Jaime Kapitulnik

Pleiotropic functions of biliverdin reductase: cellular signaling and generation of cytoprotective and cytotoxic bilirubin.

Degradation of heme requires its conversion to biliverdin (BV) by heme oxygenase, followed by reduction of BV to the free-radical quencher bilirubin (BR) by biliverdin reductase (BVR). It is now recognized that human BVR (hBVR) is a dual-specificity kinase (Ser/Thr and Tyr) upstream activator of the insulin/insulin growth factor-1 (IGF-1) and mitogen-activated protein kinase (MAPK) signaling pathways. hBVR is also a basic-leucine-zipper (bZip) DNA/chromatin-binding transcription factor, an activator and anchor protein for translocation of protein kinase C betaII and zeta isozymes within cell compartments, and a kinase kinase for their activation. hBVR is essential for MAPK-extracellular signal-regulated kinase (ERK)1/2 (MEK)-eukaryotic-like protein kinase (Elk) signaling and has been identified as the cytoplasm-nuclear heme transporter of ERK1/2 and hematin, the key components of stress-responsive gene expression. Here, we discuss the recently uncovered functions of hBVR in cell signaling and regulation of gene expression, and the role of BR in cellular signaling, cytoprotection and cytotoxicity.

Complement activation and disease: protective effects of hyperbilirubinaemia.

Complement, an important effector mechanism of the immune system, is an enzymatic cascade of approx. 30 serum proteins leading to the amplification of a specific humoral response. It can be activated through the classical or alternative pathways, or through the mannose-binding lectin pathway. The activation of the classical pathway is initiated by the binding of the C1 component to antigen-bound antibodies, known as immunocomplexes. C1 is a complex of one molecule of C1q, two molecules of C1r and two molecules of C1s. C1q contains three copies of a Y-shaped fundamental unit with globular heads included in its structure, which play a major role in the interaction with the Fc portion of immunoglobulins. Deficient or exacerbated activation of the complement system leads to diseases of variable severity, and pharmacological inhibition of the complement system is considered as a therapeutic strategy to ameliorate the inflammatory effects of exacerbated complement activation. Bilirubin is a product of haem degradation by the concerted action of haem oxygenase, which converts haem into biliverdin, and biliverdin reductase, which reduces biliverdin to UCB (unconjugated bilirubin). UCB exerts both cytoprotective and cytotoxic effects in a variety of tissues and cells, acting either as an antioxidant at low concentrations or as an oxidant at high concentrations. In the present review, we describe in detail the anti-complement properties of bilirubin, occurring at levels above the UCB concentrations found in normal human serum, as a beneficial effect of potential clinical relevance. We provide evidence that UCB interferes with the interaction between C1q and immunoglobulins, thus inhibiting the initial step in the activation of complement through the classical pathway. A molecular model is proposed for the interaction between UCB and C1q.

Local Kinetics and Dynamics of Xenobiotics

The linkage point between kinetics and dynamics of drugs and other foreign chemicals is at the active site of the target. The response is determined within certain limits by the concentration of the substance at the target and its mechanism of action. However, the concentration of the substance at the active site under in vivo conditions cannot be easily measured, and surrogates such as plasma concentration are used instead. The emergence of in vitro approaches to study some primary pharmacokinetic (PK) and pharmacodynamic (PD) processes in simpler experimental setups could provide ways to estimate the concentration at the active site, which then can be used in a more reliable extrapolation and prediction of in vivo behavior and action of the substance. Imaging and other novel analytical techniques are providing direct or indirect measurements of in vivo concentrations in tissues. In this review, processes such as transport, metabolism, etc. affecting the concentration of the substance at the active site are discussed, analytical possibilities to measure directly or estimate indirectly the concentration at the active site are illustrated, the overall design and performance of physiologically based PK–PD models are described, and some examples concerning blood–brain barrier, liver, and lungs are presented. In conclusion, several important recommendations are made: (1) In vitro systems should be characterized in a much more detailed way than has been done so far, especially with respect to unbound concentrations at the sites of disposition and action; (2) kinetics of the compound in the in vitro system have to be more thoroughly characterized; (3) local models encompassing the in vitro system should be developed and used; and (4) PBPK/PD models should incorporate these local models.

Monitoring of cytochrome P-450 1A activity by determination of the urinary pattern of caffeine metabolites in Wistar and hyperbilirubinemic Gunn rats.

Various studies suggest that induction of cytochrome P-450 1A (CYP1A) might be a valuable therapeutic modality for reducing the hyperbilirubinemia of infants with Crigler-Najjar syndrome type I (CNS-I), a severe form of congenital jaundice. To evaluate inducers of CYP1A as possible tools in the treatment of hyperbilirubinemia, a novel assay was established, based on the analysis of the urinary pattern of caffeine metabolites in rats. Wistar rats received [1-Me-(14)C]-caffeine (10 mg/kg i.p.), before and 48h after administration of the potent CYP1A inducer 5,6-benzoflavone (BNF) (80 mg/kg, i.p.). A substantial increase in the fractions of the terminal caffeine metabolites 1-methyluric acid (1-U), 1-methylxanthine (1-X), and a concomitant decrease in the caffeine demethylation product 1,7-dimethylxanthine (1,7-X) was observed after application of BNF. The ratio of the caffeine metabolites (1-U+1-X)/1,7-X may serve as an index of CYP1A activity in rats in vivo. Hyperbilirubinemic, homozygous (jj) Gunn rats are an accepted model for human CNS-I. In male jj Gunn rats treated with BNF or with indole-3-carbinol (I3C, 80 mg/kg, oral gavage), the inducing effect of BNF and 13C on CYP1A activity was confirmed by the urinary pattern of caffeine metabolites, and was parallelled by a decrease in plasma bilirubin levels. These data demonstrate the usefulness of the established caffeine assay for the evaluation of inducers of CYP1A as tools for reducing hyperbilirubinemia and further confirm the potential value of I3C in the treatment of CNS-I.

Extrahepatic drug metabolizing enzymes

Drug metabolizing enzymes are known to be present and active in most extrahepatic tissues. In this review, evidence is presented that expression and activity of several extrahepatic drug‐metabolizing enzymes are regulated in unique ways, which may be associated with tissue functions and activities. In several instances evidence is offered that hormonal effects may be regulated through tissue specific distribution and/or responses of transcription factors.

Bilirubin Increases the Expression of Glucose Transporter-1 and the Rate of Glucose Uptake in Vascular Endothelial Cells

BACKGROUND AND OBJECTIVES The close contact between the endothelial cell monolayer in blood vessels and blood plasma allows free diffusion of the hydrophobic unconjugated bilirubin (BR) into these cells. BR can exert both anti- and pro-oxidative effects in various types of cells in a dose-dependent manner. High glucose levels downregulate the expression of the glucose transporter-1 (GLUT-1) and the rate of glucose uptake in vascular endothelial cell (VEC). Pro-oxidants, on the other hand, up-regulate this system in VEC. We aimed to investigate potential effects of BR on the glucose transport system in VEC. METHODS Primary cultures of bovine aortic endothelial cells were exposed to BR, and the rate of hexose transport, GLUT-1 expression and plasma membrane localization were determined. RESULTS BR induced oxidative stress in VEC, and significantly augmented the rate of glucose transport and GLUT-1 expression and plasma membrane localization in these cells. BR also reversed the high glucose-induced downregulation of the glucose transport system in VEC. CONCLUSION The pro-oxidative properties of BR are responsible for its effects on the regulation of glucose transport in vascular endothelium. Pathological concentrations of BR in the vascular compartment (jaundice) may influence the cellular handling of glucose in diabetes.

Endothelial Cells Derived from the Blood-Brain Barrier and Islets of Langerhans Differ in their Response to the Effects of Bilirubin on Oxidative Stress Under Hyperglycemic Conditions.

Unconjugated bilirubin (UCB) is a neurotoxic degradation product of heme. Its toxic effects include induction of apoptosis, and ultimately neuronal cell death. However, at low concentrations, UCB is a potent antioxidant that may protect cells and tissues against oxidative stress by neutralizing toxic metabolites such as reactive oxygen species (ROS). High glucose levels (hyperglycemia) generate reactive metabolites. Endothelial cell dysfunction, an early vascular complication in diabetes, has been associated with hyperglycemia-induced oxidative stress. Both glucose and UCB are substrates for transport proteins in microvascular endothelial cells of the blood-brain barrier (BBB). In the current study we show that UCB (1–40 μM) induces apoptosis and reduces survival of bEnd3 cells, a mouse brain endothelial cell line which serves as an in vitro model of the BBB. These deleterious effects of UCB were enhanced in the presence of high glucose (25 mM) levels. Interestingly, the bEnd3 cells exhibited an increased sensitivity to the apoptotic effects of UCB when compared to the MS1 microcapillary endothelial cell line. MS1 cells originate from murine pancreatic islets of Langerhans, and are devoid of the barrier characteristics of BBB-derived endothelial cells. ROS production was increased in both bEnd3 and MS1 cells exposed to high glucose, as compared with cells exposed to normal (5.5 mM) glucose levels. While UCB (0.1–40 μM) did not alter ROS production in cells exposed to normal glucose, relatively low (“physiological”) UCB concentrations (0.1–5 μM) attenuated ROS generation in both cell lines exposed to high glucose levels. Most strikingly, higher UCB concentrations (20–40 μM) increased ROS generation in bEnd3 cells exposed to high glucose, but not in similarly treated MS1 cells. These results may be of critical importance for understanding the vulnerability of the BBB endothelium upon exposure to increasing UCB levels under hyperglycemic conditions.

The Role of Bile Pigments in Health and Disease: Effects on Cell Signaling, Cytotoxicity, and Cytoprotection

Degradation of heme involves its conversion to biliverdin by heme oxygenase followed by reduction of biliverdin to bilirubin by biliverdin reductase. There is ample evidence for the role of heme oxygenase in protecting cells from the toxic effects of heme, as well as for the pleiotropic functions of biliverdin reductase in cell signaling and regulation of gene expression. This enzyme plays a major role in glucose uptake and the stress response. Bilirubin has been shown to behave as a “double-edged sword.” It can exert either cytotoxic or cytoprotective effects, depending on the blood and/or tissue concentration of its free fraction, the nature of the target cell or tissue, and the cellular redox state. The central nervous system is particularly sensitive to the neurotoxic effects of bilirubin. Its antioxidant effect is the basis for the proposed cardioprotective effect of bilirubin in humans with moderate hyperbilirubinemia, as is the case in subjects with the Gilbert syndrome. This Research Topic forum is intended to serve as a platform for updating information and presenting advances in basic and clinical research in the above and related subjects. The topic is discussed by leading experts in the field of bile pigments, and presented in

Effects of pharmaceuticals and other active chemicals at biological targets: mechanisms, interactions, and integration into PB-PK/PD models.

Biologically active molecules, that is pharmaceuticals and other chemical substances, exert their therapeutic and/or toxic effects by complex interactions with their biological targets/active sites. This review discusses the factors and processes governing the kinetics and effects of active molecules at their cellular targets, the chain of events leading to clinical effects, and the crosstalk between regulatory pathways controlling these processes. Special attention is given to the discussion of effects of a single drug or other chemical on multiple targets, to the interaction of multiple ligands with a single target/receptor and the effects of single ligand–target complexes on multiple signal transduction pathways, and to the control of physiological functions, such as regulation of blood glucose levels, by numerous primary mechanisms acting on different cellular targets. Physiologically based-pharmacokinetic/pharmacodynamic (PB-PK/PD) models are of great value for the design of active principles by the pharmaceutical industry and for the optimization and individualization of patient therapy. Experimental results from in vitro and in vivo studies can be used for building such models. On the other hand, properly designed models and simulation can contribute to a better design of experiments. Much of what is presented in this article applies equally well to drugs and other chemicals. Unless specified otherwise, reference to drugs applies also to other chemicals. This review is based on an expert meeting organized by COST Action B25 held in Eilat, Israel, on 14 – 15 February 2008. The authors have prepared this article to reflect the presentations and discussions at that meeting.

Effects of prolonged lithium treatment on the water consumption and lithium content of rats.

Abstract Injection of LiC1 to rats (3.0 mmole/kg/day) caused a marked increase in water intake. The polydipsic effect was maximal after 8 – 10 days of treatment and then gradually declined. This reduction in polydipsia was accompanied by a decrease in Li + content of plasma, red blood cells, liver, spleen, kidney and brain. Furthermore, the ratio of tissue to plasma lithium concentration (lithium index) declined in all tissues examined. The decrease in polydipsia was abolished by increasing the dose of LiC1 by 50%. These results suggest that tolerance to the polydipsic effect of lithium develops in the rat during chronic treatment with LiC1 as a result of a reduction in the cells capacity to accumulate lithium.