Metabolism and effects on biliary lipid secretion of murocholic acid in the hamster

Molecular mechanisms of hepatic fibrogenesis

Liver fibrosis, a wound-healing response to a variety of chronic stimuli, is characterized by excessive deposition of extracellular matrix (ECM) proteins, of which type I collagen predominates. This alters the structure of the liver leading to organ dysfunction. The activated hepatic stellate cell (HSC) is primarily responsible for excess collagen deposition during liver fibrosis. Two important aspects are involved in mediating the fibrogenic response: first the HSC becomes directly fibrogenic by synthesizing ECM proteins; second, the activated HSC proliferates, effectively amplifying the fibrogenic response. Although the precise mechanisms responsible for HSC activation remain elusive, substantial insight is being gained into the molecular mechanisms responsible for ECM production and cell proliferation in the HSC. The activated HSC becomes responsive to both proliferative (platelet-derived growth factor) and fibrogenic (transforming growth factor-beta[TGF-beta]) cytokines. It is becoming clear that these cytokines activate both mitogen-activated protein kinase (MAPK) signaling, involving p38, and focal adhesion kinase-phosphatidylinositol 3-kinase-Akt-p70 S6 kinase (FAK-PI3K-Akt-p70(S6K)) signaling cascades. Together, these regulate the proliferative response, activating cell cycle progression as well as collagen gene expression. In addition, signaling by both TGF-beta, mediated by Smad proteins, and p38 MAPK influence collagen gene expression. Smad and p38 MAPK signaling have been found to independently and additively regulate alpha1(I) collagen gene expression by transcriptional activation while p38 MAPK, but not Smad signaling, increases alpha1(I) collagen mRNA stability, leading to increased synthesis and deposition of type I collagen. It is anticipated that by understanding the molecular mechanisms responsible for HSC proliferation and excess ECM production new therapeutic targets will be identified for the treatment of liver fibrosis.

Cytokine regulation of hepatic stellate cells in liver fibrosis

Cytokines constitute a major class of mediators responsible for "activation" of hepatic stellate cells (HSCs) in vitro and in vivo. They are largely divided into mitogenic (transforming growth factor-alpha, platelet-derived growth factor, interleukin-1, tumor necrosis factor-alpha, and insulin-like growth factor) and fibrogenic (transforming growth factor-beta and interleukin-6) cytokines. In addition to their mitogenic (stimulation of cell proliferation) and fibrogenic (induction of matrix proteins) properties, they are also shown to confer in vitro unique cellular changes known to be the key features of HSC "activation," including loss of vitamin A, stimulation of migration, enhanced cellular contractility, and matrix metalloproteinase and tissue inhibitor of metalloproteinase induction. Potential cellular sources of the cytokines consist of hepatic macrophages, endothelial cells, biliary epithelial cells, lymphocytes, platelets, hepatocytes, and activated HSCs. To better understand the mode of actions and the pathogenetic significance of cytokines/chemokines involved in "activation" of HSCs, the following four questions need to be addressed: (1) What other cytokines are expressed by HSCs to establish critical autocrine stimulation? (2) What are endogenous or exogenous priming factors for HSC stimulation? (3) What is the mechanism of activation for transforming growth factor-beta, the pivotal fibrogenic cytokine? (4) How important are HSC-derived proinflammatory mediators in liver fibrosis? This review will discuss these questions, along with the current understanding of the role of cytokines in HSC activation.

Macrophage and hepatic stellate cell responses during experimental hepatocarcinogenesis

The aim of the study was to assess the monocyte/macrophage and hepatic stellate cell responses during experimental diethylnitrosamine (DEN)-induced hepatocarcinogenesis. Diethylnitrosamine (50mg/L) was administered to 39 rats for 10 weeks; liver tissue was obtained at weeks 10, 16 and 19. In this model, necroinflammatory damage occurs during the period of DEN administration but thereafter subsides; dysplastic nodules and carcinomas subsequently develop. Monocytes/ macrophages were detected immunohistochemically using ED1 and ED2 monoclonal antibodies; hepatic stellate cells (HSC) were detected using antibodies to alpha-smooth muscle actin (alpha-SMA) (activated HSC) and glial fibrillary acidic protein (GFAP). Parenchymal ED1- and ED2-positive monocytes/macrophages and alpha-SMA-positive HSC increased at week 10 when there was ongoing DEN-induced necroinflammatory activity. ED1- and ED2-positive cells were also prominent at weeks 16 and 19, particularly around the periphery of dysplastic and carcinomatous nodules, with occasional macrophages between dysplastic hepatocytes. alpha-SMA-positive HSC were present within sinusoids between dysplastic cells and were more abundant at weeks 16 and 19 than in control or week 10 animals. Activated HSC were prominent in fibrous septa around and within dysplastic and carcinomatous nodules at weeks 16 and 19. In contrast, GFAP-positive HSC did not accumulate in developing septa or within dysplastic and carcinomatous nodules. We have demonstrated changes in the monocyte/ macrophage and HSC populations during the development of hepatocellular dysplasia and carcinoma at time points when there is little necroinflammatory activity; this may therefore represent a host response to hepatocyte dysplasia. The HSC activation may be mediated, in part, by monocyte/ macrophage-derived factors, but we speculate that it may also result from direct stimulation by factors released from dysplastic hepatocytes.

Effects of hyodeoxycholic acid and alpha-hyocholic acid, two 6 alpha-hydroxylated bile acids, on cholesterol and bile acid metabolism in the hamster

The effects of hyodeoxycholic (HDCA) and alpha-hyocholic acids (alpha-HCA), on cholesterol, bile acid and lipoprotein metabolism, were studied in hamsters. The animals were fed a low cholesterol control diet supplemented with 0.1% HDCA or alpha-HCA for 3 weeks. In both treated groups, the LDL-cholesterol concentration was significantly lowered and was associated with a global hypocholesterolemic effect. Moreover, hepatic cholesterol ester storage was reduced and HMGCoA reductase activity was respectively enhanced 13.5-times and 7.7-times in HDCA and alpha-HCA groups compared to controls. In contrast, cholesterol 7 alpha-hydroxylase activity and LDL-receptor activity and mass were not modified. In bile, the cholesterol saturation index was increased 5-fold (HDCA group) and 2-fold (alpha-HCA group) as a consequence of an enlarged proportion of biliary cholesterol. The two 6-hydroxylated bile acids induced an enhanced fecal excretion of neutral sterols (HDCA group: 11.6-times, alpha-HCA group: 3.2-times versus controls) which was consistent with a 59% decrease in intestinal cholesterol absorption in the HDCA group. The major effects due to bile acid treatments were a decrease in LDL-cholesterol concentration, a strong stimulation of hepatic cholesterol biosynthesis and an excessive loss of cholesterol in feces. These perturbations might be the result of the enrichment of bile with hydrophilic bile acids, leading to a limited return of endogenous cholesterol from the intestine to the liver.

Molecular biological aspects of alcohol-induced liver disease

Molecular biological investigations have become a predominant methodology applied to the study of alcohol-induced liver disease. The enzymatic pathways responsible for ethanol metabolism, and their genetic as well as environmental control, have become the focus of detailed investigation. More recently, the significance of cytokines in the pathogenesis of alcohol-induced liver disease has also become a major area of speculation. This review focuses on the advances made in studies of two important enzymes responsible for alcohol metabolism, alcohol dehydrogenase and aldehyde dehydrogenase, as well as the investigation of the proinflammatory and profibrogenic cytokines involved in the process of hepatic fibrogenesis. The quality and quantity of new discoveries made in the field of alcohol-induced liver disease is impressive, especially when one realizes that molecular biological approaches have been employed in this area for only 15 years. However, in most cases the studies have been predominantly descriptive, with little direct relevance to the therapeutics of alcoholism and alcohol-induced organ injury. Because the groundwork has been laid, one hopes that the next 15 years will rectify this failure.

Acetaldehyde-induced stimulation of collagen synthesis and gene expression is dependent on conditions of cell culture: studies with rat lipocytes and fibroblasts

Acetaldehyde has been proposed as a mediator of fibrogenesis in alcoholic liver disease, based in part on its ability to stimulate collagen synthesis by hepatic lipocytes in late primary or passaged culture. In this study, we examined the effect of acetaldehyde on rat lipocytes and fibroblasts at various stages of culture, in an effort to determine whether culture-related events influence responsiveness to this compound. Lipocytes from normal rat liver were studied in primary culture at 3 and 7 days after plating; fibroblasts were studied in subculture, at subconfluent and confluent densities. Both cell types were incubated with 100 microM acetaldehyde for 24 hr followed by measurement of collagen synthesis and type I collagen gene expression. Acetaldehyde had no effect on lipocytes at either 3 or 7 days in primary culture. The inability of acetaldehyde to stimulate collagen synthesis in primary culture was not attributable to toxicity, because cell morphology and total protein synthesis were identical in both treated and untreated cultures. Fibroblasts exhibited a variable response to acetaldehyde that was dependent on cell density: subconfluent cells contained similar amounts of type I collagen mRNA in both the presence and absence of acetaldehyde, whereas confluent cells exhibited a 2- to 3-fold increase in collagen mRNA levels upon acetaldehyde exposure. To determine whether quiescent lipocytes would respond to acetaldehyde in a culture system that mimics the hepatic environment in vivo, lipocytes were plated in coculture with hepatocytes on a basement membrane gel and incubated with 20 mM ethanol for 72 hr. Direct communication between these two cell types did not provoke lipocyte activation, even in the setting of ethanol oxidation.(ABSTRACT TRUNCATED AT 250 WORDS)

Cholesterol and bile acid biodynamics after total small bowel resection and bile diversion in humans

BACKGROUND

In humans, the patterns of cholesterol and bile acid biodynamics in the absence of the small intestine are not yet known. They are described in two parenterally fed patients several months after total enterectomy and bile diversion.

METHODS

After an intravenous pulse of [3H]cholesterol, a long-term study involved the analysis of both the decay in the specific activity of plasma cholesterol and the biliary outputs of sterols and bile acids.

RESULTS

Plasma cholesterol input reached 2-3 g/day (vs. 1 g/day in healthy patients), mostly from synthesis. As assessed by sterol balance, whole body cholesterol synthesis approximated 6 g/day (vs. 0.6-0.8 g/day). Unusually, about 60% of the newly synthesized cholesterol was eliminated, without prior transit into the bloodstream, from the liver into the bile. Bile acid conversion concerned over 90% (vs. 40%-50%) of the cholesterol meant to be excreted, issued from plasma or hepatic synthesis. In addition to cholic and chenodeoxycholic acids, one patient secreted up to 1 g/day of 7-epicholic acid.

CONCLUSIONS

The stimulation (up to 10-fold) of the cholesterol and bile acid synthesis, stronger than that observed following ileal bypass or resection or complete bile diversion, could well be partially linked to the absence of small bowel tissue per se.

Metabolism and time-course excretion of murideoxycholic acid, a 6 beta-hydroxylated bile acid, in humans

The metabolism and time-courses of urinary and fecal excretions of murideoxycholic acid (MDCA; 3 alpha,6 beta-dihydroxy-5 beta-cholanoic acid), a 6 beta-hydroxylated bile acid, was investigated in man. The study was carried out in two groups of subjects. Six cholecystectomized patients fitted with a cystic duct drain ingested 100 mg of a tracer dose of 3H-MDCA. Time-course of radioactivity in plasma was then followed for an 8-h period. Biliary, urinary and fecal excretions of radioactivity were measured for a 5-day period and excreted MDCA metabolites were identified. Five lithiasic patients with intact enterohepatic circulation ingested 500 mg of the same tracer dose of 3H-MDCA. Radioactivity in plasma was followed for a 49-h period and urinary and fecal excretions of radioactivity were measured daily for 7 days. In the first group, the excretion of the radioactivity by the three routes (bile+urine+feces) reached 97.8 +/- 1.5% of the ingested dose but dropped to 75 +/- 8.3% (urine+feces) in patients in the second group. In cholecystectomized patients, the estimation of intestinal MDCA absorption was dependent on cystic duct drain flow rate and gave values ranging from 20% to 87%. The biological half-life of MDCA in lithiasic patients averaged 3.4 +/- 0.7 days. Radioactivity appeared in the plasma in the first hour and reached a maximum 6 and 3 h after the beginning of the experiment in group I and II respectively. In the second group, another peak of radioactivity in plasma was observed just after breakfast.(ABSTRACT TRUNCATED AT 250 WORDS)