Evidence of alpha2-adrenoceptor involvement in B[alpha]P induction processes of drug-metabolizing enzymes: the effect of stress

Dopamine D2-Receptor Antagonists Down-Regulate CYP1A1/2 and CYP1B1 in the Rat Liver

Dopaminergic systems regulate the release of several hormones including growth hormone (GH), thyroid hormones, insulin, glucocorticoids and prolactin (PRL) that play significant roles in the regulation of various Cytochrome P450 (CYP) enzymes. The present study investigated the role of dopamine D₂-receptor-linked pathways in the regulation of CYP1A1, CYP1A2 and CYP1B1 that belong to a battery of genes controlled by the Aryl Hydrocarbon Receptor (AhR) and play a crucial role in the metabolism and toxicity of numerous environmental toxicants. Inhibition of dopamine D₂-receptors with sulpiride (SULP) significantly repressed the constitutive and benzo[a]pyrene (B[a]P)-induced CYP1A1, CYP1A2 and CYP1B expression in the rat liver. The expression of AhR, heat shock protein 90 (HSP90) and AhR nuclear translocator (ARNT) was suppressed by SULP in B[a]P-treated livers, whereas the AhRR expression was increased by the drug suggesting that the SULP-mediated repression of the CYP1 inducibility is due to inactivation of the AhR regulatory system. At signal transduction level, the D₂-mediated down-regulation of constitutive CYP1A1/2 and CYP1B1 expression appears to be mediated by activation of the insulin/PI3K/AKT pathway. PRL-linked pathways exerting a negative control on various CYPs, and inactivation of the glucocorticoid-linked pathways that positively control the AhR-regulated CYP1 genes, may also participate in the SULP-mediated repression of both, the constitutive and induced CYP1 expression. The present findings indicate that drugs acting as D₂-dopamine receptor antagonists can modify several hormone systems that regulate the expression of CYP1A1, CYP1A2 and CYP1B1, and may affect the toxicity and carcinogenicity outcome of numerous toxicants and pre-carcinogenic substances. Therefore, these drugs could be considered as a part of the strategy to reduce the risk of exposure to environmental pollutants and pre-carcinogens.

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.

Differences in hepatic cytochrome P450 activity correlate with the strain-specific biotransformation of medetomidine in AX/JU and IIIVO/JU inbred rabbits

Medetomidine is an alpha(2)-adrenoceptor agonist with sedative and analgesic properties. Previously we demonstrated significant differences in the response to medetomidine between two inbred rabbit strains, denoted IIIVO/JU and AX/JU. The aim of the present study was twofold: first, to compare the hepatic CYP450 enzyme activities between these rabbit strains [n = 13(male male,7 female female)/strain]. To this end, liver microsomes were incubated with known fluorescent substrates for the major drug-metabolizing CYP450 isoforms. A comparison of the obtained results indicated significant gender differences as well as differences between the two rabbit inbred strains. Secondly, the biotransformation rate of medetomidine in liver microsomes of both rabbit strains was determined using liquid chromatography coupled to tandem mass spectrometry. The rate of hydroxymedetomidine and medetomidine carboxylic acid formation was found to be significantly higher in the AX/JU strain. Specific CYP2D and CYP2E inhibitors could decrease the formation of both metabolites. Significant correlations were found between the rate of biotransformation of medetomidine and the activities of CYP2D and CYP2E, as well as between CYP450 enzyme activities and the anaesthetic response to medetomidine.

Effects of medetomidine on hepatic EROD activity in three species of fish

Medetomidine, an antifouling candidate, was investigated for its effects on cytochrome P4501A (CYP1A) activity in fish. Rainbow trout (Oncorhynchus mykiss), turbot (Psetta maxima), and Atlantic cod (Gadus morhua) were exposed to medetomidine either via i.p. injection (<5 micromol (1mg)/kg) or via water (<50 nM). Enzyme activity was measured as ethoxyresorufin-O-deethylase (EROD) activity in liver microsomes. There was a small (2-7-fold) increase in EROD activity in rainbow trout. In turbot, EROD activity increased (4-fold) after injection, while a non-significant (50%) decrease was observed after water exposure. No effects on EROD activities were observed in Atlantic cod. In vitro inhibition studies of EROD activities in liver microsomes from all three species showed that medetomidine was a very potent CYP1A inhibitor. Thus, median inhibition values (IC(50)) were 35+/-10nM for rainbow trout, 47+/-17 nM for turbot, and 111+/-70 nM for Atlantic cod. These observed effects suggest that medetomidine interferes with CYP1A-dependent metabolism of xenobiotics in these fish species.

Predominant role of peripheral catecholamines in the stress-induced modulation of CYP1A2 inducibility by benzo(alpha)pyrene

The potential involvement of catecholamines and in particular of alpha(2)-adrenoceptor-related signalling pathways, in the regulation of drug-metabolizing enzymes by stress was investigated in Wistar rats after exposure to the environmental pollutant benzo(alpha)pyrene. For this purpose, total cytochrome P450 content, the CYP1A2 mRNA levels, 7-methoxyresorufin-O-dealkylase (MROD), 7-pentoxyresorufin-O-dealkylase (PROD) and p-nitrophenol hydroxylase activity levels were determined in the livers of rats exposed to repeated restraint stress after treatment with benzo(alpha)pyrene coupled with pharmacological manipulations of peripheral and/or central catecholamines and alpha(2)-adrenoceptors. The data show that stress is a significant factor in the regulation of CYP1A2 induction and that catecholamines play a central role in the stress-mediated modulation of hepatic CYP1A2 inducibility by benzo(alpha)pyrene. The up-regulating effect of stress on benzo(alpha)pyrene-induced CYP1A2 gene expression was eliminated after a generalized catecholamine depletion with reserpine. Similarly, in a state where only peripheral catecholamines were depleted and central catecholamines remained intact after guanethidine administration, the up-regulating effect of stress was eliminated. It is apparent that stress up-regulates the induction of CYP1A2 by benzo(alpha)pyrene mainly via peripheral catecholamines, while central catecholamines hold a minor role in the regulation. Pharmacological manipulations of alpha(2)-adrenoceptors appear to interfere with the effect of stress on the regulation of CYP1A2 inducibility. Either blockade or stimulation of alpha(2)-adrenoceptors with atipamezole and dexmedetomidine respectively, eliminated the up-regulating effect of stress on CYP1A2 benzo(alpha)pyrene-induced expression, while it enhanced MROD activity. In contrast, stress and pharmacological manipulations of catecholamines and alpha(2)-adrenoceptors did not affect total P450 content, the CYP2B1/2-dependent PROD and the CYP2E1-dependent p-nitrophenol hydroxylase activities. In conclusion, stress is a significant factor in the regulation of the CYP1A2 inducibility by benzo(alpha)pyrene, which in turn is involved in the metabolism of a large spectrum of toxicants, drugs and carcinogenic agents. Although the mechanism underlying the stress effect on CYP1A2 induction has not been clearly elucidated, it appears that peripheral catecholamines hold a predominant role, while central catecholamines and in particular, central noradrenergic pathways hold a minor role.

Modification of inherent and drug-induced dopaminergic activity after exposure to benzo(alpha)pyrene

The aim of this study was to investigate the effect of benzo(alpha)pyrene (B(alpha)P), a representative polycyclic aromatic hydrocarbon (PAH), on dopaminergic activity in brain. (B(alpha)P) altered dopaminergic activity in discrete regions of the rat brain, including the hippocampus, hypothalamus, caudate putamen and nucleus accumbens. Specifically, B(alpha)P increased DA levels in the hippocampus and DA turnover in the caudate putamen. In addition, B(alpha)P suppressed DA levels in the caudate putamen and DA turnover in the nucleus accumbens. B(alpha)P also altered the effect of several dopaminergic agents, L-DOPA, sulpiride and bromocriptine, on DA activity. In particular, B(alpha)P enhanced the L-DOPA-induced increase in the DA turnover ratio in the caudate putamen and increased DA levels in the nucleus accumbens. B(alpha)P also reversed the sulpiride-induced increase of DA turnover in the nucleus accumbens and the bromocriptine-induced increase of DA turnover in the hippocampus. In addition, DA turnover was increased by B(alpha)P in the nucleus accumbens and caudate putamen and DA levels were suppressed in the nucleus accumbens of bromocriptine treated rats, though the drug alone had no effect. These changes indicate that exposure to B(alpha)P and related compounds may affect dopaminergic function in discrete brain regions that are implicated in cognitive functions, psychosis, depression and Parkinson's disease, and may possibly interfere with their pharmacological intervention.

Benzo(α)pyrene-induced up-regulation of CYP1A2 gene expression: Role of adrenoceptor-linked signaling pathways

CYP1A2, a principal catalyst for metabolism of various therapeutic drugs and carcinogens, among others, is in part regulated by the stress response. This study was designed to assess whether catecholamines and in particular adrenergic receptor-dependent pathways, modulate benzo(alpha)pyrene (B(alpha)P)-induced hepatic CYP1A2. To distinguish between the role of central and peripheral catecholamines in the regulation of CYP1A2 induction, the effect of central and peripheral catecholamine depletion using reserpine was compared to that of peripheral catecholamine depletion using guanethidine. The effects of peripheral adrenaline and L-DOPA administration were also assessed. The results suggest that alterations in central catecholamines modulate 7-methoxyresorufin O-demethylase activity (MROD), CYP1A2 mRNA and protein levels in the B(alpha)P-induced state. In particular, central catecholamine depletion, dexmedetomidine-induced inhibition of noradrenaline release and blockade of alpha(1)-adrenoceptors with prazosin, up-regulated CYP1A2 expression. Phenylephrine and dexmedetomidine-induced up-regulation may be mediated, in part, via peripheral alpha(1)- and alpha(2)-adrenoceptors, respectively. On the other hand, the L-DOPA-induced increase in central dopaminergic activity was not followed by any change in the up-regulation of CYP1A2 expression by B(alpha)P. Central noradrenergic systems appeared to counteract up-regulating factors, most likely via alpha(1)- and alpha(2)-adrenoceptors. In contrast, peripheral alpha- and beta-adrenoceptor-related signaling pathways are linked to up-regulating processes. The findings suggest that drugs that bind to adrenoceptors or affect central noradrenergic neurotransmission, as well as factors that challenge the adrenoceptor-linked signaling pathways may deregulate CYP1A2 induction. This, in turn, may result in drug-therapy and drug-toxicity complications.

Role of adrenoceptor-linked signaling pathways in the regulation of CYP1A1 gene expression

Alpha2-adrenoceptor agents as well as stress affect the activity of several hepatic monoxygenases including those related to CYP1A enzymes. This study was therefore designed to assess the role of central and/or peripheral catecholamines and, in particular, of adrenoceptors in the regulation of B(alpha)P-induced cytochrome CYP1A1 expression. In order to discriminate the role of central from that of peripheral catecholamines in the regulation of CYP1A1 induction, the effect of central and peripheral catecholamine depletion using reserpine versus only peripheral catecholamine depletion using guanethidine was assessed. By using selected agonists and antagonists, the role of alpha and beta-adrenoceptors in the regulation of CYP1A1 induction was evaluated. The results showed that the central catecholaminergic system has a negative regulatory effect on 7-ethoxyresorufin O-deethylase (EROD) inducibility by benzo(alpha)pyrene (B(alpha)P), and that this may be mediated via alpha1-, alpha2- and beta-adrenoceptors. Specifically, stimulation of alpha2-adrenoceptors with dexmedetomidine and blockade of alpha1- or beta-adrenoceptors with prazosin or propranolol respectively, resulted in a further increase of EROD inducibility. Adrenoceptors were found to be involved in the regulation of the CYP1A1 gene at mRNA level. Both, reduced noradrenaline release in central nervous system induced with dexmedetomidine and central catecholamine depletion, as well as blockade of central alpha1-adrenoceptors induced with prazosin, all were associated with up-regulation of CYP1A1 expression. In contrast, stimulation of central beta-adrenoceptors with isoprenaline resulted in a down-regulation of CYP1A1 expression. Our observations indicate that drugs, which stimulate or block adrenoceptors and catecholamine release may lead to complications in drug therapy and modulate the toxicity or carcinogenicity of drugs that are substrates for the CYP1A1.

Stress-mediated modulation of B(α)P-induced hepatic CYP1A1: role of catecholamines

The present study investigated the involvement of catecholamines in stress-mediated alterations in CYP1A1 induction by benzo(alpha)pyrene (B(alpha)P) in Wistar rats. This was achieved by measuring EROD activity and CYP1A1 mRNA levels in liver tissue from rats exposed to restraint stress and B(alpha)P coupled with pharmacological modulation of peripheral and central catecholamine levels and different adrenoceptors. In a state of reserpine-induced central and peripheral catecholamine depletion, stress strongly suppressed EROD induction. Peripheral catecholamines do not appear to play a critical role in the stress-mediated modulation of EROD inducibility by B(alpha)P. Stress did not alter EROD inducibility by B(alpha)P when peripheral catecholamines were either depleted by guanethidine or supplemented by peripheral adrenaline administration. On the other hand, central noradrenergic systems appear to have a role in the stress-mediated changes in B(alpha)P-induced EROD activity and Cyp1A1 gene expression. Stimulation or blockade of noradrenaline release with atipamezole and dexmedetomidine, respectively, significantly modified the up-regulating effect of stress. Alpha1 adrenoceptors also appear to participate in the effect of stress on EROD inducibility. Alpha1-blockade with prazosin potentiated the up-regulating effect of stress, possibly preventing the down-regulating effect of noradrenaline. Beta adrenoceptors also seem to be involved directly or indirectly in the stress-mediated modulation of Cyp1A1, as propranolol (beta-antagonist) blocked the down-regulating effect of stress on B(alpha)P-induced Cyp1A1 gene expression. Plasma corticosterone alterations after stress were not related to alterations in the B(alpha)P-induced EROD activity and Cyp1A1 gene expression. In conclusion, stress appears to interfere in the regulation of B(alpha)P-induced hepatic CYP1A1 in an unpredictable manner and via signalling pathways not always directly related to catecholamines. In particular, whenever drug treatment disrupts noradrenergic neurotransmission, other stress-stimulated factors appear to modify the induction of CYP1A1. In summary, regulation of induction of hepatic CYP1A1 during stress appears to involve various components of the stress system, including central and peripheral catecholamines, which interact in a complex manner, yet to be elucidated.

Modulation of neurotoxic behavior in F-344 rats by temporal disposition of benzo(a)pyrene

The behavioral changes caused by benzo(a)pyrene (BaP), a polycyclic aromatic hydrocarbon (PAH) compound, were monitored, and also its metabolite levels in cerebellum and cortex were measured in BaP treated rats to see if any relationship existed between these two aspects. Rats were administered 0, 25, 50, 100, and 200 mg/kg of BaP in peanut oil by oral gavage. Plasma, and brain tissue (cerebellum and cortex) samples were collected at 0, 2, 4, 6, 12, 24, 48, 72 and 96 h post administration. Neurotoxic effects peaked at 2 h after dosing and lasted 48 h after dosing for all dose groups. The metabolite levels remained the same from 2 to 4 h, reached a peak at 6 h post gavage and showed a gradual decline returning to baseline levels at 72 h when the motor activity of treatment groups also returned to control levels, indicating recovery from the effects of BaP. A significant (P<0.05) correlation between neurotoxic effects and BaP plasma, and brain metabolite concentrations suggests that metabolism plays an important role in modulating the neurobehavioral effects of BaP.