Induction of the drug metabolizing enzyme CYP2D in monkey brain by chronic nicotine treatment
Introduction
Cytochrome P450 2D6 (CYP2D6) is an enzyme that metabolizes a range of endogenous (e.g. dopamine) and exogenous (e.g. parathion) substrates. It metabolizes approximately 25% of all clinically used drugs including analgesics (e.g. codeine), neuroleptics (e.g. clozapine), antidepressants (e.g. imipramine), and drugs of abuse (e.g. ecstasy) (Zanger et al., 2004). CYPs are expressed in a number of extrahepatic tissues including the brain. CYP2D has been identified in human (Miksys et al., 2002), dog (Tyndale et al., 1991) and rat (Miksys et al., 2000, Yue et al., 2008) brain tissue. Both, CYP2D mRNA and protein have been surveyed in the human brain (Miksys et al., 2002). Brain membranes from human (Voirol et al., 2000), dog (Tyndale et al., 1991) and rat (Tyndale et al., 1999), readily metabolize CYP2D6 probe drugs (e.g. dextromethorphan), indicating a functional enzyme. Furthermore, in rat brain both mRNA and protein correlate with activity (Miksys et al., 2000). A study demonstrating the initial analgesic effects of codeine due to its in situ metabolism to morphine is additional evidence of brain CYP2D function (Chen et al., 1990).
As CYP2D6 inactivates neurotoxins such as MPTP (Herraiz et al., 2006), tetrahydroisoquinoline (Suzuki et al., 1992), harmaline, harmine (Yu et al., 2003) and pesticides (Tang et al., 2002), individuals lacking a functional CYP2D6 may be more susceptible to neurotoxicity from these compounds. The expression and function of hepatic CYP2D6 is variable amongst individuals, because it is genetically highly polymorphic (http://www.cypalleles.ki.se/cyp2d6.htm). Functional CYP2D6 protein is absent in approximately 10% of Caucasians due to genetic polymorphisms. Population studies indicate that individuals with no functional alleles (i.e. CYP2D6 poor metabolizers (PMs)) are at a higher risk for Parkinson's disease (PD) compared to those carrying one or two functional alleles (i.e. extensive metabolizers (EMs)) (McCann et al., 1997), and this risk increases with exposure to pesticides (Elbaz et al., 2004). Overall, these findings suggest that functional CYP2D6 has an important role in defense against endogenous and/or exogenous PD-causing neurotoxins.
Smokers are 50% less likely to develop PD (Alves et al., 2004). If CYP2D6 is higher in the brains of smokers it may contribute to the neuroprotection against PD observed in smokers. Therefore, we examined the levels of brain CYP2D6 in smokers and non-smokers. Nicotine, a component of cigarette smoke, has been shown in a Parkinsonian model to prevent the loss of dopaminergic markers and preserve neuronal plasticity (Quik et al., 2006a, Quik et al., 2006b). It also reduces, and maintains the reduction in, the dyskinetic side effects of levodopa (Quik et al., 2007). Chronic nicotine treatment induces brain CYPs in rats (Howard et al., 2003, Yue et al., 2008). Therefore, we investigated if nicotine could induce CYP2D in a non-human primate brain and whether these changes were comparable to those seen in human smokers. Possible implications of induced brain CYP2D are that it may (a) provide neuroprotection from both endogenous and exogenous neurotoxins inactivated by CYP2D6, (b) alter localized CNS drug metabolism and drug response, and (c) influence mental health and behaviour through the altered metabolism of neurotransmitters and neurosteroids.
Section snippets
Materials
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,
Smokers have higher brain CYP2D6 than non–smokers
In humans, there were significantly higher levels of CYP2D6 in some brain regions of smokers compared to non-smokers (Fig. 1), such as the globus pallidus (7.9-fold), substantia nigra (3.5-fold) and the cerebellum (4.3-fold). This data should be interpreted cautiously due to the limited sample size. However, the observation of higher levels of CYP2D6 in specific brain regions of smokers provided the impetus for the subsequent studies using nicotine treatments.
CYP2D is expressed in the monkey brain
There was no cross-reactivity of
Discussion
This is the first study to show the induction of CYP2D in a non-human primate brain by a commonly used drug, nicotine. The induction of CYP2D by nicotine is brain region-specific and cell-specific (in select regions), with no change observed in liver. The induction within the brain is striking as CYP2D6 is considered uninducible in the liver by drugs and xenobiotics that readily induce other CYPs (e.g. phenobarbital) (Rae et al., 2001, Edwards et al., 2003).
Here we report that smokers have
Acknowledgments
Funding for this work was provided by the Centre for Addiction and Mental Health, CIHR Grant (MOP14173) and a Canada Research Chair in Pharmacogenetics to Dr R.F. Tyndale. Additional support was provided by CIHR Tobacco Use in Special Populations Fellowship to A. Mann. Dr. R.F. Tyndale has shares in Nicogen Research Inc. Funds were not received from Nicogen for these studies, nor was this manuscript reviewed by individuals associated with Nicogen. We would like to thank Dr. Roberta Palmour for
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