Effect of gemfibrozil on apolipoprotein B secretion and diacylglycerol acyltransferase activity in human hepatoblastoma (HepG2) cells

Gemfibrozil-Induced Intracellular Triglyceride Increase in SH-SY5Y, HEK and Calu-3 Cells

Gemfibrozil is a drug that has been used for over 40 years to lower triglycerides in blood. As a ligand for peroxisome proliferative-activated receptor-alpha (PPARα), which is expressed in many tissues, it induces the transcription of numerous genes for carbohydrate and lipid-metabolism. However, nothing is known about how intracellular lipid-homeostasis and, in particular, triglycerides are affected. As triglycerides are stored in lipid-droplets, which are known to be associated with many diseases, such as Alzheimer's disease, cancer, fatty liver disease and type-2 diabetes, treatment with gemfibrozil could adversely affect these diseases. To address the question whether gemfibrozil also affects intracellular lipid-levels, SH-SY5Y, HEK and Calu-3 cells, representing three different metabolically active organs (brain, lung and kidney), were incubated with gemfibrozil and subsequently analyzed semi-quantitatively by mass-spectrometry. Importantly, all cells showed a strong increase in intracellular triglycerides (SH-SY5Y: 170.3%; HEK: 272.1%; Calu-3: 448.1%), suggesting that the decreased triglyceride-levels might be due to an enhanced cellular uptake. Besides the common intracellular triglyceride increase, a cell-line specific alteration in acylcarnitines are found, suggesting that especially in neuronal cell lines gemfibrozil increases the transport of fatty acids to mitochondria and therefore increases the turnover of fatty acids for the benefit of additional energy supply, which could be important in diseases, such as Alzheimer's disease.

Niacin regulates apolipoprotein M expression via liver X receptor‑α

Niacin is currently the most effective drug that increases HDL-C levels. Apolipoprotein M (ApoM) in humans is mainly found in plasma high-density lipoprotein (HDL). Little is known about the role played by niacin in ApoM expression. In this study, the effects of niacin on ApoM expression were assessed as well as the associated mechanism. Human liver cancer cell line HepG2 was treated with niacin alone or with liver X receptor-α (LXRα) inhibitor at multiple concentrations. The mRNA and protein expression of ApoM were assessed by qRT-PCR and western blotting. Specific LXRα shRNA was transfected into HepG2 cells to further evaluate the regulatory effects of LXRα on ApoM. An in vivo model was also established to investigate the LXRα inhibitor on the mouse ApoM levels. The comparisons among groups were evaluated using one-way ANOVA and Student-Newman-Keuls test. It was revealed that in HepG2 cells, niacin dose-dependently increased ApoM gene and protein expression levels. Niacin-induced upregulation of ApoM was attenuated by an LXRα inhibitor or LXRα shRNA, indicating that LXRα mediated this effect. Moreover, niacin treatment resulted in increased LXRα mRNA levels, in vivo and in vitro; niacin treatment resulted in increased ApoM gene and protein expression levels in mice. In conclusion, niacin upregulates ApoM expression by increasing LXRα expression in vivo and in vitro.

Simultaneous analysis of gemfibrozil, morphine, and its two active metabolites in different mouse brain structures using solid-phase extraction with ultra-high performance liquid chromatography and tandem mass spectrometry with a deuterated internal standard

A rapid and sensitive bioassay was established and validated to simultaneously determine gemfibrozil, morphine, morphine-3β-glucuronide, and morphine-6β-glucuronide in mouse cerebrum, epencephalon, and hippocampus based on ultra-high performance liquid chromatography and tandem mass spectrometry. The deuterated internal standard, M6G-d3, was mixed with the prepared samples at 10 ng/mL as the final concentration. The samples were transferred into the C18 solid-phase extraction columns with gradient elution for solid-phase extraction. The mobile phase consisted of methanol and 0.05% formic acid (pH 3.2). Multiple reaction monitoring has been applied to analyze gemfibrozil (m/z 249.0 → 121.0) in anion mode, and M6G-d3 (m/z 465.1 → 289.1), morphine (m/z 286.0 → 200.9), and M3G and M6G (m/z 462.1 → 286.1) in the positive ion mode. The method has a linear calibration range from 0.05 to 10 ng for gemfibrozil, morphine, and M3G and M6G with correlation coefficients >0.993. The lower limit of quantitation for all four analytes was 0.05 ng/mL, relative standard deviation of intra- and interday precision was less than 10.5%, and the relative error of accuracy was from -8.2 to 8.3% at low, medium, and high concentrations for all the analytes. In conclusion, gemfibrozil can influence the morphine antinociception after coronary heart disease induced chronic angina by the change in one of morphine metabolites', M3G, distribution in mouse brain.

Hepatic denervation impairs the assembly and secretion of VLDL-TAG

VLDL secretion is a regulated process that depends on the availability of lipids, apoB and MTP. Our aim was to investigate the effect of liver denervation upon the secretion of VLDL and the expression of proteins involved in this process. Denervation was achieved by applying a 85% phenol solution onto the portal tract, while control animals were treated with 9% NaCl. VLDL secretion was evaluated by the Tyloxapol method. The hepatic concentration of TAG and cholesterol, and the plasma concentration of TAG, cholesterol, VLDL-TAG, VLDL-cholesterol and HDL-cholesterol were measured, as well as mRNA expression of proteins involved in the process of VLDL assembly. Hepatic acinar distribution of MTP and apoB was evaluated by immunohistochemistry. Denervation increased plasma concentration of cholesterol (125.3 +/- 10.1 vs. 67.1 +/- 4.9 mg dL(-1)) and VLDL-cholesterol (61.6 +/- 5.6 vs. 29.4 +/- 3.3 mg dL(-1)), but HDL-cholesterol was unchanged (45.5 +/- 6.1 vs. 36.9 +/- 3.9 mg dL(-1)). Secretion of VLDL-TAG (47.5 +/- 23.8 vs. 148.5 +/- 27.4 mg dL h(-1)) and mRNA expression of CPT I and apoB were reduced (p < 0.01) in the denervated animals. MTP and apoB acinar distribution was not altered in the denervated animals, but the intensity of the reaction was reduced in relation to controls.

Evolutionary conservation of drug action on lipoprotein metabolism-related targets

Genetic analysis has shown that the slower than normal rhythmic defecation behavior of the clk-1 mutants of Caenorhabditis elegans is the result of altered lipoprotein metabolism. We show here that this phenotype can be suppressed by drugs that affect lipoprotein metabolism, including drugs that affect HMG-CoA reductase activity, reverse cholesterol transport, or HDL levels. These pharmacological effects are highly specific, as these drugs affect defecation only in clk-1 mutants and not in the wild-type and do not affect other behaviors of the mutants. Furthermore, drugs that affect processes not directly related to lipid metabolism show no or minimal activity. Based on these findings, we carried out a compound screen that identified 190 novel molecules that are active on clk-1 mutants, 15 of which also specifically decrease the secretion of apolipoprotein B (apoB) from HepG2 hepatoma cells. The other 175 compounds are potentially active on lipid-related processes that cannot be targeted in cell culture. One compound, CHGN005, was tested and found to be active at reducing apoB secretion in intestinal Caco-2 cells as well as in HepG2 cells. This compound was also tested in a mouse model of dyslipidemia and found to decrease plasma cholesterol and triglyceride levels. Thus, target processes for pharmacological intervention on lipoprotein synthesis, transport, and metabolism are conserved between nematodes and vertebrates, which allows the use of C. elegans for drug discovery.

Fibrate therapy in patients with metabolic syndrome and diabetes mellitus

Patients with metabolic syndrome and type 2 diabetes mellitus are usually in moderately high-risk, high-risk, or very high-risk cardiovascular categories and present major therapeutic challenges. The dyslipidemia in such patients is typically a disorder of the triglyceride/high-density lipoprotein axis (TG/HDL axis) characterized by an excess of triglyceride-rich lipoproteins and a reduction of HDL. Very often, lifestyle therapy and statin monotherapy fail to achieve guideline goals, necessitating combination therapies. Fibric acids (or fibrates), are agonists of peroxisome proliferator-activated receptor alpha,which have amassed significant lipid-surrogate and clinical outcome trial data, especially in insulin-resistant patients, typical of those with metabolic syndrome or type 2 diabetes mellitus. Current guidelines advocate fibrate use as an add-on to statin therapy when TG/HDL abnormalities exist in such patients. In this paper, we review pertinent and recent trial data, mechanisms of action, and the safety of fibrate therapy.

Suppression of apolipoprotein B secretion from HepG2 cells by glucosyl hesperidin

Our previous study has shown that a soluble hesperidin derivative, glucosyl hesperidin (G-hesperidin), preferentially lowers serum triglyceride (TG) level in hypertriglyceridemic subjects through the improvement of very low-density lipoprotein (VLDL) metabolic abnormality. G-Hesperidin has also been found to decrease an elevated serum apolipoprotein B (apo B) level in the hypertriglyceridemic subjects, suggesting a possibility that this compound suppresses excess VLDL secretion in the liver. In the present study, to gain a better understanding of possible mechanisms by which G-hesperidin lowers serum TG, we examined whether this derivative affects apo B secretion from HepG2 human hepatoma cells, a model of hepatic VLDL secretion. As a result, G-hesperidin significantly reduced apo B secretion from the oleate-stimulated HepG2 cells. Furthermore, G-hesperidin significantly suppressed apo B secretion only in the oleate-stimulated cells and failed to act on the cells incubated without oleate. In the oleate-stimulated cells, G-hesperidin significantly decreased cellular cholesteryl ester (CE), although it had no effect on cellular TG or free cholesterol amounts. Moreover, the oleate-stimulated cells had a decrease in cellular apo B amounts by G-hesperidin exposure. These findings indicate that G-hesperidin down-regulates the assembly of apo B-containing lipoproteins via the reduction of CE synthesis augmented with oleate and results in suppressing excess apo B secretion from the cells. This effect is speculated to be associated with the improvement of VLDL metabolic abnormality in hypertriglyceridemic subjects and considered as a mechanism of lowering serum TG.

Role of fibric acid derivatives in the management of risk factors for coronary heart disease

Although elevated low-density lipoprotein (LDL)-cholesterol is a well established coronary heart disease (CHD) risk factor, the ability to adequately discriminate high-risk individuals by this risk factor alone is limited and other metabolic risk variables are known to modulate CHD risk. For instance, it has been reported that the cluster of metabolic disturbances observed among individuals with abdominal obesity, the so-called metabolic syndrome, is associated with a substantially increased risk of CHD. Among the features of the dyslipidaemic profile observed in these individuals, the high triglyceride-low high-density lipoprotein (HDL)-cholesterol dyslipidaemia is predictive of an elevated risk of CHD. Fibric acid derivatives (fibrates) have been used in clinical practice for more than 2 decades as a class of agents known to decrease triglyceride levels while substantially increasing HDL-cholesterol levels, with a limited but significant additional lowering effect on LDL-cholesterol levels. Although the clinical benefits of HMG-CoA reductase inhibitors (statins) have been well documented by primary and secondary prevention trials that justify their widespread use, it was not until the publication of the VA-HIT (Veterans Affairs High-Density Lipoprotein Intervention Trial) that the relevance of identifying HDL-cholesterol as a therapeutic target to reduce the risk of recurrent CHD events was finally confirmed. The clinical benefits of fibrate therapy are especially important in the subpopulation of patients with low HDL-cholesterol levels with the metabolic syndrome, particularly in patients with type 2 diabetes mellitus or in abdominally obese, hyperinsulinaemic patients. Evidence also suggests that there is a 'fibrate effect' that mediates the reduction in CHD risk beyond the favourable impact of these agents on HDL-cholesterol levels. This last notion is consistent with the pleiotropic effects of fibrates which are known to be related to their mechanisms of action. Through peroxisome proliferator-activated alpha-receptors, fibrates have a significant impact on the synthesis of several apolipoproteins (apo) and enzymes of lipoprotein metabolism as well as on the expression of several genes involved in fibrinolysis and inflammation. Fibrate therapy has been reported to decrease apo CIII levels (a powerful inhibitor of lipoprotein lipase) and increase apo AI levels, as well as to increase lipoprotein lipase activity. Such changes contribute to improve the catabolism of triglyceride-rich lipoproteins, leading to a substantial increase in HDL-cholesterol levels accompanied by a shift in the size and density of LDL particles (from small, dense LDL particles to larger, more buoyant cholesteryl ester-rich LDL). It is proposed that some of these pleiotropic effects could explain some of the clinical benefits of fibrate therapy beyond its HDL-raising properties, particularly among patients with abdominal obesity, hyperinsulinaemia or type 2 diabetes with both low HDL- and low/normal LDL-cholesterol levels.

Pharmacologic treatment of type 2 diabetic dyslipidemia

Patients with diabetes mellitus have a higher risk for cardiovascular heart disease (CHD) than does the general population, and once they develop CHD, mortality is higher. Good glycemic control will reduce CHD only modestly in patients with diabetes. Therefore, reduction in all cardiovascular risks such as dyslipidemia, hypertension, and smoking is warranted. The focus of this article is on therapy for dyslipidemia in patients with type 2 diabetes. Patients with the metabolic syndrome (insulin resistance) share similarities with patients with type 2 diabetes and may have a comparable cardiovascular risk profile. Diabetic patients tend to have higher triglyceride, lower high-density lipoprotein cholesterol (HDL), and similar low-density lipoprotein cholesterol (LDL) levels compared with those levels in nondiabetic patients. However, diabetic patients tend to have a higher concentration of small dense LDL particles, which are associated with higher CHD risk. Current recommendations are for an LDL goal of less than 100 mg/dl (an option of < 70 mg/dl in very high-risk patients), an HDL goal greater than 40 mg/dl for men and greater than 50 mg/dl for women, and a triglyceride goal less than 150 mg/dl. Nonpharmacologic interventions (diet and exercise) are first-line therapies and are used with pharmacologic therapy when necessary. Lowering LDL levels is the first priority in treating diabetic dyslipidemia. Statins are the first drug choice, followed by resins or ezetimibe, then fenofibrate or niacin. If a single agent is inadequate to achieve lipid goals, combinations of the preceding Drugs may be used. For elevated triglyceride levels, hyperglycemia must be controlled first. If triglyceride or HDL levels remain uncontrolled, pharmacologic agents should be considered. Fibrates are slightly more effective than niacin in lowering triglyceride levels, but niacin increases HDL levels appreciably more than do fibrates. Unlike gemfibrozil, niacin selectively increases subfraction Lp A-I, a cardioprotective HDL. Niacin is distinct in that it has a broad spectrum of beneficial effects on lipids and atherogenic lipoprotein subfraction levels. Niacin produces additive results when used in combination therapy. Recent data suggest that lower dosages and newer formulations of niacin can be used safely in diabetic patients with good glycemic control. Current evidence and guidelines mandate that diabetic dyslipidemia be treated aggressively, and lipid goals can be achieved in most patients with diabetes when all available products are considered and, if necessary, used in combination.

Overexpression of human diacylglycerol acyltransferase 1, acyl-coa:cholesterol acyltransferase 1, or acyl-CoA:cholesterol acyltransferase 2 stimulates secretion of apolipoprotein B-containing lipoproteins in McA-RH7777 cells

The relative importance of each core lipid in the assembly and secretion of very low density lipoproteins (VLDL) has been of interest over the past decade. The isolation of genes encoding diacylglycerol acyltransferase (DGAT) and acyl-CoA:cholesterol acyltransferases (ACAT1 and ACAT2) provided the opportunity to investigate the effects of isolated increases in triglycerides (TG) or cholesteryl esters (CE) on apolipoprotein B (apoB) lipoprotein biogenesis. Overexpression of human DGAT1 in rat hepatoma McA-RH7777 cells resulted in increased synthesis, cellular accumulation, and secretion of TG. These effects were associated with decreased intracellular degradation and increased secretion of newly synthesized apoB as VLDL. Similarly, overexpression of human ACAT1 or ACAT2 in McA-RH7777 cells resulted in increased synthesis, cellular accumulation, and secretion of CE. This led to decreased intracellular degradation and increased secretion of VLDL apoB. Overexpression of ACAT2 had a significantly greater impact upon assembly and secretion of VLDL from liver cells than did overexpression of ACAT1. The addition of oleic acid (OA) to media resulted in a further increase in VLDL secretion from cells expressing DGAT1, ACAT1, or ACAT2. VLDL secreted from DGAT1-expressing cells incubated in OA had a higher TG:CE ratio than VLDL secreted from ACAT1- and ACAT2-expressing cells treated with OA. These studies indicate that increasing DGAT1, ACAT1, or ACAT2 expression in McA-RH7777 cells stimulates the assembly and secretion of VLDL from liver cells and that the core composition of the secreted VLDL reflects the enzymatic activity that is elevated.

Pharmacologic elevation of high-density lipoproteins: recent insights on mechanism of action and atherosclerosis protection

PURPOSE OF REVIEW

Despite the best efforts in reduction of low-density lipoprotein cholesterol, most cardiovascular events are not being prevented. Because high-density lipoprotein (HDL) promotes reverse cholesterol transport and other antiatherogenic effects, interventions aimed at raising HDL cholesterol or mimicking its beneficial effects may greatly improve treatment and prevention of cardiovascular disease. This article reviews the antiatherogenic effects of HDL, recent insights into the mechanisms of action of currently available, and emerging HDL-based therapies.

RECENT FINDINGS

New insights into the basic science of HDL function and metabolism (such as the discovery of beta-chain ATP synthase as a hepatic catabolic HDL receptor) are further characterizing the importance of HDL in atheroprotection and identifying novel targets of drug development. Nicotinic acid, fibrates, statins, and thiazolidinediones not only increase HDL cholesterol but also alter HDL subpopulation size and composition. Furthermore, these drugs promote direct antiatherogenic effects of HDL (antioxidation, anti-inflammation, antithrombotic effects, endothelial stabilization). Emerging HDL-raising therapies (such as cholesteryl ester transfer protein inhibitors and 1,2-dimyristoyl-sn-glycero-phosphocholine) and novel interventions that mimic HDL's beneficial effects (such as apolipoprotein AImilano and apolipoprotein AI mimetic peptides) are proving beneficial in animal and human studies.

SUMMARY

An understanding of the atheroprotective mechanisms of HDL is essential for the rational use of currently available drugs and directed development of new drugs. Increasing total HDL cholesterol may not be as important as increasing the functional properties of HDL. Cardiovascular disease treatment and prevention can be improved by combining current low-density lipoprotein-based strategies with effective HDL-based interventions.

Downregulation of hepatic acyl-CoA:diglycerol acyltransferase in chronic renal failure

Chronic renal failure (CRF) is associated with hypertriglyceridemia and elevated plasma VLDL and IDL concentrations. These events can be due to either increased production or depressed catabolism of triglyceride-rich lipoproteins. Several studies have documented downregulation of lipoprotein lipase, hepatic triglyceride lipase, and the VLDL receptor, leading to depressed clearance and elevated plasma concentration of triglyceride-rich lipoproteins and their remnants in CRF. However, the effect of CRF on the triglyceride biosynthetic pathway has not been explored. Diglycerol acyltransferase (DGAT) is a microsomal enzyme that joins acyl-CoA to 1,2 diacylglycerol and, as such, constitutes the final step in triglyceride biosynthesis. Two distinct forms of DGAT (DGAT-1 and -2) have thus far been identified. The present study was undertaken to examine the effect of CRF on DGAT gene expression and activity in the liver, which is the source of endogenous triglycerides in the circulation. Male Sprague-Dawley rats were studied 8 wk after 5/6 nephrectomy (CRF) or sham operation. DGAT-1 and DGAT-2 mRNA abundance and DGAT activity were quantified. The CRF group showed reduced creatinine clearance, elevated plasma triglycerides, and VLDL concentrations. This was accompanied by significant reductions in hepatic DGAT-2 mRNA abundance (P < 0.01) and total DGAT activity (P < 0.1), pointing to diminished hepatic triglyceride production capacity in CRF animals. In conclusion, CRF results in significant downregulation of hepatic DGAT gene expression and activity. Given the critical role of DGAT in triglyceride biosynthesis, the present study points to diminished, not increased, hepatic triglyceride synthetic capacity in CRF rats.

The role of acyl-CoA:diacylglycerol acyltransferase (DGAT) in energy metabolism

Acyl-CoA:diacylglycerol acyltransferase (DGAT, EC2.3.1.20), a key enzyme in triglyceride (TG) biosynthesis, not only participates in lipid metabolism but also influences metabolic pathways of other fuel molecules. Changes in the expression and/or activity levels of DGAT may lead to changes in systemic insulin sensitivity and energy homeostasis. The synthetic role of DGAT in adipose tissue, the liver, and the intestine, sites where endogenous levels of DGAT activity and TG synthesis are high, is relatively clear. Less clear is whether DGAT plays a mediating or preventive role in the development of ectopic lipotoxicity in tissues such as muscle and the pancreas, when their supply of free fatty acids (FFAs) exceeds their needs. Future studies with tissue-specific overexpression and/or knockout in these animal models would be expected to shed additional light on these issues.

Dyslipidemia treatment: current considerations and unmet needs

Monumental evidence from clinical trials indicates an approximately 30% reduction in atheroslcerotic cardiovascular disease (ASCVD) risk using monotherapy with lipid-regulating drugs in dyslipidemic patients. In order to achieve greater reductions in risk, other approaches are necessary, including improvements in technology designed to assess ASCVD risk. Recent preliminary, but encouraging evidence indicates that by combining drugs that have different mechanisms of action on lipid metabolism yields not only an additive effect on the lipoprotein spectrum, but also reduces ASCVD events. New studies indicate that niacin potently increases high-density lipoproteins (HDL) by inhibiting HDL catabolism and decreases hepatic production of atherogenic very-low- and low-density lipoproteins by inhibiting the key enzyme for triglyceride synthesis (diacylglycerol acyltransferase). Statins, fibrates, bile acid sequestrants and ezetimibe have mechanisms that are different. Combination therapy using statins and niacin not only safely corrects dyslipidemia, but also yields ASCVD risk reduction significantly in excess of the 30% seen with monotherapy. Newer drugs with different mechanisms of action or combinations of new formulations have recently become available. Drug discovery research is likely to yield additional agents. Clinical trials focused on combination therapies to reduce ASCVD risk well beyond 30% need to be conducted to establish the rationale for further reducing the incidence of the primary cause of death today.