Adipocyte differentiation of bone marrow-derived mesenchymal stem cells: cross talk with the osteoblastogenic program.

Bone marrow mesenchymal stem cells (MSCs) are multipotent cells, which among other cell lineages, give rise to adipocytes and osteoblasts. Within the bone marrow, the differentiation of MSCs into adipocytes or osteoblasts is competitively balanced; mechanisms that promote one cell fate actively suppress mechanisms that induce the alternative lineage. This occurs through the cross talk between complex signaling pathways including those derived from bone morphogenic proteins (BMPs), winglesstype MMTV integration site (Wnt) proteins, hedgehogs, delta/jagged proteins, fibroblastic growth factors (FGF), insulin, insulin-like growth factors (IGF), and transcriptional regulators of adipocyte and osteoblast differentiation including peroxisome proliferator-activated receptor-gamma (PPAR gamma) and runt-related transcription factor 2 (Runx2). Here, we discuss the molecular regulation of bone marrow adipogenesis with emphasis on signals that interact with osteoblastogenic pathways and highlight the possible therapeutic implications of these interactions.

Evidence for non-genomic transmission of ecological information via maternal behavior in female rats.

Maternal behavior is flexible and programs offspring development. Using a novel manipulation, we demonstrate that rat maternal behavior is sensitive to ecologically relevant stimuli. Long-Evans hooded rat dams (F0) and pups were exposed to a predator condition (cat odor) or a control condition (no odor) for 1 h on the day of parturition. Predator-exposed F0 dams displayed significantly more maternal behavior (licking/grooming, arched-back nursing) relative to control-exposed dams across five subsequent observation days. Female offspring (F1) were raised to adulthood, bred and maternal behavior was observed. F1 dams reared by a predator-exposed F0 dam displayed significantly higher maternal behavior relative to F1 dams reared by a control-exposed F0 dam across 5 days of observation. Increased levels of maternal behavior in predator-reared (PR) F1 dams were evident even in F1 females that had been cross-fostered (CF) from a control-exposed F0 dam, suggesting a non-genomic transmission of increased levels of maternal behavior. Lactating PR F1 dams had significantly elevated estrogen receptor alpha and beta mRNA in the medial preoptic area relative to control-reared (CR) F1 dams. Furthermore, among CR F1 dams, there was no significant difference between those dams that had been CF from predator-exposed F0 dams and those that had been sham CF. These results support the hypothesis that flexible rat maternal behavior can shape offspring development according to current environmental conditions. The results also suggest that estrogen signaling may be part of an epigenetic mechanism by which changes in maternal behavior are passed from F0 to F1 dams.

Sex differences in the chloride cotransporters, NKCC1 and KCC2, in the developing hypothalamus.

In immature neurones, high basal [Cl(-)](i) results in membrane depolarisation following GABA(A) receptor activation, which is critical for various developmental processes including steroid-mediated sexual differentiation of the hypothalamus. Previously, we demonstrated that oestradiol enhances GABA-mediated Ca(2+) influx in neonate hypothalamus and that Ca(2+) induced activation of the transcription factor, cyclicAMP response element binding protein (CREB), was higher in male (high oestradiol) relative to female neonate hypothalamus. Based on these results, we hypothesised that expression of developmentally regulated chloride cotransporters may be sexually dimorphic. Here, we investigate the expression of the chloride cotransporters, NKCC1 (Na-K-2Cl(-)) and KCC2 (K-Cl(-)) in neonate mediobasal hypothalamus of male and female rats. The NKCC1 transporter moves Cl(-) into cells and helps maintain depolarising GABA action while the KCC2 transporter has the opposite effect by moving Cl(-) out of cells. NKCC1 mRNA levels were higher in males than females on the day of birth (postnatal day 0; PND 0) and total NKCC1 protein levels were significantly higher in males than females on embryonic day (ED) 20 and PND0. Levels of activated phosphorylated NKCC1 (pNKCC1) were not sexually dimorphic. Females were treated with a masculinising dose of oestradiol benzoate (EB; 100 microg; EB-females) on PND0. Total NKCC1 protein levels in tissue processed on PND1 and PND2 were similar in EB-females and oil-treated PND0 males and females. However, pNKCC1 protein levels measured on PND2 (but not PND1) were significantly higher in EB-treated females relative to oil-treated males and females. By contrast, KCC2 mRNA levels were significantly lower in males relative to females on PND0. KCC2 protein was not detectable on ED20 or PND0 but was significantly lower in males relative to females on PND5. These results suggest a complex relationship between KCC2 and NKCC1 mRNA and protein in developing brain that is not easily linked to regulation by oestradiol.

Conditional disruption of the aryl hydrocarbon receptor nuclear translocator (Arnt) gene leads to loss of target gene induction by the aryl hydrocarbon receptor and hypoxia-inducible factor 1alpha.

To determine the function of the aryl hydrocarbon receptor nuclear translocator (ARNT), a conditional gene knockout mouse was made using the Cre-loxP system. Exon 6, encoding the conserved basic-helix-loop-helix domain of the protein, was flanked by loxP sites and introduced into the Arnt gene by standard gene disruption techniques using embryonic stem cells. Mice homozygous for the floxed allele were viable and had no readily observable phenotype. The Mx1-Cre transgene, in which Cre is under control of the interferon-gamma promoter, was introduced into the Arnt-floxed mouse line. Treatment with polyinosinic-polycytidylic acid to induce expression of Cre resulted in complete disruption of the Arnt gene and loss of ARNT messenger RNA (mRNA) expression in liver. To determine the role of ARNT in gene control in the intact animal mouse liver, expression of target genes under control of an ARNT dimerization partner, the aryl hydrocarbon receptor (AHR), was monitored. Induction of CYP1A1, CYP1A2, and UGT1*06 mRNAs by the AHR ligand 2,3,7,8-tetrachlorodibenzo-p-dioxin was absent in livers of Arnt-floxed/Mx1-Cre mice treated with polyinosinic-polycytidylic. These data demonstrate that ARNT is required for AHR function in the intact animal. Partial deletion of the Arnt allele was found in kidney, heart, intestine, and lung. Despite more than 80% loss of the ARNT expression in lung, maximal induction of CYP1A1 was found, indicating that the expression level of ARNT is not limiting to AHR signaling. Cobalt chloride induction of the glucose transporter-1 and heme oxygenase-1 mRNAs was also markedly abrogated in mice lacking ARNT expression, suggesting an inhibition of HIF-1alpha activity. These studies establish a critical role for ARNT in AHR and HIF-1alpha signal transduction in the intact mouse.

Farnesoid X-activated receptor induces apolipoprotein C-II transcription: a molecular mechanism linking plasma triglyceride levels to bile acids.

The farnesoid X-activated receptor (FXR; NR1H4), a member of the nuclear hormone receptor superfamily, induces gene expression in response to several bile acids, including chenodeoxycholic acid. Here we used suppression subtractive hybridization to identify apolipoprotein C-II (apoC-II) as an FXR target gene. Retroviral expression of FXR in HepG2 cells results in induction of the mRNA encoding apoC-II in response to several FXR ligands. EMSAs demonstrate that recombinant FXR and RXR bind to two FXR response elements that are contained within two important distal enhancer elements (hepatic control regions) that lie 11 kb and 22 kb upstream of the transcription start site of the apoC-II gene. A luciferase reporter gene containing the hepatic control region or two copies of the wild-type FXR response element was activated when FXR-containing cells were treated with FXR ligands. In addition, we report that hepatic expression of both apoC-II and phospholipid transfer protein mRNAs increases when mice are fed diets supplemented with cholic acid, an FXR ligand, and this induction is attenuated in FXR null mice. Finally, we observed decreased plasma triglyceride levels in mice fed cholic acid- containing diets. These results identify a mechanism whereby FXR and its ligands lower plasma triglyceride levels. These findings may have important implications in the clinical management of hyperlipidemias.

Towards an integrative approach to understanding the role of chemerin in human health and disease.

Chemerin is an adipocyte-secreted protein with autocrine/paracrine roles on adipose development and function as well as endocrine roles in metabolism and immunity. Following prochemerin secretion, protease-mediated generation of chemerin isoforms with a range of biological activities is a key regulatory mechanism controlling local, context-specific chemerin bioactivity. Together, experimental and clinical data indicate that localized and/or circulating chemerin expression and activation are elevated in numerous metabolic and inflammatory diseases including psoriasis, obesity, type 2 diabetes, metabolic syndrome and cardiovascular disease. These elevations are positively correlated with deleterious changes in glucose, lipid, and cytokine homeostasis, and may serve as a link between obesity, inflammation and other metabolic disorders. This review highlights the current state of knowledge regarding chemerin expression, processing, biological function and relevance to human disease, particularly with respect to adipose tissue development, inflammation, glucose homeostasis and cardiovascular disease. Furthermore, it discusses study variability, deficiencies in current measurement, and questions concerning chemerin function in disease, with a special emphasis on techniques and tools used to properly assess chemerin biology. An integration of basic and clinical research is key to understanding how chemerin influences disease pathobiology, and whether modulation of chemerin levels and/or activity may serve as a potential method to prevent and treat metabolic diseases.

Mice as clinically relevant models for the study of cytochrome P450-dependent metabolism.

The cytochrome P450 (CYP) gene superfamily comprises a large group of hemoproteins with diverse functions in steroid, lipid, and xenobiotic metabolism. The human genome is estimated to contain 57 genes that encode functional CYP proteins, a number of which are important for the metabolism of foreign chemicals, including carcinogens and most therapeutic drugs. Given that metabolic interactions are a major source of adverse drug interactions, a comprehensive understanding of CYP function is critically important for the development and safe clinical application of drugs. While some cross-species genetic conservation of CYPs exists, drug metabolism can differ between humans and other mammalian species. The development of humanized mice that replicate many aspects of human drug metabolism has provided invaluable experimental models that circumvent this limitation to a considerable degree. This brief review focuses on the value and limitations of mouse models for the study of drug metabolism in humans.

Kinetics and selectivity of mechanism-based inhibition of guinea pig hepatic and pulmonary cytochrome P450 by N-benzyl-1-aminobenzotriazole and N-alpha-methylbenzyl-1-aminobenzotriazole.

The time dependence for mechanism-based inactivation of cytochrome P450 (P450)-dependent 7-pentoxyresorufin O-depentylation (PROD), 7-ethoxyresorufin O-deethylation (EROD), and 7-methoxyresorufin O-demethylation (MROD) activities by N-benzyl-1-aminobenzotriazole (BBT) and N-alpha-methylbenzyl-1-aminobenzotriazole (alpha MB) was investigated in hepatic and pulmonary microsomes from phenobarbital-treated guinea pigs. In the presence of NADPH, both compounds inhibited P450-dependent catalytic activity in a time- and concentration-dependent manner. Inactivation of hepatic PROD activity was more rapid (t1/2 = 13.2 vs. 155 min) for 0.1 microM alpha MB when compared with equimolar BBT. On the other hand, hepatic EROD inactivation was more rapid (t1/2 = 8.1 vs. 11 min) with 0.1 microM BBT, compared with equimolar alpha MB. Inactivation of pulmonary PROD activity was the most rapid and potent, with an apparent half-life for inactivation of t1/2 = 0.94 and 32.2 min for 0.025 microM alpha MB and BBT, respectively. Incubation of hepatic microsomes for 45 min in the presence of NADPH and 10 microM BBT or alpha MB resulted in > 90% inhibition of PROD, EROD, and MROD activities. After washing by repeated sedimentation and resuspension, inhibition of PROD (78%; 93% for BBT and alpha MB, respectively), EROD (80% and 50%), and MROD (15% and 3%) activities was reversed to varying degrees. We conclude that BBT and alpha MB are rapidly metabolized to products that inhibit individual P450 isozymes by both mechanism-based (P4502B and P4501A1) and reversible (P4501A2) mechanisms. Of the two inhibitors, alpha MB is relatively more potent and selective for guinea pig lung P4502B isozyme(s).

Aryl hydrocarbon receptor-dependent induction of cyp1a1 by bilirubin in mouse hepatoma hepa 1c1c7 cells.

Heme metabolism normally involves enzymatic conversion to biliverdin and subsequently to bilirubin, catalyzed by heme oxygenase and biliverdin reductase, respectively. We examined the ability of exogenously added hemin, biliverdin, or bilirubin to regulate Cyp1a1, an enzyme that may be active in bilirubin elimination. A substantial dose-dependent increase in Cyp1a1 mRNA occurred after treatment of Hepa 1c1c7 cells with either of the three compounds. This increase was readily apparent 1 hr after treatment with biliverdin or bilirubin but required >/=2 hr with hemin. Treatment of Hepa 1c1c7 cells with these compounds also caused a dose-dependent increase in Cyp1a1-dependent 7-ethoxyresorufin-O-deethylase (EROD) activity. Of the three compounds, bilirubin produced the greatest maximal increase in Cyp1a1 mRNA and EROD (5.5-, 10.5-, and 15-fold for 100 microM hemin, biliverdin, and bilirubin, respectively) activity. The RNA polymerase inhibitor actinomycin D completely blocked Cyp1a1 induction by these compounds, indicating a requirement for de novo RNA synthesis via transcriptional activation. The protein synthesis inhibitor cycloheximide did not affect Cyp1a1 mRNA induction, indicating a lack of requirement for labile protein factors. In contrast, EROD induction by hemin, biliverdin, or bilirubin was completely blocked by cycloheximide treatment, indicating that the increase in enzyme activity is dependent on increased Cyp1a1 apoprotein synthesis. Aryl hydrocarbon receptor (AHR)- and AHR nuclear translocator-deficient mutant Hepa 1c1c7 cells did not exhibit increased Cyp1a1 mRNA or EROD activity after treatment with these compounds, indicating the requirement for a functional AHR for this response. Consistent with this, hemin, biliverdin, and bilirubin were able to induce expression of the dioxin-response element/luciferase reporter plasmid pGudLuc1.1 after transient transfection into wild-type Hepa 1c1c7 cells. Gel retardation assays demonstrated that bilirubin, but not hemin or biliverdin, was able to transform the AHR to a form capable of specifically binding to a 32P-labeled oligonucleotide containing a dioxin-response element sequence. These data indicate that bilirubin induces Cyp1a1 gene transcription through direct interaction with the AHR. In contrast, hemin and biliverdin seem to induce Cyp1a1 indirectly by serving as precursors to the endogenous formation of bilirubin via normal heme metabolism pathways. This is the first direct demonstration that the endogenous heme metabolite bilirubin can directly regulate Cyp1a1 gene expression and enzymatic activity in an AHR-dependent manner.

Isozyme-selective metabolic intermediate complex formation of guinea pig hepatic cytochrome P450 by N-aralkylated derivatives of 1-aminobenzotriazole.

The capacity for metabolic intermediate (MI) complex formation as a mechanism of action for the isozyme-selective cytochrome P450 (P450) inhibitors N-benzyl-1-aminobenzotriazole (BBT), N-(alpha-methylbenzyl)-1-aminobenzotriazole (alpha MB), and N-(alpha-ethylbenzyl)-1-aminobenzotriazole (alpha EB) was investigated in hepatic microsomes from untreated, phenobarbital-induced, and beta-naphthoflavone-induced guinea pigs. Similar to other complex forming amines, MI complex formation was observed as an absorbance maximum at approximately 455 nm by optical-difference spectroscopy, was dependent upon incubation with NADP(H), and was dissociable by the addition of 50 microM potassium ferricyanide. MI complexes formed by BBT, alpha MB, and alpha EB were also dissociable by sedimentation and resuspension, as well as in the presence of limiting concentrations of NADP(H). Maximal complexation with the three compounds was observed in microsomes from phenobarbital-induced guinea pigs where the initial rate of complex formation was dependent upon inhibitor concentration and apparent Km values of 108 +/- 44, 338 +/- 96, and 84 +/- 15 microM for BBT, alpha MB, and alpha EB, respectively, were found. Inclusion of 1 mM glutathione in the incubation mixtures had a significant attenuating effect upon complex formation, suggesting the involvement of an electrophilic, reactive intermediate. Complex formation was not observed with the three inhibitors in pulmonary microsomes from either guinea pigs or rabbits. MI complexation is not likely to contribute to the mechanism-based inactivation of guinea pig hepatic P450 2Bx, the homologue of rabbit P450 2B4, due to the irreversible inactivation of this isoform at very low inhibitor concentrations, the lack of glutathione attenuation of this destruction, the instability of formed MI complexes, and the absence of MI complex formation with guinea pig or rabbit pulmonary P450.

Antagonism of the actions of peroxisome proliferator-activated receptor-alpha by bile acids.

The peroxisome proliferator-activated receptor-alpha (PPARalpha) is a ligand-activated transcription factor that regulates the expression of a number of genes critical for fatty acid beta-oxidation. Because a number of substrates and intermediates of this metabolic pathway serve as ligand activators of this receptor, homeostatic control of fatty acid metabolism is achieved. Evidence also exists for PPARalpha-dependent regulation of genes encoding critical enzymes of bile acid biosynthesis. To determine whether the primary products of bile acid biosynthesis, cholic acid and chenodeoxycholic acid, were capable of modulating PPARalpha function, a variety of in vivo and in vitro approaches were utilized. Feeding a bile acid-enriched diet significantly reduced the degree of hepatomegaly and induction of target genes encoding enzymes of fatty acid beta-oxidation caused by treatment with the potent PPARalpha ligand Wyeth-14,643. Convergent data from mechanistic studies indicate that bile acids interfere with transactivation by PPARalpha at least in part by impairing the recruitment of transcriptional coactivators. The results of this study provide the first evidence in favor of the existence of compounds, normally found within the body, that are capable of antagonizing the physiological actions of PPARalpha. The impact of PPARalpha antagonism by endogenous bile acids is likely to be limited under normal conditions and to have only minimal effects on bile acid homeostasis. However, during certain pathophysiological states where intracellular bile acid concentrations are elevated, meaningful effects on PPARalpha-dependent target gene regulation are possible.

Differential in vivo effects of alpha-naphthoflavone and beta-naphthoflavone on CYP1A1 and CYP2E1 in rat liver, lung, heart, and kidney.

Male Sprague-Dawley rats were treated intraperitoneally with corn oil, the aryl hydrocarbon receptor (AHR) agonist beta-naphthoflavone (betaNF), or the relatively weak AHR agonist alpha-naphthoflavone (alphaNF). Animals treated with betaNF experienced a significant loss (12%) of total body mass over 5 days and a dramatic elevation of CYP1A1 mRNA in all of the organs studied. Treatment with alphaNF had no significant effect on body mass after 5 days and caused only minor increases of liver, kidney, and heart CYP1A1 mRNA. In contrast, lung CYP1A1 mRNA was increased by alphaNF treatment to levels comparable to that seen with betaNF treatment. CYP2E1 mRNA levels were also elevated in liver, lung, kidney, and heart in response to betaNF treatment, whereas alphaNF was without effect. Large increases of CYP1Al-dependent 7-ethoxyresorufin O-deethylation (EROD) activity occurred with microsomes prepared from the tissues of betaNF-treated animals. Comparatively small changes were associated with alphaNF treatment, with the exception of lung, where EROD activity was increased to approximately 60% of that with betaNF treatment. CYP2E1-dependent p-nitrophenol hydroxylase (PNP) activity was also increased by betaNF treatment in microsomes prepared from kidney (3.1-fold), whereas alphaNF was without effect. In contrast, alphaNF or betaNF treatment caused significant decreases of lung microsomal PNP (72% and 27% of corn oil control, respectively) and 7-pentoxyresorufin O-deethylation (48% and 17% of corn oil control, respectively) activities, indicating that PNP activity may be catalyzed by P450 isoforms other than CYP2E1 in rat lung. We conclude that betaNF and alphaNF have differential effects on the expression and catalytic activity of CYP1A1 and CYP2E1, depending upon the organ studied. These changes most likely occur as a result of the direct actions of these compounds as AHR agonists, in addition to secondary effects associated with AHR-mediated toxicity.

Liver transplantation induces cytochrome P450 1A1 dependent monooxygenase activity in rat lung and kidney.

Although liver transplantation has been the subject of intensive investigation, comparatively little is known regarding the effects of this procedure on the metabolism of xenobiotics. The objective of the present study was to examine the effect of orthotopic liver transplantation on rat hepatic, pulmonary, and renal microsomal cytochrome P450 (P450) monooxygenase activity through the use of isozyme-selective substrates. Pulmonary microsomal P450 1A1 dependent 7-ethoxyresorufin O-deethylation (ERFD) activity increased over time in recipient rats, with maximal induction (750% of donor) observed after 21 days. Similarly, ERFD activity in renal microsomes was increased (200% of donor) after 21 days. Both pulmonary and renal microsomal P450 2B dependent 7-pentoxyresorufin O-depentylation (PRFD) activity was decreased (50 and 75% of donor) 1 day after transplantation but was essentially unchanged 3, 7, and 21 days after transplantation. Pulmonary and renal microsomal heme oxygenase activities were not significantly affected by liver transplantation. In contrast, total hepatic microsomal P450 concentrations were decreased maximally (to 45% of donor concentration) 7 days after transplantation and remained low (55% of donor) up to 21 days. Similarly, hepatic P450 1A dependent ERFD and P450 2B dependent PRFD activities were maximally depressed (20 and 25% of donor activities) after 7 days and remained low (75 and 30% of donor) up to 21 days after transplantation. The decreases in rates of hepatic P450 monooxygenation were accompanied by significant increases in microsomal heme oxygenase activity. The data presented in this study suggest the existence of generalized stress responses to inflammation that result in tissue- and isozyme-selective modulation of P450 monooxygenase activity.(ABSTRACT TRUNCATED AT 250 WORDS)

Selective increase of rat lung cytochrome P450 1A1 dependent monooxygenase activity after acute sodium arsenite administration.

Arsenic is a known pulmonary, hepatic, and skin carcinogen in humans and a known inducer of stress proteins. Consequently, the ability of arsenite (As3+) to modulate isozyme-selective cytochrome P450 (P450) dependent monooxygenase activities was investigated in microsomes prepared from lung, liver, and kidney of male, adult Sprague-Dawley rats treated subcutaneously (s.c.) with sodium arsenite (75 mumol/kg body weight) 24 h before death. In the lung, the activity of P450 1A1 catalyzed 7-ethoxyresorufin O-deethylation (ERFD) was markedly (approximately 5-fold) increased in treated versus control rats, whereas the activity of P450 2B catalyzed 7-pentoxyresorufin O-depentylation (PRFD) was unchanged. Pulmonary ERF activity remained elevated for at least 48 h after As3+ treatment. In contrast, As3+ inhibited hepatic microsomal ERFD and PRFD activity by approximately 20 and 35%, respectively, 24 h after treatment. ERFD activity was also decreased in kidney microsomes of As(3+)-treated rats, but the inhibition was greater than in liver (50 vs. 35%) 24 h after injection. These effects are almost certainly not due to a direct action of As3+ on P450-dependent catalysis, as in vitro addition of sodium As3+ at concentrations up to 1 mM had no effect on ERFD activity of control rat lung microsomes. In addition, pretreatment of rats with Zn (153 mumol.kg-1.day-1 for 2 days, s.c.) had no effect on control or As(3+)-mediated changes in P450-dependent ERFD activity of rat lung or kidney microsomes. These results demonstrate that As3+ is an isozyme-selective modulator of P450 monooxygenase activity (i.e., significant increase of P450 1A1 catalyzed activity but not P450 2B catalyzed activity) in rat lung.(ABSTRACT TRUNCATED AT 250 WORDS)

Enantioselective, mechanism-based inactivation of guinea pig hepatic cytochrome P450 by N-(alpha-methylbenzyl)-1-aminobenzotriazole.

N-Aralkylated derivatives of 1-aminobenzotriazole are well-established, mechanism-based inhibitors of cytochrome P450 (CYP or P450). In this study, the kinetics of inactivation of CYP2B-dependent 7-pentoxyresorufin O-depentylation (PROD) and CYP1A-dependent 7-ethoxyresorufin O-deethylation (EROD) activities by enantiomers of N-(alpha-methylbenzyl)-1-aminobenzotriazole (alphaMB) were compared. The racemic mixture (+/-)-alphaMB, as well as the enantiomers (-)-alphaMB and (+)-alphaMB, produced a time-, concentration-, and NADPH-dependent loss of PROD and EROD activity in hepatic microsomes from phenobarbital-treated guinea pigs. The rates of PROD inactivation by (-)-alphaMB were significantly faster than for (+)-alphaMB. Consistent with this, the derived maximal kinact was also significantly greater for (-)-alphaMB than for (+)-alphaMB (0.49 vs. 0.35 min-1). In contrast, the concentrations required for the half-maximal rate of inactivation (Ki) were equivalent for (-)-alphaMB and (+)-alphaMB, whereas the degree of competitive inhibition of PROD activity was greater for (+)-alphaMB. No significant differences were found among (-)-alphaMB, (+)-alphaMB, and (+/-)-alphaMB with respect to mechanism-based inactivation (kinact = 0.18, 0.16, and 0.17 min-1, respectively) or competitive inhibition of EROD activity. No differences were found for the maximal extent of PROD or EROD inhibition or the loss of spectral P450 after an extended 30-min incubation with the inhibitors. We conclude that mechanism-based inactivation of guinea pig CYP2B, but not CYP1A, isozymes by alphaMB occurs in a stereoselective manner, most likely as a result of a difference in the balance between metabolic activation and deactivation for the alphaMB enantiomers.

Targeted disruption of the nuclear receptor FXR/BAR impairs bile acid and lipid homeostasis.

Mice lacking the nuclear bile acid receptor FXR/BAR developed normally and were outwardly identical to wild-type littermates. FXR/BAR null mice were distinguished from wild-type mice by elevated serum bile acid, cholesterol, and triglycerides, increased hepatic cholesterol and triglycerides, and a proatherogenic serum lipoprotein profile. FXR/BAR null mice also had reduced bile acid pools and reduced fecal bile acid excretion due to decreased expression of the major hepatic canalicular bile acid transport protein. Bile acid repression and induction of cholesterol 7alpha-hydroxylase and the ileal bile acid binding protein, respectively, did not occur in FXR/BAR null mice, establishing the regulatory role of FXR/BAR for the expression of these genes in vivo. These data demonstrate that FXR/BAR is critical for bile acid and lipid homeostasis by virtue of its role as an intracellular bile acid sensor.

Targeted disruption of soluble epoxide hydrolase reveals a role in blood pressure regulation.

Renal microsomal cytochrome P-450 monooxygenase-dependent metabolism of arachidonic acid generates a series of regioisomeric epoxyeicosatrienoic acids that can be further metabolized by soluble epoxide hydrolase to the corresponding dihydroxyeicosatrienoic acids. Evidence exists that these metabolites affect renal function and, in particular, blood pressure regulation. To examine this possibility, blood pressure and renal arachidonic acid metabolism were examined in mice with a targeted disruption of the soluble epoxide hydrolase gene. Systolic blood pressure of male soluble epoxide hydrolase-null mice was lower compared with wild-type mice in both the absence and presence of dietary salt loading. Both female soluble epoxide hydrolase-null and wild-type female mice also had significantly lower systolic blood pressure than male wild-type mice. Renal formation of epoxyeicosatrienoic and dihydroxyeicosatrienoic acids was markedly lower for soluble epoxide hydrolase-null versus wild-type mice of both sexes. Although disruption of soluble epoxide hydrolase in female mice had minimal effects on blood pressure, deletion of this gene feminized male mice by lowering systolic blood pressure and altering arachidonic acid metabolism. These data provide the first direct evidence for a role for soluble epoxide hydrolase in blood pressure regulation and identify this enzyme as a novel and attractive target for therapeutic intervention in hypertension.

Disrupted bile acid homeostasis reveals an unexpected interaction among nuclear hormone receptors, transporters, and cytochrome P450.

Sister of P-glycoprotein (SPGP) is the major hepatic bile salt export pump (BSEP). BSEP/SPGP expression varies dramatically among human livers. The potency and hierarchy of bile acids as ligands for the farnesyl/bile acid receptor (FXR/BAR) paralleled their ability to induce BSEP in human hepatocyte cultures. FXR:RXR heterodimers bound to IR1 elements and enhanced bile acid transcriptional activation of the mouse and human BSEP/SPGP promoters. In FXR/BAR nullizygous mice, which have dramatically reduced BSEP/SPGP levels, hepatic CYP3A11 and CYP2B10 were strongly but unexpectedly induced. Notably, the rank order of bile acids as CYP3A4 inducers and activators of pregnane X receptor/steroid and xenobiotic receptor (PXR/SXR) closely paralleled each other but was markedly different from their hierarchy and potency as inducers of BSEP in human hepatocytes. Moreover, the hepatoprotective bile acid ursodeoxycholic acid, which reverses hydrophobic bile acid hepatotoxicity, activates PXR and efficaciously induces CYP3A4 (a bile-metabolizing enzyme) in primary human hepatocytes thus providing one mechanism for its hepatoprotection. Because serum and urinary bile acids increased in FXR/BAR -/- mice, we evaluated hepatic transporters for compensatory changes that might circumvent the profound decrease in BSEP/SPGP. We found weak MRP3 up-regulation. In contrast, MRP4 was substantially increased in the FXR/BAR nullizygous mice and was further elevated by cholic acid. Thus, enhanced hepatocellular concentrations of bile acids, due to the down-regulation of BSEP/SPGP-mediated efflux in FXR nullizygous mice, result in an alternate but apparent compensatory up-regulation of CYP3A, CYP2B, and some ABC transporters that is consistent with activation of PXR/SXR by bile acids.