Hepatic Apo E mRNA levels in BALB/c and C57BL/6 mice fed different levels and types of oil
Described here is the first known application of differential display to isolate genes regulated by level of dietary lipids in female BALB/c mice. Differences were revealed with all five primer pairs tested. Nine fragments were isolated, cloned, partially sequenced and compared to the Genbank data base. Sequence analyses identified Apolipoprotein E, P450-15-$\alpha$-hydroxylase (Cyp 2a4) and seven unique genes. Apolipoprotein E is found on most lipoproteins and binds primarily to the LDL and Apo E receptors in the liver, thereby clearing lipoproteins from the plasma. Cyp 2a4 hydroxylates testosterone, progesterone and androstenedione at the 15-$\alpha$ position for excretion. Northern analyses revealed that Cyp 2a4 was not regulated by our dietary treatments. Four feeding studies examined hepatic mRNA for Apo E in BALB/c mice fed diets varying in level and source of fat. All diets contained the same ratio of calories to other nutrients except carbohydrates and fat. Northern analyses of Apo E mRNA abundance in mice fed diets containing 0, 5, 10, 15 or 20% corn oil for 3 weeks confirmed that increasing dietary corn oil decreased abundance of Apo E in mice fed 20% corn oil compared to 5% (p $<$ 0.01). A third study investigated the hepatic mRNA level of Apo E in BALB/c and C57BL/6 mice fed semipurified diets containing 3 or 20% corn oil. Although differences were not statistically significant, C57BL/6 mice fed 20% corn oil showed increased expression of Apo E while BALB/c fed 20% showed decreased expression. A fourth study examined the relationship between hepatic Apo E mRNA levels and dietary fat level, serum lipid responses, stage of estrus and fast/refeeding. Mice were fed 3, 10 or 20% corn oil diets for 3 weeks. Northern analyses revealed that hepatic expression of Apo E mRNA was lower in mice fed 20% oil compared with 3% and 10% (p $<$ 0.0001) and was higher postprandially (p $<$ 0.0001). Serum triglycerides decreased with fasting and increased postprandially (p $<$ 0.0027). Serum HDL cholesterol increased with fasting and decreased postprandially (p $<$ 0.0066) and total serum cholesterol did not differ significantly during fasting and refeeding. No differences in serum lipid levels were seen with feeding of different fat levels. Mice in metestrus had less mRNA for Apo E than those in diestrus (p $<$ 0.04). The fifth study compared Apo E mRNA and serum lipids in female BALB/c and C57BL/6 mice after consuming semipurified diets containing 4 or 20% corn or coconut oil for 16 weeks. More hepatic mRNA for Apo E was seen in C57BL/6 mice in all dietary treatment groups. Total serum cholesterol, HDL cholesterol and serum triglycerides were higher in BALB/c mice than C57BL/6 (p $<$ 0.005). Both strains of mice consuming 20% fat showed higher triglycerides as did all mice consuming coconut oil. No atherosclerotic lesions were observed in mice that had consumed these diets for 16 weeks. This method employing feeding of comparable caloric intake to two strains of mice with defined genotypes revealed that genes responding to different levels of fat can be identified.
Biology, Animal Physiology, Health Sciences, Nutrition
University of Illinois - Urbana-Champaign
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University of Illinois at Urbana-Champaign