CYP2B6 is expressed in African Green monkey brain and is induced by chronic nicotine treatment
Introduction
Cytochrome P450 (CYP) 2B6 metabolizes a wide variety of substances including endogenous compounds such as testosterone (Gervot et al., 1999) and the neurotransmitter serotonin (Fradette et al., 2004). It metabolizes clinical drugs such as efavirenz (Ward et al., 2003) and cyclophosphamide (Huang et al., 2000) as well as drugs of abuse such as cocaine (Aoki et al., 2000), ecstasy (Kreth et al., 2000) and phencyclidine (Jushchyshyn et al., 2003). The expression and activity of CYP2B6 in smokers is of interest as it inactivates nicotine (Yamazaki et al., 1999), activates tobacco smoke pro-carcinogens such as 4-methylnitrosamino-1-3-pyridyl-1-butanone (NNK) (Hecht, 1999, Smith et al., 2003) and activates the smoking cessation drug bupropion (Faucette et al., 2000).
CYP2B6 is expressed in human brain at very low levels compared to hepatic CYP2B6; however, brain CYP2B6 is highly expressed in specific cells such as cortical pyramidal cells and astrocytes (Miksys et al., 2003). Metabolism within brain cells is of interest because it may alter the levels of therapeutic drugs, drugs of abuse, endogenous compounds and toxic compounds in localized brain areas. Brain CYPs also have different regulation compared to hepatic CYPs, for example rat brain CYP2B1 is induced by nicotine but hepatic CYP2B1 is not (Miksys et al., 2000).
Smokers have higher CYP2B6 protein levels in the hippocampus, caudate, putamen and cerebellum compared to non-smokers (Miksys et al., 2003). CYP2B6 expression in smokers also occurs in different cells in brain regions such as the hippocampus and cerebellum. The cause of the higher CYP2B6 levels in smokers has not been identified. Nicotine is a likely candidate because chronic nicotine treatment induces rat brain CYP2B1 (Miksys et al., 2000); however, a rat model of nicotine exposure in human brain is limited because nicotine-treated rats have a different pattern of brain CYP2B1 induction compared to that seen in smokers. The difference in brain CYP2B expression between nicotine-treated rats and human smokers may be due to many factors including different inducers (smoking compared to nicotine treatment), different regulatory factors or brain structures between rats and humans, different lifestyle factors associated with smoking or other unknown components.
African Green monkeys (Cercopithecus aethiops), also named vervet monkeys, are better models of human neuroanatomy and enzyme metabolism compared to rats. African Green monkey neuroanatomy is closely related to human whereas rat neuroanatomy has some substantial differences relative to primate neuroanatomy. For example, one of the brain regions in rats that showed a nicotine-induced increase in CYP2B1 protein levels is the olfactory bulb and tubercle (Miksys et al., 2000); however, this is a minor brain region in primates. Enzyme metabolic pathways frequently differ between rats and humans. For example, the main nicotine-metabolizing enzymes in rats are CYP2B1 and CYP2B2 (Hammond et al., 1991) whereas in monkeys and humans, CYP2A6 is the main nicotine-metabolizing enzyme while CYP2B6 plays a minor role (Schoedel et al., 2003, Yamazaki et al., 1999). Rats also differ from primates in the metabolism of other substrates. Coumarin is mainly metabolized by CYP2A6 to 7-hydroxycoumarin in humans and the crab-eating macaque, which are closely related to African Green monkeys, whereas rats produce negligible amounts of 7-hydroxycoumarin and instead metabolize coumarin via alternative pathways (Pearce et al., 1992). African Green monkey CYP2B6 protein (CYP2B6agm) has been sequenced (accession number AY485344Q7M3F2, GI: 75053416) (Ohmori et al., 2004) and shares 91% amino acid homology with human CYP2B6. The basal expression of CYP2B6agm in monkey brain has not yet been investigated; therefore, our first objective was to determine the regional and cellular distribution of CYP2B6agm protein in monkey brain. The effects of nicotine on primate brain are of particular interest as nicotine is widely used as a smoking cessation agent. Nicotine is also in clinical trials for treatment of some neurological disorders such as age-associated memory impairments (White and Levin, 2004) and schizophrenia-associated cognitive defects (Harris et al., 2004). Our second objective was to determine if chronic nicotine treatment induced monkey brain CYP2B6agm and if so where the induction occurs.
Section snippets
Chemicals
Nicotine bitartrate was purchased from Sigma–Aldrich Canada Ltd. (Oakville, ON, Canada). All other chemical reagents were obtained from standard commercial sources. Protein assay dye reagent was purchased from Bio-Rad Laboratories (Hercules, CA, USA). Pre-stained molecular weight protein markers were purchased from MBI Fermentas (Flamborough, ON, Canada). Nitrocellulose membrane was purchased from Pall Life Sciences (Pensacola, FL, USA). Human cDNA-expressed CYP1A1, CYP1A2, CYP2A6, CYP2B6,
Nicotine pharmacokinetics
All pharmacokinetic parameters were calculated for each individual monkey. The pre-treatment nicotine challenge dose of 0.1 mg/kg resulted in an average maximum plasma concentration of 87 ± 69 ng/ml reached in 0.5 ± 0.4 h. The area under the curve was 118 ± 52 ng × h/ml (extrapolated to infinity) and 99 ± 47 ng × h/ml (calculated to the last measurable number). The average half-life was 2.6 ± 1.5 h and the average elimination constant was 0.4 ± 0.2 per hour. The concentration–time curve for the pre-treatment
Discussion
The distribution of CYP2B6 in brain is important because of its potential effect on localized drug metabolism. We have developed a monkey model to investigate the regional and cellular distribution of basal brain CYP2B6agm and to determine whether nicotine can induce primate brain CYP2B6agm.
The peak plasma nicotine levels in human smokers range from 19 to 50 ng/ml (Schneider et al., 2001). In monkeys, the 0.1 mg/kg pre-treatment nicotine challenge dose resulted in an average peak plasma nicotine
Acknowledgements
We gratefully thank Lyndon Lacaya and Helma Nolte for excellent technical assistance. This work was supported by the Centre for Addiction and Mental Health and CIHR Grant MT14173. We are grateful for additional support from the CIHR Tobacco Use in Special Populations Fellowship to AML and a Canadian Research Chair in Pharmacogenetics to RFT.
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