Immunochemical measurement of apolipoprotein synthesis in cell and organ culture

Regulation of the apolipoprotein B in heterozygous hypobetalipoproteinemic knock-out mice expressing truncated apoB, B81. Low production and enhanced clearance of apoB cause low levels of apoB

Low levels of cholesterol are protective against development of coronary artery disease. Heterozygous hypobetalipoproteinemic individuals expressing truncated apolipoprotein (apo)B as a result of mutation in the apob gene have low levels of cholesterol and apoB in their plasma. To study the molecular mechanism of low levels of apoB in these individuals, we employed a previously reported knock out mouse model generated by targeted modification of the apob gene. The heterozygous, apoB-100/B-81, mice express full length and truncated apoB, B-81, and have 20 and 35% lower levels of total cholesterol and apoB, respectively, when compared to WT (apoB-100/B-100) mice. The majority of the truncated apoB, B-81, fractionated in the VLDL- density range. The mechanism of low levels of apoB in B-100/B-81 mice was examined. Total hepatic apoB mRNA levels decreased by 15%, primarily due to lower levels of apoB-81 mRNA. Since apoB mRNA transcription rates were similar in B-100/B-100 and B-100/B-81 mice, low levels of mutant apoB-81 mRNA occurred by enhanced degradation of apoB mRNA transcript containing premature translational stop codon. ApoB synthesis measured on isolated hepatocytes decreased in B-100/B-81 mice by 35%, while apoB-48, apoE, and apoAI syntheses remained unchanged. Metabolic studies using whole animal showed a 32% decrease in triglyceride secretion rates, consistent with the apoB secretion rates. Inhibition of receptor-mediated clearance of apoB-81-containing particles resulted in greater relative accumulation of apoB-81 in plasma than apoB-100, suggesting enhanced clearance of apoB-81-containing particles. These results demonstrate that low levels of apoB in heterozygous hypobetalipoproteinemic mice occurs by low rates of apoB secretion, and increased clearance of truncated apoB. Similar mechanisms appear to contribute to low levels of apoB in hypobetalipoproteinemic humans.

Dietary fat increases high density lipoprotein (HDL) levels both by increasing the transport rates and decreasing the fractional catabolic rates of HDL cholesterol ester and apolipoprotein (Apo) A-I. Presentation of a new animal model and mechanistic studies in human Apo A-I transgenic and control mice

In humans, diets high in saturated fat and cholesterol raise HDL-cholesterol (HDL-C) levels. To explore the mechanism, we have devised a mouse model that mimics the human situation. In this model, HuAITg and control mice were studied on low fat (9% cal)-low cholesterol (57 mg/1,000 kcal) (chow) and high fat (41% cal)-high cholesterol (437 mg/1,000 kcal) (milk-fat based) diets. The mice responded to increased dietary fat by increasing both HDL-C and apo A-I levels, with a greater increase in HDL-C levels. This was compatible with an increase in HDL size observed by nondenaturing gradient gel electrophoresis. Turnover studies with doubly labeled HDL showed that dietary fat both increase the transport rate (TR) and decreased the fractional catabolic rate of HDL cholesterol ester (CE) and apo A-I, with the largest effect on HDL CE TR. The latter suggested that dietary fat increases reverse cholesterol transport through the HDL pathway, perhaps as an adaptation to the metabolic load of a high fat diet. The increase in apo A-I TR by dietary fat was confirmed by experiments showing increased apo A-I secretion from primary hepatocytes isolated from animals on the high fat diet. The increased apo A-I production was not associated with any increase in hepatic or intestinal apo A-I mRNA, suggesting that the mechanism of the dietary fat effect was posttranscriptional, involving either increased translatability of the apo A-I mRNA or less intracellular apo A-I degradation. The dietary fat-induced decrease in HDL CE and apo A-I fractional catabolic rate may have been caused by the increase in HDL particle size, as was suggested by our previous studies in humans. In summary, a mouse model has been developed and experiments performed to better understand the paradoxical HDL-raising effect of a high fat diet.

N-3 fatty acids stimulate intracellular degradation of apoprotein B in rat hepatocytes

When rat hepatocytes were incubated with albumin complexed to the n-3 fatty acids, eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), rather than to oleic acid (OA), the secretion of newly synthesized apoprotein B100 (apoB100) or B48 (apoB48) was reduced, despite stimulation of cellular triglyceride synthesis by all three fatty acids. When pulse-chase studies of apoB synthesis and secretion were performed in the presence of OA, EPA, or DHA, there were no significant changes in the initial synthetic rates of either apoB species. However, during the chase period, the total recovery of labeled apoB100 and apoB48 from the cell and medium was less in the n-3 fatty acid groups, so that by 150 min, approximately half as much labeled apoB was recovered as in the OA group. Overall, the decreased accumulation in medium of labeled apoB in the presence of EPA and DHA could be quantitatively accounted for by increased degradation of intracellular apoB. Thus, in the primary hepatocyte, apoB degradation is not constitutive, but can be regulated by n-3 fatty acids.

Dietary fatty acids and dietary cholesterol differ in their effect on the in vivo regulation of apolipoprotein A-I and A-II gene expression in inbred strains of mice

Dietary cholesterol and dietary saturated fatty acids affected the plasma concentrations of various HDL components and the hepatic and intestinal expression of the apolipoprotein (apo) A-I gene and the hepatic expression of the A-II gene differently in three inbred strains of female mice. Thus, the HC diet (0.5% cholesterol, no added fatty acids) decreased HDL-cholesterol in C57BL and SWR strains but not in the C3H strain; plasma apo A-I and apo A-II concentrations decreased in all three strains. HDL-C/apo A-I and apo A-I/apo A-II mass ratios increased, suggesting that the HC diet altered both the concentrations and the compositions of HDL particles. In contrast, the HF diet (20% hydrogenated coconut oil, no added cholesterol) increased HDL cholesterol and apo A-I concentrations. The combination diet (HF/C, 20% coconut oil plus 0.5% cholesterol) increased HDL cholesterol and decreased triacylglycerols. Apo A-I concentrations were unaltered except for a significant increase in SWR mice. Apo A-II concentrations decreased in all strains. To examine molecular events that could lead to the changes in plasma apo A-I and apo A-II, we measured transcription rates in hepatic nuclei and steady state mRNA concentrations in liver and intestine and apo A-I synthetic rates in liver. Dietary cholesterol and fatty acids produced differing effects at transcriptional as well as post-transcriptional loci and the changes differed according to mouse strain. The most pronounced strain-related differences for both apo A-I and apo A-II occurred at post-transcriptional loci of apoprotein production. These could represent altered rates of translation in, or secretion from liver and/or intestine, or altered rates of clearance from plasma. In conclusion, the regulation of apo A-I and apo A-II gene expression by diet occurs at several steps of their production and perhaps also in catabolic pathways. This study identifies potential loci of regulation and forms the basis for future studies investigating specific genetic and molecular regulatory mechanisms.

Expression of the human apolipoprotein E gene suppresses steroidogenesis in mouse Y1 adrenal cells

The lipid transport protein, apolipoprotein E (apoE), is expressed in many peripheral tissues in vivo including the adrenal gland and testes. To investigate the role of apoE in adrenal cholesterol homeostasis, we have expressed a human apoE genomic clone in the Y1 mouse adrenocortical cell line. Y1 cells do not express endogenous apoE mRNA or protein. Expression of apoE in Y1 cells resulted in a dramatic decrease in basal steroidogenesis; secretion of fluorogenic steroid was reduced 7- to greater than 100-fold relative to Y1 parent cells. Addition of 5-cholesten-3 beta,25-diol failed to overcome the suppression of steroidogenesis in these cells. Cholesterol esterification under basal conditions, as measured by the production of cholesteryl [14C]oleate, was similar in the Y1 parent and the apoE-transfected cell lines. Upon incubation with adrenocorticotropin or dibutyryl cAMP, production of cholesteryl [14C]oleate decreased 5-fold in the Y1 parent cells but was unchanged in the apoE-transfected cell lines. These results suggest that apoE may be an important modulator of cholesterol utilization and steroidogenesis in adrenal cells.

Systemic distribution of apolipoprotein E secreted by grafts of epidermal keratinocytes: implications for epidermal function and gene therapy

In the present study, human apolipoprotein E (apoE) was monitored in the circulation of athymic mice and rats bearing human epidermal grafts. Human apoE was detected in the systemic circulation of graft-bearing animals as long as the graft remained on the animal. Within 24 hr of graft removal, human apoE was not detectable in plasma, indicating that apoE resulted from continuous production of the protein by grafted keratinocytes. These results show that proteins as large as apoE (299 amino acids) traverse the epidermal-dermal barrier and achieve systemic distribution where they may produce effects on distal tissues. The feasibility of using grafts of genetically-altered keratinocytes for the delivery of secreted proteins is clearly reinforced by the demonstration that an epidermally derived protein exhibits systemic distribution. Finally, by virtue of its systemic distribution, apoE produced in a peripheral tissue such as skin, may function in the reverse transport of cholesterol from peripheral tissues to the liver.

Expression of the murine apolipoprotein E gene is coupled to the differentiated state of F9 embryonal carcinoma cells

Apolipoprotein E (apoE) expression was studied in F9 embryonal carcinoma cells that differentiate in response to retinoic acid into cells resembling either parietal endoderm or visceral endoderm of the parietal or visceral yolk sac. F9 cells secreted newly synthesized apoE when incubated with radiolabeled amino acid. Upon differentiation to parietal endoderm-like cells, apoE synthesis and secretion were markedly down-regulated. In contrast, apoE secretion was up-regulated upon differentiation to visceral endoderm-like cells. These changes in apoE expression were due, at least in part, to regulation of apoE mRNA abundance since visceral endoderm-like cells contained 5- to 7-fold more apoE mRNA than parietal endoderm-like cells. The apoE phenotype reflected the differentiated state of the F9 cell since either the up-regulated or down-regulated pattern was stable to the removal of the retinoic acid inducer. To determine how well apoE regulation in F9 cells reflects regulation in visceral and parietal yolk sacs, apoE mRNA was measured in yolk sac tissues of the 12-day mouse embryo. Visceral yolk sac contained 109 +/- 11 pg of apoE mRNA per micrograms of RNA while parietal yolk sac contained 14.3 +/- 3.1 pg/micrograms of RNA. This 7- to 8-fold difference is similar to the difference in the apoE mRNA contents of visceral endoderm-like and parietal endoderm-like F9 cells. RNA gel blot analysis showed that apoE mRNA is the same size in F9 cells as in yolk sac tissues and fetal or adult liver. In addition, primer-extension analysis showed that transcription is initiated at or near the same site on the apoE gene in F9 cells and mouse tissues. These data suggest that both quantitative and qualitative features of apoE gene expression in development are retained in the F9 cell. The F9 cell should provide a useful system to study the developmental activation of endogenous apolipoprotein genes as well as exogenous apolipoprotein genes introduced by transfection.