Cellular and secreted pro-protein convertase subtilisin/kexin type 9 catalytic activity in hepatocytes

OBJECTIVES

Pro-protein convertase subtilisin/kexin type 9 (PCSK9) impairs the low density lipoprotein receptor (LDLr) recyling. To reach the LDLr, the pro-protein must cleave itself in the endoplasmic reticulum. Using a fluorogenic peptide corresponding to the cleavage site, we directly monitored for the first time the cleavage activity of purified human PCSK9 and that of endogenous human wild-type PCSK9 and naturally occurring variants in hepatocytes.

METHODS

Validation of the assay was performed with wild type or PCSK9 deficient primary mouse hepatocytes and immortalized human hepatocytes transfected with antiPCSK9 siRNA. An analysis of the cleaved peptide was performed using mass spectrometry. Pharmacological regulation of the enzyme was studied in human hepatocytes. Expression vectors coding for the variants S127R, D374Y, F216L, S386A were transfected in primary hepatocytes from PCSK9 deficient mice.

RESULTS

PCSK9 activity was measured in cell lysates and media, at levels 100 times higher than with the human purified recombinant protein. The assay is highly specific for PCSK9 in cell lysate and cell culture media but not in plasma. Pharmacological up- or down-regulation of PCSK9 expression produced paralleled effects on the activity. The catalytic activity of gain-of-function variants S127R, D374Y recapitulated roughly the maturation efficiency estimated by western blots, in contrast with the F216L variant that presented with a 54% lower catalytic activity than the wild-type protein, despite similar proPCSK9 to PCSK9 ratios. Thus, other factors might be involved in the maturation of PCSK9.

CONCLUSION

All together, these results shed a new light on PCSK9 enzymatic activity and could help identifying proPCSK9 inhibitors.

Effect of low-fat, fermented milk enriched with plant sterols on serum lipid profile and oxidative stress in moderate hypercholesterolemia

BACKGROUND

Plant sterol (PS)-enriched foods have been shown to reduce plasma LDL-cholesterol concentrations. In most studies, however, PSs were incorporated into food products of high fat content.

OBJECTIVE

We examined the effect of daily consumption of PS-supplemented low-fat fermented milk (FM) on the plasma lipid profile and on systemic oxidative stress in hypercholesterolemic subjects.

DESIGN

Hypercholesterolemic subjects (LDL-cholesterol concentrations >or=130 and <or= 190 mg/dL; n = 194) consumed 2 low-fat portions of FM in the same meal daily for 6 wk. Subjects were randomly assigned to 2 groups: low-fat FM enriched with 0.8 g PS ester per portion or control FM. Plasma concentrations of lipids, oxidized LDL, beta-carotene, beta-sitosterol, campesterol, and high-sensitivity C-reactive protein were measured during the trial.

RESULTS

Plasma LDL-cholesterol concentrations were reduced by 9.5% and 7.8% after 3 and 6 wk, respectively, in the 1.6-g/d PS group compared with the control group, whereas plasma triacylglycerol and HDL-cholesterol concentrations were not significantly affected. In addition, there were no significant changes in serum beta-carotene on normalization to LDL cholesterol during the study period in both groups, whereas plasma concentrations of oxidized LDL were reduced significantly in the PS group compared with the control group (-1.73 compared with 1.40 U/L, respectively; P < 0.05). Plasma sitosterol concentrations were increased by 35% (P < 0.001 compared with control); however, campesterol concentrations did not change during the study period.

CONCLUSION

Daily consumption of 1.6 g PS in low-fat FM efficiently lowers LDL cholesterol in subjects with moderate hypercholesterolemia without deleterious effects on biomarkers of oxidative stress.

Hepatic PCSK9 expression is regulated by nutritional status via insulin and sterol regulatory element-binding protein 1c

Familial autosomal dominant hypercholesterolemia is associated with high risk for cardiovascular accidents and is related to mutations in the low density lipoprotein receptor or its ligand apolipoprotein B (apoB). Mutations in a third gene, proprotein convertase subtilisin kexin 9 (PCSK9), were recently associated to this disease. PCSK9 acts as a natural inhibitor of the low density lipoprotein receptor pathway, and both genes are regulated by depletion of cholesterol cell content and statins, via sterol regulatory element-binding protein (SREBP). Here we investigated the regulation of PCSK9 gene expression during nutritional changes. We showed that PCSK9 mRNA quantity is decreased by 73% in mice after 24 h of fasting, leading to a 2-fold decrease in protein level. In contrast PCSK9 expression was restored upon high carbohydrate refeeding. PCSK9 mRNA increased by 4-5-fold in presence of insulin in rodent primary hepatocytes, whereas glucose had no effect. Moreover, insulin up-regulated hepatic PCSK9 expression in vivo during a hyperinsulinemic-euglycemic clamp in mice. Adenoviral mediated overexpression of a dominant or negative form of SREBP-1c confirmed the implication of this transcription factor in insulin-mediated stimulation of PCSK9 expression. Liver X receptor agonist T0901317 also regulated PCSK9 expression via this same pathway (a 2-fold increase in PCSK9 mRNA of primary hepatocytes cultured for 24 h in presence of 1 microm T0901317). As our last investigation, we isolated PCSK9 proximal promoter and verified the functionality of a SREBP-1c responsive element located from 335 bp to 355 bp upstream of the ATG. Together, these results show that PCSK9 expression is regulated by nutritional status and insulinemia.

Kinetics of prebeta1 HDL and alphaHDL in type II diabetic patients

BACKGROUND

The aim of this study was to analyze the recycling of high density lipoprotein (HDL) in six type II diabetic patients compared with six control subjects by endogenous labelling of apolipoprotein A-I (Apo A-I) with stable isotope Apo A.

MATERIALS AND METHODS

The -I-HDL kinetics were performed by infusion of (5.5.5-(2)H3)-leucine for 14 h. The prebeta1 and alphaHDL were separated by gel filtration fast protein liquid chromatrography system (FPLC). Kinetics of isotopic enrichment of Apo A-I were analyzed with a multi-compartmental model software (SAAM II, SAAM Institute, Seattle, WA).

RESULTS

Plasma Apo A-I concentration was decreased in patients with type II diabetes as a result of a decrease in Apo A-I-alphaHDL (P < 0.05). Diabetic patients were also characterized by an increased relative contribution of Apo A-I in prebeta1 HDL (18.3 +/- 2.8% vs 11.9 +/- 3.7%, P < 0.01). The synthetic rate of prebeta1 HDL was slightly increased in diabetic patients compared with control (NS) and an increase of recycling rate of alpha to prebeta1 HDL was observed (11.67 +/- 3.14 d(-1) vs 7.09 +/- 4.51 d(-1), P < 0.05). The clearance rate of Apo A-I was higher in diabetic patients (P < 0.05 for Apo A-I-prebeta1 HDL and P < 0.005 for Apo A-I-alphaHDL).

CONCLUSION

This study suggests that the usual increase in prebeta1 HDL in type II diabetic patients is mainly related to an increased conversion rate of alpha to prebeta1 HDL.

Human mucosa/submucosa interactions during intestinal inflammation: involvement of the enteric nervous system in interleukin-8 secretion

Interleukin-8 (IL-8) is a key chemokine upregulated in various forms of intestinal inflammation, especially those induced by bacteria such as Clostridium difficile (C. difficile). Although interactions between different mucosal and submucosal cellular components have been reported, whether such interactions are involved in the regulation of IL-8 secretion during C. difficile infection is unknown. Moreover, whether the enteric nervous system, a major component of the submucosa, is involved in IL-8 secretion during an inflammatory challenge remains to be determined. In order to investigate mucosa/submucosa interactions that regulate IL-8 secretion, we co-cultured human intestinal mucosa and submucosa. In control condition, IL-8 secretion in co-culture was lower than the sum of the IL-8 secretion of both tissue layers cultured alone. Contrastingly, IL-8 secretion increased in co-culture after mucosal challenge with toxin B of C. difficile through an IL-1 beta-dependent pathway. Moreover, we observed that toxin B of C. difficile increased IL-8 immunoreactivity in submucosal enteric neurones in co-culture and in intact preparations of mucosa/submucosa, through an IL-1 beta-dependent pathway. IL-1 beta also increased IL-8 secretion and IL-8 mRNA expression in human neuronal cell lines (NT2-N and SH-SY5Y), through p38 and ERK1/2 MAP kinase-dependent pathways. Our results demonstrate that mucosa/submucosa interactions regulate IL-8 secretion during inflammatory processes in human through IL-1 beta-dependent pathways. Finally we observed that human submucosal neurones synthesize IL-8, whose production in neurones is induced by IL-1 beta via MAPK-dependent pathways.

Wild-type PCSK9 inhibits LDL clearance but does not affect apoB-containing lipoprotein production in mouse and cultured cells

Mutations in Proprotein Convertase Subtilisin Kexin 9 (PCSK9) have been associated with autosomal dominant hypercholesterolemia. In vivo kinetic studies indicate that LDL catabolism was impaired and apolipoprotein B (apoB)-containing lipoprotein synthesis was enhanced in two patients presenting with the S127R mutation on PCSK9. To understand the physiological role of PCSK9, we overexpressed human PCSK9 in mouse and cellular models as well as attenuated the endogenous expression of PCSK9 in HuH7 hepatoma cells using RNA interference. Here, we show that PCSK9 dramatically impairs the expression of the low density lipoprotein receptor (LDLr) and, in turn, LDL cellular binding as well as LDL clearance from the plasma compartment in C57BL6/J mice but not in LDLr-deficient mice, establishing a definitive role for PCSK9 in the modulation of the LDLr metabolic pathway. In contrast to data obtained in S127R-PCSK9 patients presenting with increased apoB production, our study indicates that wild-type PCSK9 does not significantly alter the production and/or secretion of VLDL apoB in either cultured cells or mice. Finally, we show that unlike PCSK9 overexpression in mice, the S127R mutation in patients led to increased VLDL apoB levels, suggesting a potential gain of function for S127R-PCSK9 in humans.

Retinoic acid receptor-mediated induction of ABCA1 in macrophages

ABCA1, the mutant molecule in Tangier Disease, mediates efflux of cellular cholesterol to apoA-I and is induced by liver X receptor (LXR)/retinoid X receptor (RXR) transcription factors. Retinoic acid receptor (RAR) activators (all-trans-retinoic acid [ATRA] and TTNPB) were found to increase ATP-binding cassette transporter 1 (ABCA1) mRNA and protein in macrophages. In cellular cotransfection assays, RARgamma/RXR activated the human ABCA1 promoter, via the same direct repeat 4 (DR4) promoter element as LXR/RXR. Chromatin immunoprecipitation analysis in macrophages confirmed the binding of RARgamma/RXR to the ABCA1 promoter DR4 element in the presence of ATRA, with weaker binding of RARalpha/RXR, and no binding of RARbeta/RXR. However, in macrophages from RARgamma(-/-) mice, TTNPB still induced ABCA1, in association with marked upregulation of RARalpha, suggesting that high levels of RARalpha can compensate for the absence of RARgamma. Dose-response experiments with ATRA in mouse primary macrophages showed that other LXR target genes were weakly induced (ABCG1 and SREBP-1c) or not induced (apoE and LXRalpha). The more specific RAR activator TTNPB did not induce SREBP-1c in mouse primary macrophages or liver. These studies indicate a direct role of RARgamma/RXR in induction of macrophage ABCA1.

Phospholipid transfer protein deficiency protects circulating lipoproteins from oxidation due to the enhanced accumulation of vitamin E

Vitamin E is a lipophilic anti-oxidant that can prevent the oxidative damage of atherogenic lipoproteins. However, human trials with vitamin E have been disappointing, perhaps related to ineffective levels of vitamin E in atherogenic apoB-containing lipoproteins. Phospholipid transfer protein (PLTP) promotes vitamin E removal from atherogenic lipoproteins in vitro, and PLTP deficiency has recently been recognized as an anti-atherogenic state. To determine whether PLTP regulates lipoprotein vitamin E content in vivo, we measured alpha-tocopherol content and oxidation parameters of lipoproteins from PLTP-deficient mice in wild type, apoE-deficient, low density lipoprotein (LDL) receptor-deficient, or apoB/cholesteryl ester transfer protein transgenic backgrounds. In all four backgrounds, the vitamin E content of very low density lipoprotein (VLDL) and/or LDL was significantly increased in PLTP-deficient mice, compared with controls with normal plasma PLTP activity. Moreover, PLTP deficiency produced a dramatic delay in generation of conjugated dienes in oxidized apoB-containing lipoproteins as well as markedly lower titers of plasma IgG autoantibodies to oxidized LDL. The addition of purified PLTP to deficient plasma lowered the vitamin E content of VLDL plus LDL and normalized the generation of conjugated dienes. The data show that PLTP regulates the bioavailability of vitamin E in atherogenic lipoproteins and suggest a novel strategy for achieving more effective concentrations of anti-oxidants in lipoproteins, independent of dietary supplementation.

Modulation of the phospholipid transfer protein-mediated transfer of phospholipids by diacylglycerols

Previous studies have shown that diacylglycerols (DAG) are formed during triglyceride hydrolysis in very low density lipoproteins (VLDL), a process that is accompanied by an elevated phospholipid transfer protein (PLTP)-mediated transfer of phospholipids (PL) from VLDL to high density lipoprotein. Because PLTP has been also shown to transfer DAG, we hypothesized that DAG might modulate PL transfer through a mechanism of competition with respect to PLTP. To address this question we performed in vitro PL transfer assays using specifically designed PL donor particles. These were single bilayer vesicles (SBV) and large (EM-L) or small (EM-S) lipid emulsions, containing various proportions of DAG. The PLTP-mediated transfers of PL decreased as the volumes of the particle cores increased (SBV > EM-S > EM-L). In all cases, these transfers were inhibited by DAG in a concentration-dependent manner. We determined the core-to-surface distribution of DAG and we measured their relative affinity for PLTP by comparison with that of PL. From these parameters, we calculated the theoretical effects of DAG on PL transfers that would result from a competition mechanism. The experimental data showed that the inhibiting effects of DAG on PL transfers were much more important than those predicted from our calculations. Additional data showed that a large part of DAG effects was in fact due to their ability to increase the viscosity of the particle PL surfaces, as calculated from electron spin resonance experiments. These results show that DAG can modulate the PLTP-dependent PL transfers, both by competition with PL and by increasing the viscosity of the particle surfaces. These findings might be physiopathologically relevant in situations where elevated plasma concentrations of DAG might result from hypertriglyceridemia.-Lalanne, F., C. Motta, Y. Pafumi, D. Lairon, and G. Ponsin. Modulation of the phospholipid transfer protein-mediated transfer of phospholipids by diacylglycerols. J. Lipid Res. 2001. 42: 142;-149.

Mechanism of the phospholipid transfer protein-mediated transfer of phospholipids from model lipid vesicles to high density lipoproteins

To study the effects of the phospholipid transfer protein (PLTP) on the thermodynamic parameters governing the transfer of phospholipids (PL) from single bilayer vesicles (SBV) to high density lipoprotein (HDL), we performed transfer measurements at various temperatures between 4 and 65 degrees C, using a pyrenylphosphatidylcholine (Pyr-PC) as probe. The proportion of excimer (E) to monomer (M) fluorescence of a pyrenyl moiety constitutes a direct measure of its local concentration. The transfers of Pyr-PC were monitored by following the decrease of E/M. The data were used to calculate the rate constants K(+1) for the transfer from SBV to HDL and to generate the corresponding Arrhenius plots. The equilibrium constants, K(eq), for the same reactions were also determined and used to generate Van't Hoff plots. From these data, we calculated the thermodynamic parameters for both the whole transfer reaction and the transition state. Both K(+1) and K(eq) values clearly varied with temperature. PLTP induced very similar decreases in the free energy for the whole reaction (DeltaG) and in that for the transition state (DeltaG(#)). At 37 degrees C, the decreases were of 0.37 and 0.29 kcal/mol, respectively. We studied the thermal denaturation of PLTP between 37 and 65 degrees C, and the effects of denatured PLTP samples on the PL transfer reaction were then determined. In all cases, the changes of DeltaG remained comparable to those of DeltaG(#). Thus the essential action of PLTP is to facilitate the first step of the reaction, which can be considered as the desorption of PL molecules from the surface of donor particles.

VLDL-bound lipoprotein lipase facilitates the cholesteryl ester transfer protein-mediated transfer of cholesteryl esters from HDL to VLDL

In recent years, it has been established that lipoprotein lipase (LPL) is partly associated with circulating lipoproteins. This report describes the effects of physiological amounts of very low density lipoprotein (VLDL)-bound LPL on the cholesteryl ester transfer protein (CETP)-mediated cholesteryl ester transfer (CET) from high density lipoprotein (HDL) to VLDL. Three patients with severe LPL deficiency exhibited a strong decrease in net mass CET that was more than 80% lower than that of common hypertriglyceridemic subjects. Recombination experiments showed that this was due to an abnormal behavior of the VLDL fraction. Replacement of the latter by normal VLDL totally normalized net mass CET. We therefore prepared VLDL containing controlled amounts of bound LPL that we used as CE acceptors in experiments involving unidirectional radioisotopic CET measurements. These were carried out either in the absence or in the presence of inhibitors of LPL lipolytic activity. When LPL-induced lipolysis was totally blocked, the stimulating effect of the enzyme on the CETP-dependent CET was only reduced by about 50%, showing that it did not entirely result from its lipolytic action. These data were dependent upon neither the type of LPL inhibitor (E600 or THL) nor the source of CETP (delipidated plasma or partially purified CETP). Thus, in addition to the well-known stimulating effect of LPL-dependent lipolysis on CET, our work demonstrates that physiological amounts of VLDL-bound LPL may facilitate CET through a mechanism partially independent of its lipolytic activity.

Distribution of diacylglycerols among plasma lipoproteins in control subjects and in patients with non-insulin-dependent diabetes

BACKGROUND

Diacylglycerols (DAGs), which are well-known components of insect lipophorins, have been recently recognized as a major glyceride of human high-density lipoprotein (HDL). Moreover, DAGs are good substrates for hepatic lipase and for the phospholipid transfer protein (PLTP). The present work was undertaken to determine the lipoprotein concentrations of DAGs, in control subjects, in non-insulin-dependent diabetic (NIDD) patients and in patients with severe hypertriglyceridaemia.

MATERIALS AND METHODS

Lipoproteins were isolated from 11 control subjects, 17 diabetic patients and three hypertriglyceridaemic patients, using a combination of ultracentrifugation and precipitation. After lipid extraction, DAGs were separated by thin-layer chromatography and quantified by a glyceride assay.

RESULTS

DAGs were detectable in all lipoprotein fractions of the three groups of subjects. Total DAGs were correlated with total triglycerides (TGs) and even more strikingly with very low-density lipoprotein triglycerides. Although the majority of DAG was recovered in apo B-containing lipoproteins, the proportion of DAG with respect to TG was most elevated in HDL.

CONCLUSION

These findings indicate that DAGs are probably formed from TG during lipolysis and that they can be transported to HDL through the action of PLTP. This raises the question whether DAG might act as an inhibitor of phospholipid transfer by competition for binding to PLTP.