Cytochrome P450 4F subfamily: At the crossroads of eicosanoid and drug metabolism

Abstract

The cytochrome P450 4F (CYP4F) subfamily has over the last few years come to be recognized for its dual role in modulating the concentrations of eicosanoids during inflammation as well as in the metabolism of clinically significant drugs. The first CYP4F was identified because it catalyzed the hydroxylation of leukotriene B₄ (LTB₄) and since then many additional members of this subfamily have been documented for their distinct catalytic roles and functional significance. Recent evidence emerging in relation to the temporal change of CYP4F expression in response to injury and infection supports an important function for these isozymes in curtailing inflammation. Their tissue-dependent expression, isoform-based catalytic competence and unique response to the external stimuli imply a critical role for them to regulate organ-specific functions. From this standpoint variations in relative CYP4F levels in humans may have direct influence on the metabolic outcome through their ability to generate and/or degrade bioactive eicosanoids or therapeutic agents. This review covers the enzymatic characteristics and regulatory properties of human and rodent CYP4F isoforms and their physiological relevance to major pathways in eicosanoid and drug metabolism.

Introduction

Cytochrome P450s (CYP), phase I drug metabolizing enzymes, comprise a superfamily of heme-thiolate proteins that play critical roles in endogenous as well as xenobiotic metabolism (Guengerich, 1991, Oritz de Montellano, 1995, Anzenbacher and Anzenbacherova, 2001, Nebert and Russell, 2002). CYP-dependent monooxygenases are found predominantly in the microsomal fraction of tissues, but such activities can also be seen in the mitochondria (Anandatheerthavarada et al., 1997, Anandatheerthavarada et al., 1999, Robin et al., 2002). The microsomal CYP monooxygenase enzyme system, which is our interest here, has 3 major components: (a) the heme protein, CYP, (b) the flavoprotein, NADPH CYP-reductase and (c) phospholipids (Strobel et al., 1970, Coon, 2002, Coon, 2005). The reductase transfers 2 electrons to the CYP supplying reducing equivalents to the heme iron for reduction of molecular oxygen and product formation during catalysis (Strobel et al., 1995a).

Although first identified in liver, the presence and activity of CYP have been extended to extrahepatic tissues including kidney, colon, lung and brain (Guengerich and Mason, 1979, Strobel et al., 1997, Strobel et al., 2001, Ding and Kaminsky, 2003, Zhao and Imig, 2003, Kroetz and Xu, 2004). Currently, > 700 CYP have been characterized, inclusive of the many different species of organisms that have been studied (Nelson, 1999). These proteins are conveniently arranged into families and subfamilies on the basis of percent amino acid sequence identity (Nebert & Gonzalez, 1987). The individual CYP are represented by an Arabic number denoting the family, followed by an alphabetical letter designating the sub-family and an Arabic numeral representing the individual gene within the subfamily (Nelson et al., 1993, Nelson, 1999). This nomenclature is followed throughout this review.

Apart from having multiple forms, a fascinating fact about CYP is their broad substrate spectrum. While some CYP isoforms are fairly specific in their choice of substrate, such as those involved in steroidogenesis, the rest of them catalyze a large number of reactions.

Beyond their toxicological and pharmacological relevance, CYP enzymes play a central role in metabolizing endogenous products such as cholesterol, steroids, vitamins, eicosanoids and fatty acids (Capdevila et al., 1982, Andersson et al., 1989, McGiff, 1991, Kikuta et al., 1993, Kawashima et al., 1997, Lund et al., 1999, Node et al., 1999, Bylund et al., 2000, Capdevila et al., 2000, Thompson et al., 2000, Oliw et al., 2001, Nebert and Russell, 2002, Sontag and Parker, 2002, Cheng et al., 2004). Of particular interest is their role in the arachidonic acid (AA) cascade. AA, a polyunsaturated long-chain fatty acid esterified to cellular phospholipids, can be metabolized into various active and inactive products based on their cellular and physiological context. Prostaglandins (PG) and leukotrienes (LT) are the most widely studied metabolites of AA; however, several CYP in the subfamilies 2 and 4 catalyze conversion of AA into 20-hydroxyeicosatetraenoic acid (HETE) and epoxyeicosatrienoic acids (EET) (Fisslthaler et al., 1999, Capdevila et al., 2000, Lasker et al., 2000, Zeldin, 2001, Chuang et al., 2004, Kroetz and Xu, 2004). The focus of this review is the CYP4F subfamily which plays a pivotal role in metabolism of AA, its derivatives and many clinically important drugs.