Studies on rat liver phosphatidate phosphohydrolase and plasma lipids: effect of HMG-CoA reductase inhibitors

Increase in hepatic expression of SREBP-2 by gemfibrozil administration to rats

It is well known that gemfibrozil increases the biliary output of cholesterol and phospholipids, but we have little knowledge about the impact these changes have on liver cholesterol and phospholipid biosynthetic pathways. In the present study, no changes were detected in liver lipids and CTP:phosphocholine cytidylyltransferase after gemfibrozil administration to rats. On the contrary, 3-hydroxy-3-methylglutaryl-CoA reductase mRNA (9.9-fold) and Rd activity (16.7-fold) and phosphatidate phosphohydrolase activity (1.7-fold) increased, while plasma apo B-cholesterol (40%) and triglyceride (43%) levels decreased. As a part of a compensatory homeostatic response, we report for the first time that gemfibrozil administration to rats increased the hepatic sterol regulatory element binding protein-2 (SREBP-2) mRNA (2.9-fold) and mature protein (2.2-fold) levels. An early increase in the transcriptional activity of SREBP-2 elicited by gemfibrozil administration might be responsible for the observed changes in HMG-CoA reductase, phosphatidate phosphohydrolase, and SREBP-2 expression.

Use of a nonionic detergent (Triton WR 1339) in healthy cats to assess hepatic secretion of triglyceride

OBJECTIVE

To determine whether a nonionic detergent (Triton WR 1339) can be used in cats to assess hepatic secretion of triglyceride.

ANIMALS

28 healthy cats.

PROCEDURE

Triton WR 1339 was administered IV according to the following schedule: 5, 50, 150, and 250 mg/kg of body weight. Control cats did not receive an injection or received 0.9% NaCl or PBS solutions at the same osmolarity and volume as the 250 mg/kg group. Blood samples were collected throughout the 48-hour period after administration for determination of triglyceride and cholesterol concentrations and for RBC morphology and osmotic fragility studies.

RESULTS

Administration of Triton WR 1339 at 150 and 250 mg/kg caused profound hypertriglyceridemia. Triglyceride concentrations increased in a curvilinear fashion for the first 2 hours and remained increased for approximately 24 hours. Area under the time-concentration curve for triglyceride at 5 hours differed significantly among groups. At 12 and 24 hours, cholesterol was significantly higher in cats receiving 250 mg/kg. The most dramatic changes in osmotic fragility and RBC morphology were in cats receiving 250 mg/kg; 1 of these cats developed severe icterus and died 5 days later. Feeding rice and casein before administering Triton WR 1339 at 150 mg/kg did not appear to affect the hypertriglyceridemia response.

CONCLUSIONS AND CLINICAL RELEVANCE

Triton WR 1339 can be administered IV to cats at a rate of 150 mg/kg to assess hepatic triglyceride secretion, although some cats may have increased RBC osmotic fragility. Higher dosages caused substantial adverse effects, whereas lower dosages did not alter plasma triglyceride concentration.

Atorvastatin in the treatment of primary hypercholesterolemia and mixed dyslipidemias

OBJECTIVE

To review the efficacy and safety of atorvastatin in the treatment of dyslipidemias.

DATA SOURCES

A MEDLINE search (January 1960-April 1998), Current Contents search, additional references listed in articles, and unpublished data obtained from the manufacturer were used to identify data from scientific literature. Studies evaluating atorvastatin (i.e., abstracts, clinical trials, proceedings, data on file with the manufacturer) were considered for inclusion.

STUDY SELECTION

English-language literature was reviewed to evaluate the pharmacology, pharmacokinetics, therapeutic use, and adverse effects of atorvastatin. Additional relevant citations were used in the introductory material and discussion.

DATA EXTRACTION

Open and controlled animal and human clinical studies published in the English-language literature were reviewed and evaluated. Clinical trials selected for inclusion were limited to those in human subjects and included data from animals if human data were not available.

DATA SYNTHESIS

Atorvastatin is a recent hydroxymethylglutaryl-coenzyme A (HMG-CoA) reductase inhibitor for the treatment of primary hypercholesterolemia, mixed dyslipidemias, and homozygous familial hypercholesterolemia. In patients who have not met the low-density lipoprotein cholesterol (LDL-C) goal as recommended by the National Cholesterol Education Program Adult Treatment Panel II guidelines, atorvastatin 10-80 mg/d may be used as monotherapy or as an adjunct to other lipid-lowering agents and dietary modifications. In placebo-controlled clinical trials, atorvastatin 10-80 mg/d lowered LDL-C by 35-61% and triglyceride (TG) concentrations by 14-45%. In comparative trials, atorvastatin 10-80 mg/d showed a greater reduction of serum total cholesterol (TC), LDL-C, TG concentrations, and apolipoprotein B-100 (apo B) compared with pravastatin, simvastatin, or lovastatin. In comparison, currently available HMG-CoA reductase inhibitors (lovastatin, simvastatin, pravastatin, fluvastatin, cerivastatin) lower LDL-C concentrations by approximately 20-40% and TG concentrations by approximately 10-30%. In pooled placebo-controlled clinical trials of up to a duration of 52 weeks, atorvastatin in dosages up to 80 mg/d appeared to be well tolerated. The most common adverse effect of atorvastatin was gastrointestinal upset. The incidence of elevated serum hepatic transaminases may be greater at higher dosages of atorvastatin. The risk of myopathy and/or rhabdomyolysis is increased when an HMG-CoA reductase inhibitor is taken concomitantly with cyclosporine, gemfibrozil, niacin, erythromycin, or azole antifungals.

CONCLUSIONS

Atorvastatin appears to reduce TC, LDL-C, TG concentrations, and apo B to a greater extent than do currently available HMG-CoA reductase inhibitors. Atorvastatin may be preferred in patients requiring greater than a 30% reduction in LDL-C or in patients with both elevated LDL-C and TG concentrations, which may obviate the need for combination lipid-lowering therapy. Adverse effects of atorvastatin appear to be similar to those of other HMG-CoA reductase inhibitors and should be routinely monitored. Long-term safety data (> 1 y) on atorvastatin compared with other HMG-CoA reductase inhibitors are still needed. Cost-effectiveness studies comparing atorvastatin with other HMG-CoA reductase inhibitors remain a subject for further investigation. Published clinical studies evaluating the impact of atorvastatin on cardiovascular morbidity and mortality are still needed. Additionally, clinical studies evaluating the impact of lipid-lowering therapy in a larger number of women, the elderly (> 70 y), and patients with diabetes for treatment of primary and secondary prevention of coronary heart disease are needed.

Lack of predictability of classical animal models for hypolipidemic activity: a good time for mice?

Hypolipidemic drugs that are efficacious in man are not always active in classical animal models of dyslipidemia. Inhibitors of HMG-CoA reductase (statins) do not lower plasma cholesterol in rats, but yet this species was alone in providing activity for fibrate-type drugs. Nicotinic acid possesses many desirable features with regard to clinical use, but most of these actions are lacking in rats and monkeys. The metabolism of low density lipoproteins in hamsters is widely thought to be similar to that in humans, yet neither statins or fibrates lower plasma lipids in these species. With the advent of mouse models expressing specific human genes (or disruption of genes) it is now possible to re-examine the effect of established drugs and to characterize new hypolipidemic compounds with respect to site and mechanism of action. Drug responses observed in humans are now being seen in such mouse models (e.g. HDL elevation with fenofibrate in mice with the human apo A-I gene). Moreover, mice are now being screened for compounds that lower plasma (human) Lp(a), or lower plasma cholesterol in the absence of LDL receptors. It is proposed that these new genetic mouse models may afford a more focused examination of drug action and provide, for new compounds, better prediction of the human response.

Effect of hypolipidemic drugs on key enzyme activities related to lipid metabolism in normolipidemic rabbits

The effect of atorvastatin (3 mg kg(-1) day(-1)), simvastatin (3 mg kg(-1) day(-1)) and bezafibrate (100 mg kg(-1) day(-1)) administered for 4 weeks to male New Zealand white rabbits on enzyme activities related to lipid metabolism has been studied. Only the statins reduced plasma cholesterol values, while none of the drugs modified plasma triglyceride or high density lipoprotein (HDL)-cholesterol concentrations, nor the activity of enzymes such as hepatic diacylglycerol acyltransferase, lipoprotein lipase or hepatic lipase, directly involved in triglyceride metabolism. Both statins elicited similar increases in the hepatic microsomal 3-hydroxy-3-methyl-glutaryl Coenzyme A (CoA) reductase activity (147 and 109% induction for simvastatin and atorvastatin, respectively), and none of the drugs assayed modified hepatic acyl-coenzyme A:cholesterol acyltransferase activity significantly. Only bezafibrate induced a significant 57% reduction in the activity of hepatic microsomal cholesterol 7alpha-hydroxylase. Regarding the rate limiting enzyme of phosphatidylcholine biosynthesis, CTP:phosphocholine cytidylyl transferase, atorvastatin and bezafibrate behaved similarly, decreasing the enzyme activity in the liver by 45% and 54%, respectively; simvastatin induced no modification of this activity. The reduction of CTP:phosphocholine cytidylyl transferase activity is not caused by a direct inhibition of the enzyme by bezafibrate and atorvastatin. Further, the inhibitory effect of atorvastatin appears to be unrelated to the inhibition of 3-hydroxy-3-methyl-glutaryl CoA reductase elicited in vivo.

Effects of statins on triglyceride metabolism

Hepatic 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase inhibitors, or "statins," have been extremely efficacious in decreasing low-density lipoprotein (LDL) cholesterol levels in patients with hypercholesterolemia and in treating patients with dysbetalipoproteinemia, a relatively rare form of hyperlipidemia. Clinical trials have indicated that statins can significantly lower very-low-density lipoprotein (VLDL) triglyceride levels, although the central mechanism of action of statins-i.e., increasing the number of LDL receptors-would appear to suggest that they would have no significant effect on VLDL levels. Through a review of published data from animal and human studies, this article addresses the important clinical question of how drugs that inhibit the rate-limiting enzyme for cholesterol can affect triglyceride metabolism.

Increased cellular triglyceride levels in human monocytic and rat smooth muscle cells after lovastatin

Beta-hydroxy-beta-methyl-glutaryl-coenzyme A (HMG-CoA) reductase inhibitors reduce plasma LDL cholesterol by upregulating hepatic LDL receptors. However, their effects on lipid metabolism in extrahepatic cells may also contribute to their therapeutic benefit. We examined the effects of lovastatin (LOV) on cellular lipid levels in the human monocytic Mono Mac 6sr and cultured rat smooth muscle cells. In both cell types, LOV produced a dose-dependent increase in cellular triglycerides. This increase was observed in cells grown in the absence of exogenous lipids in the culture medium, but was more pronounced after additions of oleic acid (50 to 200 microM) and VLDL (50 to 200 microg ml-1). In Mono Mac 6sr cells grown in medium containing 10% delipidated FCS for the last 16 h, the LOV-induced rise in triglyceride levels was completely reversed by 2 mM mevalonic acid and was associated with a decrease in cellular cholesterol. However, when cells were maintained in lipoprotein-replete medium, the LOV-induced rise in triglycerides did not correlate with cellular cholesterol. LOV also reduced cellular cholesterol esterification and increased the synthesis of fatty acids and their incorporation into triglycerides and phospholipids. Increased triglyceride levels were also seen in Mono Mac 6sr cells treated with the lanosterol demethylase inhibitor RS-21607 and the acylcoenzyme A:cholesterol acyltransferase inhibitor SaH 58035. Our findings suggest that the LOV-induced triglyceride accumulation involves changes in intracellular cholesterol pools regulating cellular fatty acid concentrations. Although decreased cholesterol levels in cells participating in plaque formation are beneficial, the impact of the herein described shift in intracellular neutral lipid metabolism on other cellular functions warrants further investigation.

Difficulties in the assay of phosphatidate phosphohydrolase activity. Influence of ionic strength, detergent, and selection of substrate

In the present paper, problems in connection with assay of the activity of magnesium-dependent rat liver phosphatidate phosphohydrolase (PAP) are discussed. PAP activity is usually measured by following the production of diacylglycerol or inorganic phosphate from the substrate phosphatidate. These two methods may give widely different results due to a number of factors that may affect the assay. One such factor is the composition of the substrate. Higher apparent enzyme activity was observed with dioleoyl-phosphatidate than with dipalmitoyl-phosphatidate. This substrate-dependent difference in apparent PAP activity was 2-2.5-fold in the absence and 10-fold in the presence of Triton X-100, respectively. Triton X-100 reduced the activity as measured with the dipalmitoyl-phosphatidate substrate. In contrast, the activity of PAP as measured with dioleoyl-phosphatidate was stimulated by Triton X-100. The stimulatory effect of Triton was reduced or abolished when the ionic strength in the assay mixture was increased. Assays based on 32P-labeled substrate are rapid and sensitive. It is shown here that 33P can be used as an alternative. This radionuclide has a longer half-life and also emits particles with lower energy, thus posing less potential health hazards for the user.

Lipid-lowering activity of atorvastatin and lovastatin in rodent species: triglyceride-lowering in rats correlates with efficacy in LDL animal models

Since inhibitors of HMG-CoA reductase lower plasma triglycerides rather than cholesterol in rats, we compared the triglyceride-lowering activity of lovastatin in rats to that of atorvastatin, a more potent synthetic inhibitor, prior to evaluating these drugs in established animal models in which low density lipoproteins (LDL) rather than high density lipoproteins (HDL) are the major transporters of plasma cholesterol. Atorvastatin was more efficacious than lovastatin in normal, chow-fed rats, and more potent in rats with endogenous hypertriglyceridemia (sucrose-fed). In hypertriglyceridemic rats plasma apoB concentrations decreased only with atorvastatin (30 mg/kg), and VLDL-triglyceride secretion (Triton method) was also decreased more by atorvastatin. The inactive enantiomer of atorvastatin did not lower plasma triglycerides. Thus, triglyceride-lowering was dependent upon inhibition of HMG-CoA reductase. Liver unesterified cholesterol and cholesteryl esters (mg/g) were increased by both drugs in normal rats but remained unchanged in hypertriglyceridemic rats. In normal, chow-fed guinea pigs atorvastatin was a more potent cholesterol-lowering drug, and unlike lovastatin, lowered plasma triglycerides and VLDL-cholesterol. In casein-fed rabbits with endogenous hypercholesterolemia and in chow-fed rabbits atorvastatin lowered LDL-cholesterol more potently than lovastatin, but in chow-fed rabbits neither drug had an effect on the in vivo rate of VLDL-lipid secretion, suggesting that efficacy was due to inhibition of direct LDL production and/or enhanced LDL clearance. We conclude that normal rats can be used as a preclinical tool to assess the efficacy of HMG-CoA reductase inhibitors since triglyceride-lowering correlates with cholesterol-lowering in LDL animal models. In this regard atorvastatin is a more potent hypolipidemic agent than lovastatin in animals. A common but not sole mechanism for these drugs may be direct inhibition of the hepatic production of the major apoB-containing lipoprotein in a given species, e.g. VLDL in rats and LDL in guinea pigs and rabbits.