Statin Resistance Research Articles

Insensitivity to atorvastatin is associated with increased accumulation of intracellular lipid droplets and fatty acid metabolism in breast cancer cells

Apart from the relevant lipid-lowering effects, statins have demonstrated significant, although heterogeneous, anti-tumor activities in preventing breast cancer (BC) progression. To characterize the critical pathways behind the diverse responses to therapy, we investigated statin-induced changes in regulation of lipid metabolism and abundance of neutral lipid-containing cytoplasmic lipid droplets (LDs) in BC cells displaying different sensitivity to atorvastatin. Following atorvastatin treatment, accumulated LD levels inversely mirrored the marginal anti-proliferative effects in a dose and time-dependent manner in the less-sensitive BC cells. Transcriptional profiling excluded dysregulation of lipid uptake and efflux as specific mechanisms associated with differences in LD accumulation and anti-proliferative effects of atorvastatin. Notably, significant upregulation of genes involved in unsaturated fatty acid metabolism [stearoyl-CoA desaturase (SCD)] and cholesterol biosynthesis [3-hydroxy-3-methylglutaryl-CoA reductase (HMGCR)], were associated with atorvastatin insensitivity. Taken together, the increased ability to store neutral lipids in LDs as consequence of atorvastatin treatment likely confers a proliferative advantage to BC cells and may serve as potential biomarker of statin resistance in BC. Contributions of cholesterol biosynthesis and unsaturated fatty acid metabolism to LD formation should be thoroughly explored for better understanding of the molecular mechanisms underlying statin-induced effects against BC progression.

Treatment Strategy for Dyslipidemia in Cardiovascular Disease Prevention: Focus on Old and New Drugs.

Prevention and treatment of dyslipidemia should be considered as an integral part of individual cardiovascular prevention interventions, which should be addressed primarily to those at higher risk who benefit most. To date, statins remain the first-choice therapy, as they have been shown to reduce the risk of major vascular events by lowering low-density lipoprotein cholesterol (LDL-C). However, due to adherence to statin therapy or statin resistance, many patients do not reach LDL-C target levels. Ezetimibe, fibrates, and nicotinic acid represent the second-choice drugs to be used in combination with statins if lipid targets cannot be reached. In addition, anti-PCSK9 drugs (evolocumab and alirocumab) provide an effective solution for patients with familial hypercholesterolemia (FH) and statin intolerance at very high cardiovascular risk. Recently, studies demonstrated the effects of two novel lipid-lowering agents (lomitapide and mipomersen) for the management of homozygous FH by decreasing LDL-C values and reducing cardiovascular events. However, the costs for these new therapies made the cost–effectiveness debate more complicated.

The lipid nephrotoxicity hypothesis

In 1982, Moorhead introduced the “lipid nephrotoxicity hypothesis” which suggests 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 original hypothesis that lipid abnormalities contribute to the glomerulosclerosis. In this Review, we discuss recent developments that are relevant to the lipid nephrotoxicity hypothesis, especially the important role of inflammation accompanying chronic kidney disease on lipid homeostasis and kidney injury. In addition, inflammatory stress causes a degree of statin resistance. These phenomena alter traditional understanding of the pathogenesis of lipid-mediated renal injury and have significant clinical implications.

Cholesterol-lowering drugs: science and marketing.

Long-term use of statin therapy is essential to obtain clinical benefits, but adherence is often suboptimal and some patients are also reported to fail because of 'statin resistance'. The identification of PCSK9 as a key factor in the LDL clearance pathway has led to the development of new monoclonal antibodies. Here we critically review the economic evaluations published in Europe and focused on statins. We searched the PubMed database to select the studies published from July 2006 to June 2016 and finally selected 19 articles. Overall, the majority of studies were conducted from a third-party payer's viewpoint and recurred to modelling. Most studies were sponsored by industry and funding seemed to play a pivotal role in the study design. Patients resistant to LDL-C level reduction were considered only in a few studies. The place in therapy of the new class of biologic should be considered a kind of 'third line' for cholesterol-lowering, after patients have failed with restricted dietary regimens and then with current drug therapies. Otherwise they could result in hardly sustainable expenses even for developed countries.

YM155 Reverses Statin Resistance in Renal Cancer by Reducing Expression of Survivin.

The purpose of the present study was to clarify whether treatment with YM155, a novel small-molecule inhibitor of survivin, reverses statin resistance in statin-resistant renal cell cancer (RCC). We induced simvastatin resistance in a renal clear cell carcinoma cell line (Caki-1-staR). In vitro and in vivo models were used to test the efficacy of YM155 and simvastatin. Survivin gene expression was significantly stronger in Caki-1-staR cells than in its parent cells (Caki-1). In Caki-1-staR cells, YM155 significantly reduced expression of survivin gene and cell proliferation in a dose-dependent manner. Treatment with YM155 significantly reversed simvastatin resistance in Caki-1-staR cells. YM155 significantly inhibited the growth of Caki-1-staR tumors in a nude mouse tumor xenograft model. Furthermore, YM155 significantly enhanced the antitumor effects of simvastatin on Caki-1-staR tumors. Our results indicate that inhibition of survivin by YM155 overcomes statin resistance in RCC cells.

Genetic and Non-Genetic Determinants of the Pharmacological Activity of Statins.

Statins are cholesterol-lowering agents which belong to the group of the most commonly prescribed drugs. The use of statins has become the standard treatment in patients with an increased risk of cardiovascular and coronary heart diseases. However, many clinical studies have shown that 13 - 75% of patients fail to achieve LDL-cholesterol and total cholesterol target levels. The clinical implications of insufficient response include cardiovascular complications caused by atherosclerosis leading to acute myocardial infarction, stroke and death. The mechanism underlying statin resistance has been associated with genetic polymorphisms and nongenetic factors (e.g. concomitant diseases, drug-drug interactions, interactions with food and dietary supplements). The article provides a comprehensive update of the current knowledge regarding the role of genetic polymorphism and non-genetic determinants of cholesterol-lowering effect of statins. Dietary aspects of statin efficacy were also presented. The Pubmed search was performed to identify relevant papers from the last ten years which were included in the review. Consideration of the genetic and non-genetic determinants of pharmacological action of statins as well as mechanisms of drug-drug interactions may be useful in clinical practice for improving safety and efficacy of statin treatment.

A Mutation in Caenorhabditis elegans NDUF-7 Activates the Mitochondrial Stress Response and Prolongs Lifespan via ROS and CED-4.

The mevalonate pathway is responsible for the synthesis of cholesterol, coenzyme Q, and prenyl groups essential for small GTPase modification and function, and for the production of dolichols important for protein glycosylation. Statins, i.e., cholesterol-lowering drugs that inhibit the rate-limiting enzyme in the mevalonate pathway, HMG-CoA reductase, are lethal to Caenorhabditis elegans even though this animal lacks the branch of the mevalonate pathway that leads to cholesterol synthesis. To better understand the effects of statins that are not related to cholesterol, we have adopted the strategy of isolating statin-resistant C. elegans mutants. Previously, we showed that such mutants often have gain-of-function mutations in ATFS-1, a protein that activates the mitochondrial unfolded protein response. Here, we describe the isolation of a statin-resistant mutant allele of the NDUF-7 protein, which is a component of complex I in the mitochondrial electron transport chain. The novel nduf-7(et19) mutant also exhibits constitutive and ATFS-1-dependent activation of the mitochondrial unfolded protein response (UPRmt) and prolonged life span, both of which are mediated through production of ROS. Additionally, lifespan extension, but not activation, of the mitochondrial unfolded protein response was dependent on the pro-apoptotic gene ced-4. We conclude that the nduf-7(et19) mutant allele causes an increase in reactive oxygen species that activate ATFS-1, hence UPRmt-mediated statin resistance, and extends life span via CED-4.

Understanding the molecular mechanism of host-based statin resistance in hepatitis C virus replicon containing cells

A number of statins, the cholesterol-lowering drugs, inhibit the in vitro replication of hepatitis C virus (HCV). In HCV-infected patients, addition of statins to the earlier standard of care therapy (pegIFN-α and ribavirin) resulted in increased sustained virological response rates. The mechanism by which statins inhibit HCV replication has not yet been elucidated. In an attempt to gain insight in the underlying mechanism, hepatoma cells carrying an HCV replicon were passaged in the presence of increasing concentrations of fluvastatin. Fluvastatin-resistant replicon containing cells could be generated and proved ∼8-fold less susceptible to fluvastatin than wild-type cultures. The growth efficiency of the resistant replicon containing cells was comparable to that of wild-type replicon cells. The fluvastatin-resistant phenotype was not conferred by mutations in the viral genome but is caused by cellular changes. The resistant cell line had a markedly increased HMG-CoA reductase expression upon statin treatment. Furthermore, the expression of the efflux transporter P-gp was increased in fluvastatin-resistant replicon cells (determined by qRT-PCR and flow cytometry). This increased expression resulted also in an increased functional transport activity as measured by the P-gp mediated efflux of calcein AM. In conclusion, we demonstrate that statin resistance in HCV replicon containing hepatoma cells is conferred by changes in the cellular environment.

Statin-induced mevalonate pathway inhibition attenuates the growth of mesenchymal-like cancer cells that lack functional E-cadherin mediated cell cohesion.

The cholesterol reducing drugs, statins, exhibit anti-tumor effects against cancer stem cells and various cancer cell lines, exert potent additivity or synergy with existing chemotherapeutics in animal models of cancer and may reduce cancer incidence and cancer related mortality in humans. However, not all tumor cell lines are sensitive to statins, and clinical trials have demonstrated mixed outcomes regarding statins as anticancer agents. Here, we show that statin-induced reduction in intracellular cholesterol levels correlate with the growth inhibition of cancer cell lines upon statin treatment. Moreover, statin sensitivity segregates with abundant cytosolic vimentin expression and absent cell surface E-cadherin expression, a pattern characteristic of mesenchymal-like cells. Exogenous expression of cell surface E-cadherin converts statin- sensitive cells to a partially resistant state implying that statin resistance is in part dependent on the tumor cells attaining an epithelial phenotype. As metastasizing tumor cells undergo epithelial to mesenchymal transition during the initiation of the metastatic cascade, statin therapy may represent an effective approach to targeting the cells most likely to disseminate.

Abstract 17751: PCSK9 is Elevated in HIV+ Patients

Purpose: Proprotein convertase subtilisin kexin 9 (PCSK9) is induced by inflammation and inhibits LDL receptor expression. Increased PCSK9 might explain the dyslipidemia observed in HIV patients, which has been previously attributed to HIV medication, HIV disease, and/or chronic inflammation. We aimed to compare PCSK9 levels in HIV patients and uninfected controls and identify predictors of elevated PCSK9 in HIV. Methods: We measured PCSK9 levels in 567 participants (495 HIV, 72 controls) from an outpatient cohort in San Francisco using a high affinity ELISA assay. Poisson regression models were used to determine factors associated with PCSK9. Results: The median age of all participants was 50 years (IQR 43-55) and 89% were male; 34% were smokers, 25% had hypertension, the median LDL was 103 mg/dL (IQR 80-127), and 21% were on statins. HIV patients and controls had similar rates of traditional risk factors except prior CAD was more common in HIV patients. Half of the HIV patients were treated and suppressed on HIV medication. Unadjusted PCSK9 levels were 11% higher in HIV patients vs. controls [mean 430 ng/ml (SD 166) vs. 386 ng/ml (SD134), p=0.015] with some HIV patients having extremely elevated levels >1000 ng/ml. After adjustment for demographics and statin use, HIV remained independently associated with 10% higher PCSK9 levels (p=0.03). In addition, older age, other race, statin use, Hepatitis C (HCV), higher triglycerides, current smoking and Lp(a)>90 nmol/L were all independently associated with higher PCSK9 levels in adjusted analysis. In contrast, neither inflammatory markers (IL-6, hs-CRP) nor CD4+ count or HIV viral load were associated with higher PCSK9. Conclusion: PCSK9 is increased in HIV-infected individuals. Traditional risk factors, HCV, and Lp(a)>90 nmol/L were also independently associated with PCSK9. Future studies should explore whether PCSK9 inhibition may be used to treat dyslipidemia and statin resistance in HIV+ patients.

Inflammatory stress reduces the effectiveness of statins in the kidney by disrupting HMGCoA reductase feedback regulation.

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. Inflammatory stress was induced by adding cytokines and lipopolysaccharide (LPS) to human mesangial cells (HMCs) in vitro, and in vivo by subcutaneous casein injection in apolipoprotein E, scavenger receptors class A and CD36 triple knockout mice. Inflammatory stress exacerbated cholesterol accumulation and was accompanied in vitro and in vivo by increased HMGCoA reductase (HMGCoA-R) mRNA and protein expression mediated via activation of the sterol regulatory element-binding protein cleavage-activating protein (SCAP)/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 15 µM inhibited HMGCoA-R activity by 50% (IC50) in HMCs, but the same concentration in the presence of interleukin (IL)-1β resulted in only 30% inhibition of HMGCoA-R activity in HMCs. Knocking down SCAP prevented statin resistance induced by IL-1β, and overexpression of SCAP-induced statin resistance even without inflammatory stress. In vivo, the amount of atorvastatin required to lower serum cholesterol and decrease kidney lipid accumulation rose from 2 to 10 mg/kg/day in the presence of inflammatory stress. Inflammatory stress can disrupt HMGCoA-R-mediated cholesterol synthesis resulting in intracellular lipid accumulation and statin resistance.

Inflammatory Stress Induces Statin Resistance by Disrupting 3-Hydroxy-3-Methylglutaryl-CoA Reductase Feedback Regulation

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.

The mitochondrial unfolded protein response activator ATFS-1 protects cells from inhibition of the mevalonate pathway

Statins are cholesterol-lowering drugs that inhibit 3-hydroxy-3-methyl-glutaryl-CoA (HMG-CoA) reductase, the rate-limiting enzyme in the synthesis of cholesterol via the mevalonate pathway. This pathway also produces coenzyme Q (a component of the respiratory chain), dolichols (important for protein glycosylation), and isoprenoids (lipid moieties responsible for the membrane association of small GTPases). We previously showed that the nematode Caenorhabditis elegans is useful to study the noncholesterol effects of statins because its mevalonate pathway lacks the sterol synthesis branch but retains all other branches. Here, from a screen of 150,000 mutagenized genomes, we isolated four C. elegans mutants resistant to statins by virtue of gain-of-function mutations within the first six amino acids of the protein ATFS-1, the key regulator of the mitochondrial unfolded protein response that includes activation of the chaperones HSP-6 and HSP-60. The atfs-1 gain-of-function mutants are also resistant to ibandronate, an inhibitor of an enzyme downstream of HMG-CoA reductase, and to gliotoxin, an inhibitor acting on a subbranch of the pathway important for protein prenylation, and showed improved mitochondrial function and protein prenylation in the presence of statins. Additionally, preinduction of the mitochondrial unfolded protein response in wild-type worms using ethidium bromide or paraquat triggered statin resistance, and similar observations were made in Schizosaccharomyces pombe and in a mammalian cell line. We conclude that statin resistance through maintenance of mitochondrial homeostasis is conserved across species, and that the cell-lethal effects of statins are caused primarily through impaired protein prenylation that results in mitochondria dysfunction.

An update on the lipid nephrotoxicity hypothesis

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.

Effect of Fibrates on Lipid Profiles and Cardiovascular Outcomes: A Systematic Review

Objective Fibrates might represent a viable treatment option for patients who do not meet their target low-density lipoprotein levels on statins or who are resistant or intolerant to statins. New data from fibrate trials can be synthesized with the existing literature to better estimate their effects. Methods We systematically searched the literature to identify randomized, double-blind, placebo-controlled trials examining the effect of fibrates on lipid profiles or cardiovascular outcomes. We estimated the effect of fibrates on the incidence of nonfatal myocardial infarction and all-cause mortality using random effects models. Results Compared with placebo, fibrates were associated with greater reductions in total cholesterol (range: −101.3 mg/dL to −5.0 mg/dL) and triglycerides (range: −321.3 mg/dL to −20.8 mg/dL), and a greater increase in high-density lipoprotein (range: +1.1 mg/dL to +17.9 mg/dL) in all trials. Fibrates tended to be associated with a greater reduction in low-density lipoprotein (range: −76.3 mg/dL to +38.7 mg/dL) than placebo, although these results were not consistent across all trials. Fibrates were more efficacious than placebo at preventing nonfatal myocardial infarction (odds ratio = 0.78; 95% confidence interval, 0.69-0.89), but not all-cause mortality (odds ratio = 1.05; 95% confidence interval, 0.95-1.15). Conclusion In addition to improving lipid profiles, fibrates are associated with an important decrease in nonfatal myocardial infarction, but do not substantially affect all-cause mortality. Potential applications include treatment for patients with statin resistance or isolated hypertriglyceridemia, or as an adjunct to other lipid-lowering therapies.

Effect of LDL-apheresis on plasma lipids, chitotriosidase and anti-oxLDL antibodies in heterozygous familial hypercholes-terolemia

Forty four consecutive subjects aged 29-58 years (21 males and 23 females) with a clinical diagnosis of heterozygous familial hypercholesterolemia period- ically treated every 30 days with LDL-apheresis for statin resistance, were enrolled in this study. A lipid profile was obtained immediately before starting LDL-apheresis, a second profile was obtained within four hours after LDL-apheresis. Chit activity and anti-oxLDL levels were determined with appropriate methods in all patients before and after LDL- apheresis. Total cholesterol, LDL-cholesterol, HDL- cholesterol and triglycerides decreased significantly after LDL-apheresis, while the variations of Chit activity and anti-oxLDL were not significant after LDL-apheresis. The correlation between Chit and total cholesterol was negative (r= -0.44 and -0.50 res- pectively) before and after LDL-apheresis as between Chit and LDL-cholesterol (r= -0.45 and -0.55 respectively). Anti-oxLDL concentration before and after LDL-apheresis positively correlated with Chit activity (r= 0.52 and r = 0.63 respectively), negatively with total cholesterol (r= -0.33 and r = -0.35 res- pectively) and with LDL (r = -0.32 and r = -0.21 respectively). We think that removing LDL with LDL-apheresis the anti-oxLDL/oxLDL ratio could increase and the excess of anti-oxLDL could induce macrophage activation through the surface Fc receptors. Alternatively with high levels of LDL- cholesterol, the deposition of foam cells represent the characteristic evolution of atherosclerosis process. Macrophage activation in the heterozygous familial

Effect of LDL-apheresis on plasma lipids, chitotriosidase and anti-oxLDL antibodies in heterozygous familial hypercholes-terolemia

Forty four consecutive subjects aged 29-58 years (21 males and 23 females) with a clinical diagnosis of heterozygous familial hypercholesterolemia periodically treated every 30 days with LDL-apheresis for statin resistance, were enrolled in this study. A lipid profile was obtained immediately before starting LDL-apheresis, a second profile was obtained within four hours after LDL-apheresis. Chit activity and anti-oxLDL levels were determined with appropriate methods in all patients before and after LDL- apheresis. Total cholesterol, LDL-cholesterol, HDL- cholesterol and triglycerides decreased significantly after LDL-apheresis, while the variations of Chit activity and anti-oxLDL were not significant after LDL-apheresis. The correlation between Chit and total cholesterol was negative (r= –0.44 and –0.50 res- pectively) before and after LDL-apheresis as between Chit and LDL-cholesterol (r= –0.45 and –0.55 respectively). Anti-oxLDL concentration before and after LDL-apheresis positively correlated with Chit activity (r= 0.52 and r = 0.63 respectively), negatively with total cholesterol (r= –0.33 and r = –0.35 res- pectively) and with LDL (r = –0.32 and r = –0.21 respectively). We think that removing LDL with LDL-apheresis the anti-oxLDL/oxLDL ratio could increase and the excess of anti-oxLDL could induce macrophage activation through the surface Fc receptors. Alternatively with high levels of LDL- cholesterol, the deposition of foam cells represent the characteristic evolution of atherosclerosis process. Macrophage activation in the heterozygous familial hypercholesterolemia could represent an attempt for re-modeling the vessel wall, reducing the growth of lipid plaques.

131: Statin Resistance; Progression of Coronary Atherosclerosis and Continued Vascular Dysfunction Despite Optimal LDL Lowering

131: Statin Resistance; Progression of Coronary Atherosclerosis and Continued Vascular Dysfunction Despite Optimal LDL Lowering

Inflammatory cytokines disrupt LDL-receptor feedback regulation and cause statin resistance: a comparative study in human hepatic cells and mesangial cells

LDL receptor (LDLr) is widely expressed in both liver and peripheral tissue. We aimed to clarify tissue-specific regulation of LDLr in hepatic cell line (HepG2) cells and human kidney mesangial cells (HMCs) under physiological and inflammatory conditions. We have demonstrated that the concentration of LDL required for 50% inhibition of LDLr mRNA expression (IC50) in HepG2 was 75 microg/ml, but only 30 microg/ml in HMCs. The concentration of mevastatin, a 3-hydroxy-3-methylglutaryl coenzyme A reductase inhibitor, which achieved 200% upregulation of LDLr (UC200) in HepG2 cells, was 0.7 microM, which is much lower than 2.8 microM in HMCs. Inflammatory stress increased IC50 to 80 and 75 microg/ml of LDL, UC(200) to 2.8 microM, and 4.2 microM of mevastatin in HepG2 and HMCs. There was obvious sterol-regulatory element binding protein cleavage-activating protein accumulation in the Golgi in HepG2 cells, but not in HMCs in the presence of high concentration of LDL. IL-1beta further increased sterol-regulatory element binding protein cleavage-activating protein accumulation in HepG2 and HMCs in the presence of high concentration of LDL. These results indicate that LDLr in HepG2 cells have a relative resistant phenotype for downregulation, while LDLr in HMCs is very sensitive for downregulation. Inflammatory cytokine disrupts LDLr negative feedback regulation induced by intracellular cholesterol in both cell types, to a greater degree in HMCs, which could be one reason why HMCs are more prone to become foam cells under inflammatory stress. Inflammation also causes statin resistance; therefore, a high concentration of statin may be required to achieve the same biological effect.

Impaired Regulation of 3-Hydroxy-3-methylglutaryl-Coenzyme A Reductase Degradation in Lovastatin-resistant Cells

L-90 cells were selected to grow in the presence of serum lipoproteins and 90 microM lovastatin, an inhibitor of 3-hydroxy-3-methylglutaryl-coenzyme A reductase (HMGR). L-90 cells massively accumulate HMGR, a result of >10-fold amplification of the gene and 40-fold rise in mRNA, and also overexpress other enzymes of the mevalonate pathway. Western blot and promoter-luciferase analyses indicate that transcriptional regulation of sterol-responsive genes by 25-hydroxycholesterol or mevalonate is normal. Yet, none of these genes is regulated by lipoproteins, a result of severe impairment in the low density lipoprotein receptor pathway. Moreover, L-90 cells do not accelerate the degradation of HMGR or transfected HMGal chimera in response to 25-hydroxycholesterol or mevalonate. This aberrant phenotype persists when cells are grown without lovastatin for up to 37 days. The inability to regulate HMGR degradation is not due to its overproduction since in LP-90 cells, which were selected for lovastatin resistance in lipoprotein-deficient serum, HMGR is overexpressed, yet its turnover is regulated normally. Also, the rapid degradation of transfected alpha subunit of T cell receptor is markedly retarded in L-90 cells. These results show that in addition to gene amplification and overexpression of cholesterogenic enzymes, statin resistance can follow loss of regulated HMGR degradation.