The role of CYP2A and CYP2E1 in the metabolism of 3-methylindole in primary cultured porcine hepatocytes

Functional Characterization of Cytochromes P450 Linked to Herbicide Detoxification and Selectivity in Winter Wheat and the Problem Competing Weed Blackgrass

The selective chemical control of wild grasses in wheat is primarily determined by the relative rates of herbicide metabolism, with the superfamily of cytochromes P450 (CYPs) playing a major role in catalyzing phase 1 detoxification reactions. This selectivity is enhanced by herbicide safeners, which induce CYP expression in cereals, or challenged by the evolution of nontarget site resistance (NTSR) in weeds such as blackgrass. Using transcriptomics, proteomics, and functional expression in recombinant yeast, CYPs linked to safener treatment and NTSR have been characterized in wheat and blackgrass. Safener treatment resulted in the induction of 13 families of CYPs in wheat and 5 in blackgrass, with CYP71, CYP72, CYP76, and CYP81 members active toward selective herbicides in the crop. Based on their expression and functional activities, three inducible TaCYP81s were shown to have major roles in safening in wheat. In contrast, a single AmCYP81 that was enhanced by NTSR, but not by safening, was found to dominate herbicide detoxification in blackgrass.

Nuclear Receptor Pathways Mediating the Development of Boar Taint

The nuclear receptors PXR, CAR, and FXR are activated by various ligands and function as transcription factors to control the expression of genes that regulate the synthesis and metabolism of androstenone and skatole. These compounds are produced in entire male pigs and accumulate in the fat to cause the development of a meat quality issue known as boar taint. The extent of this accumulation is influenced by the synthesis and hepatic clearance of androstenone and skatole. For this reason, PXR, CAR, and FXR-mediated signaling pathways have garnered interest as potential targets for specialized treatments designed to reduce the development of boar taint. Recent research has also identified several metabolites produced by gut microbes that act as ligands for these nuclear receptors (e.g., tryptophan metabolites, short-chain fatty acids, bile acids); however, the connection between the gut microbiome and boar taint development is not clear. In this review, we describe the nuclear receptor signaling pathways that regulate the synthesis and metabolism of boar taint compounds and outline the genes involved. We also discuss several microbial-derived metabolites and dietary additives that are known or suspected nuclear receptor ligands and suggest how these compounds could be used to develop novel treatments for boar taint.

Establishment of a p-nitrophenol oxidation-based assay for the analysis of CYP2E1 activity in intact hepatocytes in vitro

CYP2E1 is a mammalian cytochrome P450 enzyme, which oxidizes a structurally diverse class of endogenous and exogenous (xenobiotic) compounds. Best studied is the role of CYP2E1 in phase I metabolism of xenobiotics including alcohol. CYP2E1 metabolizes ethanol and is active in generating reactive oxygen species (ROS) and subsequent oxidative stress in the hepatic tissues. Several studies have shown and discussed the importance of CYP2E1 in the hepatotoxic actions of alcohol. However, the vast majority assessed the CYP2E1 activity only in isolated microsomes. Here, we aimed to develop and optimize a fast and easy method to assess alcohol-induced CYP2E1 activity in hepatocytes in vitro applying oxidation of para-nitrophenol to para-nitrocatechol as specific substrate probe. Using hepatoma cells with and without stable CYP2E1 expression and primary human hepatocytes, we established specific methodology to assess CYP2E1 catalytic activity and its induction by ethanol in a small number of cells and in a very short time.

Skatole metabolites in urine as a biological marker of pigs with enhanced hepatic metabolism

Boar taint is a quality defect in meat, related to accumulation of skatole and androstenone in male pigs. The levels of skatole and its main metabolites in plasma and urine samples were measured with a validated liquid chromatography-MS method and related to activity of hepatic cytochrome P450 (CYP450) in order to identify 'fast metabolizing' pigs. Urine (n=46), blood (n=12), liver (n=25) and adipose tissue (n=46) were sampled from a total of 46 entire male pigs. Skatole levels in fat were negatively correlated to CYP2E1 activity and positively to 3-hydroxy-3-methyloxindole (HMOI), indole-3-carboxylic acid (ICA) and 2-aminoacetophenone in urine. HMOI and ICA levels in urine were the best predictors of high skatole levels in fat. In summary, the present study provided further evidence for the key role of CYP2E1 in skatole metabolism and suggested that measurement of HMOI and/or ICA in urine might provide information about skatole levels in live pigs.

The role of CYP2A5 in liver injury and fibrosis: chemical-specific difference

Liver injuries induced by carbon tetrachloride (CCL4) or thioacetamide (TAA) are dependent on cytochrome P450 2E1 (CYP2E1). CYP2A5 can be induced by TAA but not by CCL4. In this study, liver injury including fibrosis induced by CCL4 or TAA were investigated in wild-type (WT) mice and CYP2A5 knockout (cyp2a5 (-/-) ) mice as well as in CYP2E1 knockout (cyp2e1 (-/-) ) mice as a comparison. Acute and subchronic liver injuries including fibrosis were induced by CCL4 and TAA in WT mice but not in cyp2e1 (-/-) mice, confirming the indispensable role of CYP2E1 in CCL4 and TAA hepatotoxicity. WT mice and cyp2a5 (-/-) mice developed comparable acute liver injury induced by a single injection of CCL4 as well as subchronic liver injury including fibrosis induced by 1 month of repeated administration of CCL4, suggesting that CYP2A5 does not affect CCL4-induced liver injury and fibrosis. However, while 200 mg/kg TAA-induced acute liver injury was comparable in WT mice and cyp2a5 (-/-) mice, 75 and 100 mg/kg TAA-induced liver injury were more severe in cyp2a5 (-/-) mice than those found in WT mice. After multiple injections with 200 mg/kg TAA for 1 month, while subchronic liver injury as indicated by serum aminotransferases was comparable in WT mice and cyp2a5 (-/-) mice, liver fibrosis was more severe in cyp2a5 (-/-) mice than that found in WT mice. These results suggest that while both CCL4- and TAA-induced liver injuries and fibrosis are CYP2E1 dependent, under some conditions, CYP2A5 may protect against TAA-induced liver injury and fibrosis, but it does not affect CCL4 hepatotoxicity.

Regulation of porcine hepatic cytochrome p450 - implication for boar taint

Cytochrome P450 (CYP450) is the major family of enzymes involved in the metabolism of several xenobiotic and endogenous compounds. Among substrates for CYP450 is the tryptophan metabolite skatole (3-methylindole), one of the major contributors to the off-odour associated with boar-tainted meat. The accumulation of skatole in pigs is highly dependent on the hepatic clearance by CYP450s. In recent years, the porcine CYP450 has attracted attention both in relation to meat quality and as a potential model for human CYP450. The molecular regulation of CYP450 mRNA expression is controlled by several nuclear receptors and transcription factors that are targets for numerous endogenously and exogenously produced agonists and antagonists. Moreover, CYP450 expression and activity are affected by factors such as age, gender and feeding. The regulation of porcine CYP450 has been suggested to have more similarities with human CYP450 than other animal models, including rodents. This article reviews the available data on porcine hepatic CYP450s and its implications for boar taint.

Regulation of cytochrome P450 mRNA expression in primary porcine hepatocytes by selected secondary plant metabolites from chicory (Cichorium intybus L.)

Chicory (Cichorium intybus) has been shown to induce enzymes of pharmacokinetic relevance (cytochrome P450; CYP). The aim of this study was to investigate the effects of selected secondary plant metabolites with a global extract of chicory root, on the expression of hepatic CYP mRNA (1A2, 2A19, 2C33, 2D25, 2E1 and 3A29), using primary porcine hepatocytes. Of the tested secondary plant metabolites, artemisinin, scoparone, lactucin and esculetin all induced increased expression of specific CYPs, while esculin showed no effect. In contrast, a global extract of chicory root decreased the expression of CYP1A2, 2C33, 2D25 and 3A29 at high concentrations. The results suggest that purified secondary metabolites from chicory affect CYP expression and thereby might affect detoxification in general, and that global extracts of plants can have effects different from individual components.

Biochemical, nutritional and genetic effects on boar taint in entire male pigs

Pork odour is to a great extent affected by the presence of malodorous compounds, mainly androstenone and skatole. The present review outlines the current state of knowledge about factors involved in the regulation of androstenone and skatole in entire male pigs. Androstenone is a pheromonal steroid synthesised in the testes and metabolised in the liver. Part of androstenone accumulates in adipose tissue causing a urine-like odour. Skatole is produced in the large intestine by bacterial degradation of tryptophan and metabolised by hepatic cytochrome P450 enzymes and sulphotransferase. The un-metabolised part accumulates in adipose tissue, causing faecal-like odour. Androstenone levels are mostly determined by genetic factors and stage of puberty, whereas skatole levels in addition to genetic background and hormonal status of the pigs are also controlled by nutritional and environmental factors. To reduce the risk of tainted carcasses entering the market, male pigs are surgically castrated in many countries. However, entire males compared to castrates have superior production characteristics: higher growth rate, better feed efficiency and leaner carcasses. Additionally, animal welfare aspects are currently of particular importance in light of increasing consumers' concerns. Nutrition, hormonal status, genetic influence on boar taint compounds and the methods to develop genetic markers are discussed. Boar taint due to high levels of skatole and androstenone is moderately heritable and not all market weight entire males have boar taint; it should thus be possible to select for pigs that do not have boar taint. In these studies, it is critical to assess the steroidogenic potential of the pigs in order to separate late-maturing pigs from those with a low genetic potential for boar taint. A number of candidate genes for boar taint have been identified and work is continuing to develop genetic markers for low boar taint. More research is needed to clarify the factors involved in the development of boar taint and to develop additional methods to prevent the accumulation of high concentrations of skatole and androstenone in fat. This review proposes those areas requiring further research.

Nutritional Influences on Skatole Formation and Skatole Metabolism in the Pig

Skatole is a tryptophan (TRP) metabolite with fecal odor. Together with the testicular steroid androstenone it is regarded as a main determinant of boar taint, even if elevated concentrations of skatole occur occasionally in gilts and barrows. Skatole concentrations in adipose tissue result from a complex process, which includes the availability of TRP and the presence of specialized bacteria in the gut in need of TRP for energy production, as well as absorption, transport and accumulation of skatole in adipose tissue. Several steps of this process are influenced by diet and specific feed compounds. In the present paper the current knowledge about physiological mechanisms of skatole dynamics is summarized. Additionally mechanisms are discussed, by which effective feeding strategies and feed additives exert their influence in the prevention of high skatole concentrations in adipose pig tissue. It was concluded that the most effective measures are those which influence several steps of skatole formation. Despite the numerous studies carried out in the field of skatole physiology, interesting aspects still need clarification, such as the effect of adipose tissue turnover. Reliable control of skatole accretion in fat of boars is one of the main prerequisites for pork production with entire males.

Gender-related differences in cytochrome P450 in porcine liver--implication for activity, expression and inhibition by testicular steroids

In pigs, the hepatic cytochrome P450 (CYP) 1A2, 2A and 2E1 activity is important in the regulation of skatole accumulation in adipose tissue. This study investigated gender-related differences in CYP1A2, 2A and 2E1 dependent activity, protein and mRNA expression. This study also investigated the gonadal steroid dependent inhibition of CYP activity in relation to gender and dietary composition. Microsomes were prepared from the liver of female and entire male pigs (Landrace × Yorkshire sire and Duroc boars) reared under similar conditions and slaughtered at an age of 164 days. A group of entire male pigs fed dried chicory root for 16 days prior to slaughter were included in the study. CYP activities were assessed by the use of probe substrates, whilst mRNA and protein expression were analysed by RT-PCR and Western blotting. Furthermore inhibition of CYP dependent activity by gonadal steroids was assessed in vitro. Microsomes from female pigs had greater CYP1A2 and 2A activity, as well as mRNA expression compared to entire male pigs. The antibodies used did not detected differences in protein expression. In vitro inhibition by 17β-oestradiol, oestrone, androstenone and 3β-OH androstenol of CYP2E1 activity in microsomes from entire male pigs as well as inhibition of CYP1A activity in chicory fed entire male pigs was observed. Apart from that no effect of steroids was shown. In conclusion, female pigs show greater CYP activity and mRNA expression. Including chicory in the diet for 16 days changed the gonadal steroid dependent inhibition of CYP activity in entire male pigs.

Gene expression profiles in liver of pigs with extreme high and low levels of androstenone

BACKGROUND

Boar taint is the unpleasant odour and flavour of the meat of uncastrated male pigs that is primarily caused by high levels of androstenone and skatole in adipose tissue. Androstenone is a steroid and its levels are mainly genetically determined. Studies on androstenone metabolism have, however, focused on a limited number of genes. Identification of additional genes influencing levels of androstenone may facilitate implementation of marker assisted breeding practices. In this study, microarrays were used to identify differentially expressed genes and pathways related to androstenone metabolism in the liver from boars with extreme levels of androstenone in adipose tissue.

RESULTS

Liver tissue samples from 58 boars of the two breeds Duroc and Norwegian Landrace, 29 with extreme high and 29 with extreme low levels of androstenone, were selected from more than 2500 individuals. The samples were hybridised to porcine cDNA microarrays and the 1% most significant differentially expressed genes were considered significant. Among the differentially expressed genes were metabolic phase I related genes belonging to the cytochrome P450 family and the flavin-containing monooxygenase FMO1. Additionally, phase II conjugation genes including UDP-glucuronosyltransferases UGT1A5, UGT2A1 and UGT2B15, sulfotransferase STE, N-acetyltransferase NAT12 and glutathione S-transferase were identified. Phase I and phase II metabolic reactions increase the water solubility of steroids and play a key role in their elimination. Differential expression was also found for genes encoding 17beta-hydroxysteroid dehydrogenases (HSD17B2, HSD17B4, HSD17B11 and HSD17B13) and plasma proteins alpha-1-acid glycoprotein (AGP) and orosomucoid (ORM1). 17beta-hydroxysteroid dehydrogenases and plasma proteins regulate the availability of steroids by controlling the amount of active steroids accessible to receptors and available for metabolism. Differences in the expression of FMO1, NAT12, HSD17B2 and HSD17B13 were verified by quantitative real competitive PCR.

CONCLUSION

A number of genes and pathways related to metabolism of androstenone in liver were identified, including new candidate genes involved in phase I oxidation metabolism, phase II conjugation metabolism, and regulation of steroid availability. The study is a first step towards a deeper understanding of enzymes and regulators involved in pathways of androstenone metabolism and may ultimately lead to the discovery of markers to reduce boar taint.

A gel-based reference map of the porcine hepatocyte proteome

The overall goal of our research is to characterize and identify gene expression profiles of porcine hepatic cells. In this study, we have prepared two-dimensional electrophoresis maps of cytosol and membrane fractions from freshly prepared hepatocytes which were pooled from three crossbred pigs (35-69kg). Following isoelectric focusing with three pH range immobilized pH gradient strips (pH 3-6, 5-8 and 7-10) and staining the second dimension gels with colloidal Coomassie blue, 728 protein spots were picked and digested with trypsin. Extracted tryptic peptides were initially subjected to matrix-assisted laser desorption/ionization-time of flight mass spectrometry (MALDI-TOF-MS) analysis for identification of proteins by peptide mass fingerprinting (PMF). Proteins which were not identified by PMF were analyzed by liquid chromatography-tandem MS. Utilizing publicly available databases [NCBInr, Swiss Prot and expressed sequence tags (EST)], 648 proteins were identified. Of those, 282 were unique proteins and greater than 90% of proteins spots contained single proteins. These data represent the first comprehensive proteomic analysis of porcine hepatocytes and will provide a database for future investigations of endocrine regulation of gene expression and metabolic processes in vitro.

Regulation of CYP2A6 protein expression by skatole, indole, and testicular steroids in primary cultured pig hepatocytes

CYP2A6 is one of the enzymes involved in the hepatic metabolism of a naturally produced compound, skatole, in the pig. Low CYP2A6 activity has been linked to excessive accumulation of skatole in pig adipose tissue and development of the phenomenon "boar taint." CYP2A6 activity varies between male and female animals, suggesting the involvement of sex hormones in regulation of the enzyme activity and/or expression. The present study investigated whether pig hepatic CYP2A6 protein expression is regulated by the testicular steroids testosterone, androstenone, or estrone sulfate using primary cultured hepatocytes as a model system. The study has also examined whether CYP2A6 expression can be modulated by the boar taint compounds skatole and indole. The research has established that androstenone inhibits CYP2A6 protein expression at the concentration of 1, 10, and 100 nM by 55, 37, and 44%, respectively. In contrast to androstenone, skatole and indole (final concentrations, 1, 10, and 100 nM) had a stimulatory effect on CYP2A6 expression. The effect of indole was more pronounced than that of skatole (maximum induction by 145 and 70%, respectively). Estrone sulfate and testosterone did not have a significant effect on CYP2A6 protein level. This is, as far as we know, the first communication to report the regulation of pig hepatic CYP2A6 expression by steroids and boar taint compounds. The hormonal modulation of CYP2A6 expression might contribute to gender-related differences in pig hepatic CYP2A6 activity and skatole accumulation in pig adipose tissue.