Effects Of Cereal Dietary Fiber On Cholesterol Metabolism Of Intestine In C57BL/6J Mice Fed By A High-fat/cholesterol Diet

Cardiovascular disease(CVD) has become the leading cause of death worldwide.Amount of death from cardiovascular diseases is more than death from other diseases.Elevated low density lipoprotein cholesterol(LDL-c) is a major risk factor for cardiovascular disease. Therefore, reducing serum cholesterol levels is the main methods to prevent CVD. Meta analysis proved that, a dietary fiber intake of 10 g/day reduces mortality of CVD by an additional 9%. It indicated that dietary fiber have an important role on prevention and control of CVD. Recently, a large number of epidemiological studies have shown that, cereal fiber was more effective in inducing the risk of chronic metabolic disease than other fibers. Previously, research on the biological effect of dietary fiber mainly concentrated in the liver, while intestine plays an important role in regulating lipid balance. Intestine have related enzymes involved in lpid metabolism regulation, but research of cereal dietary fiber in regulation cholesterol metabolism in the intestine is limited.ObjectiveTo explore the effects and mechanism of cereal dietary fiber on cholesterol absorption and metabolism of intestine in C57BL/6J mice fed a high-fat/cholesterol diet.MethodsAfter one week acclimatization to the laboratory conditions, seven-week-old C57BL/6J mice(N=48) were weighted and randomly divided into 4 groups: reference chow(RC) diet group, high fat/choleserol(HFC) diet group, HFC plus oatwell fiber(Oat Well) group and HFC plus wheat bran fiber(Wheat). The experiment lasted for24 weeks. The animals were given a reference chow diet, or a high fat/choleserol(HFC)diet supplemented with or without oatwell fiber or wheat bran fiber. Body weights and average food intakes were routinely recorded weekly. At the end of the experiment,overnight fasted animals were killed after collecting the blood sample. Serum lipids,glucose, insulin, free fatty acids(FFAs) and adipocytokine were measured, as well as liver homogenate TG and FFAs. Histological examination of the liver and intestinal tissue samples were carried out with HE staining method. The protein expressions involved in lipid metabolism in liver and intestine tissues were detected by western blot,which included sterol regulatory element binding protein 2(SBEBP-2), low-density lipoproteins receptor(LDLR), SREBP-1, fatty acid synthase(FAS), acetyl-co A carboxylase(ACC), GTP binding protein Sar1 B, niemann pick C 1 like 1(NPC1L1),scavenger receptor B1(SR-B1), ATP-binding cassette A1(ABCA1), ABCG1, ABCG8,peroxisome proliferator activatived receptors α( PPAR-α) and liver X receptor(LXR-α), were detected by western blot in the liver or intestine tissues.Results1.Effects of cereal dietary fiber on glucose and lipid metabolism and histological analysis of the liver :(1) Throughout the experimental period, animals in Wheat diet group showed a considerably higher food intake than animals in the two groups of HFC and Oat Well diet groups. There were no significant differences on average energy intake in the three group of RC, HFC and Oat Well diet groups, by contrast, Wheat diet group exhibited a considerably higher energy intake than the three diet groups during the whole experiment.(2)After 24 weeks of feeding, compared with HFC group, two dietary fiber groups significantly decreased body weight, serum TC、TG、FFAs and LDL-c concentrations. Oat Well diet-fed mice showed significantly lower TC concentration than Wheat diet-fed mice.(3) Compared with HFC group, dietary fiber significantly reduced the blood glucose, fasting insulin, HOMA-IR index and leptin,IL-6.(4)With abundant and large lipid droplets in the cytoplasm were observed in the livers of HFC diet-fed mice, the liver cell appeared serious fatty degeneration.While the mice were fed with cereal dietary fiber for 24 weeks, liver cell degeneration was lessened.2.Effects of dietary fiber on lipid metabolism and cholesterol metabolism of theliver(1) Liver homogenate TG and FFAs concentrations were significantly inhibited in Oat Well and Wheat diet-fed mice after 24 weeks of feeding, compared with those in HFC diet-fed mice. Furthermore, a significantly lower liver homogenate TG was observed in the OatWell diet group than that in the Wheat diet group.(2) Protein expressions of SREBP-1, FAS, and ACC in the liver were significantly reduced in the two cereal dietary fiber diet groups than those in the HFC diet group. While protein expressions of SREBP-2 and LDLR in the liver were significantly increased in the two cereal dietary fiber diet groups than those in the HFC diet group.3. Effects of cereal dietary fiber on cholesterol metabolism in the intestine tissues:(1) Histological analysis of the intestine tissue: The intestinal villus became short evidently and fall off severely, and the edema of lamina propria edema was thickened in the HFC diet group. Compared with mice from HFC group, the intestinal villus length increased lightly in mice from Oat Well and Wheat fiber group, and the boundaries between epithelium and lamina propria was clear, as well as the degree of edema was mitigated. And the effect oat fiber was more significant.(2) Protein expressions involved in cholesterol metabolism: Effects of cereal dietary fiber on the protein expressions involved in cholesterol metabolism largely varied in different part of intestine,especially in the middle part of intestine. Compared with HFC group, oat fiber and wheat bran fiber significantly increased protein expression of PPAR-α、LXR-α、SREBP-2 、 LDLR 、 ABCA1 and ABCG1 after 24 weeks of feeding. While protein expression of NPC1L1、SREBP-1、FAS and ACC were significantly reduced in Oat Well and Wheat diet groups. The variation in protein expressions in the Oat Well diet group was more notable than that in the Wheat diet group.Besides, protein expression of Sar1 B was upregulated in mice from OatWell diet group. Cereal dietary fiber have no influence on protein expression of ABCG8 and SR-B1. Oat fiber was more notable on cholesterol absorption and metabolism in the tissue than wheat bran fiber.Conclusion:1. Cereal dietary fiber can reduce the body weight and blood lipids, as well as insulin resistance and release of adipocytokines, and improve liver steatosis in C57BL/6J mice fed a high-fat/cholesterol diet. Wheat bran fiber was more effective inweight gain than oat dietary fiber. Oat fiber was more effective in reducing serum TC than the wheat bran fiber.2. Cereal dietary fiber can alleviate hepatic lipid accumulation. Oat fiber exhibited better effect in reducing liver TG accumulation than wheat bran fiber. Cereal dietary fiber can increase liver cholesterol removal and improve lipid metabolism disorders by raising the protein expression of SREBP- 2 and LDLR, reducing the protein expression involved in fatty acid synthesis, including SREBP-1 and FAS, ACC.3. Cereal dietary fiber can relieve injury of intestinal cell caused by a high fat/cholesterol diet. Mechanism of cereal dietary fiber on lowering blood lipids may include three pathways:(1)Cereal dietary fiber promoted the intestinal cholesterol efflux through activating LXR-α and then increasing the protein expression of ABCA1 and ABCG1;(2) Cereal dietary fiber increased the intestinal cholesterol removal by upregulating SREBP-2 and then increasing the protein expression of LDLR;(3) Cereal dietary fiber reduced fatty acid synthesis by downregulating SREBP-1 and then decreasing the protein expression of FAS and ACC. In addition, oat fiber can reduce intestinal cholesterol absorption by raising Sar1 B and lowering NPC1L1 expression. We speculated that oat fiber was more effective in improving cholesterol absorption and metabolism than wheat bran fiber.