Immune-regulation of the apolipoprotein A-I/C-III/A-IV gene cluster in experimental inflammation

Apolipoprotein A-IV is a member of the apo A-I/C-III/A-IV gene cluster. In order to investigate its hypothetical coordinated regulation, an acute phase was induced in pigs by turpentine oil injection. The hepatic expression of the gene cluster as well as the plasma levels of apolipoproteins were monitored at different time periods. Furthermore, the involvement of the inflammatory mediators' interleukins 1 and 6 and tumor necrosis factor in the regulation of this gene cluster was tested in cultured pig hepatocytes, incubated with those mediators and apo A-I/C-III/A-IV gene cluster expression at the mRNA level was measured. In response to turpentine oil-induced inflammation, a decreased hepatic apo A-IV mRNA expression was observed (independent of apo A-I and apo C-III mRNA) not correlating with the plasma protein levels. The distribution of plasma apo A-IV experienced a shift from HDL to larger particles. In contrast, the changes in apo A-I and apo C-III mRNA were reflected in their corresponding plasma levels. Addition of cytokines to cultured pig hepatocytes also decreased apo A-IV and apo A-I mRNA levels. All these results show that the down-regulation of apolipoprotein A-I and A-IV messages in the liver may be mediated by interleukin 6 and TNF-alpha. The well-known HDL decrease found in many different acute-phase responses also appears in the pig due to the decreased expression of apolipoprotein A-I and the enlargement of the apolipoprotein A-IV-containing HDL.     

Adaptation of intestinal production of apolipoprotein A-IV during chronic feeding of lipid

We examined the effect of daily fat supplementation on intestinal gene expression and protein synthesis and plasma levels of apolipoprotein A-IV (apo A-IV). Rats were fasted overnight and then given intragastric bolus infusion of either saline or fat emulsion after 0, 1, 2, 4, 8, or 16 days of similar daily feedings. Four hours after the final saline or fat infusion, plasma and jejunal mucosa were harvested; plasma levels of apo A-IV, triglycerides, and leptin were measured, as well as mucosal apo A-IV mRNA levels and biosynthesis of apo A-IV protein. In response to fat, plasma apo A-IV showed an initial 40% increase compared with saline-injected control rats; with continued daily fat feeding, the plasma A-IV response showed rapid and progressive diminution such that by 4 days, plasma A-IV was not different between fat- and saline-fed groups. Jejunal mucosal apo A-IV synthesis and mRNA levels also showed time-dependent refractoriness to fat feeding. However, the kinetics of this effect were considerably slower than in the case of plasma, requiring 16 days for completion. There was no correlation between plasma leptin or triglyceride levels and intestinal apo A-IV synthesis or plasma apo A-IV. These results indicate rapid, fat-induced, posttranslational adapation of plasma apo A-IV levels and a slower, but similarly complete pretranslational adaptation of intestinal apo A-IV production, which are independent of plasma levels of leptin.     

The effect of dietary copper on rat plasma apolipoprotein B,E plasma levels,and apolipoprotein gene expression in liver and intestine

The plasma levels of apo B and apo E, and the level of hepatic and intestinal mRNA coding for these apolipoproteins were investigated in weanling male rats pair-fed for 6 wk with a control or copperdeficient diet. Plasma cholesterol, triglycerides, and phospholipids were significantly increased, and plasma apo B and apo E levels were also markedly increased in copper-deficient rats as compared to control rats. Copper deficiency significantly increased triglyceride levels and decreased cholesterol levels in the liver. No major differences in the levels of hepatic and intestinal apo B and apo E mRNA occurred between control and copper-deficient rats. These data imply that hypertriglyceridemia dn hypercholesterolemia owing to the copper deficiency are not accompanied by modifications in the gene expression at the mRNA level in the liver and intestine of the apolipoproteins studied.     

Induction of liver apolipoprotein A-IV mRNA in porphyric mice.

We have isolated cDNA clones for mRNAs that are induced by porphyria from a mouse liver library. Of the three inducible clones isolated, we have identified one as being apolipoprotein A-IV (apo A-IV) by its extensive homology with a rat apolipoprotein A-IV cDNA sequence. The level of liver apo A-IV mRNA increases rapidly in response to either of two porphyrogenic drugs. When the ferrochelatase-inhibited drug, 3,5-dicarbethoxy-1,4-dihydrocollidine (DDC) is used, a 6 and 28 fold induction of liver apo A-IV mRNA is observed in male and female mice, respectively. If the heme-destroying porphyrogenic drug, allylisopropylacetamide (AIA) is the inducing agent, liver apo A-IV mRNA levels increase 2-3 fold in both males and females. The level of apo A-IV mRNA reaches a maximum within 6-10 hr. after drug administration. Intestine apo A-IV mRNA levels do not change during either of these drug-induced porphyrias. RNA from acute-phase responsive liver or liver from mice treated with bilirubin, porphobilinogen, or protoporphyrin IX show no increase in apo A-IV mRNA. These results indicate that apo A-IV induction is tied to a disruption in porphyrin-heme biosynthesis but is not directly affected by several heme intermediates nor by the major heme degradation product, bilirubin.     

Regulation of intestinal and hypothalamic apolipoprotein A-IV

This review discusses the regulation of the intestinal and hypothalamic apolipoprotein A-IV (apo A-IV) gene and protein expression. Apo A-IV is a glycoprotein secreted together with triglyceride-rich lipoproteins by the small intestine. Intestinal apo A-IV synthesis is stimulated by fat absorption, probably mediated by chylomicron formation. This stimulation of intestinal apo A-IV synthesis is attenuated by intravenous leptin infusion. Chronic ingestion of a high-fat diet blunts the intestinal apo A-IV in response to dietary lipid. Intestinal apo A-IV synthesis is also stimulated by members of the pancreatic polypeptide family, including peptide YY (PYY), neuropeptide Y (NPY), and pancreatic polypeptide (PP). Recently, apo A-IV was demonstrated to be present in the hypothalamus as well. Hypothalamic apo A-IV level was reduced by food deprivation and restored by lipid feeding. Intracerebroventricular administration of apo A-IV antiserum stimulated feeding and decreased the hypothalamic apo A-IV mRNA level, implying that feeding is intimately regulated by endogenous hypothalamic apo A-IV. Central administration of NPY significantly increased hypothalamic apo A-IV mRNA levels in a dose-dependent manner.     

Peptide YY stimulates the expression of apolipoprotein A-IV gene in Caco-2 intestinal cells

The effect of peptide YY, a gastrointestinal hormone, on the expression of the apolipoprotein A-IV gene in the intestinal epithelial cell line Caco-2 was examined by semiquantitative RT-PCR followed by Southern hybridization with an inner oligonucleotide probe. Apolipoprotein A-IV mRNA levels were increased in response to peptide YY in a dose- and time-dependent fashion. Western blotting revealed that the exogenous peptide YY increased the intracellular concentration of apolipoprotein A-IV. In contrast, apolipoprotein A-I, B, and C-III mRNA did not respond to peptide YY. Differentiated Caco-2 cells expressed Y1- but not Y2- and Y5-receptor subtype mRNA. The present results suggest that peptide YY modulates apolipoprotein A-IV gene expression, likely via the Y1-receptor subtype in intestinal epithelial cells.     

The fatty liver dystrophy (fld) mutation. A new mutant mouse with a developmental abnormality in triglyceride metabolism and associated tissue-specific defects in lipoprotein lipase and hepatic lipase activities

An autosomal recessive mutation, termed fatty liver dystrophy (fld), can be identified in neonatal mice by their enlarged and fatty liver (Sweet, H. O., Birkenmeier, E. H., and Davisson, M. T. (1988) Mouse News Letter 81, 69). We have examined the underlying metabolic abnormalities in fld/fld mice from postnatal days 3-40. Serum and hepatic triglyceride levels were elevated 5-fold in suckling fld/fld mice compared to their +/? littermates but abruptly resolved at the suckling/weaning transition. Blot hybridization analysis of liver and intestinal RNAs revealed a liver-specific increase in apolipoprotein (apo) A-IV and C-II mRNA concentrations (100- and 6-fold, respectively) that was limited to the suckling and early weaning stages in fld/fld mice. Resolution of these differences during the weaning period could not be delayed by prolonging suckling to the 20th postnatal day nor could the mutant phenotype be elicited in young adult animals with a high fat diet. Lipoprotein lipase (LPL) activity was reduced 16-fold in the white adipose tissue of fld/fld mice until the onset of weaning. Heart activity was decreased less than 2-fold, but there were no deficits in brown adipose tissue or liver. Hepatic lipase (HL) mRNA levels and activity were significantly reduced in fld/fld livers and sera, respectively, during the suckling period. Mapping studies show the fld locus to be distinct from loci encoding LPL, HL, and apoA-IV, and those responsible for the combined lipase deficiencies in cld/cld and W/Wv mice. These data suggest that the fld mutation is associated with developmentally programmed tissue-specific defects in the neonatal expression of LPL and HL activities and provide evidence for a new regulatory locus which affects these lipase activities. This mutation could serve as a useful model for (i) analyzing the homeostatic mechanisms controlling lipid metabolism in newborn mice and (ii) understanding and treating certain inborn errors in human triglyceride metabolism.     

Development, food intake, and ethinylestradiol influence hepatic triglyceride lipase and LDL-receptor mRNA levels in rats

The influence of development and ethinylestradiol on low density lipoprotein (LDL)-receptor mRNA and hepatic triglyceride lipase (HTGL) activity and mRNA levels was studied in rat liver and intestine. Intestinal LDL-receptor mRNA levels are maximal in the perinatal period, whereas liver LDL-receptor and HTGL mRNA levels are highest after weaning in adult life. All mRNA levels reach a maximum between day 15 and 20 when rats still consume a lipid-rich diet, and increase twofold during weaning. Liver and intestinal LDL-receptor mRNA levels are not influenced by ovariectomy, but increase after ethinylestradiol treatment. Liver LDL-receptor mRNA shows a dose-dependent increase after ethinylestradiol and a sevenfold rise in liver LDL-receptor mRNA is attained with a dose of 2000 micrograms/day. Intestinal LDL-receptor mRNA increases slightly more than twofold after ethinylestradiol and this increase is not dose-dependent. Changes in LDL-receptor mRNA are independent of changes in food intake induced by ethinylestradiol treatment, since they are still observed after pair-feeding. The ethinylestradiol-induced increases in LDL-receptor mRNA levels are reflected by decreased serum apoB levels. HTGL mRNA levels increase after ovariectomy and show a dose-dependent decrease after ethinylestradiol. Pair-feeding abolishes the increase seen after ovariectomy, while the estrogen-mediated decrease is attenuated. These alterations in HTGL mRNA are reflected by similar changes in liver HTGL activity.(ABSTRACT TRUNCATED AT 250 WORDS)     

Influence of development, estrogens, and food intake on apolipoprotein A-I, A-II, and E mRNA in rat liver and intestine

The influence of development and ethinylestradiol (EE) on apolipoprotein (apo) A-I, A-II, and E mRNA in rat liver and intestine was studied by dot blot hybridization and Northern blot analysis. ApoA-I mRNA levels were maximal in the perinatal period and declined after day 15. An opposite trend was noted for the apoA-II mRNA levels, whereas apoE mRNA remained fairly constant. Liver apoA-I mRNA levels increased after ovariectomy (OVX). A further rise was observed when EE was given at 2000 micrograms/day. When the influence of OVX and EE was controlled for food intake by pair-feeding, OVX still increased hepatic apoA-I mRNA. The rise in liver apoA-I mRNA after EE, however, was no longer significant. Under the same conditions OVX slightly increased intestinal apoA-I mRNA. EE (2000 micrograms/day) decreased intestinal apoA-I mRNA to 80% of the pair-fed controls. Liver apoA-II mRNA levels did not change after OVX when the animals were fed ad libitum, but decreased slightly when the rats were pair-fed. EE caused a dose-dependent decrease in liver apoA-II mRNA, irrespective of food intake. None of these treatments caused any change in liver apoE mRNA levels. Serum apoA-I levels increased upon OVX, while serum apoE did not change. EE provoked a dose-dependent decrease of both apolipoproteins in serum. In conclusion: 1) Changes in food intake play an important role in the in vivo effects of estrogens on apolipoprotein mRNA levels. 2) The stimulatory effect of OVX on hepatic apoA-I mRNA as well as the inhibitory effect of EE on hepatic apoA-II mRNA are independent of food intake.(ABSTRACT TRUNCATED AT 250 WORDS)     

Diurnal changes in intestinal apolipoprotein A-IV and its relation to food intake and corticosterone in rats

To further investigate the role of intestinal aplipoprotein A-IV (apo A-IV) in the management of daily food intake, we examined the diurnal patterns in apo A-IV gene and protein expression in freely feeding (FF) and food-restricted (FR; food provided 4 h daily for 4 wk) rats that were killed at 3-h intervals throughout the 24-h diurnal cycle. In FF rats, the intestinal apo A-IV mRNA and protein levels showed a circadian rhythm concomitant with the feeding pattern. The daily pattern of fluctuation of apo A-IV, however, was altered in FR rats, which had a marked increase in intestinal apo A-IV levels during the 4-h feeding period of light phase. In both FF and FR rats, increased plasma corticosterone (Cort) levels temporally coincided with the increasing phase of intestinal apo A-IV mRNA and protein expression. Depletion of Cort by adrenalectomy abolished the diurnal rhythm by decreasing the apo A-IV expression during the dark period but did not change the feeding rhythm. Exposure of adrenalectomized rats to consistent Cort level (50-mg continuous release Cort pellet) resulted in fixed apo A-IV levels throughout the day. These results indicate that intestinal apo A-IV exhibits a diurnal rhythm, which can be regulated by endogenous Cort independently of the light-dark cue. The fact that intestinal apo A-IV levels were consistent with the food intake during the normal diurnal cycle as well as during the cycle of 4-h feeding each day suggests that intestinal apo A-IV is involved in the regulation of daily food intake.     

Binding of human apolipoprotein A-IV to human hepatocellular plasma membranes

We have investigated the binding of human apolipoprotein A-IV (apo A-IV) to human hepatocellular plasma membranes. Addition of increasing concentrations of radiolabeled apo A-IV to hepatic plasma membranes, in the presence and absence of a 25-fold excess of unlabeled apo A-IV, revealed saturation binding to the membranes with a KD of 154 nM and a binding maximum of 1.6 ng/microgram of membrane protein. The binding was temperature-insensitive, partially calcium-dependent, abolished when apo A-IV was denatured by guanidine hydrochloride or when the membranes were treated with Pronase and decreased when apo A-IV was incorporated into phospholipid/cholesterol proteoliposomes. In displacement studies using purified apolipoproteins and isolated lipoproteins, only unlabeled apo A-IV, apo A-I and high-density lipoproteins effectively competed with radiolabeled apo A-IV for membrane binding sites. We conclude that human apo A-IV exhibits high-affinity binding to isolated human hepatocellular plasma membranes which is saturable, reversible and specific.     

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 [3]. This results into the elevation of plasma levels of apolipoprotein (apo) B and apoE, and lowering of apoA-I-containing particles. The mechanisms of apoB and apoE elevation by estrogen have been elucidated [6], 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.