| For medical practitioners and the general public - Cholesterol Journal Article Catalog. |
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| Role of multidrug resistance P-glycoproteins in cholesterol biosynthesis Metherall, J. E., H. Li, et al. (1996), J Biol Chem 271(5): 2634-40. Abstract: Multidrug resistance (MDR) P-glycoproteins were first recognized for their ability to catalyze ATP-dependent efflux of cytotoxic agents from tumor cells when overexpressed. Despite extensive study, little is known about the normal substrate(s) and normal cellular function of these proteins. In the accompanying manuscript (Metherall, J. E., Waugh, K., and Li, H. (1996) J. Biol. Chem. 271, 2627-2633), we demonstrate that progesterone inhibits cholesterol biosynthesis, causing the accumulation of a number of cholesterol precursors. In the current manuscript, we use several criteria to show that the progesterone receptor is not involved in this inhibition. Rather, we demonstrate that progesterone inhibits cholesterol biosynthesis by interfering with MDR activity. We show that a steroid hormone's ability to inhibit cholesterol biosynthesis is correlated with: 1) its general hydrophobicity and 2) its ability to inhibit MDR activity. The only exception to this finding is beta-estradiol, which is a more potent inhibitor of cholesterol biosynthesis than expected based solely on hydrophobicity and MDR inhibition. We further demonstrate that nonsteroidal inhibitors of MDR also inhibit cholesterol biosynthesis. Since MDR activity is required for esterification of LDL-derived cholesterol (P. DeBry and J. E. Metherall, submitted for publication), we investigated the relationship between these phenomena and show that inhibition of cholesterol esterification does not cause inhibition of cholesterol biosynthesis and that inhibition of cholesterol biosynthesis does not cause inhibition of cholesterol esterification. We propose a model in which MDR is required for transport of sterols from the plasma membrane to the endoplasmic reticulum (ER). Inhibiting this transport prevents cholesterol esterification and cholesterol biosynthesis by preventing sterol substrates from reaching ER-resident enzymes. |
| Role of multidrug resistance P-glycoproteins in cholesterol esterification Debry, P., E. A. Nash, et al. (1997), J Biol Chem 272(2): 1026-31. Abstract: Cholesterol esterification, catalyzed by acyl-CoA:cholesterol acyltransferase (ACAT), plays a central role in cellular cholesterol homeostasis and in physiologic processes that lead to coronary heart disease. Although ACAT resides in the endoplasmic reticulum (ER), the cholesterol substrate for esterification originates in the plasma membrane and must be transported to the ER for esterification. Progesterone inhibits esterification, possibly by blocking the transport of cholesterol to the ER. Recent studies suggest that progesterone acts by inhibiting the activity of one or more of the multidrug-resistant (MDR) P-glycoproteins. In the current manuscript, we demonstrate that progesterone's ability to inhibit esterification is not mediated through the progesterone receptor. We evaluate a series of steroid hormones and find a strong correlation between a steroid hormone's hydrophobicity and its ability to inhibit both cholesterol esterification and MDR-catalyzed drug efflux. We also find that cholesterol esterification is inhibited by nonsteroidal MDR inhibitors, and that this inhibition specifically affects the esterification of cholesterol derived from the plasma membrane. MDR inhibitors also inhibit cholesterol esterification in a wide range of cultured human cell lines. These observations suggest that MDR activity normally functions in a general process of intracellular cholesterol transport. |
| Role of newly synthesized cholesterol or its metabolites on the regulation of bile acid biosynthesis after short-term biliary diversion in the rat Vlahcevic, Z. R., W. M. Pandak, et al. (1993), Hepatology 18(3): 660-8. Abstract: Cholesterol 7 alpha-hydroxylase, the rate-limiting enzyme in the bile acid biosynthetic pathway, is thought to be regulated by hydrophobic bile acids through negative feedback control. The role of cholesterol in the regulation of cholesterol 7 alpha-hydroxylase is more controversial, in part because of incomplete understanding of the relationship between the pathways of cholesterol synthesis and degradation. The main objective of this study was to define the interaction between these two pathways in an experimental model in which the supply of newly synthesized cholesterol was interrupted by sustained infusion of mevinolin (lovastatin), an inhibitor of 3-hydroxy-3-methylglutaryl coenzyme A reductase (HMG-CoA reductase) or accelerated by a continuous infusion of mevalonate, a cholesterol precursor. The study was carried out in rats subjected to short-term bile fistula. In one set of experiments, rats were treated postoperatively with mevinolin (5 mg/kg loading dose followed by 2 mg/kg/hr infusion), mevalonate (180 mumol/hr infusion) or both for up to 96 hr. In a separate set of experiments, rats were infused intraduodenally with taurocholate (36 mumol/100 gm/hr for up to 96 hr). We determined cholesterol 7 alpha-hydroxylase- and HMG-CoA reductase specific activities at those time intervals, whereas bile acid synthesis rates were determined throughout the study. Compared with rats not subjected to surgery, rats with short-term biliary diversion had increases in cholesterol 7 alpha-hydroxylase activity of 259% and 827% at 48 and 96 hr, respectively. The increase in bile acid biosynthesis was less pronounced. Continuous infusion of mevinolin completely prevented increases in cholesterol 7 alpha-hydroxylase specific activity and bile acid biosynthesis at both time intervals.(ABSTRACT TRUNCATED AT 250 WORDS) |
| Role of Niemann-Pick type C1 protein in intracellular trafficking of low density lipoprotein-derived cholesterol Cruz, J. C., S. Sugii, et al. (2000), J Biol Chem 275(6): 4013-21. Abstract: Niemann-Pick type C (NPC) is a disease that affects intracellular cholesterol-trafficking pathways. By cloning the hamster ortholog of NPC1, we identified the molecular lesions in two independently isolated Chinese hamster ovary cell mutants, CT60 and CT43. Both mutants lead to premature translational terminations of the NPC1 protein. Transfecting hamster NPC1 cDNA complemented the defects of the mutants. Investigation of the CT mutants, their parental cells, and an NPC1-stable transfectant allow us to present evidence that NPC1 is involved in a post-plasma membrane cholesterol-trafficking pathway. We found that the initial movement of low density lipoprotein (LDL)-derived cholesterol to the plasma membrane (PM) did not require NPC1. After reaching the PM and subsequent internalization, however, cholesterol trafficking back to the PM did involve NPC1. Both LDL-derived cholesterol and cholesterol originating from the PM accumulated in a dense, intracellular compartment in the CT mutants. Cholesterol movement from this compartment to the PM or endoplasmic reticulum was defective in the CT mutants. Our results functionally distinguish the dense, intracellular compartment from the early endocytic hydrolytic organelle and imply that NPC1 is involved in sorting cholesterol from the intracellular compartment back to the PM or to the endoplasmic reticulum. |
| Role of N-linked glycosylation of lecithin:cholesterol acyltransferase in lipoprotein substrate specificity O, K., J. S. Hill, et al. (1995), Biochim Biophys Acta 1254(2): 193-7. Abstract: Lecithin:cholesterol acyltransferase (LCAT) is responsible for the formation of cholesteryl ester in plasma. LCAT is a glycoprotein which has a carbohydrate content estimated to be approx. 25% of its total mass. Previous studies of recombinant LCAT have characterized the function of the four N-linked glycosylation sites of LCAT with respect to reconstituted HDL analogue substrates. In order to investigate the relationship between N-linked glycosylation and the ability of LCAT to esterify cholesterol in native plasma lipoproteins, we have expressed a series of mutant LCAT cDNAs in which each of the four glycosylation consensus sequences was eliminated individually. All mutant LCAT proteins were secreted by stably transfected baby hamster kidney cells. The ability of mutant LCATs to esterify cholesterol in purified native lipoproteins indicated that the elimination of the carbohydrate chain at position 20 of recombinant LCAT was associated with a lower activity than the wild type enzyme when HDL was used as a substrate, but no inhibitory effect was observed when LDL was used as a substrate. A mutant enzyme with a substitution of Asn-84-->Gln or Asn-272-->Gln displayed a decreased ability to esterify cholesterol in either HDL or LDL. In contrast, the loss of a carbohydrate chain at position 384 was associated with an increase in enzyme activity for both HDL (1.5-fold) and LDL (2.5-fold) substrates. Kinetic analysis of these recombinant enzymes indicated that the apparent Km values for cholesterol in either HDL or LDL were not affected, but that the differences in activities were due to changes in the apparent Vmax. Heat inactivation studies were performed to assess the role of specific carbohydrate groups in enzyme stability. Loss of a carbohydrate chain at position 20, 272 or 384 decreased thermostability of LCAT whereas a mutation at position 84 did not affect thermostability. These results suggest that individual carbohydrate chains confer specific structural and functional properties to LCAT. |
| Role of non-esterified fatty acids in regulating plasma cholesterol transport Barter, P. J. (1991), Clin Exp Pharmacol Physiol 18(2): 77-9. Abstract: 1. The cholesteryl ester transfer protein (CETP) promotes an equimolar exchange of cholesteryl esters between the high density lipoproteins (HDL) and low density lipoproteins (LDL) in human plasma. 2. Sodium oleate converts this CETP-mediated process of exchange into one of net mass transfer of cholesteryl esters from HDL to LDL. 3. Thus, conditions which increase the concentration of non-esterified fatty acids in plasma may favour the redistribution of cholesterol from the non-atherogenic HDL to the atherogenic LDL fraction. |
| Role of non-high-density lipoprotein cholesterol in prevention of cardiovascular disease: updated evidence from clinical trials Xydakis, A. M. and C. M. Ballantyne (2003), Curr Opin Cardiol 18(6): 503-9. Abstract: PURPOSE OF REVIEW: Although current clinical practice and treatment guidelines focus on low-density lipoprotein cholesterol as the basis for diagnosing and treating atherogenic dyslipidemias, many persons who develop cardiovascular disease do not have substantially elevated low-density lipoprotein cholesterol levels but may have low levels of high-density lipoprotein cholesterol and elevated triglycerides. Assessment of non-high-density lipoprotein cholesterol level provides a measure of cholesterol contained in all atherogenic particles, including low-density lipoprotein, lipoprotein(a), and triglyceride-rich particles such as very low density lipoprotein, very low density lipoprotein remnants, and intermediate-density lipoprotein. In the third Adult Treatment Panel guidelines of the US National Cholesterol Education Program, non-high-density lipoprotein cholesterol was introduced as a secondary target of therapy in persons with high triglyceride levels. RECENT FINDINGS: A growing number of epidemiological studies and clinical trials have examined the relation between non-high-density lipoprotein cholesterol level and cardiovascular disease events. Non-high-density lipoprotein cholesterol has been shown to be a better predictor for cardiovascular events than low-density lipoprotein cholesterol, and trials of statin therapy have demonstrated reductions in non-high-density lipoprotein cholesterol as well as low-density lipoprotein cholesterol. SUMMARY: Clinical trial evidence indicates the importance of incorporating all atherogenic lipoprotein particles in risk stratification. Non-high-density lipoprotein cholesterol is the most readily available surrogate marker for assessment of these particles and may also be a potential target of lipid-altering therapy. Statin therapy, which has already been demonstrated to decrease cardiovascular morbidity and mortality, provides significant reductions in non-high-density lipoprotein cholesterol. |
| Role of phospholipase A2 in cholesterol gallstone formation is associated with biliary phospholipid species selection at the site of hepatic excretion: indirect evidence Hattori, Y., S. Tazuma, et al. (2000), Dig Dis Sci 45(7): 1413-21. Abstract: Phospholipase A2 plays a role in cholesterol gallstone development by hydrolyzing bile phospholipids into lysolecithin and free fatty acids. Lysolecithin and polyunsaturated free fatty acids are known to stimulate the synthesis and/or secretion of gallbladder mucin via a prostanoid pathway, leading to enhancing cholesterol crystal nucleation and growth, and therefore, the action of phospholipase A2 is associated, in part, with bile phospholipid fatty acid. To clarify this hypothesis, we evaluated the effect on bile lipid metastability in vitro of replacing phospholipids with lysolecithin and various free fatty acids. Supersaturated model biles were created with an identical composition (cholesterol saturation index, 1.8; egg yolk lecithin, 34 mM; taurocholate, 120 mM; cholesterol, 25 mM) except for 5%, 10%, or 20% replacement of egg yolk lecithin with a combination of palmitoyl-lysolecithin and a free fatty acid (palmitate, stearate, oleate, linoleate, or arachidonate), followed by time-sequentially monitoring of vesicles and cholesterol crystals using spectrophotometer and video-enhanced differential contrast microscopy. Replacement with hydrophilic fatty acids (linoleate and arachidonate) reduced vesicle formation and promoted cholesterol crystallization, whereas an enhanced cholesterol-holding capacity was evident after replacement with hydrophobic fatty acids (palmitate and stearate). These results indicate that the effect of phospholipase A2 on bile lithogenecity is modulated by the fatty acid species in bile phospholipids, and therefore, that the role of phospholipase A2 in cholesterol gallstone formation is dependent, in part, on biliary phospholipid species selection at the site of hepatic excretion. |
| Role of phospholipid transfer protein and prebeta-high density lipoproteins in maintaining cholesterol efflux from Fu5AH cells to plasma from insulin-resistant subjects Dullaart, R. P. and A. van Tol (2001), Scand J Clin Lab Invest 61(1): 69-74. Abstract: Plasma phospholipid transfer protein (PLTP) enhances the generation of prebeta-high density lipoproteins (HDL) that may act as initial acceptors of cellular cholesterol, and are likely to play an important role in the antiatherogenic process of reverse cholesterol transport. We examined the interrelationships between insulin resistance, the ability of plasma to stimulate cellular cholesterol efflux, HDL cholesterol, plasma PLTP activity and prebeta-HDL in 12 non-diabetic, non-smoking, normotriglyceridaemic men. Cholesterol efflux from Fu5AH cells to plasma, plasma lipoproteins, PLTP activity and prebeta-HDL formation as measured in incubated plasma were determined after a 12-h fast. Insulin sensitivity was assessed by a euglycaemic, hyperinsulinaemic clamp (M-value). HDL cholesterol was positively correlated with the M-value (r=0.65, p< 0.05), whereas plasma PLTP activity (r= -0.59, p <0.05) and prebeta-HDL in incubated plasma (r= -0.66, p<0.05) were negatively correlated with the M-value. Thus, the lower the insulin sensitivity, the lower was HDL cholesterol and the higher were plasma PLTP activity and prebeta-HDL. Cellular cholesterol efflux tended to be correlated with HDL cholesterol (r=0.55, p < 0.10) as well as with plasma PLTP activity (r=0.56, p<0.10) and was positively correlated with prebeta-HDL in incubated plasma (r=0.74, p<0.01). No positive correlation between the M-value and cellular cholesterol efflux was found (r= -0.34, ns). These preliminary results support the hypothesis that, despite a lower HDL cholesterol, the ability of plasma from insulin-resistant subjects to promote cellular cholesterol efflux is not impaired, as a consequence of a higher plasma PLTP activity and enhanced prebeta-HDL formation. |
| Role of pleural fluid cholesterol in differentiating transudative from exudative pleural effusion Guleria, R., S. R. Agarwal, et al. (2003), Natl Med J India 16(2): 64-9. Abstract: BACKGROUND: Pleural fluid cholesterol has been reported to be useful in distinguishing between transudative and exudative pleural effusion. However, the difference in lipid profile between tubercular and non-tubercular pleural effusion has not been studied. METHODS: The lipid profile of pleural fluid in 50 patients with exudative (25 tubercular and 25 non-tubercular) and 25 with transudative effusion was studied. The diagnosis was based on clinical criteria and/or a positive diagnosis from another site. RESULTS: The criteria that best identified an exudative pleural effusion were pleural fluid cholesterol > or = 60 mg/dl, pleural fluid to serum cholesterol ratio > or = 0.4, pleural fluid triglyceride > or = 40 mg/dl and pleural fluid to serum triglyceride ratio > or = 0.3. Pleural fluid cholesterol had a sensitivity of 88% and a specificity of 100% for exudates with an accuracy of 92%. Pleural fluid to serum cholesterol ratio had a sensitivity of 98% and a specificity of 84%. These results were superior to the criteria proposed by Light et al. (sensitivity 98% and specificity 80%). CONCLUSION: Pleural fluid cholesterol estimation is an effective and cost-efficient method of differentiating exudative from transudative pleural effusion. The lipid profile does not help in diagnosing tubercular effusion. |
| Role of proteins and cholesterol in human senile cataractogenesis Yadav, S., K. P. Mistry, et al. (1991), Indian J Ophthalmol 39(1): 17-9. Abstract: Cataracts are a major cause of blindness in man with far reaching personal, social and economic consequences. The clarity of the lens is dependent upon the maintenance of the integrity of the fiber cell plasma membrane whose important component is cholesterol. In the present study, we have demonstrated that cataract formation influences the cholesterol and protein distribution within the lens. |
| Role of selenium in cytoprotection against cholesterol oxide-induced vascular damage in rats Huang, K., H. Liu, et al. (2002), Atherosclerosis 162(1): 137-44. Abstract: Wistar rats were fed Se-deficient (0.038 mg/kg diet) and adequate (0.326 mg/kg diet) diets for 13 weeks. The blood Se content, blood and vascular wall glutathione peroxidase (GPx) activity, serum high-density lipoprotein cholesterol (HDL-C) level and plasma prostacyclin (PGI(2)) concentration were decreased significantly, and the blood lipid peroxide (LPO) concentration, serum low-density lipoprotein cholesterol (LDL-C), total cholesterol (TC) level and plasma thromboxane A(2) (TXA(2)), content were increased significantly in Se-deficient rats compared with Se-adequate group, respectively. Furthermore the Se-deficient and adequate rats were given 5 mg/kg of cholestane-3 beta, 5 alpha, 6 beta -triol (3-triol) or vehicle only. Twenty four hours after treatment, the plasma PGI(2) level was decreased in Se-adequate rats infused 3-triol (+3 triol), meanwhile, the level in Se- deficient +3-triol group was much lower than that in Se-adequate +3-triol group. Compared with Se-adequate group, plasma TXA(2) content in Se-adequate +3-triol group had no significantly difference, but in Se- deficient rats infused 3-triol, plasma TXA(2) content was much higher than that in Se-adequate +3-triol group. The plasma ET concentration in Se-deficient group decreased slightly, but the concentration in Se-adequate +3-triol group increased significantly with respect to Se-deficient group. Although plasma ET concentration in Se-deficient group +3-triol did not increase, it was significantly lower than that in Se-adequate +3-triol group. The luminal surfaces of aorta thoracica of experimental rats were examined by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Observed by SEM, the luminal surface of aorta of Se-deficient rats showed few crater-like defects due to the disruption of endothelial cell. Se-adequate +3-triol group showed some crater-like defects on the their aorta luminal surface, but the luminal surface of Se-deficient +3-triol group exhibited numerous crater-like defects and appeared sponge-like as well as platelets adhering followed by thrombus formation in focal area of extensive endothelial damage. TEM studies also showed that the endothelium of aorta of Se deficient +3-triol group had more frequent lesion where endothelial cell plasma were swelling with profuse intracellular edema and some vacuoles were seen in cytoplasm. In severely injured areas, endothelial integrity was completely destroyed and smooth muscle cells were proliferating and migrated to the endomembrane. Thus, we can conclude that Se or selenoproteins in the vascular wall plays an important role in cytoprotection against cholesterol oxide-induced vascular damage in rats. |
| Role of small dense low-density lipoprotein in coronary artery disease patients with normal plasma cholesterol levels Koba, S., T. Hirano, et al. (2000), J Cardiol 36(6): 371-8. Abstract: OBJECTIVES: The relationship between plasma low-density lipoprotein (LDL) cholesterol and the risk of coronary artery disease (CAD) is known, but the other characteristics of LDL, particularly particle size and density, are unclear. The relationship between small dense LDL phenotype and non-diabetic, normocholesterolemic CAD was investigated in 70 patients with angiographically documented CAD, and 38 age-matched control subjects. METHODS: Peak LDL particle diameter was determined by using 2-16% polyacrylamide gradient gel electrophoresis. Small dense LDL phenotype was defined as particle diameter equal to or less than 255 A. RESULTS: LDL particle diameters in patients with CAD were significantly smaller than those in controls (252.4 +/- 6.9 vs 259.3 +/- 8.8 A, mean +/- SD, p < 0.0001). Prevalence of small dense LDL was markedly higher in patients with CAD (72%) than in subjects without CAD (24%). CAD patients had significantly lower high-density lipoprotein (HDL)-cholesterol and apolipoprotein A-I levels (39.3 +/- 8.8 vs 49.8 +/- 12.0, 108.1 +/- 20.6 vs 122.9 +/- 20.1 mg/dl), and higher lipoprotein (a) and apolipoprotein B levels (28.8 +/- 30.4 vs 16.8 +/- 18.8, 96.5 +/- 21.8 vs 80.2 +/- 14.9 mg/dl) than non-CAD subjects, whereas total cholesterol, LDL-cholesterol, triglyceride, remnant-like particle cholesterol and insulin levels were not increased in CAD patients compared with non-CAD subjects. Stepwise regression analysis revealed that LDL particle size was the most powerful independent determinant of CAD (F value = 20.04, p < 0.0001). Logistic regression analysis revealed that small dense LDL phenotype relative risk (RR) of 7.0, 95% confidence interval (95% CI) 2.4-20.1, low HDL-cholesterol (RR of 5.6, 95% CI 2.1-15.2), and increased apolipoprotein B (RR of 5.8, 95% CI 1.8-18.5) were independently associated with incidence of CAD. CONCLUSIONS: High prevalence of small dense LDL is a leading cause of CAD with even normal cholesterol levels. |
| Role of smoking in the U-shaped relation of cholesterol to mortality in men. The Framingham Study D'Agostino, R. B., A. J. Belanger, et al. (1995), Am J Epidemiol 141(9): 822-7. Abstract: Elevated mortality has been reported at extremes of the serum total cholesterol distribution, with increased coronary mortality reported at high total cholesterol levels and increased cancer and non-cardiovascular/non-cancer mortality at low total cholesterol levels. The authors used data collected on 1,959 males aged 35-69 years from the fourth Framingham Study examination to analyze the relations between total serum cholesterol levels and 409 coronary deaths, 325 cancer deaths, and 534 other deaths for a 32-year follow-up. Age- and risk factor-adjusted Cox regressions were computed. Nonlinear (U-shaped) relations were investigated with the use of quadratic regression and with dummy variables using the 160-199 mg/dl group as the comparison group. Subset analyses investigated the relation in smokers and men who drank > or = 14 alcoholic drinks per week. All analyses were repeated removing those with existing cardiovascular disease and cancer and those who died during the first 5 years of follow-up. A significant U-shaped relation with all-cause mortality was noted, as were an inverse relation to cancer mortality and a monotonic increasing relation with coronary disease mortality. In subset analyses, the association of low serum cholesterol (< 160 mg/dl) with cancer mortality was observed in men who smoked cigarettes. Compared with the 160-199 mg/dl group, the relative risk was 3.72 (p = 0.0001, 95% confidence interval 1.91-7.25). Studies of the relation of low total serum cholesterol levels, cigarette smoking, and cancer are needed. |
| Role of soy protein in cholesterol-lowering: how good is it? Nestel, P. (2002), Arterioscler Thromb Vasc Biol 22(11): 1743-4. |
| Role of sphingomyelin in the regulation of cholesterol esterification in the plasma lipoproteins. Inhibition of lecithin-cholesterol acyltransferase reaction Subbaiah, P. V. and M. Liu (1993), J Biol Chem 268(27): 20156-63. Abstract: In order to determine whether sphingomyelin (SPH) affects the rate of cholesterol esterification by plasma lecithin-cholesterol acyltransferase (LCAT), we studied the effects of its incorporation in to defined proteoliposome substrates containing phosphatidyl choline (PC), unesterified cholesterol, and apoprotein A-I, on the activity of purified LCAT. Cholesterol esterification was inhibited by up to 90% in the presence of SPH, and this inhibition was reversed by treatment with bacterial sphingomyelinase. The inhibition could be overcome by increasing the concentration of PC, but not unesterified cholesterol or apoprotein A-I, in the substrate. The effect of SPH was not related to the alterations in the size of the substrate particle and was not dependent on the type of acyl donor or apoprotein activator employed. The lysolecithin acyltransferase and phospholipase reactions carried out by LCAT were also inhibited by SPH. Kinetic studies suggested that: 1) LCAT binds better to substrate vesicles which contain SPH; 2) SPH competes with PC in binding to the active site of the enzyme; and 3) SPH is a more powerful competitive inhibitor than a diether analog of PC. The ability of various lipoproteins to act as substrates for purified LCAT varied inversely with the SPH/PC ratio. Treatment of the lipoproteins with sphingomyelinase activated the LCAT reaction, the percent activation being directly proportional to the SPH concentration in the native lipoprotein. Enrichment of high density lipoproteins with SPH inhibited cholesterol esterification in them by 50%, and this inhibition could be reversed by the degradation of SPH. These results show that SPH is a physiological inhibitor of cholesterol esterification in the plasma, by virtue of its competition with PC, the acyl donor for the reaction. |
| Role of the cholesterol biosynthetic pathway in osteoblastic differentiation of marrow stromal cells Parhami, F., N. Mody, et al. (2002), J Bone Miner Res 17(11): 1997-2003. Abstract: Cholesterol is an important molecule that plays a key role in regulating cellular differentiation and function. Although the possible role of lipids has been implicated in regulating osteoblastic cells, the role of cholesterol in that process is not well defined. In this study we have examined the role of the cellular cholesterol biosynthetic pathway on osteoblastic differentiation of marrow stromal cells (MSCs). Treatment of pluripotent mouse MSCs M2-10B4 with inhibitors of the cholesterol biosynthetic pathway mevastatin or mevinolin inhibited the maturation of these cells into functional osteoblastic cells. This was determined by the inhibition of the activity and expression of alkaline phosphatase (ALP), a key enzyme involved in differentiation and mineralization of osteoblastic cell cultures, as well as inhibition of mineralization. Mevastatin treatment did not affect expression of the osteoblast-specific gene osteocalcin (OCN). Furthermore, promoter-reporter studies in MSCs showed that mevastatin inhibited activity of the ALP gene promoter, suggesting regulation by derivatives of the cholesterol biosynthetic pathway. The effects of mevastatin and mevinolin were reversed by mevalonate but not by geranylgeraniol or farnesol, intermediates in the cholesterol biosynthetic pathway. Altogether, these results suggest that products of the cholesterol biosynthetic pathway are important for proper development of MSCs into functional osteoblastic cells capable of forming a mineralized matrix. Identification of those molecules may provide new therapeutic approaches to prevent the decline in osteoblastic activity in osteoporosis and aging. |
| Role of the cyclic AMP-dependent pathway in free radical-induced cholesterol accumulation in vascular smooth muscle cells Gesquiere, L., N. Loreau, et al. (2000), Free Radic Biol Med 29(2): 181-90. Abstract: We have previously reported that free radical-treated vascular smooth muscle cells (SMC) lead to cholesterol accumulation in vitro. In the current study, we investigated the effects of oxidative stress on cyclic AMP concentration and cAMP-dependent enzymes involved in cholesterol homeostasis in A7r5 cells. Under our conditions of a mild oxidative stress, namely with no change in cell viability, we found that free radicals, initiated using azobis-amidinopropane dihydrochloride (AAPH), resulted in a dose-dependent decrease in cellular cAMP which was opposed by vitamin E preincubation. Although the addition of adenylate cyclase activators (carbacyclin and forskolin) increased cAMP levels it did not succeed in restoring the AAPH-induced decrease. The oxidative stress-induced increase in activities of 3-hydroxy-3-methylglutaryl coenzyme A reductase and of acyl coenzyme A: cholesterol acyltransferase and the decrease in neutral cholesteryl ester hydrolase activity were suppressed by addition of dibutyryl cAMP. Taken together, these results strongly suggest that free radicals reduce cAMP concentrations by altering cell membrane adenylate cyclase activity. The changes of cAMP-dependent enzymes induced by oxidative stress resulting in cholesterol accumulation might be one of the processes leading to SMC-derived foam cells depicted in atheroma plaque. Moreover, if extrapolated to in vivo, these data may explain in part the beneficial effects of antioxidants in the reduction of cardiovascular diseases. |
| Role of the hepatic ABCA1 transporter in modulating intrahepatic cholesterol and plasma HDL cholesterol concentrations Basso, F., L. Freeman, et al. (2003), J Lipid Res 44(2): 296-302. Abstract: The current model for reverse cholesterol transport proposes that HDL transports excess cholesterol derived primarily from peripheral cells to the liver for removal. However, recent studies in ABCA1 transgenic mice suggest that the liver itself may be a major source of HDL cholesterol (HDL-C). To directly investigate the hepatic contribution to plasma HDL-C levels, we generated an adenovirus (rABCA1-GFP-AdV) that targets expression of mouse ABCA1-GFP in vivo to the liver. Compared with mice injected with control AdV, infusion of rABCA1-GFP-AdV into C57Bl/6 mice resulted in increased expression of mouse ABCA1 mRNA and protein in the liver. ApoA-I-dependent cholesterol efflux was increased 2.6-fold in primary hepatocytes isolated 1 day after rABCA1-GFP-AdV infusion. Hepatic ABCA1 expression in C57Bl/6 mice (n = 15) raised baseline levels of TC, PL, FC, HDL-C, apoE, and apoA-I by 150-300% (P < 0.05 all). ABCA1 expression led to significant compensatory changes in expression of genes that increase hepatic cholesterol, including HMG-CoA reductase (3.5-fold), LDLr (2.1-fold), and LRP (5-fold) in the liver. These combined results demonstrate that ABCA1 plays a key role in hepatic cholesterol efflux, inducing pathways that modulate cholesterol homeostasis in the liver, and establish the liver as a major source of plasma HDL-C. |
| Role of the interfacial binding domain in the oxidative susceptibility of lecithin:cholesterol acyltransferase Wang, K. and P. V. Subbaiah (2002), Biochem J 365(Pt 3): 649-57. Abstract: We had previously shown that the cholesterol esterification activity of lecithin:cholesterol acyltransferase (LCAT) is destroyed by oxidation, but still it retains the ability to hydrolyse water-soluble substrates. This suggested that the inactivation of the enzyme is not due to its catalytic function, but due to a loss of its hydrophobic binding. Since recent studies have shown that a tryptophan residue in the putative interfacial domain (Trp(61)) is critical for the activity, we determined the possible role of this residue in the oxidative susceptibility and substrate specificity of LCAT by site-directed mutagenesis. Deletion of Trp(61) resulted in a 56% loss of cholesterol esterification (LCAT) activity, but the phospholipase A(2) (PLA(2)) and the esterase activities of the enzyme were stimulated slightly. Replacing Trp(61) with another aromatic residue Trp(61)-->Tyr (W61Y) resulted in an increase in all activities (14-157%), whereas replacing it with an aliphatic residue Trp(61)-->Gly (W61G) caused a dramatic loss of LCAT (-90%) and PLA(2) (-82%) activities, but not the esterase activity (-5%). W61Y was the most sensitive to oxidation, whereas W61G was the most resistant, with respect to the LCAT and PLA(2) activities. However, the activities which do not involve interfacial binding, namely the esterase activity and the transesterification of short-chain phospholipids, were more resistant to oxidation in all LCATs, indicating a selective loss of the interfacial binding by oxidation. Furthermore, replacing the two cysteines (Cys(31) and Cys(184)) in the Trp(61) deletion mutant caused additional resistance of the enzyme to oxidizing agents, showing that both domains of the enzyme contribute independently to its oxidative susceptibility. Since the hydrolysis of truncated phospholipids, generated during the oxidation of low-density lipoproteins, does not require the interfacial-binding domain, our results suggest that LCAT may take part in the detoxification of these compounds even after the loss of its cholesterol esterification function. |