In vivo regulation of low-density lipoprotein receptor and apolipoprotein B gene expressions by dietary fat and cholesterol in inbred strains of mice

Two proteins that may be important in the hypercholesterolemia and atherosclerosis produced by dietary fat and/or cholesterol are apoB and the LDL-receptor. We evaluated the molecular and genetic regulation of these two proteins by two important components of atherogenic diets: dietary fatty acids and dietary cholesterol. The control diet (C) contained 5% corn oil; the high cholesterol (HC) diets, 5% corn oil plus 0.5% or 2% cholesterol; the high fat diet (HF) 1% corn oil and 20% hydrogenated coconut oil; the fat plus cholesterol diets (HF/C) were the same as HF diet plus either 0.5% or 2% cholesterol. Ten strains of inbred mice were fed the C and HF/C (2% cholesterol) diets. Three strains; C3H, C57BL and SWR, were studied in greater detail. In them the effects of dietary fat and cholesterol were assessed separately and together. These three strains were fed all six diets. Lipoprotein profiles of plasma and indexes of lipoprotein composition were obtained by gel filtration chromatography and in selected strains by gradient ultracentrifugation. Relative rates of transcription of LDL-receptor mRNA and apoB mRNA were measured in purified mouse liver nuclei and levels of LDL-receptor mRNA and apoB mRNA in liver and intestine were quantified by RNA excess solution hybridization assays. The HF/C diet produced rises in plasma total-, VLDL- and LDL-cholesterol and apoB concentrations in the ten strains. VLDL and LDL became cholesterol-enriched and the proportion of total cholesterol transported in VLDL and LDL rose at the expense of HDL. This general pattern of HF/C diet-induced changes was similar in all strains, but there were marked quantitative differences between strains with respect to lipid and lipoprotein concentrations, and compositions and the distribution of cholesterol on both the HC and HF/C diets. The strain-related differences were not due to differences in absorption of dietary cholesterol because, for any given diet, hepatic cholesterol levels increased to the same extent in all strains. Nor were the strain-related differences related to alleles of the apoB gene as determined by RFLP analyses. In the three strains, hepatic LDL-receptor mRNA transcription was suppressed by all diets. But, LDL-receptor mRNA levels in both intestine and liver were suppressed only by the HC and HF/C diets and not by the HF diet. Thus, dietary cholesterol decreased LDL-receptor mRNA levels by mechanisms operating at the transcriptional level, while dietary fatty acids, in addition to inhibiting transcription also appeared to enhance mRNA stability.(ABSTRACT TRUNCATED AT 400 WORDS)

In vivo regulation of apolipoprotein A-I gene expression by estradiol and testosterone occurs by different mechanisms in inbred strains of mice

We tested the hypothesis that testosterone and estrogen modulate apoA-I gene expression and metabolism by different mechanisms that may be influenced by genetic factors. Male and female C3H/HeJ (atherosclerosis-resistant) and C57BL/6J (atherosclerosis-susceptible) mice (n = 5/group) were castrated (Placebo). Castrates were given 17 beta-estradiol (E2) at 0.16 microgram/g (E2L) or 5 micrograms/g (E2H) body weight per day, or testosterone (Testo) 1 microgram/g per day, 14 days after surgery, for 14 days. Plasma total cholesterol concentrations (TC) were higher in male Placebo mice than in females. Testosterone altered TC and high density lipoprotein (HDL) cholesterol by gender and strain; however (HDL-C)/TC ratios and apoA-I concentrations were unaltered. Testosterone did reduce HDL particle diameters in both genders of C3H mice only. Low density lipoprotein-cholesterol (LDL-C)/TC ratios remained constant and apoB increased in males only. E2L and E2H decreased TC, HDL-C/TC ratios, and apoA-I. Decrements varied by strain. HDL diameters decreased in both genders in C3H mice only; however, HDL size distributions were altered in both strains. LDL-C/TC ratios increased in all groups. E2L mice showed variable responses of apoB, but apoB rose uniformly in all E2H groups. Testosterone increased and E2H decreased hepatic apoA-I synthesis. ApoA-I mRNA concentrations remained stable in both Testo and E2 groups. ApoA-I gene transcription varied by strain and gender, but all changes were less than twofold. Testosterone did not affect hepatic apoB or LDL receptor mRNA, however, E2H increased both mRNAs in males but not in females. On Western blotting of liver membranes, E2H had little effect on mouse LDL receptor protein mass; by contrast, E2H increased LDL receptor approximately threefold in rats. In summary, responsiveness of mouse lipids to testosterone and E2 vary by strain and gender. Testosterone and E2 differ in their regulation of apoA-I production mainly at the level of translation. Hormones operate at several levels of gene regulation, suggesting that complex mechanisms are involved. Mice differ from rats and rabbits in their LDL receptor responsiveness to estradiol treatment.

Expression of LDL receptor, apolipoprotein B, apolipoprotein A-I and apolipoprotein A-IV mRNA in various mouse organs as determined by a novel RNA-excess solution hybridization assay

We report expression of LDL receptor, apolipoprotein B (apoB), apolipoprotein A-I (apoA-I) and apolipoprotein A-IV (apoAIV) mRNA in various mouse organs. These mRNA were quantified by an RNA-excess solution hybridization assay. For preparing specific probes, we cloned cDNA fragments of rat LDL receptor, apoB and apoA-I and mouse apoA-IV into the polylinker region of pGEM3Zf(+) and used the recombinant vectors for preparing 32P-labeled cRNA probes as well as RNA standards using the T7 and SP6 promoters flanking the polylinker regions. Preparation of cRNA probes and RNA standards is faster and more convenient than preparing cDNA probes and ssDNA standards. Absolute levels of mRNA were quantified in the liver, intestine, kidney, heart, lung, spleen and adrenals of females of two mouse strains. C3H/HeJ and C57BL/6J. ApoB, apoA-I and apoA-IV genes in mice are expressed in the liver and intestine and LDL receptor gene is expressed mainly in liver, intestine and adrenals. ApoA-I mRNA levels were found to be 730 and 1039 molecules per cell in liver and intestine, respectively, in C3H mice and 762 and 952 molecules per cell in C57BL mice. ApoB mRNA levels were 66 and 170 molecules per cell in the liver and intestine of C3H and 83 and 243 molecules per cell in C57BL, respectively. ApoA-IV mRNA was found to be 3525 and 2964 molecules per cell in the liver and intestine of female C57BL mice, respectively. LDL receptor mRNA levels were 39, 32 and 14 molecules per cell in the liver, intestine and adrenals of C3H.