ApoB Mice Research Articles

Changes in arterial function in a mouse model of human familial hypercholesterolaemia

Atherosclerosis is the most common cause of cardiovascular disease. The ApoB mouse is a model for human familial hypercholesterolaemia and has a lipoprotein profile similar to that of humans with atherosclerosis. Therefore, it is a suitable model to investigate the changes in vasoreactivity during atherogenesis. This study investigates contractile and dilatative properties of arteries in this model in relation to age. Male ApoB mice and B6, wild-type (WT), mice were examined at age four or 18 months. Isometric measurements of 2-mm ring preparations of the aorta thoracica were performed using a wire myograph. Histological and biochemical methods served to determine atherosclerosis, lipid status and endothelial markers respectively. Morphometric analysis showed that all old ApoB mice had severe atherosclerosis in the aorta. Atherosclerotic alteration of the aorta of the ApoB mice coincided with a diminished vasodilatation to acetylcholine. The phenylephrine response was significantly attenuated already to the same degree in the non-atherosclerotic aorta of the young ApoB mice as in the atherosclerotic aorta of the older ApoB mice. Serum parameters showed a rise in total cholesterol and triglycerides in the ApoB strain compared to WT mice. Soluble intercellular adhesion molecule (sICAM)-1 and soluble vascular adhesion molecule (sVCAM)-1 were increased in old compared to young ApoB mice. The study shows that reduced acetylcholine-induced dilatation is related to the presence of atherosclerosis in old ApoB mice. Remarkably, the impaired vessel reactivity to phenylephrine already in young ApoB mice indicates early changes in vascular function in this model.

Abstract 406: In Vivo Measurement of Cholesterol Deposition in Peripheral Macrophages

Cholesterol accumulation in macrophages contained in peripheral tissue is an initial step in the initiation and progression of atherosclerosis. Due to the difficulty in harvesting atherosclerotic macrophages and the complexity of atherosclerotic plaques, it is difficult to obtain reliable measurements of cholesterol mass contained in these cells in vivo. We describe a method for measuring cholesterol accumulation in vivo in peripheral macrophages and demonstrate rapid cholesterol deposition in macrophages when transplanted in a mouse with an atherosclerotic genetic background. Previously, we described a novel method for measuring in vivo cholesterol mass changes in macrophages by entrapping macrophages from donor mice into porous hollow fibers and then implanting the fibers into the peritoneal cavity of a recipient mouse. This procedure allows for the recovery of the cells after recovery of the fibers. We now show that subcutaneous implantation of macrophage-containing fibers results in cholesterol mass changes when the fibers are implanted for 24h. Using this system, we implanted fibers containing wild type peritoneal macrophages subcutaneously into mice with three different genetic backgrounds: 1) wild type, 2) LDLr apoBec dKO and 3) LDLr apoBec dKO/human apoB background. Serum cholesterol levels for the LDLr apoBec dKO and LDLr apoBec dKO/human apoB was 484±101 and 917±12mg/dl respectively, whereas serum cholesterol content of in wild-type controls was 108±6mg/dl. Significant cholesterol deposition in the macrophages was detected as early as 4h after implantation. Following 24h of implantation there was a 3-fold increase in cholesterol mass in the macrophages implanted in the LDLr apoBec dKO, and a five-fold increase in cholesterol content in the macrophages implanted in the LDLr apoBec dKO/human apoB mice. This increase in cell cholesterol mass was a result of expansion of both the unesterified and esterified cholesterol pools, resulting in a 50% / 50% distribution after 24h.

Overexpression of apolipoprotein B attenuates pathologic cardiac remodeling and hypertrophy in response to catecholamines and after myocardial infarction in mice

Introduction. The heart produces apolipoprotein (apo) B-containing lipoproteins which enables cardiac export of potentially cardiotoxic lipids. We hypothesized that overexpression of apoB attenuates the pathologic cardiac remodeling and hypertrophic response following pathological stimuli such as chronic adrenergic overstimulation and myocardial infarction (MI). Methods. Cardiac hypertrophy was induced by a chronic infusion of isoproterenol (ISO) 15 mg/kg/day for 3 weeks in human apoB transgenic mice (n = 9) and in non-transgenic wild-type mice (n = 10). As controls, apoB transgenic (n = 10) and wild-type mice (n = 10) saline infusions were used. Transthoracic echocardiography was performed at baseline and after 3 weeks of treatment to evaluate left ventricular (LV) function and morphology. To investigate the effects of expression on postinfarct hypertrophic response we induced MI in apoB transgenic mice (n = 8) and in wild-type controls (n = 11). The hearts were explanted and weighed 6 weeks post MI. Results. At baseline, WT mice had higher BW and LV mass (LVM) compared to the apoB mice. The increase in LV mass and dimensions after 3 weeks of treatment with ISO was significantly lower while systolic function was significantly better in the apoB group. Six weeks post MI the apoB mice had significantly lower heart weight and heart weight to body weight ratio. The infarct size was similar in both groups. Conclusion. Overexpression of apoB attenuates the pathologic remodeling and hypertrophic response to chronic adrenergic stimulation and MI. Our results indicate that cardiac expression of apoB-containing lipoproteins might be an important regulator of myocardial structure and function.

Overexpression of apolipoprotein-B improves cardiac function and increases survival in mice with myocardial infarction

BackgroundThe heart produces apolipoprotein-B containing lipoproteins (apoB) whose function is not well understood. The aim of this study was to evaluate importance of myocardial apoB for cardiac function, structure and survival in myocardial infarction (MI) and heart failure (HF). Methods and resultsMI was induced in mice (n=137) and myocardial apoB content was measured at 30min, 3, 6, 24, 48, 120h and 8weeks post-MI. Transgenic mice overexpressing apoB (n=27) and genetically matched controls (n=27) were used to study the effects of myocardial apoB on cardiac function, remodeling, arrhythmias and survival after MI. Echocardiography was performed at rest and stress conditions at baseline, 2, 4 and 6week post-MI and cumulative survival rate was registered. The myocardial apoB content increased both in the injured and the remote myocardium (p<0.05) in response to ischemic injury. ApoB mice had 2-fold higher survival rate (p<0.05) and better systolic function (p<0.05) post-MI. ConclusionOverexpression of apoB in the heart increases survival and improves cardiac function after acute MI. Myocardial apoB may be an important cardioprotective system in settings such as myocardial ischemia and HF.

Aberrant Hepatic Expression of PPARγ2 Stimulates Hepatic Lipogenesis in a Mouse Model of Obesity, Insulin Resistance, Dyslipidemia, and Hepatic Steatosis

Insulin-resistant apoB/BATless mice have hypertriglyceridemia because of increased assembly and secretion of very low density apolipoprotein B (apoB) and triglycerides compared with mice expressing only apoB (Siri, P., Candela, N., Ko, C., Zhang, Y., Eusufzai, S., Ginsberg, H. N., and Huang, L. S. (2001) J. Biol. Chem. 276, 46064-46072). Despite increased very low density lipoprotein secretion, apoB/BATless mice have fatty livers. We found that hepatic mRNA levels of key lipogenic enzymes, acetyl-CoA carboxylase, fatty-acid synthase, and stearoyl-CoA desaturase-1 were increased in apoB/BATless mice compared with levels in apoB mice, suggesting increased lipogenesis in apoB/BATless mice. This was confirmed by determining incorporation of tritiated water into fatty acids. Neither the hepatic mRNA of the lipogenic transcription factor, SREBP-1c (sterol-response element-binding protein 1c), nor the nuclear levels of the mature form of SREBP-1 protein were elevated in apoB/BATless mice. By contrast, hepatic levels of peroxisomal proliferator-activated receptor 2 (PPARgamma2) mRNA and protein were specifically increased in apoB/BATless mice, as were hepatic mRNA levels of two targets of PPARgamma, CD36 and aP2. Treatment of apoB/BATless mice for 4 weeks with intraperitoneal injections of a PPARgamma antisense oligonucleotide resulted in dramatic reductions of both PPARgamma1 and PPARgamma2 mRNA, PPARgamma2 protein, and mRNA levels of fatty-acid synthase and acetyl-CoA carboxylase. These changes were associated with decreased hepatic de novo lipogenesis and hepatic triglyceride concentrations. We conclude that hepatic steatosis in apoB/BATless mice is associated with elevated rates of hepatic lipogenesis that are linked directly to increased hepatic expression of PPARgamma2. The mechanism whereby hepatic Ppargamma2 gene expression is increased and how PPARgamma2 stimulates lipogenesis is under investigation.

Post-transcriptional Stimulation of the Assembly and Secretion of Triglyceride-rich Apolipoprotein B Lipoproteins in a Mouse with Selective Deficiency of Brown Adipose Tissue, Obesity, and Insulin Resistance

A mouse model of insulin resistance and its associated dyslipidemia was generated by crossing mice expressing human apolipoprotein B (apoB) with mice lacking only brown adipose tissue (BATless). On a high fat diet, male apoB/BATless mice became obese, hypercholesterolemic, hypertriglyceridemic, and hyperinsulinemic compared with control apoB mice. Fast performance liquid chromatography revealed increased triglyceride concentrations in intermediate density lipoprotein/low density lipoprotein (LDL) and reduced high density lipoprotein cholesterol concentrations. Inhibition of lipolysis by the drug, tetrahydrolipostatin, demonstrated that very low density lipoprotein-sized particles were initially secreted. Metabolic studies employing Triton WR-1339 and either [(3)H]glycerol or [(3)H]palmitate showed that the hypertriglyceridemia in apoB/BATless mice was due to the increased synthesis and secretion of triglyceride. Furthermore, lipoprotein lipase and hepatic lipase activities were not defective. ApoB was also secreted at increased rates in the apoB/BATless mice. Similar levels of apoB mRNA in apoB and apoB/BATless mice indicated that apoB secretion was regulated post-transcriptionally. LDL receptor mRNA was increased in the apoB/BATless mice, indicating that the observed increase in apoB-lipoprotein secretion was not due to their decreased reuptake. Finally, mRNA levels of the large subunit of microsomal triglyceride transfer protein, a required component for very low density protein assembly, were not different between apoB and apoB/BATless mice. This rodent model should prove useful in exploring mechanisms underlying the regulation of apoB secretion in the context of insulin resistance.

Expression of human apolipoprotein B90 in transgenic mice. Demonstration that apolipoprotein B90 lacks the structural requirements to form lipoprotein.

Lipoprotein(a) (Lp(a)) is a lipoprotein formed by the disulfide linkage of apolipoprotein(a) (apo(a)) to the apoB100 of a low density lipoprotein particle. Earlier site-directed mutagenesis studies of apo(a) demonstrated that apo(a) cysteine 4057 is required for the disulfide linkage; however, the cysteine residue within apoB100 that is involved in the disulfide bond has not been identified. We previously demonstrated that the apoB100 produced by human apoB transgenic mice binds to apo(a) and forms Lp(a) (Linton, M.F., Farese, R. V., Jr., Chiesa, G., Grass, D. S., Chin, P., Hammer, R. E., Hobbs, H.H., and Young, S.G. (1993) J. Clin. Invest. 92, 3029-3037). To further explore the structural features of human apoB that are required for the formation of Lp(a), we used a transposon-interrupted human apoB gene clone to develop transgenic mice that express high levels of a truncated form of human apoB, apoB90, which contains the amino-terminal 4084 amino acids of apoB. In vitro incubation of apo(a) with the plasma of human apoB90 transgenic mice did not yield Lp(a), as judged by Western blots of SDS-polyacrylamide gels or by a monoclonal antibody-based radioimmunoassay. In contrast, incubation of apo(a) with the plasma of a mouse that expressed an equivalent amount of the full-length apoB100 did yield Lp(a). In addition to these in vitro incubation studies, no Lp(a) could be detected in the plasma of a "double transgenic" mouse expressing both human apoB90 and apo(a). These data indicate that the carboxyl-terminal 10% of apoB100 contains amino acid sequences that are essential for the formation of Lp(a).