Expression levels of ACAT1 and ACAT2 genes in the liver and intestine of baboons with high and low lipemic responses to dietary lipids.

Acyl-coenzyme A:cholesterol acyltransferase (ACAT) 1 and ACAT2 play an important role in cellular cholesterol esterification and thus modulate intestinal cholesterol absorption and hepatic lipoprotein secretion. The relative expression levels of ACAT1 and ACAT2 in human tissues differ from those in other animals, including nonhuman primates. The present study compared the relative expression levels of ACAT1 and ACAT2 in baboons with high and low lipemic responses to dietary lipids. We isolated RNA and prepared cDNA from frozen liver and small intestine from high- and low-responding pedigreed baboons necropsied after consuming a high-cholesterol and high-fat diet for 18 months. The expression of ACAT1 and ACAT2 was measured by TaqMan real-time quantitative PCR normalized to 18s ribosomal RNA. The expression of ACAT1 was higher than that of ACAT2 in the liver, whereas the expression of ACAT2 was higher than that of ACAT1 in the duodenum and jejunum. There was no difference in the expression of ACAT1 or ACAT2 in the liver and intestine between high- and low-responding baboons except that the expression of ACAT1 was higher in the duodenum of high responders than in that of low responders. Western blot analysis also showed a higher level of ACAT1 protein in the duodenum of high responders than in that of low responders. There was a significant correlation between duodenal ACAT expression levels and total plasma cholesterol concentration in baboons. These results suggest that differences in ACAT1 expression may affect plasma cholesterol concentration and partly affect diet-induced hyperlipidemia.

Low-density lipoprotein apolipoprotein B production differs between laboratory opossums exhibiting high and low lipemic responses to dietary cholesterol and fat.

Two partially inbred strains of laboratory opossums exhibit extremely high or low levels of low-density lipoprotein (LDL) cholesterol concentrations, respectively, when challenged with a high-cholesterol and high-fat (HCHF) diet. The present studies were conducted to determine whether the catabolism or the production of LDL apolipoprotein B (apoB) is responsible for the variability in plasma LDL cholesterol and apoB concentrations. Iodinated LDL prepared from plasma of donor opossums consuming HCHF diet was injected into high- and low-responding recipients maintained on the HCHF diet. Blood was drawn at intervals beginning at 3 minutes and ending at 24 hours. At the end of the study, animals were necropsied, and livers were removed for isolation of RNA. Plasma LDL apoB was separated by sodium dodecyl sulfate-electrophoresis, and the level of radioactivity was determined. Hepatic LDL receptor and apoB mRNA levels were measured by Northern blotting. Radioactivity decay curves were plotted by using the radioactivity at each time point as percentage of the radioactivity recovered at 3 minutes. Fractional catabolic rates (FCRs) were calculated by the curve peeling technique. Steady-state production rates were calculated by multiplying the FCR values with apoB concentrations. LDL apoB FCR was slightly higher (1.63-fold) in low responders than in high responders. On the other hand, LDL apoB production was much higher (5.5-fold) in high responders than in low responders. There was no difference in hepatic mRNA levels for either the LDL receptor or apoB. The differences in LDL apoB FCR may be explained on the basis of differences in pool size between the 2 strains. Therefore, LDL apoB production is the major determinant of diet-induced hyperlipidemia in laboratory opossums. Because LDL apoB production was not associated with hepatic mRNA levels, the production of LDL apoB is regulated posttranscriptionally or posttranslationally.

Cholesterol absorption and hepatic acyl-coenzyme A:cholesterol acyltransferase activity play major roles in lipemic response to dietary cholesterol and fat in laboratory opossums.

Partially inbred lines of laboratory opossums differ considerably in their low-density lipoprotein (LDL) cholesterol responses to dietary cholesterol and fat. Genetic analysis suggested that a single major gene is responsible for the variation in LDL cholesterol on the high cholesterol and high fat (HCHF) diet. We measured cholesterol absorption and acyl-coenzyme A:cholesterol acyltransferase (ACAT) activity in intestine and liver to narrow the search for the major gene. We measured plasma lipoproteins and percent cholesterol absorption by the fecal isotope ratio method in high and low responding lines of opossums on basal and HCHF diets. We also measured lipids in liver and ACAT activity in liver and intestine on the HCHF diet. High and low lines exhibited no differences in percent cholesterol absorption on the basal diet. However, high responding opossums had significantly higher percent cholesterol absorption, hepatic free and esterified cholesterol, and hepatic ACAT activity than low responding opossums on the HCHF diet. Hepatic ACAT activity but not the intestinal ACAT activity was associated with hepatic cholesterol concentration and percent cholesterol absorption. Cholesterol absorption is a major determinant of diet-induced hyperlipidemia in opossums. Hepatic ACAT activity but not the intestinal ACAT may also play a role in diet-induced hyperlipidemia in opossums.