Cellular localization and functional significance of CYP3A4 in the human epileptic brain

Drug resistance in epilepsy is not fully understood. The complexity of the drug-resistant phenotype reflects the nature of the pathophysiologic process, its possible evolution over time, and the different individual sensitivity to drugs (Loscher et al., 2009). In order to elucidate the mechanisms underlying this condition, a pharmacodynamic hypothesis and a pharmacokinetic hypothesis have been proposed (Dresser et al., 2000; Oby & Janigro, 2006; Galanopoulou et al., 2009; Granata et al., 2009). The latter hypothesis suggests that the bioavailability of drug in epileptic brain regions could be diminished by the overexpression of transporter proteins (e.g., MDR1) at the blood–brain barrier (BBB; Dombrowski et al., 2001; Loscher & Potschka, 2005; Marchi et al., 2004, 2010). A new interpretation of the role of drug transporters in drug-resistant epilepsy was proposed when MDR1 overexpression was discovered in parenchymal astrocytes and neurons (Potschka et al., 2002, 2004; Marchi et al., 2006; Rambeck et al., 2006). Therefore, although the localization of drug-extrusion pumps at the BBB is consistent with reduced brain penetration of drugs, neuroglial MDR1 expression was proposed to be associated with protective mechanisms favoring cell survival (Marchi et al., 2004). Although most studies linking the BBB to drug-resistant epilepsy have focused on the role of multidrug transporter proteins, we have recently shown that transcript levels of several P450 enzymes (CYPs) are elevated in brain endothelial cells derived from temporal lobe resections of drug-resistant patients with epilepsy (Ghosh et al., 2010). Among these enzymes, CYP3A4 was further explored (Levy, 1995; Dresser et al., 2000; Ghosh et al., 2010). Interestingly, in non-CNS organs (e.g., liver and gut) CYP3A4 and MDR1 are transcriptionally coregulated (Xie et al., 2000; Bauer et al., 2004; Cerveny et al., 2007; Chen et al., 2007). CYP3A4 and MDR1 share substrates and inducers and, in non-CNS tissues, constitute the machinery responsible for detoxification and drug processing (Yasuda et al., 2002; Pal & Mitra, 2006). It is unknown whether the same applies to the drug-resistant epilepsy. Based on these considerations we have done the following: (1) evaluated the pattern of CYP3A4 expression in brain specimens obtained from surgical resections (temporal lobe epilepsy, TLE; tuberous sclerosis, TS; and cavernous angioma, CA) associated with seizures; (2) evaluated CYP3A4 and MDR1 colocalization; and (3) investigated the association between CYP3A4 expression and cell survival.