Origins of fecal neutral steroids in rats

Frozen Autoclaved Sorghum Enhanced Colonic Fermentation and Lower Visceral Fat Accumulation in Rats

As raw sorghum is not able to influence considerable colonic fermentation despite its higher resistant starch (RS) content, our study aimed to investigate the effects of frozen autoclaved sorghum on colonic fermentation. Fischer 344 rats were fed frozen cooked refined (S-Rf) and whole (S-Wh) sorghum diets and were compared against α-corn starch (CON) and high amylose starch (HAS) fed rats for zoometric parameters, cecal biochemical and microbiological parameters. Sorghum fed rats exhibited significantly lower feed intake and visceral adipose tissue mass compared to CON. Bacterial alpha diversity was significantly higher in the sorghum fed rats compared to HAS and the two sorghum fed groups clustered together, separately from HAS and CON in the beta diversity plot. Serum non-High Density Lipoprotein cholesterol and total cholesterol in S-Rf group were significantly lower compared to CON, while total fecal bile excretion was also significantly higher in the two sorghum fed groups. Lower visceral adiposity was correlated with lower feed intake, RS content ingested and cecal short chain fatty acid (SCFA) contents. Thus, higher RS inflow to the colon via frozen autoclaved sorghum might have influenced colonic fermentation of RS and the resultant SCFA might have influenced lower adiposity as manifested by the lower body weight gain.

Rethinking reverse cholesterol transport and dysfunctional high-density lipoproteins

Human plasma high-density lipoprotein cholesterol concentrations are a negative risk factor for atherosclerosis-linked cardiovascular disease. Pharmacological attempts to reduce atherosclerotic cardiovascular disease by increasing plasma high-density lipoprotein cholesterol have been disappointing so that recent research has shifted from HDL quantity to HDL quality, that is, functional vs dysfunctional HDL. HDL has varying degrees of dysfunction reflected in impaired reverse cholesterol transport (RCT). In the context of atheroprotection, RCT occurs by 2 mechanisms: one is the well-known trans-hepatic pathway comprising macrophage free cholesterol (FC) efflux, which produces early forms of FC-rich nascent HDL (nHDL). Lecithin:cholesterol acyltransferase converts HDL-FC to HDL-cholesteryl ester while converting nHDL from a disc to a mature spherical HDL, which transfers its cholesteryl ester to the hepatic HDL receptor, scavenger receptor B1 for uptake, conversion to bile salts, or transfer to the intestine for excretion. Although widely cited, current evidence suggests that this is a minor pathway and that most HDL-FC and nHDL-FC rapidly transfer directly to the liver independent of lecithin:cholesterol acyltransferase activity. A small fraction of plasma HDL-FC enters the trans-intestinal efflux pathway comprising direct FC transfer to the intestine. SR-B1^-/- mice, which have impaired trans-hepatic FC transport, are characterized by high plasma levels of a dysfunctional FC-rich HDL that increases plasma FC bioavailability in a way that produces whole-body hypercholesterolemia and multiple pathologies. The design of future therapeutic strategies to improve RCT will have to be formulated in the context of these dual RCT mechanisms and the role of FC bioavailability.

Natural products as modulators of the nuclear receptors and metabolic sensors LXR, FXR and RXR

Nuclear receptors (NRs) represent attractive targets for the treatment of metabolic syndrome-related diseases. In addition, natural products are an interesting pool of potential ligands since they have been refined under evolutionary pressure to interact with proteins or other biological targets. This review aims to briefly summarize current basic knowledge regarding the liver X (LXR) and farnesoid X receptors (FXR) that form permissive heterodimers with retinoid X receptors (RXR). Natural product-based ligands for these receptors are summarized and the potential of LXR, FXR and RXR as targets in precision medicine is discussed.

Cross-talk between liver and intestine in control of cholesterol and energy homeostasis

A major hurdle for organisms to dispose of cholesterol is the inability to degrade the sterol nucleus which constitutes the central part of the molecule. Synthesis of the sterol nucleus requires a complex, energy costly, metabolic pathway but also generates a diverse array of intermediates serving crucial roles in cellular energy metabolism and signal transduction. This may be the reason why this complex pathway has survived evolutionary pressure. The only way to get rid of substantial amounts of cholesterol is conversion into bile acid or direct excretion of the sterol in the feces. The lack of versatility in disposal mechanisms causes a lack of flexibility to regulate cholesterol homeostasis which may underlie the considerable human pathology linked to cholesterol removal from the body. Export of cholesterol from the body requires an intricate communication between intestine and the liver. The last decade this inter-organ cross talk has been focus of intense research leading to considerable new insight. This novel information on particular the cross-talk between liver and intestine and role of bile acids as signal transducing molecules forms the focus of this review.

Transintestinal cholesterol efflux

PURPOSE OF REVIEW

Regulation of cholesterol homeostasis is a complex interplay of a multitude of metabolic pathways situated in different organs. The liver plays a central role and has received most attention of the research community. In this review, we discuss recent progress in the understanding of the emerging role of the intestine in cholesterol transport.

RECENT FINDINGS

In recent years, insight in the transport systems that mediate intestinal cholesterol excretion has deepened considerably. Evidence is emerging that the proximal part of the small intestine is able to secrete cholesterol actively, a pathway called transintestinal cholesterol efflux (TICE). In mice, TICE accounts for up to 70% of fecal neutral sterol excretion.

SUMMARY

The small intestine plays a significant role in the regulation of body cholesterol homeostasis. Active processes control both absorption and excretion of the sterol and the pathways involved are being elucidated. TICE might provide an attractive target for therapy aiming at reduction of atherosclerosis.

Changes in endogenous lipid excretion in rats fed diets containing non-heated and thermally oxidized olive oils

The objective of this study was to determine the effects of diets containing non-heated and thermally oxidized olive oils on fecal endogenous lipids. Male Wistar rats were fed fat-free diets and diets supplemented with 12% non-heated, heated, and a 1:1 mixture of non-heated/heated olive oils. After a 15-day experimental period two groups of fecal lipids from major endogenous sources were quantitated: neutral sterols and fatty acids associated with intestinal microflora action. Fecal endogenous sterols, particularly cholesterol, were significantly higher when diets contained oil, and excretion increased as the dietary oil alteration increased. Similar results were obtained for endogenous fatty acids. Increments of fecal sterols, dependent on oil alteration, could be explained by impairments in triglyceride hydrolysis and subsequent effect on cholesterol micellar solubilization. Moreover, high concentrations of poorly digestible lipids may have led to intestinal microbial modifications.

Cholesterol metabolism by Treponema hyodysenteriae

The sterol content of cellular lipids of Treponema hyodysenteriae, the agent of swine dysentery, was determined. When cultured in lipid-depleted brain heart infusion broth containing vesicles made from [4-14C]cholesterol and phosphatidylcholine, T. hyodysenteriae cells incorporated radioactive label. Most (95%) of this radioactivity was associated with bacterial membrane preparations. Lipids were extracted from radiolabeled cells and fractionated by silicic acid column chromatography. Components of the neutral lipid fraction were separated by reversed-phase high-performance liquid chromatography and were detected by monitoring both radioactivity and UV absorption (210 nm) of the column effluent. Cholesterol represented only about 5% of the total radioactivity in the bacterial neutral lipids. The remaining radioactivity was associated with a compound that did not absorb light at 210 nm. This lipid was purified and, on the basis of results from thin-layer chromatography and mass spectrometry, was identified as cholesterol (5 alpha-cholestan-3 beta-ol), a sterol lacking the unsaturated bond of cholesterol. Cholestanol was also present in cell-free culture broth, but only after growth of the spirochete. These results are evidence that cholesterol is used by T. hyodysenteriae for membrane synthesis. Cholesterol is converted to cholestanol in T. hyodysenteriae cultures and cholestanol is a major component (approximately 9% by weight) of T. hyodysenteriae cell lipids.

Cholesterol homeostasis in lean and obese male Zucker rats

Studies were performed in male Zucker rats to determine the metabolic effect of genetic obesity on whole body cholesterol homeostasis. Lean and obese mature Zucker rats were studied during intake of either a chow diet or a semisynthetic diet containing 10% corn oil; in addition growing animals were studied during constant body weight gain on a chow diet. Under all conditions the obese Zucker rats had significantly higher levels of total plasma cholesterol and triglyceride; however, measurements of the specific activity of hepatic 3-hydroxy-3-methylglutaryl coenzyme A reductase and of the rate of whole body cholesterol synthesis by sterol balance techniques demonstrated that the lean and obese animals did not differ in their endogenous rates of cholesterol synthesis. When sterol balance data were calculated per kilogram body weight, lean male Zucker rats synthesized a greater amount of cholesterol per day than obese animals. These studies demonstrate that the obese male Zucker rat, in many ways a model of human obesity, does not overproduce cholesterol and thus fails to exhibit one of major characteristics of the obese human.

Cholesterol homeostasis in the guinea pig. The importance of quantitating net tissue accumulation of cholesterol in sterol balance studies

Sterol balance measurements of whole body cholesterol synthesis were performed in guinea pigs to determine the relative quantitative importance of dietary cholesterol intake, endogenous cholesterol synthesis, fecal steroid excretion and net tissue accumulation in cholesterol homeostasis of a rapidly growing animal. Sterol inputs were from diet (33%) and endogenous synthesis (67%); sterol outputs, as fecal neutral and acidic steroids, accounted for 60% of the total input, the remainder being used for the demands of tissue growth. The data demonstrate that the measurement of total body cholesterol synthesis can be grossly underestimated in this rapidly growing animal if net tissue accumulation of cholesterol is not considered in the balance measurement.

Sterol synthesis from biliary squalene in the jejunal mucosa of the rat in vivo

Because bile contains substantial amounts of cholesterol precursors, e.g., squalene and different methyl sterols, the fate of biliary squalene was studied by incubating isolated jejunal loops of the rat in vivo with bile containing 3H-squalene and 14C-cholesterol. After 90 min, no radioactivity was found in plasma lipids. Closer analysis of gut epithelium revealed that both labeled compounds were preferentially taken up by the villous cells. Biliary 3H-squalene was absorbed almost completely and was further cyclized to free and esterified methyl sterols and cholesterol. Whereas squalene not cyclized to sterols stayed in the mucosa, the newly synthesized sterols were transferred to lumen. The lipid patterns of gut lumen and mucosal cells were quite different, suggesting that the transfer of the newly synthesized lipid to intestinal lumen was not due to the desquamation of epithelial cells alone. The results suggest that biliary cholesterol precursors can contribute to the cholesterol production of the jejunal villous cells bypassing the rate-limiting step of the cholesterol synthesis pathway, and to the "nonexchanging" fecal neutral sterols of the rat.

Modification of dimethylhydrazine-induced carcinogenesis in rats by dietary cholesterol

Diets capable of affecting serum cholesterol levels were examined for effects on tumorigenesis in male Fischer 344 rats given 1,2-dimethylhydrazine dihydrochloride (DMH). Four regimens were used: Group A was maintained on a semipurified diet; Group B was maintained on the same diet with cholesterol and bile salts added; Group C was fed the cholesterol and bile salt diet and then switched to the semipurified diet; and Group D was fed the cholesterol and bile salt diet and then placed on a semipurified diet containing bile salts. Six months after DMH administration, Group B and D exhibited similar tumor incidence (77% and 83%, respectively) and yield (2.3 and 2.4 tumors/tumor-bearing rat). These yields were significantly greater than those seen in Group A (70% incidence and 1.4 tumors/tumor-bearing rat) or Group C (52% incidence and 1.7 tumors/tumor-bearing rat). Tumors in Groups B and D were larger in size and closer to the rectum than those seen in the other groups. In each of Groups A, C, and D, approximately 26% of the tumors were malignant; in Group B, 42% were malignant. Colonic epithelial cell kinetics in rats from Groups C and D not given DMH were determined using autoradiographic study of [3H]thymidine incorporation. Group D showed enhanced cell proliferation and expansion of the crypt cell population. These results suggest that dietary bile salts increase cell turnover and frequency of tumors, and that the addition of cholesterol to a cocarcinogenic diet can be associated with a significantly higher percentage of invasive tumors.

Cholesterol homeotasis in rats fed a purified diet

The rate of whole bodyb cholesterol synthesis was measured in male Sprague-Dawley rats fed either a standard chow, cereal-based diet or a semi-synthetic purified diet consisting of casein, sucrose and lard. The purified diet significantly decreased daily fecal excretion of neutral and acidic sterols, the specific acitvity of hepatic 3-hydroxy-3-methylglutaryl coenzyme A reductase, the bile acid pool size, and total daily cholesterol synthesis in the rat, while increasing plasma cholesterol concentrations and the total body content of cholesterol. The increased body content of cholesterol occurred primarily in muscle and connective tissue and not in the liver. The data demonstrate the importance of quantitating the net tissue accumulatin of cholesterol for accurate measurement of daily sterol synthesis in growing animals when sterol balance measurements are used. Tissue accumulation accounted for 7% of total daily cholesterol synthesis in rats fed the cereal diet, and 20% of daily synthesis in animals fed the purified diet.