Current Drug Metabolism - Volume 12, Issue 2, 2011

The expression and activity of drug-metabolizing enzymes (DME) is influenced by variety of physiological factors (e.g. age, sex, pregnancy, menstrual cycle) and patho-physiological states of human organism (e.g. inflammation, infection, cancer, diabetes). Environmental factors (chemicals, radiation, microorganisms etc.) and gene polymorphisms have a great impact on activity of DME. Regulation of DME at molecular level is very complex and involves multiple mechanisms, including proteasome-mediated degradation, posttranslational regulation, transcriptional and post-transcriptional regulation. The principles of transcriptional regulation of DME were elucidated mainly in 90s'. In particular, the discovery of xenosensors AhR (aryl hydrocarbon receptor), PXR (pregnane X receptor) and CAR (constitutive androstane receptor), was a breakpoint in that research. Xenosensors were initially considered as orphan receptors that regulate only DME. However, all xenosensors were “deorphanized” at this time and demonstrated to regulate many other genes besides DME. For instance, endogenous ligands for AhR are metabolic intermediates such as bilirubin, biliverdin, indirubin, indole (tryptophan derivatives) and arachidonic acid derivatives. It is known that AhR plays many endogenous functions such as regulating the cell cycle and proliferation, immune response, circadian rhythm, tumor promotion etc. Examples of AhR-driven genes other than DME are AhRR, TGFβ, p27, IL-1β, Jun and Bax. Similarly PXR and CAR have many endogenous functions and they regulate metabolism of lipids, carbohydrates, cholesterol and bile acids. Consistently, examples of target genes for PXR and CAR, others than DME, are ApoA1 and amino levulinate synthase 1 (ALAS-1), respectively. Putative endogenous ligands for PXR (vitamin E, vitamin K, beta-carotene, bile acids) and CAR (farnesol) were identified as far. Taken together, deorphanization of xenosensors and identification of their role in intermediary metabolism and cell physiology, defined a relation between metabolism of xenobiotics and intermediary metabolism at molecular level. In the course of time, it was more and more clear that important regulators of DME are also typical regulators of intermediary metabolism, such as GR (glucocorticoid receptor), VDR (vitamin D receptor), ER (estrogen receptor), RAR (retinoic acid receptor) and RXR (retinoic X receptor), via their natural and synthetic ligands. In addition, many other endogenous compounds including inflammatory cytokines, growth factors, lipids, neurotransmitters, eicosanoids etc. influence the expression of DMEs. Hence, a reciprocal relation between endogenous compounds and metabolism of xenobiotics is defined in that way. There exist mutual interactions between xenoreceptors, steroid receptors and nuclear receptors by several mechanisms, usually called cross-talks. Cross-interactions or cross-talks between signaling pathways for two or more receptors result in functional molecular and physiological consequences. Cross-talks between two receptor-controlled pathways may occur by several mechanisms, comprising “sharing the ligand” (e.g. miconazole is agonist for PXR but at the same time antagonist for GR), “sharing the response element” (e.g. PXR and CAR share several response elements in CYP3A4 promoter), “regulatory cascade” (e.g. GR up-regulates transcriptionally PXR and CAR), “sharing the co-activator” (e.g. AhR that is transcriptional co-activator of ER) or “metabolic cascade” (e.g. CAR up-regulates UGT1A1, which in turn inactivates thyroid receptor ligands T3 and T4). The present issue of Current Drug Metabolism focuses on several aspects of mutual interaction between endobiotics and xenobiotics regulatory pathways.

Retinoic acid receptors (RARs), retinoid X receptors (RXRs) and thyroid hormone receptors (TRs) are nuclear receptors that are crucial transcriptional regulators of many cellular processes such as differentiation, development, apoptosis, carbohydrate and lipid metabolism, homeostasis etc. In addition, RXRs are common heterodimerization partners for several receptors including vitamin D receptor, pregnane X receptor (PXR), constitutive androstane receptor (CAR) etc. In the course of 90s', PXR and CAR were discovered as key xenosensors regulating drug-metabolizing enzymes. Since there exist various cross-talks between cell signaling pathways, this was not surprising that RXRs, RARs and TRs were identified as regulators of human drug-metabolizing cytochromes P450 and cytochromes P450 involved in metabolism of endogenous compounds. Hence, a link between regulation of xenobiotic metabolizing enzymes and regulatory pathways of intermediary metabolism was established. Additionally, several drug-metabolizing enzymes are involved in metabolism of retinoids, rexinoids and thyroid hormones. In the current paper, we summarize the knowledge on the role of RARs, RXRs and TRs in the regulation of drug metabolizing cytochromes P450, and vice versa on the role of P450s in homeostasis of retinoids, rexinoids and thyroid hormone.

The aryl hydrocarbon receptor (AhR) is a ligand-activated transcription factor, which plays a major role in toxic effects of environmental pollutants. It is a pivotal regulator of several xenobiotic-metabolizing enzymes (XMEs), and is now considered to play an important role also in control of cell cycle, apoptosis and cell differentiation. The accumulating evidence suggests that there exists a multiple crosstalk between AhR activation and the signaling pathways activated by inflammatory mediators, such as nuclear factor-κB, a pleiotropic transcription factor controlling the immune/inflammatory responses. In this review, we summarize the current knowledge about the interactions of AhR with inflammatory mediators leading to deregulation of the AhR-dependent XMEs, as well as the evidence pointing to the role of AhR in modulation of inflammatory signals. These include altered expression of proinflammatory cytokines, such as tumor necrosis factor-alpha or interleukin-6, and deregulation of expression/activity of principle enzymes producing inflammatory mediators, such as cyclooxygenase-2. Recent studies also indicate that various classes of AhR ligands may differentially modulate AhRdependent toxic responses and inflammation, which opens an interesting opportunity for a targeted synthesis of AhR ligands with antiinflammatory properties. Although the role of activated AhR in the regulation of inflammation is still far from being completely understood, the close interactions between AhR and inflammatory signaling evidently can play a significant role in immune dysfunctions, metabolism of xenobiotics or carcinogenesis. The current review will focus mostly on the interaction of AhR and inflammation relative to mechanisms associated with the pathology of carcinogenesis.

Phase II biotransformation enzymes conjugate xenobiotics using small, organic donor molecules, such as glutathione, UDPglucuronic acid, S-adenosyl-L-methionine, acetyl coenzyme A and amino acids (primarily glycine). These reactions generally resulted in detoxification by the loss of pharmacological activity and by quickening the elimination of xenobiotics from the body, however bioactivation is also known to be occured. Historically, it was placed more emphasis on research of phase I than of phase II enzymes. Nevertheless, it is well known that conjugation enzymes play an important role in drug and toxicant disposition since they can dramatically alter pharmacokinetics and therefore therapeutic efficacy and toxicity of drugs. In this context surprisingly, the exact regulation mechanism of phase II conjugation enzymes expression is not fully understood. However, available experimental data suggest that several transcriptional factors are involved in this process. In the current review, we characterize and summarize our knowledge about regulation of the most important phase II enzymes, such as UDP-glucuronosyltransferases (UGTs), sulfotransferases (SULTs), glutathione S-transferases (GSTs), arylamine N-acetyltransferases (NATs), catechol O-methyltransferase (COMT) and thiopurine S-methyltransferase (TPMT) by different steroid hormones.

The central and peripheral nervous systems are important factors influencing the functioning of liver cytochrome P450 (CYP). It has been shown that changes in the brain monoaminergic systems affect liver cytochrome P450 (CYP) expression (CYP1A, CYP2B, CYP2C11 and CYP3A). The brain dopaminergic system has been established as an important center regulating the liver CYP. This regulation proceeds via the tuberoinfundibular pathway and the dopaminergic D2 receptors of the pituitary, as well as the mesolimbic pathway engaging the D2 receptors of the nucleus accumbens (conveying a message to the paraventricular nucleus of the hypothalamus). These two dopaminergic pathways stimulate the secretion of pituitary hormones, which directly (GH) or indirectly (ACTH, TSH) activate hepatic nuclear/ cytosolic receptors controlling CYP genes. Recent preliminary studies with selective noradrenaline or serotonin neurotoxins suggest also involvement of the brain noradrenergic and serotonergic systems in the regulation of liver CYP. Moreover, the influence of the peripheral nervous system involving several neurotransmitters (acetylcholine, noradrenaline, adrenaline, dopamine, serotonin) on liver function may also be important for the physiological regulation of hepatic CYP activity. The hypothalamus controls liver function not only by releasing hormones from the pituitary gland but also by stimulating the autonomic sympathetic and parasympathetic projections to the liver. In addition to direct neural connections, the autonomic nervous system can indirectly affect liver function via the hypothalamus-adrenal axis and the hypothalamus- pancreas axis. Therefore, the estimation of neuroactive drug action on hepatic CYP requires an in vivo model which allows the central neuroendocrine and peripheral autonomic regulation of genes coding for CYP isoforms.

Eleven members of the human organic anion transporter (OATP) family (grouped into six families) facilitate the Na+- independent transmembrane transport of various endo- and xenobiotics (bile acids, bilirubin, steroid hormone conjugates, thyroid hormones, prostaglandins, clinically used drugs, and toxins). OATPs are 12-transmembrane glycoproteins (643-722 amino acids) and contain many conserved structural features, for example, eleven cysteines in the large extracellular loop 5. They are important for proper transport, for which translocation of substrates through a central, positively-charged pore in a rocker-switch-type mechanism has been proposed. Although OATPs are expressed in various cells and tissues, some members show a more restricted pattern (well-studied OATP1B1/OATP1B3 in liver, OATP4C1 in kidney, and OATP6A1 in testis). In cancer, the distribution pattern is no longer maintained, and OATPs, like OATP1B3, become upregulated in malignant tissues (colon, breast, prostate). Studies in cell lines and animal models further revealed that the expression of OATPs is regulated in a cell- and tissue-specific way by cytokines and activation of nuclear receptors (LXR, FXR, PXR, CAR, HNF4). Also epigenetic mechanisms and postranslational modifications influence their expression and function. Therefore, changes in the expression of OATPs under pathological conditions will influence transport processes causing an altered accumulation of OATP substrates in cells of excretory organs (intestine, liver, kidney) and on various blood/organ barriers (such as brain, testis, placenta). For drugs, this may result in increased toxicity and adverse drug reactions. Therefore, it is important to improve the knowledge on the regulation and function of individual OATPs, and to apply it for therapeutic considerations.

Cytochrome P450 (P450) monooxygenases are capable of catalyzing metabolism of various endogenous and exogenous compounds, such as bile acids, fatty acids, retinoids, steroids, drugs and other xenobiotics. The enzymes, belonging to CYP1, CYP2 and CYP3 families are primarily involved in the metabolism of drugs and xenobiotics. P450-mediated defense mechanism protects organisms from the potentially toxic effects of xenobiotics to which they are exposed. The adaptive transcriptional induction of P450s by xenobiotics is mediated by aromatic hydrocarbon receptor of Per-ARNT-Sim family, and nuclear hormone receptors, including pregnane X receptor, constitutive androstane receptor and glucocorticoid receptor. In addition to the receptor-mediated induction, endogenous factors (developmental, sex or hormonal factors) can also modulate P450 expression. Steroid hormones are biologically active compounds, controlling many physiological processes via endocrine signaling pathways and contributing to the transcriptional regulation of drugmetabolizing P450s. Any change in P450 activities influences the rate of activation or inactivation of drugs. Exposure to xenobiotics (drugs, environmental pollutants) can exert changes in endocrine function both directly as hormone agonists/antagonists or indirectly altering the rates of hormone metabolism and consequently the circulating levels of hormones. Modulation of P450 expression by xenobiotics can affect the subsequent metabolism of not only foreign chemicals, but also steroid hormones. Perturbation in hormone metabolism leads to the imbalance in sexual and reproductive development, and in glucose, lipid and salt/water homeostasis. The purpose of this review is to highlight the interplay between drug-metabolizing P450s and steroid hormones as well as the interactions of xenosensor with steroid signaling pathways.

Cytochromes P450 of the liver are involved in maintenance of lipid homeostasis (cholesterol, vitamin D, oxysterol and bile acid metabolism) and in detoxification processes of endogenous compounds (i. e. bile acids) and xenochemicals (drugs). This review describes the roles of various CYPs in production of cholesterol related endogenous metabolites. These metabolites (oxysterols, bile acids, vitamin D3) act as activators of a battery of nuclear receptors, in particular liver X receptor (LXR), pregnane X receptor (PXR), constitutive androstane receptor (CAR), farnesoid X receptor (FXR) and vitamin D receptor (VDR). Nuclear receptors that are activated by cholesterol metabolites or by drugs (i. e. PXR or CAR) bind to promoter regions of responsive genes. The downstream genes include CYPs from cholesterol metabolism and/or from drug metabolism whose transcription is activated in a feedback manner. Cholesterol metabolites are thus major actors of the cross-talk that is mediated by nuclear receptors and activated CYPs. This results in a simultaneous regulation of genes from cholesterol metabolism, drug metabolism and also other pathways. The interplay between metabolism of endogenous and exogenous compounds is a frequent cause of drug failures that can now be explained at the molecular level.

Unlike most cytochrome P450 (CYP) enzymes, murine hepatic CYP2A5 is induced during pathological conditions that result in liver injury including hepatotoxicity mediated by xenobiotics, hepatitis caused by various microbial agents and liver neoplasia. Since CYP2A5 metabolizes various important xenobiotics including nicotine and pro-carcinogens such as nitrosamines and aflatoxin B1, altered gene expression could affect tobacco addiction, hepatotoxicity and hepatocarcinogenesis. This article synthesizes the current knowledge concerning hepatic expression of Cyp2a5 including the transcriptional and post-transcriptional regulatory mechanisms, pathophysiological conditions associated with enzyme induction such as oxidative and endoplasmic reticulum stress and altered lipid and energy homeostasis as well as the known exogenous and putative endogenous substrates. Knowledge of the stimuli responsible for the unique overexpression of CYP2A5 during liver injury may provide clues to a functional role for this enzyme and the impact of variable CYP2A5 expression on xenobiotic metabolism and toxicity, disease development and the adaptive response to hepatocellular stress.

Aryl hydrocarbon receptor (AhR) is an important transcriptional regulator of drug metabolizing enzymes that dominantly controls the expression of cytochrome P450 CYP1 family genes and some phase II enzymes. AhR also has many endogenous functions including cell cycle control, immune response, and cell differentiation. In addition, AhR is well-known to be involved in chemicallyinduced carcinogenesis. AhR is activated by a variety of endogenous and exogenous ligands. While exogenous activation of AhR has deleterious effects on human organism, sustained activation of AhR by endogenous ligands is indispensable for proper cell functions. Therefore, the effects of exogenous and endogenous ligands on AhR resemble the Dr. Jekyll and Mr. Hyde story. The aim of the current paper is to summarize and update the knowledge on exogenous and endogenous AhR ligands.