microsomal or mitochondrial oxidations, usually involving cytochrome P-450 species. Oxysterols are variously metabolized to esters, bile acids, steroid hormones, cholesterol, or other sterols through pathways that may differ according to the type of cell and mode of experimentation (in vitro, in vivo, cell culture). Reliable measurements of oxysterol levels and activities are hampered by low physiological concentrations (approximately 0.01-0.1 microM plasma) relative to cholesterol (approximately 5,000 microM) and by the susceptibility of cholesterol to autoxidation, which produces artifactual oxysterols that may also have potent activities. Reports describing the occurrence and levels of oxysterols in plasma, low-density lipoproteins, various tissues, and food products include many unrealistic data resulting from inattention to autoxidation and to limitations of the analytical methodology. Because of the widespread lack of appreciation for the technical difficulties involved in oxysterol research, a rigorous evaluation of the chromatographic and spectroscopic methods used in the isolation, characterization, and quantitation of oxysterols has been included. This review comprises a detailed and critical assessment of current knowledge regarding the formation, occurrence, metabolism, regulatory properties, and other activities of oxysterols in mammalian systems.

Acyl-CoA binding protein (ACBP) regulates acyl-CoA:cholesterol acyltransferase (ACAT) in human mononuclear phagocytes

It is demonstrated that the acyl-CoA:cholesterol acyltransferase (ACAT) enzyme activity in rough endoplasmatic reticulum membranes is regulated by the acyl-CoA binding protein (ACBP). The ACAT activity is strongly inhibited by different ACBP/oleoyl-CoA complexes depending from the molar ratio of protein and fatty acid-CoA. Other lipid binding proteins such as bovine serum albumin and the liver fatty acid binding protein do not show any effects on ACAT activity. In addition, we can show that cholesterol loading with acetylated low density lipoproteins does not lead to an increase of the ACBP mRNA level. Consequently, the increase of the intracellular concentration of fatty acids because of the cholesteryl ester accumulation renders ACAT more active for cholesterol esterification. In binding studies we have characterized binding sites on microsomal membranes for the ACAT substrate oleoyl-CoA and the ACAT inhibitor diazepam. Diazepam competes with oleoyl-CoA and vice versa for its binding to microsomal membranes. This common binding site is suggested to be responsible for the transfer from ACBP-bound oleoyl-CoA to ACAT and, therefore, to be essential for the microsomal cholesterol esterification.

Regulation of rat liver microsomal cholesterol ester hydrolase by reversible phosphorylation

The regulation of neutral cholesterol ester hydrolase activity by changes in its phosphorylation state was studied in rat liver microsomes. Treatment with cAMP-dependent protein kinase resulted in increased enzyme activity, which was further enhanced by the addition of cAMP and MgATP. Consistent activations were also achieved with MgCl2 and MgATP, the magnesium effect being abolished by ethylenediaminetetraacetic acid and adenosine triphosphate. Cholesterol ester hydrolase was activated twofold by free calcium and Ca2+/calmodulin; this latter effect was blocked by the chelator ethylene-glycol-bis(beta-aminoethyl ether)N,N,N',N'-tetraacetic acid and the calmodulin antagonist trifluoperazine. The phosphatase inhibitors pyrophosphate and glycerophosphate led to marked and dose-dependent increases in esterase activity, whereas okadaic acid elicited no effect. Furthermore, pyrophosphate and okadaic acid did not change the increases in enzyme activity promoted by Ca2+, Ca2+/calmodulin, Mg2+ and MgATP. Cholesterol ester hydrolase was inactivated in a concentration-dependent manner by nonspecific alkaline phosphatases. In cAMP-dependent protein kinase/cAMP- or Ca2+/calmodulin-activated microsomes, a time-dependent loss of activation in cholesteryl oleate hydrolysis was caused by alkaline phosphatase. These findings suggest that microsomal cholesterol ester hydrolase is activated through cAMP and Ca2+/calmodulin phosphorylation, whereas enzyme deactivation is dependent on phosphatase action.

Simultaneous assay of the activities of two key enzymes in cholesterol metabolism by gas chromatography-mass spectrometry

A very sensitive and specific method for the simultaneous assay of the activities of two key regulatory enzymes in cholesterol metabolism, 3-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) reductase (EC 1.1.1.34), and cholesterol 7 alpha-hydroxylase (EC 1.14.13.7), is described. The assay is based on the measurement of [2H3]mevalonolactone and 7 alpha-hydroxycholesterol produced by the incubation of [2H3]HMG-CoA and endogenous cholesterol with hamster liver microsomes using isotope dilution mass spectrometry. The incubation mixture was purified by means of solid extraction cartridges, and the extract was treated with benzylamine followed by dimethylethylsilyl imidazole. The resulting ether derivatives of the mevalonylbenzylamide and 7 alpha-hydroxycholesterol were quantified by gas chromatography-mass spectrometry with selected-ion monitoring in a high resolution mode. The method made it possible to assay simultaneously the activities of HMG-CoA reductase and cholesterol 7 alpha-hydroxylase in hamster liver microsomes with high sensitivity and accuracy.

Distal small bowel resection does not modify the intestinal 3-hydroxy-3-methylglutaryl CoA reductase activity

The activity of 3-hydroxy-3-methylglutaryl coenzyme-A reductase (HMG-CoA red.) has been evaluated in rat jejunal, caecal and colonic mucosa adapting to 50% or 75% distal small bowel resection (SBR). Six wk after surgery, the total activity and the activity state (active form) of the enzyme were determined. The results show that HMG-CoA red. activity was stimulated per organ but not per unit mass in the mucosa adapting to SBR. As a side result, we noted regional differences in the active/total reductase ratio (RA/RT) for HMG-CoA red. that was lower in small intestine compared with that of the large intestine.

An improved assay for cholesterol 7 alpha-hydroxylase activity using phospholipid liposome solubilized substrate

A persistent problem in measurement of cholesterol 7 alpha-hydroxylase (7 alpha-OHase) activity by isotope incorporation has been solubilization of cholesterol substrate. Solubilization with Tween 20, for example, resulted in a 75% reduction in 7 alpha-OHase activity after a 60 min incubation of substrate with microsomes. Incorporation of cholesterol substrate into small, unilamellar phospholipid vesicles (liposomes) prevented this effect, resulting in a 50% increase in activity over the same 60 min incubation at optimal concentrations. Using cholesterol in liposomes as substrate, standard assay conditions were determined to be: preparation of liposomes with 180 microM cholesterol substrate and 0.5 mg phospholipid/assay; incubation of these liposomes with 0.5 mg microsomal protein at 37 C for 60 min; addition of a NADPH generating system to start the reaction, and incubation at 37 C for 30 min before stopping the reaction and determining the amount of 7 alpha-hydroxycholesterol formed. In addition to preventing the detergent-related inhibition of the enzyme, liposome-solubilized substrate also reduced the variation among replicates from a coefficient of 45% with Tween 20 to 4.2% with phospholipid. This method provides a sensitive and reliable alternative to methods which require more sophisticated equipment and allows total control of substrate concentration in a form readily accessible to the enzyme.

Conditions that may result in (de-)phosphorylation of hepatic acyl-CoA:cholesterol acyltransferase result also in modulation of substrate supply in vitro

The present experiments were designed to study intervesicular transfer of cholesterol in rat liver microsomal fraction and modulation of the activity of acyl-CoA:cholesterol acyltransferase (ACAT) under conditions that are expected to result in the covalent modification (phosphorylation/dephosphorylation) of the enzyme. Preincubation of rat liver microsomal fraction followed by assay of ACAT showed a time-dependent increase in activity. This rate was temperature-dependent. Preincubation in the presence of cholesterol/phospholipid liposomes resulted in a time-dependent transfer of cholesterol from liposomal to the microsomal vesicles and in an increase in the rate of ACAT change owing to the preincubation. Both these rates were dependent on liposomal cholesterol concentration and on temperature. The presence of cytosol in the preincubation mixture increased the rate of change of ACAT activity in the absence or in the presence of cholesterol/phospholipid liposomes. In the latter case the presence of cytosol also increased the rate of transfer of cholesterol from liposomal to the microsomal vesicles. Activation energies of the rate of this transfer and of the rate of increase of ACAT activity were similar in the presence and in the absence of cytosol. Both in the absence and in the presence of cytosol, the presence of NaF (50 mM) in the preincubation mixture considerably decreased the rate of transfer of cholesterol from liposomal to microsomal vesicles and the rate of increase of ACAT activity. The presence of Mg2+ in the preincubation mixture produced no effect on the rate of transfer of cholesterol from liposomal to the microsomal vesicles, although under most conditions it decreased the rate of increase of ACAT activity caused by the preincubation. These results are discussed in relation to the molecular mechanism involved in this intervesicular transfer of cholesterol and to the modulation of ACAT activity by substrate supply, and also in relation to the hypothesis that ACAT activity can be modulated by a mechanism involving the phosphorylation/dephosphorylation of the enzyme.

Effects of dietary proteins on the regulation of liver lipoprotein receptors in rats

Female rats fed a 1.2% cholesterol diet with animal proteins (casein) develop a significant hypercholesterolemia, with a marked increase of very low density lipoprotein (VLDL)-associated cholesterol. Substitution of soy proteins for casein in the diet counteracts the increase of both total and VLDL cholesterol. Studies of liver receptor activity were carried out with both casein and soybean-cholesterol diets, to define the site of action of soy proteins. Binding of a cholesterol-rich lipoprotein fraction (beta-VLDL) to hepatic membranes is normal when a soybean-cholesterol diet is administered, and markedly reduced with casein-cholesterol. The activity of receptor-linked enzymes, HMG-CoA reductase, cholesterol 7 alpha-hydroxylase and acyl-CoA:cholesterol O-acyltransferase (ACATase), is differently affected by the two diets. HMG-CoA reductase activity is reduced by both diets with, however, significantly higher enzyme activities in the soybean-cholesterol-fed group. Both 7 alpha-hydroxylase and ACATase activity levels are significantly raised by casein-cholesterol but are in a normal range with soybean-cholesterol. These findings suggest that the hepatic receptor regulation of cholesterol metabolism is differently affected by animal and vegetable proteins in the diet.

Cholesterol esterification and hydrolysis in the adrenal cortex--the role of acyl-CoA:cholesterol acyltransferase

The regulation of cholesterol esterification and hydrolysis of cholesteryl esters in the adrenal cortex appear to be complementary to each other. The esterification of cholesterol, catalysed by acyl-CoA:cholesterol acyltransferase, is regulated in a different manner in the adrenal compared with the liver.