Initial cholesterol uptake by everted sacs of rat small intestine: kinetic and thermodynamic aspects

Lipid-lowering effects of Coffea arabica pulp aqueous extract in Caco-2 cells and hypercholesterolemic rats

BACKGROUND

Coffea arabica pulp (CP) is the first by-product obtained from coffee berries during coffee processing. The major constituents of CP, including chlorogenic acid, caffeine, and epicatechin exhibit anti-hyperlipidemic effects in in vitro and in vivo models. Whether Coffea arabica pulp aqueous extract (CPE) has a lipid-lowering effect remains unknown.

PURPOSE

This study examined the effect of CPE on cholesterol absorption, and identified the mechanisms involved in lowered cholesterol in in vitro and in vivo models.

METHODS

Uptake of [³H]-cholesterol micelles and the mode of CPE inhibition were determined using human intestinal Caco-2 cells, and subsequently, confirmed using isolated rat jejunal loops. In addition, the 12-week high-fat diet-induced hypercholesterolemic rats (HF) received either CPE (1000  mg/kg BW), a sole and high dose which was selected because it contained approximately 12  mg of CGA that was previously shown to have lipid-lowering effects, or ezetimibe (10  mg/kg BW), a cholesterol inhibitor. The rats were divided into HF, HF  ++ CPE, and HF  ++ ezetimibe groups for the next 12 weeks. Normal rats received a normal diet (ND) and CPE (ND  +  CPE). Body weights and lipid profiles were evaluated. Cholesterol transporter, Niemann-Pick C1-Like 1 (NPC1L1), protein expression and liver X receptor alpha (LXRα) mRNA expression were determined. In vitro micellar complex properties were also investigated.

RESULTS

CPE inhibited [³H]-cholesterol micelle transport in Caco-2 cells and rat jejunal loops in a dose-dependent, non-competitive manner partly by decreasing membrane NPC1L1 expression. Congruently, CPE and its major constituents activated LXRα which, in turn, down-regulated NPC1L1. Furthermore, CPE interfered with physicochemical characteristics of cholesterol mixed micelles. These data were consistent with decreased body weight and slowed body weight gain and improved lipid profiles by CPE in hypercholesterolemic rats while no change occurred in these parameters in normal rats. Down-regulated intestinal NPC1L1 expression mediated by increased LXRα mRNA were also observed in HF  ++ CPE and ND  +  CPE rats.

CONCLUSION

CPE has a cholesterol-lowering effect in in vitro and in vivo via inhibition of intestinal cholesterol absorption by down-regulating NPC1L1 mediated LXRα activation and interfering with micellar complex formation. Accordingly, CPE could be developed as nutraceutical product to prevent dyslipidemia-induced obesity and insulin resistance.

Effect of oat gum on the physical properties of the gastrointestinal contents and on the uptake of D-galactose and cholesterol by rat small intestine in vitro

Recent reports indicate that oats have a relatively low glycaemic effect in comparison with other carbohydrate food, and that their consumption leads to a reduction in plasma-cholesterol levels in man. These properties may be due to a soluble non-starch polysaccharide in oats. The present study was undertaken to explore the physiological properties of this material. Three groups of male Wistar rats were meal-fed on a control diet free of soluble dietary fibre for 10 d before being given a 10 g meal of either the control diet, a diet containing oat gum (beta-glucan), or finely ground rolled oats. The contents of the stomach, small intestine and caecum were later recovered and the weight, water content and viscosity were measured. The small intestinal contents from oat-gum-fed or oat-fed rats had a higher wet: dry weight ratio than that of the controls, and a higher viscosity. In in vitro studies the rate of uptake of D-galactose by jejunal rings was reduced in the presence of oat gum. The estimated Michaelis-Menten constant for the carrier-mediated component in the presence of oat gum was higher than that for controls, but the maximum transport rates were similar. Cholesterol uptake by everted jejunal sacs was progressively inhibited by increasing concentrations of oat gum in the mucosal medium. It is concluded that increased viscosity of the contents of the small intestine may contribute to the low glycaemic index and hypocholesterolaemic effects of oats in man. Oats appear to be amongst the few palatable sources of viscous dietary fibre in the conventional Western diet.

Characteristics of cholesterol absorption by human gall bladder: relevance to cholesterolosis

The characteristics of cholesterol uptake by 83 human gall bladders (obtained at cholecystectomy) were studied with a modified Ussing technique. Real and artificial biles labelled with 14C-cholesterol and 3H-dextran (the latter to correct for adherent mucosal bile) were used; all gall bladders absorbed cholesterol (average 3.5 nmol/cm2/minute). Recovery of the absorbed cholesterol from the tissue showed that about 4% was esterified over 60 minutes. In artificial bile the rate of absorption of cholesterol increased as the bile saturation index rose, but became constant once supersaturation was achieved. In contrast, supersaturated real bile permitted greater absorption of cholesterol, possibly due to enhanced cholesterol solubilisation. Preincubation of gall bladder tissue in sodium cyanide (5 mM) caused a 30% reduction in cholesterol uptake indicating that, although absorption is predominantly a "passive" process, there is a partial "active" component. There were no pronounced differences in the rate of cholesterol absorption as gall bladders became more diseased, but there was a reduction in the amount of cholesterol ester formed.

The influence of guar gum on intestinal cholesterol transport in the rat

Everted sacs of rat proximal small intestine were used to determine the effect of guar gum (5 g/l) on the uptake of cholesterol (0.1 mM) from a solution of micelles. The uptake of cholesterol was found to be linear both in the presence and absence of guar gum. When guar was present throughout the whole of the incubation medium, the uptake of cholesterol was reduced to approximately 40% of control values. Sacs which had been pre-incubated in guar gum before exposure to cholesterol in a guar-free medium also showed a reduction in cholesterol uptake but this was less pronounced. A two-stage perfusion technique, previously described (Blackburn & Johnson, 1981), was used to determine the effect of a guar layer adsorbed to the mucosal surface on cholesterol absorption in vivo. Such a layer leads to a reduction of approximately 36%; it was concluded that guar slows the absorption of cholesterol from micelles by a mechanism, or mechanisms, involving an increased resistance to diffusion in the aqueous medium. Groups of rats were meal-fed for at least 30 d on semi-synthetic diets with or without the inclusion of guar gum (20 g/kg). Rates of intestinal absorption of cholesterol, glucose and fluid were then determined by the perfusion technique in vivo. There was no reduction in absorption in the test animals compared with the controls. It is proposed that guar gum is able to slow the intestinal transport of cholesterol from a suspension of pre-formed micelles, but only when both are present in the lumen together. No evidence was obtained to suggest that the consumption by rats of a diet containing guar gum, at a level similar to that used in human studies, leads to any adaptive reduction in their rates of cholesterol or glucose absorption.

Some aspects of mechanism of inhibition of cholesterol absorption by beta-sitosterol

Mixed bile salt micelle solubilized either cholesterol or beta-sitosterol to a comparable extent. When added simultaneously, beta-sitosterol restricted the micellar solubility of cholesterol. beta-Sitosterol also reduced the cholesterol content in the aqueous (micellar) phase of the intestinal contents of rats, the extent of reduction being comparable with that observed in vitro. The intestinal uptake of cholesterol in vivo was equivalent to the micellar incorporation of cholesterol both in vitro and in vivo. beta-Sitosterol had no inhibitory effect on cholesterol absorption from the micellar solution in jejunal loops in situ, whereas the rate of beta-sitosterol uptake was only about one-fifth that of cholesterol. The intestinal uptake of beta-sitosterol intubated into the stomach of rats was about one-fifth that of cholesterol. The intestinal brush-border membrane discriminated these sterols. These results suggest that the restriction of the micellar solubility of cholesterol, rather than the inhibition of uptake from brush-border membrane, is the major determinant for the interference of beta-sitosterol with cholesterol absorption.

Effect of beta-sitostanol (5 alpha-stigmastan-3 beta-ol) on cholesterol absorption from micellar solutions in jejunal loops in situ

In situ jejunal loops were infused with micellar solutions of cholesterol with or without beta-sitostanol (5 alpha-stigmastan-3 beta-ol), and the uptake of 14C-cholesterol by the loop was followed for 20 minutes. It was found that beta-sitostanol, given as a 'solution-mix' (a solution resulting from the mixture of two separate micellar solutions of cholesterol and beta-sitostanol), at a concentration of 0.30 mM reduced cholesterol uptake. Substituting cholesterol for beta-sitostanol in the 'solution-mix' had no effect on cholesterol uptake by the loop. beta-Sitostanol at a concentration of 0.30 mM in the 'pre-mix' (a solution resulting from pre-mixing of the two sterols prior to preparation of the micellar solution) condition, had no effect on cholesterol absorption. Taken together, these results suggest that the concentration of beta-sitostanol-containing micelles is the important factor in its suppression of cholesterol absorption.