NMR characterization of structure, backbone dynamics, and glutathione binding of the human macrophage migration inhibitory factor (MIF)

Therapeutic Opportunities in Breast Cancer by Targeting Macrophage Migration Inhibitory Factor as a Pleiotropic Cytokine

As a heterogeneous disease, breast cancer (BC) has been characterized by the uncontrolled proliferation of mammary epithelial cells. The tumor microenvironment (TME) also contains inflammatory cells, fibroblasts, the extracellular matrix (ECM), and soluble factors that all promote BC progression. In this sense, the macrophage migration inhibitory factor (MIF), a pleiotropic pro-inflammatory cytokine and an upstream regulator of the immune response, enhances breast tumorigenesis through escalating cancer cell proliferation, survival, angiogenesis, invasion, metastasis, and stemness, which then brings tumorigenic effects by activating key oncogenic signaling pathways and inducing immunosuppression. Against this background, this review was to summarize the current understanding of the MIF pathogenic mechanisms in cancer, particularly BC, and address the central role of this immunoregulatory cytokine in signaling pathways and breast tumorigenesis. Furthermore, different inhibitors, such as small molecules as well as antibodies (Abs) or small interfering RNA (siRNA) and their anti-tumor effects in BC studies were examined. Small molecules and other therapy target MIF. Considering MIF as a promising therapeutic target, further clinical evaluation of MIF-targeted agents in patients with BC was warranted.

Underrepresented Impurities in 4-Hydroxyphenylpyruvate Affect the Catalytic Activity of Multiple Enzymes

Macrophage migration inhibitory factor (MIF) is a key immunostimulatory protein with regulatory properties in several disorders, including inflammation and cancer. All the reported inhibitors that target the biological activities of MIF have been discovered by testing against its keto/enol tautomerase activity. While the natural substrate is still unknown, model MIF substrates are used for kinetic experiments. The most extensively used model substrate is 4-hydroxyphenyl pyruvate (4-HPP), a naturally occurring intermediate of tyrosine metabolism. Here, we examine the impact of 4-HPP impurities in the precise and reproducible determination of MIF kinetic data. To provide unbiased evaluation, we utilized 4-HPP powders from five different manufacturers. Biochemical and biophysical analyses showed that the enzymatic activity of MIF is highly influenced by underrepresented impurities found in 4-HPP. Besides providing inconsistent turnover results, the 4-HPP impurities also influence the accurate calculation of ISO-1's inhibition constant, an MIF inhibitor that is broadly used for in vitro and in vivo studies. The macromolecular NMR data show that 4-HPP samples from different manufacturers result in differential chemical shift perturbations of amino acids in MIF's active site. Our MIF-based conclusions were independently evaluated and confirmed by 4-hydroxyphenylpyruvate dioxygenase (HPPD) and D-dopachrome tautomerase (D-DT); two additional enzymes that utilize 4-HPP as a substrate. Collectively, these results explain inconsistencies in previously reported inhibition values, highlight the effect of impurities on the accurate determination of kinetic parameters, and serve as a tool for designing error-free in vitro and in vivo experiments.

A Cross-Kingdom View on the Immunomodulatory Role of MIF/D-DT Proteins in Mammalian and Plant Pseudomonas Infections

Gram-negative Pseudomonas bacteria are largely harmless saprotrophs, but some species can be potent pathogens of both plants and mammals. Macrophage migration inhibitory factor (MIF) and its homolog D-dopachrome tautomerase (D-DT, also referred to as MIF-2) are multifunctional proteins that in addition to their intracellular functions also serve as extracellular signaling molecules (cytokines) in orchestrating mammalian immune responses. It recently emerged that plants also possess MIF-like proteins, termed MIF/D-DT-like (MDL) proteins. We here provide a comparative cross-kingdom view on the immunomodulatory role of MIF and MDL proteins during Pseudomonas infections in mammals and plants. Although in both kingdoms the lack of MIF/MDL proteins is associated with a reduction in bacterial load and disease symptoms, the underlying molecular principles seem to be different. We provide a perspective for future research activities to unravel additional commonalities and differences in the MIF/MDL-mediated adjustment of antibacterial immune activities.

Probing the Functional Interaction Interface of Lipopolysaccharide and Antimicrobial Peptides: A Solution-State NMR Perspective

Antimicrobial peptides (AMPs) have been a topic of substantial research as the next-generation antibiotics. They have been extensively studied for the selectivity and action against microbial membrane lipids in imparting their targeted functioning. To determine the effectivity of the peptides against the Gram-negative pathogens, it is imperative to elucidate their role in interacting with the lipopolysaccharide moieties. Lipopolysaccharide is a major component of the outer membrane of the Gram-negative bacteria. It serves to protect the bacteria as well as govern the functionality of several antibacterial agents. It can prevent the access of the agents into the inner membrane of the bacteria, thus rendering them inactive. Several techniques have been employed to study the interaction for better designing of peptides; NMR spectroscopy is one of the most widely used techniques in determining the interactive properties of peptides with LPS as it can provide the details in atomistic level. NMR spectroscopy provides information about the structural and conformational changes as well as the dynamics of the interactions.

Tetralone derivatives are MIF tautomerase inhibitors and attenuate macrophage activation and amplify the hypothermic response in endotoxemic mice

Macrophage migration inhibitory factor (MIF) is a pro-inflammatory cytokine playing crucial role in immunity. MIF exerts a unique tautomerase enzymatic activity that has relevance concerning its multiple functions and its small molecule inhibitors have been proven to block its pro-inflammatory effects. Here we demonstrate that some of the E-2-arylmethylene-1-tetralones and their heteroanalogues efficiently bind to MIF's active site and inhibit MIF tautomeric (enolase, ketolase activity) functions. A small set of the synthesised derivatives, namely compounds (4), (23), (24), (26) and (32), reduced inflammatory macrophage activation. Two of the selected compounds (24) and (26), however, markedly inhibited ROS and nitrite production, NF-κB activation, TNF-α, IL-6 and CCL-2 cytokine expression. Pre-treatment of mice with compound (24) exaggerated the hypothermic response to high dose of bacterial endotoxin. Our experiments suggest that tetralones and their derivatives inhibit MIF's tautomeric functions and regulate macrophage activation and thermal changes in severe forms of systemic inflammation.

Molecular Level Insights Into the Structural and Dynamic Factors Driving Cytokine Function

Cytokines are key mediators of cellular communication and regulators of biological advents. The timing, quantity and localization of cytokines are key features in producing specific biological outcomes, and thus have been thoroughly studied and reviewed while continuing to be a focus of the cytokine biology community. Due to the complexity of cellular signaling and multitude of factors that can affect signaling outcomes, systemic level studies of cytokines are ongoing. Despite their small size, cytokines can exhibit structurally promiscuous and dynamic behavior that plays an equally important role in biological activity. In this review using case studies, we highlight the recent insight gained from observing cytokines through a molecular lens and how this may complement a system-level understanding of cytokine biology, explain diversity of downstream signaling events, and inform therapeutic and experimental development.

Identification and Structure-Activity Relationships of Dietary Flavonoids as Human Macrophage Migration Inhibitory Factor (MIF) Inhibitors

Dietary flavonoids are known to have anti-inflammatory and anticancer effects, but their influences on human macrophage migration inhibitory factor (MIF), a vital proinflammatory cytokine recognized as a therapeutic target for infectious diseases and cancers, have been rarely reported. Here, we identified 24 dietary flavonoids that could inhibit the tautomerase activity of MIF, five of which exerted IC₅₀ values lower than the positive control ISO-1 in the micromolar range: morin (IC₅₀ = 11.01 ± 0.45 μM) and amentoflavone (IC₅₀ = 13.32 ± 0.64 μM) exhibited the most potent efficacy followed by apigenin (IC₅₀ = 42.74 ± 4.20 μM), naringin (IC₅₀ = 51.38 ± 2.12 μM), and fisetin (IC₅₀ = 51.99 ± 0.63 μM). X-ray crystallography, molecular docking, and cellular experiments were utilized to illustrate the molecular binding details and structure-activity relationships. Scaffold modifications of flavonoids significantly influenced the potency. What stands out for morin is the unique 2'-OH substitution. In addition, amentoflavone situated at the MIF trimer pore may impact MIF-CD74 signaling. The results also showed that flavonoids could suppress cell chemotaxis and nitric oxide production in RAW264.7 cells. Our results elucidate the molecular mechanism of flavonoids acting on MIF and shed light on developing lead compounds against MIF-involved diseases.

Macrophage Migration Inhibitory Factor (MIF) Expression Increases during Myocardial Infarction and Supports Pro-Inflammatory Signaling in Cardiac Fibroblasts

Macrophage migration inhibitory factor (MIF) is a pleiotropic cytokine known to play a major role in inflammatory diseases such as myocardial infarction (MI), where its expression increases. Cardio-protective functions of MIF during ischemia have been reported. Recently, the structurally related MIF-2 was identified and similar effects are assumed. We wanted to further investigate the role of MIF and MIF-2 on inflammatory processes during MI. Therefore, we subjected mice to experimentally induced MI by coronary occlusion for one and five days. During the acute phase of MI, the gene expression of Mif was upregulated in the infarct zone, whereas Mif-2 was downregulated, suggesting a minor role of MIF-2. Simulating ischemic conditions or mechanical stress in vitro, we demonstrated that Mif expression was induced in resident cardiac cells. To investigate possible auto-/paracrine effects, cardiomyocytes and cardiac fibroblasts were individually treated with recombinant murine MIF, which in turn induced Mif expression and the expression of pro-inflammatory genes in cardiac fibroblasts. Cardiomyocytes did not respond to recombinant MIF with pro-inflammatory gene expression. While MIF stimulation alone did not change the expression of pro-fibrotic genes in cardiac fibroblasts, ischemia reduced their expression. Mimicking the increased MIF levels during MI, we exposed cardiac fibroblasts to simulated ischemia in the presence of MIF, which led to further reduced expression of pro-fibrotic genes. The presented data show that MIF was expressed by resident cardiac cells during MI. In vitro, Mif expression was induced by different external stimuli in cardiomyocytes and cardiac fibroblasts. Addition of recombinant MIF protein increased the expression of pro-inflammatory genes in cardiac fibroblasts including Mif expression itself. Thereby, cardiac fibroblasts may amplify Mif expression during ischemia promoting cardiomyocyte survival.

Identification of an Arg-Leu-Arg tripeptide that contributes to the binding interface between the cytokine MIF and the chemokine receptor CXCR4

MIF is a chemokine-like cytokine that plays a role in the pathogenesis of inflammatory and cardiovascular disorders. It binds to the chemokine-receptors CXCR2/CXCR4 to trigger atherogenic leukocyte migration albeit lacking canonical chemokine structures. We recently characterized an N-like-loop and the Pro-2-residue of MIF as critical molecular determinants of the CXCR4/MIF binding-site and identified allosteric agonism as a mechanism that distinguishes CXCR4-binding to MIF from that to the cognate ligand CXCL12. By using peptide spot-array technology, site-directed mutagenesis, structure-activity-relationships, and molecular docking, we identified the Arg-Leu-Arg (RLR) sequence-region 87–89 that – in three-dimensional space – ‘extends’ the N-like-loop to control site-1-binding to CXCR4. Contrary to wildtype MIF, mutant R87A-L88A-R89A-MIF fails to bind to the N-terminal of CXCR4 and the contribution of RLR to the MIF/CXCR4-interaction is underpinned by an ablation of MIF/CXCR4-specific signaling and reduction in CXCR4-dependent chemotactic leukocyte migration of the RLR-mutant of MIF. Alanine-scanning, functional competition by RLR-containing peptides, and molecular docking indicate that the RLR residues directly participate in contacts between MIF and CXCR4 and highlight the importance of charge-interactions at this interface. Identification of the RLR region adds important structural information to the MIF/CXCR4 binding-site that distinguishes this interface from CXCR4/CXCL12 and will help to design MIF-specific drug-targeting approaches.

MIF, a controversial cytokine: a review of structural features, challenges, and opportunities for drug development

INTRODUCTION

Macrophage migration inhibitory factor (MIF) has emerged as a promising drug target in diseases including sepsis, rheumatoid arthritis, and cancer. MIF has multiple properties that favor development of specific, targeted therapies: it is expressed broadly among human cells, has noted roles in diverse inflammatory and oncological processes, and has intrinsic enzymatic activity amenable to high-throughput screening. Despite these advantages, anti-MIF therapy remains well behind other cytokine-targeted therapeutics, with no small molecules in the pipeline for clinical development and anti-MIF antibodies only recently beginning clinical trials. Areas covered: In this review we summarize current literature regarding MIF structure and function-including challenges and controversies that have arisen in studies of anti-MIF therapeutics-and propose a strategy for development of clinically relevant anti-MIF drugs. Expert opinion: We believe that the field of anti-MIF therapeutics would benefit from capitalizing on the protein's multiple assets while acknowledging their flaws. The tautomerase enzymatic site of MIF may not be active biologically, but can nonetheless offer a high-throughput method to highlight molecules of interest that can affect its other, frequently intertwined bioactivities. Future work should also focus on developing more robust assays for MIF bioactivity that can be used for second-pass screening and specificity studies.

Limiting cardiac ischemic injury by pharmacological augmentation of macrophage migration inhibitory factor-AMP-activated protein kinase signal transduction

BACKGROUND

Macrophage migration inhibitory factor (MIF) exerts a protective effect on ischemic myocardium by activating AMP-activated protein kinase (AMPK). Small molecules that increase the affinity of MIF for its receptor have been recently designed, and we hypothesized that such agonists may enhance AMPK activation and limit ischemic tissue injury.

METHODS AND RESULTS

Treatment of cardiomyocytes with the candidate MIF agonist, MIF20, augmented AMPK phosphorylation, increased by 50% the surface localization of glucose transporter, and enhanced by 25% cellular glucose uptake in comparison with MIF alone. In mouse hearts perfused with MIF20 before no-flow ischemia and reperfusion, postischemic left ventricular function improved commensurately with an increase in cardiac MIF-AMPK activation and an augmentation in myocardial glucose uptake. By contrast, small-molecule MIF agonism was not effective in cells or tissues genetically deficient in MIF or the MIF receptor, verifying the specificity of MIF20 for MIF-dependent AMPK signaling. The protective effect of MIF20 also was evident in an in vivo regional ischemia model. Mice treated with MIF20 followed by left coronary artery occlusion and reperfusion showed a significant reduction in infarcted myocardium.

CONCLUSIONS

These data support the pharmacological utility of small-molecule MIF agonists in enhancing AMPK activation and reducing cardiac ischemic injury.

Characterization of molecular determinants of the conformational stability of macrophage migration inhibitory factor: leucine 46 hydrophobic pocket

Macrophage Migration Inhibitory Factor (MIF) is a key mediator of inflammatory responses and innate immunity and has been implicated in the pathogenesis of several inflammatory and autoimmune diseases. The oligomerization of MIF, more specifically trimer formation, is essential for its keto-enol tautomerase activity and probably mediates several of its interactions and biological activities, including its binding to its receptor CD74 and activation of certain signaling pathways. Therefore, understanding the molecular factors governing the oligomerization of MIF and the role of quaternary structure in modulating its structural stability and multifunctional properties is crucial for understanding the function of MIF in health and disease. Herein, we describe highly conserved intersubunit interactions involving the hydrophobic packing of the side chain of Leu46 onto the β-strand β3 of one monomer within a hydrophobic pocket from the adjacent monomer constituted by residues Arg11, Val14, Phe18, Leu19, Val39, His40, Val41, Val42, and Pro43. To elucidate the structural significance of these intersubunit interactions and their relative contribution to MIF’s trimerization, structural stability and catalytic activity, we generated three point mutations where Leu46 was replaced by glycine (L46G), alanine (L46A) and phenylalanine (L46F), and their structural properties, stability, oligomerization state, and catalytic activity were characterized using a battery of biophysical methods and X-ray crystallography. Our findings provide new insights into the role of the Leu46 hydrophobic pocket in stabilizing the conformational state of MIF in solution. Disrupting the Leu46 hydrophobic interaction perturbs the secondary and tertiary structure of the protein but has no effect on its oligomerization state.

Allosteric inhibition of macrophage migration inhibitory factor revealed by ibudilast

AV411 (ibudilast; 3-isobutyryl-2-isopropylpyrazolo-[1,5-a]pyridine) is an antiinflammatory drug that was initially developed for the treatment of bronchial asthma but which also has been used for cerebrovascular and ocular indications. It is a nonselective inhibitor of various phosphodiesterases (PDEs) and has varied antiinflammatory activity. More recently, AV411 has been studied as a possible therapeutic for the treatment of neuropathic pain and opioid withdrawal through its actions on glial cells. As described herein, the PDE inhibitor AV411 and its PDE-inhibition-compromised analog AV1013 inhibit the catalytic and chemotactic functions of the proinflammatory protein, macrophage migration inhibitory factor (MIF). Enzymatic analysis indicates that these compounds are noncompetitive inhibitors of the p-hydroxyphenylpyruvate (HPP) tautomerase activity of MIF and an allosteric binding site of AV411 and AV1013 is detected by NMR. The allosteric inhibition mechanism is further elucidated by X-ray crystallography based on the MIF/AV1013 binary and MIF/AV1013/HPP ternary complexes. In addition, our antibody experiments directed against MIF receptors indicate that CXCR2 is the major receptor for MIF-mediated chemotaxis of peripheral blood mononuclear cells.

A new class of isothiocyanate-based irreversible inhibitors of macrophage migration inhibitory factor

Macrophage migration inhibitory factor (MIF) is a homotrimeric multifunctional proinflammatory cytokine that has been implicated in the pathogenesis of several inflammatory and autoimmune diseases. Current therapeutic strategies for targeting MIF focus on developing inhibitors of its tautomerase activity or modulating its biological activities using anti-MIF neutralizing antibodies. Herein we report a new class of isothiocyanate (ITC)-based irreversible inhibitors of MIF. Modification by benzyl isothiocyanate (BITC) and related analogues occurred at the N-terminal catalytic proline residue without any effect on the oligomerization state of MIF. Different alkyl and arylalkyl ITCs modified MIF with nearly the same efficiency as BITC. To elucidate the mechanism of action, we performed detailed biochemical, biophysical, and structural studies to determine the effect of BITC and its analogues on the conformational state, quaternary structure, catalytic activity, receptor binding, and biological activity of MIF. Light scattering, analytical ultracentrifugation, and NMR studies on unmodified and ITC-modified MIF demonstrated that modification of Pro1 alters the tertiary, but not the secondary or quaternary, structure of the trimer without affecting its thermodynamic stability. BITC induced drastic effects on the tertiary structure of MIF, in particular residues that cluster around Pro1 and constitute the tautomerase active site. These changes in tertiary structure and the loss of catalytic activity translated into a reduction in MIF receptor binding activity, MIF-mediated glucocorticoid overriding, and MIF-induced Akt phosphorylation. Together, these findings highlight the role of tertiary structure in modulating the biochemical and biological activities of MIF and present new opportunities for modulating MIF biological activities in vivo.

The conformational flexibility of the carboxy terminal residues 105-114 is a key modulator of the catalytic activity and stability of macrophage migration inhibitory factor

Macrophage migration inhibitory factor (MIF) is a multifunctional protein and a major mediator of innate immunity. Although X-ray crystallography revealed that MIF exists as a homotrimer, its oligomerization state in vivo and the factors governing its oligomerization and stability remain poorly understood. The C-terminal region of MIF is highly conserved and participates in several intramolecular interactions that suggest a role in modulating the stability and biochemical activity of MIF. To determine the importance of these interactions, point mutations (A48P, L46A), insertions (P107) at the monomer-monomer interfaces, and C-terminal deletion (Delta 110-114NSTFA and Delta 105-114NVGWNNSTFA) variants were designed and their structural properties, thermodynamic stability, oligomerization state, catalytic activity and receptor binding were characterized using a battery of biophysical methods. The C-terminal deletion mutants DeltaC5 huMIF 1-109 and DeltaC10 huMIF 1-104 were enzymatically inactive and thermodynamically less stable than wild type MIF. Analytical ultracentrifugation studies demonstrate that both C-terminal mutants sediment as trimers and exhibit similar binding to CD74 as the wild type protein. Disrupting the conformation of the C-terminal region 105-114 and increasing its conformational flexibility through the insertion of a proline residue at position 107 was sufficient to reproduce the structural, biochemical and thermodynamic properties of the deletion mutants. P107 MIF forms an enzymatically inactive trimer and exhibits reduced thermodynamic stability relative to the wild type protein. To provide a rationale for the changes induced by these mutations at the molecular level, we also performed molecular dynamics simulations on these mutants in comparison to the wild type MIF. Together, our studies demonstrate that intersubunit interactions involving the C-terminal region 105-114, including a salt-bridge interaction between Arg73 of one monomer and the carboxy terminus of a neighboring monomer, play critical roles in modulating tertiary structure stabilization, enzymatic activity, and thermodynamic stability of MIF, but not its oligomerization state and receptor binding properties. Our results suggest that targeting the C-terminal region could provide new strategies for allosteric modulation of MIF enzymatic activity and the development of novel inhibitors of MIF tautomerase activity.

Chaperone-like activity of macrophage migration inhibitory factor

Macrophage migration inhibitory factor is a ubiquitous multifunctional cytokine having diverse immunological and neuroendocrine properties. Although this protein is known to be released into the circulation from the secretory granules of anterior pituitary or directly from immune cells as a consequence of stress, its participation in heat stress-induced aggregation of proteins has not yet been reported. We provide here the first evidence that the macrophage migration inhibitory factor possesses chaperone-like properties. It was shown to exist in the form of a mixture of low and high molecular weight oligomers. At heat stress temperatures the large oligomers dissociate into monomers that bind and stabilize thermally denatured malate dehydrogenase and glycogen phosphorylase b and thus prevent aggregation of the model proteins. Similar chaperone-like effects were also observed in the presence of partially purified brain extract containing besides the macrophage migration inhibitory factor a number of ubiquitous hydrophobic low molecular weight proteins identified by N-terminal microsequence analysis. Being highly stable and hydrophobic, the macrophage migration inhibitory factor in combination with other proteins of similar properties may comprise a family of constitutively expressed "small chaperones" that counteract the early onset of stress, around physiological conditions, when heat shock proteins are not abundant.

Some Insights into the Stereochemistry of Inhibition of Macrophage Migration Inhibitory Factor with 2-Fluoro-p-hydroxycinnamate and Its Analogues from Molecular Dynamics Simulations

Macrophage migration inhibitory factor (MIF) exhibits tautomerase activity on phenylpyruvate and has E-stereochemistry preference. To investigate the binding modes of its competitive inhibitors and evaluate their binding affinities, molecular dynamics simulations together with MM-PBSA (molecular mechanics Poisson-Boltzmann surface area) analysis were performed on MIF complexed with (E)-2-fluoro-p-hydroxycinnamate and five analogues. Pro-1 was discovered to form a bifurcated hydrogen bond between its protonated nitrogen and carboxylate oxygens of E-ligands and Tyr-36. No hydrogen bonds were found between Pro-1 and Z-ligands. This distinct binding characteristic of E- and Z-ligands with Pro-1 may be the main factor for the large difference in their binding affinities, which is consistent with the previous report that Pro-1 is essential for the catalytic activity of MIF. MM-PBSA analysis revealed that energy components including van der Waals, electrostatic, and hydrophobic interactions are in favor of binding, among which electrostatic interactions are predominant to the binding affinity difference.

Link between macrophage migration inhibitory factor and cellular redox regulation

Macrophage migration inhibitory factor (MIF) is an evolutionary conserved 12.5-kDa protein mediator with multiple functions in innate and acquired immunity. Upon leaderless secretion, MIF acts as a typical inflammatory cytokine, but there is no structural homology between MIF and any of the known cytokine protein families. Also, MIF is unique among cytokines in that it exhibits certain endocrine properties and has enzymatic activity. The catalytic thiol-protein oxidoreductase (TPOR) activity of MIF is mediated by a Cys-Ala-Leu-Cys active site between residues 57 and 60 that can undergo reversible intramolecular disulfide formation. Such a redox motif is typically found in TPORs of the thioredoxin (Trx) family of proteins. MIF seems to act as a disulfide reductase, and structure-function analyses of the redox site indicate that this activity is not only observed in vitro, but plays a role in cellular redox homeostasis, apoptosis inhibition, MIF-mediated monocyte/macrophage activation, and possibly the modulation of the activity of MIF-binding proteins. In this Forum review, the biochemical and biological evidence for a role of the TPOR activity for various MIF functions is summarized and discussed. In particular, the marked functional homologies with Trx proteins, the MIF redox/MHC II link, and recent attempts to discern the intra- versus extracellular roles of the MIF TPOR activity are dealt with.

Charge heterogeneity of bovine brain macrophage migration inhibitory factor

Macrophage migration inhibitory factor (MIF) is known as a ubiquitous pluripotent cytokine originally identified for its capacity to inhibit the random migration of macrophages in vitro. It is recognized as an important regulator of the immunological, neuroendocrine and enzymatic processes. MIF is widely expressed in brain, but its role in the nervous system is not yet understood. In the course of the study of the primary structure of bovine brain MIF we have previously identified a number of MIF-related proteins having identical N-terminal sequences. In this paper we report the results of isoelectric focusing of MIF isolated to a homogeneous state from bovine brain that revealed MIF charge heterogeneity. We have detected isoelectric forms of MIF with pI values of 6.9, 7.0, 7.3, and 7.8. The diverse actions of MIF within the immuno-neuroendocrine system is suggested to be a result of its occurrence in different isoforms and oligomerization states.

NMR structural characterization of the N-terminal domain of the adenylyl cyclase-associated protein (CAP) from Dictyostelium discoideum

Cyclase-associated proteins (CAPs) are highly conserved, ubiquitous actin binding proteins that are involved in microfilament reorganization. The N-termini of CAPs play a role in Ras signaling and bind adenylyl cyclase; the C-termini bind to G-actin. We report here the NMR characterization of the amino-terminal domain of CAP from Dictyostelium discoideum (CAP(1-226)). NMR data, including the steady state (1)H-(15)N heteronuclear NOE experiments, indicate that the first 50 N-terminal residues are unstructured and that this highly flexible serine-rich fragment is followed by a stable, folded core starting at Ser 51. The NMR structure of the folded core is an alpha-helix bundle composed of six antiparallel helices, in a stark contrast to the recently determined CAP C-terminal domain structure, which is solely built by beta-strands.

Internal dynamics and ionization states of the macrophage migration inhibitory factor: comparison between wild-type and mutant forms

The macrophage migration inhibitory factor (MIF) is a cytokine that shares a common structural architecture and catalytic strategy with three isomerases: 4-oxalocrotonate tautomerase, 5-carboxymethyl-2-hydroxymuconate isomerase, and D-dopachrome tautomerase. A highly conserved N-terminal proline acts as a base-acid during the proton transfer reaction catalyzed by these enzymes. Such unusual catalytic strategy appears to be possible only due to the N-terminal proline pK(a) shifted to 5.0-6.0 units. Mutations of this residue result in a significant decrease of the catalytic activity of MIF. Two hypotheses have been proposed to explain the catalytic inefficiency of MIF: the lower basicity of primary amines with regard to secondary ones and the increased flexibility resulting from the replacement of a proline by residues like glycine. To investigate that, we have performed molecular dynamics simulations of MIF wild-type and its mutant P1G, as well as calculated the protonation properties of several mutant forms. It was found that the N-terminal glycine does not show larger fluctuations compared to proline, but the former residue is more exposed to the solvent throughout the simulations. The apparent pK(a) of these residues displays very little change (as expected from the structural rigidity of MIF) and is not significantly affected by the surrounding ionizable residues. Instead, the hydrophobic character of the active site seems to be the main factor in determining the pKa of the N-terminal residue and the catalytic efficiency of MIF.

Macrophage migration inhibitory factor (MIF): mechanisms of action and role in disease

Macrophage migration inhibitory factor (MIF) is a unique cytokine and critical mediator of host defenses with a role in septic shock and chronic inflammatory and autoimmune diseases. Its mechanism of action is incompletely understood. Here, we attempt to correlate current knowledge on the molecular pathways of MIF activity with its functions in immunity and disease.

NMR 15N relaxation of the insulin-like growth factor (IGF)-binding domain of IGF binding protein-5 (IGFBP-5) determined free in solution and in complex with IGF-II

15N NMR relaxation rates of mini-IGFBP-5, an N-terminal insulin-like growth factor binding domain of the insulin-like growth factor binding protein 5 (IGFBP-5), were analysed at three field strengths using the Lipari-Szabo procedure (see below) and reduced spectral density methods. Isotropic and anisotropic Lipari-Szabo models were analysed and an analytical formula for the overall correlation time for anisotropic molecules is presented. Mini-IGFBP-5 was found to be mainly rigid on fast ps time scales except for 11 unstructured flexible residues at the C-terminus. The insulin-like growth factor binding loop in the apo-protein exhibits small amounts of flexibility on fast time scales (ps to ns) but several loop residues show significant exchange broadening. These loop residues display no exchange broadening in the complex of IGF-II/mini-IGFBP-5. The isotropic overall tumbling time in solution at 31 degrees C of mini-IGFBP-5 complexed to IGF-II is tauc = 18.4 +/- 0.2 ns indicating a strong tendency for aggregation.

Ionization state and molecular docking studies for the macrophage migration inhibitory factor: the role of lysine 32 in the catalytic mechanism

The macrophage migration inhibitory factor (MIF) is a cytokine that is structurally similar to certain isomerases and for which multiple immune and catalytic roles have been proposed. Different catalytic activities have been reported for MIF, yet the exact mechanism by which MIF acts is not completely known. As a tautomerase, the enzyme uses a general acid-base mechanism of proton transfer in which the amino-terminal proline has been shown to function as the catalytic base. We report the results of molecular docking simulations of macrophage migration inhibitory factor with three substrates, D-dopachrome, L-dopachrome methyl ester and p-(hydroxyphenyl)pyruvate. Electrostatic pK(a) predictions were also performed for the free and complexed forms of the enzyme. The predicted binding mode of p-(hydroxyphenyl)pyruvate is in agreement with the recently published X-ray structure. A model for the binding mode of D-dopachrome and L-dopachrome methyl ester to MIF is proposed which offers insights into the catalytic mechanism of D-dopachrome tautomerase activity of MIF. The proposed catalytic mechanism is further supported by the pK(a) predictions, which suggest that residue Lys32 acts as the general acid for the enzymatic catalysis of D-dopachrome.

Characterization of catalytic centre mutants of macrophage migration inhibitory factor (MIF) and comparison to Cys81Ser MIF

Macrophage migration inhibitory factor (MIF) displays both cytokine and enzyme activities, but its molecular mode of action is still unclear. MIF contains three cysteine residues and we showed recently that the conserved Cys57-Ala-Leu-Cys60 (CALC) motif is critical for the oxidoreductase and macrophage-activating activities of MIF. Here we probed further the role of this catalytic centre by expression, purification, and characterization of the cysteine-->serine mutants Cys60Ser, Cys57Ser/Cys60Ser, and Cys81Ser of human MIF and of mutants Ala58Gly/Leu59Pro and Ala58Gly/Leu59His, containing a thioredoxin (Trx)-like and protein disulphide isomerase (PDI)-like dipeptide, respectively. The catalytic centre mutants formed inclusion bodies and the resultant mutant proteins Cys57Ser/Cys60Ser, Ala58Gly/Leu59Pro, and Als58Gly/Leu59His were only soluble in organic solvent or 6 m GdmHCl when reconstituted at concentrations above 1 microgram.mL-1. This made it necessary to devise new purification methods. By contrast, mutant Cys81Ser was soluble. Effects of pH, solvent, and ionic strength conditions on the conformation of the mutants were analysed by far-UV CD spectropolarimetry and mutant stability was examined by denaturant-induced unfolding. The mutants, except for mutant Cys81Ser, showed a close conformational similarity to wild-type (wt) MIF, and stabilization of the mutants was due mainly to acid pH conditions. Intramolecular disulphide bond formation at the CALC region was confirmed by near-UV CD of mutant Cys60Ser. Mutant Cys81Ser was not involved in disulphide bond formation, yet had decreased stability. Analysis in the oxidoreductase and a MIF-specific cytokine assay revealed that only substitution of the active site residues led to inactivation of MIF. Mutant Cys60Ser had no enzyme and markedly reduced cytokine activity, whereas mutant Cys81Ser was active in both tests. The Trx-like variant showed significant enzyme activity but was less active than wtMIF; PDI-like MIF was enzymatically inactive. However, both variants had full cytokine activity. Together with the low but nonzero cytokine activity of mutant Cys60Ser, this indicated that the cytokine activity of MIF may not be tightly regulated by redox effects or that a distinguishable receptor mechanism exists. This study provides evidence for a role of the CALC motif in the oxidoreductase and cytokine activities of MIF, and suggests that Cys81 could mediate conformational effects. Availability and characterization of the mutants should greatly aid in the further elucidation of the mechanism of action of the unusual cytokine MIF.

Equilibrium and transient intermediates in folding of human macrophage migration inhibitory factor

Acid, guanidinium-Cl and urea denaturations of recombinant human macrophage migration inhibitory factor (MIF) were measured using CD and fluorimetry. The acid-induced denaturation was followed by CD at 200, 222, and 278 nm and by tryptophan fluorescence. All four probes revealed an acid-denatured state below pH 3 which resembled a typical molten globule. The pH transition is not two-state as the CD data at 222 nm deviated from all other probes. Urea and guanidinium-Cl denaturations (pH 7, 25 degrees C) both gave an apparent DeltaGU app H2O of 31 +/- 3 kJ.mol-1 when extrapolated to zero denaturant concentration. However, denaturation transitions recorded by fluorescence (at the same protein concentration) occurred at lower urea or guanidinium-Cl concentrations, consistent with an intermediate in the course of MIF denaturation. CD at 222 nm was not very sensitive to protein concentration (in 10-fold range) even though size-exclusion chromatogryphy (SEC) revealed a dimer-monomer dissociation prior to MIF unfolding. Refolding experiments were performed starting from acid, guanidinium-Cl and urea-denatured states. The kinetics were multiphasic with at least two folding intermediates. The intrinsic rate constant of the main folding phase was 5.0 +/- 0.5 s-1 (36.6 degrees C, pH 7) and its energy of activation 155 +/- 12 kJ.mol-1.

Backbone dynamics of the CDK inhibitor p19(INK4d) studied by 15N NMR relaxation experiments at two field strengths

The four members of the INK4 gene family, p16(INK4a), p15(INK4b), p18(INK4c) and p19(INK4d), are known to bind to and inhibit the closely related cyclin-dependent kinases CDK4 and CDK6 as part of the regulation of the G1/S transition in the cell division cycle. Loss of INK4 gene product function, and particularly that of p16(INK4a), is found in human cancer. 15N NMR relaxation rates of p19(INK4d) were analyzed using the reduced spectral density mapping method. Most of the backbone of p19(INK4d) exists in a well-defined structure of limited conformational flexibility on the nanosecond to picosecond time-scales. Introducing appropriate scaling to account for the effects of anisotropy, a considerable amount of exchange broadening was found for several residues throughout the sequence, especially residues in the second ankyrin repeat and in the beginnings and ends of loops connecting ankyrin repeats. A possible mode of binding between p19(INK4d) and CDK4 and CDK6 could therefore involve the loop segments of p19(INK4d). The average overall correlation time taumeff was determined to be 13.6 ns, reflecting the tendency of p19(INK4d) to aggregate.

Direct link between cytokine activity and a catalytic site for macrophage migration inhibitory factor

Macrophage migration inhibitory factor (MIF) is a secreted protein that activates macrophages, neutrophils and T cells, and is implicated in sepsis, adult respiratory distress syndrome and rheumatoid arthritis. The mechanism of MIF function, however, is unknown. The three-dimensional structure of MIF is unlike that of any other cytokine, but bears striking resemblance to three microbial enzymes, two of which possess an N-terminal proline that serves as a catalytic base. Human MIF also possesses an N-terminal proline (Pro-1) that is invariant among all known homologues. Multiple sequence alignment of these MIF homologues reveals additional invariant residues that span the entire polypeptide but are in close proximity to the N-terminal proline in the folded protein. We find that p-hydroxyphenylpyruvate, a catalytic substrate of MIF, binds to the N-terminal region and interacts with Pro-1. Mutation of Pro-1 to a glycine substantially reduces the catalytic and cytokine activity of MIF. We suggest that the underlying biological activity of MIF may be based on an enzymatic reaction. The identification of the active site should facilitate the development of structure-based inhibitors.

Macrophage migration inhibitory factor interactions with glutathione and S-hexylglutathione

Macrophage migration inhibitory factor (MIF) has been reported to interact with glutathione and S-hexylglutathione and to possess glutathione S-transferase activity. However, contrary to these reports, a recent NMR study concluded that MIF shows no affinity for glutathione. Re-examination of the glutathione-MIF interactions indicates that the reported increase in fluorescence upon addition of glutathione is because of pH-induced unfolding of the protein and not to any direct interactions. Circular dichroism shows that MIF remains folded from pH 4.5-7.5 but is 50% unfolded at pH 2.9 +/- 0.2. The reported increase in fluorescence can be achieved by acid titration. Under strongly buffered conditions, no fluorescence change is observed upon addition of glutathione. In contrast to the results with glutathione, MIF binds S-hexylglutathione with a Kd of 2.5 +/- 0.6 mM. Using NMR spectroscopy, a binding site which clusters around the N-terminal proline was identified. These data indicate that the binding site for S-hexylglutathione is the same as the catalytic site for the dopachrome tautomerase activity of MIF. Consequently, the binding of S-hexylglutathione as well as hexanethiol inhibits this catalytic activity.

Cross-linking and mutational analysis of the oligomerization state of the cytokine macrophage migration inhibitory factor (MIF)

The structure of the cytokine MIF has been investigated by X-ray crystallography, NMR, and biochemical methods with conflicting results regarding the structural and functional oligomerization state of this protein. Determination of the oligomeric state(s) is important for understanding more precisely the molecular mechanism of MIF action. To address this issue, we performed cross-linking of human and mouse MIF and selected mutants by various methods and analyzed the oligomerization by SDS-PAGE and gel filtration. MIF was found to form a mixture of monomeric, dimeric, and trimeric states at physiological concentrations, with the monomer and dimer representing the major species. Similar results were obtained when the carboxy-truncated mutants MIF(1-104) and MIF(1-109) were examined, indicating that the C-terminus of MIF is not critical for trimer stabilization. Cross-linking analysis of the isosteric Cys --> Ser mutants C56S and C80S of human MIF resulted in a similar oligomer distribution, whereas substitution of Cys59 led to a significant reduction in the dimeric and trimeric forms, indicating that the hydrophobic region around Cys59 is important for the oligomerization of MIF. Together, our data argue that physiological MIF solutions contain a mixture of monomers, dimers, and trimers.

Structure activity studies of the cytokine macrophage migration inhibitory factor (MIF) reveal a critical role for its carboxy terminus

Carboxy-truncated mutants of human MIF (MIF(1-104) and MIF(1-109)) were used in structure activity studies. CD spectroscopy revealed an overall structural similarity between the mutants and MIF. Denaturant-induced unfolding demonstrated that the C-terminus contributed significantly to the conformational stability of MIF. This appears to be due to the formation of two C-terminal beta-strands. The mutants were enzymatically active, exhibiting half of the enzymatic redox activity of MIF. However, immunological analysis showed that deletion of both 5 and 10 C-terminal residues resulted in loss of the macrophage activating properties of MIF, providing functional evidence that the C-terminus is important for immunological activity and trimer formation. A more detailed study of the C-terminus may assist in identifying the molecular basis for the immunological and enzymatic activities of MIF.