Abstract 292: ETC-1002 Reduces Body Weight Gain and Hepatic Triglyceride Content and Improves Glycemic Control in a Mouse Model of Diet-Induced Obesity

ETC-1002 is an investigational drug currently in Phase 2 clinical development to treat dyslipidemia and other cardiometabolic risk factors. Previously, ETC-1002 prevented hyperlipidemia and atherosclerosis in rodent models; and improved hepatic triglycerides (TG) as well as fasting blood glucose and insulin in the KKA y insulin resistant mouse model via putative mechanisms including activation of AMP-activated protein kinase. In the present study we investigated the effect of ETC-1002 on body weight, hepatic TG and insulin sensitivity in a diet-induced obese (DIO) mouse model. C57BL/6 mice were fed a 60% high-fat diet beginning at 11 weeks of age. At 12 weeks of age mice were assigned to treatment groups and administered vehicle or ETC-1002 at 3, 10, or 30 mg/kg/day for 9 weeks. A separate cohort of mice was maintained on standard rodent chow diet throughout the study as a comparator. Food consumption, body weight, hepatic TG content, fasting blood glucose, fasting plasma insulin, insulin tolerance tests, and glucose tolerance tests were measured. Mice developed obesity, hyperinsulinemia, mild hyperglycemia and elevated hepatic triglycerides in response to the high-fat diet. ETC-1002 results were dose-dependent and statistically significant at doses of 10 and 30 mg/kg/day. ETC-1002 attenuated body weight gain 8% and 15% with no effect on food consumption. Body weight changes were associated with 12% and 32% decreases in epididymal fat pad mass. Hepatic TG content was also reduced with ETC-1002 by 34% and 46%; respectively. ETC-1002 treatment reduced fasting blood glucose 11% and 16%; plasma insulin 80% and 95%; and resulted in significant improvements in insulin tolerance tests (19% and 22% reduction in AUC) with modestly improved glucose tolerance (not significant). In an intervention study with a 12 week lead-in on high-fat diet, comparable effects on body weight, hepatic TG, and insulin sensitivity were observed. In summary, ETC-1002 reduced obesity and hepatic TG and improved glycemic parameters in a high-fat fed diet-induced mouse model of disease. The present data in the DIO mouse, combined with previously reported efficacy in rodent models supports ETC-1002 as a regulator of imbalances in lipid and carbohydrate metabolism.

Dietary cholate increases plasma levels of apolipoprotein B in mice by posttranscriptional mechanisms

To induce atherogenesis in mice, a high fat (HF) diet is supplemented with cholic acid (CA), which increases apoB-containing particles and lower apoA-I-containing particles. HF diet without CA increases levels of both HDL and LDL, suggesting that CA may be responsible for the elevation of LDL and lowering of HDL. The mechanism of dietary CA-induced lowering of apoA-I-containing particles has recently been reported. In this study, we examined the mechanism of CA- and HF-induced elevation of apoB-containing lipoproteins in mice. Mice were fed the following four diets: control chow (C), high fat high cholesterol, (HF), control and 0.5% cholate (CA), and HF+CA. Dietary CA increased the plasma levels of apoB-containing particles by approximately 2-fold when compared to control; VLDL levels increased 2-fold, and LDL levels increased 1.3-fold. On HF diet, VLDL increased by 1.4-fold, and LDL by 2-fold, suggesting that CA and HF-induced increases of apoB-containing particles occurred by different mechanisms. We investigated the potential mechanisms regulating plasma levels of apoB in CA- and HF-fed mice. Although hepatic apoB mRNA levels did not change on CA diet, apoB-100 mRNA increased relative to B-48 as a result of decreased editing of apoB mRNA. Measurements of hepatic LDL receptor mRNA suggested that CA diet down-regulated LDL receptor mRNA, possibly by increasing the levels of hepatic cholesterol. Since plasma and hepatic vitamin E levels did not show significant changes on CA-containing diets, it suggests that dietary CA did not act by increasing the absorption of dietary fat. Hepatic lipase, known to modulate plasma levels of apoB-containing particles, did not show changes in CA- or HF-fed mice. Taken together, these results suggest that dietary CA increased apoB-containing particles both in chow-fed and fat-fed mice by enhancing the relative production of apoB-100, and also by reducing LDL receptor-mediated clearance of apoB-containing particles. Thus, dietary cholate modulates plasma levels of apoB primarily by posttranscriptional mechanisms.

Estrogen increases hepatic lipase levels in inbred strains of mice: a possible mechanism for estrogen-dependent lowering of high density lipoprotein

We have shown mouse to be an useful animal model for studies on the estrogen-mediated synthesis and secretion of lipoproteins since, unlike in rats, low density lipoprotein receptors are not upregulated in mice. This results into the elevation of plasma levels of apolipoprotein (apo) B and apoE, and lowering of apoA-1-containing particles. The mechanisms of apoB and apoE elevation by estrogen have been elucidated, but the mechanism of lowering of plasma levels of HDL is still not known. Among other factors, apoA-I, cholesterol ester transfer protein (CETP), scavenger receptor B1 (SR-B1), and hepatic lipase are potential candidates that modulate plasma levels of HDL. Since estrogen treatment increased hepatic apoA-I mRNA and apoA-I synthesis, and mouse express undetectable levels of CETP, we tested the hypothesis that estradiol-mediated lowering of HDL in mice may occur through modulation of hepatic lipase (HL). Four mouse strains (C57L, C57BL, BALB, C3H) were administered supraphysiological doses of estradiol, and plasma levels of HDL as well as HL mRNA were quantitated. In all 4 strains estradiol decreased plasma levels of HDL by 30%, and increased HL mRNA 2-3 fold. In a separate experiment groups of male C57BL mouse were castrated or sham-operated, and low and high doses of estradiol administered. We found 1.4-2.5 fold elevation of HL mRNA with concomitant lowering of HDL levels. Ten other mouse strains examined also showed estradiol-induced elevation of HL mRNA, but the extent of elevation was found to be strain-specific. Based on these studies, we conclude that hepatic lipase is an important determinant of plasma levels of HDL and that HL mRNA is modulated by estrogen which in turn may participate in the lowering of plasma levels of HDL.

Dietary cholic acid lowers plasma levels of mouse and human apolipoprotein A-I primarily via a transcriptional mechanism

To induce dietary atherosclerosis in mice, high-fat/high-cholesterol (HF) diets are frequently supplemented with cholic acid (CA). This diet produces low plasma levels of high-density lipoprotein (HDL) and high levels of low-density lipoprotein (LDL). However, HF diets without any added CA, which more closely resemble human diets, increase levels of both HDL and LDL, suggesting that CA may be responsible for the lowering of HDL. Our aim was to examine the potential mechanism responsible for the lowering of HDL. Nontransgenic (NTg) C57BL mice and apoA-I-transgenic (apoAI-Tg) mice, with greatly increased basal apoA-I and HDL levels, were used. Mice were fed the following four diets: control (C), high-fat/high-cholesterol (HF), control and 1% cholate (CA) and HF + CA. Dietary CA reduced plasma HDL levels by 35% in NTg and 250% in apoAI-Tg mice, independent of the fat or cholesterol content of the diet. Hepatic apoA-I mRNA decreased 30% in NTg and 180% in apoAI-Tg mice. Hepatic apoA-I synthesis and apoA-I mRNA transcription rates also decreased in parallel with apoA-I mRNA levels, suggesting that the CA-induced decreases in plasma apoA-I levels occurred primarily via decreasing apoA-I mRNA transcription rates. An HF diet increased HDL levels 1.8-fold in NTg and 1.5-fold in apoAI-Tg mice. Addition of CA to the HF diet lowered HDL levels by 1.6-fold in NTg and 2. 5-fold in apoAI-Tg mice. Transfection studies with the apoA-I promoter suggested the presence of a putative cis-acting element responsible for the CA-mediated down-regulation of the apoA-I promoter activity. Measurements of apoA-I regulatory protein-1 (ARP-1) mRNA, a negative regulator of the apoA-I gene in the mouse liver showed that CA increased the ARP-1 mRNA levels. Because apoA-I gene transcription alone was not sufficient to account for the lowering of plasma HDL levels, scavenger receptor-B1 (SR-B1) and hepatic lipase (HL) mRNAs levels were quantitated. The levels of SR-B1 and HL mRNA were not changed by dietary CA. These studies suggest that dietary cholate regulates plasma levels of apoA-I primarily by a transcriptional mechanism via a putative bile acid response element involving a negative regulator of apoA-I, and partly by an unidentified post-transcriptional mechanism.

Apolipoprotein E gene expression is reduced in apolipoprotein A-I transgenic mice

The levels of plasma apolipoprotein (apo) E, an anti-atherogenic protein involved in mammalian cholesterol transport, were found to be 2-3 fold lower in mice over-expressing human apoA-I gene. ApoE is mainly associated with VLDL and HDL-size particles, but in mice the majority of the apoE is associated with the HDL particles. Over-expression of the human apoA-I in mice increases the levels of human apoA-I-rich HDL particles by displacing mouse apoA-I from HDL. This results in lowering of plasma levels of mouse apoA-I. Since plasma levels of apoE also decreased in the apoA-I transgenic mice, the mechanism of apoE lowering was investigated. Although plasma levels of apoE decreased by 2-3 fold, apoB levels remained unchanged. As expected, the plasma levels of human apoA-I were almost 5-fold higher in the apoAI-Tg mice compared to mouse apoA-I in WT mice. If the over-expression of human apoA-I caused displacement of apoE from the HDL, the levels of hepatic apoE mRNA should remain the same in WT and the apoAI-Tg mice. However, the measurements of apoE mRNA in the liver showed 3-fold decreases of apoE mRNA in apoAI-Tg mice as compared to WT mice, suggesting that the decreased apoE mRNA expression, but not the displacement of the apoE from HDL, resulted in the lowering of plasma apoE in apoAI-Tg mice. As expected, the levels of hepatic apoA-I mRNA (transgene) were 5-fold higher in the apoAI-Tg mice. ApoE synthesis measured in hepatocytes also showed lower synthesis of apoE in the apoAI-Tg mice. These studies suggest that the integration of human apoA-I transgene in mouse genome occurred at a site that affected apoE gene expression. Identification of this locus may provide further understanding of the apoE gene expression.

High density lipoprotein, apolipoprotein A-I, and coronary artery disease

High density lipoproteins (HDL), one of the main lipoprotein particles circulating in plasma, is involved in the reverse cholesterol transport. Several lines of evidence suggest that elevated levels of HDL is protective against coronary heart disease. The role of HDL in the removal of body cholesterol and in the regression of atherosclerosis add to the importance of understanding the molecular-cellular processes that determine plasma levels of HDL. Factors modulating plasma levels of HDL may have influence on the predisposition of an individual to premature coronary artery disease. Apolipoprotein (apo) A-I is the main apolipoprotein component of HDL and, to a large extent, sets the plasma levels of HDL. Thus, understanding the regulation of apoA-I gene expression may provide clues to raise plasma levels of HDL. This review discusses the various pathways that alter plasma levels of HDL. Since apoA-I is the main protein component of HDL and determines the plasma levels of HDL, this review also covers the regulation of apoA-I gene expression.

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.

Molecular bases of low production rates of apolipoprotein B-100 and truncated apoB-82 in a mutant HepG2 cell line generated by targeted modification of the apolipoprotein B gene

In subjects with familial hypobetalipoproteinemia heterozygous for truncated forms of apolipoprotein B, both apoB-100 and the truncated forms are produced at lower than expected rates. We studied the mechanism of low levels of apoB in a cell model produced by targeted modification of the apob gene of HepG2 cells. One of the three alleles of apob was found to be targeted. The targeted cells expressed apoB-100 and B-82. The media of mutant cells contained 56% of the levels of apoB-100 present in the media of wild-type (WT) HepG2 cells. ApoB-82 was present at 11% of the apoB-100 levels in mutant cell media. An 85-kD protein (apoB-15) representing the N-terminal fragment of apoB was also secreted, but only in the mutant cell media. We examined the mechanism of low levels of apoB-82. Cellular apoB-82 mRNA was 11% of apoB-100 mRNA, lower than the 33% expected, but consistent with relative levels of apoB-82 in the media. ApoB mRNA transcription in WT and the mutant cells did not differ, while the levels of apoB-82 mRNA in nuclei and polysomes were 46% and 12% of the levels of apoB-100 mRNA, respectively, suggesting that the lower levels of apoB-82 mRNA were due to altered message stability. In a pulse/chase experiment with [35S] methionine, at zero time of chase, the amounts of apoB-100 in mutant cells was 66% that of WT levels, consistent with the modification of one allele. The fractions of newly synthesized apoB-100 secreted into the media at 2 h were 10% in the mutant cells and 19% in the WT cells, suggesting greater presecretory degradation of apoB-100 in the mutant cells. Thus, low levels of mutant apoB-82 mRNA gave rise to the low levels of apoB-82, while low levels of apoB-100 were due to low rates of secretion.

Estrogen up-regulates apolipoprotein E (ApoE) gene expression by increasing ApoE mRNA in the translating pool via the estrogen receptor alpha-mediated pathway

The antiatherogenic property of estrogens is mediated via at least two mechanisms: first by affecting plasma lipoprotein profiles, and second by affecting the components of the vessel wall. Raising plasma apolipoprotein E (apoE) in mice protects them against diet-induced atherosclerosis (Shimano, H., Yamada, N., Katsuki, M., Gotoda, T., Harada, K., Murase, T., Fukuzawa, C., Takaku, F., and Yazaka, Y. (1992) Proc. Natl. Acad. Sci. U. S. A. 89, 1750-1754). It is possible that estrogen may be antiatherogenic at least in part by increasing plasma apoE levels. Therefore, we studied the regulation of apoE by estrogen. A survey of 15 inbred strains of mice showed that some mouse strains responded to injections or subcutaneously implanted pellets of estradiol by raising their apoB and apoE levels and some did not. We performed detailed studies in two "responder" strains, C57L and C57BL, and two "non-responder" strains, C3H and BALBc. Responders increased their plasma apoE levels 2.5-fold. Non-responders' levels were altered +/-10%. In the responders the distribution of apoE among the plasma lipoproteins shifted from high density lipoprotein toward the apoB-containing lipoprotein fractions. In nonresponders the shift was toward high density lipoprotein. Hepatic apoE mRNA levels and relative rates of apoE mRNA transcription were unchanged in all strains, suggesting that apoE regulation occurred at posttranscriptional loci. Therefore, we measured apoE synthesis in fresh liver slices and on isolated hepatic polysomes. Two-fold increases were noted but only in responders accompanied by selective 1.5-fold increases in polysomal apoE mRNA levels. Similar increases in apoE synthesis were also observed in castrated C57BL mice given either physiological or pharmacological replacement doses of estradiol, but not testosterone, suggesting that the effect of estradiol was specific on the distribution of apoE mRNA in the translationally active polysomal pool. Next, we examined whether the effects of estrogen on apoE translation were mediated by estrogen receptors (ER). ER-alpha knock-out mice and their wild-type littermates were administered estradiol. As expected, apoE levels and hepatic apoE synthesis increased more than 2-fold in the wild-type littermates, but only 20% increases in the plasma apoE and hepatic synthesis were observed in the ER knock-out mice. Hepatic apoE mRNA levels did not change in either the wild-type or the ER knock-out mice. Thus, estradiol up-regulates apoE gene expression by increasing levels of apoE mRNA in the polysomal translating pool. Furthermore, the increased polysomal recruitment of apoE mRNA is largely mediated by estrogen receptors.

Normal intestinal dietary fat and cholesterol absorption, intestinal apolipoprotein B (ApoB) mRNA levels, and ApoB-48 synthesis in a hypobetalipoproteinemic kindred without any ApoB truncation

The purpose of this study was to characterize intestinal apolipoprotein B (apoB) metabolism in subjects with familial hypobetalipoproteinemia (FHBL), where segregation analysis supports linkage to the apoB gene but no apoB truncations are present. We investigated cholesterol and fat absorption, intestinal apoB mRNA synthesis and editing, as well as apoB-48 synthesis. Plasma triglycerides (TG) and retinyl palmitate in the chylomicron fractions were analyzed after 12 hours of fasting and then repeatedly for 14 hours after ingestion of a vitamin A-containing high-fat meal. Cholesterol absorption was assessed using a dual stable-isotope method. Mean peak times and concentrations and areas under the curve (AUCs) for fat absorption and mean percentages of cholesterol absorption were comparable in affected and nonaffected family members. Intestinal biopsies were extracted for total RNA and also incubated with 35S-methionine for measurements of apoB synthesis. Similar quantities of apoB mRNA were found to be expressed in the intestine in affected and control subjects by RNase protection assay. ApoB mRNA editing assay showed that the majority of apoB-100 mRNA was edited to the apoB-48 form to a similar extent in both groups. Virtually no apoB-100 protein was synthesized by the intestine in any subject, and apoB-48 protein synthesis was not significantly different in the affected individuals. These data are consistent with in vivo metabolism data that show normal production rates for liver-derived apoB-100 but increased apoB-100 fractional catabolic rates in affected members of this family. Thus, the molecular defect probably does not affect transcription, translation, or secretion of apoB-containing lipoproteins, but may instead affect their clearance.

Regulation of lipoprotein metabolism by estrogen in inbred strains of mice occurs primarily by posttranscriptional mechanisms

Estrogen protects against developing premature coronary artery disease. However, the mechanism of protective effects of estrogen still remains poorly understood. One mechanism by which estrogen can have protective effects appears to be through modulation of plasma lipoproteins. We showed that the mouse can be used as animal model to study estrogen-mediated synthesis and secretion of lipoproteins since, unlike the rat, the mouse does not up-regulate LDL receptors (Srivastava et al. [4]). Since inbred strains of mice differ in their genetic background and show differing responsiveness to dietary lipids, we examined how various inbred strains of mice respond to estradiol administration, and whether some mouse strains show responses similar to rats. 17beta-estradiol was administered to male mice from 15 different inbred strains, and the changes in plasma levels of lipids, apoB, apoAI, and apoE were examined. Total cholesterol decreased in all but one strain, apoAI levels decreased in all but 3 strains while apoB levels and apoB/apoAI ratios increased in all but 2 strains, suggesting that in contrast to rats, the apoB-containing lipoproteins increased relative to HDL in all strains of mice examined. Basal and estradiol-induced changes in total cholesterol were significantly correlated with changes in apoAI, but not apoB, reflecting the predominance of HDL over other lipoproteins in mouse plasma. The effects of estrogen on plasma apoE levels varied among various inbred strains of mice tested. Plasma apoE levels increased in seven strains treated with estrogen, and remained unchanged in the rest. To examine whether changes of plasma apoproteins are associated with the changes in the respective hepatic mRNA levels, apoAI, B and E mRNA were quantified by RNase protection assay. Hepatic apoE mRNA did not show correlation with either basal or post treatment plasma apoE levels in any of the strains. Similarly, most of the mouse strains did not show correlation of plasma apoAI and apoB levels with the corresponding hepatic mRNA levels. These results suggest that estrogen regulates plasma lipoprotein concentrations primarily by posttranscriptional mechanisms, and there were strain-related differences in the estrogen-mediated regulation of lipoprotein metabolism.

Alcohol increases plasma levels of cholesterol diet-induced atherogenic lipoproteins and aortic atherosclerosis in rabbits

The purpose of the present study was to reexamine the relationship between alcohol and atherosclerosis. Two experiments were performed: The first contained three groups of New Zealand White (NZW) female rabbits. The control group was fed a cholesterol-containing liquid diet and the other two groups were fed the same diet with either 20% or 30% of the calories supplied by alcohol. The second experiment had two treatments: one control group and another group fed a 10% alcohol diet. In experiment 1, alcohol at the 20% and 30% levels increased VLDL and LDL but not HDL compared with levels in control rabbits. Hepatic mRNA levels of apolipoprotein (apo) A-I, apoB, and 7 alpha-hydroxylase were not affected by alcohol. However, the LDL-receptor mRNA was decreased to half of control values by either 20% or 30% alcohol. Lesion areas and aortic cholesterols were significantly increased in the 20% and 30% alcohol-treated groups. Also, significant correlations were found between plasma cholesterol levels and total lesion area or lesion cholesterol contents. In experiment 2, the 10% alcohol-treated rabbits showed no differences in circulating lipoproteins, LDL-receptor mRNA, or lesion formation above that observed in controls. These experiments suggest that alcohol substituted at 20% or 30% of the dietary calories induces hypercholesterolemia and more aortic atherosclerotic lesions. The alcohol-induced accumulation of VLDL and LDL was accompanied by low hepatic LDL-receptor mRNA levels, suggesting that alcohol may affect LDL-receptor expression and rates of lipoprotein clearance, but more experiments are needed to evaluate this possibility.

A new apolipoprotein B truncation (apo B-43.7) in familial hypobetalipoproteinemia: genetic and metabolic studies

We describe a new truncation of apolipoprotein (apo) B in a white kindred with familial hypobetalipoproteinemia (FHBL). Apo B-43.7, found in a daughter and her father, was due to a C --> T change in base position 6162 of the apo B gene converting the arginine (residue 1986) codon CGA to a stop codon TGA. Both subjects were heterozygotes, and both apo B-43.7- and apo B-100-containing particles were present in plasma. On density gradient ultracentrifugation (DGUC), approximately 30% to 40% of apo B-43.7 floated with very-low-density lipoprotein (VLDL)/intermediate-density lipoprotein (IDL)-density particles and 60% to 70% floated with high-density lipoprotein (HDL)-density particles. To assess the metabolism of apo B, 13C-leucine was infused and its rates of appearance in and disappearance from apo B-43.7- and apo B-100-containing particles were quantified by multicompartmental kinetic analysis. Apo B-100 entered plasma via VLDL with a production rate of 30 mg x kg-1 x d-1. Fractional catabolic rates (FCRs) for apo B-100 VLDL, IDL, and low-density lipoprotein (LDL) were 20.0, 16.0, and 0.46 pools x d-1, respectively. The production rate of apo B-43.7 was 9.6 mg x kg-1 x d-1, and FCRs for apo B-43.7 VLDL- and HDL-like particles were 12.0 and 1.8 pools x d-1, respectively. Approximately 30% of apo B-43.7 in HDL-density particles was derived from VLDL apo B-43.7, and about 70% appeared to enter the plasma as HDLs. The relatively low production rate of apo B-43.7 is compatible with previous reports that apo B truncations are produced at lower rates than their apo B-100 counterparts.

The multifaceted roles of the RNA processing enzyme ribonuclease III

Ribonuclease III was initially characterized as an endoribonuclease specific for double stranded RNA. Subsequently RNase III was found to be involved in the processing and maturation of ribosomal and tRNAs. Recent studies demonstrate that RNase III also participates in the processing of small stable RNAs. A number of other biological processes in which RNase III participates are: (a), conversion of polycistronic transcript of the bacteriophage T7 early region into discrete monocistronic mRNAs, (b), controlling expression of a variety of genes by processing of gene transcripts, (c), autoregulation of its own gene and (d), regulation of mRNA stability and stimulation of translation. No single processing enzyme displays such a wide variety of roles in RNA metabolism and gene expression as RNA processing enzyme ribonuclease III. This review provides an account of the various roles of RNase III in regulating gene expression and RNA metabolism.

Expression, purification and properties of recombinant E. coli ribonuclease III

Cloned RNase III gene in a T7 RNA polymerase promoter system was expressed in Escherichia coli cells lacking endogenous RNase III, and the over-expressed recombinant RNase III was purified to homogeneity using ion exchange, exclusion and affinity column chromatography. The overexpressed RNase III was found to separate with the membrane fraction after sonication, which was solubilized, fractionated with (NH)2SO4 and the active fractions used for further purification. The properties of the purified recombinant RNase III were studied using the synthetic RNA substrate, 3[H]poly[A].poly[U], and the natural substrates, 7S and p10Sa RNAs, and compared with the partially purified RNase III from wild-type E. coli cells. The recombinant RNase III showed maximal activity at 37 degrees C and at a pH range of 6.9 to 7.4, which was similar to the RNase III purified from the wild-type cells. Recombinant RNase III efficiently hydrolyzed 3[H].poly[A].poly[U] in the presence of Mg2+. However, the recombinant RNase III cleaved natural RNA substrates efficiently and accurately in the presence of Mn2+. A concentration of Mn2+ ranging from 150 to 300 microM was found to be optimal; concentrations higher than 0.5 mM were inhibitory. Other divalent cations did not support RNase III activity. Monovalent cations, Na+, K+ and NH4+ at 20 mM were equally effective in stimulating RNase III activity although they were not absolutely required for the activity. The thermal stability of the recombinant RNase III was examined at two temperatures, 37 degrees and 50 degrees C. Incubation of RNase III at 37 degrees C for 30 min did not affect activity, but it lost almost 50% of its activity when incubated at 50 degrees C for 30 min. Thus, the recombinant RNase III prefers Mn2+ for the cleavage of natural substrates and exhibits several properties similar to the wild-type RNase III.

Regulation of the apolipoprotein E by dietary lipids occurs by transcriptional and post-transcriptional mechanisms

The aim of the present investigation was to study the regulation of apolipoprotein E by two dietary nutrients, saturated fat and cholesterol, known to raise plasma cholesterol levels. ApoE is a protein component of several classes of lipoproteins including VLDL and HDL, and dietary lipids may regulate VLDL and apoE-containing HDL particles through their effects on apoE gene. Male rats and mice were fed the following 4 diets: control diet (C); high cholesterol diet with 0.5% cholesterol (HC); high fat diet with 20% hydrogenated coconut oil (HF); and high fat plus high cholesterol diet with 0.5% cholesterol and 20% fat (HF/ C). Plasma cholesterol levels remained unchanged on HC diet, but in mice VLDL-cholesterol increased by 31%. HF diet increased VLDL and LDL by 15-17% in rats, and 21% in mice. A combination of fat and cholesterol diet showed pronounced effects on plasma lipoprotein concentrations, raising apoB-containing particles by 21% and 44% in mice and rats, respectively. Plasma apoE levels increased significantly on all diets. The mechanism of regulation of increased plasma apoB and apoE levels was examined. Quantification of hepatic apoB mRNA showed a lack of correlation between plasma apoB and hepatic apoB mRNA levels, suggesting that posttranscriptional regulation increased plasma apoB-containing lipoproteins in animals fed saturated fat diets. Hepatic apoE mRNA levels increased significantly in animals fed cholesterol-rich diets. However, despite increased plasma apoE levels on diet containing only saturated fat, hepatic apoE mRNA did not change. Synthesis of apoE on the liver polysomes increased selectively on cholesterol-rich diets. These results suggest that cholesterol-rich diets altered apoE, in part, by transcriptional mechanism, and saturated fat-rich diets increased plasma apoE levels by posttranscriptional mechanism, possibly decreased receptor-mediated uptake of apoE-containing particles. The regulation of LDL receptor was also studied since plasma apoB and E levels may be altered by LDL receptor-mediated uptake by the hepatocytes. As expected, high cholesterol diet decreased LDL receptor mRNA by 30-40%. However, the LDL receptor protein on liver membranes did not change on any of the test diets in both animal species. Hepatic cholesterol content increased several fold selectively on high cholesterol diets. These findings suggest that: 1) both transcriptional and posttranscriptional mechanisms are important in regulating plasma apoB and E containing lipoproteins; 2) dietary cholesterol regulates apoE gene by a transcriptional mechanism and dietary saturated fat by posttranscriptional mechanism; and 3) changes in the hepatic apoE and LDL receptor mRNA are associated with the changes in intracellular cholesterol concentrations.

Apolipoprotein E gene expression in various tissues of mouse and regulation by estrogen

Apolipoprotein (apo) E is associated with several classes of lipoproteins and serves as a ligand for the receptor mediated uptake of cholesterol-rich particles by hepatocytes and peripheral tissues. Variant forms of apo E is also associated with dyslipidemia and late-onset of Alzheimer's Disease (AD). We report here expression of apoE in various mouse tissues, and regulation of apoE in liver, kidney, brain and testes by supraphysiological doses of estrogen. ApoE mRNA was quantified by RNase protection assay and translatable apoE mRNA by in vitro translation. As an internal control the levels of beta-actin mRNA were also quantified. Highest levels of apoE were expressed in liver (220-280 pg/mu g RNA) with negligible levels in small intestine. Brain expressed highest levels of total (35-40 pg/mu g RNA) and translatable apoE mRNA next only to liver. Other tissues that expressed relatively higher levels of apoE were adrenals, testes and ovary. ApoE was also found to be expressed in heart, lung, kidney and spleen. Regulation of apoE gene expression by estrogen (3 mu g 17beta-estradiol/ g body weight/ day for 5 consecutive days) was studied in liver, kidney, brain and testes of 4 mouse strains. Hepatic apoE mRNA did not change significantly in any of the mouse strains following estradiol administration. Of note was significant increases in the levels of brain apoE mRNA in the strain C3H. These studies demonstrate that estrogen regulates apoE gene expression in a tissue-specific manner in mice, and increases in apoE mRNA in the brain by estrogen may have implications in late-onset of Alzheimer's Disease.

Increased apoB100 mRNA in inbred strains of mice by estrogen is caused by decreased RNA editing protein mRNA

Estrogen administration to rats diminishes all apoproteins and lipoproteins from plasma. In contrast, some inbred strains of mice raise their plasma apoB and LDL levels by more than 2-fold (Srivastava et al, 1993, Eur. J. Biochem. 216, 527-538). Further studies with 13 inbred strains of mice given 3 micrograms beta-estradiol/g body weight/day for 5 consecutive days suggest that some mouse strains increased their apoB and LDL levels and some did not. To examine the mechanism of influence of genetic factors on apoB regulation, two strains, C57L and C57BL, that increased their VLDL- and LDL-cholesterol, and 2 strains, BALB and C3H, that did not, were chosen. Estrogen increased plasma apoB levels selectively in the strains C57L and C57BL, termed as 'responders,' but did not change in BALB and C3H, termed as 'non-responders.' One of the mechanisms for increased plasma apoB levels could be through increased production of apoB-containing particles. This possibility was investigated. ApoB and REPR mRNA were quantified by RNase protection assay, and apoB-100 mRNA by apoB mRNA editing assay. Hepatic apoB mRNA increased by 30% in 'non-responders,' but decreased by 20% in the 'responders.' However, apoB-100 mRNA increased relative to apoB-48 mRNA in all the 4 strains by 50%. The mRNA for RNA editing protein (REPR) decreased in all strains, suggesting that apoB-100 mRNA increased as a result of decreased apoB mRNA editing activity. These results suggest that:(a) modulation of apoB mRNA by estrogen was strain-specific;(b) increased apoB100 mRNA in inbred strains of mice were caused by decreased apoB mRNA editing activity; and (c) the differences in the plasma apoB levels among 'responder' and 'nonresponder' strains of mice occur through mechanisms other than the apoB mRNA editing.

Regulation of low density lipoprotein receptor gene expression in HepG2 and Caco2 cells by palmitate, oleate, and 25-hydroxycholesterol

Our in vivo studies in mice have shown that LDL-receptor gene expression is regulated differently in both liver and intestine by dietary cholesterol and dietary saturated fat. While dietary cholesterol serves to regulate at transcriptional levels, dietary fatty acids do not. To study the mechanism of regulation of LDL-receptor by saturated fat and cholesterol at the cellular level, where any secondary effects of long-term feeding in vivo are minimized we used the cultured hepatoma and colon carcinoma cells, HepG2 and Caco2. LDL-receptor activity was determined by 125I-labeled LDL binding and uptake, LDL-receptor protein by Western blotting, LDL-receptor mRNA by RNase protection assay, and relative rates of LDL-receptor mRNA transcription by nuclear 'run-off' assay. Incubation of cells in lipoprotein-deficient serum (LPDS) for 48 h progressively induced LDL-receptor activity and LDL-receptor protein by 5- to 6-fold in HepG2 cells and 2- to 3-fold in Caco2 cells. Absolute levels of LDL-receptor mRNA and relative rates of LDL-receptor mRNA transcription also increased in parallel to the LDL-receptor activity and protein levels in both cell lines. These data suggest that LPDS induced the LDL-receptor gene by transcriptional mechanism. The suppressive effect of 25-hydroxycholesterol on LDL-receptor regulation was studied by incubating HepG2 and Caco2 cells grown either in 10% FCS or 10% LPDS for 24 h and then for 0-24 h with various doses of 25-hydroxycholesterol. In HepG2 cells, LDL-receptor activity and protein mass progressively decreased to 50% of zero time controls over 24 h. LDL-receptor mRNA levels and relative rates of transcription decreased in parallel. In Caco2 cells, 25-hydrocholesterol lowered LDL-receptor activity, mRNA, and transcription by approximately 35%. To examine the effects of palmitate on LDL-receptor regulation, palmitate was complexed with albumin. Palmitate decreased LDL-receptor activity by 25% in HepG2 cells without altering LDL-receptor mass, mRNA levels, or rates of mRNA transcription. Similarly, in Caco2 cells, palmitate decreased LDL-receptor activity and protein mass 30% of controls, but did not change LDL-receptor mRNA levels and/or rates of transcription. The combination of palmitate (0.8 mM) and 25-hydroxycholesterol (2.5-5 micrograms/ml) suppressed LDL-receptor activity by 65% in HepG2 cells and by 52% in Caco2 cells. However, LDL-receptor mRNA decreased by approximately 50% in HepG2 cells and 30-40% in Caco2 cells.(ABSTRACT TRUNCATED AT 400 WORDS)

Saturated fatty acid, but not cholesterol, regulates apolipoprotein AI gene expression by posttranscriptional mechanism

The aim of the present study was to investigate the regulation of the apoAI gene by dietary saturated fat and cholesterol. Saturated fatty acids and cholesterol raise low density- and high density lipoprotein particles in humans. Increased LDL is attributed to the down-regulation of LDL-receptor gene, but the mechanism of increased plasma HDL levels is unknown. To study the mechanism of HDL elevation by saturated fat, male rats and male mice were employed as animal models, since they also raise their plasma HDL levels when fed high lipid diets. Animals were divided in four groups and fed the following diets: control (5% corn oil); high cholesterol (0.5%); high fat (20% coconut oil); and high fat plus cholesterol diets. The high cholesterol diet did not alter plasma and HDL-cholesterol levels. However, the high fat diet increased HDL levels by 20% in rats and 55% in mice. A combination of saturated fat and cholesterol diet raised plasma HDL levels by 36 and 67% in rats and mice, respectively. Plasma apoAI levels increased parallel to HDL concentrations. Mechanism of HDL elevation by saturated fat was investigated. Hepatic and intestinal apoAI mRNA did not change with any of the test diets in mice. Rat hepatic apoAI mRNA was also unchanged by the high cholesterol diet, but was decreased on high fat and fat-cholesterol combination diets. These results suggest that transcriptional regulation of the apoAI gene was not responsible for increased plasma apoAI and HDL. The translational efficiency of apoAI on isolated polysomes was also measured, and it was found that apoAI synthesis increased about 20% on high fat and fat-cholesterol combination diets. This partially explains the elevated levels of plasma HDL. Additional regulation through impaired catabolism of HDL particles by high fat diet feeding may be another pathway for increased HDL levels. Unlike apoAI mRNA, the mRNA of other HDL apoproteins, apoAII and apoAIV, were increased by high fat and combination diet feeding. These results suggest that saturated fatty acids regulate plasma HDL levels by translational and posttranslational mechanisms.

Regulation of the apolipoprotein AIV gene expression by estrogen differs in rat and mouse

Previously we have shown that estrogen administration to Sprague Dawley rats and to the inbred C3H/HeJ mouse strain produced different effects on plasma lipoproteins [Srivastava, R. A. K., Baumann, D. & Schonfeld, G. (1993) Eur. J. Biochem. 216, 527-538]. While low-density lipoprotein (LDL) levels fell in rats, they rose in mice. Plasma apoprotein (apo) AI levels and high-density lipoprotein (HDL) cholesterol fell in both species but by much less in mice than in rats. Since apolipoproteins AIV and AII are two other protein constituents of HDL, we wished to test the hypothesis that estrogen would produce different effects on these apoproteins in mice and rats. Male rats and C3H/HeJ mice were administered 17 beta-estradiol at 5 micrograms.g body mass-1.day-1 for six consecutive days. In a separate experiment, castrated male C3H/HeJ mice were administered beta-estradiol [(0.16 micrograms.g body mass-1.day-1 or 5.0 micrograms.g body mass-1.day-1, or testosterone (1 microgram/g)] for 14 days. ApoAIV mRNA levels were determined in total liver, in liver nuclei and in total intestine. Rat hepatic apoAIV mRNA decreased twofold (from 16.5 +/- 3 pg/micrograms total RNA to 7.1 +/- 2.5 pg/micrograms total RNA) while mouse hepatic and nuclear apoAIV mRNA both increased 1.5-2-fold. Intestinal apoAIV mRNA decreased in mice and increased in rats. Testosterone had no effects. Nuclear apoAIV mRNA transcription rates in rat and mouse liver changed little, if at all, indicating that estrogen-induced changes in steady-state levels of apoAIV mRNA were not determined by hepatic transcriptional mechanisms. Both species possessed similar apoAIV mRNA transcription start sites. To assess whether other mouse strains also differed from rats, we surveyed 13 other inbred mouse strains. Some strains increased hepatic apoAIV mRNA, some did not change but, in contrast to rat, no strain experienced a fall in mRNA levels. Estrogen-induced changes in plasma apoAIV levels were not correlated with changes in the levels of hepatic apoAIV mRNA levels. These data indicate that (a) apoAIV mRNA levels are regulated differently by estrogen in mouse and rat livers and intestines, (b) regulation of apoAIV mRNA by estrogen is both mouse strain and tissue specific and (c) regulation of plasma apoAIV is achieved by mechanisms other than those depending on the steady-state levels of hepatic apoAIV mRNA. In contrast with apoAIV mRNA, estrogen decreased hepatic apoAII mRNA both in rat (threefold) and in mouse (twofold) and parallel changes were observed in transcription rates.(ABSTRACT TRUNCATED AT 400 WORDS)

In vivo regulation of low-density lipoprotein receptors by estrogen differs at the post-transcriptional level in rat and mouse

Rats and mice are frequently used in studies of the regulation of lipoprotein metabolism. Although the species are closely related, they differ dramatically in the responses of their lipoproteins to estrogen administration. In rats, estrogens produce profound decreases in the levels of all plasma lipoproteins and this is attributed largely to estrogen-induced increases of hepatic low-density lipoprotein receptor (LDL-receptor) activity. Estrogens affect mouse plasma lipoproteins to a much lesser extent. Therefore, one of our aims was to compare the regulation of LDL-receptor gene expression in rats and mice at several potential loci of regulation. To assess the specificity of the estrogen effect, we also compared the responses of apolipoprotein AI (apoAI), apolipoprotein B (apoB), and beta-actin to the response of the LDL-receptor. In male Sprague Dawley rats given 17 beta-estradiol or 17 alpha-ethinyl estradiol at supraphysiological doses of 5 micrograms/g body mass/day, plasma total cholesterol and triacylglycerols fell to approximately 5% and approximately 50%, and, plasma apoAI and apoB fell to approximately 12% and approximately 16% of controls, respectively. By contrast, in male C3H/HeJ mice the above parameters dropped only to approximately 65% of controls and apoB concentrations rose to approximately 200% of controls. In rats, relative rates of LDL-receptor mRNA transcription (nuclear 'run-off' assay) and total hepatic, nuclear and polysomal LDL-receptor mRNA levels (RNase protection assay) increased by 1.5-2-fold, while synthesis of LDL-receptor protein on hepatic polysomes (in a wheat-germ translation system) increased 8-fold and LDL-receptor protein mass in hepatic plasma membranes increased 10-fold (by immunoblotting). In mouse liver, too, LDL-receptor mRNA levels increased 1.5-fold and the LDL-receptor mRNA transcription start sites in rat and mouse were found to be the same, but mouse LDL-receptor protein mass did not change, i.e. LDL-receptors of mice were similar to rat with respect to transcriptional regulation, but differed in their post-transcriptional control mechanisms. In rats, estrogen administration increased apoAI mRNA transcription rates 1.6-fold and also apoAI mRNA levels in total liver homogenates, nuclei and polysomes, (2-fold for each) consistent with transcriptional regulation. However, apoAI synthesis on total RNA increased less than apoAI mRNA, indicating that apoAI translational control mechanisms, at least in part, also regulate hepatic rates of apoAI production. ApoB mRNA transcription rates and levels showed small increases following estrogen administration. Hepatic beta-actin mRNA transcription and levels did not change.(ABSTRACT TRUNCATED AT 400 WORDS)

Hormonal and nutritional stimuli modulate apolipoprotein B mRNA editing in mouse liver

Human livers produce apoB-100, a major protein of VLDL, while intestines produce apoB-48, the major protein of chylomicrons. ApoB-48 is translated from apoB-100 mRNAs that are post-transcriptionally edited at codon 2153, converting CAA (glutamine) to TAA, a stop codon. In contrast to humans, mouse and rat livers contain the apoB-100 mRNA editing mechanism. Because hormones and nutrients affect the metabolism of apoB containing lipoproteins, we studied the effects of sex hormones and diets on apoB mRNA editing. Groups of male and female C3H/HeJ mice were castrated and treated with 17 beta-estradiol at 0.16 (E2L) or at 5 micrograms (E2H), or with testosterone propionate at 1 microgram/g body weight/day for 14 days. Plasma apoB levels and ratios of apoB-100/apoB-48 both increased 2-fold, but only in the E2H group. To determine if the increased apoB-100/apoB-48 ratios were associated with altered levels of apoB-100 and apoB-48 mRNA, both forms of apoB mRNA were quantified. We found that indeed ApoB-100 mRNA increased 1.8-fold (p < 0.025) compared to apoB-48 mRNA only in the E2H group. Next, we studied the individual effects of dietary fatty acids and dietary cholesterol on the relative abundance of apoB-100 and apoB-48 mRNA. Contrary to the estrogen effect, the high fat-combination diet increased apoB-48 mRNA relative to apoB-100 mRNA. Total plasma apoB as well as apoB-48 synthesis in liver also increased. Our studies demonstrate that estrogens and high fat diet both modulate apoB editing in mouse liver, but that estrogens and fat diet affected apoB mRNA editing in opposite directions.

Expression of low density lipoprotein receptor, apolipoprotein AI, AII and AIV in various rat organs utilizing an efficient and rapid method for RNA isolation

Intact RNA from various rat organs was isolated by an efficient and rapid method. This method of RNA isolation is a modification of an earlier method that uses guanidinium isothiocynate followed by extraction in the presence of sarcosyl, acetate and phenol. The RNA obtained by the method reported here was comparable with the RNA prepared by the CsCl2 ultracentrifugation method and the commercially available kit based on published methods. The quality of RNA was found suitable for Northern blotting analysis, RNase protection assays and reverse transcriptase-polymerase chain reaction (RT-PCR). Since reverse transcriptase is active in the buffer used for Taq DNA polymerase, only one reaction needs to be set up. We also found that the use of aurintricarboxylic acid in the RNA preparation prevents the degradation of RNA during storage. Expression of low density lipoprotein (LDL) receptor, apolipoprotein (apo) AI, AII and AIV mRNAs were quantified in various rat organs. Our results indicated that rat LDL receptor mRNA is expressed in several organs whereas apoAI and AIV mRNAs were expressed mainly in the liver and intestine. However, apo AII mRNA is expressed mainly in the liver. Unlike mice and some species of monkeys, in the rat apoAI mRNA is expressed at 5-6 times higher levels in the intestine compared to liver. Apo AIV mRNA abundance was also found to be several fold higher in intestine compared to hepatic tissues. We present here, for the first time, data on the absolute amounts of LDL receptor, apoAI, AII and AIV mRNA in various rat organs which were quantified by a novel RNase protection/solution hybridization assay.

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.

Characterization of the RNA processing enzyme RNase III from wild type and overexpressing Escherichia coli cells in processing natural RNA substrates

1. A precursor to small stable RNA, 10Sa RNA, accumulates in large amounts in a temperature sensitive RNase E mutant at non-permissive temperatures, and somewhat in an rnc (RNase III-) mutant, but not in an RNase P- mutant (rnp) or wild type E. coli cells. 2. Since p10Sa RNA was not processed by purified RNase E and III in customary assay conditions, we purified p10Sa RNA processing activity about 700-fold from wild type E. coli cells. 3. Processing of p10Sa RNA by this enzyme shows an absolute requirement for a divalent cation with a strong preference for Mn2+ over Mg2+. Other divalent cations could not replace Mn2+. 4. Monovalent cations (NH+4, Na+, K+) at a concentration of 20 mM stimulated the processing of p10Sa RNA and a temperature of 37 degrees C and pH range of 6.8-8.2 were found to be optimal. 5. The enzyme retained half of its p10Sa RNA processing activity after 30 min incubation at 50 degrees C. 6. Further characterization of this activity indicated that it is RNase III. 7. To further confirm that the p10Sa RNA processing activity is RNase III, we overexpressed the RNase III gene in an E. coli cells that lacks RNase III activity (rnc mutant) and RNase III was purified using one affinity column, agarose.poly(I).poly(C). 8. This RNase III preparation processed p10Sa RNA in a similar way as observed using the p10Sa RNA processing activity purified from wild type E. coli cells, confirming that the first step of p10Sa RNA processing is carried out by RNase III.

Use of riboprobes for northern blotting analysis

We compared cDNA and riboprobes for their use in Northern blotting analysis and found that higher hybridization signal can be obtained using riboprobes. Comparison of five nylon membranes for Northern blotting using riboprobe revealed that each nylon membrane differed in their performance. Background noise was minimal for all five membranes for two riboprobes. However, because of the higher thermal stability of RNA.RNA hybrids, the riboprobes could not be stripped out from the membrane even after five hours of treatment at 80 degrees C.

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.

RNA processing enzymes RNase III, E and P in Escherichia coli are not ribosomal enzymes

We recently showed that RNase III can process a small stable RNA, precursor 10Sa RNA, that accumulates in an rne (RNase E) strain at non-permissive temperatures. Precursor 10Sa (p10Sa) RNA is processed to 10Sa RNA in two steps, the first step is catalyzed by RNase III in the presence of Mn2+ but not Mg2+. It was shown that RNase III cosediments with membrane preparation from wild type as well as RNase III overexpressing cells. However, the possibility of membrane preparation contamination with ribosomes could not be ruled out. Here we show that RNase III, E and P are not associated with ribosomes. E. coli cells were opened either by alumina grinding or by sonication and fractionated into cytosolic and pellet fractions. The characterization of membrane preparations was done by assaying NADH oxidase, a bona fide membrane enzyme. Ribosomes prepared by alumina grinding were found to be contaminated with small fragments of membrane which contained RNase III activity. RNase III and NADH oxidase activities were present in the ribosomal preparations which could be solubilized by reagents that dissolve the inner membrane. Isopycnic sucrose gradient centrifugation of the membrane and ribosomal preparations also confirmed that RNase III fractionated with the inner membrane. Similarly RNase P activity was found in the corresponding fractions when isopycnic centrifugation of membrane and ribosome preparations was carried out. RNase E activity was also found to be present mostly in the post-ribosomal supernatant. These findings show that RNase III, E and P are not ribosomal enzymes.

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.

Location of the RNA-processing enzymes RNase III, RNase E and RNase P in the Escherichia coli cell

Cells overexpressing the RNA-processing enzymes RNase III, RNase E and RNase P were fractionated into membrane and cytoplasm. The RNA-processing enzymes were associated with the membrane fraction. The membrane was further separated to inner and outer membrane and the three RNA-processing enzymes were found in the inner membrane fraction. By assaying for these enzymatic activities we showed that even in a normal wild-type strain of Escherichia coli these enzymes fractionate primarily with the membrane. The RNA part of RNase P is found in the cytosolic fraction of cells overexpressing this RNA, while the overexpressed RNase P protein sediments with the membrane fraction; this suggests that the RNase P protein anchors the RNA catalytic moiety of the enzyme to a larger entity. The implications of these findings for the cellular organization of the RNA-processing enzymes in the cell are discussed.

Effect of glycosylation of bacterial amylase on stability and active site conformation

With a view to understand the changes in the conformation of bacterial amylase, the enzyme preparation was conjugated to dextran. Glycosylation of purified bacterial amylase resulted in increased stability against heat, proteolytic enzymes and denaturing agents. Several group specific inhibitors exhibited dose-dependent inhibition and the extent of inhibition was same for native as well as for the glycosylated enzyme. The pH optima of native and glycosylated enzyme remained the same indicating that the ionization at the active site is not greatly influenced as a result of glycosylation. Although the native as well as the glycosylated enzyme bind to the substrate with the same affinity, the rate of reaction differed greatly at 90 and 100 degrees C. At 70 degrees C, the rate of reaction was similar for the conjugated as well as the unconjugated amylase. Thermostability at different temperatures clearly showed that the glycosylated enzyme had greater stability compared to the native enzyme. The divalent cation binding site in the amylase also appears to be unaltered upon glycosylation since EDTA inhibited both enzymes to the same extent and addition of calcium ion restored the activity to almost the same level. These studies showed that conjugating the amylase enzyme with a bulky molecule like dextran does not affect the conformation at the active site.

A rapid and simple method for screening large numbers of recombinant DNA clones

A simple and rapid method has been described for the isolation of plasmid, phagemid and phage DNAs. Hundreds of recombinant clones can be screened in one day employing this method. It takes half an hour to prepare plasmid DNA from ten clones, and the DNA prepared from a single colony using this method is of sufficient quality and in sufficient amount to perform at least five restriction digestions. This method eliminates the need for RNase treatment and phenol chloroform extraction if the plasmids are needed only for the restriction digestion. If needed, RNAs can be removed after restriction digestion by adding RNase and incubating for two minutes at room temperature. After RNase treatment and phenol/chloroform extraction, the plasmid DNA serves as a good template for sequencing. The DNA can be stored at -20 degrees C for over eight weeks.

Culture Conditions for Production of Thermostable Amylase by Bacillus stearothermophilus

Bacillus stearothermophilus grew better on complex and semisynthetic medium than on synthetic medium supplemented with amino acids. Amylase production on the complex medium containing beef extract or corn steep liquor was higher than on semisynthetic medium containing peptone (0.4%). The synthetic medium, however, did not provide a good yield of extracellular amylase. Among the carbohydrates which favored the production of amylase are, in order starch > dextrin > glycogen > cellobiose > maltohexaose-maltopeptaose > maltotetraose and maltotriose. The monosaccharides repressed the enzyme production, whereas inositol and d -sorbitol favored amylase production. Organic and inorganic salts increased amylase production in the order of KCI > sodium malate > potassium succinate, while the yield was comparatively lower with other organic salts of Na and K. Amino acids, in particular isoleucine, cysteine, phenylalanine, and aspartic acids, were found to be vital for amylase synthesis. Medium containing CaCl 2 2H 2 O enhanced amylase production over that on Ca 2+ -deficient medium. The detergents Tween-80 and Triton X-100 increased biomass but significantly suppressed amylase synthesis. The amylase powder obtained from the culture filtrate by prechilled acetone treatment was stable over a wide pH range and liquefied thick starch slurries at 80°C. The crude amylase, after (NH 4 ) 2 SO 4 fractionation, had an activity of 210.6 U mg −1 . The optimum temperature and pH of the enzyme were found to be 82°C and 6.9, respectively. Ca 2+ was required for the thermostability of the enzyme preparation.