[Show abstract][Hide abstract] ABSTRACT: Insulin resistance is closely related to inflammatory stress and the mammalian target of rapamycin/S6 kinase (mTOR/S6K) pathway. The present study investigated whether rapamycin, a specific inhibitor of mTOR, ameliorates inflammatory stress-induced insulin resistance in vitro and in vivo. We used tumor necrosis factor-alpha (TNF-α) and interleukin-6 (IL-6) stimulation in HepG2 hepatocytes, C2C12 myoblasts and 3T3-L1 adipocytes and casein injection in C57BL/6J mice to induce inflammatory stress. Our results showed that inflammatory stress impairs insulin signaling by reducing the expression of total IRS-1, p-IRS-1 (tyr632), and p-AKT (ser473); it also activates the mTOR/S6K signaling pathway both in vitro and in vivo. In vitro, rapamycin treatment reversed inflammatory cytokine-stimulated IRS-1 serine phosphorylation, increased insulin signaling to AKT and enhanced glucose utilization. In vivo, rapamycin treatment also ameliorated the impaired insulin signaling induced by inflammatory stress, but it induced pancreatic β-cell apoptosis, reduced pancreatic β-cell function and enhanced hepatic gluconeogenesis, thereby resulting in hyperglycemia and glucose intolerance in casein-injected mice. Our results indicate a paradoxical effect of rapamycin on insulin resistance between the in vitro and in vivo environments under inflammatory stress and provide additional insight into the clinical application of rapamycin.
[Show abstract][Hide abstract] ABSTRACT: The prevalence of nonalcoholic fatty liver disease (NAFLD) increases with increasing body mass index (BMI). However, approximately 40-50% of obese adults do not develop hepatic steatosis. The level of inflammatory biomarkers is higher in obese subjects with NAFLD compared to BMI-matched subjects without hepatic steatosis. We used a casein injection in high-fat diet (HFD)-fed C57BL/6J mice to induce inflammatory stress. Although mice on a HFD exhibited apparent phenotypes of obesity and hyperlipidemia regardless of exposure to casein injection, only the HFD+Casein mice showed increased hepatic vacuolar degeneration accompanied with elevated inflammatory cytokines in the liver and serum, compared to mice on a normal chow diet. The expression of genes related to hepatic fatty acid synthesis and oxidation were upregulated in the HFD-only mice. The casein injection further increased baseline levels of lipogenic genes and decreased the levels of oxidative genes in HFD-only mice. Inflammatory stress induced both oxidative stress and endoplasmic reticulum stress in HFD-fed mice livers. We conclude that chronic inflammation precedes hepatic steatosis by disrupting the balance between fatty acid synthesis and oxidation in the livers of HFD-fed obese mice. This mechanism may operate in obese individuals with chronic inflammation, thus making them more prone to NAFLD.
Full-text · Article · May 2015 · Scientific Reports
[Show abstract][Hide abstract] ABSTRACT: Inflammation and lipids play significant roles in the progression of chronic kidney disease. This study was designed to investigate whether inflammation disrupts cellular cholesterol homeostasis and causes the lipid nephrotoxicity in vitro and in vivo, and explored its underlying mechanisms. Inflammatory stress was induced by cytokines (interleukin-1β (IL-1β); tumor necrosis factor α (TNF-α)) to human mesangial cells (HMCs) in vitro and by subcutaneous casein injection in C57BL/6J mice in vivo. The data showed that inflammatory stress exacerbated renal cholesterol ester accumulation in vitro and in vivo. Inflammation increased cellular cholesterol uptake and synthesis via upregulating the expression of low-density lipoprotein receptor (LDLr) and 3-hydroxy-3-methylglutaryl-coenzyme A reductase (HMGCoA-R), while it decreased cholesterol efflux via downregulating the expression of liver X receptor alpha and ATP-binding cassette transporter A1. The increased lipid accumulation by inflammatory stress induced reactive oxygen species (ROS) and increased levels of endoplasmic reticulum (ER) stress markers (inositol-requiring protein 1 and activating transcription factor 6) in HMCs and kidneys of C57BL/6J mice. This study implied that inflammation promoted renal lipid accumulation and foam cell formation by disrupting cellular cholesterol homeostasis. Increased intracellular lipids under inflammatory stress caused oxidative stress and ER stress in vitro and in vivo which may contribute to renal injury and progression of chronic kidney disease.
[Show abstract][Hide abstract] ABSTRACT: Inflammatory stress is an independent risk factor for the development of non-alcoholic fatty liver disease (NAFLD). Although CD36 is known to facilitate long-chain fatty acid uptake and contributes to NAFLD progression, the mechanisms that link inflammatory stress to hepatic CD36 expression and steatosis remain unclear. As the mammalian target of rapamycin (mTOR) signalling pathway is involved in CD36 translational activation, this study was undertaken to investigate whether inflammatory stress enhances hepatic CD36 expression via mTOR signalling pathway and the underlying mechanisms. To induce inflammatory stress, we used tumour necrosis factor alpha (TNF-α) and interleukin-6 (IL-6) stimulation of the human hepatoblastoma HepG2 cells in vitro and casein injection in C57BL/6J mice in vivo. The data showed that inflammatory stress increased hepatic CD36 protein levels but had no effect on mRNA expression. A protein degradation assay revealed that CD36 protein stability was not different between HepG2 cells treated with or without TNF-α or IL-6. A polysomal analysis indicated that CD36 translational efficiency was significantly increased by inflammatory stress. Additionally, inflammatory stress enhanced the phosphorylation of mTOR and its downstream translational regulators including p70S6K, 4E-BP1 and eIF4E. Rapamycin, an mTOR-specific inhibitor, reduced the phosphorylation of mTOR signalling pathway and decreased the CD36 translational efficiency and protein level even under inflammatory stress resulting in the alleviation of inflammatory stress-induced hepatic lipid accumulation. This study demonstrates that the activation of the mTOR signalling pathway increases hepatic CD36 translational efficiency, resulting in increased CD36 protein expression under inflammatory stress.
[Show abstract][Hide abstract] ABSTRACT: Background Patients with chronic kidney disease (CKD) are unlikely to gain the same benefit from conventional doses of statins as do
patients with cardiovascular disease alone. This study investigated whether inflammation accompanying CKD causes statin resistance.
Full-text · Article · Jun 2014 · Nephrology Dialysis Transplantation
[Show abstract][Hide abstract] ABSTRACT: Rapamycin, a mammalian target of rapamycin (mTOR)-specific inhibitor, has the effect of anti-lipid deposition on non-alcoholic fatty liver disease (NAFLD), but the mechanisms that rapamycin alleviates hepatic steatosis are not fully disclosed. CD36 is known to facilitate long-chain fatty acid uptake and contribute to NAFLD progression. Hepatic CD36 expression is closely associated with hepatic steatosis, while mTOR pathway is involved in CD36 translational control. This study was undertaken to investigate whether rapamycin alleviates hepatic steatosis via the inhibition of mTOR pathway-dependent CD36 translation. Human hepatoblastoma HepG2 cells were treated with palmitate and C57BL/6J mice were fed with high fat diet (HFD) to induce hepatic steatosis. Hepatic CD36 protein expression was significantly increased with lipid accumulation in palmitate-treated HepG2 cells or HFD-fed C57BL/6J mice. Rapamycin reduced hepatic steatosis and CD36 protein expression, but it had no influence on CD36 mRNA expression. Rapamycin had no effect on CD36 protein stability, but it significantly decreased CD36 translational efficiency. We further confirmed that rapamycin inhibited the phosphorylation of mTOR and its downstream translational regulators including p70 ribosomal protein S6 kinase (p70S6K), eukaryotic initiation factor 4E-binding protein 1 (4E-BP1), and eukaryotic initiation factor 4E (eIF4E). This study demonstrates that rapamycin inhibits hepatic CD36 translational efficiency through the mTOR pathway, resulting in reduction of CD36 protein expression and alleviation of hepatic steatosis.
No preview · Article · Apr 2014 · Biochemical and Biophysical Research Communications
[Show abstract][Hide abstract] ABSTRACT: The risk of cardiovascular disease is increased by up to 33 to 50× in chronic inflammatory states and convention doses of statins may not provide the same cardiovascular protection as in noninflamed patients. This study investigated whether the increase in 3-hydroxy-3-methylglutaryl-CoA reductase (HMGCoA-R)-mediated cholesterol synthesis observed under inflammatory stress was resistant to the action of statins and if so, whether this was because of interference with the sterol regulatory element binding protein cleavage-activating protein pathway.
Inflammatory stress was induced by adding cytokines (interleukin-1β, tumor necrosis factor-α, and interleukin-6) and lipopolysaccharides to vascular smooth muscle cells in vitro and by subcutaneous casein injection in apolipoprotein E/scavenger receptors class A/CD36 triple knockout mice in vivo. Inflammatory stress exacerbated cholesterol ester accumulation and was accompanied in vitro and in vivo by increased HMGCoA-R mRNA and protein expression mediated via activation of the sterol regulatory element binding protein cleavage-activating protein/sterol regulatory element binding protein-2 pathway. Atorvastatin reduced HMGCoA-R enzymatic activity and intracellular cholesterol synthesis in vitro. However, inflammatory stress weakened these suppressive effects. Atorvastatin at concentrations of 16 μmol/L inhibited HMGCoA-R activity by 50% in vascular smooth muscle cells, but the same concentration resulted in only 30% of HMGCoA-R activity in vascular smooth muscle cells in the presence of interleukin-1β. Knocking down sterol regulatory element binding protein cleavage-activating protein prevented statin resistance induced by interleukin-1β, and overexpression of sterol regulatory element binding protein cleavage-activating protein induced statin resistance even without inflammatory stress. In vivo, the amount of atorvastatin required to lower serum cholesterol and decrease aortic lipid accumulation rose from 2 to 10 mg/kg per day in the presence of inflammatory stress.
Increased cholesterol synthesis mediated by HMGCoA-R under inflammatory stress may be one of the mechanisms for intracellular lipid accumulation and statin resistance.
No preview · Article · Nov 2013 · Arteriosclerosis Thrombosis and Vascular Biology
[Show abstract][Hide abstract] ABSTRACT: Inflammatory stress promotes foam cell formation by disrupting LDL receptor feedback regulation in macrophages. Sterol Regulatory Element Binding Proteins (SREBPs) Cleavage-Activating Protein (SCAP) glycosylation plays crucial roles in regulating LDL receptor and 3-hydroxy-3-methyl-glutaryl-CoA reductase (HMGCoAR) feedback regulation. The present study was to investigate if inflammatory stress disrupts LDL receptor and HMGCoAR feedback regulation by affecting SCAP glycosylation in THP-1 macrophages. Intracellular cholesterol content was assessed by Oil Red O staining and quantitative assay. The expression of molecules controlling cholesterol homeostasis was examined using real-time quantitative RT-PCR and Western blotting. The translocation of SCAP from the endoplasmic reticulum (ER) to the Golgi was detected by confocal microscopy. We demonstrated that exposure to inflammatory cytokines increased lipid accumulation in THP-1 macrophages, accompanying with an increased SCAP expression even in the presence of a high concentration of LDL. These inflammatory cytokines also prolonged the half-life of SCAP by enhancing glycosylation of SCAP due to the elevated expression of the Golgi mannosidase II. This may enhance translocation and recycling of SCAP between the ER and the Golgi, escorting more SREBP2 from the ER to the Golgi for activation by proteolytic cleavages as evidenced by an increased N-terminal of SREBP2 (active form). As a consequence, the LDL receptor and HMGCoAR expression were up-regulated. Interestingly, these effects could be blocked by inhibitors of Golgi mannosidases. Our results indicated that inflammation increased native LDL uptake and endogenous cholesterol de novo synthesis, thereby causing foam cell formation via increasing transcription and protein glycosylation of SCAP in macrophages. These data imply that inhibitors of Golgi processing enzymes might have a potential vascular-protective role in prevention of atherosclerotic foam cell formation.
[Show abstract][Hide abstract] ABSTRACT: Inflammatory stress is closely related to metabolic disease and insulin resistance. The precise cellular mechanism linking obesity and diabetes is largely unknown, but about 14-20% of obese individuals develop diabetes. In this study, we investigated whether chronic inflammation exacerbated glucose metabolism disorder by impairing β cell function in high fat diet (HFD)-fed C57BL/6J mice. We used subcutaneous casein injection to induce chronic inflammation in HFD-fed C57BL/6J mice; 14 weeks on a HFD resulted in weight gain, hyperlipidemia and low insulin sensitivity in these mice which nevertheless had normal blood glucose and serum inflammatory cytokines levels. Casein injection in the background of HFD elevated serum TNF-α and serum amyloid A (SAA) levels and increased TNF-α and MCP-1 expression in adipose tissue, liver and muscle of HFD-fed mice. Chronic inflammation induced by casein injection further decreased insulin sensitivity and insulin signaling, resulting in insulin deficiency and hyperglycemia in these mice. Islet mass and insulin content were markedly increased in HFD mice. However, in contrast with HFD-fed alone, chronic inflammation in HFD-fed mice decreased both islet mass and insulin content, reduced the genetic expression of insulin synthesis and secretion, and increased β cell apoptosis. We conclude that chronic inflammation exacerbated glucose metabolism disorders by impairing β cell function in HFD-fed C57BL/6J mice, suggesting that this mechanism may operate in obese individuals with chronic inflammation, making them prone to hyperglycemia.
No preview · Article · Sep 2013 · Journal of Endocrinology
[Show abstract][Hide abstract] ABSTRACT: Myeloid differentiation factor 88 (MyD88) and nuclear factor-κB (NFκB) play central roles in mediating signal transduction of the Toll like receptor (TLR) superfamily in the human macrophages. The feedback regulation of LDL receptor (LDLr) and 3-hydroxy-3-methylglutaryl coenzyme A reductase (HMGCoAR) are mediated by sterol regulatory element binding proteins (SREBPs) cleavage-activating protein (SCAP)-SREBP2 pathway and are key regulatory elements for cholesterol homeostasis in human cells. This study was designed to investigate crosstalk between TLR4-MyD88-NFκB and SCAP-SREBP2 pathways in macrophage foam cell formation. Phorbol, 12-myristate, 13-acetate (PMA) activated-THP-1 macrophages were transfected with negative control or MyD88 siRNA. The transfected cells were incubated with LPS in the absence or presence of LDL or IKK inhibitor (BMS-345541). Intracellular cholesterol content was assessed. The mRNA and protein expression of LDLr, HMGCoAR, SCAP and SREBP2 were examined by real-time RT-PCR and Western blotting. The intracellular translocation of SCAP in the organelles was detected by immunofluorecence and confocal microscopy. We demonstrated that LPS induced cholesterol accumulation was attenuated by applying siRNA against MyD88 in the absence or presence of LDL. LPS increased both gene and protein expression of LDLr and HMGCoAR by increasing expression and abnormal translocation of SCAP from the ER to the Golgi. These effects were blocked by knocking down MyD88 or blocking IKK or knocking down SCAP, suggesting that the crosstalk between NFκB and SCAP plays an important role in macrophage foam cell formation and that interfering the crosstalk might be a potential approach in preventing LPS-induced macrophage foam cell formation.
Preview · Article · Jan 2013 · AJP Heart and Circulatory Physiology
[Show abstract][Hide abstract] ABSTRACT: Both inflammation and cholesterol accumulation play important roles in the development of non-alcoholic fatty liver disease. This study was undertaken to investigate whether inflammation aggravated cholesterol accumulation via disrupting hepatic cholesterol export and we explored the underlying mechanisms.
We used casein injection in C57BL/6J mice, and tumor necrosis factor alpha (TNF-α) stimulation in human hepatoblastoma cell line (HepG2) cells to induce inflammation. Intracellular cholesterol level was examined by Oil Red O staining and quantitative analysis. Bile acid level was quantified by colorimetric analysis. (3)[H] cholesterol assay by scintillation counting was performed to evaluate the cholesterol efflux. The mRNA and protein expression was examined by real-time polymerase chain reaction and Western blot.
Inflammation increased cholesterol accumulation in livers of C57BL/6J mice and in HepG2 cells. High-fat diet in mice and low-density lipoprotein (LDL) loading in HepG2 cells increased bile acid synthesis and cholesterol efflux, enhanced the mRNA and protein expression of liver X receptor α (LXRα), peroxisome proliferator-activated receptors (PPARα, γ), cholesterol 7α-hydroxylase (CYP7A1) and ATP-binding cassette transporter A1 (ABCA1). However, inflammation reduced bile acid synthesis and cholesterol efflux even in high-fat-diet-fed mice and HepG2 cells in the presence of LDL loading. The enhanced effects of these genes and proteins expression due to high-fat diet and LDL loading were inhibited by inflammation both in vivo and in vitro.
Inflammation disrupted PPAR-LXR-CYP7A1/ABCA1-mediated bile acid synthesis and cholesterol efflux resulting in exacerbated cholesterol accumulation in livers of C57BL/6J mice and HepG2 cells.
No preview · Article · Nov 2011 · Journal of Gastroenterology and Hepatology
[Show abstract][Hide abstract] ABSTRACT: Both lipids and inflammation play important roles in the progression of kidney disease. This study was designed to investigate whether inflammation exacerbates lipid accumulation via LDL receptors (LDLr), thereby causing renal injury in C57BL/6J mice, apolipoprotein E (ApoE) knockout (KO) mice, and ApoE/CD36/scavenger receptor A triple KO mice. The mice were given a subcutaneous casein injection to induce inflammatory stress. After 14 wk, terminal blood samples were taken for renal function, lipid profiles, amyloid A (SAA), and IL-6 assays. Lipid accumulation in kidneys was visualized by oil red O staining. Fibrogenic molecule expression in kidneys was examined. There was a significant increase in serum SAA and IL-6 in the all casein-injected mice compared with respective controls. Casein injection reduced serum total cholesterol, LDL cholesterol, and HDL cholesterol and caused lipid accumulation in kidneys from three types of mice. The expression of LDLr and its regulatory proteins sterol-responsive element-binding protein (SREBP) 2 and SREBP cleavage-activating protein (SCAP) were upregulated in inflamed mice compared with controls. Casein injection induced renal fibrosis accompanied by increased expression of fibrogenic molecules in the triple KO mice. These data imply that inflammation exacerbates lipid accumulation in the kidney by diverting lipid from the plasma to the kidney via the SCAP-SREBP2-LDLr pathway and causing renal injury. Low blood cholesterol levels, resulting from inflammation, may be associated with high risk for chronic renal fibrosis.
[Show abstract][Hide abstract] ABSTRACT: Chronic systemic inflammation and abnormal free fatty acid metabolism are closely related to ectopic lipid deposition. In this study, we investigate if inflammation tissue-specifically disrupts lipogenesis and lipolysis in nonadipose tissues and adipose tissue, resulting in ectopic lipid deposition in C57BL/6J mice.
We used casein injection in C57BL/6J mice to induce a chronic systemic inflammatory stress in vivo. Serum was analyzed for free fatty acid and cytokines. Insulin sensitivities were evaluated by glucose and insulin tolerance tests. Liver, muscle, adipose tissues were taken for lipid analysis. Real-time polymerase chain reaction and western blotting were used to examine the gene and protein expression of molecules involved in adipogenesis and lipolysis in tissues.
Casein injection elevated serum levels of IL-6 and SAA in mice, which are associated with increased lipid accumulation in liver and muscle, suggesting that chronic systemic inflammation induces ectopic lipid deposition in nonadipose tissues. The inflammatory stress upregulated mRNA and protein expression of sterol regulatory element binding protein 1, fatty acid synthase, and acetyl CoA carboxylase alpha, while inhibited these molecules expression in adipose. Interestingly, in the same experimental setting, inflammation increased triglyceride lipase and hormone-sensitive lipase expression in white adipose tissue. Inflammation also induced insulin resistance and increased serum free fatty acid levels in C57BL/6J mice.
Chronic systemic inflammation increased lipogenesis in nonadipose tissues and lipolysis in white adipose tissue, resulting in ectopic lipid deposition in nonadipose tissues. This disturbed free fatty acid homeostasis and caused insulin resistance in C57BL/6J mice.
Full-text · Article · Jun 2011 · Lipids in Health and Disease
[Show abstract][Hide abstract] ABSTRACT: Purpose
Murine gamma herpes virus 68 (MHV68) is a naturally occurring mouse pathogen that is homologous to Epstein–Barr virus. This study was designed to determine the correlation between MHV68 infection and lipid accumulation and insulin resistance in livers of C57BL/6J mice, and to explore the underlying mechanisms.
C57BL/6J mice fed a high fat diet were randomly assigned to receive either MHV68 or phosphate buffered saline treatment. Insulin sensitivities were evaluated by glucose tolerance tests. Serum was analyzed for lipids and cytokines. Liver was taken for histology and lipid analysis. Quantitative RT–PCR and western blotting were used to measure expression of hepatic mammalian target of rapamycin (mTOR), ribosomal S6 kinase 1 (S6K1), insulin receptor substrate-1 (IRS-1), sterol regulatory element binding protein-1 (SREBP1), fatty acid synthase (FAS), and acetyl CoA carboxylase (ACC).
MHV68 infection promoted fatty liver, hypertriglyceridemia, insulin resistance, and hyperinsulinemia in association with elevated inflammatory cytokines. In the livers of MHV68-infected C57BL/6J mice, SREBP1, FAS, ACC levels were increased. MHV68 infection also inhibited total IRS-1 expression and increased serine phosphorylation levels of IRS-1, which is parallel to the over activation of mTOR signaling pathway. Sirolimus, a specific inhibitor of mTOR pathway, inhibited MHV68-induced hepatic expression of serine p-IRS-1, increased total IRS-1 levels and improved MHV68-induced hepatic insulin resistance.
In C57BL/6J mice, MHV68 infection promotes fatty liver formation and hepatic insulin resistance, which can be ameliorated by sirolimus.
No preview · Article · Jun 2011 · Hepatology International
[Show abstract][Hide abstract] ABSTRACT: Cholesterol accumulation plays an important role in the progression of non-alcoholic fatty liver disease. We have demonstrated that inflammation aggravated cholesterol accumulation, causing tissue injury in the vessel and kidney. This study was undertaken to investigate whether inflammatory stress exacerbates hepatic cholesterol accumulation and we explored the underlying mechanisms.
We used casein injection in C57BL/6J mice, interleukin-1β and interleukin-6 stimulation in human hepatoblastoma cell line (HepG2) cells to induce inflammatory stress. Oil Red O staining and intracellular cholesterol assay were used to quantify cellular cholesterol levels. Real-time reverse transcription polymerase chain reaction and Western blot were used to measure messenger RNA (mRNA) and protein expression of target genes. HMGCoA reductase (HMGCoA-r) enzymatic activity and cellular cholesterol synthesis were measured by radioactive methods.
We demonstrated that inflammatory stress increased hepatic cholesterol accumulation and enhanced sterol regulatory element binding protein 2 (SREBP2), low-density lipoprotein receptor (LDLr) and HMGCoA-r mRNA and protein expression in livers of C57BL/6J mice and in HepG2 cells. A high-fat diet in mice or LDL loading in HepG2 cells inhibited mRNA and protein expression of these genes. However, the suppressive effect was overridden by inflammatory stress both in vivo and in vitro. Inflammatory stress increased HMGCoA-r enzymatic activity and cellular cholesterol synthesis in HepG2 cells in the absence or presence of LDL loading.
Inflammatory stress disrupted hepatic SREBP2-mediated low-density lipoprotein receptor and HMGCoA-r feedback regulation resulting in exacerbated cholesterol accumulation in livers of C57BL/6J mice and HepG2 cells.
No preview · Article · May 2011 · Journal of Gastroenterology and Hepatology
[Show abstract][Hide abstract] ABSTRACT: Advanced glycation end products (AGEs) is one of the causative factors of diabetic nephropathy, which is associated with lipid accumulation in glomeruli. This study was designed to investigate whether N(ε)-(carboxymethyl) lysine (CML; a member of the AGEs family) increases lipid accumulation by impairing the function of sterol-regulatory element binding protein (SREBP) cleavage-activating protein (SCAP) in human mesangial cells (HMCs). Intracellular cholesterol content was assessed by Oil Red O staining and quantitative assay. The expression of molecules controlling cholesterol homeostasis was examined using real-time quantitative RT-PCR and Western blotting. The activity of Golgi-processing enzymes was determined using enzyme-based methods, and the translocation of SCAP from the endoplasmic reticulum (ER) to the Golgi was detected by confocal microscopy. CML increased cholesterol accumulation in HMCs. Exposure to CML increased expression and abnormal translocation of SCAP from the ER to the Golgi even in the presence of a high concentration of LDL. The increased SCAP translocation carried more SREBP-2 to the Golgi for activation by proteolytic cleavages, enhancing transcription of 3-hydroxy-3-methylclutaryl-CoA reductase and the LDL receptor. CML increased Golgi mannosidase activity, which may enhance glycosylation of SCAP. This prolonged the half-life and enhanced recycling of SCAP between the ER and the Golgi. The effects of CML were blocked by inhibitors of Golgi mannosidases. AGEs (CML) increased lipid synthesis and uptake, thereby causing foam cell formation via increasing transcription and protein glycosylation of SCAP in HMCs. These data imply that inhibitors of Golgi-processing enzymes might have a potential renoprotective role in prevention of mesangial foam cell formation.
[Show abstract][Hide abstract] ABSTRACT: Inflammatory stress accelerates the progression of atherosclerosis. Sirolimus, a new immunosuppressive agent, has been shown to have pleiotropic antiatherosclerotic effects. In this study we hypothesized that sirolimus inhibits 3-hydroxy-3-methylglutaryl coenzyme A reductase (HMGR)-mediated cholesterol synthesis in human vascular smooth muscle cells (VSMCs) under inflammatory stress. Using radioactive assay, we demonstrated that sirolimus inhibited the increase of interleukin-1beta (IL-1beta)-induced cholesterol synthesis in VSMCs. Further studies showed that sirolimus inhibited both the HMGR gene and protein expression in VSMCs treated with or without IL-1beta. These effects were mediated by inhibiting the gene expression of sterol regulatory element-binding protein-2 (SREBP-2) and SREBP-2 cleavage-activating protein (SCAP) as checked by real-time PCR, Western blot analysis, and confocal microscopy for the observation of decreased protein translocation of the SCAP/SREBP-2 complex from the endoplasmic reticulum (ER) to the Golgi. Insulin-induced gene-1 (Insig-1) is a key ER protein controlling the feedback regulation of HMGR at transcriptional and posttranscriptional levels. We demonstrated that sirolimus increased Insig-1 expression which may bind to the SCAP, preventing the exit of SCAP-SREBP complexes from the ER. The increased Insig-1 also accelerated HMGR protein degradation in VSMCs as shown by pulse-chase analysis. In conclusion, sirolimus inhibits cholesterol synthesis induced by inflammatory stress through the downregulation of HMGR expression and the acceleration of HMGR protein degradation. These findings may improve our understanding of the molecular mechanisms of the antiatherosclerosis properties of sirolimus.
Preview · Article · Mar 2010 · AJP Heart and Circulatory Physiology
[Show abstract][Hide abstract] ABSTRACT: When the 'lipid nephrotoxicity hypothesis' was proposed in 1982, it brought together several disparate experimental findings in hyperlipidemia and renal disease to suggest that concomitant hyperlipidemia and proteinuria would cause self-perpetuating renal disease once the initial glomerular insult was no longer present. This process would be analogous to atherosclerosis. Since 1982, increasing evidence has supported the hypothesis that lipid abnormalities contribute to both atherosclerosis and glomerulosclerosis. In this Review, we discuss research developments that are relevant to the lipid nephrotoxicity hypothesis. We describe how inflammatory stress accompanying chronic kidney disease modifies lipid homeostasis by increasing cholesterol uptake mediated by lipoprotein receptors, inhibiting cholesterol efflux mediated by the ATP-binding cassette transporter 1 and impairing cholesterol synthesis in peripheral cells. As a result of these events, cholesterol relocates to and accumulates in renal, vascular, hepatic and possibly other tissues. The combination of increased cellular cholesterol influx and reduced efflux causes injury in some tissues and lowers the plasma cholesterol level. In addition, inflammatory stress causes a degree of statin resistance via unknown mechanisms. These phenomena alter traditional understanding of the pathogenesis of lipid-mediated renal and vascular injury and could influence the clinical evaluation of renal and cardiovascular risk and the role of lipid-lowering treatment in affected patients.
No preview · Article · Oct 2009 · Nature Reviews Nephrology