Cholesterol is the major component of native lipoproteins activating the p38 mitogen-activated protein kinases

Elevated low-density lipoprotein (LDL) levels induce activation of the p38 mitogen-activated protein kinase (MAPK), a stress-activated protein kinase potentially participating in the development of atherosclerosis. The nature of the lipoprotein components inducing p38 MAPK activation has remained unclear however. We show here that both LDLs and high-density lipoproteins (HDLs) have the ability to stimulate the p38 MAPKs with potencies that correlate with their cholesterol content. Cholesterol solubilized in methyl-beta-cyclodextrin was sufficient to activate the p38 MAPK pathway. Liposomes made of phosphatidylcholine (PC) or sphingomyelin, the two main phospholipids found in lipoproteins, were unable to stimulate the p38 MAPKs. In contrast, PC liposomes loaded with cholesterol potently activated this pathway. Reducing the cholesterol content of LDL particles lowered their ability to activate the p38 MAPKs. Cell lines representative of the three main cell types found in blood vessels (endothelial cells, smooth muscle cells and fibroblasts) all activated their p38 MAPK pathway in response to LDLs or cholesterol-loaded PC liposomes. These results indicate that elevated cholesterol content in lipoproteins, as seen in hypercholesterolemia, favors the activation of the stress-activated p38 MAPK pathway in cells from the vessel wall, an event that might contribute to the development of atherosclerosis.     

High-density lipoproteins protect endothelial cells from tumor necrosis factor-alpha-induced apoptosis

High-density lipoproteins (HDL) levels have been shown to be inversely correlated with coronary heart disease, but the mechanisms of the direct protective effect of HDL on endothelial cells are not fully understood. The apoptosis of endothelial cells induced by cytokines and/or oxidized low-density lipoproteins, etc. may provide a mechanistic clue to the "response-to-injury" hypothesis of atherogenesis. Here we report that HDL prevent the apoptosis of human umbilical venous endothelial cells (HUVECs) induced by tumor necrosis factor-alpha (TNF-alpha) via an inhibition of CPP32-like protease activity. The incubation of HUVECs with TNF-alpha significantly increased the CPP32-like protease activity, and induced apoptosis. Preincubation of HUVECs with HDL before incubation with TNF-alpha significantly suppressed the increase in the CPP32-like protease activity, preventing apoptosis in a concentration-dependent manner. These results suggest that HDL prevent the suicide pathway leading to apoptosis of endothelial cells by decreasing the CPP32-like protease activity and that HDL thus play a protective role against the "response-to-injury" hypothesis of atherogenesis.     

Activated platelets contribute to oxidized low-density lipoproteins and dysfunctional high-density lipoproteins through a phospholipase A2-dependent mechanism

Plasma activity of secretory phospholipase A2 (sPLA2) increases in patients with cardiovascular disease. The present study investigated whether platelet-released sPLA2 induces low-density lipoprotein (LDL) and high-density lipoprotein (HDL) modifications that translate into changes in lipoprotein function. Activated but not resting platelets induced oxidative modifications of human native LDLs and HDLs, which render these particles dysfunctional. Platelet-incubated LDLs stimulated the incorporation of cholesterol oleate into macrophages, and modified HDLs lost their cholesterol efflux capacity and antioxidant properties. In vitro and ex vivo experiments showed that lysophophatidylcholine accumulated in the platelet-modified LDLs and HDLs of mice expressing sPLA2 (Balb/c and transgenic C57Bl/6 mice expressing human sPLA2) but not in the lipoproteins of naturally sPLA2-deficient mice (C57Bl/6). Unlike C57Bl/6 mice, Balb/c mice injected with leptin (67 μg/mouse, i.p.) as an in vivo prothrombotic agent displayed increased plasma sPLA2 activity, reduced clotting time, higher plasma levels of oxidation products, increased production of nonesterified fatty acids, and more substantial platelet-mediated modification of lipoproteins. These effects were blocked completely by injection of the platelet inhibitor ticlopidine (5 mg/kg, i.p.) or by a sPLA2 inhibitor (LY311727, 3 mg/kg, i.p.). These results demonstrate that stimulated platelets are major contributors to plasma sPLA2 activity in vivo and account to a large extent for the adverse modification of circulating lipoproteins.     

Distribution of oleoyl-estrone in rat plasma lipoproteins.

Pooled adult normal rat plasma was used for the separation of lipoprotein fractions: VLDL, LDL and HDL, from which a total lipids extract was obtained. The presence of fragments with the MW of estrone and oleoyl-estrone in the lipoprotein fractions was analyzed by HPLC-MS. The results show that oleoyl-estrone is the major estrone component in lipoproteins; this molecular species was present in all three lipoprotein lipid extracts. The lipoprotein fractions were used for the analysis of protein and lipid classes: triacylglycerols, total and esterified cholesterol and phospholipids as well as acyl-estrone. About half of the total acyl-estrone was in the HDL fraction and only about 10% in the VLDL fraction. HDLs contained about one molecule in 50 particles, LDLs one molecule per particle and VLDLs 15 molecules per particle, i.e. given their size, the larger lipoproteins contained more oleoyl-estrone than the HDLs. The distribution of this hormone suggests that oleoyl-estrone is lost with other lipids as the lipoproteins shrink. The results presented show that oleoyl-estrone is a molecule found naturally in rat lipoproteins in low concentrations - the lowest in HDLs - that are consistent with its postulated role in the control of body weight.     

HDLs inhibit endoplasmic reticulum stress and autophagic response induced by oxidized LDLs

The apoptotic effect of oxidized LDLs (oxLDLs) is mediated through a complex sequence of signaling events involving a deregulation of the cytosolic Ca(2+) homeostasis. OxLDLs also trigger ER stress that may lead to cellular dysfunction and apoptosis, through the activation of the IRE1α/c-Jun N-terminal kinase pathway. Moreover, ER stress and oxidized lipids have been shown to trigger autophagy. The antiatherogenic high-density lipoproteins (HDLs) display protective effects against oxLDLs toxicity. To more deeply investigate the mechanisms mediating the protective effects of HDLs, we examined whether ER stress and autophagy were implicated in oxLDLs-induced apoptosis and whether HDLs prevented these stress processes. We report that, in human endothelial cells, HDLs prevent the oxLDL-induced activation of the ER stress sensors IRE1α, eIF2α and ATF6 and subsequent activation of the proapoptotic mediators JNK and CHOP. OxLDLs also trigger the activation of autophagy, as assessed by LC3 processing and Beclin-1 expression. The autophagic process is independent of the proapoptotic arms of ER stress, but Beclin-1 contributes to PS exposure and subsequent phagocytosis of oxLDLs exposed cells. Induction of autophagy and PS exposure by oxLDLs is prevented by HDLs. Finally, the cytosolic Ca(2+) deregulation triggered by oxLDLs is a common signaling pathway that mediates ER stress-induced cell death and autophagy, all these events being blocked by HDLs.     

Modified LDLs induce proliferation-mediated death of human vascular endothelial cells through MAPK pathway

The ability of modified low-density lipoptoteins (LDLs) to induce both proliferation and death of endothelial cells is considered to be a mechanism of early atherosclerosis development. We previously showed that carbamylated LDL (cLDL) induces human coronary artery endothelial cell (HCAEC) death in vitro. This effect is similar to the atherogenic action of oxidized LDL (oxLDL) that induces the proliferation and death of endothelial cells. The present study was designed to analyze a potential proliferative effect of cLDL and whether proliferation caused by modified LDLs is related to cell death. Cultured HCAECs were exposed to different concentrations of modified LDL or native LDL for varying periods of time. Cell proliferation measured by bromodeoxyuridine incorporation and S-phase analysis was dose-dependently increased in the presence of cLDL (6.25-200 microg/ml). The proliferation induced by cLDL or oxLDL was associated with cell death and increased phosphorylation of extracellular signal-regulated kinase (ERK) and c-Jun NH(2)-terminal kinase (JNK). Inhibition of cLDL- or oxLDL-induced proliferation by aphidicolin (1 microg/ml) was protective against both short-term cell death measured by lactate dehydrogenase release into the medium and long-term cell viability visualized by cell multiplication. Inhibition of ERK phosphorylation led to a significant decrease of DNA synthesis and cell rescue from injury by modified LDLs, while inhibition of JNK phosphorylation had an only partial rescue effect without involvement in cell proliferation. These data are the first evidence that endothelial cell death induced by cLDL or oxLDL is mediated by cell proliferation through the mitogen-activated protein kinase pathway.     

Effect of human plasma lipoproteins on prostacyclin production by cultured endothelial cells

Prostacyclin (PGI2) production by bovine aortic or human umbilical vein endothelial cells increased when either human high density lipoproteins3 (HDL3) or low density lipoproteins (LDL) were added to a serum-free culture medium. At low concentrations and short incubation times, HDL3 produced more PGI2 than LDL, but LDL was just as effective as HDL3 in 18-hr incubations with high concentrations of lipoproteins. Neither lipoprotein was toxic to the cultures as assessed by [3H]leucine incorporation into cell protein. The stimulatory effect of HDL3 and LDL on PGI2 production decreased as growing cultures became confluent. Incubation with lipoproteins neither enhanced arachidonic acid release nor increased PGI2 formation when the cells were stimulated subsequently with ionophore A23187, indicating that the lipoproteins do not affect the intracellular processes involved in PGI2 production. The addition of albumin reduced the amount of PGI2 formation elicited by HDL3 or LDL. As compared with albumin-bound arachidonic acid, from 6- to 13-fold less PGI2 was produced during incubation with the lipoproteins. Furthermore, the amount of PGI2 formation elicited by the lipoproteins in 18 hr was 4-fold less than that produced during incubation with a fatty acid mixture containing only 5% arachidonic acid, and 3-fold less than when the cells were stimulated with the ionophore A23187 for 20 min. Taken together, our results indicate that human HDL and LDL contribute to endothelial PGI2 production only in a modest way and suggest that this process is not specific for either of these two plasma lipoproteins. In view of the greater participation of albumin-bound arachidonic acid in PGI2 production, plasma lipoproteins may not play as important a role in endothelial prostaglandin formation as has been suggested.     

Structure and motion of phospholipids in human plasma lipoproteins. A 31P NMR study

The structure and motion of phospholipids in human plasma lipoproteins have been studied by using 31P NMR. Lateral diffusion coefficients, DT, obtained from the viscosity dependence of the 31P NMR line widths, were obtained for very low density lipoprotein (VLDL), low-density lipoprotein (LDL), high-density lipoproteins (HDL2, HDL3), and egg PC/TO microemulsions at 25 degrees C, for VLDL at 40 degrees C, and for LDL at 45 degrees C. At 25 degrees C, the rate of lateral diffusion in LDL (DT = 1.4 x 10(-9) cm2/s) is an order of magnitude slower than in the HDLs (DT = 2 x 10(-8) cm2/s). At 45 degrees C, DT for LDL increases to 1.1 x 10(-8) cm2/s. In contrast, DT for VLDL increases only slightly going from 25 to 40 degrees C. The large increase in diffusion rate observed in LDL occurs over the same temperature range as the smectic to disordered phase transition of the core cholesteryl esters, and provides evidence for direct interactions between the monolayer and core. In order to prove the orientation and/or order of the phospholipid head-group, estimates of the residual chemical shift anistropy, delta sigma, have been obtained for all the lipoproteins and the microemulsions from the viscosity and field dependence of the 31P NMR line widths. For VLDL and LDL, the anisotropy is 47-50 ppm at 25 degrees C, in agreement with data from phospholipid bilayers. For the HDLs, however, significantly larger values of 69-75 ppm (HDL2) and greater than 120 ppm (HDL3) were obtained.(ABSTRACT TRUNCATED AT 250 WORDS)     

Free radical lipid oxidation affects cholesterol transfer between lipoproteins and erythrocytes

Human erythrocytes were incubated for 5 h at 37 degrees C with lipoproteins (LP), preliminary oxidized to different extent, as assessed by thiobarbituric acid (TBA) test. Cholesterol content in the cells was increased by 12-14% after incubation with low-density lipoproteins (LDL) along with augmentation of order parameter and rotational correlation time of spin-labeled stearic acids incorporated into membranes. If erythrocytes were incubated with oxidized LDL, containing 2.5-4 times more TBA-reactive material than native ones, cellular content of cholesterol was increased by 24-28%. In contrast, high-density lipoproteins (HDL2 and HDL3) removed cholesterol from cell membranes, when incubated with erythrocytes. This was followed by increased fluidity of membrane lipid phase as detected by the spin probe method. Oxidation of HDL2 and HDL3 decreased their ability to accept cholesterol from cell membranes. No detectable accumulation of TBA-reactive material was observed in the samples during the incubation. The antioxidant, butylated hydroxytoluene (BHT), in the concentration of 10(-5) M did not influence the cholesterol transfer between LP and erythrocytes. Hence, the effects of lipid peroxidation (LPO) on the cholesterol transfer seem to result from LP alterations by oxidation rather than from free radical reactions occurring during the incubation. By increasing cholesterol-donating ability of LDL and inhibition of cholesterol-accepting capacity of HDL lipid peroxidation in LP may activate cholesterol accumulation in blood vessel cells and thus contribute to atherosclerosis.     

Complementary use of counter-current chromatography and hydroxyapatite chromatography for the separation of three main classes of lipoproteins from human serum

High-density, low-density and very-low-density lipoproteins (HDLs, LDLs and VLDLs) were purified from human serum by the combined use of counter-current chromatography (CCC) and hydroxyapatite chromatography. Polymer-phase CCC of human serum using the cross-axis coil planet centrifuge yielded two lipoprotein fractions, one containing HDLs and LDLs and the other VLDLs and serum proteins. Each fraction was concentrated and subjected to hydroxyapatite chromatography to obtain three lipoprotein fractions, all free from serum proteins. Each lipoprotein was confirmed by agarose gel electrophoresis.     

Differential toxicities of air (mO-LDL) or copper-oxidized LDLs (Cu-LDL) toward endothelial cells.

In vivo low density protein (LDL) oxidation is a progressive phenomenon leading to the presence of minimally and highly oxidized LDLs in the subendothelial arterial space. Oxidized LDLs have been reported to be cytotoxic against endothelial cells. The goal of this study was to determine which of the minimally and highly oxidized LDLs were the most cytotoxic against bovine aortic endothelial cells (BAEC). Both the morphological aspect of the cells themselves, and LDH or MTT tests revealed that mO- or Cu-LDLs had similar cytotoxicity with up to 8 hours of oxidation, showing no relation with the level of LDL oxidation; for longer oxidation times, Cu-LDL cytotoxicity decreased. This phenomenon is linked to their different oxidation kinetics. Moreover, in the initial hours following BAEC incubation with mO- or Cu-LDLs, total cell glutathione dropped, whereas after 16 hours of incubation, highly oxidized Cu-LDL increased the glutathione level in the cell. The biphasic evolution of glutathione concentration corresponds to an autoprotective mechanism of cells against oxidized LDL cytotoxicity. This study suggests that the specific chemical characteristics of the different types of oxidized LDLs should always be precisely described in future assays devoted to studying the biological effects of what are known under the generic term as "oxidized LDLs". This precaution should prevent any confusion in interpreting different studies.     

Small,dense HDL 3 particles attenuate apoptosis in endothelial cells: pivotal role of apolipoprotein A‐I

Plasma high‐density lipoproteins (HDLs) protect endothelial cells against apoptosis induced by oxidized low‐density lipoprotein (oxLDL). The specific component(s) of HDLs implicated in such cytoprotection remain(s) to be identified. Human microvascular endothelial cells (HMEC‐1) were incubated with mildly oxLDL in the presence or absence of each of five physicochemically distinct HDL subpopulations fractionated from normolipidemic human plasma (n= 7) by isopycnic density gradient ultracentrifugation. All HDL subfractions protected HMEC‐1 against oxLDL‐induced primary apoptosis as revealed by nucleic acid staining, annexin V binding, quantitative DNA fragmentation, inhibition of caspase‐3 activity and reduction of cytoplasmic release of cytochrome c and apoptosis‐inducing factor. Small, dense HDL 3c displayed twofold superior intrinsic cytoprotective activity (as determined by mitochondrial dehydrogenase activity) relative to large, light HDL 2b on a per particle basis (P < 0.05). Equally, all HDL subfractions attenuated intracellular generation of reactive oxygen species (ROS); such anti‐oxidative activity diminished from HDL 3c to HDL 2b. The HDL protein moiety, in which apolipoprotein A‐I (apoA‐I) predominated, accounted for ～70% of HDL anti‐apoptotic activity. Furthermore, HDL reconstituted with apoA‐I, cholesterol and phospholipid potently protected HMEC‐1 from apoptosis. By contrast, modification of the content of sphingosine‐1‐phosphate in HDL did not significantly alter cytoprotection. We conclude that small, dense, lipid‐poor HDL 3 potently protects endothelial cells from primary apoptosis and intracellular ROS generation induced by mildly oxLDL, and that apoA‐I is pivotal to such protection.     

Quantitative comparison of lipoprotein fractions derived from human plasma and serum by liquid chromatography-tandem mass spectrometry

Background  

Lipoproteins are complex, globular molecules which play essential roles in the transport and metabolism of cholesterol. Their implication in the development of cardiovascular diseases, such as atherosclerosis, has sustained a great deal of interest in these particles. Their various functions, and their contribution to the development of atherosclerosis, are often attributed to their protein constituents, which vary greatly among the different lipoprotein classes. Recent advances in the field of mass spectrometry have provided insight into the array of proteins associated with low-density lipoproteins (LDLs) and, even more so, with high-density lipoproteins (HDLs). Plasma and serum are the most commonly used samples for the analysis of lipoproteins. Although these lipoprotein sources are unique, it was our goal to determine whether or not their inherent differences would ultimately affect a quantitative analysis of the LDL and HDL proteomes. To this end, we isolated LDL and HDL fractions with fast protein liquid chromatography-size exclusion chromatography (FPLC-SEC) from both human plasma and serum samples from the same individuals. After delipidating these samples, we performed a quantitative proteomic analysis to compare the lipoprotein-associated proteins derived from both plasma and serum.     

Psoriasis decreases the anti-oxidation and anti-inflammation properties of high-density lipoprotein

Psoriasis is a chronic inflammatory skin disease, which has been linked to dyslipidemia with potential functional impairment of lipoproteins. This cross-sectional study was designed to characterize the biological activities of plasma lipoproteins in 25 patients with psoriasis and 25 age- and sex-matched healthy controls.In the present study, we found that plasma levels of high-density lipoprotein (HDL) cholesterol were decreased in the psoriasis group compared to healthy controls. The malondialdehyde (MDA) content in plasma, in HDL3 and in low-density lipoprotein (LDL) were increased. However, the activity of plasma paraoxonase-1 (PON-1) decreased in psoriasis and negatively correlated with the psoriasis area and severity index (PASI). Moreover, plasma levels of tumor necrosis factor-α (TNF-α) and interleukin-6 (IL-6) were increased in psoriasis and positively correlated with the PASI. High-sensitivity C-reactive protein (hs-CRP) was increased in psoriasis, but did not reach significance when correlated with PASI. In vitro tests displayed that the functionalities of HDL3 isolated from psoriatic patients significantly decreased, which were assessed in four independent ways, namely (1) protection against LDL oxidation, (2) inhibition of tumor necrosis factor-α (TNF-α) induced monocyte adherence to endothelial cells, (3) prevention of oxidized low density lipoprotein (ox-LDL) induced monocyte migration, and (4) protection of endothelial cells from TNF-α induced apoptosis. Further, pro-oxidative and pro-inflammatory properties of LDL isolated from psoriatic patients were increased. In conclusion, the biological activities of psoriatic lipoproteins are impaired in both HDL and LDL, which may provide a link between psoriasis and cardiovascular disease.     

Cell biology and lipoproteins in atherosclerosis

Atherosclerosis is an inflammatory process, triggered by the presence of lipids in the vascular wall, and encompasses a complex interaction among inflammatory cells, vascular elements, and lipoproteins through expression of several adhesion molecules and cytokines. Subendothelial retention of lipoproteins is the key initiating event in atherosclerosis, provoking a cascade of events to pathogenic response. High levels of plasma lipids, particularly low-density (LDL) and very-low-density lipoproteins (VLDL) are among the pathophysiologic stimuli that induce endothelial dysfunction. Endothelial cells regulate coagulation, thrombosis and the fibrinolytic system; the endothelium modulates the activity of smooth muscle cells (vascular tone/proliferation) and controls the traffic of macromolecules and inflammatory cells to the vessel wall. Furthermore, LDLs have been implicated in the induction of changes in permeability, cell adhesion and secretion of vasoactive molecules (nitric oxide [NO]), while VLDLs seem to modulate the fibrinolytic system [tissue plasminogen activator (TPA) and plasminogen activator inhibitor-1 (PAI-1)]. In this review, we will focus on the pathophysiologic functions of lipoproteins in the vascular wall.     

Formation of methionine sulfoxide-containing specific forms of oxidized high-density lipoproteins

Atherosclerosis is characterized by the accumulation of both lipoprotein-derived lipids and inflammatory cells in the affected vascular wall that results in a state of heightened oxidative stress and that is reflected by the accumulation of oxidized lipoproteins. Circulating oxidized low-density lipoprotein (oxLDL) is used as a surrogate marker for coronary artery disease, although the 'escape' of oxLDL from the vessel wall is hindered by the large size of this lipoprotein and its specific retention by the extracellular matrix. Also, the oxidation of lipoproteins in human atherosclerotic lesions is not limited to LDL. In fact, the lipids of all classes of lipoproteins are oxidized to a comparable extent. Examining the fate of lipid hydroperoxides, the primary lipid peroxidation products, in high-density lipoproteins (HDL) undergoing oxidation, revealed that they become reduced to the corresponding alcohols by specific Met residues of apolipoprotein A-I (apoA-I) and apoA-II. As a consequence, Met residues in apoA-I and apoA-II become selectively and consecutively oxidized to their respective Met sulfoxide (MetO) forms that can be separated by HPLC. This review describes the characterization of specifically oxidized HDL with an emphasis on MetO formation, the structural and functional consequences of such oxidation, and the potential utility of specifically oxidized HDL as a surrogate marker of atherosclerosis.     

Secretion of ferritin by iron-laden macrophages and influence of lipoproteins

Increasing evidence supports a role of cellular iron in the initiation and development of atherosclerosis. We and others reported earlier that iron-laden macrophages are associated with LDL oxidation, angiogenesis, nitric oxide production and apoptosis in atherosclerotic processes. Here we have further studied perturbed iron metabolism in macrophages, their interaction with lipoproteins and the origin of iron accumulation in human atheroma. In both early and advanced human atheroma lesions, hemoglobin and ferritin accumulation correlated with the macrophage-rich areas. Iron uptake into macrophages, via transferrin receptors or scavenger receptor-mediated erythrophagocytosis, increased cellular iron and accelerated ferritin synthesis at both mRNA and protein levels. The binding activity of iron regulatory proteins was enhanced by desferrioxamine (DFO) and decreased by hemin and iron compounds. Iron-laden macrophages exocytosed both iron and ferritin into the culture medium. Exposure to oxidized low-density lipoprotein (oxLDL, >or=50 microg/mL) resulted in <20% apoptosis of iron-laden human macrophages, but cells remained impermeable after a 24 h period and an increased excretion of ferritin could be observed by immunostaining techniques. Exposure to high-density lipoprotein (HDL) significantly decreased ferritin excretion from these cells. We conclude: (i) erythrophagocytosis and hemoglobin catabolism by macrophages contribute to ferritin accumulation in human atherosclerotic lesions and; (ii) iron uptake into macrophages leads to increased synthesis and secretion of ferritin; (iii) oxidized LDL and HDL have different effects on these processes.     

Low-density lipoprotein inhibits secretion of phospholipid transfer protein in human trophoblastic BeWo cells

Human plasma phospholipid transfer protein (PLTP) plays an important role in lipoprotein metabolism. In this study, we investigated the effects of lipoproteins on the secretion of PLTP in cultured BeWo choriocarcinoma cells. Low-density lipoproteins (LDLs) decreased PLTP secretion in a dose- and time-dependent manner, whereas very low density lipoproteins and high-density lipoproteins (HDLs) had little effect. LDL suppression of PLTP secretion was not altered by the inhibition of both LDL receptor and LDL receptor-related protein with receptor-associated protein. Mitogen-activated protein kinase (MAPK) kinase (MEK) inhibitor, U0126, could abolish the LDL-mediated inhibition of PLTP secretion. Furthermore, LDL, but not HDL, could stimulate the expression of MAPK phosphatase-1 (MKP-1) in BeWo cells that resulted in the inactivation of p44/p42 extracellular signal-regulated kinase (ERK) 1 and 2, the family members of MAPKs. These results support the conclusion that LDL-mediated suppression of PLTP secretion in BeWo cells is through a LDL receptor-independent MAPK signaling pathway.     

Insulin-secreting beta-cell dysfunction induced by human lipoproteins

Diabetes is associated with significant changes in plasma concentrations of lipoproteins. We tested the hypothesis that lipoproteins modulate the function and survival of insulin-secreting cells. We first detected the presence of several receptors that participate in the binding and processing of plasma lipoproteins and confirmed the internalization of fluorescent low density lipoprotein (LDL) and high density lipoprotein (HDL) particles in insulin-secreting beta-cells. Purified human very low density lipoprotein (VLDL) and LDL particles reduced insulin mRNA levels and beta-cell proliferation and induced a dose-dependent increase in the rate of apoptosis. In mice lacking the LDL receptor, islets showed a dramatic decrease in LDL uptake and were partially resistant to apoptosis caused by LDL. VLDL-induced apoptosis of beta-cells involved caspase-3 cleavage and reduction in the levels of the c-Jun N-terminal kinase-interacting protein-1. In contrast, the proapoptotic signaling of lipoproteins was antagonized by HDL particles or by a small peptide inhibitor of c-Jun N-terminal kinase. The protective effects of HDL were mediated, in part, by inhibition of caspase-3 cleavage and activation of Akt/protein kinase B. In conclusion, human lipoproteins are critical regulators of beta-cell survival and may therefore contribute to the beta-cell dysfunction observed during the development of type 2 diabetes.