| Atherosclerosis (AS) is a complex multifactorial disease. Environmental factors such as diet play significant roles in the development of AS. It is known that diets high in saturated fat and cholesterol increase the risk of CHD when administered to mice with a susceptible background. Long-term consumption of diets high in saturated fat and cholesterol can lead to atherogenic dyslipidemia, a component of metabolic syndrome which characterized by hypercholesterolemia, hypertriglyceridemia and other risk of atherosclerotic cardiovascular disease [1,2]. The liver synthesizes and releases various kinds of enzyme for lipoprotein metabolism. As the center of endogenous and exogenous lipid metabolism pathway, it plays an important balance role for the lipid metabolism, so it is the important target organ to research the lipid metabolism.The apoE-deficient (apoE-/-) mice, which are considered to be one of the most relevant models for AS, were generated by inactivating the apoE gene by targeting. They are hypertriglyceridemia and develop spontaneous atherosclerotic lesions. The sequential events involved in lesion formation in this model are strikingly similar to those in well-established larger animal models of AS and in humans.While plasma and liver lipid abnormalities after high fat and cholesterol diet have been well characterized, their molecular basis remains unclear. The effects of high fat and cholesterol diet on these progressions and their key metabolic abnormalities are likely mediated by changes in gene expression. Several transcription factors have been shown to respond to the diet content, including peroxisome proliferater activated receptorα(PPARα) and liver X receptorα(LXRα). Now, it is unclear to what extent these transcription factors play a role in gene regulation mediated by high fat and cholesterol (HF) diet in apoE-/- mice and whether there are different regulatory mechanism in mice of different genetic background. To determine how a HF diet influences the expressions of PPARα, LXRαand their responsive genes and their roles in the control of lipid homeostasis, we used semi- and real-time quantitative RT-PCR technology assessing series of genes associated with lipid metabolism in apoE-/- and wild-type (WT) mice. The serum total cholesterol (TC), triglyceride (TG), low-density lipoprotein cholesterol (LDL-C) and high-density lipoprotein cholesterol (HDL-C) levels were assayed using COD-PAP and GPO-PAP methods. Hearts and livers were serially sectioned at -15℃, stained with Sudan IV and Red oil O, respectively.Expression profiles of 8 genes were detected by semi-quantitive RT-PCR. The levels of LXRα, fatty acid synthase (FAS), angiopoietin-like protein 3 (Angptl3) were significantly increased by HF diet at 8 and 12 weeks of age (P < 0.05) in both genotype mice. In apoE-/- mouse groups, the expressions of apolipoprotein AIV (apoAIV), stearoyl-COA desalurase-I (SCDI), apolipoprotein B100 (apoB100) were up regulated by HF diet at the age of 8 and 12 weeks (P < 0.05), while carnitine palmitoyl transferase I (CPTI), acyl coenzyme A oxidase 1(ACOX1) were down regulated at the age of 4 and 8 weeks (P < 0.05). In WT mice groups, the mRNA level of CPTI was up regulated by HF diet at the age of 8 and 12 weeks (P < 0.05), while no differences were detected in the expressions of ApoAIV, SCDI, apoB100 and ACOX1. Expression profiles of 6 genes were detected by real-time quantitative RT-PCR. The expression of PPARαwas down regulated in apoE-/- mouse groups by HF diet, while up regulated in WT mice. The expressions of apolipoproteinAI (apoAI), scavenger receptor class B type I (SR-BI) and apolipoprotein AV (apoAV) were markedly reduced by HF diet in apoE-/- mouse groups (P < 0.05). A marked up-regulation of SR-BI mRNA level was induced by HF diet in WT mice. The expressions of lipoprotein lipase (LPL) and fatty acid translocase (Fat/CD36) were highly induced by HF diet in both genotype mice (P < 0.01).In the apoE -/- mouse groups, HF feeding elevated TC, LDL-C and HDL-C levels significantly beginning at 4 weeks of age by ~3-4 folds and the trend sustained throughout the treatment period, significant up-regulation of TG level was also observed. In WT mice, TC, LDL-C and HDL-C levels were increased by HF diet beginning at 8 weeks of age with no change of TG levels observed over time.ApoE-/- mice fed with HF diet showed serious hepatic steatosis at 8 weeks of age and the phenotype got worse at 12 weeks of age. Obvious lipid accumulation was also detected in NC-fed apoE-/- mice at the age of 12 weeks but it was markedly less compared with HF-fed apoE-/- mice at corresponding time point. When referred to WT mice groups, some small lipid droplets were only detected in HF-fed group at 12 weeks of age. Administration of HF diet to apoE-/- mice exacerbated the lesion development, inducing more serious lipid deposition relative to NC-fed apoE-/- mice at corresponding age. At the age of 12 weeks, the lesion area of HF-fed apoE-/- mice was ~2 fold larger than that of NC-fed apoE-/- mice (P < 0.01). However, there was no notable lipid deposition detected in WT mice groups at any age.In conclusion, this study shows that HF diet can induce hepatic steatosis and aggravate dyslipidemia which could result in more serious atherosclerotic disease in apoE-/- mice. The different changes of lipid metabolism related genes between apoE-/- and WT mice, fed with HF or chow diet, indicated that the mechanisms of dietary effects on gene mutant mice were different from those on WT mice. We suggest that the changes of PPARα, LXRαand their responsive genes aggravate lipid metabolic disorder in the liver and further accelerate the development of AS on a stress of HF diet feeding in apoE-/- mice. It is very likely that assays for these genes will emerge as important target for therapy in atherosclerosis disease induced by HF diet.
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