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

Abstract

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. 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 posttranscriptionally. 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. Increased secretion of apolipoprotein B (apoB)1 has been associated with the hyperlipidemia that presents in a number of physiological disorders, including familial combined hyperlipidemia (1-4) and the dyslipidemia associated with insulin resistance (5, 6). As such, understanding the processes that regulate the secretion of apoB has been the subject of extensive clinical and molecular investigation.Cell culture experiments have provided convincing data that the primary regulation of apoB secretion occurs post-translationally and that the intracellular degradation of apoB is an important determinant of the amount of the protein that is ultimately secreted (7-9). In several cell systems, the availability of intracellular triglyceride has been shown to affect the rates of apoB degradation and thus secretion. Specifically, in HepG2 cells, incubation with oleate led to the increased secretion of apoB into the medium by preventing the intracellular degradation of the protein (10). This stimulatory effect of oleate on apoB secretion has also been demonstrated in McARH-7777 cells, a rat hepatoma cell line (11); perfused livers from fasted rats (12); and livers from rats fed a high carbohydrate diet (13). In contrast, apoB secretion rates were unaltered by oleate availability in primary rat hepatocytes (14) and in perfused livers (12, 15) from fed rats.We sought to create a mouse model that would allow for the in vivo investigation of mechanisms underlying the increased apoB secretion observed in humans with insulin resistance. Mice have distinctly different lipid profiles compared with humans with most cholesterol carried in HDL (16). Transgenic mice expressing human apoB100 display an accumulation of lipid in LDL (17) and have been used to investigate the regulation of apoB secre...

High-density lipoprotein metabolism in human apolipoprotein B100transgenic/brown adipose tissue deficient mice: a model of obesity-induced hyperinsulinemia

“…Male hApoBtg/BATless mice on an HF-HC diet indeed developed hyperinsulinemia with weight gain compared with control mice. However, inconsistent with the previous report by Siri et al (2001) that plasma HDL-C levels were reduced in male hApoB tg /BATless mice, we did not observe the reduction in our mice. The reason for this discrepancy is not known.…”

“…An additional noticeable difference between these 2 studies is plasma triglyceride concentrations. Compared with male hApoB tg control mice, plasma triglyceride levels were significantly increased in the study by Siri et al (2001) but there was no significance difference in our mice (p = 0.18). It is possible that because of the disparate genetic background of hApoB tg mice, apoB100-containing lipoprotein metabolism could differ between 2 studies.…”

“…The resulting hApoB tg /BATless mice on an HF-HC diet have previously been shown to be susceptible to diet-induced obesity, hypercholesterolemia, hypertriglyceridemia, and hyperinsulinemia (Siri et al 2001). In addition, the plasma lipid profile of male hApoB tg /BATless mice on an HF-HC diet has been shown to be similar to that of obese and diabetic humans in regards to lipoprotein distribution and size with reduced plasma HDL-C levels compared with hApoB tg control mice (Siri et al 2001). Our initial intention was to utilize this mouse model of obesity-induced insulin resistance and dyslipidemia for elucidation of altered HDL metabolism.…”

“…However, we speculate that it may be related to difference in the genetic background of the mice. hApoB tg mice used in the study of Siri et al (2001) had C57BL/6J background whereas ours had a mixed background. An additional noticeable difference between these 2 studies is plasma triglyceride concentrations.…”

“…We chose brown adipose tissue (BAT) deficient (BATless) mice crossed with human apoB100 transgenic mice (hApoB tg ) to generate hApoB tg /BATless mice. The hApoB tg /BATless mice on a high-fat-high-cholesterol diet showed dyslipidemia similar to that of humans with development of obesity and insulin resistance (Siri et al 2001). …”