Short-chain fatty acids (SCFAs) are the main products of dietary fiber fermentation and are believed to drive the fiber-related prevention of the metabolic syndrome. Here we show that dietary SCFAs induce a peroxisome proliferator–activated receptor-γ (PPARγ)–dependent switch from lipid synthesis to utilization. Dietary SCFA supplementation prevented and reversed high-fat diet–induced metabolic abnormalities in mice by decreasing PPARγ expression and activity. This increased the expression of mitochondrial uncoupling protein 2 and raised the AMP-to-ATP ratio, thereby stimulating oxidative metabolism in liver and adipose tissue via AMPK. The SCFA-induced reduction in body weight and stimulation of insulin sensitivity were absent in mice with adipose-specific disruption of PPARγ. Similarly, SCFA-induced reduction of hepatic steatosis was absent in mice lacking hepatic PPARγ. These results demonstrate that adipose and hepatic PPARγ are critical mediators of the beneficial effects of SCFAs on the metabolic syndrome, with clearly distinct and complementary roles. Our findings indicate that SCFAs may be used therapeutically as cheap and selective PPARγ modulators.

Acetate, propionate, and butyrate are the main short-chain fatty acids (SCFAs) that arise from the fermentation of fibers by the colonic microbiota. While many studies focus on the regulatory role of SCFAs, their quantitative role as a catabolic or anabolic substrate for the host has received relatively little attention. To investigate this aspect, we infused conscious mice with physiological quantities of stable isotopes [1-(13)C]acetate, [2-(13)C]propionate, or [2,4-(13)C2]butyrate directly in the cecum, which is the natural production site in mice, and analyzed their interconversion by the microbiota as well as their metabolism by the host. Cecal interconversion, pointing to microbial cross-feeding, was high between acetate and butyrate, low between butyrate and propionate, and almost absent between acetate and propionate. As much as 62% of infused propionate was used in whole body glucose production, in line with its role as gluconeogenic substrate. Conversely, glucose synthesis from propionate accounted for 69% of total glucose production. The synthesis of palmitate and cholesterol in the liver was high from cecal acetate (2.8 and 0.7%, respectively) and butyrate (2.7 and 0.9%, respectively) as substrates, but low or absent from propionate (0.6 and 0.0%, respectively). Label incorporation due to chain elongation of stearate was approximately eightfold higher than de novo synthesis of stearate. Microarray data suggested that SCFAs exert a mild regulatory effect on the expression of genes involved in hepatic metabolic pathways during the 6-h infusion period. Altogether, gut-derived acetate, propionate, and butyrate play important roles as substrates for glucose, cholesterol, and lipid metabolism.

FGF1 is an autocrine/paracrine regulator whose binding to heparan sulfate proteoglycans effectively precludes its circulation 1,2. Though known as a mitogenic factor, FGF1 knockout mice develop insulin resistance when stressed by a high fat diet, suggesting a potential role in nutrient homeostasis 3,4. Here we show that parenteral delivery of a single dose of recombinant FGF1 (rFGF1) results in potent, insulin-dependent glucose lowering in diabetic mice that is dose-dependent, but does not lead to hypoglycemia. Chronic pharmacological rFGF1 treatment increases insulin-dependent glucose uptake in skeletal muscle and suppresses hepatic glucose production to achieve whole-body insulin sensitization. The sustained glucose lowering and insulin sensitization attributed to rFGF1 are not accompanied by the side effects of weight gain, liver steatosis and bone loss associated with current insulin sensitizing therapies. Furthermore, we demonstrate that the glucose lowering activity of FGF1 can be dissociated from its mitogenic activity and is mediated predominantly via FGF receptor 1 (FGFR1) signaling. In summary, we have uncovered an unexpected, neomorphic insulin sensitizing action for exogenous non-mitogenic human FGF1 with therapeutic potential for treatment of insulin resistance and type 2 diabetes.

Recent studies have indicated that direct intestinal secretion of plasma cholesterol significantly contributes to fecal neutral sterol loss in mice. The physiological relevance of this novel route, which represents a part of the reverse cholesterol transport pathway, has not been directly established in vivo as yet. We have developed a method to quantify the fractional and absolute contributions of several cholesterol fluxes to total fecal neutral sterol loss in vivo in mice, by assessing the kinetics of orally and intravenously administered stable isotopically labeled cholesterol combined with an isotopic approach to assess the fate of de novo synthesized cholesterol. Our results show that trans-intestinal cholesterol excretion significantly contributes to removal of blood-derived free cholesterol in C57Bl6/J mice (33% of 231 mol/kg/day) and that pharmacological activation of LXR with T0901317 strongly stimulates this pathway (63% of 706 mol/ kg/day). Trans-intestinal cholesterol excretion is impaired in mice lacking Abcg5 (؊4%), suggesting that the cholesterol transporting Abcg5/Abcg8 heterodimer is involved in this pathway. Our data demonstrate that intestinal excretion represents a quantitatively important route for fecal removal of neutral sterols independent of biliary secretion in mice. This pathway is sensitive to pharmacological activation of the LXR system. These data support the concept that the intestine substantially contributes to reverse cholesterol transport. Reverse cholesterol transport (RCT)3 is defined as the flux of excess cholesterol from peripheral tissues toward the liver followed by biliary secretion and subsequent disposal via the feces (1). Accumulation of cholesterol in macrophages in the vessel wall is considered a primary event in the development of atherosclerosis and, therefore, removal of excess cholesterol from these cells is of crucial importance for prevention and/or treatment of atherosclerotic cardiovascular diseases. It is generally accepted that HDL is the obligate transport vehicle in RCT and that plasma HDL levels reflect the capacity to accommodate this flux. In line herewith, HDL-raising therapies are currently considered as a promising strategy for prevention and treatment of atherosclerotic cardiovascular diseases (2). In the "classical" scenario, the liver has a central role in RCT (3). Biliary secretion of free cholesterol, facilitated by the heterodimeric ABC-transporter ABCG5/ABCG8 (4), and hepatic conversion of cholesterol into bile acids followed by fecal excretion are referred to as the main routes for quantitatively important elimination of cholesterol from the body. Fecal excretion of sterols is stimulated upon whole body activation of the liver X receptor (LXR, NR1H2/3), a member of the nuclear receptor family for which oxysterols have been identified as natural ligands (5). LXR regulates expression of several genes involved in RCT and activation of LXR by synthetic agonists leads to elevated plasma HDL-cholesterol levels, increased hepatobiliary cholestero...

Ursodeoxycholic acid (UDCA) is used in the treatment of cholestatic liver diseases, but its mechanism of action is not yet well defined. The aim of this study was to explore the protective mechanisms of the taurine-conjugate of UDCA (tauroursodeoxycholic acid [TUDCA]) against glycochenodeoxycholic acid (GCDCA)-induced apoptosis in primary cultures of rat hepatocytes. Hepatocytes were exposed to GCDCA, TUDCA, the glyco-conjugate of UDCA (GUDCA), and TCDCA. The phosphatidylinositol-3 kinase pathway (PI3K) and nuclear factor-B were inhibited using LY 294002 and adenoviral overexpression of dominant-negative I B, respectively. The role of p38 and extracellular signalregulated protein kinase mitogen-activated protein kinase (MAPK) pathways were investigated using the inhibitors SB 203580 and U0 126 and Western blot analysis. Transcription was blocked by actinomycin-D. Apoptosis was determined by measuring caspase-3, -9, and -8 activity using fluorimetric enzyme detection, Western blot analysis, immunocytochemistry, and nuclear morphological analysis. Our results demonstrated that uptake of GCDCA is needed for apoptosis induction. TUDCA, but not TCDCA and GUDCA, rapidly inhibited, but did not delay, apoptosis at all time points tested. However, the protective effect of TUDCA was independent of its inhibition of caspase-8. Up to 6 hours of preincubation with TUDCA before addition of GCDCA clearly decreased GCDCAinduced apoptosis. At up to 1.5 hours after exposure with GCDCA, the addition of TUDCA was still protective. This protection was dependent on activation of p38, ERK MAPK, and PI3K pathways, but independent of competition on the cell membrane, NF-B activation, and transcription. In conclusion, TUDCA contributes to the protection against GCDCA-induced mitochondria-controlled apoptosis by activating survival pathways. Supplemental material for this article can be found on the HEPATOLOGY website (http://interscience.wiley.com/jpages/0270-9139/supplmat/index.html). (HEPATOLOGY 2004;39:1563-1573

The ATP-binding cassette transporter ABCA1 is essential for high density lipoprotein (HDL) formation and considered rate-controlling for reverse cholesterol transport. Expression of the Abca1 gene is under control of the liver X receptor (LXR). We have evaluated effects of LXR activation by the synthetic agonist T0901317 on hepatic and intestinal cholesterol metabolism in C57BL/6J and DBA/1 wild-type mice and in ABCA1-deficient DBA/1 mice. In wild-type mice, T0901317 increased expression of Abca1 in liver and intestine, which was associated with a ϳ60% rise in HDL. Biliary cholesterol excretion rose 2.7-fold upon treatment, and fecal neutral sterol output was increased by 150 -300%. Plasma cholesterol levels also increased in treated Abca1 ؊/؊ mice (؉120%), but exclusively in very low density lipoproteinsized fractions. Despite the absence of HDL, hepatobiliary cholesterol output was stimulated upon LXR activation in Abca1 ؊/؊ mice, leading to a 250% increase in the biliary cholesterol/phospholipid ratio. Most importantly, fecal neutral sterol loss was induced to a similar extent (؉300%) by the LXR agonist in DBA/1 wild-type and Abca1 ؊/؊ mice. Expression of Abcg5 and Abcg8, recently implicated in biliary excretion of cholesterol and its intestinal absorption, was induced in T0901317-treated mice. Thus, activation of LXR in mice leads to enhanced hepatobiliary cholesterol secretion and fecal neutral sterol loss independent of (ABCA1-mediated) elevation of HDL and the presence of ABCA1 in liver and intestine. Reverse cholesterol transport (RCT)1 or centripetal cholesterol flux is a key process in maintenance of whole body cholesterol homeostasis (1-6). RCT involves efflux of excess cholesterol from peripheral cells toward nascent high density lipoprotein (HDL) and its transport to the liver, followed by hepatic uptake mediated by scavenger receptor class B type I (SR-BI), biliary secretion in the form of cholesterol or bile salt, and finally disposal into feces. HDL-mediated RCT is generally assumed to underlie the well known epidemiological relationship between high HDL cholesterol levels and low risk for development of atherosclerosis.Efflux of cholesterol from peripheral cells, including macrophages in the vessel wall, is now known to be mediated in part by the ATP-binding cassette transporter ABCA1 (7-10). Abca1 mRNA is widely distributed throughout the body, with high expression levels in macrophages, hepatocytes, and enterocytes (11,12). This distribution pattern has recently been confirmed for the ABCA1 protein (13). The role of ABCA1 in hepatocytes is currently unknown, but may involve formation of pre-␤-HDL particles (14). In the intestine, ABCA1 has been suggested to be involved in cholesterol efflux from enterocytes into the lumen, thereby regulating the efficiency of intestinal cholesterol absorption (15, 16).HDL is considered a major source for bile-destined cholesterol and phospholipid (17,18). Yet, we have recently demonstrated that, despite the absence of HDL, hepatobiliary cholesterol flux and fe...

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To explore mechanisms underlying triglyceride (TG) accumulation in livers of chow-fed apo E-deficient mice (Kuipers, F., J.M. van Ree, M.H. Hofker, H. Wolters, G. In't Veld, R.J. Vonk, H.M.G. Princen, and L.M. Havekes. 1996. Hepatology. 24:241-247), we investigated the effects of apo E deficiency on secretion of VLDL-associated TG (a) in vivo in mice, (b) in isolated perfused mouse livers, and (c) in cultured mouse hepatocytes. (a) Hepatic VLDL-TG production rate in vivo, determined after Triton WR1339 injection, was reduced by 46% in apo E-deficient mice compared with controls. To eliminate the possibility that impaired VLDL secretion is caused by aspecific changes in hepatic function due to hypercholesterolemia, VLDL-TG production rates were also measured in apo E-deficient mice after transplantation of wild-type mouse bone marrow. Bone marrow- transplanted apo E-deficient mice, which do not express apo E in hepatocytes, showed normalized plasma cholesterol levels, but VLDL-TG production was reduced by 59%. (b) VLDL-TG production by isolated perfused livers from apo E-deficient mice was 50% lower than production by livers from control mice. Lipid composition of nascent VLDL particles isolated from the perfusate was similar for both groups. (c) Mass VLDL-TG secretion by cultured apo E-deficient hepatocytes was reduced by 23% compared with control values in serum-free medium, and by 61% in the presence of oleate in medium (0. 75 mM) to stimulate lipogenesis. Electron microscopic evaluation revealed a smaller average size for VLDL particles produced by apo E-deficient cells compared with control cells in the presence of oleate (38 and 49 nm, respectively). In short-term labeling studies, apo E-deficient and control cells showed a similar time-dependent accumulation of [3H]TG formed from [3H]glycerol, yet secretion of newly synthesized VLDL-associated [3H]TG by apo E-deficient cells was reduced by 60 and 73% in the absence and presence of oleate, respectively. We conclude that apo E, in addition to its role in lipoprotein clearance, has a physiological function in the VLDL assembly-secretion cascade.