Atherosclerosis and apoprotein E. An enigmatic relationship

In this article, we consider the role of apoprotein E in lipoprotein metabolism and especially in the metabolism of potentially atherogenic lipoproteins. Particular consideration has been given to three features of apoprotein E involvement in lipid cell interactions. Evidence implicating free cholesterol as a mediator of apoprotein E biosynthesis in cholesterol-loaded macrophages is presented. Experiments pointing to apoprotein E as the ligand promoting the interaction of beta-very-low-density lipoprotein (beta-VLDL) with macrophages are summarized. Finally, we describe the influence of fat and cholesterol fed to rhesus monkeys and baboons on the generation of hepatogenous (from isolated liver perfusates) VLDL enriched in cholesterol ester and apoprotein E. These hepatic VLDLs, none of which exhibits beta-electrophoretic mobility, promote cholesterol esterification in macrophages in proportion to their apoprotein E content. The complex role of apoprotein E in the genesis and reversal of atherosclerosis is briefly discussed.

Uptake of chylomicron remnants by the liver: further evidence for the modulating role of phospholipids

Rat lymph chylomicrons were treated with rat heparin-releasable hepatic lipase (HL) or with bovine milk lipoprotein lipase (LPL). The ability of the resulting particles to be taken up by the liver in vivo was assessed following their infusion into the portal vein of partially hepatectomized animals. The following observations were made: a) the rate of phospholipid depletion, relative to the rate of triglyceride hydrolysis, induced by HL was two- to threefold higher than that observed for LPL; b) the depletion of at least 57% of phospholipids from the surface of HL-treated chylomicrons caused no major alterations in the apoprotein profile of the particles; c) for the same extent of triglyceride hydrolysis, HL-treated chylomicrons were taken up by liver at a rate significantly higher (P less than 0.005) than LPL-treated particles; d) the liver uptake of HL-treated chylomicrons was competitively inhibited by endogenously generated chylomicron remnants, indicating that these two types of lipoproteins share the same process of recognition and uptake by liver cells. It is concluded that the in vivo changes in phospholipid content, or composition, on the surface of chylomicrons during their transformation into remnants, modulate the differentiation of these two particles by the hepatic remnant receptor.

The characterization of yeast mitochondrial RNA polymerase. A monomer of 150,000 daltons with a transcription factor of 70,000 daltons

Transcription in organelles is regulated by both organellar and nuclear mechanisms. In order to study further the control of organellar transcription, we have purified and characterized the RNA polymerase from mitochondria of Saccharomyces cerevisiae and identified a transcription factor required for promoter recognition. The RNA polymerase can be separated into two forms, selective and nonselective. The nonselective form was purified over 11,000-fold and appears to be active as a monomer with a molecular weight of 150,000. The Mr 150,000 polypeptide acts as a core RNA polymerase, and an Mr 70,000 polypeptide appears to confer selectivity for the promoter upon this core. The Mr 70,000 transcription factor binds specifically to the mitochondrial initiation site in the absence of polymerase and decreases nonselective initiation by the polymerase. The Mr 150,000 polymerase is immunologically related to an Mr 145,000 protein purified from yeast as a primase, although it is thought to be a functional unit of mitochondrial RNA polymerase (Kelly, J. L., and Lehman, I. R. (1986) J. Biol. Chem. 261, 10340-10347). Antibodies to the Mr 145,000 protein inhibit transcription by the mitochondrial RNA polymerase purified here.

Sequential change of DNA synthesis in cultured aortic smooth muscle cells stimulated by hyperlipidemic serum

Smooth muscle cells from monkey aorta quiescent in 5% calf serum have been shown to be stimulated to renewed proliferation by hyperlipidemic serum or LDL from such serum. This proliferative response evidently is not dependent on platelet-derived growth factor present in our system in large quantities. The least exposure time required for reaction between the mitogen and the cells in order to initiate maximal DNA synthesis by this mechanism was studied using autoradiography. Stationary primary cultures and subcultures from monkey aortic media required at least 4 and 8 hr of contact with hyperlipidemic serum or LDL so that a significant number of cells reentered the mitotic cycle. Compared to the primary culture system, subcultures needed a slightly longer time of contact with serum to initiate DNA synthesis. Since there was no significant difference in labeling index between the primary cultures stimulated by serum for 8 and 48 hr and the subcultures exposed between 6 and 48 hr, it is concluded that a relatively brief stimulation commits the majority of responsive cells to reenter the cycle and initiate DNA synthesis.

Hepatic perfusate very low density lipoproteins obtained from fat-fed nonhuman primates stimulate cholesterol esterification in macrophages

The livers of both baboons and rhesus monkeys fed a high fat, high cholesterol diet secreted very low density lipoproteins (VLDL) that were enriched in cholesteryl ester and apoe as compared to VLDL secreted by the livers of chow-fed animals. Stimulation of macrophage cholesterol esterification by the experimental VLDL was compared to that produced by the standard beta-VLDL obtained from the plasma of a rhesus monkey fed 25% coconut oil plus 2% cholesterol. This standard beta-VLDL stimulated 7- to 10-fold more esterification than did the bovine albumin control. Hepatic VLDL from fat-fed animals stimulated esterification in J774 macrophages 50 to 150% as well as did the standard beta-VLDL, even though hepatic VLDL did not display beta electrophoretic mobility on agarose gel electrophoresis. Plasma VLDL from lard-fed baboons did not exhibit beta electrophoretic mobility but did stimulate esterification in macrophages. Baboons were divided into high and low responders based on the change in plasma cholesterol levels in response to a high fat, high cholesterol diet. Both plasma and hepatic VLDL from high responders stimulated cholesterol esterification, whereas hepatic VLDL obtained from low responders or chow-fed baboons did not stimulate cholesterol esterification in macrophages. There was a strong positive correlation (r = 0.866) between the number of apoE molecules per VLDL particle in VLDL obtained from chow-fed, lard-fed, or coconut oil-fed primates and the rate of cholesterol esterification in macrophages. Our results show that hepatic perfusate VLDL obtained from fat- and cholesterol-fed primates have compositional and functional properties usually ascribed to circulating beta-VLDL, without displaying beta mobility, and indicate that the liver may be an important source of atherogenic lipoproteins.

Identification of a new promoter within the tRNA gene cluster of the mitochondrial DNA of Saccharomyces cerevisiae

We have identified a new promoter within the tRNA gene cluster of Saccharomyces cerevisiae mitochondrial DNA. It is located upstream of the gene encoding the leucyl tRNA. It conforms to the consensus sequence of other yeast mitochondrial promoters, ATATAAGTA. It serves as a site for the initiation of transcription in vivo and in vitro.

In vitro characterization of the yeast mitochondrial promoter using single-base substitution mutants

A short DNA sequence, 5'ATATAAGTA(+1)3', extending from -8 to +1 nucleotides has been shown to function as a promoter in the yeast mitochondrial genome. A complete set of single site mutations of this nonanucleotide promoter sequence has been constructed, cloned, and used to promote specific in vitro transcription using a highly purified mitochondrial RNA polymerase. Each deviation from the natural promoter sequence results in a reduction or abolition of specific transcription depending on the nucleotide substituent. The nucleotide at -8 is not considered as a component of the promoter. Any nucleotide at position +1 is compatible with correct transcriptional initiation. The consensus sequence that exists in vivo is the strongest promoter since only down mutations are seen among the substitutions. The mutant analyses indicate that a very short octanucleotide sequence comprised of 5'TAT/aAA/g/cGT/a/cN(+1)3' is the minimal sequence necessary to direct accurate initiation by mitochondrial RNA polymerase.

Macrophage free cholesterol content regulates apolipoprotein E synthesis

The relationship between macrophage cholesterol content and apolipoprotein E (apoE) synthesis was studied in mouse peritoneal macrophages. Incubations in acetylated low density lipoprotein led to a concentration-dependent increase in macrophage free and esterified cholesterol content and apoE synthesis. Enhanced apoE production reflected increased apoE mRNA abundance in cholesterol-enriched cells. Including an inhibitor of acyl-CoA:cholesterol acyltransferase in incubations with acetylated low density lipoprotein did not diminish the apoE response, suggesting that increased macrophage free cholesterol content was responsible for enhancing apoE production. Incubations in 25-OH cholesterol also produced a dose-dependent stimulation of macrophage apoE synthesis. Removing free cholesterol from cells using high density lipoprotein returned apoE synthetic rates toward base line. Macrophage lysate apoE and medium apoE levels changed in parallel during cholesterol loading and efflux indicating that regulation of apoE by free cholesterol was not primarily at the level of secretion. It is concluded that (a) cholesterol enrichment of macrophages increases apoE mRNA abundance and stimulates apoE synthesis and secretion; (b) neither cholesterol esterification nor cholesteryl ester accumulation are required for increased apoE production.

Apoprotein E mediates the interaction of beta-VLDL with macrophages

beta-Very low density lipoproteins (beta-VLDL) isolated from cholesterol-fed rhesus monkeys stimulated cholesteryl ester synthesis and accumulation in mouse peritoneal macrophages. The apoprotein specificity and requirement for the cell surface uptake of beta-VLDL was investigated by treating the beta-VLDL with trypsin (beta-VLDL (T], incubating the beta-VLDL (T) with other lipoproteins or apoproteins, reisolating the beta-VLDL (T) and measuring its biological activity which, for this study, is defined as the ability of the lipoprotein to stimulate cholesterol esterification in the macrophages. Trypsin treatment of beta-VLDL abolished its biological activity. Apoprotein analysis of the beta-VLDL (T) demonstrated the absence of intact apoproteins B-100, B-48, and E. The J774 macrophage-like cell line and mouse peritoneal macrophages responded similarly with respect to cholesterol esterification following incubation with inactive and treated beta-VLDL. The J774 macrophage-like cell line was used to establish the conditions necessary for the restoration of biologic activity to the trypsinized beta-VLDL. The loss of biological activity of beta-VLDL (T) could be reversed by restoring apoprotein E-containing LDL from hyperlipemic monkeys or purified apoprotein E. Apoprotein A-I had no such effect. The restored biological activity of the beta-VLDL (T) was proportional to the amount of apoprotein E acquired by the lipoprotein. beta-VLDL particles composed of apoprotein E and either intact or degraded apoprotein B-100 had comparable biological activity. Thus, intact apoprotein E, without intact apoprotein B, is a sufficient mediator for the biological activity and metabolism of beta-VLDL by macrophages and plays a major role in receptor-lipoprotein interaction.

In vivo regulation of hepatic LDL receptor mRNA in the baboon. Differential effects of saturated and unsaturated fat

The effects of diets enriched with cholesterol and different fats upon plasma lipoproteins and hepatic low density lipoprotein (LDL) receptor mRNA levels were studied in a group of 18 normal baboons. Animals were fed diets containing 1% cholesterol and 25% fat as either coconut oil, peanut oil, or olive oil for a period of 20 weeks. Plasma total cholesterol, high density lipoprotein (HDL) cholesterol, beta-lipoprotein (LDL + very low density lipoprotein) cholesterol, apolipoprotein B and apolipoprotein A-I were measured in samples obtained at 4-week intervals. All three diet groups demonstrated a statistically significant increase in plasma cholesterol as compared to base line throughout the experiment. Hepatic LDL receptor (LDL-R) mRNA levels were quantified by dot blot hybridization in serial liver biopsies. Animals fed saturated fat sustained a significant reduction in hepatic LDL-R mRNA as compared to those fed either monounsaturated or polyunsaturated fat. A strong negative correlation between LDL-R mRNA and plasma total cholesterol (r = -0.71), HDL cholesterol (r = -0.76), and plasma apo A-I (r = -0.77) was observed only in those animals fed coconut oil. Weak negative correlations between LDL-R mRNA and other plasma parameters did not achieve statistical significance. We conclude that saturated and unsaturated oils may influence plasma cholesterol levels in part through differential effects on LDL receptor biosynthesis in baboons.

Direct determination of the specific activity of RNA uniformly labeled with 32P

A simple method for the direct determination of the specific activity of RNA uniformly labeled with 32P is described. The procedure is based on the premise that upon disintegration of 32P to 32S, the phosphodiester bond is broken. Analysis of the rate of decay of the full-length molecule by gel electrophoresis and autoradiography can accurately determine the "intramolecular specific activity" of the RNA. An equation that predicts the relative intensity of the intact RNA molecules remaining as a function of time is presented. These predictions are confirmed using in vitro-synthesized RNA labeled at a known specific activity. This procedure has been used to determine the intramolecular specific activity of RNA labeled in vivo in yeast. It can also be employed to choose the best conditions for experiments utilizing uniformly labeled RNA or single-stranded DNA and requiring the detection of intact molecules.

Temperature sensitive pet mutants in yeast Saccharomyces cerevisiae that lose mitochondrial RNA

This is a description of a new class of temperature sensitive pet mutants in Saccharomyces cereviase that lose all or part of their mitochondrial RNA at the restrictive temperature. These mutants fall into 8 different complementation groups, mna1 to mna8, and 2 different classes based on their phenotype. Class I mutations, mna1-1 through mna5-1, cause complete or partial loss of mitochondrial RNA at the restrictive temperature. The mutation, mna1-1, is especially interesting since it causes a loss of both mitochondrial DNA and RNA when the mutant is grown on a fermentable carbon source at the restrictive temperature. However, when this mutant is grown at the permissive temperature on a non-fermentable carbon source then shifted to the restrictive temperature, only the mitochondrial RNA is lost. This indicates that the primary cause for the pet phenotype is due to the loss of mitochondrial RNA and not DNA. Class II mutations, mna6-1 through mna8-1, cause complete loss of the 14S rRNA after growth at the restrictive temperature in a fermentable carbon source. This loss appears to be specific for the 14S rRNA, since all other transcripts probed by Northern analysis are normal.

Nutrition and biogenesis of plasma lipoproteins in nonhuman primates

In this article we examine the production of very low-density lipoprotein (VLDL) by perfused livers obtained from chow- and cholesterol-fed nonhuman primates. These data illustrate two important features of VLDL production. First, VLDL is secreted from the liver in a form very close to that of its plasma counterpart. Thus for chow-fed animals, plasma VLDL and liver perfusate VLDL have similar lipid compositions. Second, the composition of VLDL can be modified significantly by diet in each of two primate species, the Rhesus monkey and the baboon. Rhesus monkey livers uniformly secrete larger quantities of VLDL and show more dramatic dietary effects than do baboon livers. Nevertheless, perfused livers from both species reveal qualitatively similar responses to dietary peanut oil and to lard fed in combination with cholesterol. Both fat-containing diets induce the livers to secrete VLDL enriched in cholesteryl ester compared with control perfusates yet still cholesteryl ester deficient compared with the animals' plasma VLDL. Peanut oil diet reduces the hepatic output of VLDL-associated apoprotein B and triglyceride, whereas lard increases hepatic secretion of VLDL-associated lipids and apoprotein E. We conclude that the nature of dietary fat plays an important role in determining the profile and composition of lipoproteins formed and secreted by the primate liver. We have also briefly reviewed the production of high-density lipoprotein, which is probably formed in the plasma from many sources, with special emphasis on the possible role of newly secreted lipid-poor apolipoprotein A-I and A-II.

Differential labeling of rat hepatic Golgi and serum very low density lipoprotein apoprotein B variants

The synthesis of apoB-100 and apoB-48 by rat liver was investigated by studying the apoB complement of very low density lipoproteins (VLDL) from hepatic perfusates and Golgi fractions. The relative amounts of apoB-100 and apoB-48 in perfusate and Golgi VLDL as determined by sodium dodecyl sulfate (SDS) polyacrylamide gel electrophoresis were similar to those in serum VLDL. To investigate the relative rates of synthesis of the VLDL B proteins, rats were injected intraportally with tritiated amino acid, and hepatic Golgi and serum VLDL were isolated from 7.5 to 120 min later. In hepatic Golgi VLDL, apoB-100 and apoE were maximally labeled at 15 min after the tritiated amino acid pulse. In contrast, VLDL apoB-48 attained maximum radioactivity at 30 min after isotope injection. In serum VLDL, apoB-100 and apoE were maximally labeled at 30 min post-isotope injection, while activity in apoB-48 peaked at 60 min. The data suggest that the synthesis of the B proteins and incorporation into rat liver nascent VLDL are independently regulated. The differential labeling patterns of the VLDL B proteins may be explained by an intracellular pool of apoB-48 that is larger than that of apoB-100. An alternative explanation of the results is that apoB-100 is a precursor to apoB-48.

Steady state analysis of mitochondrial RNA after growth of yeast Saccharomyces cerevisiae under catabolite repression and derepression

The steady state levels of mitochondrial rRNAs, 5 tRNAs, the 9 S RNA, and the RNA products from the genes coding for subunits 6 and 9 of the ATP synthase, cytochrome b, and subunit 1 of cytochrome oxidase have been determined after growth of yeast under conditions of respiratory repression or derepression. The analysis indicates that the mitochondrial rRNAs are present in 2000 or 9000 copies/cell in repressed or derepressed yeast, respectively. The levels of the other RNAs also differed to a similar extent, with the exception of the level of the tRNAfMet which differs by only 1.7-fold. The levels of the individual protein coding RNAs varied from 480 copies/cell for the Oli-1 RNA to 100 copies/cell for the Oli-2 RNA under derepressive conditions and from 130 copies/cell to 33 copies/cell for the same RNAs in glucose repressive conditions. The levels of the tRNAs varied even more markedly, ranging from 4200 copies/cell for the tRNAPhe to 240 copies/cell for the tRNACys after growth in derepressive conditions and from 800 copies/cell for the tRNAfMet to 30 copies/cell for the tRNACys of glucose repressed yeast. These results indicate that glucose repression uniformly decreases the levels of the individual mitochondrial RNAs studied. This decrease is related to a lower synthesis of mitochondrial RNA in the glucose repressed cells as compared to derepressed cells.

Transcriptional regulation of the mitochondrial genome of yeast Saccharomyces cerevisiae

The relative rates of transcription of several classes of the mitochondrial genes of the yeast Saccharomyces cerevisiae have been determined. The rates were measured by pulse labeling whole yeast with [32P]O4, isolating the total RNA, hybridization to single-stranded M13 DNA probes containing segments of the gene of interest, digestion with RNase A or T1, and separation of the protected fragment by gel electrophoresis. This analysis indicated that, among the genes analyzed, transcriptional promoters varied in strength by 20-fold while the rates of transcription varied by more than 50-fold. The strengths of the promoters of the genes were ordered: tRNAMetf greater than tRNAPhe greater than 14 S rRNA greater than 21 S rRNA greater than tRNAGlu greater than Oli-1 much greater than tRNACys. In addition, transcription rates were measured within polygenic transcription units. This analysis indicated that there was transcriptional attenuation within all the polygenic transcription units with the greatest attenuation factor being as much as 17-fold, occurring after the tRNAGlu and tRNAMetf genes. This analysis indicated that regulation of the rates of transcription in the yeast mitochondrial genome occurs by two distinct mechanisms, modulation of the rate of transcriptional initiation and attenuation of transcriptional elongation.