Regulation of hepatic apolipoprotein synthesis in the 17 alpha-ethinyl estradiol-treated rat

Mouse models as tools to explore cytidine-to-uridine RNA editing

RNA editing is a process through which the nucleotide sequence specified in the genomic template is modified to produce a different nucleotide sequence in the transcript. RNA editing is an important mechanism of genetic regulation that amplifies genetic plasticity by allowing the production of alternative protein products from a single gene. There are two generic classes of RNA editing in nuclei, involving enzymatic deamination of either C-to-U or A-to-I nucleotides. The best characterized example of C-to-U RNA editing is that of apolipoprotein B (apoB), which is mediated by a holoenzyme that contains a minimal core composed of an RNA-specific cytidine deaminase apobec-1, and its cofactor apobec-1 complementation factor (ACF). C-to-U editing of apoB RNA generates two different isoforms--apoB100 and apoB48--from a single transcript. Both are important regulators of lipid transport and metabolism, and are functionally distinct. C-to-U apoB RNA editing is regulated by a range of factors including developmental, nutritional, environmental, and metabolic stimuli. Rodent models have provided a tractable system in which to study the effects of such stimuli on lipid metabolism. In addition, both transgenic and gene knockout experiments have provided important insights into gain and loss of function approaches for studying C-to-U RNA editing in a murine background.

Inhibition of the synthesis of apolipoprotein B-containing lipoproteins

Increased serum concentrations of low density lipoproteins represent a major cardiovascular risk factor. Low-density lipoproteins are derived from very low density lipoproteins secreted by the liver. Apolipoprotein (apo)B that constitutes the essential structural protein of these lipoproteins exists in two forms, the full length form apoB-100 and the carboxy-terminal truncated apoB-48. The generation of apoB-48 is due to editing of the apoB mRNA which generates a premature stop translation codon. The editing of apoB mRNA is an important regulatory event because apoB-48-containing lipoproteins cannot be converted into the atherogenic low density lipoproteins. The apoB gene is constitutively expressed in liver and intestine, and the rate of apoB secretion is regulated post-transcriptionally. The translocation of apoB into the endoplasmic reticulum is complicated by the hydrophobicity of the nascent polypeptide. The assembly and secretion of apoB-containing lipoproteins within the endoplasmic reticulum is strictly dependent on the microsomal tricylceride transfer protein which shuttles triglycerides onto the nascent lipoprotein particle. The overall synthesis of apoB lipoproteins is regulated by proteosomal and nonproteosomal degradation and is dependent on triglyceride availability. Noninsulin dependent diabetes mellitus, obesity and the metabolic syndrome are characterized by an increased hepatic synthesis of apoB-containing lipoproteins. Interventions aimed to reduce the hepatic secretion of apoB-containing lipoproteins are therefore of great clinical importance. Lead targets in these pathways are discussed.

HDL and clinical and biochemical correlates in Italian non-smoker women

High-density lipoprotein (HDL)-cholesterol levels, inversely related to the risk of myocardial infarction, are determined by genetic and environmental factors. The aim of this study was to evaluate the prevalence of low and high HDL plasma levels and the influence of environmental factors and lipid profile in an Italian non-smoker female population. HDL, apolipoprotein A-I, apolipoproteins, lipids and estrogen plasma levels were measured in a population of 1471 women with a mean age of 45±14years. HDL values ≤35mg/dl were noted in 11.2% of the subjects, showing 2.4% coronary heart disease (CHD) prevalence. The 90th percentile was characterized by HDL levels ≥66mg/dl and the absence of coronary atherosclerosis. Total cholesterol, apolipoprotein B and triglycerides (r=−0.31, p<0.0001) were the main determinants of HDL levels; apolipoprotein E, estrogen use, body mass index (BMI), alcohol consumption and age showed a weaker correlation. Apolipoprotein A-I concentration was influenced more notably by estrogen use, total cholesterol and apolipoprotein E; levels of triglycerides, apolipoprotein B, BMI, age and alcohol consumption are less important. The parameters considered here, taken together, explain HDL and apolipoprotein A-I variability of approximately 31% and 24%, respectively. A surprisingly high prevalence of very low (≤35mg/dl) and high (≥66mg/dl) HDL levels in Italian women further confirms the importance of studies on the HDL distribution in different population groups.

Developmental regulation of apolipoprotein B mRNA editing is an autonomous function of small intestine involving homeobox gene Cdx1

Apolipoprotein B mRNA editing is developmentally regulated in the human and rodent small intestine, changing from <1% at day 14 to approximately 90% by day 20 in the rat fetus. This regulation is coincident with the developmental formation of the crypt-to-villus axis functional unit, a continuous and rapidly renewing system involving cell generation, migration, and differentiation. Utilizing small intestine isografts implanted into the subcutaneous tissue of adult recipients, apolipoprotein B mRNA editing was developmentally up-regulated, parallel to that seen with an intact control. In contrast, apoB mRNA expression remains nearly constant in the isograft, unlike the normal intact small intestine. Immunohistochemical analyses demonstrated that apoB-48 protein existed predominantly in well differentiated enterocytes along the villus surface whereas apoB-100 was in the lamina propria and crypts. ApoB mRNA editing levels were very low in the crypt-like rat intestinal cell line, IEC-6 ( approximately 0.3%), but very high in well differentiated enterocytes ( approximately 91.5%). The expression of homeobox gene Cdx1 increased 18-fold in small intestine in vivo during the same time course when apoB mRNA editing increased from approximately 2 to approximately 90%. The overexpression of Cdx1 in IEC-6 cells increased apoB mRNA editing over 10-fold compared with the vector control. This increase was associated with a significant increase of activating factor ACF, a component of the apoB mRNA editing complex. Taken together, these data suggest that the developmental regulation of apoB mRNA editing is an autonomous cytodifferentiation function of small intestine for which homeobox gene Cdx1 may play an important role.

Effects of apolipoprotein A-I genetic variations on plasma apolipoprotein, serum lipoprotein and glucose levels

The present authors investigated the individual and combined associations of the apolipoprotein (apo) A-I -75 bp and +83 bp polymorphisms with plasma lipid, lipoprotein and apolipoprotein levels in 734 Caucasian men and women. The frequency of the A allele at position -75 bp (G-->A) was 0.14 in women and 0.17 in men. The frequencies for the rare M2 allele at position +83 bp and/or 84 bp (C-->T and G-->A, respectively) were 0.04 and 0.05 in women and men, respectively. In women, the A allele was associated with significantly higher levels of apo B (P = 0.016), total cholesterol (TC) (P = 0.005), low-density lipoprotein cholesterol (LDL-C) (P = 0.018) and TC:high-density lipoprotein (HDL) ratio (P = 0.026) compared to the G/G subjects. In men, no significant associations were detected between the -75 bp polymorphism and any lipid trait examined. The M2 allele for the +83 bp polymorphism was significantly associated in men with higher levels of apo A-I (P = 0.002) and TC (P = 0.046). In women, a significant effect was observed for TC (P = 0.036), with M2+/- subjects having lower levels than M2+/+ subjects. Significant linkage disequilibrium (P = 0.037) between the apo A-I -75 bp and +83 bp polymorphisms was detected. Women carrying both rare alleles (G/A M2+/-) had significantly higher TC:HDL ratios (P = 0.031) compared to the other haplotypes. In men, significant differences were observed for apo A-I (P = 0.021) and TC (P = 0.044), with carriers of the G/G M2+/- haplotype having the highest values compared to other genotype combinations. In conclusion, the -75 bp (G/A) polymorphism appears to have a significant effect on levels of apo B, plasma TC and LDL-C in women, while the +83 bp polymorphism seems to affect the apo A-I levels in men, and the plasma cholesterol levels in both genders.

Calcium increases apolipoprotein B mRNA editing

ApoB-100 and apoB-48 are major components of chylomicrons, very low-density lipoprotein (VLDL) and low-density lipoprotein (LDL). The two proteins are generated from a single apoB mRNA by apoB mRNA editing which induces an in-frame stop codon in apoB mRNA. Apolipoprotein B (apoB) mRNA editing is an important determinant of the proportion of full-length (apoB-100) and truncated (apoB-48) proteins in total apoB metabolism. Calcium is involved in the regulation of secretion and synthesis of VLDL and apoB. In this paper, we demonstrate for the first time that the amount of edited apoB mRNA in the cultured cells Caco-2 and McA7777 is markedly increased by calcium. Increasing extracellular calcium concentration, calcium ionophore (A23187 and ionomycin) treatment, and depleting calcium stores and raising cytoplasmic calcium concentration by thapsigargin increase apoB mRNA editing up to threefold in a dose dependent manner. Calcium has no direct stimulative effect on apoB mRNA editing in an in vitro editing system. The editing increase by extracellular calcium is not related to alterations of APOBEC-1 mRNA expression. These data suggest that calcium is not only involved in the regulation of apolipoprotein metabolism but also apoB mRNA editing.

Apolipoprotein B mRNA editing and apolipoprotein gene expression in the liver of hyperinsulinemic fatty Zucker rats: relationship to very low density lipoprotein composition

We previously demonstrated increased apolipoprotein B (apoB) mRNA editing, elevated levels of mRNA for the catalytic component of the apoB mRNA editing complex, apobec-1, and increased secretion of the product of the edited mRNA, apoB48, in very low density lipoproteins (VLDL) in primary cultures of Sprague-Dawley rat hepatocytes following insulin treatment. In order to determine the effect of in vivo hyperinsulinemia on these processes, we determined apoB mRNA editing, apobec-1 expression, hepatic expression of mRNA for apoB and other VLDL apoproteins, and the quantity and composition of plasma VLDL in the hyperinsulinemic fatty Zucker rat. Total apoB mRNA content of the livers of the fatty rats and lean littermates did not differ; however, edited apoB message coding for hepatic apo B48, and abundance of mRNA for the catalytic subunit of the apoB mRNA editing complex, apobec-1, was increased by 1.7- and 3.3-fold, respectively, in fatty rats. ApoCIII mRNA abundance was increased in livers of fatty rats as well, but the abundance of hepatic apoE mRNA in the fatty animal was not different from that of the lean rat. Hepatic apoAI mRNA abundance was also increased in the fatty rats. Associated with increased apoB mRNA editing, was the 1.7-fold increase in the fraction of apoB in plasma as apoB48 in fatty rats. VLDL-triglyceride and -apoB in plasma were 15- and 3-fold higher, respectively, in fatty Zucker rats compared to lean littermates, indicating both enrichment of VLDL with triglycerides and increased accumulation of VLDL particles. Increased hepatic expression of mRNA for apoCIII and apoAI was associated with increased content of apoC (and relative depletion of apoE) in VLDL of fatty rats, and plasma apoAI was increased in fatty Zucker rats, primarily in the HDL fraction. The current study provides further evidence that chronic exposure to high levels of insulin influences both the quantity of and lipid/apoprotein composition of VLDL in plasma. The increased apoC and decreased apoE (as well as increased triglyceride) content of VLDL in the fatty Zucker rat observed in the current study may affect VLDL clearance and therefore may be a factor in the observed accumulation of VLDL in the plasma of the fatty hyperinsulinemic Zucker rats.

Estrogen receptor null mice: what have we learned and where will they lead us?

All scientific investigations begin with distinct objectives: first is the hypothesis upon which studies are undertaken to disprove, and second is the overall aim of obtaining further information, from which future and more precise hypotheses may be drawn. Studies focusing on the generation and use of gene-targeted animal models also apply these goals and may be loosely categorized into sequential phases that become apparent as the use of the model progresses. Initial studies of knockout models often focus on the plausibility of the model based on prior knowledge and whether the generation of an animal lacking the particular gene will prove lethal or not. Upon the successful generation of a knockout, confirmatory studies are undertaken to corroborate previously established hypotheses of the function of the disrupted gene product. As these studies continue, observations of unpredicted phenotypes or, more likely, the lack of a phenotype that was expected based on models put forth from past investigations are noted. Often the surprising phenotype is due to the loss of a gene product that is downstream from the functions of the disrupted gene, whereas the lack of an expected phenotype may be due to compensatory roles filled by alternate mechanisms. As the descriptive studies of the knockout continue, use of the model is often shifted to the role as a unique research reagent, to be used in studies that 1) were not previously possible in a wild-type model; 2) aimed at finding related proteins or pathways whose existence or functions were previously masked; or 3) the subsequent effects of the gene disruption on related physiological and biochemical systems. The alpha ERKO mice continue to satisfy the confirmatory role of a knockout quite well. As summarized in Table 4, the phenotypes observed in the alpha ERKO due to estrogen insensitivity have definitively illustrated several roles that were previously believed to be dependent on functional ER alpha, including 1) the proliferative and differentiative actions critical to the function of the adult female reproductive tract and mammary gland; 2) as an obligatory component in growth factor signaling in the uterus and mammary gland; 3) as the principal steroid involved in negative regulation of gonadotropin gene transcription and LH levels in the hypothalamic-pituitary axis; 4) as a positive regulator of PR expression in several tissues; 5) in the positive regulation of PRL synthesis and secretion from the pituitary; 6) as a promotional factor in oncogene-induced mammary neoplasia; and 7) as a crucial component in the differentiation and activation of several behaviors in both the female and male. The list of unpredictable phenotypes in the alpha ERKO must begin with the observation that generation of an animal lacking a functional ER alpha gene was successful and produced animals of both sexes that exhibit a life span comparable to wild-type. The successful generation of beta ERKO mice suggests that this receptor is also not essential to survival and was most likely not a compensatory factor in the survival of the alpha ERKO. In support of this is our recent successful generation of double knockout, or alpha beta ERKO mice of both sexes. The precise defects in certain components of male reproduction, including the production of abnormal sperm and the loss of intromission and ejaculatory responses that were observed in the alpha ERKO, were quite surprising. In turn, certain estrogen pathways in the alpha ERKO female appear intact or unaffected, such as the ability of the uterus to successfully exhibit a progesterone-induced decidualization response, and the possible maintenance of an LH surge system in the hypothalamus. [ABSTRACT TRUNCATED]

Insulin increases expression of apobec-1, the catalytic subunit of the apolipoprotein B mRNA editing complex in rat hepatocytes

We have previously shown that chronic insulin treatment of rat hepatocytes increases the fraction of edited apolipoprotein B (apoB) mRNA from approximately 50% to as much as 90%. We have now examined the effect of insulin on apobec-1 mRNA abundance and demonstrate that increased editing of apoB mRNA following insulin treatment is accompanied by elevated apobec-1 mRNA levels in primary rat hepatocytes. Time-course measurements of the effects of insulin on apoB mRNA editing and apobec-1 mRNA abundance showed that both were elevated almost maximally within 48 hours and sustained for at least 5 days of insulin treatment.

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.

Opposite effects of bezafibrate and gemfibrozil in both normal and hypertriglyceridemic rats

Chow and sucrose-fed rats were used as animal models to study the dose-responses of bezafibrate and gemfibrozil in normolipidemic and hypertriglyceridemic states, respectively. Although both drugs lowered plasma triglycerides (TG) to about the same extent in chow-fed rats, gemfibrozil lowered liver TG as well as plasma total and LDL-cholesterol (LDL-C), but elevated HDL-cholesterol (HDL-C) and plasma apo E concentrations. Bezafibrate produced opposite effects, namely, decreased HDL-C, apo E and liver TG, and tended to increase LDL-C. TG lowering for both drugs in chow-fed rats was not due to changes in TG secretion (production) in normal rats but was associated with enhanced LPL activity. In hypertriglyceridemic rats both drugs modestly reduced TG secretion rates about 40% at a dose producing maximal TG lowering, but again, gemfibrozil elevated and bezafibrate lowered HDL-C and apo E. Unlike gemfibrozil, bezafibrate induced the appearance of LDL-C in hypertriglyceridemic rats which was not detected in control animals, and also tended to increase rather than decrease plasma apo B levels. Finally, changes in liver TG concentration (mg/g) in hypertriglyceridemic rats were opposite for these drugs, resulting in significant drug-related differences in liver TG content (mg/organ). From these data we postulate that, although similar with regard to TG lowering activity and mechanisms thereof, gemfibrozil and bezafibrate produce fundamentally different effects on LDL, HDL and apolipoprotein metabolism (apo B and apo E) in rats which may relate to potential differential effects on reverse cholesterol transport and atherogenesis.

Dexamethasone enhanced by insulin, but not by thyroid hormones stimulates apolipoprotein B mRNA editing in cultured rat hepatocytes depending on the developmental stage

The increase of hepatic apolipoprotein B (apoB) mRNA editing during rat development was not affected by hypothyroidism. Furthermore, the addition of 3,3',5'-triiodothyronine (T3) to cultured hepatocytes taken from fetal, neonatal and adult rats had no effect on apoB mRNA editing. In contrast, dexamethasone markedly stimulated apoB mRNA editing in hepatocytes taken from neonates. This effect was enhanced by the addition of insulin. For the first time our data provide evidence that glucocorticoids together with insulin are important for the regulation of apoB mRNA editing during postnatal development, whereas thyroid hormones are not critical for this process.

Reduction of intestinal apo A-IV mRNA levels in the cirrhotic rat

In the present study, intestinal apo A-IV synthesis was investigated using a carbon tetrachloride (CCl4)-induced cirrhosis rat model. Triglyceride (TG) content in rat cirrhotic liver was increased markedly by 170% (P < 0.001) and apo B was increased by 20% (P < 0.05) compared with control levels. These results reflected the steatotic change in the liver. In contrast, TG levels in the small intestine of cirrhotic rats decreased significantly (P < 0.01). In addition, intestinal apo A-IV (jejunum P < 0.001; ileum P < 0.01) and its mRNA levels (jejunum P < 0.01; ileum P < 0.05) were also reduced. The decreased apo A-IV content in the jejunum was confirmed by immunohistochemical analysis. These results indicate that intestinal apo A-IV synthesis in cirrhosis is suppressed, at least under the condition of an overnight fast. Therefore, decreased intestinal apo A-IV synthesis may relate to the decreased ability to absorb fat in cirrhosis, but a fat-loading study will be necessary to confirm this hypothesis. It is unknown from the present study why serum apo A-IV level is not significantly decreased, despite a reduction in apo A-IV synthesis. The clearance of apo A-IV by the liver may be delayed or apo A-IV synthesis may be rather markedly enhanced during fat absorption in liver cirrhosis.

Control of apolipoprotein AI gene expression through synergistic interactions between hepatocyte nuclear factors 3 and 4

Apolipoprotein AI (apoAI) gene expression in liver depends on synergistic interactions between transcription factors bound to three distinct sites (A, B, and C) within a hepatocyte-specific enhancer in the 5'-flanking region of the gene. In this study, we showed that a segment spanning sites A and B retains substantial levels of enhancer activity in hepatoblastoma HepG2 cells and that sites A and B are occupied by the liver-enriched hepatocyte nuclear factors (HNFs) 4 and 3, respectively, in these cells. In non-hepatic CV-1 cells, HNF-4 and HNF-3beta activated this minimal enhancer synergistically. This synergy was dependent upon simultaneous binding of these factors to their cognate sites, but it was not due to cooperativity in DNA binding. Separation of these sites by varying helical turns of DNA did not affect simultaneous binding of HNF-3beta and HNF-4 nor did it influence their functional synergy. The synergy was, however, dependent upon the cell type used for functional analysis. In addition, this synergy was further potentiated by estrogen treatment of cells cotransfected with the estrogen receptor. These data indicate that a cell type-restricted intermediary factor jointly recruited by HNF-4 and HNF-3 participates in activation of the apoAI enhancer in liver cells and suggest that the activity of this factor is regulated by estrogen.

Cyclical variation of plasma lipids, apolipoproteins, and lipoprotein(a) during menstrual cycle of normal women

We report herein the effects of cyclical variations of endogenous sex steroids during the menstrual cycle on plasma lipids and apolipoproteins (apo) in normal women. We examined 16 normal women (age range 25-36 yr) with normal menstrual cycles of 28-31 days. The study covered the period from the 1st day of a menstrual phase (basal) until the 1st day of the following menstrual phase. During the study all women maintained a normolipidic diet (30% fat). Plasma total cholesterol and low-density-lipoprotein cholesterol were significantly higher than basal in the preovulatory phase until progesterone started to increase in the postovulatory phase [day +8 from luteinizing hormone (LH) surge]. High-density-lipoprotein cholesterol was significantly higher than basal from day -1 to the day after LH surge, whereas plasma apoAI levels were significantly higher from day -8 to day +8 (from LH surge). Plasma apo(a) increased significantly during the luteal phase in four women characterized by a single S4 band and lower basal plasma levels of apo(a). Our results indicate that endogenous female sex steroids have significant effects on the circulating levels of plasma lipids and apolipoproteins, including apo(a). More work needs to be done to elucidate the significance of the observed apo(a) changes, and the different phases of the menstrual cycle must be taken into account when evaluating the lipidic risk profile in premenopausal women.

Estrogen-induced alterations in lipoprotein metabolism in autoimmune MRL/lpr mice

Estrogen replacement therapy has been demonstrated to shift the lipoprotein profile toward a less atherogenic one with concomitant increases in HDL and reductions in LDL cholesterol and serum triglycerides. Estrogen, however, has also been implicated in playing a significant role in autoimmune disease and may be involved with disease incidence and progression. The MRL/lpr mouse strain represents an autoimmune disease model with features resembling systemic lupus erythematosus including high-titer autoantibodies, glomerulonephritis, and vasculitis. In the present study, the effects of estrogen treatment on serum lipoprotein profiles were investigated by fast protein liquid chromatography in female MRL/lpr mice, in the MRL/++ strain with a milder form of disease, and in control Balb/c mice. Treatment of MRL/lpr mice for periods of 1 week or longer with pharmacologic doses of estrogen resulted in a significant increase in the amount of cholesterol carried on LDL particles. The up to eightfold increase in LDL cholesterol was less significant in the MRL/++ or Balb/c mice. Maximal increases were observed at 1 to 2 mg/kg of estrogen agonists, and the effect on LDL cholesterol increases was inhibited by tamoxifen. The HDL-to-LDL shift in cholesterol observed in estrogen-treated autoimmune mice correlated with an increase in apolipoprotein E, primarily on larger HDL particles. In addition to the increase in LDL cholesterol, hormonal treatment also resulted in a shift in triglycerides from the VLDL to the LDL fraction in both normal and autoimmune mice. These results suggest that pharmacologic doses of estrogen may contribute to cardiovascular disease progression by shifting the relative distribution of cholesterol from HDL to LDL in this murine model of lupus.

Hepatic apolipoprotein and LDL receptor gene expression in the genetically hypercholesterolemic (RICO) rat

The present study was designed to examine apolipoprotein and LDL receptor gene expression in genetically hypercholesterolemic RICO rats. In the plasma of RICO rats as compared to SW (control) rats, the hypercholesterolemia (+41%) was associated with a significant increase in plasma apo B (+23%) and apo E (+68%) concentrations. Study of apolipoprotein synthesis in the liver has shown that this increase in plasma apo B and apo E concentrations was not associated with modification in their synthesis and mRNA levels. Study of apo E mRNA level in various tissues has shown only the modification in adrenals in RICO as compared to SW rats (2.7-fold increase). Study of LDL binding, LDL receptor mass and LDL receptor mRNA level in the liver of RICO and SW rats has shown no significant differences between these two strains. EDTA-resistant binding of rat LDL was lower in RICO than in SW rats suggesting that binding sites others than the LDL receptor are present in lesser amount in this hypercholesterolemic strain.

Magnesium deficiency modulates hepatic lipogenesis and apolipoprotein gene expression in the rat

The present study was performed to determine the effects of magnesium (Mg) deficiency upon plasma lipoproteins and hepatic apolipoprotein gene expression in the rat. The most obvious effect of Mg-deficiency on plasma lipids is a marked increase in post-prandial triacylglycerol concentration. This increased triglyceridemia persists in fasted rats. Density gradient ultracentrifugation analysis revealed marked alterations in the distribution of plasma lipoproteins in Mg-deficient rats. An increase in triacylglycerol-rich lipoproteins (TGRLP) was associated with a significant increase in plasma apolipoprotein B (apo B) concentration and was accompanied by selective accumulation of apo B-48. A decrease in high-density lipoproteins (HDL) was accompanied by a corresponding decrease in plasma apo E concentration and a concordant decrease in hepatic apo E mRNA abundance and biosynthesis. Hepatic apo B-100 synthesis was reduced by over 75% in Mg-deficient animals despite an increase in hepatic apo B mRNA abundance. However, this change in hepatic apo B gene expression was not associated with alterations in posttranscriptional apo B mRNA editing. These changes in apolipoprotein gene expression were associated with increased hepatic lipogenesis, despite the observation that net triacylglycerol secretion rates were not different between Mg-deficient and control animals. Taken together, the data demonstrate a complex pattern of alterations in hepatic lipid metabolism and apolipoprotein gene expression in the Mg-deficient rat and suggest a defect in the catabolism rather than secretion of TGLRP as the major factor underlying the altered plasma lipoprotein profile.

Apolipoprotein A-I, A-IV and E synthesis in the liver of copper-deficient rats

Copper deficiency induces hypercholesterolemia in the rat. This hypercholesterolemia is mainly due to an increase in apo E-rich high density lipoproteins (HDL1). The present study was undertaken to determine whether the HDL increase could be explained by altered low-molecular weight apolipoprotein (apo) synthesis in the liver. The effect of copper deficiency on apo A-I, apo A-IV and apo E concentrations in plasma, as well as on respective mRNA levels and synthesis in the liver, were therefore investigated. We observed that the increased HDL1 levels in the plasma of copper-deficient rats were associated with a significant rise in plasma apo E concentrations; however, plasma apo A-I and apo A-IV concentrations remained unchanged. Liver apo synthesis and respective apo mRNA levels were not significantly altered in copper-deficient animals when compared to control rats. No changes in apo E mRNA levels in various tissues from copper-deficient, as compared to control rats, were noted. Based on the data obtained, it was concluded that the observed changes in plasma lipoprotein and apo concentrations are not related to changes in low-molecular weight apo synthesis in the liver. The mechanisms of the impaired catabolism of HDL1 should be further evaluated to possibly explain the observed increase in this fraction in copper-deficient rats.

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)

Regulation of apo B mRNA expression in liver and intestine during liver regeneration induced by CCl4

Acute liver damage in CCl4-treated rats is accompanied by a decrease of the lipoprotein particles HDL and VLDL. However, once the regenerative process has occurred both lipoproteins increase to values higher than control. In this study we have further analyzed the molecular mechanism involved in the induction of the hyperlipidemia during liver regeneration in rats treated with CCl4. The expression of apolipoprotein (apo) B gene at the apo B mRNA editing level has been analyzed during the regenerative process of the liver. The percent of apo B-48 and apo B-100 mRNA was determined and compared to the plasma levels of LDL and VLDL. These results confirm the existence of two different hyperlipidemic stages during liver regeneration. Total apo B mRNA is increased in liver and intestine during both hyperlipidemic stages. The ratio of apo B-100/apo B-48 mRNA remains constant during the first hyperlipidemic stage but the serum levels of VLDL are decreased. Then once the regenerative process has occurred, the steady-state levels of total apo B mRNA remaining, increased with higher amounts of apo B-100 than apo B-48. At this experimental time point the high levels of total apo B mRNA correlate with serum levels of VLDL that are significantly higher than control. These results indicate that apo B gene expression is induced at the mRNA level in liver and intestine during liver regeneration affecting the apo B mRNA editing mechanism in a complex manner and suggest further regulation at the translational or post-translational level.

Effect of the interruption of enterohepatic circulation of bile acids by cholestyramine on apolipoprotein gene expression in the rat

The aim of the study was to assess how the interruption of enterohepatic circulation may affect the regulation of apolipoprotein synthesis. For this purpose, apolipoprotein mRNA levels were measured in the liver and small intestine of male Wistar rats fed for 3 weeks diets with or without cholestyramine (5% w/w). In order to modulate intestinal lipid flux, we used diets with 2 or 10% lipid content. Cholestyramine treatment decreased plasma triglyceride and cholesterol concentrations (by 63% and 32%, respectively) in rats fed 10% corn oil containing diet but had no significant effect on plasma lipids in rats fed a low-fat diet. Plasma apo B and apo A-IV concentrations were lowered by cholestyramine treatment in both 10 and 2% oil fed groups; however plasma apo E concentration was only affected in rats receiving 10% corn oil in the diet. Cholestyramine treatment had no significant effect on plasma apo A-I concentration. Hepatic apo B, apo E and apo A-IV mRNA levels were similar among dietary groups. Cholestyramine administration caused an increase in hepatic apo A-I mRNA level only in rats fed low-fat diet. In the intestine cholestyramine treatment decreased apolipoprotein mRNA levels in jejunum but had a few effect on apolipoprotein mRNA levels in ileum. These experiments have shown that long-term decrease in the intraluminal availability of bile acids may alter not only lipid and lipoprotein metabolism but also the regulation of apolipoprotein synthesis.

Effect of simvastatin treatment on plasma apolipoproteins and hepatic apolipoprotein mRNA levels in the genetically hypercholesterolemic rat (RICO)

The effects of long-term treatment with simvastatin on plasma lipoproteins, plasma apolipoproteins, and on hepatic apolipoprotein gene expression were evaluated in genetically hypercholesterolemic (RICO) rats. Simvastatin administration caused a decrease in plasma triglyceride and phospholipid concentrations. Plasma cholesterol concentration was not changed by simvastatin, but cholesterol distribution among plasma lipoproteins was altered. Plasma apo B, apo A-I, and apo A-IV concentrations were lowered by simvastatin treatment whereas plasma apo E concentration was not affected by this drug. In the liver, simvastatin treatment induced a significant decrease of apo E mRNA level but had no effect on apo B, apo A-I, and apo A-IV mRNA abundances. It appears that simvastatin may modify plasma apolipoprotein concentrations by influencing their hepatic synthesis at both pre- and posttranscriptional levels.

Compensatory increase in intestinal apolipoprotein A-IV mRNA levels in the experimental nephrotic rat

Using experimental nephrotic rats, we investigated the potential feedback regulation of apolipoproteins (apos) at their hepatic and intestinal synthetic sites. Nephrotic syndrome (NS) was induced in rats by puromycin aminonucleoside (PAN) with a single intraperitoneal injection (120 mg/kg). In nephrotic rats, we observed a 60% reduction in serum apo A-IV levels despite a 3.4-fold increase in jejunum and a 1.5-fold increase in ileum apo A-IV mRNA levels, although hepatic apo A-IV levels were unchanged compared with those in pair-fed control rats. A strikingly positive correlation was observed between daily urinary excretion of apo A-IV and its mRNA levels in jejunum (r = .856, P < .01; n = 10) and ileum (r = .710, P < .05; n = 10). On the other hand, nephrotic rats had an 8.2-fold increase in serum apo A-I level associated with a 4.6-fold increase in hepatic and a small but significant increase in jejunum apo A-I mRNA levels. Compared with the fractional catabolic loss of albumin or apo A-IV, that of apo A-I was small and suggests a diminished level of glomerular filtration, leading to a further elevation in serum apo A-I level. Barring nonspecific effects of PAN, these data suggest that reduction of serum apo A-IV level due to urinary loss may directly upregulate mRNA levels in the small intestine. Alternatively, it may be the result of an effective filtration of a serum component unassociated with lipoproteins that normally and site-specifically reduces apo A-I and apo A-IV mRNA transcription.

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)

RNA editing: exploring one mode with apolipoprotein B mRNA

RNA editing is a newly described genetic phenomenon. It encompasses widely different molecular mechanisms and events. According to the specific RNA modification, RNA editing can be broadly classified into six major types. Type II RNA editing occurs in plants and mammals; it consists predominantly in cytidine to uridine conversions resulting from deamination/transamination or transglycosylation, although in plants other mechanisms have not been excluded. Apolipoprotein B mRNA editing is the only well-documented editing phenomenon in mammals. It is an intranuclear event that occurs posttranscriptionally, coincident with splicing and polyadenylation. Recent observations indicate that the tissue- and sequence-specific process is mediated by an enzyme that has separate domains for editing and sequence recognition. The presence of apolipoprotein B mRNA editing activity in tissues that do not produce the protein suggests that other RNAs may be edited and RNA editing may be a genetic phenomenon of general biological importance to the cell.

Effects of estrogen replacement on plasma lipoproteins and apolipoproteins in postmenopausal, dyslipidemic women

The effects of oral estrogen replacement (ethinyl estradiol 0.02 mg/d) on plasma triglyceride, total cholesterol, very-low-density lipoprotein (VLDL) cholesterol, low-density lipoprotein (LDL) cholesterol, high-density lipoprotein (HDL) cholesterol, and apolipoprotein (apo) A-I and B levels and LDL particle size were assessed in 20 postmenopausal women with a previous hysterectomy and various forms of dyslipidemia (LDL cholesterol > or = 4.14 mmol/L [160 mg/dL] and/or HDL cholesterol < or = 1.03 mmol/L [40 mg/dL]). All subjects were studied while on a standard cholesterol-lowering diet, and were sampled in the fasting state before beginning estrogen therapy and after a mean of 13 weeks of estrogen therapy. Lipids were measured by standardized enzymatic techniques, apos were measured by enzyme-linked immunoassays, and LDL particle size was measured by gradient gel electrophoresis. Mean values for plasma lipid parameters (mmol/L) at baseline and during estrogen replacement were as follows: triglyceride, 2.11 and 2.75 (30% increase); total cholesterol, 7.45 and 6.52 (13% decrease); VLDL cholesterol, 1.09 and 1.22 (12% increase); LDL cholesterol, 5.09 and 3.70 (27% decrease); and HDL cholesterol, 1.27 and 1.58 (24% increase). Mean values for apo A-I were 163 and 254 mg/dL (56% increase), and for apo B they were 170 and 148 mg/dL (13% decrease). The LDL particle score was 4.09 and 4.52 (11% smaller). Changes in all parameters were statistically significant (P = .05) except for VLDL cholesterol. These data indicate that estrogen replacement is effective in decreasing LDL cholesterol and apo B concentrations and increasing HDL cholesterol and apo A-I concentrations in dyslipidemic postmenopausal women, but it should not be used in patients with baseline fasting triglyceride levels higher than 2.82 mmol/L (250 mg/dL) unless it is accompanied by a progestin. Our data indicate that this form of estrogen replacement could lower the risk of coronary artery disease (CAD) by more than 50% in these women, based on favorable alterations in plasma lipoproteins.