Crystal structure of rat biliverdin reductase

Metabolic Functions of Biliverdin IXβ Reductase in Redox-Regulated Hematopoietic Cell Fate

Cytoprotective heme oxygenases derivatize heme to generate carbon monoxide, ferrous iron, and isomeric biliverdins, followed by rapid NAD(P)H-dependent biliverdin reduction to the antioxidant bilirubin. Recent studies have implicated biliverdin IXβ reductase (BLVRB) in a redox-regulated mechanism of hematopoietic lineage fate restricted to megakaryocyte and erythroid development, a function distinct and non-overlapping from the BLVRA (biliverdin IXα reductase) homologue. In this review, we focus on recent progress in BLVRB biochemistry and genetics, highlighting human, murine, and cell-based studies that position BLVRB-regulated redox function (or ROS accumulation) as a developmentally tuned trigger that governs megakaryocyte/erythroid lineage fate arising from hematopoietic stem cells. BLVRB crystallographic and thermodynamic studies have elucidated critical determinants of substrate utilization, redox coupling and cytoprotection, and have established that inhibitors and substrates bind within the single-Rossmann fold. These advances provide unique opportunities for the development of BLVRB-selective redox inhibitors as novel cellular targets that retain potential for therapeutic applicability in hematopoietic (and other) disorders.

Cellular Stress Response (Hormesis) in Response to Bioactive Nutraceuticals with Relevance to Alzheimer Disease

Significance: Alzheimer's disease (AD) is the most common form of dementia associated with aging. As the large Baby Boomer population ages, risk of developing AD increases significantly, and this portion of the population will increase significantly over the next several decades. Recent Advances: Research suggests that a delay in the age of onset by 5 years can dramatically decrease both the incidence and cost of AD. In this review, the role of nuclear factor erythroid 2-related factor 2 (Nrf2) in AD is examined in the context of heme oxygenase-1 (HO-1) and biliverdin reductase-A (BVR-A) and the beneficial potential of selected bioactive nutraceuticals. Critical Issues: Nrf2, a transcription factor that binds to enhancer sequences in antioxidant response elements (ARE) of DNA, is significantly decreased in AD brain. Downstream targets of Nrf2 include, among other proteins, HO-1. BVR-A is activated when biliverdin is produced. Both HO-1 and BVR-A also are oxidatively or nitrosatively modified in AD brain and in its earlier stage, amnestic mild cognitive impairment (MCI), contributing to the oxidative stress, altered insulin signaling, and cellular damage observed in the pathogenesis and progression of AD. Bioactive nutraceuticals exhibit anti-inflammatory, antioxidant, and neuroprotective properties and are potential topics of future clinical research. Specifically, ferulic acid ethyl ester, sulforaphane, epigallocatechin-3-gallate, and resveratrol target Nrf2 and have shown potential to delay the progression of AD in animal models and in some studies involving MCI patients. Future Directions: Understanding the regulation of Nrf2 and its downstream targets can potentially elucidate therapeutic options for delaying the progression of AD. Antioxid. Redox Signal. 38, 643-669.

Biliverdin reductase bridges focal adhesion kinase to Src to modulate synaptic signaling

Synapses connect discrete neurons into vast networks that send, receive, and encode diverse forms of information. Synaptic function and plasticity, the neuronal process of adapting to diverse and variable inputs, depend on the dynamic nature of synaptic molecular components, which is mediated in part by cell adhesion signaling pathways. Here, we found that the enzyme biliverdin reductase (BVR) physically links together key focal adhesion signaling molecules at the synapse. BVR-null (BVR^-/-) mice exhibited substantial deficits in learning and memory on neurocognitive tests, and hippocampal slices in which BVR was postsynaptically depleted showed deficits in electrophysiological responses to stimuli. RNA sequencing, biochemistry, and pathway analyses suggested that these deficits were mediated through the loss of focal adhesion signaling at both the transcriptional and biochemical level in the hippocampus. Independently of its catalytic function, BVR acted as a bridge between the primary focal adhesion signaling kinases FAK and Pyk2 and the effector kinase Src. Without BVR, FAK and Pyk2 did not bind to and stimulate Src, which then did not phosphorylate the N-methyl-d-aspartate (NMDA) receptor, a critical posttranslational modification for synaptic plasticity. Src itself is a molecular hub on which many signaling pathways converge to stimulate NMDAR-mediated neurotransmission, thus positioning BVR at a prominent intersection of synaptic signaling.

HutW from Vibrio cholerae Is an Anaerobic Heme-Degrading Enzyme with Unique Functional Properties

Increasing antibiotic resistance, and a growing recognition of the importance of the human microbiome, demand that new therapeutic targets be identified. Characterization of metabolic pathways that are unique to enteric pathogens represents a promising approach. Iron is often the rate-limiting factor for growth, and Vibrio cholerae, the causative agent of cholera, has been shown to contain numerous genes that function in the acquisition of iron from the environment. Included in this arsenal of genes are operons dedicated to obtaining iron from heme and heme-containing proteins. Given the persistence of cholera, an important outstanding question is whether V. cholerae is capable of anaerobic heme degradation as was recently reported for enterohemorrhagic Escherichia coli O157:H7. In this work, we demonstrate that HutW from V. cholerae is a radical S-adenosylmethionine methyl transferase involved in the anaerobic opening of the porphyrin ring of heme. However, in contrast to the enzyme ChuW, found in enterohemorrhagic E. coli O157:H7, there are notable differences in the mechanism and products of the HutW reaction. Of particular interest are data that demonstrate HutW will catalyze ring opening as well as tetrapyrrole reduction and can utilize reduced nicotinamide adenine dinucleotide phosphate as an electron source. The biochemical and biophysical properties of HutW are presented, and the evolutionary implications are discussed.

Subarachnoid Hemorrhage Increases Level of Heme Oxygenase-1 and Biliverdin Reductase in the Choroid Plexus

Subarachnoid hemorrhage is a specific, life-threatening form of hemorrhagic stroke linked to high morbidity and mortality. It has been found that the choroid plexus of the brain ventricles forming the blood-cerebrospinal fluid barrier plays an important role in subarachnoid hemorrhage pathophysiology. Heme oxygenase-1 and biliverdin reductase are two of the key enzymes of the hemoglobin degradation cascade. Therefore, the aim of present study was to investigate changes in protein levels of heme oxygenase-1 and biliverdin reductase in the rat choroid plexus after experimental subarachnoid hemorrhage induced by injection of non-heparinized autologous blood to the cisterna magna. Artificial cerebrospinal fluid of the same volume as autologous blood was injected to mimic increased intracranial pressure in control rats. Immunohistochemical and Western blot analyses were used to monitor changes in the of heme oxygenase-1 and biliverdin reductase levels in the rat choroid plexus after induction of subarachnoid hemorrhage or artificial cerebrospinal fluid application for 1, 3, and 7 days. We found increased levels of heme oxygenase-1 and biliverdin reductase protein in the choroid plexus over the entire period following subarachnoid hemorrhage induction. The level of heme oxygenase-1 was the highest early (1 and 3 days) after subarachnoid hemorrhage indicating its importance in hemoglobin degradation. Increased levels of heme oxygenase-1 were also observed in the choroid plexus epithelial cells at all time points after application of artificial cerebrospinal fluid. Biliverdin reductase protein was detected mainly in the choroid plexus epithelial cells, with levels gradually increasing during subarachnoid hemorrhage. Our results suggest that heme oxygenase-1 and biliverdin reductase are involved not only in hemoglobin degradation but probably also in protecting choroid plexus epithelial cells and the blood-cerebrospinal fluid barrier from the negative effects of subarachnoid hemorrhage.

Recent Advances in the Understanding of the Reaction Chemistries of the Heme Catabolizing Enzymes HO and BVR Based on High Resolution Protein Structures

In mammals, catabolism of the heme group is indispensable for life. Heme is first cleaved by the enzyme Heme Oxygenase (HO) to the linear tetrapyrrole Biliverdin IXα (BV), and BV is then converted into bilirubin by Biliverdin Reductase (BVR). HO utilizes three Oxygen molecules (O2) and seven electrons supplied by NADPH-cytochrome P450 oxidoreductase (CPR) to open the heme ring and BVR reduces BV through the use of NAD(P)H. Structural studies of HOs, including substrate-bound, reaction intermediate-bound, and several specific inhibitor-bound forms, reveal details explaining substrate binding to HO and mechanisms underlying-specific HO reaction progression. Cryo-trapped structures and a time-resolved spectroscopic study examining photolysis of the bond between the distal ligand and heme iron demonstrate how CO, produced during the HO reaction, dissociates from the reaction site with a corresponding conformational change in HO. The complex structure containing HO and CPR provides details of how electrons are transferred to the heme-HO complex. Although the tertiary structure of BVR and its complex with NAD+ was determined more than 10 years ago, the catalytic residues and the reaction mechanism of BVR remain unknown. A recent crystallographic study examining cyanobacterial BVR in complex with NADP+ and substrate BV provided some clarification regarding these issues. Two BV molecules are bound to BVR in a stacked manner, and one BV may assist in the reductive catalysis of the other BV. In this review, recent advances illustrated by biochemical, spectroscopic, and crystallographic studies detailing the chemistry underlying the molecular mechanism of HO and BVR reactions are presented.

On the structure and function of Escherichia coli YjhC: An oxidoreductase involved in bacterial sialic acid metabolism

Human pathogenic and commensal bacteria have evolved the ability to scavenge host-derived sialic acids and subsequently degrade them as a source of nutrition. Expression of the Escherichia coli yjhBC operon is controlled by the repressor protein nanR, which regulates the core machinery responsible for the import and catabolic processing of sialic acid. The role of the yjhBC encoded proteins is not known-here, we demonstrate that the enzyme YjhC is an oxidoreductase/dehydrogenase involved in bacterial sialic acid degradation. First, we demonstrate in vivo using knockout experiments that YjhC is broadly involved in carbohydrate metabolism, including that of N-acetyl-d-glucosamine, N-acetyl-d-galactosamine and N-acetylneuraminic acid. Differential scanning fluorimetry demonstrates that YjhC binds N-acetylneuraminic acid and its lactone variant, along with NAD(H), which is consistent with its role as an oxidoreductase. Next, we solved the crystal structure of YjhC in complex with the NAD(H) cofactor to 1.35 Å resolution. The protein fold belongs to the Gfo/Idh/MocA protein family. The dimeric assembly observed in the crystal form is confirmed through solution studies. Ensemble refinement reveals a flexible loop region that may play a key role during catalysis, providing essential contacts to stabilize the substrate-a unique feature to YjhC among closely related structures. Guided by the structure, in silico docking experiments support the binding of sialic acid and several common derivatives in the binding pocket, which has an overall positive charge distribution. Taken together, our results verify the role of YjhC as a bona fide oxidoreductase/dehydrogenase and provide the first evidence to support its involvement in sialic acid metabolism.

Bilirubin Links Heme Metabolism to Neuroprotection by Scavenging Superoxide

Bilirubin is one of the most frequently measured metabolites in medicine, yet its physiologic roles remain unclear. Bilirubin can act as an antioxidant in vitro, but whether its redox activity is physiologically relevant is unclear because many other antioxidants are far more abundant in vivo. Here, we report that depleting endogenous bilirubin renders mice hypersensitive to oxidative stress. We find that mice lacking bilirubin are particularly vulnerable to superoxide (O₂^⋅-) over other tested reactive oxidants and electrophiles. Whereas major antioxidants such as glutathione and cysteine exhibit little to no reactivity toward O₂^⋅-, bilirubin readily scavenges O₂^⋅-. We find that bilirubin's redox activity is particularly important in the brain, where it prevents excitotoxicity and neuronal death by scavenging O₂^⋅- during NMDA neurotransmission. Bilirubin's unique redox activity toward O₂^⋅- may underlie a prominent physiologic role despite being significantly less abundant than other endogenous and exogenous antioxidants.

Bilin-metabolizing enzymes: site-specific reductions catalyzed by two different type of enzymes

In mammals, the green heme metabolite biliverdin is converted to a yellow anti-oxidant by NAD(P)H-dependent biliverdin reductase (BVR), whereas in O₂-dependent photosynthetic organisms it is converted to photosynthetic or light-sensing pigments by ferredoxin-dependent bilin reductases (FDBRs). In NADP⁺-bound and biliverdin-bound BVR-A, two biliverdins are stacked at the binding cleft; one is positioned to accept hydride from NADPH, and the other appears to donate a proton to the first biliverdin through a neighboring arginine residue. During the FDBR-catalyzed reaction, electrons and protons are supplied to bilins from ferredoxin and from FDBRs and waters bound within FDBRs, respectively. Thus, the protonation sites of bilin and catalytic residues are important for the analysis of site-specific reduction. The neutron structure of FDBR sheds light on this issue.

In silico and crystallographic studies identify key structural features of biliverdin IXβ reductase inhibitors having nanomolar potency

Heme cytotoxicity is minimized by a two-step catabolic reaction that generates biliverdin (BV) and bilirubin (BR) tetrapyrroles. The second step is regulated by two non-redundant biliverdin reductases (IXα (BLVRA) and IXβ (BLVRB)), which retain isomeric specificity and NAD(P)H-dependent redox coupling linked to BR's antioxidant function. Defective BLVRB enzymatic activity with antioxidant mishandling has been implicated in metabolic consequences of hematopoietic lineage fate and enhanced platelet counts in humans. We now outline an integrated platform of in silico and crystallographic studies for the identification of an initial class of compounds inhibiting BLVRB with potencies in the nanomolar range. We found that the most potent BLVRB inhibitors contain a tricyclic hydrocarbon core structure similar to the isoalloxazine ring of flavin mononucleotide and that both xanthene- and acridine-based compounds inhibit BLVRB's flavin and dichlorophenolindophenol (DCPIP) reductase functions. Crystallographic studies of ternary complexes with BLVRB-NADP⁺-xanthene-based compounds confirmed inhibitor binding adjacent to the cofactor nicotinamide and interactions with the Ser-111 side chain. This residue previously has been identified as critical for maintaining the enzymatic active site and cellular reductase functions in hematopoietic cells. Both acridine- and xanthene-based compounds caused selective and concentration-dependent loss of redox coupling in BLVRB-overexpressing promyelocytic HL-60 cells. These results provide promising chemical scaffolds for the development of enhanced BLVRB inhibitors and identify chemical probes to better dissect the role of biliverdins, alternative substrates, and BLVRB function in physiologically relevant cellular contexts.

Absence of the biliverdin reductase-a gene is associated with increased endogenous oxidative stress

Bilirubin, a byproduct of heme catabolism, has been shown to be an effective lipid-soluble antioxidant in vitro. Bilirubin is able to inhibit free radical chain reactions and protects against oxidant-induced damage in vitro and ex vivo. However, direct evidence for bilirubin's antioxidant effects in vivo remains limited. As bilirubin is formed from biliverdin by biliverdin reductase, we generated global biliverdin reductase-a gene knockout (Bvra^-/-) mice to assess the contribution of bilirubin as an endogenous antioxidant. Bvra^-/- mice appear normal and are born at the expected Mendelian ratio from Bvra^+/- x Bvra^+/- matings. Compared with corresponding littermate Bvra^+/+ and Bvra^+/- animals, Bvra^-/- mice have green gall bladders and their plasma concentrations of biliverdin and bilirubin are approximately 25-fold higher and 100-fold lower, respectively. Naïve Bvra^-/- and Bvra^+/+ mice have comparable plasma lipid profiles and low-molecular weight antioxidants, i.e., ascorbic acid, α-tocopherol and ubiquinol-9. Compared with wild-type littermates, however, plasma from Bvra^-/- mice contains higher concentrations of cholesteryl ester hydroperoxides (CE-OOH), and their peroxiredoxin 2 (Prx2) in erythrocytes is more oxidized as assessed by the extent of Prx2 dimerization. These data show that Bvra^-/- mice experience higher oxidative stress in blood, implying that plasma bilirubin attenuates endogenous oxidative stress.

Mechanisms of redox interactions of bilirubin with copper and the effects of penicillamine

Toxic effects of unconjugated bilirubin (BR) in neonatal hyperbilirubinemia have been related to redox and/or coordinate interactions with Cu²⁺. However, the development and mechanisms of such interactions at physiological pH have not been resolved. This study shows that BR reduces Cu²⁺ to Cu¹⁺ in 1:1 stoichiometry. Apparently, BR undergoes degradation, i.e. BR and Cu²⁺ do not form stable complexes. The binding of Cu²⁺ to inorganic phosphates, liposomal phosphate groups, or to chelating drug penicillamine, impedes redox interactions with BR. Cu¹⁺ undergoes spontaneous oxidation by O₂ resulting in hydrogen peroxide accumulation and hydroxyl radical production. In relation to this, copper and BR induced synergistic oxidative/damaging effects on erythrocytes membrane, which were alleviated by penicillamine. The production of reactive oxygen species by BR and copper represents a plausible cause of BR toxic effects and cell damage in hyperbilirubinemia. Further examination of therapeutic potentials of copper chelators in the treatment of severe neonatal hyperbilirubinemia is needed.

A substrate-bound structure of cyanobacterial biliverdin reductase identifies stacked substrates as critical for activity

Biliverdin reductase catalyses the last step in haem degradation and produces the major lipophilic antioxidant bilirubin via reduction of biliverdin, using NAD(P)H as a cofactor. Despite the importance of biliverdin reductase in maintaining the redox balance, the molecular details of the reaction it catalyses remain unknown. Here we present the crystal structure of biliverdin reductase in complex with biliverdin and NADP+. Unexpectedly, two biliverdin molecules, which we designated the proximal and distal biliverdins, bind with stacked geometry in the active site. The nicotinamide ring of the NADP+ is located close to the reaction site on the proximal biliverdin, supporting that the hydride directly attacks this position of the proximal biliverdin. The results of mutagenesis studies suggest that a conserved Arg185 is essential for the catalysis. The distal biliverdin probably acts as a conduit to deliver the proton from Arg185 to the proximal biliverdin, thus yielding bilirubin.

Enzymatic Activity and Thermodynamic Stability of Biliverdin IXβ Reductase Are Maintained by an Active Site Serine

Biliverdin reductase IXβ (BLVRB) is a crucial enzyme in heme metabolism. Recent studies in humans have identified a loss-of-function mutation (Ser111Leu) that unmasks a fundamentally important role in hematopoiesis. We have undertaken experimental and thermodynamic modeling studies to provide further insight into the role of the cofactor in substrate accessibility and protein folding properties regulating BLVRB catalytic mechanisms. Site-directed mutagenesis with molecular dynamic (MD) simulations establish the critical role of NAD(P)H-dependent conformational changes on substrate accessibility by forming the "hydrophobic pocket", along with identification of a single key residue (Arg35) modulating NADPH/NADH selectivity. Loop80 and Loop120 block the hydrophobic substrate binding pocket in apo BLVRB (open), whereas movement of these structures after cofactor binding results in the "closed" (catalytically active) conformation. Both enzymatic activity and thermodynamic stability are affected by mutation(s) involving Ser111, which is located in the core of the BLVRB active site. This work 1) elucidates the crucial role of Ser111 in enzymatic catalysis and thermodynamic stability by active site hydrogen bond network; 2) defines a dynamic model for apo BLVRB extending beyond the crystal structure of the binary BLVRB/NADP⁺ complex; 3) provides a structural basis for the "encounter" and "equilibrium" states of the binary complex, which are regulated by NAD(P)H.

Interaction of human biliverdin reductase with Akt/protein kinase B and phosphatidylinositol-dependent kinase 1 regulates glycogen synthase kinase 3 activity: a novel mechanism of Akt activation

Biliverdin reductase A (BVR) and Akt isozymes have overlapping pleiotropic functions in the insulin/PI3K/MAPK pathway. Human BVR (hBVR) also reduces the hemeoxygenase activity product biliverdin to bilirubin and is directly activated by insulin receptor kinase (IRK). Akt isoenzymes (Akt1-3) are downstream of IRK and are activated by phosphatidylinositol-dependent kinase 1 (PDK1) phosphorylating T(308) before S(473) autophosphorylation. Akt (RxRxxSF) and PDK1 (RFxFPxFS) binding motifs are present in hBVR. Phosphorylation of glycogen synthase kinase 3 (GSK3) isoforms α/β by Akts inhibits their activity; nonphosphorylated GSK3β inhibits activation of various genes. We examined the role of hBVR in PDK1/Akt1/GSK3 signaling and Akt1 in hBVR phosphorylation. hBVR activates phosphorylation of Akt1 at S(473) independent of hBVR's kinase competency. hBVR and Akt1 coimmunoprecipitated, and in-cell Förster resonance energy transfer (FRET) and glutathione S-transferase pulldown analyses identified Akt1 pleckstrin homology domain as the interactive domain. hBVR activates phosphorylation of Akt1 at S(473) independent of hBVR's kinase competency. Site-directed mutagenesis, mass spectrometry, and kinetic analyses identified S(230) in hBVR (225)RNRYLSF sequence as the Akt1 target. Underlined amino acids are the essential residues of the signaling motifs. In cells, hBVR-activated Akt1 increased both GSK3α/β and forkhead box of the O class transcription class 3 (FoxO3) phosphorylation and inhibited total GSK3 activity; depletion of hBVR released inhibition and stimulated glucose uptake. Immunoprecipitation analysis showed that PDK1 and hBVR interact through hBVR's PDK1 binding (161)RFGFPAFS motif and formation of the PDK1/hBVR/Akt1 complex. sihBVR blocked complex formation. Findings identify hBVR as a previously unknown coactivator of Akt1 and as a key mediator of Akt1/GSK3 pathway, as well as define a key role for hBVR in Akt1 activation by PDK1.-Miralem, T., Lerner-Marmarosh, N., Gibbs, P. E. M., Jenkins, J. L., Heimiller, C., Maines, M. D. Interaction of human biliverdin reductase with Akt/protein kinase B and phosphatidylinositol-dependent kinase 1 regulates glycogen synthase kinase 3 activity: a novel mechanism of Akt activation.

Transition metal complexes of phyllobilins - a new realm of bioinorganic chemistry

Phyllobilins may function as natural ligand molecules for biologically important transition metal ions, giving complexes with remarkable chemical and photophysical properties.

Natural cyclic tetrapyrroles feature outstanding capacity for binding transition metal ions, furnishing Nature with the important metallo-porphyrinoid ‘Pigments of Life’, such as heme, chlorophyll (Chl) and vitamin B₁₂. In contrast, linear tetrapyrroles are not generally ascribed a biologically relevant ability for metal-binding. Indeed, when heme or Chl are degraded to natural linear tetrapyrroles, their central Fe- or Mg-ions are set free. Some linear tetrapyrroles are, however, effective multi-dentate ligands and their transition metal complexes have remarkable chemical properties. The focus of this short review is centred on such complexes of the linear tetrapyrroles derived from natural Chl-breakdown, called phyllobilins. These natural bilin-type compounds are massively produced in Nature and in highly visible processes. Colourless non-fluorescing Chl-catabolites (NCCs) and the related dioxobilin-type NCCs, which typically accumulate in leaves as ‘final’ products of Chl-breakdown, show low affinity for transition metal-ions. However, NCCs are oxidized in leaves to give less saturated coloured phyllobilins, such as yellow or pink Chl-catabolites (YCCs or PiCCs). YCCs and PiCCs are ligands for various biologically relevant transition metal-ions, such as Zn(ii)-, Ni(ii)- and Cu(ii)-ions. Complexation of Zn(ii)- and Cd(ii)-ions by the effectively tridentate PiCC produces blue metal-complexes that exhibit an intense red fluorescence, thus providing a tool for the sensitive detection of these metal ions. Outlined here are fundamental aspects of structure and metal coordination of phyllobilins, including a comparison with the corresponding properties of bilins. This knowledge may be valuable in the quest of finding possible biological roles of the phyllobilins. Thanks to their capacity for metal-ion coordination, phyllobilins could, e.g., be involved in heavy-metal transport and detoxification, and some of their metal-complexes could act as sensitizers for singlet oxygen or as plant toxins against pathogens.

Structural and functional features of the NAD(P) dependent Gfo/Idh/MocA protein family oxidoreductases

The Gfo/Idh/MocA protein family contains a number of different proteins, which almost exclusively consist of NAD(P)-dependent oxidoreductases that have a diverse set of substrates, typically pyranoses. In this study, to clarify common structural features that would contribute to their function, the available crystal structures of the members of this family have been analyzed. Despite a very low sequence identity, the central features of the three-dimensional structures of the proteins are surprisingly similar. The members of the protein family have a two-domain structure consisting of a N-terminal nucleotide-binding domain and a C-terminal α/β-domain. The C-terminal domain contributes to the substrate binding and catalysis, and contains a βα-motif with a central α-helix carrying common essential amino acid residues. The β-sheet of the α/β-domain contributes to the oligomerization in most of the proteins in the family.

Nanoparticle Delivered Human Biliverdin Reductase-Based Peptide Increases Glucose Uptake by Activating IRK/Akt/GSK3 Axis: The Peptide Is Effective in the Cell and Wild-Type and Diabetic Ob/Ob Mice

Insulin's stimulation of glucose uptake by binding to the IRK extracellular domain is compromised in diabetes. We have recently described an unprecedented approach to stimulating glucose uptake. KYCCSRK (P2) peptide, corresponding to the C-terminal segment of hBVR, was effective in binding to and inducing conformational change in the IRK intracellular kinase domain. Although myristoylated P2, made of L-amino acids, was effective in cell culture, its use for animal studies was unsuitable. We developed a peptidase-resistant formulation of the peptide that was efficient in both mice and cell culture systems. The peptide was constructed of D-amino acids, in reverse order, and blocked at both termini. Delivery of the encapsulated peptide to HepG2 and HSKM cells was confirmed by its prolonged effect on stimulation of glucose uptake (>6 h). The peptide improved glucose clearance in both wild-type and Ob/Ob mice; it lowered blood glucose levels and suppressed glucose-stimulated insulin secretion. IRK activity was stimulated in the liver of treated mice and in cultured cells. The peptide potentiated function of IRK's downstream effector, Akt-GSK3-(α, β) axis. Thus, P2-based approach can be used for improving glucose uptake by cells. Also, it allows for screening peptides in vitro and in animal models for treatment of diabetes.

Biliverdin reductase: a target for cancer therapy?

Biliverdin reductase (BVR) is a multifunctional protein that is the primary source of the potent antioxidant, bilirubin. BVR regulates activities/functions in the insulin/IGF-1/IRK/PI3K/MAPK pathways. Activation of certain kinases in these pathways is/are hallmark(s) of cancerous cells. The protein is a scaffold/bridge and intracellular transporter of kinases that regulate growth and proliferation of cells, including PKCs, ERK and Akt, and their targets including NF-κB, Elk1, HO-1, and iNOS. The scaffold and transport functions enable activated BVR to relocate from the cytosol to the nucleus or to the plasma membrane, depending on the activating stimulus. This enables the reductase to function in diverse signaling pathways. And, its expression at the transcript and protein levels are increased in human tumors and the infiltrating T-cells, monocytes and circulating lymphocytes, as well as the circulating and infiltrating macrophages. These functions suggest that the cytoprotective role of BVR may be permissive for cancer/tumor growth. In this review, we summarize the recent developments that define the pro-growth activities of BVR, particularly with respect to its input into the MAPK signaling pathway and present evidence that BVR-based peptides inhibit activation of protein kinases, including MEK, PKCδ, and ERK as well as downstream targets including Elk1 and iNOS, and thus offers a credible novel approach to reduce cancer cell proliferation.

Two structures of a thiazolinyl imine reductase from Yersinia enterocolitica provide insight into catalysis and binding to the nonribosomal peptide synthetase module of HMWP1

The thiazolinyl imine reductase from Yersinia enterocolitica (Irp3) catalyzes the NADPH-dependent reduction of a thiazoline ring in an intermediate for the formation of the siderophore yersiniabactin. Two structures of Irp3 were determined in the apo (1.85 Å) and NADP(+)-bound (2.31 Å) forms. Irp3 is structurally homologous to sugar oxidoreductases such as glucose-fructose oxidoreductase and 1,5-anhydro-d-fructose reductase, as well as to biliverdin reductase. A homology model of the thiazolinyl imine reductase from Pseudomonas aeruginosa (PchG) was generated. Extensive loop insertions are observed in the C-terminal domain that are unique to Irp3 and PchG and not found in the structural homologues that recognize small molecular substrates. These loops are hypothesized to be important for binding of the nonribosomal peptide synthetase modules (found in HMWP1 and PchF, respectively) to which the substrate of the reductase is covalently attached. A catalytic mechanism for the donation of a proton from a general acid (either histidine 101 or tyrosine 128) and the donation of a hydride from C4 of nicotinamide of the NADPH cofactor is proposed for reduction of the carbon-nitrogen double bond of the thiazoline.

Role of propionates in substrate binding to heme oxygenase from Neisseria meningitidis: a nuclear magnetic resonance study

Heme oxygenase (HO) cleaves hemin into biliverdin, iron, and CO. For mammalian HOs, both native hemin propionates are required for substrate binding and activity. The HO from the pathogenic bacterium Neisseria meningitidis (NmHO) possesses a crystallographically undetected C-terminal fragment that by solution (1)H nuclear magnetic resonance (NMR) is found to fold and interact with the active site. One of the substrate propionates has been proposed to form a salt bridge to the C-terminus rather than to the conventional buried cationic side chain in other HOs. Moreover, the C-terminal dipeptide Arg208His209 cleaves spontaneously over ~24 h at a rate dependent on substituent size. Two-dimensional (1)H NMR of NmHO azide complexes with hemins with selectively deleted or rearranged propionates shows that all bind to NmHO with a structurally conserved active site as reflected in optical spectra and NMR nuclear Overhauser effect spectroscopy cross-peak and hyperfine shift patterns. In contrast to mammalian HOs, NmHO requires only a single propionate interacting with the buried terminus of Lys16 to exhibit full activity and tolerates the existence of a propionate at the exposed 8-position. The structure of the C-terminus is qualitatively retained upon deletion of the 7-propionate, but a dramatic change in the 7-propionate carboxylate (13)C chemical shift upon C-terminal cleavage confirms its role in the interaction with the C-terminus. The stronger hydrophobic contacts between pyrroles A and B with NmHO contribute more substantially to the substrate binding free energy than in mammalian HOs, "liberating" one propionate to stabilize the C-terminus. The functional implications of the C-terminus in product release are discussed.

Molecular modeling to provide insight into the substrate binding and catalytic mechanism of human biliverdin-IXα reductase

Human biliverdin-IXα reductase (hBVR-A) catalyzes the conversion of biliverdin-IXα to bilirubin-IXα in the last step of heme degradation and is a key enzyme in regulating a wide range of cellular responses. Though the X-ray structure of hBVR-A is available including cofactor, a crystal structure with a bound substrate would be even more useful as a starting point for protein-structure-based inhibitor design, but none have been reported. The present study employed induced fit docking (IFD) to study the substrate binding modes to hBVR-A of biliverdin-IXα and four analogues. The proposed substrate binding modes were examined further by performing molecular dynamics (MD) simulations followed by molecular mechanics Poisson-Boltzmann surface area (MM-PBSA) calculations. The predicted binding free energies for the five biliverdin-IXα analogues match well with the relative potency of their reported experimental binding affinities, supporting that the proposed binding modes are reasonable. Furthermore, the ternary complex structure of hBVR-A binding with biliverdin-IXα and the electron donor cofactor NADPH obtained from MD simulations was exploited to investigate the catalytic mechanism, by calculating the reaction energy profile using the quantum mechanics/molecular mechanics (QM/MM) method. On the basis of our calculations, the energetically preferred pathway consists of an initial protonation of the pyrrolic nitrogen on the biliverdin substrate followed by hydride transfer to yield the reduction product. This conclusion is consistent with a previous mechanistic study on human biliverdin IXβ reductase (hBVR-B).

Biliverdin reductase: more than a namesake - the reductase, its Peptide fragments, and biliverdin regulate activity of the three classes of protein kinase C

The expanse of human biliverdin reductase (hBVR) functions in the cells is arguably unmatched by any single protein. hBVR is a Ser/Thr/Tyr-kinase, a scaffold protein, a transcription factor, and an intracellular transporter of gene regulators. hBVR is an upstream activator of the insulin/IGF-1 signaling pathway and of protein kinase C (PKC) kinases in the two major arms of the pathway. In addition, it is the sole means for generating the antioxidant bilirubin-IXα. hBVR is essential for activation of ERK1/2 kinases by upstream MAPKK-MEK and by PKCδ, as well as the nuclear import and export of ERK1/2. Small fragments of hBVR are potent activators and inhibitors of the ERK kinases and PKCs: as such, they suggest the potential application of BVR-based technology in therapeutic settings. Presently, we have reviewed the function of hBVR in cell signaling with an emphasis on regulation of PKCδ activity.

Expression, purification and preliminary X-ray crystallographic analysis of cyanobacterial biliverdin reductase

Biliverdin reductase (BVR) catalyzes the conversion of biliverdin IX α to bilirubin IX α with concomitant oxidation of an NADH or NADPH cofactor. This enzyme also binds DNA and enhances the transcription of specific genes. Recombinant cyanobacterial BVR was overexpressed in Escherichia coli, purified and crystallized. A native data set was collected to 2.34 Å resolution on beamline BL38B1 at SPring-8. An SeMet data set was collected from a microcrystal (300×10×10 µm) on the RIKEN targeted protein beamline BL32XU and diffraction spots were obtained to 3.0 Å resolution. The native BVR crystal belonged to space group P2(1)2(1)2(1), with unit-cell parameters a=58.8, b=88.4, c=132.6 Å. Assuming that two molecules are present in the asymmetric unit, VM (the Matthews coefficient) was calculated to be 2.37 Å3 Da(-1) and the solvent content was estimated to be 48.1%. The structure of cyanobacterial BVR may provide insights into the mechanisms of its enzymatic and physiological functions.

Characterization of the human biliverdin reductase gene structure and regulatory elements: promoter activity is enhanced by hypoxia and suppressed by TNF-alpha-activated NF-kappaB

hBVR is a Ser/Thr/Tyr kinase/scaffold protein/transcription factor/intracellular transporter of regulators. hBVR is an upstream activator of the insulin/IGF-1/MAPK/PI3K signaling pathway, and of NF-kappaB. As a reductase, it converts biliverdin to the antioxidant, bilirubin. hBVR gene has 8 exons; exon 1 is not translated. We report the characterization of hBVR promoter and its negative and positive regulation, respectively, by TNF-alpha and hypoxia. The 5' end of exon 1 was defined by primer extension analyses; deletion of an inhibitor sequence 350-425 bp upstream of this exon enhanced the promoter activity. One of two NF-kappaB binding sites in the 836-bp promoter was functional; the P65 subunit of NF-kappaB and TNF-alpha acted as inhibitors. On the basis of EMSA and ChIP assays, TNF-alpha treatment increases binding of NF-kappaB to its regulatory element. Overexpression of IkappaB increased hBVR mRNA. Biliverdin, but not bilirubin, was as effective as TNF-alpha in inhibiting hBVR promoter activity. Only one of 4 hypoxia responsive elements (HREs) bound to HIF-1alpha and ARNT expressed in HEK293A cells. An abasic site was introduced at the 3' G of the HRE. This element bound HIF-1 in the gel shift and in in-cell luciferase assays. hBVR was detected in the nucleus at 1, 2, and 4 h after hypoxia (1% O(2)), at which times its kinase and reductase activities were increased. Because hypoxia positively influences hBVR promoter and phosphorylation and TNF-alpha activated NF-kappaB inhibits the promoter, while biliverdin inhibits both NF-kappaB activity and hBVR promoter, we propose a regulatory mechanism for NF-kappaB by hypoxia and TNF-alpha centered on hBVR/biliverdin.

Human biliverdin reductase suppresses Goodpasture antigen-binding protein (GPBP) kinase activity: the reductase regulates tumor necrosis factor-alpha-NF-kappaB-dependent GPBP expression

The Ser/Thr/Tyr kinase activity of human biliverdin reductase (hBVR) and the expression of Goodpasture antigen-binding protein (GPBP), a nonconventional Ser/Thr kinase for the type IV collagen of basement membrane, are regulated by tumor necrosis factor (TNF-alpha). The pro-inflammatory cytokine stimulates kinase activity of hBVR and activates NF-kappaB, a transcriptional regulator of GPBP mRNA. Increased GPBP activity is associated with several autoimmune conditions, including Goodpasture syndrome. Here we show that in HEK293A cells hBVR binds to GPBP and down-regulates its TNF-alpha-stimulated kinase activity; this was not due to a decrease in GPBP expression. Findings with small interfering RNA to hBVR and to the p65 regulatory subunit of NF-kappaB show the hBVR role in the initial stimulation of GPBP expression by TNF-alpha-activated NF-kappaB; hBVR was not a factor in mediating GPBP mRNA stability. The interacting domain was mapped to the (281)CX(10)C motif in the C-terminal 24 residues of hBVR. A 7-residue peptide, KKRILHC(281), corresponding to the core of the consensus D(delta)-Box motif in the interacting domain, was as effective as the intact 296-residue hBVR polypeptide in inhibiting GPBP kinase activity. GPBP neither regulated hBVR expression nor TNF-alpha dependent NF-kappaB expression. Collectively, our data reveal that hBVR is a regulator of the TNF-alpha-GPBP-collagen type IV signaling cascade and uncover a novel biological interaction that may be of relevance in autoimmune pathogenesis.

A novel mutation in the biliverdin reductase-A gene combined with liver cirrhosis results in hyperbiliverdinaemia (green jaundice)

BACKGROUND

Hyperbiliverdinaemia is a poorly defined clinical sign that has been infrequently reported in cases of liver cirrhosis or liver carcinoma, usually indicating a poor long-term prognosis.

AIMS

To clarify the pathogenesis of hyperbiliverdinaemia in an extended case report.

METHODS

A 64-year-old man with alcoholic cirrhosis was admitted to hospital with severe bleeding from oesophageal varices. Ultrasonography showed ascites, but no dilatation of the biliary tree. The skin, sclerae, plasma, urine and ascites of the patient showed a greenish appearance. Bilirubin levels were normal, and there were no signs of haemolysis. Biliverdin was analysed in plasma and urine with liquid chromatography coupled to mass spectrometry. The seven exonic regions of the biliverdin reductase-A (BVR-A) gene was amplified by polymerase chain reaction and sequenced.

RESULTS

Biliverdin was present in plasma and urine. In nucleotide 52 of exon I of the DNA isolated from the hyperbiliverdinaemic patient, we discovered a novel heterozygous C-->T nonsense mutation converting an arginine (CGA) in position 18 into a stop codon (TGA) (R18Stop) predicted to truncate the protein N-terminally to the active site Tyr97. Two children of the proband were heterozygous for the identical mutation in the BVR-A gene, but had no clinical signs of liver disease and had normal levels of biliverdin. The BVR-A gene mutation was not found in 200 healthy volunteers or nine patients with end-stage liver cirrhosis.

CONCLUSION

Hyperbiliverdinaemia (green jaundice) with green plasma and urine may be caused by a genetic defect in the BVR-A gene in conjunction with decompensated liver cirrhosis.

The effect of pH on the initial rate kinetics of the dimeric biliverdin-IXalpha reductase from the cyanobacterium Synechocystis PCC6803

Biliverdin-IXalpha reductase from Synechocystis PCC6803 (sBVR-A) is a stable dimer and this behaviour is observed under a range of conditions. This is in contrast to all other forms of BVR-A, which have been reported to behave as monomers, and places sBVR-A in the dihydrodiol dehydrogenase/N-terminally truncated glucose-fructose oxidoreductase structural family of dimers. The cyanobacterial enzyme obeys an ordered steady-state kinetic mechanism at pH 5, with NADPH being the first to bind and NADP(+) the last to dissociate. An analysis of the effect of pH on k(cat) with NADPH as cofactor reveals a pK of 5.4 that must be protonated for effective catalysis. Analysis of the effect of pH on k(cat)/K(m)(NADPH) identifies pK values of 5.1 and 6.1 in the free enzyme. Similar pK values are identified for biliverdin binding to the enzyme-NADPH complex. The lower pK values in the free enzyme (pK 5.1) and enzyme-NADPH complex (pK 4.9) are not evident when NADH is the cofactor, suggesting that this ionizable group may interact with the 2'-phosphate of NADPH. His84 is implicated as a crucial residue for sBVR-A activity because the H84A mutant has less than 1% of the activity of the wild-type and exhibits small but significant changes in the protein CD spectrum. Binding of biliverdin to sBVR-A is conveniently monitored by following the induced CD spectrum for biliverdin. Binding of biliverdin to wild-type sBVR-A induces a P-type spectrum. The H84A mutant shows evidence for weak binding of biliverdin and appears to bind a variant of the P-configuration. Intriguingly, the Y102A mutant, which is catalytically active, binds biliverdin in the M-configuration.

Cell surface biliverdin reductase mediates biliverdin-induced anti-inflammatory effects via phosphatidylinositol 3-kinase and Akt

Biliverdin reductase A (BVR) catalyzes the reduction of biliverdin (BV) to bilirubin (BR) in all cells. Others and we have shown that biliverdin is a potent anti-inflammatory molecule, however, the mechanism by which BV exerts its protective effects is unclear. We describe and elucidate a novel finding demonstrating that BVR is expressed on the external plasma membrane of macrophages (and other cells) where it quickly converts BV to BR. The enzymatic conversion of BV to BR on the surface by BVR initiates a signaling cascade through tyrosine phosphorylation of BVR on the cytoplasmic tail. Phosphorylated BVR in turn binds to the p85alpha subunit of phosphatidylinositol 3-kinase and activates downstream signaling to Akt. Using bacterial endotoxin (lipopolysaccharide) to initiate an inflammatory response in macrophages, we find a rapid increase in BVR surface expression. One of the mechanisms by which BV mediates its protective effects in response to lipopolysaccharide is through enhanced production of interleukin-10 (IL-10) the prototypical anti-inflammatory cytokine. IL-10 regulation is dependent in part on the activation of Akt. The effects of BV on IL-10 expression are lost with blockade of Akt. Inhibition of surface BVR with RNA interference attenuates BV-induced Akt signaling and IL-10 expression and in vivo negates the cytoprotective effects of BV in models of shock and acute hepatitis. Collectively, our findings elucidate a potentially important new molecular mechanism by which BV, through the enzymatic activity and phosphorylation of surface BVR (BVR)(surf) modulates the inflammatory response.

Biliverdin reductase is a transporter of haem into the nucleus and is essential for regulation of HO-1 gene expression by haematin

hBVR (human biliverdin reductase) is an enzyme that reduces biliverdin (the product of haem oxygenases HO-1 and HO-2 activity) to the antioxidant bilirubin. It also functions as a kinase and as a transcription factor in the MAPK (mitogen-activated protein kinase) signalling cascade. Fluorescence correlation spectroscopy was used to investigate the mobility of hBVR in living cells and its function in the nuclear transport of haematin for induction of HO-1. In transiently transfected HeLa cells only kinase-competent hBVR translocates to the nucleus. A reduced mobility in the nucleus of haematin-treated cells suggests formation of an hBVR-haematin complex and its further association with large nuclear components. The binding of haematin is specific, with the formation of a 1:1 molar complex, and the C-terminal 7-residue fragment KYCCSRK(296) of hBVR contributes to the binding. The following data suggest formation of dynamic complexes of hBVR-haematin with chromatin: (i) the reduction of hBVR mobility in the presence of haematin is greater in heterochromatic regions than in euchromatic domains and (ii) hBVR mobility is not retarded by haematin in nuclear lysates that contain only soluble factors. Moreover, hBVR kinase activity is stimulated in the presence of double-stranded DNA fragments corresponding to HO-1 antioxidant and HREs (hypoxia response elements), as well as by haematin. Experiments with nuclear localization, export signal mutants and si-hBVR [siRNA (small interfering RNA) specific to hBVR] indicate that nuclear localization of hBVR is required for induction of HO-1 by haematin. Because gene regulation is energy-dependent and haematin regulates gene expression, our data suggest that hBVR functions as an essential component of the regulatory mechanisms for haem-responsive transcriptional activation.

Human biliverdin reductase is an ERK activator; hBVR is an ERK nuclear transporter and is required for MAPK signaling

Activation of the MEK/ERK/Elk-signaling cascade is a mechanism for relaying mitogenic and stress stimuli for gene activation. MEK1 is the proximate kinase for activation of ERK1/2, and nuclear targeting of ERK1/2 is obligatory for Elk1 transcriptional activity. Human biliverdin reductase (hBVR) is a recently described Ser/Thr/Tyr kinase in the MAPK insulin/insulin-like growth factor 1 (IGF1)-signaling cascade. Using 293A cells and in vitro experiments, we detail the formation of a ternary complex of MEK/ERK/hBVR, activation of MEK1 and ERK1/2 kinase activities by hBVR, and phosphorylation of hBVR by ERK1/2. hBVR is nearly as effective as IGF1 in activating ERK; intact hBVR ATP-binding domain is necessary for Elk1 activation, whereas protein-protein interaction is the basis for hBVR activation of MEK1 and ERK. The two MAPK docking consensus sequences present in hBVR, F(162)GFP and K(275)KRILHCLGL (C- and D-box, respectively), are ERK interactive sites; interaction at each site is critical for ERK/Elk1 activation. Transfection with mutant hBVR-P(165) or peptides corresponding to the C- or D-box blocked activation of ERK by IGF1. Transfection with D-box mutant hBVR prevented the activation of ERK by wild-type protein and dramatically decreased Elk1 transcriptional activity. hBVR is a nuclear transporter of ERK; experiments with hBVR nuclear export signal (NES) and nuclear localization signal (NLS) mutants demonstrated its critical role in the nuclear localization of IGF-stimulated ERK for Elk1 activation. These findings, together with observations that si-hBVR blocked activation of ERK and Elk1 by IGF1 and prevented formation of ternary complex between MEK/ERK/hBVR, define the critical role of hBVR in ERK signaling and nuclear functions of the kinase.

Mass spectrometric identification of lysine residues of heme oxygenase-1 that are involved in its interaction with NADPH-cytochrome P450 reductase

The lysine residues of rat heme oxygenase-1 (HO-1) were acetylated by acetic anhydride in the absence and presence of NADPH-cytochrome P450 reductase (CPR) or biliverdin reductase (BVR). Nine acetylated peptides were identified by MALDI-TOF mass spectrometry in the tryptic fragments obtained from HO-1 acetylated without the reductases (referred to as the fully acetylated HO-1). The presence of CPR prevented HO-1 from acetylation of lysine residues, Lys-149 and Lys-153, located in the F-helix. The heme degradation activity of the fully acetylated HO-1 in the NADPH/CPR-supported system was significantly reduced, whereas almost no inactivation was detected in HO-1 in the presence of CPR, which prevented acetylation of Lys-149 and Lys-153. On the other hand, the presence of BVR showed no protective effect on the acetylation of HO-1. The interaction of HO-1 with CPR or BVR is discussed based on the acetylation pattern and on molecular modeling.

Biliverdin reductase: PKC interaction at the cross-talk of MAPK and PI3K signaling pathways

Biliverdin reductase (BVR) was characterized some 25 years ago as a unique dual-cofactor/pH-dependent enzyme that catalyzes the reduction of biliverdin-IXa. Our knowledge of functions of BVR has increased enormously in recent years. hBVR functions in the IR/IGF-1-controlled regulation of the MAPK and PI3K cascades that are linked by the PKC enzymes. The first of the two culminates in the activation of transcription factors for oxidative stress-responsive genes, including ho-1, where BVR functions as both a bZip (basic leucine zipper) transcription factor and a kinase. The second pathway amplifies the insulin/growth-factor signal for protein/DNA synthesis and glucose transport downstream of PI3K. hBVR is a transactivator of PKC-betaII, and thus an integral component of the "activation loop" linking MAPK, PKC-betaII, and PI3K to insulin/growth-factor signaling. The emergence of biliverdin and bilirubin as a newly defined category of modulators of cell signaling and kinase activity further underscores the critical input of hBVR in the response of intracellular pathways into the external environment. Structural features of BVR and recent findings relevant to its function in cell-signaling pathways are reviewed here and are intended to complement a recent commentary on the role of BVR in linking heme metabolism and cell signaling.

Biliverdin inhibits activation of NF-kappaB: reversal of inhibition by human biliverdin reductase

hBVR functions in the cell as a reductase and as a kinase. In the first capacity, it reduces biliverdin, the product of HO activity, to the effective intracellular antioxidant, bilirubin; as a dual-specificity kinase (S/T/Y) it activates the MAPK and IGF/IRK receptor signal transduction pathways. NF-kappaB and the MAPK pathway are activated by ROS, which results in the activation of stress-inducible genes, including ho-1. Presently, we report on the negative effect of biliverdin on NF-kappaB activation and the converse effect of hBVR. Biliverdin, in a concentration- and time-dependent manner, inhibited transcriptional activity of NF-kappaB in HEK293A cells. Nuclear extracts from biliverdin-treated cells show reduced DNA binding of NF-kappaB in an electromobility shift assay, whereas extracts from cells treated with TNF-alpha showed enhanced binding. Coimmunoprecipitation data show hBVR binds to the 65 kDa subunit of NF-kappaB, and that this is dependent on activation by TNF-alpha. Overexpression of hBVR enhanced both the basal and TNF-alpha-mediated activation of NF-kappaB and also that of the NF-kappaB-activated iNOS gene. Also, overexpression of hBVR arrested the cell cycle in the G(1)/G(0) phase and reduced the number of cells in S phase. Similar results were observed with MCF-7 cells. Because of the Janus nature of NF-kappaB activity in the cell and the inhibitory action of biliverdin, the present findings provide a foundation for therapeutic intervention in inflammatory diseases and cancer that may be attained by preventing reduction of biliverdin. On the other hand, by increasing BVR levels beneficial functions of NF-kappaB might be augmented.

Heme-oxygenase-1-induced protection against hypoxia/reoxygenation is dependent on biliverdin reductase and its interaction with PI3K/Akt pathway

Heme-oxygenase-1 (HO-1), a stress-inducible protein, is an important cytoprotective agent against ischemia/reperfusion (I/R) injury. However, the role of downstream mediators involved in HO-1-induced cytoprotection is not clear. In the current study we investigated the role of biliverdin reductase, an enzyme involved in the conversion of HO-1-derived biliverdin into bilirubin and the PI3K/Akt pathway in mediating the cytoprotective effects of HO-1 against hypoxia and reoxygenation (H/R) injury in vitro and in vivo. H9c2 cardiomyocytes were transfected with a plasmid expressing HO-1 or LacZ and exposed to 24 h of hypoxia followed by 12 h of reoxygenation. At the end of reoxygenation, reactive oxygen species generation was determined using CM-H(2)DCFDA dye and apoptosis was assessed by TUNEL, caspase activity and Bad phosphorylation. p85 and Akt phosphorylation were determined using cell-based ELISA and phospho-specific antibodies, respectively. HO-1 overexpression increased phosphorylation of the regulatory subunit of the PI3K (p85alpha) and downstream effector Akt in H9c2 cells, leading to decreased ROS and apoptosis. Furthermore, cardiac expression of HO-1 increased basal phosphorylated Akt levels and decreased infarct size in response to LAD ligation and release induced I/R injury. Conversely, PI3K inhibition reversed the effects of HO-1 on Akt phosphorylation, cell death and infarct size. In addition, knockdown of biliverdin reductase (BVR) expression with siRNA attenuated HO-1-induced Akt phosphorylation and increased H/R-induced apoptosis of H9c2 cells. Co-immunoprecipitation revealed protein-protein interaction between BVR and the phosphorylated p85 subunit of the PI3 kinase. Taken together, these results suggest that the enzyme biliverdin reductase plays an important role in mediating cytoprotective effects of HO-1. This effect is mediated, at least in part, via interaction with and activation of the PI3K/Akt pathway.

Activation of biliverdin-IXalpha reductase by inorganic phosphate and related anions

The effect of pH on the initial-rate kinetic behaviour of BVR-A (biliverdin-IXalpha reductase) exhibits an alkaline optimum with NADPH as cofactor, but a neutral optimum with NADH as cofactor. This has been described as dual cofactor and dual pH dependent behaviour; however, no mechanism has been described to explain this phenomenon. We present evidence that the apparent peak of activity observed at neutral pH with phosphate buffer and NADH as cofactor is an anion-dependent activation, where inorganic phosphate apparently mimics the role played by the 2'-phosphate of NADPH in stabilizing the interaction between NADH and the enzyme. The enzymes from mouse, rat and human all exhibit this behaviour. This behaviour is not seen with BVR-A from Xenopus tropicalis or the ancient cyanobacterial enzyme from Synechocystis PCC 6803, which, in addition to being refractory to activation by inorganic phosphate, are also differentiated by an acid pH optimum with both nicotinamide nucleotides.

Regulation of TNF-alpha-activated PKC-zeta signaling by the human biliverdin reductase: identification of activating and inhibitory domains of the reductase

Human biliverdin reductase (hBVR) is a dual function enzyme: a catalyst for bilirubin formation and a S/T/Y kinase that shares activators with protein kinase C (PKC) -zeta, including cytokines, insulin, and reactive oxygen species (ROS). Presently, we show that hBVR increases PKC-zeta autophosphorylation, stimulation by TNF-alpha, as well as cytokine stimulation of NF-kappaB DNA binding and promoter activity. S149 in hBVR S/T kinase domain and S230 in YLS230F in hBVR's docking site for the SH2 domain of signaling proteins are phosphorylation targets of PKC-zeta. Two hBVR-based peptides, KRNRYLS230F (#1) and KKRILHC281 (#2), but not their S-->A or C-->A derivatives, respectively, blocked PKC-zeta stimulation by TNF-alpha and its membrane translocation. The C-terminal-based peptide KYCCSRK296 (#3), enhanced PKC-zeta stimulation by TNF-alpha; for this, Lys296 was essential. In metabolically 32P-labeled HEK293 cells transfected with hBVR or PKC-zeta, TNF-alpha increased hBVR phosphorylation. TNF-alpha did not stimulate PKC-zeta in cells infected with small interfering RNA for hBVR or transfected with hBVR with a point mutation in the nucleotide-binding loop (G17), S149, or S230; this was similar to the response of "kinase-dead" PKC-zeta(K281R). We suggest peptide #1 blocks PKC-zeta-docking site interaction, peptide #2 disrupts function of the PKC-zeta C1 domain, and peptide #3 alters ATP presentation to the kinase. The findings are of potential significance for development of modulators of PKC-zeta activity and cellular response to cytokines.

Molecular mechanism of phase I and phase II drug-metabolizing enzymes: implications for detoxification

Enzymes that catalyze the biotransformation of drugs and xenobiotics are generally referred to as drug-metabolizing enzymes (DMEs). DMEs can be classified into two main groups: oxidative or conjugative. The NADPH-cytochrome P450 reductase (P450R)/cytochrome P450 (P450) electron transfer systems are oxidative enzymes that mediate phase I reactions, whereas the UDP-glucuronosyltransferases (UGTs) are conjugative enzymes that mediate phase II enzymes. Both enzyme systems are localized to the endoplasmic reticulum (ER) where a number of drugs are sequentially metabolized. DMEs, including P450s and UGTs, generally have a highly plastic active site that can accommodate a wide variety of substrates. The P450 and UGT genes constitute a supergene family, in which UGT proteins are encoded by distinct genes and a complex gene. Both the P450 and UGT genes have evolved to diversify their functions. This chapter reviews advances in understanding the structure and function of the P450R/P450 and UGT enzyme systems. In particular, the coordinate biotransformation of xenobiotics by phase I and II enzymes in the ER membrane is examined.

Human biliverdin reductase, a previously unknown activator of protein kinase C betaII

Human biliverdin reductase (hBVR), a dual specificity kinase (Ser/Thr/Tyr) is, as protein kinase C (PKC) betaII, activated by insulin and free radicals (Miralem, T., Hu, Z., Torno, M. D., Lelli, K. M., and Maines, M. D. (2005) J. Biol. Chem. 280, 17084-17092; Lerner-Marmarosh, N., Shen, J., Torno, M. D., Kravets, A., Hu, Z., and Maines, M. D. (2005) Proc. Natl. Acad. Sci. U. S. A. 102, 7109-7114). Here, by using 293A cells co-transfected with pcDNA3-hBVR and PKC betaII plasmids, we report the co-immunoprecipitation of the proteins and co-purification in the glutathione S-transferase (GST) pulldown assay. hBVR and PKC betaII, but not the reductase and PKC zeta, transphosphorylated in assay systems supportive of activity of only one of the kinases. PKC betaII K371R mutant protein ("kinase-dead") was also a substrate for hBVR. The reductase increased the Vmax but not the apparent Km values of PKC betaII for myelin basic protein; activation was independent of phospholipids and extended to the phosphorylation of S2, a PKC-specific substrate. The increase in substrate phosphorylation was blocked by specific inhibitors of conventional PKCs and attenuated by sihBVR. The effect of the latter could be rescued by subsequent overexpression of hBVR. To a large extent, the activation was a function of the hBVR N-terminal chain of valines and intact ATP-binding site and the cysteine-rich C-terminal segment. The cobalt protoporphyrin-activated hBVR phosphorylated a threonine in a peptide corresponding to the Thr500 in the human PKC betaII activation loop. Neither serine nor threonine residues in peptides corresponding to other phosphorylation sites of the PKC betaII nor PKC zeta activation loop-derived peptides were substrates. The phosphorylation of Thr500 was confirmed by immunoblotting of hBVR.PKC betaII immunocomplex. The potential biological relevance of the hBVR activation of PKC betaII was suggested by the finding that in cells transfected with the PKC betaII, hBVR augmented phorbol myristate acetate-mediated c-fos expression, and infection with sihBVR attenuated the response. Also, in cells overexpressing hBVR and PKC betaII, as well as in untransfected cells, upon treatment with phorbol myristate acetate, the PKC translocated to the plasma membrane and co-localized with hBVR. hBVR activation of PKC betaII underscores its potential function in propagation of signals relayed through PKCs.

Spectroscopic characterization of a higher plant heme oxygenase isoform-1 from Glycine max (soybean)--coordination structure of the heme complex and catabolism of heme

Heme oxygenase converts heme into biliverdin, CO, and free iron. In plants, as well as in cyanobacteria, heme oxygenase plays a particular role in the biosynthesis of photoreceptive pigments, such as phytochromobilins and phycobilins, supplying biliverdin IX(alpha) as a direct synthetic resource. In this study, a higher plant heme oxygenase, GmHO-1, of Glycine max (soybean), was prepared to evaluate the molecular features of its heme complex, the enzymatic activity, and the mechanism of heme conversion. The similarity in the amino acid sequence between GmHO-1 and heme oxygenases from other biological species is low, and GmHO-1 binds heme with 1 : 1 stoichiometry at His30; this position does not correspond to the proximal histidine of other heme oxygenases in their sequence alignments. The heme bound to GmHO-1, in the ferric high-spin state, exhibits an acid-base transition and is converted to biliverdin IX(alpha) in the presence of NADPH/ferredoxin reductase/ferredoxin, or ascorbate. During the heme conversion, an intermediate with an absorption maximum different from that of typical verdoheme-heme oxygenase or CO-verdoheme-heme oxygenase complexes was observed and was extracted as a bis-imidazole complex; it was identified as verdoheme. A myoglobin mutant, H64L, with high CO affinity trapped CO produced during the heme degradation. Thus, the mechanism of heme degradation by GmHO-1 appears to be similar to that of known heme oxygenases, despite the low sequence homology. The heme conversion by GmHO-1 is as fast as that by SynHO-1 in the presence of NADPH/ferredoxin reductase/ferredoxin, thereby suggesting that the latter is the physiologic electron-donating system.

Understanding a transcriptional paradigm at the molecular level. The structure of yeast Gal80p

In yeast, the GAL genes encode the enzymes required for normal galactose metabolism. Regulation of these genes in response to the organism being challenged with galactose has served as a paradigm for eukaryotic transcriptional control over the last 50 years. Three proteins, the activator Gal4p, the repressor Gal80p, and the ligand sensor Gal3p, control the switch between inert and active gene expression. Gal80p, the focus of this investigation, plays a pivotal role both in terms of repressing the activity of Gal4p and allowing the GAL switch to respond to galactose. Here we present the three-dimensional structure of Gal80p from Kluyveromyces lactis and show that it is structurally homologous to glucose-fructose oxidoreductase, an enzyme in the sorbitol-gluconate pathway. Our results clearly define the overall tertiary and quaternary structure of Gal80p and suggest that Gal4p and Gal3p bind to Gal80p at distinct but overlapping sites. In addition to providing a molecular basis for previous biochemical and genetic studies, our structure demonstrates that much of the enzymatic scaffold of the oxidoreductase has been maintained in Gal80p, but it is utilized in a very different manner to facilitate transcriptional regulation.

New insights into biliverdin reductase functions: linking heme metabolism to cell signaling

Biliverdin reductase (BVR) functions in cell signaling through three distinct tracks: a dual-specificity kinase that functions in the insulin receptor/MAPK pathways (25, 29, 51); a bzip-type transcription factor for ATF-2/CREB and HO-1 regulation (1, 25); and a reductase that catalyzes the conversion of biliverdin to bilirubin (27). These, together with the protein's primary and secondary features, intimately link BVR to the entire spectrum of cell-signaling cascades.

Human biliverdin reductase: a member of the insulin receptor substrate family with serine/threonine/tyrosine kinase activity

We describe here the tyrosine kinase activity of human biliverdin reductase (BVR) and its potential role in the insulin-signaling pathway. BVR is both a substrate for insulin receptor (IR) tyrosine kinase (IRK) activity and a kinase for serine phosphorylation of IR substrate 1 (IRS-1). Our previous studies have revealed serine/threonine kinase activity of BVR. Y198, in the YMKM motif found in the C-terminal domain of BVR, is shown to be a substrate for insulin-activated IRK. This motif in IRS proteins provides a docking site for proteins that contain a Src homology 2 domain. Additionally, Y228 in the YLSF sequence and Y291 are IRK substrates; the former sequence provides optimum recognition motif in the tyrosine phosphatase, SHP-1, and for SHC (Src homology 2 domain containing transfroming protein 1). BVR autophosphorylates N-terminal tyrosines Y72 and Y83. Serine residues in IRS-1 are targets for BVR phosphorylation, and point mutation of serine residues in the kinase domain of the reductase inhibits phosphotransferase activity. Because tyrosine phosphorylation of IRS-1 activates the insulin signaling pathway and serine phosphorylation of IRS-1 blocks insulin action, our findings that insulin increases BVR tyrosine phosphorylation and that there is an increase in glucose uptake in response to insulin when expression of BVR is "knocked down" by small interfering RNA suggest a potential role for BVR in the insulin signaling pathway.

Protein expressed by the ho2 gene of the cyanobacterium Synechocystis sp. PCC 6803 is a true heme oxygenase. Properties of the heme and enzyme complex

Two isoforms of a heme oxygenase gene, ho1 and ho2, with 51% identity in amino acid sequence have been identified in the cyanobacterium Synechocystis sp. PCC 6803. Isoform-1, Syn HO-1, has been characterized, while isoform-2, Syn HO-2, has not. In this study, a full-length ho2 gene was cloned using synthetic DNA and Syn HO-2 was demonstrated to be highly expressed in Escherichia coli as a soluble, catalytically active protein. Like Syn HO-1, the purified Syn HO-2 bound hemin stoichiometrically to form a heme-enzyme complex and degraded heme to biliverdin IXalpha, CO and iron in the presence of reducing systems such as NADPH/ferredoxin reductase/ferredoxin and sodium ascorbate. The activity of Syn HO-2 was found to be comparable to that of Syn HO-1 by measuring the amount of bilirubin formed. In the reaction with hydrogen peroxide, Syn HO-2 converted heme to verdoheme. This shows that during the conversion of hemin to alpha-meso-hydroxyhemin, hydroperoxo species is the activated oxygen species as in other heme oxygenase reactions. The absorption spectrum of the hemin-Syn HO-2 complex at neutral pH showed a Soret band at 412 nm and two peaks at 540 nm and 575 nm, features observed in the hemin-Syn HO-1 complex at alkaline pH, suggesting that the major species of iron(III) heme iron at neutral pH is a hexa-coordinate low spin species. Electron paramagnetic resonance (EPR) revealed that the iron(III) complex was in dynamic equilibrium between low spin and high spin states, which might be caused by the hydrogen bonding interaction between the distal water ligand and distal helix components. These observations suggest that the structure of the heme pocket of the Syn HO-2 is different from that of Syn HO-1.

Crystal structure of a pH-regulated luciferase catalyzing the bioluminescent oxidation of an open tetrapyrrole

The luciferase of Lingulodinium polyedrum, a marine bioluminescent dinoflagellate, consists of three similar but not identical domains in a single polypeptide. Each encodes an active luciferase that catalyzes the oxidation of a chlorophyll-derived open tetrapyrrole (dinoflagellate luciferin) to produce blue light. These domains share no sequence similarity with any other in the GenBank database and no structural or motif similarity with any other luciferase. We report here the 1.8-A crystal structure of the third domain, D3, at pH 8, and a mechanism for its activity regulation by pH. D3 consists of two major structural elements: a beta-barrel pocket putatively for substrate binding and catalysis and a regulatory three-helix bundle. N-terminal histidine residues previously shown to regulate activity by pH are at the interface of the helices in the bundle. Molecular dynamics calculations indicate that, in response to changes in pH, these histidines could trigger a large molecular motion of the bundle, thereby exposing the active site to the substrate.

On how the conformation of biliverdins influences their reduction to bilirubins: a biological and molecular modeling study

The cyclic 2,18-bridged biliverdin (2) is excreted in rat bile without reduction to the corresponding bilirubin. Conformational analysis, employing an optimized Monte Carlo method and a mixed Monte Carlo/stochastic dynamics, reveals that biliverdin IXalpha (1) and the cyclic analogue 2 adopt 'lock washer' conformations, stabilized by the presence of intramolecular hydrogen bonds between N23...H22N and, to a lesser extent, between N23...H24N. Although 2 is very similar in overall shape to 1, the former adopts a 'locked lock washer' conformation unable to undergo fluctuations, thus possibly hampering a proper recognition by biliverdin reductase.

Unique features of recombinant heme oxygenase of Drosophila melanogaster compared with those of other heme oxygenases studied

We cloned a cDNA for a Drosophila melanogaster homologue of mammalian heme oxygenase (HO) and constructed a bacterial expression system of a truncated, soluble form of D. melanogaster HO (DmDeltaHO). The purified DmDeltaHO degraded hemin to biliverdin, CO and iron in the presence of reducing systems such as NADPH/cytochrome P450 reductase and sodium ascorbate, although the reaction rate was slower than that of mammalian HOs. Some properties of DmHO, however, are quite different from other known HOs. Thus DmDeltaHO bound hemin stoichiometrically to form a hemin-enzyme complex like other HOs, but this complex did not show an absorption spectrum of hexa-coordinated heme protein. The absorption spectrum of the ferric complex was not influenced by changing the pH of the solution. Interestingly, an EPR study revealed that the iron of heme was not involved in binding heme to the enzyme. Hydrogen peroxide failed to convert it into verdoheme. A spectrum of the ferrous-CO form of verdoheme was not detected during the reaction from hemin under oxygen and CO. Degradation of hemin catalyzed by DmDeltaHO yielded three isomers of biliverdin, of which biliverdin IXalpha and two other isomers (IXbeta and IXdelta) accounted for 75% and 25%, respectively. Taken together, we conclude that, although DmHO acts as a real HO in D. melanogaster, its active-site structure is quite different from those of other known HOs.