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.     

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.     

Gastrointestinal satiety signals IV. Apolipoprotein A-IV

The focus of this article is to review evidence that apolipoprotein A-IV (apo A-IV) acts as a satiety factor. Additionally, information regarding the general involvement of apo A-IV in the regulation of food intake and body weight is stated. Apo A-IV is a glycoprotein synthesized by the human intestine. In rodents, both the small intestine and liver secrete apo A-IV, but the small intestine is the major organ responsible for circulating apo A-IV. There is now solid evidence that the hypothalamus, especially the arcuate nucleus, is another active site of apo A-IV expression. Intestinal apo A-IV synthesis is markedly stimulated by fat absorption and does not appear to be mediated by the uptake or reesterification of fatty acids to form triglycerides. Rather, the local formation of chylomicrons acts as a signal for the induction of intestinal apo A-IV synthesis. Intestinal apo A-IV synthesis is also enhanced by a factor from the ileum, probably peptide tyrosine-tyrosine (PYY). The inhibition of food intake by apo A-IV is mediated centrally. The stimulation of intestinal synthesis and secretion of apo A-IV by lipid absorption are rapid; thus apo A-IV likely plays a role in the short-term regulation of food intake. Other evidence suggests that apo A-IV may also be involved in the long-term regulation of food intake and body weight, as it is regulated by both leptin and insulin. Chronic ingestion of a high-fat diet blunts the intestinal as well as the hypothalamic apo A-IV response to lipid feeding. It also suppresses apo A-IV gene expression in the hypothalamus. Whereas it is tempting to speculate that apo A-IV may play a role in diet-induced obesity, we believe the confirmation of such a proposal awaits further experimental evidence.     

Apolipoprotein A-IV interacts synergistically with melanocortins to reduce food intake

Apolipoprotein (apo) A-IV is an anorexigenic gastrointestinal peptide that is also synthesized in the hypothalamus. The goal of these experiments was to determine whether apo A-IV interacts with the central melanocortin (MC) system in the control of feeding. The third ventricular (i3vt) administration of a subthreshold dose of apo A-IV (0.5 microg) potentiated i3vt MC-induced (metallothionein-II, 0.03 nmol) suppression of 30-min feeding in Long-Evans rats. A subthreshold dose of the MC antagonist (SHU9119, 0.1 nmol, i3vt) completely attenuated the anorectic effect of i3vt apo A-IV (1.5 microg). The i3vt apo A-IV significantly elevated the expression of c-Fos in neurons of the paraventricular nucleus of the hypothalamus, but not in the arcuate nucleus or median eminence. In addition, c-Fos expression was not colocalized with proopiomelanocortin-positive neurons. These data support a synergistic interaction between apo A-IV and melanocortins that reduces food intake by acting downstream of the arcuate.     

Hormonal triggering of the diurnal variation of sterol carrier protein

Rat liver sterol carrier protein (SCP) is a major intracellular protein regulating lipid metabolism and transport. During a dark-light cycle, SCP undergoes a dramatic diurnal variation in synthesis and level, reflecting translational events. Several hormones participate in the control of SCP synthesis. Insulin was implicated when the circadian rhythm of SCP was lost in both diabetes and fasting, states where insulin is low. After a 12-h fast the amplitude of the diurnal rhythm is diminished; after a 48-h fast it disappears, although SCP synthesis and level remain high. When endogenous insulin secretion is increased in fasted rats by glucose administration, SCP increases 2-fold in less than 30 min. When food intake is manipulated, but the dark-light cycle is unchanged, the circadian rhythm of SCP corresponds to feeding patterns and not light cycling. During feeding, increases in SCP are triggered following the expected increase in serum insulin. However, SCP is rapidly and significantly elevated in response to insulin only when glucocorticoids are normally high or increased by injection of the synthetic glucocorticoid, dexamethasone. Hepatocyte SCP levels are also induced by a combination of insulin and dexamethasone (2.3-fold) or insulin alone (1.3-fold). Dexamethasone alone causes a striking depression of SCP (2.4-fold). Thus, insulin is a major regulator of the diurnal variation of SCP synthesis. Glucocorticoids and other hormones (e.g. triiodothyronine) are also essential for maximum induction of SCP but play permissive roles.     

Complex interaction between circadian rhythm and diet on bile acid homeostasis in male rats

Desynchronization between the master clock in the brain, which is entrained by (day) light, and peripheral organ clocks, which are mainly entrained by food intake, may have negative effects on energy metabolism. Bile acid metabolism follows a clear day/night rhythm. We investigated whether in rats on a normal chow diet the daily rhythm of plasma bile acids and hepatic expression of bile acid metabolic genes is controlled by the light/dark cycle or the feeding/fasting rhythm. In addition, we investigated the effects of high caloric diets and time-restricted feeding on daily rhythms of plasma bile acids and hepatic genes involved in bile acid synthesis. In experiment 1 male Wistar rats were fed according to three different feeding paradigms: food was available ad libitum for 24 h (ad lib) or time-restricted for 10 h during the dark period (dark fed) or 10 h during the light period (light fed). To allow further metabolic phenotyping, we manipulated dietary macronutrient intake by providing rats with a chow diet, a free choice high-fat-high-sugar diet or a free choice high-fat (HF) diet. In experiment 2 rats were fed a normal chow diet, but food was either available in a 6-meals-a-day (6M) scheme or ad lib. During both experiments, we measured plasma bile acid levels and hepatic mRNA expression of genes involved in bile acid metabolism at eight different time points during 24 h. Time-restricted feeding enhanced the daily rhythm in plasma bile acid concentrations. Plasma bile acid concentrations are highest during fasting and dropped during the period of food intake with all diets. An HF-containing diet changed bile acid pool composition, but not the daily rhythmicity of plasma bile acid levels. Daily rhythms of hepatic Cyp7a1 and Cyp8b1 mRNA expression followed the hepatic molecular clock, whereas for Shp expression food intake was leading. Combining an HF diet with feeding in the light/inactive period annulled CYp7a1 and Cyp8b1 gene expression rhythms, whilst keeping that of Shp intact. In conclusion, plasma bile acids and key genes in bile acid biosynthesis are entrained by food intake as well as the hepatic molecular clock. Eating during the inactivity period induced changes in the plasma bile acid pool composition similar to those induced by HF feeding.     

Obesity induced by a high-fat diet downregulates apolipoprotein A-IV gene expression in rat hypothalamus

Apolipoprotein A-IV (apo A-IV) is an anorectic protein produced in the intestine and brain that has been proposed as a satiety signal. To determine whether diet-induced obesity alters apo A-IV gene expression in the intestine and hypothalamus, rats were fed a high-fat (HF), low-fat (LF), or standard chow (CHOW) diet for 2, 4, 6, 8, or 10 wk. Rats fed the HF diet had significantly greater body weights than rats given the LF and CHOW diets. Intestinal and plasma apo A-IV levels were comparable across dietary groups and time. LF and CHOW rats had comparable hypothalamic apo A-IV mRNA across the course of the experiment. However, HF rats had a slow and progressive diminution in hypothalamic apo A-IV mRNA over time that became significantly lower than that of LF or CHOW rats by 10 wk. Intragastric infusion of lipid emulsion to animals that were fasted overnight significantly stimulated hypothalamic apo A-IV mRNA in LF and CHOW rats but had no effect in HF rats. These results demonstrate that chronic consumption of a HF diet significantly reduces apo A-IV mRNA levels and the response of apo A-IV gene expression to dietary lipids in the hypothalamus. This raises the possibility that dysregulation of hypothalamic apo A-IV could contribute to diet-induced obesity.     

Daily Changes of Plasma Corticosterone by an 8-h Daytime Feeding Occur Without Body Weight Loss or Severe Food Restriction in the Rat

Different studies have reported that daytime feeding entrains the circadian rhythm of corticosterone secretion in rats. However, it remained unclear whether calorie restriction or daytime feeding access have an effect. The aim of our study is to evaluate the effect of an 8-h daytime feeding access on the circadian rhythm of plasma corticosterone. Eleven adult male Wistar rats were assigned to two different conditions of access to food: ad lib feeding for one week and daytime feeding for the following two weeks. On the 7th, 14th and 21st day, blood samples were collected every 4 h from 08:00 to 04:00. Food intake and body weight were recorded daily. During daytime feeding, rats ingested 88% of the amount of food ingested over 24 h in the ad lib feeding period. However, body weight increased significantly from the first day to the end of experiment. Peak plasma corticosterone was 12 h shifted during daytime feeding period compared to the ad lib condition. This study showed that an 8-h daytime feeding entrained the circadian rhythm of plasma corticosterone without body weight loss or severe food restriction.     

Daily Changes of Plasma Corticosterone by an 8-h Daytime Feeding Occur Without Body Weight Loss or Severe Food Restriction in the Rat

Different studies have reported that daytime feeding entrains the circadian rhythm of corticosterone secretion in rats. However, it remained unclear whether calorie restriction or daytime feeding access have an effect. The aim of our study is to evaluate the effect of an 8-h daytime feeding access on the circadian rhythm of plasma corticosterone. Eleven adult male Wistar rats were assigned to two different conditions of access to food: ad lib feeding for one week and daytime feeding for the following two weeks. On the 7th, 14th and 21st day, blood samples were collected every 4 h from 08:00 to 04:00. Food intake and body weight were recorded daily. During daytime feeding, rats ingested 88% of the amount of food ingested over 24 h in the ad lib feeding period. However, body weight increased significantly from the first day to the end of experiment. Peak plasma corticosterone was 12 h shifted during daytime feeding period compared to the ad lib condition. This study showed that an 8-h daytime feeding entrained the circadian rhythm of plasma corticosterone without body weight loss or severe food restriction.     

Rhythms of ghrelin, leptin, and sleep in rats: effects of the normal diurnal cycle, restricted feeding, and sleep deprivation

To determine the relationships among plasma ghrelin and leptin concentrations and hypothalamic ghrelin contents, and sleep, cortical brain temperature (Tcrt), and feeding, we determined these parameters in rats in three experimental conditions: in free-feeding rats with normal diurnal rhythms, in rats with feeding restricted to the 12-h light period (RF), and in rats subjected to 5-h of sleep deprivation (SD) at the beginning of the light cycle. Plasma ghrelin and leptin displayed diurnal rhythms with the ghrelin peak preceding and the leptin peak following the major daily feeding peak in hour 1 after dark onset. RF reversed the diurnal rhythm of these hormones and the rhythm of rapid-eye-movement sleep (REMS) and significantly altered the rhythm of Tcrt. In contrast, the duration and intensity of non-REMS (NREMS) were hardly responsive to RF. SD failed to change leptin concentrations, but it promptly stimulated plasma ghrelin and induced eating. SD elicited biphasic variations in the hypothalamic ghrelin contents. SD increased plasma corticosterone, but corticosterone did not seem to influence either leptin or ghrelin. The results suggest a strong relationship between feeding and the diurnal rhythm of leptin and that feeding also fundamentally modulates the diurnal rhythm of ghrelin. The variations in hypothalamic ghrelin contents might be associated with sleep-wake activity in rats, but, unlike the previous observations in humans, obvious links could not be detected between sleep and the diurnal rhythms of plasma concentrations of either ghrelin or leptin in the rat.     

Intestinal satiety protein apolipoprotein AIV is synthesized and regulated in rat hypothalamus

Apolipoprotein AIV (apo AIV) is a satiety protein secreted by the small intestine. We demonstrate for the first time that apo AIV protein and apo AIV mRNA are present in rat hypothalamus, a site intimately involved in the integration of signals for regulation of food intake and energy metabolism. We further characterized the regulation of hypothalamic apo AIV mRNA levels. Food-deprived animals showed a pronounced decrease in gene expression of apo AIV in the hypothalamus, with a concomitant decrease in the jejunum. Refeeding fasted rats with standard laboratory chow for 4 h evokes a significant increase of apo AIV mRNA in jejunum but not in hypothalamus. However, lipid refeeding to the fasted animals restored apo AIV mRNA levels both in hypothalamus and jejunum. Intracerebroventricular administration of apo AIV antiserum not only stimulated feeding, but also decreased apo AIV mRNA level in the hypothalamus. These data further confirm the central role of apo AIV in the regulation of food intake.     

The effects of probucol on lipoprotein metabolism in the rat

The effects of probucol on liver and intestinal apolipoprotein, LDL-receptor and hepatic lipase gene expression, as well as plasma lipid and apolipoprotein levels and liver lipase activity were evaluated in male rats. Administration of probucol decreased plasma triacylglycerols, without affecting plasma cholesterol. Plasma apo E and apo B concentrations increased after probucol. Since liver and intestinal apo B and apo E mRNA levels remained unchanged, this increase could be attributed to a delayed clearance by the LDL-receptor, whose mRNA levels dropped by 50% in the liver. For the HDL-apolipoproteins, only liver apo A-IV mRNA levels decreased after probucol, which was reflected by a fall of plasma apo A-IV. Neither hepatic lipase activity nor mRNA levels were significantly influenced by probucol.     

Diurnal rhythm of rat liver mRNAs encoding 3-hydroxy-3-methylglutaryl coenzyme A reductase. Correlation of functional and total mRNA levels with enzyme activity and protein

Rat liver 3-hydroxy-3-methylglutaryl coenzyme A reductase exhibits a diurnal rhythm of activity which coincides with a diurnal rhythm of reductase protein and reductase mRNA levels. This diurnal rhythm of reductase activity, polypeptide mass, and mRNA exists in rats fed a normal diet (unsupplemented rat chow) and in rats fed a diet supplemented with cholestyramine plus or minus mevinolin. Levels of reductase protein were determined by 8 M urea/sodium dodecyl sulfate-polyacrylamide gel electrophoresis and immunoblotting. Reductase mRNA was measured by in vitro translation or blot hybridization of liver RNA. Functional reductase mRNA levels in rats fed a normal diet were approximately 10-fold higher during the middle of the dark cycle than during the middle of the light cycle. Maximum induction of functional reductase mRNA was observed in rats fed cholestyramine and mevinolin. This latter level was 157-fold higher than the level measured at the diurnal low point in rats fed a normal diet. Blot hybridization of liver RNA showed two predominant mRNAs of 4.6 and 4.2 kilobase pairs and a minor species at 6.9 kilobase pairs. These mRNAs exhibited a diurnal rhythm for rats on all three diets and reached peak levels during the 12-h dark period. These data indicate that the diurnal rhythm of reductase mass and activity is closely paralleled by the level of its mRNA.     

Suppression of food intake and body weight gain by naloxone in rats

The effect of acute and chronic administration of naloxone on food acquisition and weight gain in rats was studied in 3 experiments. One injection of a sparingly-soluble salt of naloxone in slow-release vehicle markedly lowered mean food intake over that of control rats injected with the vehicle only. Mean body weight of the naloxone-injected rats was significantly lower than that of the control group for one week.Repeated evening injections (2000 h) of naloxone hydrochloride in saline tended to reduce the night-time feeding below control levels throughout the 10-day period of naloxone administration. Food intake was significantly lower in the 4- and 8-h periods after the first injection of naloxone than that on the preceding saline control night. The initial decreases were offset by increased day-time feeding so that total daily food intake was not significantly altered over the 10 days. When saline was substituted for naloxone, food intake increased.Rats given naloxone following 24 h of fasting consumed significantly less food and gained less weight during 4 h of access to food compared to those receiving saline. After a 48-h fast naloxone-treated rats also gained significantly less body weight than those given saline, but the reduction in food intake was not statistically significant. These results suggest the possibility that endorphins may have a modulating effect on feeding activity.     

CART peptide diurnal variations in blood and brain

The central role of CART peptide in feeding, drug abuse and stress has been widely researched however, CART's role in the peripheral system are less explored. CART peptide is present in a variety of peripheral tissues including sympathetic ganglion neurons, adrenal glands, gut, pancreas and blood. Studies that examined circulating CART demonstrated that the active fragment with a molecular weight of CART55-102 is present in the blood of rats and rhesus macaques. Interestingly, CART expression in these species exhibits a distinctive diurnal rhythm which correlates with the respective daily rhythms of corticosterone and feeding. In the rat, adrenalectomy significantly reduces blood CART levels and abolishes its daily rhythm while corticosterone replacement reinstates CART expression to control levels. In addition, direct administration of corticosterone significantly increases CART blood levels while administration of corticosterone synthesis blocker metyrapone, inhibits CART blood levels. These data suggest that the adrenal gland could be a source of blood CART and that glucocorticoids may play a role in the generation of CART's diurnal rhythm. Moreover, fuel availability may be important in the control of CART levels and its daily rhythm, since 24 h food restriction alters CART levels and abolishes its rhythm. In addition to blood, both CART peptide and mRNA exhibit food-dependent diurnal rhythm in discrete rat brain areas including the nucleus accumbens, amygdala and hypothalamus. Altogether, these findings suggest that CART is influenced by hypothalamic-pituitary-adrenal interactions and that it may play a role in multiple physiological processes possibly involving feeding, stress, reward and motivation.