Estrogen receptor-alpha mediates gender differences in atherosclerosis induced by HIV protease inhibitors

As part of highly active antiretroviral therapy, protease inhibitor treatment has significantly increased the lifespan of human immunodeficiency virus (HIV)-infected individuals. Many patients, however, develop negative side effects, including premature atherosclerosis. We have previously demonstrated that in male low density lipoprotein receptor (LDL-R) null mice, HIV protease inhibitors induce atherosclerotic lesions and cholesterol accumulation in macrophages in the absence of changes in plasma lipid levels. We determined that these increases were due to an up-regulation of the scavenger receptor, CD36. In the present study, we examined the effects of HIV protease inhibitors in female LDL-R null mice. Female mice given ritonavir and amprenavir (23 and 10 microg/mouse/day, respectively) developed fewer atherosclerotic lesions than males. Furthermore, peritoneal macrophages isolated from ritonavir-treated females had reduced levels of cholesterol accumulation as compared with males, and CD36 protein levels were increased to a significantly lesser degree in females than in males. To investigate the molecular mechanisms of this gender difference, we examined the effect of genetically removing estrogen receptor-alpha (ERalpha). In female mice lacking both LDL-R and ERalpha, the protective effect of gender was lost. Additionally, the reduced levels of cholesterol accumulation in macrophages observed in females was reversed. Furthermore, the absence of ERalpha resulted in increased expression of CD36 protein in a macrophage-specific manner in mice treated with ritonavir. These data demonstrate that ERalpha is directly involved in the regulation of cholesterol metabolism in macrophages and plays an important role in the gender differences observed in HIV protease inhibitor-induced atherosclerosis.     

HIV protease inhibitors-induced atherosclerosis: prevention by alpha-tocopherol

Prolonged treatments with inhibitors of human immunodeficiency(HIV)-encoded protease (ARPI) have been reported to induce early atherosclerotic events. Our in vitro study indicates that alpha-tocopherol may prevent drug-induced premature atherosclerosis since it interferes with CD36 scavenger receptor over-expression induced by ritonavir in monocytes. The mechanism of CD36 upregulation by ritonavir involves inhibition of the ubiquitin-proteasome system and alpha-tocopherol is able to normalize proteasome activity. These findings suggest that ARPI combined with early alpha-tocopherol supplementation may decrease the drug-induced atherosclerotic risk.     

How HIV protease inhibitors promote atherosclerotic lesion formation

PURPOSE OF REVIEW: One of the aims of this review is to summarize recent clinical approaches used to determine the role of HIV protease inhibitors in the development of cardiovascular disease. Another aim is to discuss possible molecular mechanisms whereby HIV protease inhibitors may promote atherogenesis. RECENT FINDINGS: Several clinical studies have recently used ultrasonography to demonstrate increased intimal medial thickness and alterations in the structural characteristics of epi-aortic lesions in patients receiving HIV protease inhibitors. Molecular studies have indicated that several mechanisms are likely involved in mediating the effects of protease inhibitors. Possible mechanisms include inhibition of the proteasome, increased CD36 expression in macrophage, inhibition of lipoprotein lipase-mediated lipolysis, decreased adiponectin levels, and dysregulation of the NF-kappaB pathway. SUMMARY: The currently available data strongly suggest that HIV protease inhibitors negatively impact the cardiovascular system. As is often the case with complex diseases like atherosclerosis it appears that HIV protease inhibitors affect the cardiovascular system through several distinct mechanisms by affecting various components of the arterial wall directly or indirectly by influencing lipoprotein and glucose metabolism of the body.     

Modulation of the LDL receptor and LRP levels by HIV protease inhibitors

Inhibitors of the human immunodeficiency virus (HIV)-1 protease have proven to be effective antiretroviral drugs. However, patients receiving these drugs develop serious metabolic abnormalities, including hypercholesterolemia. The objective of the present study was to identify mechanisms by which HIV protease inhibitors increase plasma cholesterol levels. We hypothesized that HIV protease inhibitors may affect gene regulation of certain LDL receptor (LDLR) family members, thereby altering the catabolism of cholesterol-containing lipoproteins. In this present study we investigated the effect of several HIV protease inhibitors (ABT-378, Amprenavir, Indinavir, Nelfinavir, Ritonavir, and Saquinavir) on mRNA, protein, and functional levels of LDLR family members. Our results demonstrate that one of these drugs, Nelfinavir, significantly decreases LDLR and LDLR-related protein (LRP) mRNA and protein levels, resulting in the reduced functional activity of these two receptors. Nelfinavir exerts its effect by reducing levels of active SREBP1 in the nucleus. The finding that Nelfinavir reduces the levels of two key receptors (LRP and LDLR) involved in lipoprotein catabolism and maintenance of vessel wall integrity identifies a mechanism that causes hypercholesterolemia complications in HIV patients treated with this drug and raises concerns about the atherogenic nature of Nelfinavir.     

Antagonistic effects of oxidized low density lipoprotein and alpha-tocopherol on CD36 scavenger receptor expression in monocytes: involvement of protein kinase B and peroxisome proliferator-activated receptor-gamma

Vitamin E deficiency increases expression of the CD36 scavenger receptor, suggesting specific molecular mechanisms and signaling pathways modulated by alpha-tocopherol. We show here that alpha-tocopherol down-regulated CD36 expression (mRNA and protein) in oxidized low density lipoprotein (oxLDL)-stimulated THP-1 monocytes, but not in unstimulated cells. Furthermore, alpha-tocopherol treatment of monocytes led to reduction of fluorescent oxLDL-3,3'-dioctadecyloxacarbocyanine perchlorate binding and uptake. Protein kinase C (PKC) appears not to be involved because neither activation of PKC by phorbol 12-myristate 13-acetate nor inhibition by PKC412 was affected by alpha-tocopherol. However, alpha-tocopherol could partially prevent CD36 induction after stimulation with a specific agonist of peroxisome proliferator-activated receptor-gamma (PPARgamma; troglitazone), indicating that this pathway is susceptible to alpha-tocopherol action. Phosphorylation of protein kinase B (PKB) at Ser473 was increased by oxLDL, and alpha-tocopherol could prevent this event. Expression of PKB stimulated the CD36 promoter as well as a PPARgamma element-driven reporter gene, whereas an inactive PKB mutant had no effect. Moreover, coexpression of PPARgamma and PKB led to additive induction of CD36 expression. Altogether, our results support the existence of PKB/PPARgamma signaling pathways that mediate CD36 expression in response to oxLDL. The activation of CD36 expression by PKB suggests that both lipid biosynthesis and fatty acid uptake are stimulated by PKB.     

Genetic deletion of low density lipoprotein receptor impairs sterol-induced mouse macrophage ABCA1 expression. A new SREBP1-dependent mechanism

Low density lipoprotein receptor (LDLR) mutations cause familial hypercholesterolemia and early atherosclerosis. ABCA1 facilitates free cholesterol efflux from peripheral tissues. We investigated the effects of LDLR deletion (LDLR(-/-)) on ABCA1 expression. LDLR(-/-) macrophages had reduced basal levels of ABCA1, ABCG1, and cholesterol efflux. A high fat diet increased cholesterol in LDLR(-/-) macrophages but not wild type cells. A liver X receptor (LXR) agonist induced expression of ABCA1, ABCG1, and cholesterol efflux in both LDLR(-/-) and wild type macrophages, whereas expression of LXRalpha or LXRbeta was similar. Interestingly, oxidized LDL induced more ABCA1 in wild type macrophages than LDLR(-/-) cells. LDL induced ABCA1 expression in wild type cells but inhibited it in LDLR(-/-) macrophages in a concentration-dependent manner. However, lipoproteins regulated ABCG1 expression similarly in LDLR(-/-) and wild type macrophages. Cholesterol or oxysterols induced ABCA1 expression in wild type macrophages but had little or inhibitory effects on ABCA1 expression in LDLR(-/-) macrophages. Active sterol regulatory element-binding protein 1a (SREBP1a) inhibited ABCA1 promoter activity in an LXRE-dependent manner and decreased both macrophage ABCA1 expression and cholesterol efflux. Expression of ABCA1 in animal tissues was inversely correlated to active SREBP1. Oxysterols inactivated SREBP1 in wild type macrophages but not in LDLR(-/-) cells. Oxysterol synergized with nonsteroid LXR ligand induced ABCA1 expression in wild type macrophages but blocked induction in LDLR(-/-) cells. Taken together, our studies suggest that LDLR is critical in the regulation of cholesterol efflux and ABCA1 expression in macrophage. Lack of the LDLR impairs sterol-induced macrophage ABCA1 expression by a sterol regulatory element-binding protein 1-dependent mechanism that can result in reduced cholesterol efflux and lipid accumulation in macrophages under hypercholesterolemic conditions.     

Ritonavir stimulates foam cell formation by activating PKC

Alpha-subtype protein kinase C (PKCα) is closely related to cardiovascular disease. Ritonavir (RTV), which is a human immunodeficiency virus (HIV) protease inhibitor, can induce atherosclerosis in a PKC-dependent manner. However, it remains unclear how RTV acts on PKCα to induce pathological phenotypes. In this study, we obtained mouse peritoneal macrophages from adult female Kunmin mice. The results of Oil Red O staining and immunofluorescence using confocal laser scanning microscope demonstrated that RTV could induce foam cell formation and plasma membrane translocation of PKCα like phorbol-12-myristate-13-acetate (PMA, a PKC activator). Computational modeling also exhibited similar docking of RTV and PMA to PKCα and similar patterns of hydrophobic interaction and hydrogen bond formation. Further in vitro kinase activity studies revealed that RTV could elevate PKC activity. These data provided insight into the PKC-dependent induction of atherosclerosis and useful information for more in-depth toxicity research of HIV protease inhibitor (PI). In addition, western blot analysis proved RTV also up-regulate PKCα expression, which may be related to its influence on estrogen responsiveness in target cells and needs further prove.     

Inhibition of the NLRP3 inflammasome attenuates foam cell formation of THP-1 macrophages by suppressing ox-LDL uptake and promoting cholesterol efflux

The NOD-like receptor family, pyrin domain–containing protein 3 (NLRP3) inflammasome plays an important role in the development of atherosclerosis. The activated NLRP3 inflammasome has been reported to promote macrophage foam cell formation, but not all studies have obtained the same result, and how NLRP3 inflammasome is involved in the formation of foam cells remains elusive. We used selective NLRP3 inflammasome inhibitors and NLRP3-deficient THP-1 cells to assess the effect of NLRP3 inflammasome inhibition on macrophage foam cell formation, oxidized low-density lipoprotein (ox-LDL) uptake, esterification, and cholesterol efflux, as well as the expression of associated proteins. Inhibition of the NLRP3 inflammasome attenuated foam cell formation, diminished ox-LDL uptake, and promoted cholesterol efflux from THP-1 macrophages. Moreover, it downregulated CD36, acyl coenzyme A: cholesterol acyltransferase-1 and neutral cholesterol ester hydrolase expression; upregulated ATP-binding cassette transporter A1 (ABCA1) and scavenger receptor class B type I (SR-BI) expression; but had no effect on the expression of scavenger receptor class A and ATP-binding cassette transporter G1. Collectively, our findings show that inhibition of the NLRP3 inflammasome decreases foam cell formation of THP-1 macrophages via suppression of ox-LDL uptake and enhancement of cholesterol efflux, which may be due to downregulation of CD36 expression and upregulation of ABCA1 and SR-BI expression, respectively.     

Absence of ACAT-1 attenuates atherosclerosis but causes dry eye and cutaneous xanthomatosis in mice with congenital hyperlipidemia

Acyl-CoA:cholesterol acyltransferase (ACAT) catalyzes esterification of cellular cholesterol. To investigate the role of ACAT-1 in atherosclerosis, we have generated ACAT-1 null (ACAT-1-/-) mice. ACAT activities were present in the liver and intestine but were completely absent in adrenal, testes, ovaries, and peritoneal macrophages in our ACAT-1-/- mice. The ACAT-1-/- mice had decreased openings of the eyes because of atrophy of the meibomian glands, a modified form of sebaceous glands normally expressing high ACAT activities. This phenotype is similar to dry eye syndrome in humans. To determine the role of ACAT-1 in atherogenesis, we crossed the ACAT-1-/- mice with mice lacking apolipoprotein (apo) E or the low density lipoprotein receptor (LDLR), hyperlipidemic models susceptible to atherosclerosis. High fat feeding resulted in extensive cutaneous xanthomatosis with loss of hair in both ACAT-1-/-:apo E-/- and ACAT-1-/-:LDLR-/- mice. Free cholesterol content was significantly increased in their skin. Aortic fatty streak lesion size as well as cholesteryl ester content were moderately reduced in both double mutant mice compared with their respective controls. These results indicate that the local inhibition of ACAT activity in tissue macrophages is protective against cholesteryl ester accumulation but causes cutaneous xanthomatosis in mice that lack apo E or LDLR.     

HIV protease inhibitor induces fatty acid and sterol biosynthesis in liver and adipose tissues due to the accumulation of activated sterol regulatory element-binding proteins in the nucleus

The mechanism by which human immunodeficiency virus (HIV) protease inhibitor therapy adversely induces lipodystrophy and hyperlipidemia has not been defined. This study explored the mechanism associated with the adverse effects of the prototype protease inhibitor ritonavir in mice. Ritonavir treatment increased plasma triglyceride and cholesterol levels through increased fatty acid and cholesterol biosynthesis in adipose and liver. Ritonavir treatment also resulted in hepatic steatosis and hepatomegaly. These abnormalities, which were especially pronounced after feeding a Western type high fat diet, were due to ritonavir-induced accumulation of the activated forms of sterol regulatory binding protein (SREBP)-1 and -2 in the nucleus of liver and adipose, resulting in elevated expression of lipid metabolism genes. Interestingly, protease inhibitor treatment did not alter SREBP mRNA levels in these tissues. Thus, the adverse lipid abnormalities associated with protease inhibitor therapy are caused by the constitutive induction of lipid biosynthesis in liver and adipose tissues due to the accumulation of activated SREBP in the nucleus.     

Endoplasmic reticulum stress controls M2 macrophage differentiation and foam cell formation

Macrophages are essential in atherosclerosis progression, but regulation of the M1 versus M2 phenotype and their role in cholesterol deposition are unclear. We demonstrate that endoplasmic reticulum (ER) stress is a key regulator of macrophage differentiation and cholesterol deposition. Macrophages from diabetic patients were classically or alternatively stimulated and then exposed to oxidized LDL. Alternative stimulation into M2 macrophages lead to increased foam cell formation by inducing scavenger receptor CD36 and SR-A1 expression. ER stress induced by alternative stimulation was necessary to generate the M2 phenotype through JNK activation and increased PPARγ expression. The absence of CD36 or SR-A1 signaling independently of modified cholesterol uptake decreased ER stress and prevented the M2 differentiation typically induced by alternative stimulation. Moreover, suppression of ER stress shifted differentiated M2 macrophages toward an M1 phenotype and subsequently suppressed foam cell formation by increasing HDL- and apoA-1-induced cholesterol efflux indicating suppression of macrophage ER stress as a potential therapy for atherosclerosis.     

Atherogenic diet-induced bone loss is primarily due to increased osteoclastogenesis in mice

Atherogenic diet (AD) decreased bone density and increased serum cholesterol level in male mice, implying that cholesterol participates in bone loss. The aim of the present study was to identify the cells responsible for bone loss and evaluate the involved mechanism. AD resulted in increased number and surface of osteoclasts (OCs) with in vivo tartrate-resistant acid phosphatase (TRAP) staining, suggesting a critical role of OCs in cholesterol-induced bone loss. In vitro, cholesterol loading by oxidized low-density lipoprotein (oxLDL) increased the size and number of OCs as well as bone resorption activity, suggesting that cholesterol loading affects the number and activity of OCs. In contrast, cholesterol depletion by simvastatin decreased osteoclastogenesis and bone resorption. oxLDL stimulated osteoblasts (OBs) to increase expression of receptor activator of nuclear factor kappa-Β ligand (RANKL), resulting in increased OC formation when OBs were co-cultured with bone marrow derived macrophages. oxLDL increased expression of CD36 and liver X receptors (LXRα) in OCs as well as low density lipoprotein receptor (LDLR) and LXRα in OBs. These results suggest that CD36 and LXRα mediate the effect of oxLDL in OCs, whereas LDLR and LXRα mediate the effect of oxLDL in OBs. These findings demonstrate cholesterol-induced bone loss with increasing number and activity of OCs in mice, suggesting another harmful effect of cholesterol, a major cause of atherosclerosis.     

CD9 tetraspanin interacts with CD36 on the surface of macrophages: a possible regulatory influence on uptake of oxidized low density lipoprotein

CD36 is a type 2 scavenger receptor with multiple functions. CD36 binding to oxidized LDL triggers signaling cascades that are required for macrophage foam cell formation, but the mechanisms by which CD36 signals remain incompletely understood. Mass spectrometry analysis of anti-CD36 immuno-precipitates from macrophages identified the tetraspanin CD9 as a CD36 interacting protein. Western blot showed that CD9 was precipitated from mouse macrophages by anti-CD36 monoclonal antibody and CD36 was likewise precipitated by anti-CD9, confirming the mass spectrometry results. Macrophages from cd36 null mice were used to demonstrate specificity. Membrane associations of the two proteins on intact cells was analyzed by confocal immunofluorescence microscopy and by a novel cross linking assay that detects proteins in close proximity (<40 nm). Functional significance was determined by assessing lipid accumulation, foam cell formation and JNK activation in wt, cd9 null and cd36 null macrophages exposed to oxLDL. OxLDL uptake, lipid accumulation, foam cell formation, and JNK phosphorylation were partially impaired in cd9 null macrophages. The present study demonstrates that CD9 associates with CD36 on the macrophage surface and may participate in macrophage signaling in response to oxidized LDL.     

Lysosomal cholesterol accumulation in macrophages leading to coronary atherosclerosis in CD38−/− mice

The disruption in transportation of oxLDL‐derived cholesterol and the subsequent lipid accumulation in macrophages are the hallmark events in atherogenesis. Our recent studies demonstrated that lysosomal Ca²⁺ messenger of nicotinic acid adenine dinucleotide phosphate (NAADP), an enzymatic product of CD38 ADP‐ribosylcyclase (CD38), promoted lipid endocytic trafficking in human fibroblast cells. The current studies are designed to examine the functional role of CD38/NAADP pathway in the regulation of lysosomal cholesterol efflux in atherosclerosis. Oil red O staining showed that oxLDL concentration‐dependently increased lipid buildup in bone marrow‐derived macrophages from both wild type and CD38^?/?, but to a significant higher extent with CD38 gene deletion. Bodipy 493/503 fluorescence staining found that the deposited lipid in macrophages was mainly enclosed in lysosomal organelles and largely enhanced with the blockade of CD38/NAADP pathway. Filipin staining and direct measurement of lysosome fraction further revealed that the free cholesterol constituted a major portion of the total cholesterol segregated in lysosomes. Moreover, in situ assay disclosed that both lysosomal lumen acidity and the acid lipase activity were reduced upon cholesterol buildup in lysosomes. In CD38^?/? mice, treatment with Western diet (12 weeks) produced atherosclerotic damage in coronary artery with striking lysosomal cholesterol sequestration in macrophages. These data provide the first experimental evidence that the proper function of CD38/NAADP pathway plays an essential role in promoting free cholesterol efflux from lysosomes and that a defection of this signalling leads to lysosomal cholesterol accumulation in macrophages and results in coronary atherosclerosis in CD38^?/? mice.     

Resistin increases lipid accumulation and CD36 expression in human macrophages

Excessive lipid accumulation in macrophages plays an important role in the development of atherosclerosis. Recently, several studies have implied that resistin, an adipocytokine which is mainly expressed in human peripheral blood monocytes, may take part in the pathogenesis of atherosclerosis. In this study, we investigated the effects of resistin on lipid accumulation as well as oxLDL on resistin expression in human macrophages. Treatment of macrophages with oxLDL significantly increased resistin mRNA expression, whereas native LDL had no such effect. Resistin pre-treated macrophages contained more and larger lipid droplets stained by Nile red. Resistin increased the expression of CD36 at both mRNA and protein levels, without affecting those of class A macrophage scavenger receptor (SR-A). These results suggest that resistin promotes lipid accumulation in human macrophages through its upregulating CD36 cell surface expression. Also, it is suggested that resistin may act as a modulator for macrophage-to-foam cell transformation.