BACKGROUND: Loss of function variants of LIPG gene encoding endothelial lipase (EL) are associated with primary hyperalphalipoproteinemia (HALP), a lipid disorder characterized by elevated plasma levels of high density lipoprotein cholesterol (HDL-C). OBJECTIVE: Aim of the study was the phenotypic and genotypic characterization of a family with primary HALP. METHODS: HDL subclasses distribution was determined by polyacrylamide gradient gel electrophoresis. Serum content of prebeta-HDL was assessed by (2D)-electrophoresis. Cholesterol efflux capacity (CEC) of serum mediated by ABCA1, ABCG1 or SR-BI was assessed using cells expressing these proteins. Cholesterol loading capacity (CLC) of serum was assayed using cultured human macrophages. Next generation sequencing was used for DNA analysis. Plasma EL mass was determined by ELISA. RESULTS: Three family members had elevated plasma HDL-C, apoA-I and total phospholipids, as well as a reduced content of prebeta-HDL. These subjects were heterozygous carriers of a novel variant of LIPG gene [c.526 G>T, p.(Gly176Trp)] found to be deleterious in silico. Plasma EL mass in carriers was lower than in controls. CEC of sera mediated by ABCA1 and ABCG1 transporters was substantially reduced in the carriers. This effect was maintained after correction for serum HDL concentration. The sera of carriers were found to have a higher CLC in cultured human macrophages than control sera. CONCLUSION: The novel p.(Gly176Trp) variant of endothelial lipase is associated with changes in HDL composition and subclass distribution as well as with functional changes affecting cholesterol efflux capacity of serum which suggest a defect in the early steps of revere cholesterol transport.
AIMS: It has been hypothesized that the activity of lysosomal acid lipase (LAL), a key enzyme involved in lipid metabolism, is involved in the NAFLD phenotype. To clarify the role of LAL in NAFLD, we studied 164 consecutive patients with biopsy-proven NAFLD and fat-loaded HepG2 cells. METHODS: LAL activity was measured (i) on dried blood spots (DBS) from NAFLD patients and dyslipidemic subjects without fatty liver and (ii) on liver biopsies from NAFLD patients. LAL activity and expression were evaluated in HepG2 cells cultured in the presence of free fatty acids (FAs), with or without a PPAR-alpha agonist. RESULTS: LAL activity was significantly reduced in patients with NAFLD compared to dyslipidemic subjects. LAL activity measured in liver biopsies from NAFLD patients was highly correlated to that measured on DBS and was independent of LAL expression in the liver. In a fully adjusted model, LAL activity on DBS was associated only with platelets and, when normalized by platelet count, it did not differ according to fibrosis stage. In vitro, FA loading of HepG2 fully replicated the impairment of LAL activity observed in NALFD patients. In these cells, the activation of PPAR-alpha receptors prevented and corrected FA-induced LAL impairment, by stimulating FA oxidation and LAL expression. CONCLUSIONS: LAL activity is reduced in NAFLD patients, independently from disease progression. In vitro, impaired LAL activity induced by FA loading was rescued by PPAR-alpha activation. These data suggest that the pharmacological modulation of LAL should be explored in the management of NAFLD patients.
Lysosomal acid lipase (LAL) is responsible for the hydrolysis of cholesteryl esters (CE) and triglycerides (TG) within the lysosomes; generated cholesterol and free fatty acids (FFA) are released in the cytosol where they can regulate their own synthesis and metabolism. When LAL is not active, as in case of genetic mutations, CE and TG accumulate in the lysosomal compartment, while the lack of release of cholesterol and FFA in the cytosol leads to an upregulation of their synthesis. Thus, LAL plays a central role in the intracellular homeostasis of lipids. Since there are no indications about the effect of different lipid-lowering agents on LAL activity, aim of the study was to address the relationship between LAL activity and the type of lipid-lowering therapy in a cohort of dyslipidemic patients. LAL activity was measured on dried blood spot from 120 patients with hypercholesterolemia or mixed dyslipidemia and was negatively correlated to LDL-cholesterol levels. Among enrolled patients, ninety-one were taking one or more lipid-lowering drugs, as statins, fibrates, ezetimibe and omega-3 polyunsaturated fatty acids. When patients were stratified according to the type of lipid-lowering treatment, i.e. untreated, taking statins or taking fibrates, LAL activity was significantly higher in those with fibrates, even after adjustment for sex, age, BMI, lipid parameters, liver function, metabolic syndrome, diabetes and statin use. In a subset of patients tested after 3 months of treatment with micronized fenofibrate, LAL activity raised by 21%; the increase was negatively correlated with baseline LAL activity. Thus, the use of fibrates is independently associated with higher LAL activity in dyslipidemic patients, suggesting that the positive effects of PPAR-alpha activation on cellular and systemic lipid homeostasis can also include an improved LAL activity.
Lecithin:cholesterol acyltransferase (LCAT) catalyzes plasma cholesteryl ester formation and is defective in familial lecithin:cholesterol acyltransferase deficiency (FLD), an autosomal recessive disorder characterized by low high-density lipoprotein, anemia, and renal disease. This study aimed to investigate the mechanism by which compound A [3-(5-(ethylthio)-1,3,4-thiadiazol-2-ylthio)pyrazine-2-carbonitrile], a small heterocyclic amine, activates LCAT. The effect of compound A on LCAT was tested in human plasma and with recombinant LCAT. Mass spectrometry and nuclear magnetic resonance were used to determine compound A adduct formation with LCAT. Molecular modeling was performed to gain insight into the effects of compound A on LCAT structure and activity. Compound A increased LCAT activity in a subset (three of nine) of LCAT mutations to levels comparable to FLD heterozygotes. The site-directed mutation LCAT-Cys31Gly prevented activation by compound A. Substitution of Cys31 with charged residues (Glu, Arg, and Lys) decreased LCAT activity, whereas bulky hydrophobic groups (Trp, Leu, Phe, and Met) increased activity up to 3-fold (P < 0.005). Mass spectrometry of a tryptic digestion of LCAT incubated with compound A revealed a +103.017 m/z adduct on Cys31, consistent with the addition of a single hydrophobic cyanopyrazine ring. Molecular modeling identified potential interactions of compound A near Cys31 and structural changes correlating with enhanced activity. Functional groups important for LCAT activation by compound A were identified by testing compound A derivatives. Finally, sulfhydryl-reactive beta-lactams were developed as a new class of LCAT activators. In conclusion, compound A activates LCAT, including some FLD mutations, by forming a hydrophobic adduct with Cys31, thus providing a mechanistic rationale for the design of future LCAT activators.
The aim of this study was to evaluate the vasoprotective effects of HDL isolated from carriers of LCAT deficiency, which are characterized by a selective depletion of LpA-I:A-II particles and predominance of prebeta migrating HDL. HDLs were isolated from LCAT-deficient carriers and tested in vitro for their capacity to promote NO production and to inhibit vascular cell adhesion molecule-1 (VCAM-1) expression in cultured endothelial cells. HDLs from carriers were more effective than control HDLs in promoting eNOS activation with a gene-dose-dependent effect (PTrend = 0.048). As a consequence, NO production induced by HDL from carriers was significantly higher than that promoted by control HDL (1.63 +/- 0.24-fold vs. 1.34 +/- 0.07-fold, P = 0.031). HDLs from carriers were also more effective than control HDLs in inhibiting the expression of VCAM-1 (homozygotes, 65.0 +/- 8.6%; heterozygotes, 53.1 +/- 7.2%; controls, 44.4 +/- 4.1%; PTrend = 0.0003). The increased efficiency of carrier HDL was likely due to the depletion in LpA-I:A-II particles. The in vitro findings might explain why carriers of LCAT deficiency showed flow-mediated vasodilation and plasma-soluble cell adhesion molecule concentrations comparable to controls, despite low HDL-cholesterol levels. These results indicate that selective depletion of apoA-II-containing HDL, as observed in carriers of LCAT deficiency, leads to an increased capacity of HDL to stimulate endothelial NO production, suggesting that changes in HDL apolipoprotein composition may be the target of therapeutic interventions designed to improve HDL functionality.
LCAT synthesizes most of the plasma cholesteryl esters, and plays a major role in HDL metabolism. Mutations in the LCAT gene cause two syndromes, familial LCAT deficiency (FLD) and fish-eye disease (FED), both characterized by severe alterations in plasma lipoprotein profile. Renal disease is the major cause of morbidity and mortality in FLD cases, but an established therapy is not currently available. The present therapy of LCAT deficiency is mainly aimed at correcting the dyslipidemia associated with the disease and at delaying evolution of chronic nephropathy. LCAT deficiency represents a candidate disease for enzyme replacement therapy. In vitro and in vivo studies proved the efficacy of recombinant human LCAT (rhLCAT) in correcting dyslipidemia, and rhLCAT is presently under development.
Human familial lecithin:cholesterol acyltransferase (LCAT) deficiency (FLD) is characterized by low HDL, accumulation of an abnormal cholesterol-rich multilamellar particle called lipoprotein-X (LpX) in plasma, and renal disease. The aim of our study was to determine if LpX is nephrotoxic and to gain insight into the pathogenesis of FLD renal disease. We administered a synthetic LpX, nearly identical to endogenous LpX in its physical, chemical and biologic characteristics, to wild-type and Lcat-/- mice. Our in vitro and in vivo studies demonstrated an apoA-I and LCAT-dependent pathway for LpX conversion to HDL-like particles, which likely mediates normal plasma clearance of LpX. Plasma clearance of exogenous LpX was markedly delayed in Lcat-/- mice, which have low HDL, but only minimal amounts of endogenous LpX and do not spontaneously develop renal disease. Chronically administered exogenous LpX deposited in all renal glomerular cellular and matrical compartments of Lcat-/- mice, and induced proteinuria and nephrotoxic gene changes, as well as all of the hallmarks of FLD renal disease as assessed by histological, TEM, and SEM analyses. Extensive in vivo EM studies revealed LpX uptake by macropinocytosis into mouse glomerular endothelial cells, podocytes, and mesangial cells and delivery to lysosomes where it was degraded. Endocytosed LpX appeared to be degraded by both human podocyte and mesangial cell lysosomal PLA2 and induced podocyte secretion of pro-inflammatory IL-6 in vitro and renal Cxl10 expression in Lcat-/- mice. In conclusion, LpX is a nephrotoxic particle that in the absence of Lcat induces all of the histological and functional hallmarks of FLD and hence may serve as a biomarker for monitoring recombinant LCAT therapy. In addition, our studies suggest that LpX-induced loss of endothelial barrier function and release of cytokines by renal glomerular cells likely plays a role in the initiation and progression of FLD nephrosis.
Lecithin:cholesterol acyltransferase (LCAT) deficiency is associated with hypoalphalipoproteinemia, generally a predisposing factor for premature coronary heart disease. The evidence of accelerated atherosclerosis in LCAT-deficient subjects is however controversial. In this study, the effect of LCAT deficiency on vascular tone and endothelial function was investigated in LCAT knockout mice, which reproduce the human lipoprotein phenotype. Aortas from wild-type (Lcat(wt)) and LCAT knockout (Lcat(KO)) mice exposed to noradrenaline showed reduced contractility in Lcat(KO) mice (P<0.005), whereas acetylcholine exposure showed a lower NO-dependent relaxation in Lcat(KO) mice (P<0.05). Quantitative PCR and Western blotting analyses suggested an adequate eNOS expression in Lcat(KO) mouse aortas. Real-time PCR analysis indicated increased expression of beta2-adrenergic receptors vs wild-type mice. Aorta stimulation with noradrenaline in the presence of propranolol, to abolish the beta-mediated relaxation, showed the same contractile response in the two mouse lines. Furthermore, propranolol pretreatment of mouse aortas exposed to L-NAME prevented the difference in responses between Lcat(wt) and Lcat(KO) mice. The results indicate that LCAT deficiency leads to increased beta2-adrenergic relaxation and to a consequently decreased NO-mediated vasodilation that can be reversed to guarantee a correct vascular tone. The present study suggests that LCAT deficiency is not associated with an impaired vascular reactivity.
OBJECTIVE: Serum lipoproteins influence cell cholesterol content by delivering and removing cholesterol to/from cells, functions mainly exerted by LDL and HDL, respectively. Especially in the case of HDL, structure and composition are crucial for function, beyond serum levels. Cholesteryl ester storage disease (CESD) is caused by LIPA gene mutations and reduced activity of lysosomal acid lipase (LAL), the enzyme responsible for hydrolysis of cholesteryl esters and TG. CESD patients typically present dyslipidaemia, liver damage and premature atherosclerosis. The objective of this work was to evaluate serum HDL cholesterol efflux capacity (CEC) and serum cholesterol loading capacity (CLC) in CESD pediatric patients and to study lipoprotein qualitative modifications. METHODS: HDL CEC was evaluated by radioisotopic techniques, serum CLC was measured by a fluorimetric assay, HDL subclasses were determined by two-dimensional electrophoresis. RESULTS: CESD patients (n = 3) displayed on average increased LDL cholesterol (+163%; p = 0.019), TG (+203; p = 0.012), phospholipids (+40%; p = 0.024) and lower HDL cholesterol (-57%; p = 0.012) compared to controls (n = 9). CESD HDL CEC was impaired both as a whole (average reduction of 26%; p < 0.0001) and with respect to specific membrane cholesterol transporters (-23% for aqueous diffusion; p = 0.005; -32% for ABCA1-efflux; p = 0.0002; -60% for SR-BI-efflux; p < 0.0001; -42% for ABCG1-efflux p = 0.0003). A marked reduction in the pre-beta HDL concentration (-69%; p = 0.012) was detected. Finally, CESD serum CLC was significantly increased (+21%; p = 0.0007). CONCLUSION: These new data demonstrate that the pro-atherogenic modifications of serum include disturbances in lipoprotein functions involved in cell cholesterol homeostasis occurring from very early age in CESD patients.
LCAT (lecithin:cholesterol acyltransferase) catalyzes the transacylation of a fatty acid of lecithin to cholesterol, generating a cholesteryl ester and lysolecithin. The knowledge of LCAT atomic structure and the identification of the amino acids relevant in controlling its structure and function are expected to be very helpful to understand the enzyme catalytic mechanism, as involved in HDL cholesterol metabolism. However - after an early report in the late '90 s - no recent advance has been made about LCAT three-dimensional structure. In this paper, we propose an LCAT atomistic model, built following the most up-to-date molecular modeling approaches, and exploiting newly solved crystallographic structures. LCAT shows the typical folding of the alpha/beta hydrolase superfamily, and its topology is characterized by a combination of alpha-helices covering a central 7-strand beta-sheet. LCAT presents a Ser/Asp/His catalytic triad with a peculiar geometry, which is shared with such other enzyme classes as lipases, proteases and esterases. Our proposed model was validated through different approaches. We evaluated the impact on LCAT structure of some point mutations close to the enzyme active site (Lys218Asn, Thr274Ala, Thr274Ile) and explained, at a molecular level, their phenotypic effects. Furthermore, we devised some LCAT modulators either designed through a de novo strategy or identified through a virtual high-throughput screening pipeline. The tested compounds were proven to be potent inhibitors of the enzyme activity.
Lecithin:cholesterol acyltransferase (LCAT) is the enzyme responsible for cholesterol esterification in plasma. Mutations in the LCAT gene leads to two rare disorders, familial LCAT deficiency and fish-eye disease, both characterized by severe hypoalphalipoproteinemia associated with several lipoprotein abnormalities. No specific treatment is presently available for genetic LCAT deficiency. In the present study, recombinant human LCAT was expressed and tested for its ability to correct the lipoprotein profile in LCAT deficient plasma. The results show that rhLCAT efficiently reduces the amount of unesterified cholesterol (-30%) and promotes the production of plasma cholesteryl esters (+210%) in LCAT deficient plasma. rhLCAT induces a marked increase in HDL-C levels (+89%) and induces the maturation of small prebeta-HDL into alpha-migrating particles. Moreover, the abnormal phospholipid-rich particles migrating in the LDL region were converted in normally sized LDL.
The lecithin:cholesterol acyltransferase (LCAT) enzyme is responsible for the synthesis of cholesteryl esters in human plasma and plays a critical role in high density lipoprotein (HDL) metabolism. Genetic LCAT deficiency is a rare metabolic disorder characterized by low HDL cholesterol levels. This paper reviews the genetic and biochemical features of LCAT deficiency, highlighting the absence of enhanced preclinical atherosclerosis in carriers, despite the remarkably low HDL cholesterol.
BACKGROUND: Lecithin:cholesterol acyltransferase (LCAT) is responsible for cholesterol esterification in plasma. Mutations of LCAT gene cause familial LCAT deficiency, a metabolic disorder characterized by hypoalphalipoproteinemia. Apolipoprotein B (apoB) is the main protein component of very-low-density lipoproteins and low-density lipoprotein (LDL). Mutations of APOB gene cause familial hypobetalipoproteinemia, a codominant disorder characterized by low plasma levels of LDL cholesterol and apoB. OBJECTIVE: This was a genetic and biochemical analysis of an Italian kindred with hypobetalipoproteinemia whose proband presented with hypoalphalipoproteinemia and severe chronic kidney disease. METHODS: Plasma lipids and apolipoproteins, cholesterol esterification, and high-density lipoprotein (HDL) subclass distribution were analyzed. LCAT and APOB genes were sequenced. RESULTS: The proband had severe impairment of plasma cholesterol esterification and high prebeta-HDL content. He was heterozygote for the novel LCAT P406L variant, as were two other family members. The proband's wife and children presented with familial hypobetalipoproteinemia and were heterozygotes for the novel apoB H1401R variant. Cholesterol esterification rate of apoB H1401R carriers was reduced, likely attributable to the low amount of circulating LDL. After renal transplantation, proband's lipid profile, HDL subclass distribution, and plasma cholesterol esterification were almost at normal levels, suggesting a mild contribution of the LCAT P406L variant to his pretransplantation severe hypoalphalipoproteinemia and impairment of plasma cholesterol esterification. CONCLUSION: LCAT P406L variant had a mild effect on lipid profile, HDL subclass distribution, and plasma cholesterol esterification. ApoB H1401R variant was identified as possible cause of familial hypobetalipoproteinemia and resulted in a reduction of cholesterol esterification rate.
A genetic mendelian autosomal recessive condition of deficiency of lecithin- cholesterol acyltransferase (LCAT) can produce two different diseases: one highly interesting nephrologic picture of complete enzymatic deficiency (lecithin:cholesterol acyltransferase deficiency; OMIM ID #245900; FLD), characterized by the association of dyslipidemia, corneal opacities, anemia and progressive nephropathy; and a partial form (fish eye disease; OMIM ID #136120; FED) with dyslipidemia and progressive corneal opacities only. The diagnosis of FLD falls first of all under the competence of nephrologists, because end-stage renal disease appears to be its most severe outcome. The diagnostic suspicion is based on clinical signs (corneal opacities, more severe anemia than expected for the degree of chronic renal failure, progressive proteinuric nephropathy) combined with histology obtained by kidney biopsy (glomerulopathy evolving toward sclerosis with distinctive lipid deposition). However, the final diagnosis, starting with a finding of extremely low levels of HDL-cholesterol, requires collaboration with lipidology Centers that can perform sophisticated investigations unavailable in common laboratories. To be heterozygous for a mutation of the LCAT gene is one of the monogenic conditions underlying primary hypoalphalipoproteinemia (OMIM ID #604091). This disease, which is characterized by levels of HDL-cholesterol below the 5th percentile of those of the examined population (<28 mg/dL for Italians), has heritability estimates between 40% and 60% and is considered to be a predisposing condition for coronary artery disease. Nevertheless, some monogenic forms, and especially those associated with LCAT deficiency, seem to break the rule, confirming once more the value of a proper diagnosis before drawing prognostic conclusions from a laboratory marker. As in many other rare illnesses, trying to discover all the existing cases will contribute to allow studies broad enough to pave the way for further therapies, in this case also fostering the production by industries of the lacking enzyme by genetic engineering. Epidemiological studies, although done on selected populations such as hypoalphalipoproteinemia patients on dialysis and with the effective genetic tools of today, have been disappointing in elucidating the disease. Spreading the clinical knowledge of the disease and its diagnostic course among nephrologists seems to be the best choice, and this is the aim of our work.
OBJECTIVE: To better understand the role of lecithin:cholesterol acyltransferase (LCAT) in lipoprotein metabolism through the genetic and biochemical characterization of families carrying mutations in the LCAT gene. METHODS AND RESULTS: Thirteen families carrying 17 different mutations in the LCAT gene were identified by Lipid Clinics and Departments of Nephrology throughout Italy. DNA analysis of 82 family members identified 15 carriers of 2 mutant LCAT alleles, 11 with familial LCAT deficiency (FLD) and 4 with fish-eye disease (FED). Forty-four individuals carried 1 mutant LCAT allele, and 23 had a normal genotype. Plasma unesterified cholesterol, unesterified/total cholesterol ratio, triglycerides, very-low-density lipoprotein cholesterol, and pre-beta high-density lipoprotein (LDL) were elevated, and high-density lipoprotein (HDL) cholesterol, apolipoprotein A-I, apolipoprotein A-II, apolipoprotein B, LpA-I, LpA-I:A-II, cholesterol esterification rate, LCAT activity and concentration, and LDL and HDL3 particle size were reduced in a gene-dose-dependent manner in carriers of mutant LCAT alleles. No differences were found in the lipid/lipoprotein profile of FLD and FED cases, except for higher plasma unesterified cholesterol and unesterified/total cholesterol ratio in the former. CONCLUSIONS: In a large series of subjects carrying mutations in the LCAT gene, the inheritance of a mutated LCAT genotype causes a gene-dose-dependent alteration in the plasma lipid/lipoprotein profile, which is remarkably similar between subjects classified as FLD or FED.
Title: An omega-3 polyunsaturated fatty acid concentrate increases plasma high-density lipoprotein 2 cholesterol and paraoxonase levels in patients with familial combined hyperlipidemia Calabresi L, Villa B, Canavesi M, Sirtori CR, James RW, Bernini F, Franceschini G Ref: Metabolism, 53:153, 2004 : PubMed
A remarkable reduction of plasma concentrations of high-density lipoproteins (HDL), especially of the HDL(2) subfraction, is one of the typical lipoprotein alterations found in patients with familial combined hyperlipidemia (FCHL). Fourteen FCHL patients received 4 capsules daily of Omacor (an omega-3 polyunsaturated fatty acid [omega3 FA] concentrate providing 1.88 g of eicosapentaenoic acid [EPA] and 1.48 g of docosahexaenoic acid [DHA] per day; Pronova Biocare, Oslo, Norway) or placebo for 8 weeks in a randomized, double-blind, crossover study. Plasma triglycerides were 44% lower, and LDL cholesterol and apoliporpotein (apo)B were 25% and 7% higher after Omacor than placebo. HDL cholesterol was higher (+8%) after Omacor than placebo, but this difference did not achieve statistical significance. Omacor caused a selective increase of the more buoyant HDL(2) subfraction; plasma HDL(2) cholesterol and total mass increased by 40% and 26%, respectively, whereas HDL(3) cholesterol and total mass decreased by 4% and 6%. Both HDL(2) and HDL(3) were enriched in cholesteryl esters and depleted of triglycerides after Omacor. No changes were observed in the plasma concentration of major HDL apolipoproteins, LpA-I and LpA-I:A-II particles, lecithin:cholesterol acyltransferase (LCAT), and cholesteryl ester transfer protein (CETP). The plasma concentration of the HDL-bound antioxidant enzyme paraoxonase increased by 10% after Omacor. Omacor may be helpful in correcting multiple lipoprotein abnormalities and reducing cardiovascular risk in FCHL patients.
Title: Enzymatically active paraoxonase-1 is located at the external membrane of producing cells and released by a high affinity, saturable, desorption mechanism Deakin S, Leviev I, Gomaraschi M, Calabresi L, Franceschini G, James RW Ref: Journal of Biological Chemistry, 277:4301, 2002 : PubMed
Paraoxonase-1 (PON1) is a high density lipoprotein (HDL)-associated serum enzyme that protects low density lipoproteins from oxidative modifications. There is a relative lack of information on mechanisms implicated in PON1 release from cells. The present study focused on a model derived from stable transfection of CHO cells, to avoid co-secretion of apolipoprotein (apo) A-I and lipids, which could lead to formation of HDL-like complexes. Our results indicate that, in the absence of an appropriate acceptor, little PON1 is released. The results designate HDL as the predominant, physiological acceptor, whose efficiency is influenced by size and composition. Neither lipid-poor apoA-I or apoA-II nor low density lipoproteins could substitute for HDL. Protein-free phospholipid complexes promoted PON1 release. However, the presence of both apolipoprotein and phospholipid were necessary to promote release and stabilize the enzyme. Immunofluorescence studies demonstrated that PON1 was inserted into the external membrane of CHO cells, where it was enzymatically active. Accumulation of PON1 in the cell membrane was not influenced by the ability of the cell to co-secrete of apoA-I. Release appeared to involve desorption by HDL; human and reconstituted HDL promoted PON1 release in a saturable, high affinity manner (apparent affinity 1.59 +/- 0.3 microg of HDL protein/ml). Studies with PON1-transfected hepatocytes (HuH-7) revealed comparable structural features with the peptide located in a punctate pattern at the external membrane and enzymatically active. We hypothesize that release of PON1 involves a docking process whereby HDL transiently associate with the cell membrane and remove the peptide from the external membrane. The secretory process may be of importance for assuring the correct lipoprotein destination of PON1 and thus its functional efficiency.
Patients with familial lecithin-cholesterol acyltransferase (LCAT) deficiency very often show progressive glomerulosclerosis with evolution to end-stage disease. High levels of an abnormal lipoprotein (lipoprotein X) cause glomerular capillary endothelial damage. The ultrastructural study of renal biopsy specimens shows characteristic glomerular deposits of membrane-like, cross-striated structures and vacuole structures. The gene encoding for LCAT has been mapped to chromosome 16q22.1, and several mutations of this gene cause LCAT deficiency which is inherited as an autosomal recessive trait and which is characterized by corneal opacities, normochromic normocytic anemia, and renal dysfunction. Herein we report clinical features and renal histological findings concerning a 24-year-old male patient with classical familial LCAT deficiency due to two different allelic mutations: a nonsense mutation inherited from the father and a missense mutation inherited from the mother. Moreover, the patient showed glomerular histological lesions and an immunofluorescent glomerular pattern typical of hypocomplementemic membranoproliferative type II glomerulonephritis (dense-deposit disease). The nature of electron-dense material that characterizes dense-deposit disease is still unknown, but there are suggestions that some chemical modifications might occur in the renal basement membranes. Therefore, this clinical case might induce to consider possible relations between disorders of the lipoprotein metabolism and renal dense-deposit disease.
Paraoxonase is a high density lipoprotein (HDL) associated enzyme with a hypothesised role in the protection of low density lipoproteins (LDL) from oxidative stress. The present study examined paraoxonase in several genetically distinct HDL deficiency states. Despite reduction or even absence of detectable HDL, enzyme activity was present in sera from A-I-Pisa, A-I-Helsinki, A-I-Milano and Tangier patients. Both enzyme activities and peptide concentrations were modulated (reduced) but specific activities were broadly similar to controls, suggesting an impact on peptide concentration rather than an inhibition of enzyme activity. Despite the absence of HDL in A-I-Pisa and Tangier subjects, there was no association of paraoxonase with very low density lipoproteins or LDL. Paraoxonase function is maintained in HDL deficient states. It implies that certain HDL-associated anti-atherogenic processes may not be entirely compromised by HDL deficiency. This has important implications for the cardiovascular risk associated with modulated HDL concentrations.
