Production of small high-density lipoprotein particles after stimulation of in vivo lipolysis in hypertriglyceridemic individuals: studies before and after triglyceride-lowering therapy

In hypertriglyceridemic states, triglyceride enrichment of high-density lipoprotein (HDL) may play an important role in decreasing the HDL cholesterol and apolipoprotein (apo) A-1 plasma concentration. We have shown previously that HDL particles are transformed into small HDLs when lipolysis is stimulated in vivo or in vitro, and this process is more marked if the HDL is triglyceride-rich. The present study was conducted to determine whether the susceptibility of HDL to transformation can be altered by triglyceride-lowering therapy in humans. Seventeen moderately hypertriglyceridemic individuals (nine with type II diabetes mellitus and eight moderately hypertriglyceridemic nondiabetic subjects) were studied before and after 3 months of triglyceride-lowering therapy with gemfibrozil. Since no significant differences in postprandial and postheparin HDL metabolism were detected between type II diabetic and nondiabetic subjects, results are reported for the two groups combined (N = 17). Fasting HDL was triglyceride-rich with a preponderance of HDL3, and became more enriched with triglycerides postprandially. Heparin administration resulted in a rapid decrease in plasma and HDL triglycerides and an increase in plasma and HDL free fatty acids (FFAs). Postheparin, there was a reduction in HDL size and an increase in the proportion of small (HDL3c) HDL particles (HDL3c constituted 7.1% +/- 1.8% of total HDL preheparin and 26.6% +/- 3.8% postheparin, P < .001). Triglyceride-lowering treatment resulted in a decrease in fasting triglycerides (-54%, P < .001) and HDL triglyceride content (-36%, P = .002), an increase in fasting HDL cholesterol (19%, P = .004), and proportionately fewer (13.2% +/- 2.1%, P < .001) HDL3c particles formed postheparin. Postheparin HDL size correlated inversely with the fasting triglyceride level (r = -.55, P < .001) and HDL triglyceride concentration (r = -.34, P = .02). These results show that the postprandial increase in triglyceride levels in hypertriglyceridemic subjects is associated with increased production of small HDL particles when lipolysis is stimulated, and that lipid-lowering therapy can contribute to favorably reduce this postprandial production of small HDL particles. Further studies are needed to clarify how these abnormalities ultimately lead to a decrease of plasma HDL cholesterol and apo A-1 in hypertriglyceridemic states.

Position-specific inhibition of yeast mitochondrial transcription by a poly(T) sequence

The 3' flanking nucleotide(s) of the octanucleotide promoter sequence regulates transcriptional efficiency of some mitochondrial genes in Saccharomyces cerevisiae. To understand this regulation the in vitro transcriptional activity of various synthetic mitochondrial promoters carrying different 3' flanking sequences was examined. The results presented here demonstrate that consecutive thymidine residues, but no other polynucleotides or secondary structure, in the promoter-proximal non-transcribed DNA strand inhibited mitochondrial transcription. The location and the number of T residues in the cluster as well as the concentration of UTP in the transcription reaction are the important factors determining this transcriptional inhibition. For example, a pair of thymidine nucleotides at positions +2 and +3 is sufficient for inactivation of mitochondrial transcription, whereas more than three consecutive thymidine nucleotides beyond these positions are required for inhibition of mitochondrial transcription. However, a cluster of six to 12 thymidine residues beyond position +11, a point where mtRNA polymerase has been shown to form a stable transcription complex, did not interfere with mitochondrial transcription. Interestingly, at low UTP concentration the mtRNA polymerase generates a large quantity of aborted initiation products on a template carrying promoter-proximal poly(T) sequence probably due to the inability of the polymerase to clear this promoter. On the other hand at high UTP concentration the same mtRNA polymerase on the same mitochondrial promoter produces a higher level of productive initiation complex. These observations suggest that the mechanism of poly(T) inhibition of mitochondrial transcription is a UTP-limited transcriptional attenuation at the promoter site, which might occur under specific physiological conditions (i.e. glucose repression-derepression, switching of aerobic-anaerobic conditions).

Isoform-specific effect of apolipoprotein E on cell survival and beta-amyloid-induced toxicity in rat hippocampal pyramidal neuronal cultures

Although the genetic link between the epsilon4 allele of apolipoprotein E (apoE) and Alzheimer's disease is well established, the isoform-specific activity of apoE underlying this correlation remains unclear. To determine whether apoE influences the neurotoxic actions of beta-amyloid (Abeta), we examined the effect of native preparations of apoE3 and E4 on Abeta-induced toxicity in primary cultures of rat hippocampal pyramidal neurons. The source of apoE was conditioned medium from HEK-293 cells stably transfected with human apoE3 or E4 cDNA. ApoE4 (10 microg/ml) alone was toxic to the cultures, whereas apoE3 had no effect. ApoE3 treatment prevented the toxicity induced by 10 microM Abeta(1-40) or Abeta(25-35). The apoE3 protective effect appears to be specific to Abeta-induced toxicity, because apoE3 did not protect against the cytotoxicity produced by NMDA or staurosporine, nor did apoE3 affect the increase in intracellular calcium induced by either NMDA or KCl. ApoE3 had no effect on the toxicity produced by Abeta in the presence of receptor-associated protein, an inhibitor of apoE receptors, particularly the LDL-receptor-related protein. Interaction with apoE receptors may not mediate the toxic actions of apoE4, because receptor-associated protein did not affect apoE4-induced neurotoxicity. Consistent with our previous biochemical experiments, analysis of the culture medium revealed that SDS-stable apoE3:Abeta complex is present in greater abundance than apoE4:Abeta complex. Thus, the protection from Abeta-induced neurotoxicity afforded by apoE3 treatment may result from clearance of the peptide by apoE3:Abeta complex formation and uptake by apoE receptors.

Serum amyloid A and high density lipoprotein participate in the acute phase response of Kawasaki disease

In this study we report changes in HDL concentration and composition in acute and convalescent Kawasaki disease. Notable reductions in plasma HDL-cholesterol (0.54 +/- 0.2 mmol/L, normal level 0.7-1.81 mmol/L) and apolipoprotein A-I (apoA-I) (56 +/- 28 mg/dL, normal level 141 +/- 22 mg/dL) were observed in all 24 patients studied during the acute phase of Kawasaki disease. These changes were accompanied by the marked appearance of serum amyloid A (SAA) protein in the plasma, associated with HDL3-like lipoprotein particles. The distribution of apoA-I was analyzed in five patients and showed a significant increase in lipid-free apoA-I in the bottom fraction (28.8 +/- 4.1%, normal range 10-15%), suggesting displacement of apoA-I from the HDL particles by SAA. Within 2 wk after acute Kawasaki disease, levels of HDL-cholesterol and apoA-I returned to the normal range, and SAA disappeared from the plasma. The HDL of patients with Kawasaki disease was markedly enriched in triglyceride even in the absence of changes in total plasma triglyceride. The core composition of HDL returned to the normal range more slowly than the plasma HDL-cholesterol and apoA-I levels. This suggests that Kawasaki disease has a profound effect on the lipoprotein profile acutely and a more subtle sustained effect on the HDL composition. We interpret these changes as manifestations of the acute phase response in Kawasaki disease.

Association of human, rat, and rabbit apolipoprotein E with beta-amyloid

In humans, apolipoprotein E (apoE) has three major isoforms, E2 (Cys112, Cys158), E3 (Cys112, Arg158), and E4 (Arg112, Arg158). While epsilon4 is a genetic risk factor for Alzheimer's disease (AD), epsilon2 may protect against late-onset AD. Using native preparations of apoE from conditioned tissue culture media or plasma lipoproteins, we have previously shown that when equivalent amounts of apoE3 or E4 were incubated with beta-amyloid (A beta), apoE3 formed 20 times as much SDS-stable complex with the peptide as apoE4. This preferential binding of A beta to apoE3 was abolished when apoE was purified by a process which includes delipidation and denaturation. Here we expand these observations to include A beta binding to lipoprotein-associated and purified apoE2. Lipoproteins isolated from the plasma of individuals homozygous for either epsilon2 or epsilon3 were incubated with A beta(1-40). SDS-stable complex formation was analyzed by a non-reducing gel shift assay, followed by immunoblotting with either A beta or apoE antibodies. ApoE2:A beta complex formation was comparable to apoE3:A beta in both native and purified preparations of apoE. In addition, lipoprotein-associated rat apoE (Arg112, Arg158), like human apoE4, did not form complex with A beta, while lipoprotein-associated rabbit apoE (Cys112, Arg158) did bind the peptide. These binding studies provide one possible explanation for protective effects of both apoE2 and E3 against the development of Alzheimer's disease.

Effect of apolipoprotein E on neurite outgrowth and beta-amyloid-induced toxicity in developing rat primary hippocampal cultures

The correlation between the epsilon 4 allele of apolipoprotein E (apoE) and Alzheimer's disease is well established. However, the role of apoE in normal as well as pathological brain processes remains unclear. We evaluated the effect of apoE treatment on development and beta-amyloid (A beta)-induced toxicity using primary cultures of developing rat hippocampal neurons. The source of apoE was conditioned media from HEK cells stably transfected with human apoE3 or apoE4 cDNA, a preparation where apoE is lipid-associated. Morphological and biochemical changes in the cultures were assessed at 1 and 3 days following low- and high-density plating with either apoE3 or E4 with or without A beta. Both apoE isoforms were neurotrophic, as measured by increased neurite length. Aged A beta(1-42), a peptide preparation exhibiting extensive fibril and aggregate formation, is toxic to these cultures. Addition of apoE3 and E4 significantly and comparably attenuated the A beta-induced reduction in both neurite length and cell viability. The level of protection against this toxicity was proportional to the neurotrophic actions of the two apoE isoforms. Thus, apoE acts as a potent growth factor in both the absence and the presence of A beta, supporting a potentially important role for apoE in neurobiology.

HDL content and composition in acute phase response in three species: triglyceride enrichment of HDL a factor in its decrease

High density lipoprotein (HDL) levels decrease during the acute phase response (APR). We have used the APR model of rabbit, baboon, and mouse to study the factors that influence HDL level. In the baboons and rabbits there was massive hypertriglyceridemia, triglyceride enrichment of HDL (60-80% of core lipids), decreases of HDL-cholesterol and apolipoprotein (apo)A-I (to 10% of baseline), and increases of apoA-I in the non-lipoprotein bottom fraction suggesting dissociation of apoA-I from the particles. Detailed analyses of serum amyloid A (SAA)-rich HDL done in the rabbit revealed large, triglyceride-enriched (> 60% of core lipids) particles containing > 95% SAA. These particles had a high surface to core ratio (13.4 +/- 1.94, control = 3.0 +/- 0.12) and a very high protein (79.71 +/- 5.25 weight %, control = 37.2 +/- 0.43) proportion, large (r = 5.95 nm) when examined by non-denaturing gradient electrophoresis but small when examined by electron microscopy (r = 4.2 nm). In the mouse there was no hypertriglyceridemia, no triglyceride enrichment of HDL, no decrease of HDL cholesterol. ApoA-I decreased to about 61.4% of baseline but did not increase in the bottom fraction although large but dense SAA-enriched HDL particles were also produced. These results suggest that hypertriglyceridemia, triglyceride-enrichment of HDL, and dissociation of apoA-I from the particles, possibly by displacement of apoA-I by SAA, are important factors in the decline of HDL during the APR. Whether differences in triglyceride metabolism account for the differences in the HDL response in the species studied requires further experimentation.

Lipoprotein association of human apolipoprotein E/A-I chimeras. Expression in transfected hepatoma cells

Both apolipoprotein (apo) E and apoA-I are associated with lipoproteins, although with different particle classes. ApoE is associated with very low density lipoprotein (VLDL) and with the larger high density lipoprotein (HDL) subspecies, while apoA-I is found predominantly in association with most HDL subclasses. The genes encoding these proteins have a similar overall structure with the nucleotide sequences of the third and fourth exons coding for the mature protein. In an effort to understand the difference in lipoprotein association patterns of these two apoproteins, we have constructed and expressed chimeric apoproteins using cDNAs in which the third (n) and fourth (c) exons of human apoE and apoA-I are exchanged. McArdle rat hepatoma cells (McA-RH7777), which secrete VLDL- and HDL-like particles, were stably transfected with these cDNAs, and the cDNAs for human apoE and human apoA-I. Single spin NaBr gradient fractions of lipoprotein deficient serum-treated cell medium from transfected McA-RH7777 cells were analyzed. The distributions of transfected human apoE and apoA-I and endogenous rat apoE and apoA-I were compared with those of the chimeras. Among HDL subspecies, human apoE expressed by these cells is associated with particles of density 1.108 g/ml. Similarly, chimera apoA-InEc (exon 3 of apoA-I and exon 4 of apoE) is found in particles of density 1.111 g/ml. Human apoA-I, however, distributes in a broader range of particles with peak densities of 1.111 g/ml and 1.164 g/ml. The distribution of the complementary chimera, apoEnA-Ic, follows this same pattern, with peak particle densities of 1.098 and 1.137 g/ml. This is in contrast to the narrow distributions of endogenous rat apoE and apoA-I, which were found in particles of density 1.099 and 1.089 g/ml, respectively. When metabolically labeled medium was fractionated via gel filtration column chromatography, apoA-InEc was found to associate with the VLDL fractions; apoEnA-Ic was absent from these same fractions. These results suggest that the fourth exon largely determines the distinctive lipoprotein distribution patterns of these two human apoproteins and that the human apoA-I fourth exon sequence may account for the polydisperse HDL pattern as observed by others in transgenic mice expressing human apoA-I.

Neurons of the human frontal cortex display apolipoprotein E immunoreactivity: implications for Alzheimer's disease

Apolipoprotein E (apoE) is a plasma protein that regulates lipid transport and cholesterol homeostasis. In humans, apoE occurs as 3 major isoforms (apoE2, E3, and E4). Genetic evidence demonstrates an overrepresentation of the apoE epsilon 4 allele in Alzheimer's disease (AD). While apoE immunoreactivity (IR) is associated with the amyloid plaques and neurofibrillary tangles of AD, few studies have characterized the localization of apoE in normal human brains. We examined the distribution of apoE in the cerebral cortex of normal aged individuals and compared the results to clinically diagnosed and pathologically confirmed AD cases. In addition, we characterized the apoE IR in brains from high plaque non-demented (HPND) cases. We observed consistent and widespread apoE staining in cortical neurons from normal and HPND individuals. This finding was confirmed by double immunostaining which colocalized apoE with microtubule-associated protein-2, as well as low density lipoprotein receptor-related protein, an apoE receptor found on neurons. In contrast, AD brains displayed apoE IR in plaques and neurofibrillary tangles with little neuronal staining. These data clearly establish the presence of apoE in normal neurons, supporting an intracellular role for apoE. Moreover, the results suggest that this function of apoE is disrupted in AD, where apoE staining of neurons was drastically reduced.

Relationships of plasma and hepatic variables with rates of plasma low-density lipoprotein apolipoprotein B metabolism in baboons fed low- and high-fat diets

These studies were conducted to determine relationships of plasma low-density lipoprotein (LDL) cholesterol concentrations and hepatic mRNA levels for apolipoprotein (apo) B, LDL receptor, and hepatic hydroxymethyl glutaryl coenzyme A (HMG CoA) synthase with plasma LDL apo B production and catabolic rates in baboons maintained on a low-cholesterol, low-fat chow diet and on a high-cholesterol, high-fat (HCHF) diet. Twelve baboons with LDL cholesterol levels ranging from low to high on the HCHF diet but with similar high-density lipoprotein (HDL) cholesterol levels were selected from a colony of selectively bred pedigreed baboons. LDL apo B turnover and hepatic mRNA concentrations for apo B, LDL receptor, and HMG CoA synthase were measured on a chow diet and again on a HCHF diet fed for 14 weeks. LDL apo B fractional catabolic rates decreased and production rates increased on the HCHF diet. Hepatic mRNA concentrations for apo B were not affected by the HCHF diet. Hepatic LDL receptor and HMG CoA synthase mRNA concentrations decreased on the HCHF diet as compared with the chow diet. LDL apo B fractional catabolic rate was negatively correlated with plasma cholesterol, LDL cholesterol, LDL apo B, and LDL apo B production and positively correlated with hepatic LDL receptor and HMG CoA synthase mRNA concentrations and with plasma LDL triglyceride to cholesterol ratio on the chow diet but not on the HCHF diet. LDL apo B production was positively correlated with plasma cholesterol, LDL cholesterol, and LDL apo B on the HCHF diet and negatively correlated with LDL triglyceride to cholesterol ratio on both chow and HCHF diets.(ABSTRACT TRUNCATED AT 250 WORDS)

Import of transcription factor MTF1 into the yeast mitochondria takes place through an unusual pathway

We report studies of the import into mitochondria of MTF1, a nucleus-encoded factor that markedly stimulates the specific transcription of mitochondrial DNA. Unlike most of the mitochondrial precursor proteins studied so far, import of MTF1 does not appear to require a receptor on the outer membrane, membrane potential across the inner membrane, or ATP hydrolysis. Its import is not affected by low temperature. It lacks a cleavable presequence but translocates across the inner membrane through its amino terminus; its sorting is independent of hsp60. Our results indicate an unusual and distinct import pathway for MTF1 into the yeast mitochondria.

Purification of apolipoprotein E attenuates isoform-specific binding to beta-amyloid

Apolipoprotein E (apoE), particularly the e4 allele, is genetically linked to the incidence of Alzheimer's disease. In vitro, apoE has been shown to bind beta-amyloid (A beta), an amyloidogenic peptide that aggregates to form the primary component of senile plaques. In previous work, we demonstrated that apoE3 from tissue culture medium binds to A beta with greater avidity than apoE4 (LaDu, M. J., Falduto, M. T., Manelli, A. M., Reardon, C. A., Getz, G. S., and Frail, D. E. (1994) J. Biol. Chem. 269, 23403-23406). This is in contrast to data using purified apoE isoforms as substrate for A beta (Strittmatter, W. J., Weisgraber, K. H., Huang, D. Y., Dong, L.-M., Salvesen, G. S., Pericak-Vance, M., Schmechel, D., Saunders, A. M., Goldgaber, D., and Roses, A. D. (1993) Proc. Natl. Acad. Sci. U. S. A. 90, 8098-8102). Here we resolve this apparent discrepancy by demonstrating that the preferential binding of A beta to apoE3 is attenuated and even abolished with purification, a process that includes delipidation and denaturation. We compared the A beta binding capacity of unpurified apoE isoforms from both tissue culture medium and intact human very low density lipoproteins with that of apoE purified from these two sources. The interaction of human A beta-(1-40)-peptide and apoE was analyzed by nonreducing SDS-polyacrylamide gel electrophoresis followed by Western immunoblotting for either A beta or apoE immunoreactivity. While the level of the apoE3.A beta complex was approximately 20-fold greater compared with the apoE4.A beta complex in unpurified conditioned medium, apoE3 and apoE4 purified from this medium bound to A beta with comparable avidity. Moreover, using endogenous apoE on very low density lipoproteins from plasma of apoE3/3 and apoE4/4 homozygotes, apoE3 was again a better substrate for A beta than apoE4. However, apoE purified from these plasma lipoproteins exhibited little isoform specificity in binding to A beta. These results suggest that native preparations of apoE may be a more physiologically relevant substrate for A beta binding than purified apoE and further underscore the importance of subtle differences in apoE conformation to its biological activity.

Heat shock protein HSP60 can alleviate the phenotype of mitochondrial RNA-deficient temperature-sensitive mna2 pet mutants

mna2, which belongs to the class I temperature-sensitive pet mutants that lose mitochondrial (mt)RNA at restrictive temperature, was shown by complementation and sequence determination to correspond to the gene coding for HSP60. Both mna2-1 and mna2-2, the two available alleles of mna2, have conservative single amino acid substitutions in the HSP60 gene. Valine substitutes for an alanine (position 47) in mna2-1, and an isoleucine substitutes for a valine (position 77) in mna2-2. These substitutions result in defects in respiration and in steady-state mtRNA accumulation. Wild-type hsp60 alleviates the mtRNA phenotype completely, while partially relieving the respiratory deficiency.

Heparin-induced lipolysis in hypertriglyceridemic subjects results in the formation of atypical HDL particles

This study reports on the characterization of high density lipoprotein (HDL) in normotriglyceridemic and hypertriglyceridemic (HTG) subjects, after a fat meal and heparin-induced release of lipases. Samples for detailed analysis of HDL by density gradient ultracentrifugation and nondenaturing gradient gel electrophoresis were collected at 0 h and 5 h after the meal and 15 min after the administration of heparin. The normotriglyceridemic subjects were subdivided into two groups: those who remained normotriglyceridemic 5 h after the meal (NTG) or those who were hypertriglyceridemic at this time point (NTG-HTG). At the outset of the study, mean triglyceride levels were significantly higher (P < 0.001) and HDL cholesterol levels lower (P < 0.02) in the HTG group. The HDL particles in this group were enriched with triglyceride (P < 0.001). Serum triglyceride levels rose in all three groups after the fat meal and this was associated with further triglyceride enrichment of the HDL particles. In all groups, rapid lipolysis induced by heparin caused a significant decrease in plasma triglycerides and increase in free fatty acid levels, these changes being greatest in the HTG group. HDL density profiles of the study groups prior to the administration of heparin demonstrated two distinct peaks at density 1.09 g/ml (HDL2) and 1.13 g/ml (HDL3). However, after the administration of heparin to the HTG group, only a single peak in the HDL profile was evident that was located at the density region corresponding to HDL2 (1.09 g/ml). Upon gradient gel electrophoresis of this peak, there was an increased number (P < 0.005 vs. NTG) of small particles (< 4.37 nm) whose size was similar to the size range normally associated with HDL3b and HDL3c. Similar changes in HDL density and size after the administration of heparin were observed in the NTG-HTG group who were also hypertriglyceridemic postprandially. By contrast, the density gradient profiles and sizes of the HDL particles did not change after the administration of heparin to NTG subjects. Thus, the activation of lipolysis in HTG subjects leads to the generation of atypical HDL particles that are small but of reduced density. Rapid clearance of such particles could account for the inverse relationship between triglyceride and HDL cholesterol in this population subgroup.

Isoform-specific binding of apolipoprotein E to beta-amyloid

Apolipoprotein E (apoE), particularly the e4 allele, is genetically linked to the incidence of Alzheimer's disease. ApoE is present in the extracellular senile plaques and intracellular neurofibrillary tangles associated with Alzheimer's disease. In vitro, apoE has been shown to bind beta-amyloid (A beta), an amyloidogenic proteolytic product of amyloid precursor protein. To analyze the interaction of A beta and apoE, we used Western immunoblotting of human A beta-(1-40)-peptide incubated with conditioned medium from HEK-293 cells transfected with either human apoE3 or apoE4 (products of the e3 and e4 alleles, respectively) cDNA. Nonreducing SDS-polyacrylamide gel electrophoresis revealed the presence of an approximately 45-kDa complex with both A beta and apoE immunoreactivity. The level of the apoE3.A beta complex was approximately 20-fold greater than that of the apoE4.A beta complex. This apoE isoform-specific binding pattern was maintained from pH 5.0 to 9.0, from 2 min to 24 h of peptide incubation, and at concentrations of apoE from 5 to 100 micrograms/ml and of A beta from 10 microM to 1 mM. The higher level of apoE3 binding to A beta is in contrast to previously published data using purified apoE (Strittmatter, W. J., Weisgraber, K.H., Huang, D. Y., Dong, L.-M., Salvesen, G. S., Pericak-Vance, M., Schmechel, D., Saunders, A. M., Goldgaber, D., and Roses, A.D. (1993) Proc. Natl. Acad. Sci. U. S. A. 90, 8098-8102). Factors responsible for the isoform-specific interactions between apoE and A beta will require further study before the apparent discrepancy between these data can be reconciled.

Effect of dietary lipids on plasma activity and hepatic mRNA levels of cholesteryl ester transfer protein in high- and low-responding baboons (Papio species)

We compared the effects of dietary cholesterol, type of fat (coconut oil v corn oil), and phenotype (low low-density lipoprotein [LDL] response v high LDL response) on the plasma activity and hepatic mRNA levels of cholesteryl ester transfer protein (CETP). In a crossover design, eight high- and eight low-LDL-responding baboons were fed a series of diets with increasing amounts of cholesterol (0.05, 0.15, 0.45, and 1.35 mg/kcal) with either coconut oil or corn oil. All diets were fed for 7 weeks each. plasma and lipoprotein cholesterol concentrations, plasma lecithin:cholesterol acyltransferase (LCAT) and CETP activity, and hepatic mRNA levels for CETP and apolipoprotein (apo) A-I were measured after 6 weeks on each diet. Data were analyzed in two steps, ie, the effect of the initial change from chow to 0.05 mg cholesterol with each fat and the effect of the stepwise increase in cholesterol from 0.05 to 1.35 mg/kcal with each fat. High-responding baboons, as expected, showed a more pronounced increment in plasma LDL cholesterol at all dietary cholesterol levels, particularly with coconut oil as the dietary fat. Plasma high-density lipoprotein 2 (HDL2) and HDL3 cholesterol increased as dietary cholesterol increased on both the coconut and corn oil diets, with a greater increase in high-responding baboons than in low-responding baboons. The stepwise increase in dietary cholesterol increased plasma LCAT activity in both high- and low-responding baboons fed the coconut oil diet, but not in those fed the corn oil diet. Dietary cholesterol, regardless of type of fat, increased plasma CETP activity.(ABSTRACT TRUNCATED AT 250 WORDS)

Metabolic mechanisms for responses to dietary cholesterol and fat in high and low LDL responding baboons (Papio sp.)

These studies were conducted to determine how plasma low density lipoprotein (LDL) cholesterol levels respond to dietary cholesterol, fed in increasing amounts with either corn oil or coconut oil diets, in high as compared to low LDL responding baboons; and to determine how apolipoprotein (apo) B transcription levels are modulated in response to dietary lipids. Eight high and eight low LDL responding pedigreed adult baboons, balanced for sire, age, sex, and weight, were challenged for successive 7-week periods with increasing levels of dietary cholesterol combined with either coconut oil or corn oil. At the end of each dietary period, plasma and lipoprotein lipids, apoB, apoA-I, and hepatic mRNA levels for apolipoproteins were measured. As dietary cholesterol increased, plasma cholesterol concentrations (mostly LDL cholesterol) increased in both phenotypes and with both types of fat, but phenotypic differences were greater with coconut oil. There was not a consistent dose-response relationship of plasma or LDL cholesterol levels to increasing intakes of dietary cholesterol. Neither dietary cholesterol, type of dietary fat, nor LDL phenotype affected hepatic apoB or apoE mRNA levels. In a second experiment to resolve the inconsistent dose-response to dietary cholesterol, we fed the animals varying levels of dietary cholesterol combined with coconut oil, and separated the challenge periods with intervening 12-week chow periods. Plasma and LDL cholesterol and apoB concentrations rose consistently with increasing dietary cholesterol, and the slope of the increase diminished at the higher doses. The results suggest that genetic differences in the initial response of LDL cholesterol to dietary cholesterol and saturated fatty acids are not due to the differences in hepatic transcription of apoB, and that the preceding dietary intake of cholesterol and saturated fatty acids is a major determinant of the response of plasma lipids and the associated metabolic processes to a dietary challenge. The response of baboon plasma LDL cholesterol concentrations to dietary cholesterol, when fed with saturated fatty acids, is similar to that of humans.

Characterization of cholesteryl ester transfer protein inhibitor from plasma of baboons (Papio sp.)

Selective breeding has produced families of baboons that accumulate large high density lipoproteins (HDL1) when challenged with a high cholesterol and high fat (HCHF) diet. In the plasma isolated from these high HDL1 baboons there is a factor that decreases the transfer of cholesteryl ester from HDL to lower density lipoproteins. The purpose of these studies was to identify and characterize this inhibitor of cholesteryl ester transfer. A protein with molecular mass of approximately 4 kDa was detected in greater amounts in the plasma lipoproteins of high HDL1 baboons fed the HCHF diet than in plasma lipoproteins of low HDL1 baboons. This 4 kDa protein appeared to associate with apolipoprotein (apo) A-I, resulting in modified apoA-I with an apparent molecular mass of 31 kDa. A small amount of modified apoE was also identified with a molecular mass of 41 kDa. N-terminal amino acid sequencing of the 4 kDa peptide identified it as an N-terminal fragment of apoC-I. Like apoC-I, the fragment is also a slightly basic protein (pI 7.1). The apoC-I fragment and modified apoA-I presented at equimolar concentrations exhibited similar inhibition of cholesteryl ester transfer protein (CETP) activity in HDL of low HDL1 baboons. On the basis of baboon apoC-I amino acid sequence and the molecular mass of the inhibitor peptide, a peptide corresponding to the N-terminal 38 amino acids of apoC-I was synthesized chemically. This synthetic peptide also inhibited CETP activity in vitro. Rabbit polyclonal antisera prepared against the 38 amino acid synthetic peptide recognized the 4 kDa molecular mass inhibitor protein, apoC-I (6.6 kDa), and the modified apoA-I protein (31 kDa molecular mass) in the plasma lipoproteins of high HDL1 baboons. On the other hand, the antibody detected only apoC-I in the plasma lipoproteins of low HDL1 baboons. The IgG fraction isolated from antiserum raised against the synthetic inhibitor peptide increased cholesteryl ester transfer from HDL of high HDL1 baboons, whereas the IgG antibody against CETP decreased cholesteryl ester transfer from HDL of both high and low HDL1 baboons. These studies suggest that the CETP inhibitor is an N-terminal fragment of apoC-I, and this fragment also modifies apoA-I and apoE in the plasma.

Identification of a new nuclear gene (CEM1) encoding a protein homologous to a beta-keto-acyl synthase which is essential for mitochondrial respiration in Saccharomyces cerevisiae

We have analysed a new gene, CEM1, from Saccharomyces cerevisiae. Inactivation of this gene leads to a respiratory-deficient phenotype. The deduced protein sequence shows strong similarities with beta-keto-acyl synthases or condensing enzymes. Typically, enzymes of this class are involved in the synthesis of fatty acids or similar molecules. An analysis of the mitochondrial lipids and fatty acids shows no major difference between the wild type and deleted strains, implying that the CEM1 gene product is not involved in the synthesis of the bulk fatty acids. Thus it is possible that the CEM1 protein is involved in the synthesis of a specialized molecule, probably related to a fatty acid, which is essential for mitochondrial respiration.

Induction of (omega-1)-oxidation of monocarboxylic acids by acetylsalicylic acid

Monocarboxylic acids may be oxidized at the omega- and (omega-1)- positions to form dicarboxylic acids (DCAs) and (omega-1)-hydroxy- or (omega-1)-oxoacids. The significance of this pathway under normal conditions is unknown, but DCAs and (omega-1)-hydroxyacids are prominent features of disease states. The stimulation of this pathway has been linked to induction of fatty acid-binding protein and peroxisomal proliferation. In this study, we examined the effect of acetylsalicylic acid (ASA) on (omega-1)-oxidation. (Omega-1)-oxidation was assessed in subcellular fractions of rat liver. Rats were fed a normal diet or an ASA-supplemented diet. Products were identified by gas chromatography-mass spectrometry (GC-MS) and by comparison with the properties of authentic synthetic standards. Doses of ASA that produced relatively low serum concentrations (12-24 mg/dl) resulted in as much as a 20-fold increase in the capacity for (omega-1)-oxidation of medium (C12-C15) and long chain (C16-C20) monocarboxylic acids. Normal rat liver oxidizes monocarboxylic acids to (omega-1)-oxoacids, while liver from ASA-treated rats converts these substrates to (omega-1)-oxodicarboxylic acids and (omega-1)-oxoacids. The formation of oxoacids and oxodicarboxylic acids may be due to different enzymes. The formation of oxodicarboxylic acids appears to be more labile than the formation of oxoacids. These two processes also are differentially induced by ASA and have different substrate specificities. These results demonstrate that ASA is a potent stimulant of (omega-1)-oxidation and induces the formation of products that can be shortened in peroxisomes to key metabolic intermediates.

Inhibition of apolipoprotein E degradation in a post-Golgi compartment by a cysteine protease inhibitor

In our prior studies on lipoprotein stimulation of apolipoprotein E (apoE) secretion in HepG2 cells, it became clear that a proportion of the newly synthesized apoE was degraded intracellularly (Ye, S. Q., Olson, L. M., Reardon, C. A., and Getz, G. S. (1992) J. Biol. Chem. 267, 21961-21966). The present study was designed to determine the nature of the proteases and the intracellular sites involved in newly synthesized apoE degradation. The effect of seven protease inhibitors on total apoE levels was examined. Only N-acetyl-leucyl-leucyl-norleucinal (ALLN), a cysteine protease inhibitor, significantly blocked apoE degradation in HepG2 cells. The amount of total apoE from cells chased with ALLN for 4 h was increased by 1.58 +/- 0.05-fold relative to the controls (n = 11, p < 0.01). ALLN extended the half-life of apoE from 2.61 h to 4.38 h (p < 0.01). This effect occurs in a post-Golgi compartment since in the presence of brefeldin A, ALLN had no effect on intracellular apoE levels. Chloroquine and NH4Cl significantly reduced apoE degradation; however, ALLN plus either of these reagents appear to have an additive effect. The amount of apoE in cells chased in Ca(2+)-free medium was significantly higher than that in cells chased in Ca(2+)-containing medium (1.70 +/- 0.07-fold, n = 6, p < 0.01). ALLN plus Ca(2+)-free medium had no additive effect. ALLN had no significant influence on the degradation of albumin but had a similar effect on transfected apoE in Chinese hamster ovary cells. Overall, these data suggest that apoE may be degraded in a post-Golgi compartment of HepG2 and Chinese hamster ovary cells by lysosomal enzymes and cytosolic Ca(2+)-dependent cysteine proteases. ALLN inhibits the latter.