HIV protease inhibitor lopinavir-induced TNF-alpha and IL-6 expression is coupled to the unfolded protein response and ERK signaling pathways in macrophages

HIV protease inhibitor (PI)-associated cardiovascular risk, especially atherosclerosis, has become a major concern in the clinic. Macrophages are key players in the inflammatory response and atherosclerosis formation. We have previously shown that HIV PIs induce endoplasmic reticulum (ER) stress, activate the unfolded protein response (UPR), and increase the synthesis of the inflammatory cytokines, TNF-alpha and IL-6, by regulating the intracellular translocation of RNA binding protein HuR in macrophages. However, the underlying signaling mechanisms remain unclear. We show here that the HIV PI lopinavir significantly activated the extracellular-signal regulated protein kinase (ERK), but not c-Jun N-terminal kinase (JNK) and p38 MAPK. Lopinavir-induced cytosolic translocation of HuR and TNF-alpha and IL-6 synthesis was attenuated by specific chemical inhibitor of MEK (PD98058) or over-expression of dominant negative mutant of MEK1. In addition, we demonstrated that lopinavir-induced ERK activation and TNF-alpha and IL-6 expression were completely inhibited in macrophages from CHOP null mice. Taken together, these results indicate activation of the UPR plays an essential role in HIV PI-induced inflammatory cytokine synthesis and release by activating ERK, which increases the cytosolic translocation of HuR and subsequent binding to the 3'UTR of TNF-alpha and IL-6 mRNAs in macrophages.

25-Hydroxycholesterol-3-sulfate regulates macrophage lipid metabolism via the LXR/SREBP-1 signaling pathway

The oxysterol receptor LXR is a key transcriptional regulator of lipid metabolism. LXR increases expression of SREBP-1, which in turn regulates at least 32 genes involved in lipid synthesis and transport. We recently identified 25-hydroxycholesterol-3-sulfate (25HC3S) as an important regulatory molecule in the liver. We have now studied the effects of 25HC3S and its precursor, 25-hydroxycholesterol (25HC), on lipid metabolism as mediated by the LXR/SREBP-1 signaling in macrophages. Addition of 25HC3S to human THP-1-derived macrophages markedly decreased nuclear LXR protein levels. 25HC3S administration was followed by dose- and time-dependent decreases in SREBP-1 mature protein and mRNA levels. 25HC3S decreased the expression of SREBP-1-responsive genes, acetyl-CoA carboxylase-1, and fatty acid synthase (FAS) as well as HMGR and LDLR, which are key proteins involved in lipid metabolism. Subsequently, 25HC3S decreased intracellular lipids and increased cell proliferation. In contrast to 25HC3S, 25HC acted as an LXR ligand, increasing ABCA1, ABCG1, SREBP-1, and FAS mRNA levels. In the presence of 25HC3S, 25HC, and LXR agonist T0901317, stimulation of LXR targeting gene expression was repressed. We conclude that 25HC3S acts in macrophages as a cholesterol satiety signal, downregulating cholesterol and fatty acid synthetic pathways via inhibition of LXR/SREBP signaling. A possible role of oxysterol sulfation is proposed.

Prevention of free fatty acid-induced hepatic lipotoxicity by 18beta-glycyrrhetinic acid through lysosomal and mitochondrial pathways

Nonalcoholic fatty liver disease (NAFLD) is the most common liver disease and affects millions of people worldwide. Despite the increasing prevalence of NAFLD, the exact molecular/cellular mechanisms remain obscure and effective therapeutic strategies are still limited. It is well-accepted that free fatty acid (FFA)-induced lipotoxicity plays a pivotal role in the pathogenesis of NAFLD. Inhibition of FFA-associated hepatic toxicity represents a potential therapeutic strategy. Glycyrrhizin (GL), the major bioactive component of licorice root extract, has a variety of pharmacological properties including anti-inflammatory, antioxidant, and immune-modulating activities. GL has been used to treat hepatitis to reduce liver inflammation and hepatic injury; however, the mechanism underlying the antihepatic injury property of GL is still poorly understood. In this report, we provide evidence that 18 beta-glycyrrhetinic acid (GA), the biologically active metabolite of GL, prevented FFA-induced lipid accumulation and cell apoptosis in in vitro HepG2 (human liver cell line) NAFLD models. GA also prevented high fat diet (HFD)-induced hepatic lipotoxicity and liver injury in in vivo rat NAFLD models. GA was found to stabilize lysosomal membranes, inhibit cathepsin B expression and enzyme activity, inhibit mitochondrial cytochrome c release, and reduce FFA-induced oxidative stress. These characteristics may represent major cellular mechanisms, which account for its protective effects on FFA/HFD-induced hepatic lipotoxicity.

CONCLUSION

GA significantly reduced FFA/HFD-induced hepatic lipotoxicity by stabilizing the integrity of lysosomes and mitochondria and inhibiting cathepsin B expression and enzyme activity.

Synthesis and anti-inflammatory activities of mono-carbonyl analogues of curcumin

Curcumin has been extensively studied for its anti-inflammatory activities. However, its potential beneficial effects on various disease preventions and treatments are limited by its unstable structure. The beta-diketone moiety renders curcumin to be rapidly metabolized by aldo-keto reductase in liver. In the present study, a series of curcumin analogues with more stable chemical structures were synthesized and several compounds showed an enhanced ability to inhibit lipopolysaccharide (LPS)-induced TNF-alpha and IL-6 synthesis in macrophages.

Clostridium scindens baiCD and baiH genes encode stereo-specific 7alpha/7beta-hydroxy-3-oxo-delta4-cholenoic acid oxidoreductases

Secondary bile acids, formed by intestinal bacteria, are suggested to play a significant role in cancers of the gastrointestinal tract in humans. Bile acid 7alpha/beta-dehydroxylation is carried out by a few species of intestinal clostridia which harbor a multi-gene bile acid inducible (bai) operon. Several genes encoding enzymes in this pathway have been cloned and characterized. However, no gene product(s) has yet been assigned to the production of 3-oxo-Delta4-cholenoic acid intermediates of cholic acid (CA), chenodeoxycholic acid (CDCA) or ursodeoxycholic acid (UDCA). We previously reported that the baiH gene encodes an NADH:flavin oxidoreductase (NADH:FOR); however, the role of this protein in bile acid 7-dehydroxylation is unclear. Homology searches and secondary structural alignments suggest this protein to be similar to flavoproteins which reduce alpha/beta-unsaturated carbonyl compounds. The baiH gene product was expressed in Escherichia coli, purified and discovered to be a stereo-specific NAD(H)-dependent 7beta-hydroxy-3-oxo-Delta4-cholenoic acid oxidoreductase. Additionally, high sequence similarity between the baiH and baiCD gene products suggests the baiCD gene may encode a 3-oxo-Delta4-cholenoic acid oxidoreductase specific for CDCA and CA. We tested this hypothesis using cell extracts prepared from E. coli overexpressing the baiCD gene and discovered that it encodes a stereo-specific NAD(H)-dependent 7alpha-hydroxy-3-oxo-Delta4-cholenoic acid oxidoreductase.

Activation of bile acid biosynthesis by the p38 mitogen-activated protein kinase (MAPK): hepatocyte nuclear factor-4alpha phosphorylation by the p38 MAPK is required for cholesterol 7alpha-hydroxylase expression

Bile acids are required for intestinal absorption and biliary solubilization of cholesterol and lipids. In addition, bile acids play a crucial role in cholesterol homeostasis. One of the key enzymes in the bile acid biosynthetic pathways is cholesterol 7alpha-hydroxylase/cytochrome P450 7alpha-hydroxylase (7alpha-hydroxylase), which is the rate-limiting and regulatory step of the "classic" pathway. Transcription of the 7alpha-hydroxylase gene is highly regulated. Two nuclear receptors, hepatocyte nuclear factor 4alpha (HNF-4alpha) and alpha(1)-fetoprotein transcription factor, are required for both transcription and regulation by different physiological events. It has been shown that some mitogen-activated protein kinases, such as the c-Jun N-terminal kinase and the ERK, play important roles in the regulation of 7alpha-hydroxylase transcription. In this study, we show evidence that the p38 kinase pathway plays an important role in 7alpha-hydroxylase expression and hence in bile acid synthesis. Inhibition of p38 kinase activity in primary hepatocytes results in approximately 5-10-fold reduction of 7alpha-hydroxylase mRNA. This suppression is mediated, at least in part, through HNF-4alpha. Inhibition of p38 kinase activity diminishes HNF-4alpha nuclear protein levels and its phosphorylation in vivo and in vitro, and it renders a less stable protein. Induction of the p38 kinase pathway by insulin results in an increase in HNF-4alpha protein and a concomitant induction of 7alpha-hydroxylase expression that is blocked by inhibiting the p38 pathway. These studies show a functional link between the p38 signaling pathway, HNF-4alpha, and bile acid synthesis.

HIV protease inhibitors increase TNF-alpha and IL-6 expression in macrophages: involvement of the RNA-binding protein HuR

HIV protease inhibitors (PIs) have been associated with the serious Metabolic Syndrome, which is the major risk factor of atherosclerotic cardiovascular disease. Atherosclerosis is widely considered to be a chronic inflammatory disease. Macrophages are the most prominent cell type present in atherosclerotic lesions and play essential roles in both early lesion development and late lesion complications. We previously reported that HIV PIs induced accumulation of intracellular free cholesterol and lipids, decreased endoplasmic reticulum (ER) calcium stores, activated the unfolded protein response (UPR), significantly increased apoptosis, and promoted foam cell formation in macrophages. HIV PI-induced ER stress and subsequent activation of the UPR, represents an important cell signaling mechanism of HIV PI-induced metabolic syndromes. Here we show that all HIV PIs, except amprenavir, increased expression of the major mediators of inflammatory response, TNF-alpha and IL-6, to varying degrees. Furthermore, we show that the RNA-binding protein, HuR, plays an important role in HIV PI-induced expression of TNF-alpha and IL-6. Atazanavir increased the cytoplasmic levels of HuR and enhanced the binding of HuR to 3'-UTR of TNF-alpha and IL-6. Down regulation of HuR expression by siRNA prevented atazanavir-induced increase of TNF-alpha and IL-6. These results suggest that HuR might have an impact on pathophysiological processes of HIV PI-induced atherosclerosis.

HIV protease inhibitors activate the unfolded protein response and disrupt lipid metabolism in primary hepatocytes

Treatment of human immunodeficiency virus (HIV)-infected patients with HIV protease inhibitors (PIs) has been associated with serious lipid disturbances. However, the incidence and degree of impaired lipid metabolism observed in the clinic vary considerably between individual HIV PIs. Our previous studies demonstrated that HIV PIs differ in their ability to increase the levels of transcriptionally active sterol regulatory element-binding proteins (SREBPs), activate the unfolded protein response (UPR), induce apoptosis, and promote foam cell formation in macrophages. In the present study, we examined the effects of three HIV PIs, including amprenavir, atazanavir, and ritonavir, on the UPR activation and the expression of key genes involved in lipid metabolism in primary rodent hepatocytes. Both atazanavir and ritonavir activated the UPR, induced apoptosis, and increased nuclear SREBP levels, but amprenavir had no significant effect at the same concentrations. In rat primary hepatocytes, cholesterol 7alpha-hydroxylase (CYP7A1) mRNA levels were significantly decreased by atazanavir (38%) and ritonavir (56%) but increased by amprenavir (90%); 3-hydroxy-3-methyl-glutaryl-coenzyme A reductase mRNA levels were increased by amprenavir (23%) but not by ritonavir and atazanavir; low-density lipoprotein receptor mRNA was increased by atazanavir (20%) but not by amprenavir and ritonavir. Similar results were obtained in mouse primary hepatocytes. Atazanavir and ritonavir also decreased CYP7A1 protein levels and bile acid biosynthesis, while amprenavir had no significant effect. The current results may help provide a better understanding of the cellular mechanisms of HIV PI-induced dyslipidemia and also provide useful information to help predict clinical adverse effects in the development of new HIV PIs.

Localization of StarD5 cholesterol binding protein

Human StarD5 belongs to the StarD4 subfamily of START (for steroidogenic acute regulatory lipid transfer) domain proteins. We previously reported that StarD5 is located in the cytosolic fraction of human liver and binds cholesterol and 25-hydroxycholesterol. After overexpression of the gene encoding StarD5 in primary rat hepatocytes, free cholesterol accumulated in intracellular membranes. These findings suggested StarD5 to be a directional cytosolic sterol transporter. The objective of this study was to determine the localization of StarD5 in human liver. Western blot analysis confirmed StarD5's presence in the liver but not in human hepatocytes. Immunohistochemistry studies showed StarD5 localized within sinusoidal lining cells in the human liver and colocalized with CD68, a marker for Kupffer cells. Western blot analyses identified the presence of StarD5 in monocytes and macrophages as well as mast cells, basophils, and promyelocytic cells, but not in human hepatocytes, endothelial cells, fibroblasts, osteocytes, astrocytes, or brain tissue. Cell fractionation and immunocytochemistry studies on THP-1 macrophages localized StarD5 to the cytosol and supported an association with the Golgi. The presence of this cholesterol/25-hydroxycholesterol-binding protein in cells related to inflammatory processes provides new clues to the role of this protein in free sterol transport in the cells and in lipid-mediated atherogenesis.

Mitochondrial cholesterol transport: A possible target in the management of hyperlipidemia

Sterol 27-hydroxylase (CYP27A1) may defend cells against accumulation of excess cholesterol, making this enzyme a possible target in the management of hyperlipidemia. The study objective was to analyze cholesterol homeostatic responses to increases in CYP27A1 activity in HepG2 cells and primary human hepatocytes. Increasing CYP27A1 activity by increasing enzyme expression led to significant increases in bile acid synthesis with compensatory increases in HMG-CoA reductase (HMGR) activity/protein, LDL receptor (LDLR) mRNA, and LDLR-mediated cholesterol uptake. Under these conditions, only a small increase in cellular 27-hydroxycholesterol (27OH-Chol) concentration was observed. No changes were detected in mature sterol regulatory element-binding proteins (SREBP) 1 or 2. Increasing CYP27A1 activity by increasing mitochondrial cholesterol transport (i.e., substrate availability) led to greater increases in bile acid synthesis with significant increases in cellular 27OH-Chol concentration. Mature SREBP 2 protein decreased significantly with compensatory decreases in HMGR protein. No change was detected in mature SREBP 1 protein. Despite increasing 27OH-Chol and lowering SREBP 2 protein concentrations, LDLR mRNA increased significantly, suggesting alternative mechanisms of LDLR transcriptional regulation. These findings suggest that regulation of liver mitochondrial cholesterol transport represents a potential therapeutic strategy in the treatment of hyperlipidemia and atherosclerosis.

Bile salt biotransformations by human intestinal bacteria

Secondary bile acids, produced solely by intestinal bacteria, can accumulate to high levels in the enterohepatic circulation of some individuals and may contribute to the pathogenesis of colon cancer, gallstones, and other gastrointestinal (GI) diseases. Bile salt hydrolysis and hydroxy group dehydrogenation reactions are carried out by a broad spectrum of intestinal anaerobic bacteria, whereas bile acid 7-dehydroxylation appears restricted to a limited number of intestinal anaerobes representing a small fraction of the total colonic flora. Microbial enzymes modifying bile salts differ between species with respect to pH optima, enzyme kinetics, substrate specificity, cellular location, and possibly physiological function. Crystallization, site-directed mutagenesis, and comparisons of protein secondary structure have provided insight into the mechanisms of several bile acid-biotransforming enzymatic reactions. Molecular cloning of genes encoding bile salt-modifying enzymes has facilitated the understanding of the genetic organization of these pathways and is a means of developing probes for the detection of bile salt-modifying bacteria. The potential exists for altering the bile acid pool by targeting key enzymes in the 7alpha/beta-dehydroxylation pathway through the development of pharmaceuticals or sequestering bile acids biologically in probiotic bacteria, which may result in their effective removal from the host after excretion.

Human StarD5, a cytosolic StAR-related lipid binding protein

Recently identified StarD5 belongs to the StarD4 subfamily, a subfamily of steroidogenic acute regulatory related lipid transfer (START) domain proteins that includes StarD4 and StarD6, proteins whose functions remain unknown. The objective of this study was to confirm StarD5's protein localization and sterol binding capabilities as measures to pursue function. Using rabbit polyclonal antibody against newly purified human histidine-tagged/StarD5 protein, StarD5 was detected in human liver. In parallel studies, increased expression of StarD5 in primary hepatocytes led to a marked increase in microsomal free cholesterol. Cell fractionation studies demonstrated StarD5 protein in liver cytosolic fractions only, suggesting StarD5 as a directional cytosolic sterol carrier. Supportive in vitro binding assays demonstrated a concentration-dependent binding of cholesterol by StarD5 similar to that of the cholesterol binding START domain protein StarD1. In contrast to selective cholesterol binding by StarD1, StarD5 bound the potent regulatory oxysterol, 25-hydroxycholesterol, in a concentration-dependent manner. StarD5 binding appeared selective for cholesterol and 25-hydroxycholesterol, as no binding was observed for other tested sterols. The ability of StarD5 to bind not only cholesterol but also 25-hydroxycholesterol, a potent inflammatory mediator and regulatory oxysterol, raises basic fundamental questions about StarD5's role in the maintenance of cellular cholesterol homeostasis.

HIV protease inhibitors activate the unfolded protein response in macrophages: implication for atherosclerosis and cardiovascular disease

Human immunodeficiency virus (HIV) protease inhibitors have been successfully used in highly active antiretroviral therapy for HIV-1 infection. Treatment of patients infected with HIV with HIV protease inhibitors is unfortunately associated with a number of clinically significant metabolic abnormalities and an increased risk of premature atherosclerosis and myocardial infarction. However, the cellular/molecular mechanisms of the HIV protease inhibitor-induced lipid dysregulation and atherosclerosis remain elusive. Macrophages are the most prominent cell type present in atherosclerotic lesions and play essential roles in both early lesion development and late lesion complications. In this study, we demonstrate that three different HIV protease inhibitors (ritonavir, indinavir, and atazanavir) induce endoplasmic reticulum stress and activate the unfolded protein response in mouse macrophages. Furthermore, at therapeutic concentrations (5-15 microM), these HIV protease inhibitors were found to increase the levels of transcriptionally active sterol regulatory element binding proteins, decrease endogenous cholesterol esterification, cause the accumulation of free cholesterol in intracellular membranes, deplete endoplasmic reticulum calcium stores, activate caspase-12, and increase apoptosis in macrophages. These findings provide possible cellular mechanisms by which HIV protease inhibitors promote atherosclerosis and cardiovascular disease in HIV-1 infected patients treated with HIV protease inhibitors.

Development of methods for the detection and quantification of 7alpha-dehydroxylating clostridia, Desulfovibrio vulgaris, Methanobrevibacter smithii, and Lactobacillus plantarum in human feces

BACKGROUND

Mounting evidence suggests a relationship between bacterial metabolism of certain dietary and endogenous factors and the development of colorectal cancer. Deoxycholic acid (DCA) is a well studied co-carcinogen and bio-transformation product of 7alpha-dehydroxylating Clostridia. H2S is a cytotoxic metabolite produced primarily by sulfate-reducing bacteria (SRB). The production of methane indicates low levels of active SRB. Lactic acid bacteria (LAB) have received attention recently due to their putative anti-tumor properties.

METHOD

Human stool was spiked with pure cultures of bacteria and diluted in several enriched media. Each dilution titer was analyzed for the presence of the organism by PCR and biochemical assays. Duplicate stool aliquots were stored under various conditions for a 1 month period at -20 degrees C to test viability and detection.

RESULTS

Growth and enumeration of each spiked organism was confirmed by PCR and biochemical assays. The combination of bead beating and chemical lysis steps produced the greatest DNA yields. PCR assays detected as low as 75 fg target DNA. The ability to detect Methanobrevibacter smithii, and Desulfovibrio vulgaris by either PCR or biochemical assay declined significantly after storage at -20 degrees C for 1 month.

CONCLUSIONS

Accurate detection and quantification of each bacterium using the described methods resulted when stool was processed immediately after collection. Storage of some members of the gut flora results in decrease in or loss of viability.

Detection of the steroidogenic acute regulatory protein, StAR, in human liver cells

Overexpressing StAR (a mitochondrial cholesterol transporter) increases (>5-fold) the rate of 27-hydroxylation of cholesterol and the rates of bile acid synthesis in primary rat hepatocytes; suggesting that the transport of cholesterol into mitochondria is rate-limiting for bile acid biosynthesis via the CYP27A1 initiated 'acidic' pathway. Our objective was to determine the level of StAR expression in human liver and whether changes in StAR would correlate with changes in CYP27A1 activity/bile acid synthesis rates in human liver tissues. StAR mRNA and protein were detected in primary human hepatocytes and HepG2 cells by RT-PCR/Northern analysis and by Western analysis, respectively. In immunocompetition assays, liver StAR was competed away with the addition of purified human adrenal StAR. Overexpressing CYP27A1 in both cell types led to >2-fold increases in liver StAR concentration. StAR protein levels also increased approximately 2-fold with the addition of 27-hydroxycholesterol to HepG2 cell culture medium. Overexpressing StAR increased the rates of 27-hydroxylation of cholesterol/bile acid synthesis in both cell lines and increased intracellular levels of 27-hydroxycholesterol. In conclusion, human liver cells contain regulable StAR protein whose level of expression appears capable of regulating cellular cholesterol homeostasis, representing a potential therapeutic target in the management of hyperlipidemia.

Diet, anaerobic bacterial metabolism, and colon cancer: a review of the literature

Epidemiologic studies have demonstrated variations in the incidence of colon cancer between populations and socioeconomic groups. Differences in dietary habits have been implicated in the risk of developing colon cancer. Diet appears to influence our colonic microflora. Such variations may allow for future utilization of the fecal flora as markers for screening and diagnosis of colon cancer. The composition of the diet not only dictates the available substrates for the flora but also helps to establish predictable and competitive relationships between intestinal bacteria. To appreciate the significance of populations deemed high and low risk based on host flora, an understanding of several dynamic microbial relationships and metabolites produced is necessary. In this review, we explore the critical relationships between bile acid 7 alpha-dehydroxylation, sulfidogenesis, methanogenesis, and how they relate to carbohydrate and bile acid metabolism. We summarize the chemopreventative, anticarcinogenic, and detoxifying activity of probiotics and prebiotics, as well as potential mechanisms for protection against colon cancer.

Over-expression of hepatic neutral cytosolic cholesteryl ester hydrolase in mice increases free cholesterol and reduces expression of HMG-CoAR, CYP27, and CYP7A1

Hepatic neutral cytosolic cholesteryl ester hydrolase (hncCEH) is a key enzyme in the regulation of hepatic free cholesterol (FC). In examining the effects of over-expression of this enzyme on cholesterol homeostasis, mice were infected with a recombinant adenovirus construct (AdCEH) of the rat hncCEH cDNA driven by the human cytomegalovirus promoter. Cholesteryl esterase and p-nitrophenylcaprylate (PNPC) esterase activities were measured in liver postmitochondrial supernatants at 1, 3, 7, and 11 d after infection with AdCEH or a control virus expressing beta-galactosidase (AdbetaGAL). The PNPC esterase activity of AdCEH mice peaked threefold higher than controls on day 2, declining on subsequent days. In contrast, cholesteryl esterase peaked eightfold higher than controls on day 3, indicating a shift in substrate selectivity of hncCEH. Hepatic FC peaked at 144% of controls, 7 d postinfection. The mRNAs for cholesterol 7alpha-hydroxylase, sterol 27-hydroxylase, and HMG-CoA reductase decreased to 47, 46, and 58% of controls, respectively, on day 7, coinciding with peak FC concentrations. Coinciding with increased cholesteryl esterase activity, hepatic esterified cholesterol dropped precipitously from day 3 onward, to 11% of controls by day 11. Hepatic TAG levels also declined, consistent with the reported TAG lipase activity of hncCEH. These results demonstrate elevation of FC and depletion of cholesteryl esters by over-expression of hncCEH, which were resistant to compensatory responses by other enzymes of cholesterol homeostasis.

Bile acids induce mitochondrial ROS, which promote activation of receptor tyrosine kinases and signaling pathways in rat hepatocytes

Previous studies have demonstrated in hepatocytes that deoxycholic acid (DCA) promotes inactivation of protein tyrosine phosphatases (PTPases) and activation of ERBB1 and the extracellular-regulated kinase (ERK) 1/2 pathway. The present studies have determined the biochemical mechanism(s) through which these events occur. DCA and taurodeoxycholic acid (TDCA) (100 micromol/L) caused activation of ERBB1, insulin receptor, and the ERK1/2 and AKT pathways in primary rodent hepatocytes. DCA- and TDCA-induced receptor and signaling pathway activations were blocked by the reactive oxygen species (ROS) scavengers N-acetyl cysteine (NAC) and Trolox (TX), as well as by cyclosporin A (CsA) and bongkrekic acid (BKA). DCA activated the ERK1/2 pathway in HuH7 human hepatoma cells that was blocked by the incubation of cells with an ERBB1 inhibitor, NAC, TX, CsA, or BKA. DCA did not activate the ERK1/2 pathway in mitochondria-defective HuH7 Rho 0 cells. In HuH7 cells and primary hepatocytes, DCA enhanced the production of ROS, an effect that was abolished in Rho 0 cells and by prior incubation of cells with CsA or BKA. In hepatocytes and HuH7 cells, DCA inhibited PTPase activity. Incubation of hepatocytes with either CsA or BKA prevented DCA-induced inhibition of PTPase activity. Loss of mitochondrial function in Rho 0 cells also abolished the inhibitory effects of DCA on PTPase activity. In conclusion, DCA and TDCA cause ROS generation in hepatocytes that is dependent on metabolically active mitochondria. The generation of ROS is essential for PTPase inactivation, receptor tyrosine kinase activation, and enhanced signaling down the ERK1/2 and AKT pathways.

Overexpression of cholesterol transporter StAR increases in vivo rates of bile acid synthesis in the rat and mouse

Bile acid synthesis (BAS) occurs mainly via two pathways: the "neutral" pathway, which is initiated by highly regulated microsomal CYP7A1, and an "acidic" pathway, which is initiated by mitochondrial CYP27A1. Previously, we have shown that overexpression of the steroidogenic acute regulatory protein (StAR), a mitochondrial cholesterol transport protein, increases bile acid biosynthesis more than 5-fold via the acidic pathway in primary rat hepatocytes. This observation suggests that mitochondrial cholesterol transport is the rate-limiting step of BAS via this pathway. The objective of this study was to determine the effect of increased StAR on rates of BAS in vivo. Overexpression of StAR and CYP7A1 were mediated via infection with recombinant adenoviruses. BAS rates were determined in chronic biliary-diverted rats and mice, and in mice with an intact enterohepatic circulation. The protein/messenger RNA levels of StAR and CYP7A1 increased dramatically following overexpression. Overexpression of StAR or CYP7A1 led to a similar 2-fold (P <.01) increase in BAS over up-regulated (approximately 2-fold) 3-day chronic biliary-diverted control rats. Additionally, overexpression of StAR led to more than 3- and 6-fold increases over controls in the rates of BAS in biliary-diverted and intact mice, respectively (P <.01). In conclusion, in both rats and mice in vivo, overexpression of StAR led to a marked increase in the rates of BAS initiated by delivery of cholesterol to mitochondria containing CYP27A1.

Overexpression of cholesterol transporter StAR increases in vivo rates of bile acid synthesis in the rat and mouse

Bile acid synthesis (BAS) occurs mainly via two pathways: the "neutral" pathway, which is initiated by highly regulated microsomal CYP7A1, and an "acidic" pathway, which is initiated by mitochondrial CYP27A1. Previously, we have shown that overexpression of the steroidogenic acute regulatory protein (StAR), a mitochondrial cholesterol transport protein, increases bile acid biosynthesis more than 5-fold via the acidic pathway in primary rat hepatocytes. This observation suggests that mitochondrial cholesterol transport is the rate-limiting step of BAS via this pathway. The objective of this study was to determine the effect of increased StAR on rates of BAS in vivo. Overexpression of StAR and CYP7A1 were mediated via infection with recombinant adenoviruses. BAS rates were determined in chronic biliary-diverted rats and mice, and in mice with an intact enterohepatic circulation. The protein/messenger RNA levels of StAR and CYP7A1 increased dramatically following overexpression. Overexpression of StAR or CYP7A1 led to a similar 2-fold (P <.01) increase in BAS over up-regulated (approximately 2-fold) 3-day chronic biliary-diverted control rats. Additionally, overexpression of StAR led to more than 3- and 6-fold increases over controls in the rates of BAS in biliary-diverted and intact mice, respectively (P <.01). In conclusion, in both rats and mice in vivo, overexpression of StAR led to a marked increase in the rates of BAS initiated by delivery of cholesterol to mitochondria containing CYP27A1.

Indinavir alters sterol and fatty acid homeostatic mechanisms in primary rat hepatocytes by increasing levels of activated sterol regulatory element-binding proteins and decreasing cholesterol 7α-hydroxylase mRNA levels

Human immunodeficiency virus protease inhibitors induce hyperlipidemia in many patients treated with these drugs. We examined the effects of indinavir on cholesterol and bile acid homeostatic mechanisms in a primary rat hepatocyte (PRH) culture model. In PRH, indinavir up-regulated (2.5-fold) 3-hydroxy-3-methylglutaryl-Coenzyme A reductase mRNA levels 24hr after drug addition. In these same experiments, cholesterol 7alpha-hydroxylase (CYP7A1) mRNA levels, the rate-limiting enzyme in bile acid biosynthesis, was decreased up to 10-fold. Fatty acid synthase mRNA levels were up-regulated more than 3-fold under these conditions. Indinavir did not alter CYP7A1 transcriptional activity, but decreased CYP7A1 mRNA half-life in PRH from 1.5hr to less than 0.5hr. Sterol regulatory element-binding protein-1 (SREBP-1) mature form was increased approximately 6-fold by this drug. Indinavir-induced mRNA changes and SREBP-1 mature protein levels were significantly abated by the addition of cholesterol, solubilized in beta-cyclodextrin, to culture medium. Indinavir markedly decreased endogenous cholesterol esterification and increased cholesterol in intracellular membranes in primary hepatocytes. Indinavir gavaged into intact mice also markedly increased SREBP-1 and SREBP-2 (mature forms) in hepatic nuclei. CYP7A1 mRNA was also decreased approximately 52% in indinavir-treated animals. We propose that indinavir disrupts cellular cholesterol homeostasis by increasing SREBP's and decreasing CYP7A1 mRNA.

Deoxycholic acid activates the c-Jun N-terminal kinase pathway via FAS receptor activation in primary hepatocytes. Role of acidic sphingomyelinase-mediated ceramide generation in FAS receptor activation

We have shown previously that bile acids can activate the JNK pathway and down-regulate cholesterol 7alpha-hydroxylase (CYP7A1), the rate-limiting enzyme in the neutral pathway of bile acid biosynthesis. In this study, the mechanism(s) by which deoxycholic acid (DCA) activates the JNK pathway were examined. FAS receptor (FAS-R) and acidic sphingomyelinase (ASM)-deficient hepatocytes were resistant to DCA-induced activation of the JNK pathway. Activation of the JNK pathway (2-3-fold) in response to tumor necrosis factor-alpha was similar in both wild-type and FAS-R(-/-) hepatocytes. In wild-type and FAS-R(-/-) hepatocytes, ceramide elevation was detected as early as 2 min and peaked at 10 min after DCA treatment. In contrast, ASM(-/-) hepatocytes were defective in DCA-induced ceramide generation. Treatment with DCA resulted in movement of FAS-R to the cell surface, which was blocked upon treatment with brefeldin A. However, brefeldin A failed to block DCA-mediated JNK activation in wild-type hepatocytes. DCA-induced JNK activation was independent of either the epidermal growth factor receptor activation or free radical generation. Addition of ASM to rat hepatocytes activated JNK and down-regulated CYP7A1 mRNA levels. In conclusion, these results show that DCA activates JNK and represses CYP7A1 mRNA levels in primary hepatocytes via an ASM/FAS-R-dependent mechanism that is independent of either the epidermal growth factor receptor or free radical generation.

Regulation of oxysterol 7alpha-hydroxylase (CYP7B1) in the rat

Cholesterol metabolized to 7alpha-hydroxylated bile acids is a principle pathway of cholesterol degradation. Cholesterol 7alpha-hydroxylase (CYP7A1) is the initial and rate-determining enzyme in the "classic pathway" of bile acid synthesis. An "alternative" pathway of bile acid synthesis begins with 27-hydroxylation of cholesterol by 27-hydroxylase (CYP27), followed by 7alpha-hydroxylation by oxysterol 7alpha-hydroxylase (CYP7B1). The aim of the current study was to investigate the regulation of CYP7B1 by bile acids, cholesterol, and thyroid hormone in a previously well-studied in vivo model of bile acid synthesis, and to compare its regulation to that of CYP7A1. Three study groups were examined. In the first, male Sprague-Dawley rats with intact enterohepatic circulations were fed normal chow (controls), cholestyramine (CT), cholic acid (CA), chenodeoxycholic acid (CDCA), deoxycholic acid (DCA), or cholesterol (Chol). In the second group, taurocholate (TCA) was continuously intraduodenally infused for 48 hours to chronic biliary diverted rats. In a third set of studies, squalestatin, an inhibitor of cholesterol synthesis, was intravenously infused for 48 hours. In a fourth set of studies, the diurnal variation in CYP7B1 was compared to that of CYP7A1. At the end of each study livers were harvested, and CYP7B1 and CYP7A1 activities and mRNA levels were determined. Complete biliary diversion significantly increased the specific activity (SA) of both CYP7B1 ( upward arrow 212%; P <.002) and CYP7A1 ( upward arrow 212%; P <.007). Intraduodenal infusion of TCA to rats with biliary diversion decreased SA of both CYP7B1 ( downward arrow 29%; P <.001) and CYP7A1 ( downward arrow 46%; P <.01). The addition of CA, CDCA, or DCA to rat chow led to downregulation of CYP7B1 SAs by 42% (P <.003), 51% (P <.009), and 47% (P <.003), and CYP7A1 SAs by 32% +/- 6% (P <.003), 73% +/- 9% (P <.002), and 60% +/- 13% (P <.004), respectively. CT feeding upregulated both CYP7B1 ( upward arrow 136%; P <.004) and CYP7A1 ( upward arrow 216%; P <.001) SAs. While Chol feeding significantly upregulated CYP7A1 SA, no significant increase in CYP7B1 SA was found. Conversely, as previously shown in vitro, inhibition of cholesterol synthesis significantly suppressed both CYP7A1 and CYP7B1 activity and mRNA levels. Both CYP7B1 and CYP7A1 underwent diurnal variation, with peak and trough values for CYP7B1 lagging approximately 6 hours behind CYP7A1. We conclude that, in the rat, like CYP7A1, CYP7B1 demonstrates diurnal rhythm and is regulated by bile acids and cholesterol.

Development and application of a polymerase chain reaction assay for the detection and enumeration of bile acid 7alpha-dehydroxylating bacteria in human feces

BACKGROUND

Secondary bile acids are synthesized in the human colon from the bacterial 7alpha-dehydroxylation of primary bile acids. Increased levels of secondary bile acids have been correlated with an increased risk of colon cancer and cholesterol gallstone disease. Based on 16s rDNA sequence analysis, stock cultures of bacterial strains with bile acid 7alpha-dehydroxylation all belong to the genus Clostridium.

METHODS

We developed a polymerase chain reaction (PCR)-based assay that can specifically amplify the baiCD gene, specific to bile acid 7alpha-dehydroxylation pathway. The PCR primer pair was designed using known nucleotide sequences from two different baiCD genes described for Clostridium scindens VPI 12708 and Clostridium hiranonis TO931. Although the DNA sequences of these genes were <70% identical, several regions were conserved enough to design primers with little or no redundancy.

RESULTS

The PCR assay was effective in detecting the baiCD gene in several strains known to exhibit bile acid 7alpha-dehydroxylation activity. The PCR assay also detected the baiCD gene in DNA extracted from fecal dilution series and correlated with the levels of cholic acid 7alpha-dehydroxylating bacteria detected by activity assays. The PCR assay was sensitive enough to detect the baiCD gene in DNA samples extracted directly from as little as 0.5 mg feces.

CONCLUSIONS

This new method should be useful for the monitoring of levels of bile acid 7alpha-dehydroxylating bacteria in human fecal samples.

Suppression of cholesterol 7alpha-hydroxylase transcription and bile acid synthesis by an alpha1-antitrypsin peptide via interaction with alpha1-fetoprotein transcription factor

alpha1-Antitrypsin (alpha1-AT) is a serum protease inhibitor that is synthesized mainly in the liver, and its rate of synthesis markedly increases in response to inflammation. This increase in alpha1-AT synthesis results in an increase in peptides, like its carboxyl-terminal C-36 peptide (C-36), resulting from alpha1-AT cleavage by proteases. Atherosclerosis is a form of chronic inflammation, and one of the risk factors is elevated plasma cholesterol levels. Because of the correlation between atherosclerosis, plasma cholesterol content, inflammation, and alpha1-AT rate of synthesis, we investigated the effect of the C-36 serpin peptide on hepatic bile acid biosynthesis. We discovered that C-36 is a powerful and specific transcriptional down-regulator of bile acid synthesis in primary rat hepatocytes, through inhibition of the cholesterol 7alpha-hydroxylase/CYP7A1 (7alpha-hydroxylase) promoter. Mice injected with the C-36 peptide also showed a decrease in 7alpha-hydroxylase mRNA. A mutated but very similar peptide did not have any effect on 7alpha-hydroxylase mRNA or its promoter. The sterol 12alpha-hydroxylase/CYP8B1 (12alpha-hydroxylase) promoter is also down-regulated by the C-36 peptide in HepG2 cells but not by the mutated peptide. The DNA element involved in the C-36-mediated regulation of 7alpha- and 12alpha-hydroxylase promoters mapped to the alpha1-fetoprotein transcription factor (FTF) site in both promoters. The C-36 peptide prevented binding of FTF to its target DNA recognition site by direct interaction with FTF. We hypothesize that the C-36 peptide specifically interacts with FTF and induces a conformational change that results in loss of its DNA binding ability, which results in suppression of 7alpha- and 12alpha-hydroxylase transcription. These results suggest that peptides derived from specific serum proteins may alter hepatic gene expression in a highly specific manner.

Regulation of oxysterol 7alpha-hydroxylase (CYP7B1) in primary cultures of rat hepatocytes

Conversion of cholesterol into 7alpha-hydroxylated bile acids is a principal pathway of cholesterol disposal. Cholesterol 7alpha-hydroxylase (CYP7A1) is the initial and rate-determining enzyme in the "classic" pathway of bile acid synthesis. An "alternative" pathway of bile acid synthesis is initiated by sterol 27-hydroxylase (CYP27) with subsequent 7alpha-hydroxylation of 27-hydroxycholesterol by oxysterol 7alpha-hydroxylase (CYP7B1). The regulation of CYP7B1, possibly a rate-determining enzyme in the alternative pathway, has not been thoroughly studied. The aims of this study were to (1) study the regulation of liver CYP7B1 by bile acids, cholesterol, adenosine 3', 5'-cyclic monophosphate (cAMP), and phorbol myristate acetate (PMA) in primary rat hepatocytes and (2) determine the effect of CYP7B1 overexpression on rates of bile acid synthesis. The effects of different bile acids (3-150 micromol/L), cAMP (50 micromol/L), PMA (100 nmol/L; protein kinase C stimulator), cholesterol (200 micromol/L), and squalestatin (1 micromol/L; cholesterol synthesis inhibitor) on CYP7B1 expression in primary rat hepatocytes were studied. Taurocholic acid and taurodeoxycholic acid decreased CYP7B1 activity by 45% +/- 10% and 36% +/- 7%, respectively. Tauroursodeoxycholic acid and taurochenodeoxycholic acid did not alter CYP7B1 activity. Inhibition of cholesterol synthesis with squalestatin decreased CYP7B1 activity by 35%, whereas addition of cholesterol increased activity by 39%. Both PMA and cAMP decreased CYP7B1 activity by 60% and 34%, respectively, in a time-dependent fashion. Changes in CYP7B1 messenger RNA (mRNA) levels correlated with changes in specific activities. Overexpression of CYP7B1 led to a marked increase in CYP7B1 mRNA levels and specific activity but no change in rates of bile acid synthesis. In conclusion, in the rat, CYP7B1 specific activity is highly regulated but does not seem to be rate limiting for bile acid synthesis.

LXR alpha is the dominant regulator of CYP7A1 transcription

Cholesterol 7 alpha-hydroxylase (CYP7A1) catalyzes the rate-limiting step in the classic pathway of bile acid biosynthesis. Dietary cholesterol stimulates CYP7A1 transcription via activation of oxysterol receptor, LXR alpha, whereas bile acids repress transcription through FXR-mediated induction of SHP protein. The aim of this study was to determine the quantitative role that LXR- and FXR-regulated pathways play in regulating CYP7A1 and SHP in both rat and hamster models. In rats fed a 2% cholesterol diet, both SHP and CYP7A1 mRNA levels were elevated. The inability to induce CYP7A1 mRNA levels by cholesterol feeding in hamsters led to a decline in SHP mRNA levels. Elimination of hepatic bile acid flux by cholestyramine or bile fistula resulted in a marked repression of rat SHP mRNA levels. These results suggest that under conditions of both SHP and LXR alpha activation, stimulatory effect of LXR alpha overrides the inhibitory effect of FXR and results in an induction of rat CYP7A1 mRNA levels.

Activation of the Raf-1/MEK/ERK cascade by bile acids occurs via the epidermal growth factor receptor in primary rat hepatocytes

Bile acids have been reported to activate several different cell signaling cascades in rat hepatocytes. However, the mechanism(s) of activation of these pathways have not been determined. This study aims to determine which bile acids activate the Raf-1/MEK/ERK cascade and the mechanism of activation of this pathway. Taurodeoxycholic acid (TDCA) stimulated (+235%) the phosphorylation of p(74) Raf-1 in a time (5 to 20 minutes) and concentration-dependent (10 to 100 micromol/L) manner. Raf-1 and ERK activities were both significantly increased by most bile acids tested. Deoxycholic acid (DCA) was the best activator of ERK (3.6-fold). A dominant negative Ras (N17) construct expressed in primary hepatocytes prevented the activation of ERK by DCA. The epidermal growth factor receptor (EGFR)-specific inhibitor (AG1478) significantly inhibited (approximately 81%) the activation of ERK by DCA. DCA rapidly (30 to 60 seconds) increased phosphorylation of the EGFR (approximately 2-fold) and Shc (approximately 4-fold). A dominant negative mutant of the EGFR (CD533) blocked the ability of DCA to activate ERK. In conclusion, these results show that DCA activates the Raf-1/MEK/ERK signaling cascade in primary hepatocytes primarily via an EGFR/Ras-dependent mechanism.

Bile acid metabolism by fresh human colonic contents: a comparison of caecal versus faecal samples

BACKGROUND

Deoxycholic acid (DCA), implicated in the pathogenesis of gall stones and colorectal cancer, is mainly formed by bacterial deconjugation (cholylglycine hydrolase (CGH)) and 7 alpha-dehydroxylation (7 alpha-dehydroxylase (7 alpha-DH)) of conjugated cholic acid (CA) in the caecum/proximal colon. Despite this, most previous studies of CGH and 7 alpha-DH have been in faeces rather than in caecal contents. In bacteria, CA increases 7 alpha-DH activity by substrate-enzyme induction but little is known about CA concentrations or CA/7 alpha-DH induction in the human colon.

AIMS AND METHODS

Therefore, in fresh "faeces", and in caecal aspirates obtained during colonoscopy from 20 patients, we: (i) compared the activities of CGH and 7 alpha-DH, (ii) measured 7 alpha-DH in patients with "low" and "high" percentages of DCA in fasting serum (less than and greater than the median), (iii) studied CA concentrations in the right and left halves of the colon, and examined the relationships between (iv) 7 alpha-DH activity and CA concentration in caecal samples (evidence of substrate-enzyme induction), and (v) 7 alpha-DH and per cent DCA in serum.

RESULTS

Although mean CGH activity in the proximal colon (18.3 (SEM 4.40) x10(-2) U/mg protein) was comparable with that in "faeces" (16.0 (4.10) x10(- 2) U/mg protein), mean 7 alpha-DH in the caecum (8.54 (1.08) x10(-4) U/mg protein) was higher (p<0.05) than that in the left colon (5.72 (0.85) x10(-4) U/mg protein). At both sites, 7 alpha-DH was significantly greater in the "high" than in the "low" serum DCA subgroups. CA concentrations in the right colon (0.94 (0.08) micromol/ml) were higher than those in the left (0.09 (0.03) micromol/ml; p<0.001) while in the caecum (but not in the faeces) there was a weak (r=0.58) but significant (p<0.005) linear relationship between 7 alpha-DH and CA concentration. At both sites, 7 alpha-DH was linearly related (p<0.005) to per cent DCA in serum. INTERPRETATION/SUMMARY: These results: (i) confirm that there are marked regional differences in bile acid metabolism between the right and left halves of the colon, (ii) suggest that caecal and faecal 7 alpha-DH influence per cent DCA in serum (and, by inference, in bile), and (iii) show that the substrate CA induces the enzyme 7 alpha-DH in the caecum.

Hepatitis B virus X protein increases expression of p21(Cip-1/WAF1/MDA6) and p27(Kip-1) in primary mouse hepatocytes, leading to reduced cell cycle progression

Previously, we have linked prolonged intense mitogen-activated protein kinase (MAP kinase; MAPK) signaling in hepatocytes to increased expression of p21(Cip-1/WAF1/MDA6) (p21) and p16(INK4a) (p16), that leads to a p21-dependent growth arrest. In this study, we investigated the impact of hepatitis B virus X protein (pX) expression on MAPK-modulated cell cycle progression in primary mouse hepatocytes. In hepatocytes, expression of pX enhanced protein levels of p21 and p27, but not of p16. The elevated levels of p21 and p27 correlated with reduced DNA synthesis in wild-type (+/+) hepatocytes and with a weak stimulation of DNA synthesis in p21 null (-/-) cells. Antisense p27 messenger RNA (mRNA) (p27as) increased DNA synthesis in +/+ and p21 -/- cells, and pX blunted this effect in +/+ cells. In p21 -/- cells, however, p27as permitted pX to further stimulate DNA synthesis. These data argue that a reduced ability to enhance expression of both p21 and p27 is required to fully reveal the growth-potentiating properties of pX. This finding also implies that depending on the functional status of the p21 and p27 genes, expression of pX can have 2 very different effects on hepatocyte proliferation. Prolonged intense MAPK signaling reduced DNA synthesis in +/+ cells and enhanced DNA synthesis in p21 -/- cells. The enhancement of DNA synthesis in p21 -/- cells was blocked by pX, and the effect of pX was abrogated by p27as. Furthermore in p21 -/- cells, overexpression of p16 blocked MAPK-stimulated DNA synthesis, and this effect was partially reversed by p27as. These data argue that p27 can also cooperatively interact with p16 to inhibit DNA synthesis in hepatocytes. Collectively, our findings show that reduced expression of p16, p21, and p27, which can occur during hepatocellular carcinoma, enhances the ability of MAPK signaling and pX to cause proliferation in hepatocytes. Thus loss of cyclin kinase inhibitor function may play an important role in the process of tumor progression after chronic hepatitis B virus infection.

Effects of CYP7A1 overexpression on cholesterol and bile acid homeostasis

The initial and rate-limiting step in the classic pathway of bile acid biosynthesis is 7alpha-hydroxylation of cholesterol, a reaction catalyzed by cholesterol 7alpha-hydroxylase (CYP7A1). The effect of CYP7A1 overexpression on cholesterol homeostasis in human liver cells has not been examined. The specific aim of this study was to determine the effects of overexpression of CYP7A1 on key regulatory steps involved in hepatocellular cholesterol homeostasis, using primary human hepatocytes (PHH) and HepG2 cells. Overexpression of CYP7A1 in HepG2 cells and PHH was accomplished by using a recombinant adenovirus encoding a CYP7A1 cDNA (AdCMV-CYP7A1). CYP7A1 overexpression resulted in a marked activation of the classic pathway of bile acid biosynthesis in both PHH and HepG2 cells. In response, there was decreased HMG-CoA-reductase (HMGR) activity, decreased acyl CoA:cholesterol acyltransferase (ACAT) activity, increased cholesteryl ester hydrolase (CEH) activity, and increased low-density lipoprotein receptor (LDLR) mRNA expression. Changes observed in HMGR, ACAT, and CEH mRNA levels paralleled changes in enzyme specific activities. More specifically, LDLR expression, ACAT activity, and CEH activity appeared responsive to an increase in cholesterol degradation after increased CYP7A1 expression. Conversely, accumulation of the oxysterol 7alpha-hydroxycholesterol in the microsomes after CYP7A1 overexpression was correlated with a decrease in HMGR activity.

Deoxycholic acid (DCA) causes ligand-independent activation of epidermal growth factor receptor (EGFR) and FAS receptor in primary hepatocytes: inhibition of EGFR/mitogen-activated protein kinase-signaling module enhances DCA-induced apoptosis

Previous studies have argued that enhanced activity of the epidermal growth factor receptor (EGFR) and the mitogen-activated protein kinase (MAPK) pathway can promote tumor cell survival in response to cytotoxic insults. In this study, we examined the impact of MAPK signaling on the survival of primary hepatocytes exposed to low concentrations of deoxycholic acid (DCA, 50 microM). Treatment of hepatocytes with DCA caused MAPK activation, which was dependent upon ligand independent activation of EGFR, and downstream signaling through Ras and PI(3) kinase. Neither inhibition of MAPK signaling alone by MEK1/2 inhibitors, nor exposure to DCA alone, enhanced basal hepatocyte apoptosis, whereas inhibition of DCA-induced MAPK activation caused approximately 25% apoptosis within 6 h. Similar data were also obtained when either dominant negative EGFR-CD533 or dominant negative Ras N17 were used to block MAPK activation. DCA-induced apoptosis correlated with sequential cleavage of procaspase 8, BID, procaspase 9, and procaspase 3. Inhibition of MAPK potentiated bile acid-induced apoptosis in hepatocytes with mutant FAS-ligand, but did not enhance in hepatocytes that were null for FAS receptor expression. These data argues that DCA is causing ligand independent activation of the FAS receptor to stimulate an apoptotic response, which is counteracted by enhanced ligand-independent EGFR/MAPK signaling. In agreement with FAS-mediated cell killing, inhibition of caspase function with the use of dominant negative Fas-associated protein with death domain, a caspase 8 inhibitor (Ile-Glu-Thr-Asp-p-nitroanilide [IETD]) or dominant negative procaspase 8 blocked the potentiation of bile acid-induced apoptosis. Inhibition of bile acid-induced MAPK signaling enhanced the cleavage of BID and release of cytochrome c from mitochondria, which were all blocked by IETD. Despite activation of caspase 8, expression of dominant negative procaspase 9 blocked procaspase 3 cleavage and the potentiation of DCA-induced apoptosis. Treatment of hepatocytes with DCA transiently increased expression of the caspase 8 inhibitor proteins c-FLIP-(S) and c-FLIP-(L) that were reduced by inhibition of MAPK or PI(3) kinase. Constitutive overexpression of c-FLIP-(s) abolished the potentiation of bile acid-induced apoptosis. Collectively, our data argue that loss of DCA-induced EGFR/Ras/MAPK pathway function potentiates DCA-stimulated FAS-induced hepatocyte cell death via a reduction in the expression of c-FLIP isoforms.

Expression of sterol 12alpha-hydroxylase alters bile acid pool composition in primary rat hepatocytes and in vivo

BACKGROUND & AIMS

The rate of 12alpha-hydroxylation of bile acid intermediates is believed to determine the ratio of cholic acid (CA) to chenodeoxycholic acid (CDCA) biosynthesis and the overall hydrophobicity of the bile acid pool. The aim of this study was to determine the effects of the level of expression of sterol 12alpha-hydroxylase (CYP8b1) and cholesterol 7alpha-hydroxylase (CYP7a1) on rates of CA biosynthesis and bile acid pool composition.

METHODS

Expression of CYP8b1 and CYP7a1 was accomplished through infection of primary rat hepatocytes (PRH) or intact male SD rats with replication-defective recombinant adenoviruses encoding either CYP8b1 or CYP7a1.

RESULTS

Increased expression of CYP7a1 over basal levels in PRH dramatically increased bile acid biosynthesis (586% +/- 82%, P < 0.001) but did not alter the ratio of CA to CDCA. Conversely, increased expression of CYP8b1 in vitro had no significant effect on the rates of total bile acid synthesis but significantly increased (4.1-fold) the rates of CA biosynthesis, resulting in an increase in the CA-CDCA ratio from 1:6.6 to 2.8:1. In whole rats, increased CYP8b1 expression over basal levels markedly increased the CA in the bile acid pool from 36% +/- 3.4% to 50% +/- 2.9% in 5 days. CDCA and its muricholic acid derivatives decreased from 64% +/- 3.4% to 50% +/- 2.9%.

CONCLUSIONS

Increased expression of CYP8b1 led to a marked increase in CA biosynthesis both in PRH and in whole animals. CYP8b1 is capable of 12alpha-hydroxylating bile acid intermediates from both the classic and acidic pathways.

Down-regulation of cholesterol 7alpha-hydroxylase (CYP7A1) gene expression by bile acids in primary rat hepatocytes is mediated by the c-Jun N-terminal kinase pathway

Cholesterol 7alpha-hydroxylase (CYP7A1), the rate-limiting enzyme in the neutral pathway of bile acid biosynthesis, is feedback-inhibited at the transcriptional level by hydrophobic bile acids. Recent studies show that bile acids are physiological ligands for farnesoid X receptor (FXR). Activated FXR indirectly represses CYP7A1 transcription through induction of small heterodimer protein (SHP-1). In this study, we provide evidence that bile acids rapidly down-regulate CYP7A1 transcription via activation of the JNK/c-Jun pathway. Furthermore, we demonstrate that SHP-1 is also a direct target of activated c-Jun. In primary rat hepatocyte cultures, taurocholate (TCA) strongly activated JNK in a time- and concentration-dependent manner. Tumor necrosis factor-alpha, a potent activator of JNK, also rapidly activated JNK and down-regulated CYP7A1 mRNA levels. Overexpression of dominant-negative JNK1 or a transactivating domain mutant of c-Jun significantly blocked the ability of TCA to down-regulate CYP7A1 mRNA. In contrast, overexpression of wild-type c-Jun (c-Jun(wt)) enhanced the repression of CYP7A1 by TCA. Moreover, overexpression of c-Jun(wt) resulted in increased SHP-1 promoter activity. Mutation of a putative AP-1 (c-Jun) element suppressed c-Jun-mediated activation of the SHP-1 promoter construct. These results indicate that the bile acid-activated JNK pathway plays a pivotal role in regulating CYP7A1 levels in primary rat hepatocytes.

A role for both Ets and C/EBP transcription factors and mRNA stabilization in the MAPK-dependent increase in p21 (Cip-1/WAF1/mda6) protein levels in primary hepatocytes

In primary hepatocytes and HepG2 hepatoma cells, prolonged activation of the p42/44 mitogen-activated protein kinase (MAPK) pathway is associated with a reduction in DNA synthesis, mediated by increased expression of the cyclin-dependent kinase inhibitor protein p21 (Cip-1/WAF1/mda6) (p21). This study was performed to evaluate the contribution of transcriptional and post-transcriptional regulation in this response. Prolonged activation of the MAPK pathway in wild-type or p21 null hepatocytes caused a large decrease and increase, respectively, in DNA synthesis. Prolonged activation of the MAPK pathway in either wild-type or p21 antisense HepG2 cells also caused large decreases and increases, respectively, in DNA synthesis. MAPK signaling increased the phosphorylation of the transcription factors Ets2, C/EBPalpha, and C/EBPbeta, and rapidly increased transcription from the p21 promoter via multiple Ets- and C/EBP-elements within the enhancer region. Eight hours after MAPK activation, loss of C/EBPbeta or Ets2 function significantly reduced MAPK-stimulated transcription from the p21 promoter and abolished increased p21 protein expression. At this time, MAPK signaling increased both p21 mRNA and p21 protein stabilities that were also demonstrated to be essential for a profound increase in p21 protein levels. Thirty-six hours after MAPK activation, transcription from the p21 promoter was still significantly reduced in cells without either C/EBPbeta or Ets2 function; however, these cells were now capable of exhibiting a partial increase in p21 protein expression. In contrast, loss of C/EBPalpha function modestly reduced MAPK-stimulated transcription from the p21 promoter but strongly inhibited the ability of prolonged MAPK activation to increase protein levels of p21. This data suggested that prolonged enhancement of p21 protein levels may be under posttranscriptional control. In agreement with this hypothesis, prolonged MAPK signaling further increased p21 mRNA stability at 36 h, compared with the 8-h time point. Our data argue that MAPK signaling increased p21 promoter activity via multiple transcription factors, which alone were insufficient for a robust prolonged increase in p21 protein levels in primary hepatocytes, and that to increase p21 protein levels also required enhanced stabilization of p21 mRNA and p21 protein. Collectively, these data suggest that loss of transcription factor and mRNA/protein stabilization functions correlates with an inability of MAPK signaling to cause growth arrest versus proliferation in primary hepatocytes.

Regulation of multidrug resistance 2 P-glycoprotein expression by bile salts in rats and in primary cultures of rat hepatocytes

Biliary phospholipid secretion is tightly coupled to the secretion of free cholesterol and bile salts. The secretion of phospholipids across the canalicular membrane of hepatocytes occurs via the multidrug resistance 2 (mdr2) P-glycoprotein (Pgp). The mechanism underlying the coupling of bile salt and phospholipid secretion has not been elucidated. The aims of this study were to determine the effects of bile acid structure on the expression of mdr2 in vitro and in vivo. Under optimal culture conditions, taurine-conjugated bile acids (50 micromol/L) increased mdr2 messenger RNA (mRNA) levels in the following order: taurocholate (TCA) (288 +/- 36%, P <. 005) = taurodeoxycholate (TDCA) (276 +/- 36%, P <.025) > taurochenodeoxycholate (TCDCA) (216 +/- 34%, P <.025) > tauroursodeoxycholate (TUDCA) (175 +/- 28%, P <.05) of control levels. The increase in mdr2 mRNA levels by TCA was both time and concentration dependent. Cholate feeding to rats with intact enterohepatic circulation increased mdr2 transcriptional activity by 4-fold and protein mass by 1.9-fold. Chronic biliary diversion (CBD) decreased mdr2 mRNA levels to 66 +/- 9% (P <.025) of sham-operated controls. Intraduodenal infusion of TCA for 48 hours in CBD rats caused a significant increase in mdr2 mRNA levels (224%) as compared with CBD controls. A diet high in cholesterol (4%) decreased mdr2 mRNA levels to 57% +/- 2 (P <.001) of pair-fed controls. Squalestatin (1 micromol/L), an inhibitor of cholesterol biosynthesis, increased mdr2 mRNA levels by 8.8-fold (P <.005) in hepatocyte cultures after 24 hours. In conclusion, in the rat, bile acids up-regulated mdr2 transcriptional activity whereas cholesterol decreased mdr2 mRNA both in vitro and in vivo.

Identification and characterization of a bile acid 7alpha-dehydroxylation operon in Clostridium sp. strain TO-931, a highly active 7alpha-dehydroxylating strain isolated from human feces

Clostridium sp. strain TO-931 can rapidly convert the primary bile acid cholic acid to a potentially toxic compound, deoxycholic acid. Mixed oligonucleotide probes were used to isolate a gene fragment encoding a putative bile acid transporter from Clostridium sp. strain TO-931. This DNA fragment had 60% nucleotide sequence identity to a known bile acid transporter gene from Eubacterium sp. strain VPI 12708, another bile acid-7alpha-dehydroxylating intestinal bacterium. The DNA (9.15 kb) surrounding the transporter gene was cloned from Clostridium sp. strain TO-931 and sequenced. Within this larger DNA fragment was a 7.9-kb region, containing six successive open reading frames (ORFs), that was encoded by a single 8.1-kb transcript, as determined by Northern blot analysis. The gene arrangement and DNA sequence of the Clostridium sp. strain TO-931 operon are similar to those of a Eubacterium sp. strain VPI 12708 bile acid-inducible operon containing nine ORFs. Several genes in the Eubacterium sp. strain VPI 12708 operon have been shown to encode products required for bile acid 7alpha-dehydroxylation. In Clostridium sp. strain TO-931, genes potentially encoding bile acid-coenzyme A (CoA) ligase, 3alpha-hydroxysteroid dehydrogenase, bile acid 7alpha-dehydratase, bile acid-CoA hydrolase, and a bile acid transporter were similar in size and exhibited amino acid homology to similar gene products from Eubacterium sp. strain VPI 12708 (encoded by baiB, baiA, baiE, baiF, and baiG, respectively). However, no genes similar to Eubacterium sp. strain VPI 12708 biaH or baiI were found in the Clostridium sp. strain TO-931 bai operon, and the two putative Eubacterium sp. strain VPI 12708 genes, baiC and baiD, were arranged in one continuous ORF in Clostridium sp. strain TO-931. Intergene regions showed no significant DNA sequence similarity, but primer extension analysis identified a region 115 bp upstream from the first ORF that exhibited 58% identity to a bai operator/promoter region identified in Eubacterium sp. strain VPI 12708. These results indicate that the gene organization, gene product amino acid sequences, and promoters of the bile acid-inducible operons of Clostridium sp. strain TO-931 and Eubacterium sp. strain VPI 12708 are highly conserved.

Regulation of sterol 12alpha-hydroxylase and cholic acid biosynthesis in the rat

BACKGROUND & AIMS

Sterol 12alpha-hydroxylase (CYP8b1) is required for the biosynthesis of cholic acid (CA) and hence helps determine the ratio of CA to chenodeoxycholic acid (CDCA) in bile. This study examined the in vivo regulation of CYP8b1 in the rat by bile acids, cholesterol, and thyroxine.

METHODS

The specific activities (SAs), messenger RNA (mRNA) levels, and transcriptional activities of CYP8b1 were determined in intact rats and rats with biliary diversion.

RESULTS

CA, CDCA, and deoxycholic acid (DCA), fed as a supplement to the diet, down-regulated CYP8b1 SAs by 99% +/- 0%, 72% +/- 10%, and 98% +/- 1%, respectively. Under these same conditions, mRNA levels decreased by 93% +/- 7%, 60% +/- 11%, and 93% +/- 4%, respectively. Intraduodenal infusion of taurocholate (36 micromol/h. 100 g rat(-1)) decreased SAs and mRNA levels by 63% +/- 8% and 74% +/- 8%, respectively. Ursodeoxycholic acid (UDC) and hyocholic acid (HC) feeding increased CYP8b1 SAs by 119% +/- 21% and 65% +/- 18%, respectively. CA feeding decreased CYP8b1 transcriptional activity by 72%. Complete biliary diversion increased CYP8b1 SAs and mRNA levels by 150% +/- 30% and 287% +/- 51%, respectively. Cholesterol feeding decreased CYP8b1 mRNA by 39% +/- 8%. In intact rats, a single injection of thyroid hormone eliminated CYP8b1 activity.

CONCLUSIONS

CYP8b1 is transcriptionally down-regulated by hydrophobic but not hydrophilic bile acids. Cholesterol feeding and a single thyroid hormone injection repressed CYP8b1 in the face of induction of cholesterol 7alpha-hydroxylase (CYP7a1 by the new nomenclature) SAs. These results suggest that cholesterol, thyroid hormone, and hydrophobic bile acids are important regulators of CYP8b1 and consequently of the bile acid pool composition.

Isolation and characterization of cholic acid 7alpha-dehydroxylating fecal bacteria from cholesterol gallstone patients

BACKGROUND/AIMS

The development of cholesterol gallstones, in some patients, has been associated with increased proportions of deoxycholic acid in the bile acid pool. Deoxycholic acid is a microbial product of cholic acid 7alpha-dehydroxylation in the intestines. The levels and activities of bile acid 7alpha-dehydroxylating bacteria have been reported to be increased in gallstone patients. The aim of the current study was to isolate 7alpha-dehydroxylating bacteria from gallstone patients and determine if these individuals are colonized by similar bacterial species.

METHODS

The levels of 7alpha-dehydroxylating bacteria in fecal samples were determined by fecal dilutions in 24 gallstone patients and 10 controls. 7alpha-Dehydroxylating bacteria were isolated by a non-selective streak plate technique and 7alpha-dehydroxylation activity was determined by measuring the conversion of [14C]-cholic acid to [14C]-deoxycholic acid using thin-layer chromatography.

RESULTS

Gallstone patients had >42-fold (p<0.01) higher levels of 7alpha-dehydroxylating bacteria than patients who had not developed gallstones. Eighteen strains of 7alpha-dehydroxylating bacteria were isolated from eight gallstone patients. Attempts to isolate 7alpha-dehydroxylating bacteria from ten control patients were unsuccessful using identical isolation techniques. Surprisingly, all strains of bacteria isolated from gallstone patients appear to belong to the genus Clostridium.

CONCLUSION

Gallstone patients have higher levels of 7alpha-dehydroxylating fecal bacteria and appear to harbor only members of the genus Clostridium with this activity.

Down-regulation of the rat hepatic sterol 27-hydroxylase gene by bile acids in transfected primary hepatocytes: possible role of hepatic nuclear factor 1alpha

In vitro and in vivo studies have shown that the sterol 27-hydroxylase (CYP27) gene is transcriptionally repressed by hydrophobic bile acids. The molecular mechanism(s) of repression of CYP27 by bile acids is unknown. To identify the bile acid responsive element (BARE) and transcription factor(s) that mediate the repression of CYP27 by bile acids, constructs of the CYP27 5'-flanking DNA were linked to either the CAT or luciferase reporter gene and transiently transfected into primary rat hepatocytes. Taurocholate (TCA), taurodeoxycholate (TDCA) and taurochenodeoxycholate (TCDCA) significantly reduced CAT activities of the -840/+23, -329/+23, and -195/+23 mCAT constructs. A -76/+23 construct showed no regulation by bile acids. When a DNA fragment (-110/-86) from this region was cloned in front of an SV 40 promoter it showed down-regulation by TDCA. 'Super'-electrophoretic mobility shift assays (EMSA) indicated that both HNF1alpha and C/EBP bind to the -110 to -86 bp DNA fragment. Recombinant rat HNF1alpha and C/EBPalpha competitively bound to this DNA fragment. 'Super'-EMSA showed that TDCA addition to hepatocytes in culture decreased HNF1alpha, but not C/EBP, binding to the -110/-86 bp DNA fragment. A four base pair substitution mutation (-103 to -99) in this sequence eliminated TCA and TDCA regulation of the (-840/+23) construct. The substitution mutation also eliminated (>95%) HNF1alpha, but not C/EBP, binding to this DNA fragment. We conclude that bile acids repress CYP27 transcription through a putative BARE located between -110 and -86 bp of the CYP27 promoter. The data suggest that bile acids repress CYP27 transcriptional activity by decreasing HNF1alpha binding to the CYP27 promoter.

Use of a short A/T-rich cassette for enhanced expression of cloned genes in Escherichia coli

A short (43-bp) A/T-rich stretch of DNA located in the intergenic region between the baiA2 and baiF genes from Eubacterium sp. strain VPI 12708 was amplified by polymerase chain reaction (PCR) and inserted in front of the Shine-Dalgarno (SD) sequences of three inefficiently-expressed Eubacterium sp. strain VPI 12708 genes cloned in Escherichia coli plasmids. Insertion of this A/T-rich cassette increased gene expression in all cases tested. Deletion of part of the A/T-rich region from a baiF clone in pUC19 resulted in decreased gene expression. Synthesis of specific mRNA was increased with addition of the A/T-rich cassette to constructs containing the baiC gene from Eubacterium sp. strain VPI 12708, but mRNA synthesis was not significantly changed in cells containing plasmid constructs with the baiF and baiG genes. Enhanced translation resulting from a decrease in mRNA secondary structure in the ribosome binding site region is discussed as a possible reason for increased gene expression with the A/T-rich cassette.

The bile acid-inducible baiF gene from Eubacterium sp. strain VPI 12708 encodes a bile acid-coenzyme A hydrolase

The human intestinal Eubacterium sp. strain VPI 12708 has been shown to have a multistep biochemical pathway for bile acid 7alpha-dehydroxylation. A bile acid-inducible operon encoding 9 open reading frames has been cloned and sequenced from this organism. Several of the genes in this operon have been shown to catalyze specific reactions in the 7alpha-dehydroxylation pathway. The baiF gene from this operon was cloned, expressed in Escherichia coli, and found to encode a novel bile acid-coenzyme A (CoA) hydrolase. The subunit molecular mass of the purified bile acid-CoA hydrolase was calculated to be 47,466 daltons and the native enzyme had a relative molecular weight of 72,000. The K m and Vmax for cholyl-coenzyme A (CoA) hydrolysis was approximately 175 microm and 374 micromol/min per mg protein, respectively. The enzyme used cholyl-CoA, 3-dehydrocholyl-CoA, and chenodeoxycholyl-CoA as substrates. No hydrolytic activity was detected using acetyl-CoA, isovaleryl-CoA, palmitoyl-CoA, or phenylacetyl-CoA as substrates. Amino acid sequence database searches showed no significant similarity of bile acid-CoA hydrolase to other thioesterases, but significant amino acid sequence identity was found with Escherichia coli carnitine dehydratase. The characteristic thioesterase active site Gly-X-Ser-X-Gly motif was not found in the amino acid sequence of this enzyme. Bile acid-CoA hydrolase from Eubacterium sp. strain VPI 12708 may represent a new family of thioesterases.

Biotransformation of monoterpenes, bile acids, and other isoprenoids in anaerobic ecosystems

Isoprenoic compounds play a major part in the global carbon cycle. Biosynthesis and mineralization by aerobic bacteria have been intensively studied. This review describes our knowledge on the anaerobic metabolism of isoprenoids, mainly by denitrifying and fermentative bacteria. Nitrate-reducing beta-Proteobacteria were isolated on monoterpenes as sole carbon source and electron donor. Thauera spp. were obtained on the oxygen-containing monoterpenes linalool, menthol, and eucalyptol. Several strains of Alcaligenes defragrans were isolated on unsaturated monoterpenes as growth substrates. A novel denitrifying beta-Proteobacterium, strain 72Chol, mineralizes cholesterol completely to carbon dioxide. Physiological studies showed the presence of several oxidative pathways in these microorganisms. Investigations by organic geochemists indicate possible contributions of anaerobes to early diagenetic processes. One example, the formation of p-cymene from monoterpenes, could indeed be detected in methanogenic enrichment cultures. In man, cholic acid (CA) and chenodeoxycholic acid (CDCA), are synthesized in the liver from cholesterol. During their enterohepatic circulation, bile acids are biotransformed by the intestinal microflora into a variety of metabolites. Known bacterial biotranformations of conjugated bile acids include: deconjugation, oxidation of hydroxy groups at C-3, C-7 and C-12 with formation of oxo bile acids and reduction of these oxo groups to either alpha- or beta-configuration. Quantitatively, the most important bacterial biotransformation is the 7 alpha-dehydroxylation of CA and CDCA yielding deoxycholic acid and lithocholic acid, respectively. The 7 alpha-dehydroxylation of CA occurs via a novel six-step biochemical pathway. The genes encoding several enzymes that either transport bile acids or catalyze various reactions in the 7 alpha-dehydroxylation pathway of Eubacterium sp. strain VPI 12708 have been cloned, expressed in Escherichia coli, purified, and characterized.

Regulation of hepatic neutral cholesteryl ester hydrolase by hormones and changes in cholesterol flux

To understand molecular events in regulation of hepatic neutral cholesteryl ester hydrolase (EC3.1.1.13; CEH), catalytic activity, protein mass, and mRNA levels were measured in rats with various perturbations of hepatic cholesterol metabolism. Cholesterol feeding decreased activity (56 +/- 2%), mass (44 +/- 2%), and mRNA (14 +/- 3%). The cholesterol precursor mevalonate also decreased activity (42 +/- 6%), mass (76 +/- 3%), and mRNA (23 +/- 16%). Inhibition of cholesterol biosynthesis by lovastatin increased activity (65 +/- 12%) and mRNA (31 +/- 24%). Stimulation of cholesterol efflux by chronic biliary diversion increased activity (138 +/- 34%), mass (29 +/- 7%), and mRNA (146 +/- 28%). Chenodeoxycholate feeding decreased activity (46 +/- 6%) and mRNA (26 +/- 12%). These data suggest rational regulation of CEH in response to changes in cholesterol flux through the liver. In primary hepatocytes, steady-state mRNA markedly decreased during 72-h cultures and addition of L-thyroxine and dexamethasone synergistically maintained mRNA levels near control values. Lovastatin increased mRNA levels by 103 +/- 15%. Taurocholate and phorbol 12-myristate 13-acetate suppressed mRNA (61 +/- 4% and 49 +/- 13%, respectively), suggesting that protein kinase C mediated effects of bile acids on CEH mRNA levels. These data suggest regulation of CEH by hormones and signal transduction in addition to changes in cholesterol flux.