Cytochrome P450 Enzymes and Nuclear Receptors for Metabolism of Steroids and Xenobiotics --- New Venues for Novel Drug Development

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1 August, 2000

Cytochrome P450 Enzymes and Nuclear Receptors for Metabolism of Steroids and Xenobiotics --- New Venues for Novel Drug Development

Cytochrome P450 mono-oxygenases (hydroxylases), often referred simply to as CPY enzymes, catalyze oxygen insertion into many different kinds of substrates, including natural steroids, fatty acids and foreign compounds, in a step of their metabolism. They serve as the enzymes for drug metabolism, toxification as well as detoxication of xenobiotic (non-biological) compounds prevalent in the environment. Several classes of xenobiotic compounds thus elicit characteristic biological effects in vivo due often to intermediate metabolites. These enzymes are expressed in both the gastrointestinal tract and hepatic microsomes, and the activity may vary between 6- and 30-fold among individuals due to genetic and nongenetic factors such as drug therapy.

There have been remarkable progress in recent years in the study of mechanisms through which steroids and xenobiotics induce the expression of hepatic P450 enzymes that belong to families of CYP2, CYP3, and CYP4. The induction of CYP2, CYP3 and CYP4 are mediated by nuclear receptor superfamily members designated CAR (constitutive androstane receptor), PXR (pregnane X receptor), and PPAR (peroxisome proliferator-activated receptor). The prototypical inducers of P450 enzymes are phenobarbital, pregnenolone 16alpha-carbonitrile and rifampicin, and clofibric acid, respectively, through the activation of receptors CAR, PXR and PPAR. In addition, two other nuclear receptors, designated LXR and FXR, have been discovered which are respectively activated by oxysterols and bile acids. These receptors also play a role in liver P450 expression, in this case regulation of cholesterol balance. LXR induces reverse a cholesterol transporter and FXR induces P450 cholesterol 7alpha-hydroxylase (CYP7A1), a key enzyme of bile acid biosynthesis from cholesterol. All these receptors share another nuclear receptor, retinoid X-receptor (RXR), as the heterodimerization partner in the binding to a hexameric DNA direct repeat, and interact with still other nuclear receptors, and with factors in intracellular signaling pathways.

Among the P450 enzymes, the induction of CYP3A has been studied extensively. CYP3A is essential for the metabolism of endogenous steroid hormones, and of almost two-thirds of drugs and other xenobiotic compounds. These compounds include peroxisome proliferators, thiazolidinediones (TZDs), and an additional set of compounds. All three classes of xenobiotics are now known to exert their actions through activation of PPARs. The peroxisome proliferators include the fibrate class of triglyceride- and cholesterol-lowering drugs that activate PPARalpha. TZDs that activate PPARgamma sensitize tissues such as skeletal muscle, liver, and adipose to the actions of insulin and lower serum glucose and lipid levels in type 2 diabetics. Thus they are used as anti-glycemic agents to combat insulin resistance in type 2 diabetes. Various naturally occurring fatty acids (n-6 and n-3 families) and their respective eicosanoids may also be PPAR ligands, whereby dietary lipids and their metabolites can regulate gene transcription in cell differentiation and tumorigenesis, and influence overall energy balance.

The induction of CYP3A enzymes appears to be species-specific. There are marked differences in the compounds that induce CYP3A gene expression between species, and the difference may define susceptibility of mammals to xenobiotics in the environment. It is currently postulated that the induction involves nuclear receptors that serve as specific sensors. In addition to PPARs, one of the candidate sensors is PXR. Many of the PXR activators are widely-used drugs such as dexamethasone, lovastatin and rifampicin. In some cases, the induction of CYP3A through PXR may promote the metabolism of other drugs, leading to toxfication often with adverse consequences. The extent of this toxification differs in different animals.

The cloning and characterization of PXR showed sequence diversity of amino acids in the ligand-binding domain. Rat PXR exhibits a 95% sequence identity with mouse PXR, but only 79% identity with the human and rabbit PXRs (Zhang H et al, 1999: uid=10415106; Jones SA et al, 2000: uid=10628745). This sequence divergence may be reflected in pharmacological differences in PXR activation profiles. The macrolide antibiotic rifampicin and the anti-diabetic drug troglitazone were effective activators of the human and rabbit PXRs but had little activity on the rat and mouse PXRs. Conversely, pregnenalone 16alpha-carbonitrile was a more potent activator of the rat and mouse PXRs than the human and rabbit receptors.

Zie W et al (2000: uid=10935643) showed recently that targeted disruption of the mouse PXR gene abolished induction of CYP3A by dexamethasone or pregnenalone-16alpha-carbonitrile. In this case, when steroid and xenobiotic receptor (SXR) that is a human homolog to mouse PXR was transduced into the mouse, the transgenics expressed CYP3A gene constitutively and enhanced protection of the mice against toxic compounds. In particular, the transgenics were responsive to human-specific toxic inducers such as rifampicin in the human-specific pattern. Thus there seems to be species-specific pattern of CYP3A inducibility, and PXR is possibly a specific zeno-sensor that mediates the adaptive hepatic response. Many of the compounds that induce CYP3A expression bind directly to human PXR (SXR), and the nuclear receptor as being a sensor may represent the critical biochemical mechanism of xenoprotection. Further, the induction of CYP3A enzymes can be molecular basis of drug-drug interactions, and may in some cases reduce the efficacy of prescribed drugs by having prior induction of the enzymes by other xenobiotics and natural food constituents.

Finally to mention briefly, another xeno-sensor candidate is CAR (constitutive androstane receptor). Xenobiotics induce the transcription of CYP2B through CAR, and when systematically compared with a series of xenobiotics and natural steroids, CAR for CYP2B exhibited marked pharmacological differences from those of PXR for CYP3A. Several of the compounds were also shown to bind directly to CAR (Moore LB et al, 2000: uid=10748001). Thus CAR essential for CYP2B induction is also a receptor that is capable of recognizing structurally diverse compounds, and may serve as a sensor for xenobiotics that are not taken care of by PXR.

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