Differential regulation of endobiotic-oxidizing cytochromes P450 in vitamin A-deficient male rat liver

Stress is a critical player in CYP3A, CYP2C, and CYP2D regulation: role of adrenergic receptor signaling pathways

Stress is a critical player in the regulation of the major cytochrome P-450s (CYPs) that metabolize the majority of the prescribed drugs. Early in life, maternal deprivation (MD) stress and repeated restraint stress (RS) modified CYP expression in a stress-specific manner. In particular, the expression of CYP3A1 and CYP2C11 was increased in the liver of MD rats, whereas RS had no significant effect. In contrast, hepatic CYP2D1/2 activity was increased by RS, whereas MD did not affect it. The primary effectors of the stress system, glucocorticoids and epinephrine, highly induced CYP3A1/2. Epinephrine also induced the expression of CYP2C11 and CYP2D1/2. Further investigation indicated that AR-agonists may modify CYP regulation. In vitro experiments using primary hepatocyte cultures treated with the AR-agonists phenylephrine, dexmedetomidine, and isoprenaline indicated an AR-induced upregulating effect on the above-mentioned CYPs mediated by the cAMP/protein kinase A and c-Jun NH₂-terminal kinase signaling pathways. Interestingly though, in vivo pharmacological manipulations of ARs using the same AR-agonists led to a suppressed hepatic CYP expression profile, indicating that the effect of the complex network of central and peripheral AR-linked pathways overrides that of the hepatic ARs. The AR-mediated alterations in CYP3A1/2, CYP2C11, and CYP2D1/2 expressions are potentially connected with those observed in the activation of signal transducer and activator of transcription 5b. In conclusion, stress and AR-agonists may modify the expression of the major CYP genes involved in the metabolism of drugs used in a wide range of diseases, thus affecting drug efficacy and toxicity.

Hepatic drug metabolizing profile of Flinders Sensitive Line rat model of depression

The Flinders Sensitive Line (FSL) rat model of depression exhibits some behavioral, neurochemical, and pharmacological features that have been reported in depressed patients and has been very effective in screening antidepressants. Major factor that determines the effectiveness and toxicity of a drug is the drug metabolizing capacity of the liver. Therefore, in order to discriminate possible differentiation in the hepatic drug metabolism between FSL rats and Sprague-Dawley (SD) controls, their hepatic metabolic profile was investigated in this study. The data showed decreased glutathione (GSH) content and glutathione S-transferase (GST) activity and lower expression of certain major CYP enzymes, including the CYP2B1, CYP2C11 and CYP2D1 in FSL rats compared to SD controls. In contrast, p-nitrophenol hydroxylase (PNP), 7-ethoxyresorufin-O-dealkylase (EROD) and 16alpha-testosterone hydroxylase activities were higher in FSL rats. Interestingly, the wide spread environmental pollutant benzo(alpha)pyrene (B(alpha)P) induced CYP1A1, CYP1A2, CYP2B1/2 and ALDH3c at a lesser extend in FSL than in SD rats, whereas the antidepressant mirtazapine (MIRT) up-regulated CYP1A1/2, CYP2C11, CYP2D1, CYP2E1 and CYP3A1/2, mainly, in FSL rats. The drug also further increased ALDH3c whereas suppressed GSH content in B(alpha)P-exposed FSL rats. In conclusion, several key enzymes of the hepatic biotransformation machinery are differentially expressed in FSL than in SD rats, a condition that may influence the outcome of drug therapy. The MIRT-induced up-regulation of several drug-metabolizing enzymes indicates the critical role of antidepressant treatment that should be always taken into account in the designing of treatment and interpretation of insufficient pharmacotherapy or drug toxicity.

Steroid binding sites in liver membranes: interplay between glucocorticoids, sex steroids, and pituitary hormones

Steroid hormones activate target cells through specific receptors that discriminate among ligands based upon recognition of distinct structural features. For most known steroids, membrane and nuclear receptors co-exist in many target cells. However, while the structure of the nuclear receptors and their function as transcriptional activators of specific target genes is generally well understood, the identity of the membrane receptors remains elusive. Using pharmacological and biochemical approaches, we are beginning to characterize receptors for glucocorticoids and anabolic-androgenic steroids in male rat liver membranes. Male rat liver endoplasmic reticulum contains two steroid binding sites which are functionally related and associated with a 90-134 kDa oligomeric protein: (1) the low-affinity glucocorticoid binding site (LAGS), composed at least in part of two peptides (37 and 53 kDa) that bind glucocorticoids and (2) the stanozolol binding protein (STBP), composed at least in part of three peptides (22, 31, and 55 kDa) that bind the synthetic androgen stanozolol. These steroid binding proteins have many properties different from those of classical nuclear receptors, with the salient differences being a failure to recognize "classical" ligands for nuclear receptors together with marked differences in biochemical properties and physiological regulation. The mechanism of interaction of glucocorticoids with the LAGS can be clearly distinguished from that with STBP. Moreover, STBP shows an extremely narrow pharmacological profile, being selective for ST and its analog, danazol, among more than 100 steroids and non-steroidal compounds that were assayed, including those that are able to displace glucocorticoids from the LAGS. The level of LAGS activity undergoes dramatic variations following changes from the physiological serum levels of thyroid hormones, glucocorticoids, GH, vitamin A, and E2. However, neither thyroid hormones nor GH have a critical role on STBP activity. The STBP is functionally related to LAGS. We have suggested a novel mechanism for STBP whereby membrane-associated glucocorticoid binding activity is targeted by stanozolol (and 16beta-hydroxylated stanozolol): stanozolol modulates glucocorticoid activity in the liver through negative allosteric modulation of the LAGS resulting in an effective increase in classical GR-signaling by increasing glucocorticoid availability to the cytosolic GR.

Role of signalling systems in the effects of dietary factors on the expression of mammalian CYPs

Changes in mammalian diets alter the hepatic expression of CYP drug-metabolising enzymes and endobiotic oxidases. Thus, dietary constituents may significantly influence the duration of action of chemicals in tissues. Recent improvements in the mechanistic information on the regulation of constitutive and inducible expression of CYPs has facilitated our understanding as to how dietary factors modulate expression. Altered regulation appears to occur either by direct activation of transcription factors or by indirect modulation of signal transduction pathways. For example, dietary lipid directly activates PPAR-alpha, or other nuclear hormone receptors, to elicit CYP induction, and vitamin A deficiency downregulates the growth hormone-responsive CYP2C11 by perturbing Janus kinase-signal transducers and activators of transcription signalling. This article focuses on the present understanding of the regulation of CYP genes by dietary nutrients.

Regulation of the rat CYP4A2 gene promoter by c-Jun and octamer binding protein-1

The physiologically important cytochrome P450 (CYP) 4A2 arachidonic acid omega-hydroxylase gene is widely expressed in rat tissues. Although the induction of CYPs 4A by peroxisome proliferators and dietary lipids is established there is minimal information on the factors that control constitutive expression. To address this issue we cloned 1.4 kb of the CYP4A2 5'-upstream region and identified several DNA elements that resembled the activator protein-1 (AP-1) consensus sequence. Using a series of 5'-truncated reporter constructs a 42 bp region was detected that was responsive to the AP-1 factor c-Jun, which is important in basal gene regulation. The roles of two putative AP-1 elements at -47/-41 and -31/-25 were tested, with the former emerging from studies with mutagenised constructs as the functionally important site. These findings were supported by electromobility shift assay (EMSA) studies that indicated the interaction of the -47/-41 element with c-Jun. The -31/-25 element mediated the suppression of CYP4A2 transactivation by octamer binding protein-1 (oct-1). Thus, mutagenesis of this element relieved the modulatory effect of oct-1 on c-Jun-mediated transactivation. In EMSAs, the binding of nuclear proteins to the -31/-25 element was competed by an oct-1 consensus sequence and supershifted by an anti-oct-1 antibody. Overexpression of c-Jun in rat liver-derived H4IIE cells increased CYP4A2 mRNA to approximately 2-fold of control, but oct-1 overexpression was without significant effect. From chromatin immunoprecipitation assays both c-Jun and oct-1 bound to the CYP4A2 5'-upstream sequence in H4IIE cells. These findings implicate c-Jun and oct-1 as potentially important constitutive factors that modulate the transactivation of the CYP4A2 gene promoter.

Phospho-STAT5 accumulation in nuclear fractions from vitamin A-deficient rat liver

The growth hormone (GH)-responsive cytochrome P450 (CYP) 2C11 is down-regulated in vitamin A-deficient (VAD) rat liver. This study assessed the impact of a VAD diet on the hepatic Janus kinase-Signal Transducers and Activators of Transcription (JAK-STAT) system that mediates GH signalling. Nuclear tyrosine- and serine-phosphorylated STAT5 accumulated in VAD liver, whereas nuclear JAK2 tyrosine kinase and SHP-1 phosphatase were decreased. Tyrosine-phosphorylated SHP-1 was decreased to 36+/-14% of control (P<0.01), indicating its impaired activation in VAD liver. Episodic GH pulses increased nuclear phospho-STAT5, especially in control liver, but nuclear phospho-JAK2 and phospho-SHP-1 were not restored. CYP2C11 protein and testosterone 16alpha-hydroxylation were decreased in VAD liver to 67+/-16% and 76+/-19% of control, and were further decreased by GH to 32+/-8% and 30+/-14% of control. Thus, hypo-responsiveness of JAK-STAT in VAD liver is associated with impaired nuclear phospho-STAT dephosphorylation.

Down regulation of hepatic PPARalpha function by AhR ligand

Aryl hydrocarbon receptor (AhR) is a ligand-activated transcription factor that mediates a spectrum of toxic and biological effects of 2,3,7,8-tetrachloro dibenzo-p-dioxin (TCDD) and related compounds. Peroxisome proliferator activated receptor alpha (PPARalpha) is a nuclear receptor involved in the maintenance of lipid and glucose homeostasis. In this study we hypothesized that one of the possible mechanisms for the effect of TCDD and its related chemicals on fat metabolism could be through down regulation of PPARalpha functions. We treated Wistar rats with an AhR ligand, Sudan III (S.III), and/or PPARalpha ligand, Clofibric Acid (CA), for 3 days. We analysed the expression of one of the PPARalpha-target gene products, CYP4A protein and its mRNA. We also tested HepG2 cells with the afore-mentioned treatments and evaluated their effects on PPARalpha and RXRalpha protein. Treatment of Wistar rats with S.III was found to down regulates CYP4A protein expression and reduced its induction with CA. It also decreased mRNA expressions of CYP4A1, CYP4A2, CYP4A3 and PPARalpha. In HepG2 cells, PPARalpha and RXRalpha protein expression was decreased by S.III treatment in a dose dependent manner. Our results suggest that AhR has an inhibitory effect on PPARalpha function and a new pathway by which AhR ligands could disturb lipid metabolism.

Upregulation of cytochromes P450 2B in rat liver by orphenadrine

1 The alkylamine drug orphenadrine (ORPH) is an inducer and inhibitor of the microsomal cytochrome P450 (CYP) system in mammals. This study evaluated the selectivity of CYP induction by ORPH in rat liver. 2 Immunoblot analysis indicated that ORPH was a selective inducer of the phenobarbitone (PB)-inducible CYP2B in rat liver. CYP2B protein was increased to approximately 14-fold of levels in untreated rat liver. By comparison PB increased CYP2B expression 40-fold. Corresponding increases in the activity of CYP2B-dependent androstenedione 16beta-hydroxylation were measured in microsomes from ORPH and PB-induced rats. 3 Northern analysis indicated that CYP2B1/2 mRNA was increased in ORPH-induced rat liver. Consistent with this finding, ORPH was found to activate a PB-responsive enhancer module in constitutive androstane receptor (CAR)-transfected Hep G2 cells. 4 Other alkylamines like troleandomycin impair CYP turnover. We tested whether ORPH induction of CYP2B may include a post-translational component. In PB-pretreated animals ORPH administration delayed the loss of CYP2B after PB withdrawal, but no evidence for altered turnover was found. 5 These studies establish ORPH as a selective inducer of CYP2B in rat liver. Induction appears to be mediated pretranslationally by CAR activation of CYP2B gene transcription. Post-translational stabilisation by an ORPH metabolite does not elicit induction. Induction of CYP2B may influence pharmacokinetic interactions involving ORPH.