Regulation of early cholesterol biosynthesis in rat liver: effects of sterols, bile acids, lovastatin, and BM 15.766 on 3-hydroxy-3-methylglutaryl coenzyme A synthase and acetoacetyl coenzyme A thiolase activities

A proteomic approach for investigating the pleiotropic effects of statins in the atherosclerosis risk in communities (ARIC) study

BACKGROUND

Statins are prescribed to reduce LDL-c and risk of CVD. Statins have pleiotropic effects, affecting pathophysiological functions beyond LDL-c reduction. We compared the proteome of statin users and nonusers (controls). We hypothesized that statin use is associated with proteins unrelated to lipid metabolism.

METHODS

Among 10,902 participants attending ARIC visit 3 (1993-95), plasma concentrations of 4955 proteins were determined using SOMAlogic's DNA aptamer-based capture array. 379 participants initiated statins within the 2 years prior. Propensity scores (PS) were calculated based on visit 2 (1990-92) LDL-c levels and visit 3 demographic/clinical characteristics. 360 statin users were PS matched to controls. Log2-transformed and standardized protein levels were compared using t-tests, with false discovery rate (FDR) adjustment for multiple comparisons. Analyses were replicated in visit 2.

RESULTS

Covariates were balanced after PS matching, except for higher visit 3 LDL-c levels among controls (125.70 vs 147.65 mg/dL; p < 0.0001). Statin users had 11 enriched and 11 depleted protein levels after FDR adjustment (q < 0.05). Proteins related and unrelated to lipid metabolism differed between groups. Results were largely replicated in visit 2.

CONCLUSION

Proteins unrelated to lipid metabolism differed by statin use. Pending external validation, exploring their biological functions could elucidate pleiotropic effects of statins.

SIGNIFICANCE

Statins are the primary pharmacotherapy for lowering low-density lipoprotein (LDL) cholesterol and preventing cardiovascular disease. Their primary mechanism of action is through inhibiting the protein 3hydroxy-3-methylglutaryl CoA reductase (HMGCR) in the mevalonate pathway of LDL cholesterol synthesis. However, statins have pleiotropic effects and may affect other biological processes directly or indirectly, with hypothesized negative and positive effects. The present study contributes to identifying these pathways by comparing the proteome of stain users and nonusers with propensity score matching. Our findings highlight potential biological mechanisms underlying statin pleiotropy, informing future efforts to identify statin users at risk of rare nonatherosclerotic outcomes and identify health benefits of statin use independent of LDL-C reduction.

Cholesterol synthesis inhibition promotes axonal regeneration in the injured central nervous system

Neuronal regeneration in the injured central nervous system is hampered by multiple extracellular proteins. These proteins exert their inhibitory action through interactions with receptors that are located in cholesterol rich compartments of the membrane termed lipid rafts. Here we show that cholesterol-synthesis inhibition prevents the association of the Neogenin receptor with lipid rafts. Furthermore, we show that cholesterol-synthesis inhibition enhances axonal growth both on inhibitory -myelin and -RGMa substrates. Following optic nerve injury, lowering cholesterol synthesis with both drugs and siRNA-strategies allows for robust axonal regeneration and promotes neuronal survival. Cholesterol inhibition also enhanced photoreceptor survival in a model of Retinitis Pigmentosa. Our data reveal that Lovastatin leads to several opposing effects on regenerating axons: cholesterol synthesis inhibition promotes regeneration whereas altered prenylation impairs regeneration. We also show that the lactone prodrug form of lovastatin has differing effects on regeneration when compared to the ring-open hydroxy-acid form. Thus the association of cell surface receptors with lipid rafts contributes to axonal regeneration inhibition, and blocking cholesterol synthesis provides a potential therapeutic approach to promote neuronal regeneration and survival in the diseased Central Nervous System. SIGNIFICANCE STATEMENT: Statins have been intensively used to treat high levels of cholesterol in humans. However, the effect of cholesterol inhibition in both the healthy and the diseased brain remains controversial. In particular, it is unclear whether cholesterol inhibition with statins can promote regeneration and survival following injuries. Here we show that late stage cholesterol inhibition promotes robust axonal regeneration following optic nerve injury. We identified distinct mechanisms of action for activated vs non-activated Lovastatin that may account for discrepancies found in the literature. We show that late stage cholesterol synthesis inhibition alters Neogenin association with lipid rafts, thereby i) neutralizing the inhibitory function of its ligand and ii) offering a novel opportunity to promote CNS regeneration and survival following injuries.

Fetal coenzyme Q10 deficiency in intrahepatic cholestasis of pregnancy

AIM

Intrahepatic cholestasis of pregnancy (ICP) is considered a high-risk condition because it may have serious consequences for the fetus health. ICP is characterized by the accumulation of bile acids in maternal serum which contribute to an imbalance between the production of reactive oxygen species and the antioxidant defenses increasing the oxidative stress experienced by the fetus. Previously, it was reported a significant decrease in plasma coenzyme Q10 (CoQ10) in women with ICP. CoQ10 is a redox substance integrated in the mitochondrial respiratory chain and is recognized as a potent antioxidant playing an intrinsic role against oxidative damage. The objective of the present study was to investigate the levels of CoQ10 in umbilical cord blood during normal pregnancy and in those complicated with ICP, all of them compared to the maternal ones.

METHODS

CoQ10 levels and bile acid levels in maternal and umbilical cord blood levels during normal pregnancies (n=23) and in those complicated with ICP (n=13), were investigated.

RESULTS

A significant decrease in neonate CoQ10 levels corrected by cholesterol (0.105±0.010 vs. 0.069±0.011, P<0.05, normal pregnancy vs. ICP, respectively), together with an increase of total serum bile acids (2.10±0.02 vs. 7.60±2.30, P<0.05, normal pregnancy vs. ICP, respectively) was observed.

CONCLUSIONS

A fetus from an ICP mother is exposed to a greater risk derived from oxidative damage. The recognition of CoQ10 deficiency is important since it could be the starting point for a new and safe intervention strategy which can establish CoQ10 as a promising candidate to prevent the risk of oxidative stress.

The blood lipid regulation of Monascus-produced monascin and ankaflavin via the suppression of low-density lipoprotein cholesterol assembly and stimulation of apolipoprotein A1 expression in the liver

BACKGROUND/PURPOSES

Monascin (MS) and ankaflavin (AK) produced by Monascus purpureus NTU 568 were proven to show excellent hypolipidemic effects in our previous studies; however, the mechanism is still unclear.

METHODS

This study used MS, AK, and monacolin K as test substances and performed tests on rats fed high-fat and high-cholesterol diet for 8 weeks. The lipid levels and the related protein levels of the rats were assessed to understand the effects of MS, AK, and monacolin K on lipid metabolism.

RESULTS

MS and AK lowered low-density lipoprotein cholesterol (LDL-C) and preserved high-density lipoprotein cholesterol contents. MS and AK inhibited acetyl-coenzyme A acetyltransferase, microsomal triglyceride transfer protein, and apolipoprotein (apo) B-100 expression, thereby preventing LDL assembly. In addition, enhanced LDL-receptor expression increased the transport of LDL-C to the liver for metabolism. MS and AK also significantly increase apo A1 expression, which facilitates high-density lipoprotein cholesterol formation.

CONCLUSION

Monascus-fermented MS and AK can perform blood lipid regulation via the suppression of LDL-C assembly and stimulation of apo A1 expression in liver.

A metabolomic and pharmacokinetic study on the mechanism underlying the lipid-lowering effect of orally administered berberine

Clinical and animal studies demonstrated that orally administered berberine had a distinct lipid-lowering effect. However, pharmacokinetic studies showed that berberine was poorly absorbed into the body so the levels of berberine in the blood and target tissues were far below the effective concentrations revealed. To probe the underlying mechanism, the effect of berberine on the biological system was studied on a high-fat-diet-induced hamster hyperlipidemia model. Our results showed that intragastrically-administered berberine was poorly absorbed into circulation and most berberine accumulated in gut content. Although the bioavailability of intragastrically administered berberine was much lower than that of intraperitoneally administered berberine, it had a stronger lipid-lowing effect, indicating that the gastrointestinal tract is a potential target for the hypolipidemic effect of berberine. A metabolomic study on both serum and gut content showed that orally administered berberine significantly regulated molecules involved in lipid metabolism, and increased the generation of bile acids in the hyperlipidemic model. DNA analysis revealed that the orally administered berberine modulated the gut microbiota, and berberine showed a significant inhibition of the 7α-dehydroxylation conversion of cholic acid to deoxycholic acid, indicating a decreased elimination of bile acids in the gut. However, in model hamsters, elevated bile acids failed to downregulate the expression and function of CYP7A1 in a negative feedback loop. It was suggested that the hypocholesterolemic effect of orally administered berberine involves modulating the turnover of bile acids and the farnesoid X receptor signal pathway.

Developmental processes regulated by the 3-hydroxy-3-methylglutaryl-CoA reductase (HMGCR) pathway: highlights from animal studies

The 3-hydroxy-3-methylglutaryl coenzyme A reductase (HMGCR) is the rate-limiting enzyme in the biosynthesis of cholesterol and isoprenoids, which are substrates required for post-translational modification of signalling proteins that can potentially regulate various aspects of embryonic development. The HMGCR transcripts are detectable during early embryogenesis in both invertebrates and vertebrates, which suggests a conserved developmental requirement for mevalonate derivatives. Consistently, recent animal and in vitro studies have yielded valuable insights into potential morphogenic parameters that are modulated by HMGCR activity. These developmental end-points include brain and craniofacial morphogenesis, PGC migration and survival, myocardial epithelial migration and fusion, EC migration and survival, and vascular stabilization. By providing a synthesis of these studies, we hope that this review will highlight the need to comprehensively examine the entire suite of developmental processes regulated by HMGCR.

Cholesterol synthesis in the lactating cow: Induced expression of candidate genes

Despite the extensive knowledge for other species, cholesterol metabolism in ruminants is nowadays still not clear. Huge differences in milk cholesterol concentration are observed between breeds, managing strategies, individuals and moment of the lactating cycle, but the genetic actors working in the process of cholesterol secretion into milk have not been identified. As ruminant diet contains no cholesterol, understanding the mechanisms and regulation of synthesis, transport and secretion into milk is crucial when trying to reduce the amount of this metabolite in dairy products. The present work aims to study the expression of candidate genes for these processes in the liver of Bos taurus during the lactating cycle. Liver biopsies were obtained from 16 adult brown Swiss cows at different time points (2 weeks pre-partum and 0, 2, 4 and 8 weeks post-partum). After RNA extraction and reverse transcription, gene expression of candidate genes was studied using quantitative RT-PCR. Key enzymes of the cholesterol synthesis (3-hydroxy-methyglutaryl-coenzyme-A (HMG-CoA) synthase, HMG-CoA reductase and farnesyldiphosphat-farnesyltransferase (FDFT)) and gene expression feed-back regulators involved in lipid metabolism (sterol regulatory element binding proteins (SREBP1and 2) SREBP-cleavage activating protein (Scap) were selected as candidate genes. HMG-CoA-reductase and FDFT showed a huge expression increase until week 2 post-partum (p<0.01), most probably in response to the new requirements in the mammary gland. As well, and as a possible explanation for such modifications, an increase in the expression of the regulators SREBP1 and Scap was observed (p<0.01 and p<0.05 respectively). Most important, the whole synthesis machinery showed a coordinated regulation, as highly significant positive correlations were found between the expression levels of the above mentioned enzymes (p<0.01). The increase of milk and blood cholesterol levels in B. taurus after parturition might be the result of a coordinated induction in the expression of key liver enzymes and their regulating factors.

A specific inhibitor of cholesterol biosynthesis, BM15.766, reduces the expression of ?-secretase and the production of amyloid-?in vitro

We have previously shown that statins reduce the production of amyloid-beta (Abeta) by both isoprenoid- and cholesterol-dependent mechanisms. These pathways contribute to the regulation of the dimerisation of BACE into its physiologically active form. Statins reduce cellular cholesterol levels by 20-40%; therefore, it is possible that the remaining cholesterol within the cell may play a significant role in the production of Abeta. Incubation of cells with the specific cholesterol biosynthesis inhibitor BM15.766 together with 50 micromol/L simvastatin and 400 micromol/L mevalonate reduced cellular cholesterol levels in a dose-dependent manner with increasing BM15.766 concentration (r = -0.9736, p = 0.0264). Furthermore, decreases in cellular cholesterol levels correlated with reductions in total Abeta production (r = 0.9683, p = 0.0317). A total of 2.5 micromol/L BM15.766 inhibited the dimerisation of BACE, whilst the expression of BACE monomer was reduced by 5 micromol/L BM15.766. BM15.766 treatment localised BACE predominantly within the Golgi, and reduced total BACE expression per cell. Similar changes were observed in the expression of the Golgi marker golgin-97, suggesting that reduced BACE expression may arise from a decrease in protein trafficking and an increase in degradation. By targeting cholesterol synthesis using specific cholesterol biosynthesis inhibitors, it is possible to reduce Abeta production without reducing protein isoprenylation.

Effects of ursodeoxycholic acid on synthesis of cholesterol and bile acids in healthy subjects

BACKGROUND/AIMS

Ursodeoxycholic acid (UDCA) decreases biliary secretion of cholesterol and is therefore used for the dissolution of cholesterol gallstones. It remains unclear whether these changes in biliary cholesterol excretion are associated with changes in cholesterol synthesis and bile acid synthesis. We therefore studied the activities of rate-limiting enzymes of cholesterol synthesis and bile acid synthesis, 3-hydroxy-3-methylglutaryl-coenzyme A reductase and cholesterol 7alpha-hydroxylase, respectively, in normal subjects during UDCA feeding.

METHODS

UDCA was given to 8 healthy volunteers (5 men, 3 women; age 24-44 years) in a single dose of 10-15 mg/kg body weight for 40 days. Before and during (days 3, 5, 10, 20, 30 and 40) UDCA treatment, urinary excretion of mevalonic acid and serum concentrations of 7alpha-hydroxy-4-cholesten-3-one (7alpha-HCO) were determined as markers of cholesterol and bile acid synthesis, respectively. The Wilcoxon signed rank test and Spearman's rank correlation coefficient were used for statistical analysis.

RESULTS

Cholesterol synthesis and serum lipid concentrations remained unchanged during UDCA treatment for 40 days. However, synthesis of bile acids increased during long-term treatment with UDCA as reflected by an increase in 7alpha-HCO serum concentrations from 39.7 +/- 21.3 ng/ml (median 32.8 ng/ml) before treatment to 64.0 +/- 30.4 ng/ml (median 77.5 ng/ml) at days 30-40 of UDCA treatment (p < 0.05).

CONCLUSIONS

UDCA treatment does not affect cholesterol synthesis in the liver, but does increase bile acid synthesis after prolonged treatment. This may represent a compensatory change following decreased absorption of endogenous bile acids as observed with UDCA therapy.

Farnesol-induced cell death and stimulation of 3-hydroxy-3-methylglutaryl-coenzyme A reductase activity in tobacco cv bright yellow-2 cells

Growth inhibition of tobacco (Nicotiana tabacum L. cv Bright Yellow-2) cells by mevinolin, a specific inhibitor of 3-hydroxy-3-methylglutaryl-coenzyme A reductase (HMGR) could be partially overcome by the addition of farnesol. However, farnesol alone inhibited cell division and growth as measured by determination of fresh weight increase. When 7-d-old tobacco cv Bright Yellow-2 cells were diluted 40-fold into fresh culture, the cells exhibited a dose-dependent sensitivity to farnesol, with 25 microM sufficient to cause 100% cell death, as measured by different staining techniques, cytometry, and monitoring of fragmentation of genomic DNA. Cells were less sensitive to the effects of farnesol when diluted only 4-fold. Farnesol was absorbed by the cells, as examined by [1-(3)H]farnesol uptake, with a greater relative enrichment by the more diluted cells. Both mevinolin and farnesol treatments stimulated apparent HMGR activity. The stimulation by farnesol was also reflected in corresponding changes in the steady-state levels of HMGR mRNA and enzyme protein with respect to HMGR gene expression and enzyme protein accumulation.

3-Hydroxy-3-methylglutaryl-coenzyme A reductase activity is inhibited by cholesterol and up-regulated by sitosterol in sitosterolemic fibroblasts

Sitosterolemia is an inherited recessive disease characterized by abnormally increased plasma and tissue plant sterol concentrations. Patients hyperabsorb sitosterol. In addition, hepatic, ileal, and mononuclear leukocyte 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase, the rate-controlling enzyme in the cholesterol biosynthetic pathway, is markedly suppressed in this disease. It is still controversial whether the down-regulation is due to accumulated sitosterol, but the effect of sitosterol on HMG-CoA reductase activity has not been studied in sitosterolemic tissues. To investigate whether sitosterol inhibits HMG-CoA reductase activity in sitosterolemia, we measured the enzyme activities in liver and cultured skin flbroblasts from patients. Hepatic HMG-CoA reductase activities in patients were decreased 76% (P < .05) as compared with results in control subjects. In contrast, HMG-CoA reductase activities in sitosterolemic fibroblasts were not decreased as compared with results in control fibroblasts, and the activities in all cells were up-regulated similarly when they were exposed to delipidated medium. Because the cultured sitosterolemic fibroblasts contained only trace amounts of plant sterols, we added 20 microg/mL sitosterol directly to the cell medium. Raising the intracellular sitosterol concentration to 7% of cellular cholesterol level increased HMG-CoA reductase activity 23% (P < .05), while the addition of the same amount of cholesterol to the cells reduced the activity 46% (P < .05). Thus, when sitosterolemic skin fibroblasts were used, it was possible to distinguish between the effects of cholesterol and those of sitosterol on the activity of HMG-CoA reductase. These results suggest that reduced HMG-CoA reductase activity in this disease is caused by secondary effects of unknown regulator(s) other than sitosterol.

Cellular and molecular biology of the liver

This paper highlights several key issues, ideas, and findings that significantly contribute to our understanding of the organization, communication, and molecular machinery of the liver. The functional anatomy of the liver has been studied in several ways that have revealed the extent of the biliary tree within the hepatic parenchyma, including identification of the canals of Hering as their most distal ramification. The canals of Hering are also considered as the potential residence of hepatic progenitor cells. Hepatocytes can "communicate" with each other via gap junctions, but might also deliver hormones and nucleotides downstream to cholangiocytes. The interaction of inflammatory cells and inflammatory mediators with hepatocytes is of particular importance in transplant immunology, infection, inflammation, viral hepatitis, and fibrogenesis. The role of these mediators as well as certain "toxic" bile acids in apoptosis has become clearer with the discovery of the mitochondrial permeability transition. Moreover, ursodeoxycholic acid can reduce apoptosis by minimizing the mitochondrial permeability transition. Two new nuclear hormone receptors, PXR and SXR, have been identified. These are both activated by a variety of chemically distinct ligands, whose final common goal is the activation of cytochrome P450-containing drug-metabolizing enzymes. Thus, these two receptors are critical to the body's ability to metabolize a variety of compounds properly. Additional insight into the role of cytokines and cytokine receptors in liver regeneration is presented. Finally, in vivo gene therapy of liver-expressed genes by chimeric oligonucleotides appears quite promising as a means of correcting single nucleotide gene defects.