cholesterol transporters, regulatory enzymes, and transcription factors Modulation of intestinal cholesterol absorption by high glucose levels: impact on (PDF) (original) (raw)
Journal of Lipid Research, 1994
Apolipoprotein A-I (apoA-I) gene expression is known to be regulated by nutritional and hormonal factors. Experiments were conducted to determine the effects of vitamin A deficiency and retinoic acid repletion on the in vivo expression of apoA-I in rat intestine and liver. The relative abundance of apoA-I mRNA (apoA-IIP-actin ratio) in the intestine did not differ significantly between vitamin A-deficient and -sufficient rats. However, the relative abundance of hepatic apoA-I mRNA of vitamin A-deficient rats was 2.2-to 6-times that of sufficient rats. Even marginal vitamin A status resulted in a significant increase in hepatic apoA-I mRNA expression. Treatment of vitamin A-deficient rats with a single dose of retinoic acid (20 pg, 20 h before tissues were collected) reduced the hepatic apoA-I mRNAIP-actin ratio by about 40%, while further reduction (about 60-6570) was observed after two treatments with retinoic acid. By nuclear run-on assay, the increase in hepatic apoA-I mRNA in vitamin A-deficient rats was attributable to increased transcription of the apoA-I gene. However, immunoblot analysis showed no apparent differences in apoA-I protein in either liver homogenates or plasma of vitamin A-deficient and -sufficient rats. I These data indicate that apoA-I gene expression in vivo is sensitive to retinoid status and suggest that there is additional regulation of post-transcriptional events. -Zolfaghari,
Journal of Lipid Research, 1997
We showed previously that retinoids stimulate apolipoprotein A-I (apoA-I) synthesis in cultured cynomolgus hepatocytes only after a 24-h lag phase. Here we report on the biochemical background of the slow .response, the requirement for high retinoic acid concentrations, and the involvement of different retinoid receptors. The time course of the effect of 10 PM all-trans retinoic acid (at-RA) on apoA-I mRNA levels and protein secretion were comparable, i.e., minor increases were observed after a 24-h incubation and mRNA levels were increased 2.2-and 3.5-fold after 48 h and 72 h, respectively. In contrast, apoA-I gene transcription was already increased (2.6-fold) after a 4-h incubation with 10 PM at-RA. At-RA disappeared rapidly from the cultures: after 2 h of incubation 40% of the added amount was left and after 24 h only 2%. RARp mRNA and gene expression were increased after incubation with 10 ~L M at-RA, whereas RARa and RXRa mRNA levels and expression remained unchanged. No transcriptional activity and mRNA for other retinoid receptors were detectable. Both RAR-selective ('ITNPB) and RXR-selective (3-methyl-TTNEB) agonists induced apoA-I synthesis at 1 and 10 P L M . ~ These results show that i) the slow increase in apoA-I secretion is caused by a slow increase of its mRNA level; ii) the apoA-I gene transcription in cynomolgus hepatocytes is induced rapidly by retinoids; izi) the added at-RA disappeared rapidly from the cultures, explaining the necessity for high initial concentrations; iv) RARa and/or RARP and RXRa are involved in the activation of apoA-I expression by retinoids.-Kaptein, A., D. M. Neele, J. Twisk, H. F. J. Hendriks, T. Kooistra, and H. M. G. Princen. Mechanism of the slow induction of apolipoprotein A-I synthesis by retinoids in cynomolgus hepatocytes: involvement of retinoic acid and retinoid X receptors.
Journal of Clinical Investigation, 1996
The regulation of liver apolipoprotein (apo) A-I gene expression by fibrates was studied in human apo A-I transgenic mice containing a human genomic DNA fragment driving apo A-I expression in liver. Treatment with fenofibrate (0.5% wt/wt) for 7 d increased plasma human apo A-I levels up to 750% and HDL-cholesterol levels up to 200% with a shift to larger particles. The increase in human apo A-I plasma levels was time and dose dependent and was already evident after 3 d at the highest dose (0.5% wt/wt) of fenofibrate. In contrast, plasma mouse apo A-I concentration was decreased after fenofibrate in nontransgenic mice. The increase in plasma human apo A-I levels after fenofibrate treatment was associated with a 97% increase in hepatic human apo A-I mRNA, whereas mouse apo A-I mRNA levels decreased to 51%. In nontransgenic mice, a similar down-regulation of hepatic apo A-I mRNA levels was observed. Nuclear run-on experiments demonstrated that the increase in human apo A-I and the decrease in mouse apo A-I gene expression after fenofibrate occurred at the transcriptional level. Since part of the effects of fibrates are mediated through the nuclear receptor PPAR (peroxisome proliferator-activated receptor), the expression of the acyl CoA oxidase (ACO) gene was measured as a control of PPAR activation. Both in transgenic and nontransgenic mice, fenofibrate induced ACO mRNA levels up to sixfold. When transgenic mice were treated with gemfibrozil (0.5% wt/wt) plasma human apo A-I and HDL-cholesterol levels increased 32 and 73%, respectively, above control levels. The weaker effect of this compound on human apo A-I and HDL-cholesterol levels correlated with a less pronounced impact on ACO mRNA levels (a threefold increase) suggesting that the level of induction of human apo A-I gene is related to the PPAR activating potency of the fibrate used. Treatment of human primary hepatocytes with fenofibric acid (500 M) provoked an 83 and 50% increase in apo A-I secretion and mRNA levels, respectively, supporting that a direct action of fibrates on liver human apo A-I production leads to the observed increase in plasma apo A-I and HDL-cholesterol.
The Journal of Lipid Research
Retinoids are reported to stimulate apolipoprotein (apo) A-I gene promoter activity (Rottman et al. 1991. Mol. Cell. Biol. 11: 3814-3820) and apoA-I protein secretion by monkey hepatocytes . Arterioscler. Thmmb. 13: 1505-1514. In this study we have assessed the effects of retinoids on parameters of apoA-I biosynthesis in human cell lines. Caco-2 and HepG2 cells (human intestinal and hepatoma cell lines, respectively, both known to express and secrete apoA-I) were stably transfected with a reporter gene construct containing 1.3 kb of the 5-' flanking region of the human apoA-I gene linked to the firefly luciferase coding region. These cells were incubated for 48 h with 10 pM all-trans retinoic acid (RA) or 9 4 RA. The cells were then assayed for luciferase activity, for apoA-I mRNA level, and for secretion of apoA-I protein in the medium. Secretion of apoB was monitored as well. In Caco-2 cells, all-tram and 9-cis RA increased luciferase activity, mRNA content, and protein secretion by 40% to 80% above control. Strikingly, in HepG2 cells all-truns and 9 4 s RA caused a more marked stimulation of luciferase activity (by lOO-l50%) but a weaker increase of mRNA content and protein secretion (by 25-30%). In contrast, apoB secretion was inhibited by the two
Regulation of ApoA-I Gene Expression and Prospects to Increase Plasma ApoA-I and HDL Levels
High Density Lipoproteins, Dyslipidemia, and Coronary Heart Disease, 2010
C/EBP CAAT/enhancer binding protein CAT Chloramphenicol acetyl transferase EGR-1 Early growth response factor-1 FXR Farnesoid X receptor HNF-4 Hepatocyte nuclear factor-4 HDL High density lipoprotein HRE Hormone response element LRH-1 Liver receptor homolog-1 LXRs Liver X receptors PPARa Peroxisome proliferator-activated receptor a PLTP Phospholipid transfer protein RORa Retinoic acid receptor-related orphan receptor a RXRa Retinoid X receptor a SHP Small heterodimer partner SP1 Specificity protein 1 SR-BI Scavenger receptor class B type I SREBP Sterol regulatory element binding protein WT Wild type
We showed previously that retinoids stimulate apolipoprotein A-I (apoA-I) synthesis in cultured cynomolgus hepatocytes only after a 24-h lag phase. Here we report on the biochemical background of the slow .response, the requirement for high retinoic acid concentrations, and the involvement of different retinoid receptors. The time course of the effect of 10 PM all-trans retinoic acid (at-RA) on apoA-I mRNA levels and protein secretion were comparable, i.e., minor increases were observed after a 24-h incubation and mRNA levels were increased 2.2-and 3.5-fold after 48 h and 72 h, respectively. In contrast, apoA-I gene transcription was Abbreviations: apo, apolipoprotein; RAR, retinoic acid receptor; RXR, retinoid X receptor; at-RA, all-trans-retinoic acid; g-cis-RA, 9cis-retinoic acid.
cis-Acting determinants of basal and lipid-regulated apolipoprotein A-IV expression in mice
Journal of Biological Chemistry, 1989
The levels of apolipoprotein A-IV (apoA-IV) mRNA are regulated by dietary lipid in the liver of both the mouse and rat. Thirteen different inbred mouse strains were fed a high lipid diet, and the effect on apoA-IV liver mRNA levels was examined. It was found that each strain responded in one of two ways. Mice of four strains had higher liver apoA-IV mRNA levels as compared with syngeneic mice fed a normal chow diet. Mice of the other nine strains had decreased liver apoA-IV mRNA levels as compared with syngeneic mice fed a normal chow diet. Using F1 hybrids between mice from BALB/c, C3H, and C57BL/6 and between 129 and C57BL/6, as well as recombinant inbred strains derived from a cross between BALB/c and C57BL/6, we have shown that both the normal level of liver apoA-IV mRNA in the chow-fed mice and the lipid-dependent regulation of apoA-IV mRNA levels are controlled by cis-acting genetic elements. The apoA-IV mRNA levels in mice fed a normal diet varied dramatically among strains, with the largest difference (90-fold) being between the 129/J inbred strain and the C57BL/6J strain. In addition, we have examined the expression of apoA-IV during mouse development. ApoA-IV mRNA is expressed early in mouse liver (16 days postcoitum), whereas others have shown previously that rat liver apoA-IV mRNA is undetectable until 14 days after birth. ApoA-IV mRNA levels in the intestine and apoA-I mRNA levels in the liver and intestine, by contrast, mirror the pattern seen in the rat.
European Journal of Biochemistry, 1995
The regulation by fibrates and dietary fatty acids of the hepatic gene expression of apolipoproteins (apo) A-I and A-11, the major protein constituents of high-density lipoproteins, as well as of acyl-CoA oxidase, the rate-limiting enzyme of the peroxisomal P-oxidation pathway, was studied in vivu in the rat and in vitru in primary cultures of rat hepatocytes. In primary hepatocytes, different fibrates decreased apo A-I and increased acyl-CoA oxidase mRNA levels, whereas apo A-I1 mRNA only decreased in level after treatment with fenofibric acid, but not after bezafibrate, gemfibrozil or Wy-14643 treatment. Treatment with fenofibric acid counteracted the increase in apo A-I mRNA levels observed after dexamethasone or all-trans retinoic acid treatment, whereas simultaneous addition of fenofibric acid together with all-trans retinoic acid or dexamethasone resulted in a superinduction of acyl-CoA oxidase mRNA. Addition of the n-3 polyunsaturated fatty acids (PUFAs), docosanohexaenoic acid and eicosanopentaenoic acid, or the fatty acid derivative a-bromopalmitate, decreased apo A-I and increased acyl-CoA oxidase mRNA in a dose-dependent and time-dependent manner, whereas apo A-I1 mRNA did not change significantly. Nuclear run-on experiments demonstrated that fenofibric acid and a-bromopalmitate decreased apo A-I and increased acyl-CoA oxidase gene expression at the transcriptional level. When rats were fed isocaloric diets enriched in saturated fat (hydrogenated coconut oil), n-6 PUFAs (safflower oil) or n-3 PUFAs (fish oil), a significant decrease in liver apo A-I and apo A-I1 mRNA levels was only observed after fish oil feeding. Compared to feeding low fat, liver acyl-CoA oxidase mRNA increased after fat feeding, but this effect was most pronounced (twofold) in rats fed fish oil. Results from these studies indicate that fish oil feeding reduces rat liver apo A-I and apo A-I1 gene expression, similar to results obtained after feeding fenofibrate. Fibrates and n-3 fatty acids (and the fatty acid derivative, a-bromopalmitate) down-regulate apo A-I and induce acyl-CoA oxidase gene expression through a direct transcriptional action on the hepatocyte. In contrast, only fenofibric acid, but not the other fibrates or fatty acids tested, decrease apo A-I1 gene expression in vitro.
Journal of Clinical Investigation, 1994
Two transgenic mouse lines, expressing low or high amounts of human apo A-IV were created. In low and high expressor HuAIVTg mice on a chow diet, serum human apo A-IV levels were 6 and 25 times the normal human level and on a high fat diet, they were 12 and 77 times higher. Human apo A-IV was equally distributed between lipoprotein (mainly HDL) and lipid-free fractions. Intestinal absorption ofradiolabeled cholesterol and triglycerides was unaffected in HuAIVTg mice. Vitamin A, carried exclusively in chylomicrons and their remnants, was catabolized normally. When an intragastric vitamin E bolus is given to the HuAIVTg mice, the initial absorption and appearance in triglyceride-rich lipoproteins was similar to that observed in normal mice. However, elevated amounts of vitamin E were subsequently observed in the VLDL of the HuAIVTg mice. Furthermore, in the fed state, serum VLDL triglycerides were markedly elevated in HuAIVTg mice. This effect was greater in high expressor mice. Serum total cholesterol was not elevated, but the distribution was altered in the HuAIVTg mice; VLDL-C was increased at the expense of HDL-C. Kinetic studies suggested a delayed clearance of VLDL in HuAIVTg mice. Apo A-IV has been suggested to be a satiety factor, but no effect on feeding behavior or weight gain was observed in these HuAIVTg mice. In summary, our studies with HuAIVTg mice show that additional apo A-IV does not effect intestinal absorption of fat and fat-soluble vitamins, and at least chronic elevation of plasma apo A-IV does not effect feeding behavior in this model system. (J. serves a variety of functions. 1776 Aalto-Setdld et al. J. Clin. Invest.
Journal of Hepatology and Gastroenterology, 2017
A high serum level of HDL lowers the risk of myocardial infarction. The predominant protein associated with HDL is apoA-I which is synthesized in hepatocytes and enterocytes [1-3]. A high serum level of apoA-I lowers the risk of myocardial infarction [4, 5]. Although apoA-I plays a central role in the regulation of lipid metabolism and thus in prevention of myocardial infarction, little information is available on the regulation of its synthesis. ApoA-I is synthesized as a 267 amino acid precursor protein, preproapoA-I [6-13]. The presegment is 18 amino acids long and cotranslationally cleaved off, resulting in proapoA-I [11-16]. ProapoA-I is the precursor of the mature apoA-I, differing from apoA-I by a hexapeptide extension at the N-terminal end [14]. For rat apoA-I three slightly different propeptides have been described:
Gut, 1979
Isolated mucosal cells from the human jejunum and stomach, cryostat sections from the jejunum, isolated parenchymal liver cells and lymphocytes were investigated for the presence of apolipoprotein A (apoA). Antisera against purified human apoA-I and apoA-II were raised in rabbits and conjugated with fluorescein-isothiocyanate (FITC). Mucosal cells from jejunum and stomach were isolated with pronase from tissue obtained from operated patients. ApoA-I and apoA-II could be demonstrated in isolated mucosal cells as well as in cryostat sections from the jejunum. The fluorescence pattern in isolated jejunal cells was coarse granular. In the radial gel diffusion test the homogenate from mucosal cells ofjejunum showed a single precipitation line with anti-apoA-I and with anti-apoA-II, respectively. The reaction was more intensive with anti-apoA-I than with anti-apoA-II. Isolated gastric cells were negative for apoA. Hepatocytes incubated with FITC anti-apoA-I showed a fine granular fluorescence pattern in the cytoplasm. Anti-apoA-II did not react with hepatocytes. There was no evidence for an in vitro fixation of serum-apoA at the surface of isolated mucosal cells from jejunum or isolated hepatocytes. The results support the hypotheses that in man apoA is synthesised in the epithelial cells of the small intestine and in parenchymal liver cells.
Apolipoprotein A-IV: a protein intimately involved in metabolism
Journal of lipid research, 2015
The purpose of this review is to summarize our current understanding of the physiological roles of apolipoprotein A-IV (apoA-IV) in metabolism, and to underscore the potential for apoA-IV to be a focus for new therapies aimed at the treatment of diabetes and obesity-related disorders. ApoA-IV is primarily synthesized by the small intestinal synthesis and attached to chylomicrons by enterocytes and secreted into intestinal lymph during fat absorption. In circulation, apoA-IV is associated with high density lipoproteins (HDL), chylomicron remnants but a large portion is lipoprotein free. Functions of circulating apoA-IV have been related to protection from cardiovascular disease due to its anti-oxidative and anti-inflammatory properties, and because it can mediate reverse-cholesterol transport. This review, however, focuses primarily on several properties of apoA-IV that impact other metabolic functions related to food intake, obesity and diabetes. In addition to participating in trig...
Reduction of intestinal apo A-IV mRNA levels in the cirrhotic rat
Journal of Gastroenterology and Hepatology, 1996
In the present study, intestinal apo A-IV synthesis was investigated using a carbon tetrachloride (CC1,)-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, T G 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.
Mechanisms of HDL deficiency in mice overexpressing human apoA-II
The Journal of Lipid Research, 2002
To ascertain the mechanisms underlying the hypoalphalipoproteinemia present in mice overexpressing human apolipoprotein A-II (apoA-II) (line 11.1), radiolabeled HDL or apoA-I were injected into mice. Fractional catabolic rate of [ 3 H]cholesteryl oleoyl ether HDL ([ 3 H]HDL) was 2-fold increased in 11.1 transgenic mice compared with control mice and this was concomitant with increased radioactivity in liver, gonads, and adrenals. However, scavenger receptor class B, type I (SR-BI) was increased only in adrenals.