Advances in experimental dyslipidemia and atherosclerosis

No effect of dietary fat on short-term weight gain in mice treated with atypical antipsychotic drugs

RATIONALE

Atypical antipsychotic drugs (AAD) induce significant weight gain in female C57BL/6J mice. The effect of dietary fat on weight gain and serum lipids in this model is unknown.

OBJECTIVES

Test the hypothesis that the obesigenic effects of these drugs are greater in the presence of a high-fat diet.

METHODS

Female C57BL/6J mice were treated with atypical antipsychotics for 3 weeks and fed either a low-fat or high-fat diet (4.6 vs 15.6% fat by wt). Food intake (FI), body weight (BW), body composition, and serum lipids were measured during treatment with optimized doses of olanzapine, quetiapine, and risperidone. Energy intake (EI) and feed efficiency (FE) were calculated. Group differences in change were analyzed via repeated measures analysis of variance (ANOVA). Serum lipid concentrations, EI and FE were compared using two-way ANOVA.

RESULTS

AAD-treated mice gained significantly more weight than controls after 3 weeks (P<0.001). Treatment and diet had significant effects on FI and EI over time (P<0.001). AAD-treated mice had significantly higher FE than controls (P<0.05); however, there was no significant drug by diet interaction (P=0.65). Risperidone low-fat mice gained significantly more absolute fat mass than placebo low-fat mice (P<0.05). All treatment groups, except quetiapine low-fat and olanzapine high-fat, gained significantly more absolute lean mass than placebo controls (P<0.05). Cholesterol levels were significantly lower in quetiapine and risperidone than placebo (P<0.05). Risperidone low-fat mice had significantly higher triglyceride levels than placebo and risperidone high-fat mice (P<0.05).

CONCLUSIONS

A high-fat diet does not increase AAD-induced BW gain in female mice during a 3-week treatment period.

Surfactant protein D is proatherogenic in mice

Surfactant protein D (SP-D) is an important innate immune defense molecule that mediates clearance of pathogens and modulates the inflammatory response. Moreover, SP-D is involved in lipid homeostasis, and pulmonary accumulation of phospholipids has previously been observed in SP-D-deficient ( Spd−/−) mice. Atherogenesis involves both inflammation and lipid deposition, and we investigated the role of SP-D in the development of atherosclerosis. SP-D synthesis was localized to vascular endothelial cells. Atherosclerotic lesion areas were 5.6-fold smaller in the aortic roots in Spd−/− mice compared with wild-type C57BL/6N mice on an atherogenic diet. HDL cholesterol (HDL-C) was significantly elevated in Spd−/− mice. Treatment of Spd−/− mice with a recombinant fragment of human SP-D resulted in decreases of HDL-C (21%) as well as total cholesterol (26%), and LDL cholesterol (28%). Plasma TNF-α was reduced in Spd−/− mice (45% difference). SP-D was proatherogenic in the mouse model used. The effect is likely to be due to the observed disturbances of plasma lipid metabolism and alteration of the inflammatory process, which underlie the reduced susceptibility to atherosclerosis in Spd−/− mice.

Paradoxical effects of fenofibrate and nicotinic acid in apo E-deficient mice

Atherosclerosis is a complex vascular disease initiated by abnormal accumulation of plasma lipoproteins in the subendothelial space. Elevated levels of plasma triglycerides (TG) and low-density lipoprotein (LDL)-cholesterol as well as low concentrations of high-density lipoprotein (HDL) play a causal role in the development and progression of atherosclerotic lesions. We have shown that apolipoprotein E-deficient (apo E-KO) mice have elevated triglyceride levels plus diminished HDL concentrations. Drugs such as fenofibrate and nicotinic acid are well known to reduce TG and increase HDL levels in humans. In this study, we investigated the beneficial effects of fenofibrate and niacin on lipid profile and atherogenesis in apo E-KO mice and their wild-type counterparts. Animals were fed with a cholesterol-enriched diet supplemented with fenofibrate (0.1% wt/wt, n = 8) or nicotinic acid (0.5% wt/wt, n = 8) for 14 weeks. Body weights were recorded weekly, and plasma lipid profiles were determined at 4-week intervals. The hearts and aortas were collected and fixed for histologic and morphometric evaluations of atherosclerotic lesions. Fenofibrate treatment in apo E-KO mice paradoxically increased total cholesterol and TG by 65% and 44%, respectively, and decreased HDL-cholesterol levels by 35% as compared with controls. Similar effects of fenofibrate on cholesterol levels, but not on TG concentrations, were observed in C57BL/6 mice. Fenofibrate-treated mice had lower body weight as compared with controls. Niacin had no effect on body weight gain but failed to decrease TG or to increase HDL levels in either apo E-KO mice or their wild-type counterparts. Neither fenofibrate nor niacin significantly influenced atherogenesis in apo E-KO mice as compared with controls. In conclusion, this study shows that neither niacin nor fenofibrate has beneficial lipid-modifying and antiatherosclerosis activities in mice. Identification of mechanisms underlying paradoxical effects of fenofibrate on lipoprotein metabolisms in apo E-KO mice merits further investigation.

Combination of dietary phytosterols plus niacin or fenofibrate: effects on lipid profile and atherosclerosis in apo E-KO mice

Patients with mixed dyslipidemias (increased LDL cholesterol and triglyceride as well as low HDL cholesterol levels) benefit from a combination of lipid-modifying drugs such as statins, niacin, fibrates and ezetemibe. However, safety, tolerability and cost are a concern in drug combination therapy. Dietary phytosterols reduce LDL cholesterol, and niacin or fenofibrate primarily reduces triglyceride and increases HDL-cholesterol levels. Thus, we hypothesized that a combination of phytosterols with niacin or fenofibrate will synergistically impact lipoprotein profile and atherogenesis in apo E-KO mice. Phytosterols alone significantly reduced plasma total cholesterol levels (14.1 vs. 16.9 mmol/L, P < .05) and the extent of atherosclerosis (0.42 vs. 0.15 mm(2), P < .05). The addition of fenofibrate to phytosterols increased plasma total cholesterol levels by >50% (14.1 vs. 21.6 mmol/L, P < .05) and decreased HDL-cholesterol concentrations by 50% (0.8 vs. 0.4 mmol/L). These changes were accompanied by slight reductions in the extent of atherosclerosis (0.42 vs. 0.34 mm(2), P > 0.05) as compared to controls, suggesting other potential anti-atherogenic effects of fenofibrate. Unlike fenofibrate, niacin caused an increase of 150% (P < .05) in HDL-cholesterol concentrations and a decrease of 22% (P < .05) in total cholesterol levels which were associated with significant reductions (65%, P < .05) in atherosclerotic lesion size as compared to controls. Neither the addition of niacin nor of fenofibrate reduced plasma triglyceride levels. In conclusion, the addition of niacin to phytosterols synergistically increases HDL-cholesterol levels, while a combination of phytosterols and fenofibrate results in no synergistic effects in apo E-KO mice. Further studies in other animal models are needed to establish synergetic effects between these lipid-modifying dietary and pharmacological agents.

Physical activity and atherosclerosis: which animal model?

Atherosclerosis is a progressive disease that is the most important single contributor to human cardiovascular morbidity and mortality. Epidemiologic studies show that physical activity, or routine exercise, reduces the risk of developing cardiovascular disease. The mechanisms through which exercise may function in primary or secondary prevention of atherosclerosis remain largely to be established. Most studies in humans are performed after the onset of clinical signs when disease is well advanced and the prescription of exercise is based on empirical evidence of benefit in secondary prevention. Animal models per-mit the study of the initiation and progression of preclinical stages of atherosclerosis. In order to provide information relevant to treatment and prevention, these models should mimic human disease and interactions of physical activity with disease processes as closely as possible. The purpose of this review is to compare animal models of atherosclerosis and to summarize the available data in those models in regard to the effects of exercise.

Dietary cholesterol, atherosclerosis and coronary heart disease

As early as at the beginning of the last century, animal studies have pointed to a causal role of dietary cholesterol in atherogenesis. In humans, however, most observational studies have not provided convincing evidence for an impact of cholesterol intake on coronary heart disease (CHD). Rather, these studies have consistently established a close association between a certain eating pattern and the risk of CHD. This eating pattern has usually been characterized by a high intake of total fat, saturated fatty acids (SFA) and cholesterol, and a low intake of fiber and polyunsaturated fatty acids (PUFA). In typical western diets the amounts of total fat, SFA, and cholesterol are strongly correlated with each other, while they are negatively related to the intake of fiber and PUFA. Thus, it has not been possible to determine whether the association between the above mentioned eating pattern and CHD is due to the high consumption of SFA, cholesterol, both, or an insufficient supply of one or more protective factors such as fiber or PUFA. As the consumption of eggs leads to a high intake of cholesterol without necessarily resulting in high uptake levels of SFA and total fat, several groups have tried to elucidate the effect of cholesterol by investigating the relationship between the consumption of eggs and the development of CHD. Based on these studies, the association between dietary cholesterol and CHD risk is, if anything, minor in nature. This is consistent with the finding that an increase in dietary cholesterol intake results in only a minimal increase in the total/high-density lipoprotein cholesterol ratio. Taken together these studies suggest that the association between dietary cholesterol and CHD is small, as most subjects can effectively adapt to higher levels of cholesterol intake. Nevertheless, lowering dietary cholesterol content might reduce the risk of CHD considerably in a subgroup of individuals who are highly responsive to changes in cholesterol intake.

Possible involvement of enhanced intestinal microsomal triglyceride transfer protein (MTP) gene expression in acceleration of lipid absorption by a western-type diet in apolipoprotein E knockout mice

Evidence has been accumulating that triglyceride (TG)-rich lipoproteins are atherogenic. Microsomal TG transfer protein (MTP) is essential for the synthesis of both chylomicron in the intestine and very low density lipoprotein in the liver. To investigate whether a western-type diet, a so-called atherogenic diet, alters intestinal lipid absorption via change in intestinal MTP expression, the effects of two different diet regimes in apolipoprotein-E knockout (apoE KO) mice were examined. Male apoE KO mice aged 6 weeks were fed a western-type diet or a chow diet for 5 weeks. Then, measurement of plasma TG levels after oral fat-loading and analysis of jejunal MTP gene expression were performed. Both the maximum level and the 0-8 h area under the curve (AUC) of the increase in TG levels in the western-type diet-fed mice were almost three times greater than those in the chow diet-fed mice. MTP gene expression, determined by reverse transcriptase-polymerase chain reaction (RT-PCR), was obviously enhanced in the western-type diet-fed mice compared to the chow diet-fed mice. These results suggest that the enhancement of intestinal MTP gene expression is involved in the accelerated lipid absorption in the western-type diet-fed mice.

Antiatherogenic properties of metformin: the experimental evidence

Cardiovascular disease (CVD) is the major determining factor of morbidity and mortality in type 2 diabetic patients. The established relationship between type 2 diabetes and atherosclerosis has fueled suggestions that anti-diabetic drugs with beneficial effects on CV risk factors may help attenuate the atherosclerotic process in diabetic patients. Metformin is a hypoglycaemic agent widely used in the management of type 2 diabetes. In addition to its insulin-sensitising action, this drug has favourable effects on various CV risk factors and reduces macrovascular complications in obese type 2 diabetic patients. This review summarises in vivo and in vitro experimental evidence on the antiatherogenic properties of metformin.

Comparative effects of the antipsychotics sulpiride or risperidone in rats. I: bodyweight, food intake, body composition, hormones and glucose tolerance

Obesity and related metabolic disorders are important side effects of some antipsychotic drugs (APs). The currently available animal model of AP-induced bodyweight gain (BWG) in rats is based on administration of sulpiride (SUL). However, this model has important limitations. For example, SUL is a pure dopamine antagonist, whereas most APs in current clinical use interact with multiple neurotransmitter receptors involved in food intake (FI) and metabolism regulation. Therefore, we evaluated the effects of risperidone (RIS, 0.125, 0.25 or 0.5 mg/kg during 16 days) on BWG and FI in male and female rats. Comparison between RIS (0.5 mg/kg), SUL (20 mg/kg) and vehicle (VEH) during 12 days was also conducted in females. In male rats, RIS did not significantly affect BWG, FI, glucose tolerance or leptin levels, even though prolactin and corticosterone were significantly elevated. In females, both APs significantly increased BWG and FI, but the effect was stronger with SUL. The BWG was significantly associated with an increase in body fat. Serum leptin levels were increased only in SUL-treated rats. The area under the curve for glucose (AUGC) was significantly lower in the SUL group, but it was similar for insulin in all treatment groups. The area under the curve for insulin (AUIC) and BWG positively correlated only in the RIS group. Prolactin and corticosterone were significantly increased by both APs. Serum estradiol levels were significantly increased by RIS but not by SUL, but progesterone levels were similar in both groups. The observed positive correlation between BWG and the AUIC during RIS administration suggests that this agent may represent a better model of AP administration in humans. The animal model of RIS-induced obesity in rats might be improved by testing other doses, route of administration and type of diet.

Peroxisome proliferator-activated receptor agonists, hyperlipidaemia, and atherosclerosis

Dyslipidaemia is a major risk factor in the development of atherosclerosis, and lipid lowering is achieved clinically using fibrate drugs and statins. Fibrate drugs are ligands for the fatty acid receptor peroxisome proliferator-activated receptor (PPAR)alpha, and the lipid-lowering effects of this class of drugs are mediated by the control of lipid metabolism, as directed by PPARalpha. PPARalpha ligands also mediate potentially protective changes in the expression of several proteins that are not involved in lipid metabolism, but are implicated in the pathogenesis of heart disease. Clinical studies with bezafibrate and gemfibrozil support the hypothesis that these drugs may have a significant protective effect against cardiovascular disease. The thiazolidinedione group of insulin-sensitising drugs are PPARgamma ligands, and these have beneficial effects on serum lipids in diabetic patients and have also been shown to inhibit the progression of atherosclerosis in animal models. However, their efficacy in the prevention of cardiovascular-associated mortality has yet to be determined. Recent studies have found that PPARdelta is also a regulator of serum lipids. However, there are currently no drugs in clinical use that selectively activate this receptor. It is clear that all three forms of PPARs have mechanistically different modes of lipid lowering and that drugs currently available have not been optimised on the basis of PPAR biology. A new generation of rationally designed PPAR ligands may provide substantially improved drugs for the prevention of cardiovascular disease.