CaMKK2 is inactivated by cAMP-PKA signaling and 14-3-3 adaptor proteins.

The calcium-calmodulin-dependent protein kinase kinase-2 (CaMKK2) is a key regulator of cellular and whole-body energy metabolism. It is known to be activated by increases in intracellular Ca2+, but the mechanisms by which it is inactivated are less clear. CaMKK2 inhibition protects against prostate cancer, hepatocellular carcinoma, and metabolic derangements induced by a high-fat diet; therefore, elucidating the intracellular mechanisms that inactivate CaMKK2 has important therapeutic implications. Here we show that stimulation of cAMP-dependent protein kinase A (PKA) signaling in cells inactivates CaMKK2 by phosphorylation of three conserved serine residues. PKA-dependent phosphorylation of Ser495 directly impairs calcium-calmodulin activation, whereas phosphorylation of Ser100 and Ser511 mediate recruitment of 14-3-3 adaptor proteins that hold CaMKK2 in the inactivated state by preventing dephosphorylation of phospho-Ser495 We also report the crystal structure of 14-3-3ζ bound to a synthetic diphosphorylated peptide that reveals how the canonical (Ser511) and noncanonical (Ser100) 14-3-3 consensus sites on CaMKK2 cooperate to bind 14-3-3 proteins. Our findings provide detailed molecular insights into how cAMP-PKA signaling inactivates CaMKK2 and reveals a pathway to inhibit CaMKK2 with potential for treating human diseases.

The GM-CSF/IL-3/IL-5 cytokine receptor family: from ligand recognition to initiation of signaling.

Granulocyte-macrophage colony-stimulating factor (GM-CSF), interleukin-3 (IL-3), and IL-5 are members of a discrete family of cytokines that regulates the growth, differentiation, migration and effector function activities of many hematopoietic cells and immunocytes. These cytokines are involved in normal responses to infectious agents, bridging innate and adaptive immunity. However, in certain cases, the overexpression of these cytokines or their receptors can lead to excessive or aberrant initiation of signaling resulting in pathological conditions, with chronic inflammatory diseases and myeloid leukemias the most notable examples. Recent crystal structures of the GM-CSF receptor ternary complex and the IL-5 binary complex have revealed new paradigms of cytokine receptor activation. Together with a wealth of associated structure-function studies, they have significantly enhanced our understanding of how these receptors recognize cytokines and initiate signals across cell membranes. Importantly, these structures provide opportunities for structure-based approaches for the discovery of novel and disease-specific therapeutics. In addition, recent biochemical evidence has suggested that the GM-CSF/IL-3/IL-5 receptor family is capable of interacting productively with other membrane proteins at the cell surface. Such interactions may afford additional or unique biological activities and might be harnessed for selective modulation of the function of these receptors in disease.

The ßc receptor family - Structural insights and their functional implications.

Granulocyte-macrophage colony-stimulating factor (GM-CSF), interleukin-3 (IL-3) and IL-5 are members of a small family of cytokines that share a beta receptor subunit (ßc). These cytokines regulate the growth, differentiation, migration and effector function activities of many hematopoietic cells in bone marrow, blood and sites of inflammation. Excessive or aberrant signaling can result in chronic inflammatory conditions and myeloid leukemias. The crystal structures of the GM-CSF ternary complex, the IL-5 binary complex and the very recent IL-3 receptor alpha subunit build upon decades of structure-function studies, giving new insights into cytokine-receptor specificity and signal transduction. Selective modulation of receptor function is now a real possibility and the structures of the ßc receptor family are being used to discover novel and disease-specific therapeutics.

Distinct Assemblies of Heterodimeric Cytokine Receptors Govern Stemness Programs in Leukemia.

Leukemia stem cells (LSC) possess distinct self-renewal and arrested differentiation properties that are responsible for disease emergence, therapy failure, and recurrence in acute myeloid leukemia (AML). Despite AML displaying extensive biological and clinical heterogeneity, LSC with high interleukin-3 receptor (IL3R) levels are a constant yet puzzling feature, as this receptor lacks tyrosine kinase activity. Here, we show that the heterodimeric IL3Rα/ßc receptor assembles into hexamers and dodecamers through a unique interface in the 3D structure, where high IL3Rα/ßc ratios bias hexamer formation. Importantly, receptor stoichiometry is clinically relevant as it varies across the individual cells in the AML hierarchy, in which high IL3Rα/ßc ratios in LSCs drive hexamer-mediated stemness programs and poor patient survival, while low ratios mediate differentiation. Our study establishes a new paradigm in which alternative cytokine receptor stoichiometries differentially regulate cell fate, a signaling mechanism that may be generalizable to other transformed cellular hierarchies and of potential therapeutic significance. SIGNIFICANCE: Stemness is a hallmark of many cancers and is largely responsible for disease emergence, progression, and relapse. Our finding that clinically significant stemness programs in AML are directly regulated by different stoichiometries of cytokine receptors represents a hitherto unexplained mechanism underlying cell-fate decisions in cancer stem cell hierarchies. This article is highlighted in the In This Issue feature, p. 1749.

Structure of rat aldose reductase-like protein AKR1B14 holoenzyme: Probing the role of His269 in coenzyme binding by site-directed mutagenesis.

Rat aldose reductase-like protein (AKR1B14) is the ortholog of mouse vas deferens protein (AKR1B7) playing roles in detoxification of reactive aldehydes and synthesis of prostaglandin F(2α). The crystal structure of the binary complex (AKR1B14-NADPH) was determined at 1.86Å resolution, and showed that the adenine ring and the 2'-phosphate group of the coenzyme formed π-stacking and electrostatic interactions with the imidazole ring and ND1 atom, respectively, of His269, which is not conserved in other aldose reductase-like proteins. The interactions were supported by site-directed mutagenesis of His269 to Arg, Phe and Met, which increased the K(m) for NADPH by 4, 7 and 127-fold, respectively. This is the first report of the tertiary structure of a rodent AKR1B7 ortholog, which describes the role of a novel dual interaction for the non-conserved His269 in coenzyme binding.

Probing the inhibitor selectivity pocket of human 20α-hydroxysteroid dehydrogenase (AKR1C1) with X-ray crystallography and site-directed mutagenesis.

Human 20α-hydroxysteroid dehydrogenase (AKR1C1) is an important drug target due to its role in the development of lung and endometrial cancers, premature birth and neuronal disorders. We report the crystal structure of AKR1C1 complexed with the first structure-based designed inhibitor 3-chloro-5-phenylsalicylic acid (K(i)=0.86 nM) bound in the active site. The binding of 3-chloro-5-phenylsalicylic acid to AKR1C1 resulted in a conformational change in the side chain of Phe311 to accommodate the bulky phenyl ring substituent at the 5-position of the inhibitor. The contributions of the nonconserved residues Leu54, Leu306, Leu308 and Phe311 to the binding were further investigated by site-directed mutagenesis, and the effects of the mutations on the K(i) value were determined. The Leu54Val and Leu306Ala mutations resulted in 6- and 81-fold increases, respectively, in K(i) values compared to the wild-type enzyme, while the remaining mutations had little or no effects.

Studies on a Tyr residue critical for the binding of coenzyme and substrate in mouse 3(17)alpha-hydroxysteroid dehydrogenase (AKR1C21): structure of the Y224D mutant enzyme.

Mouse 3(17)alpha-hydroxysteroid dehydrogenase (AKR1C21) is the only aldo-keto reductase that catalyzes the stereospecific reduction of 3- and 17-ketosteroids to the corresponding 3(17)alpha-hydroxysteroids. The Y224D mutation of AKR1C21 reduced the K(m) value for NADP(H) by up to 80-fold and completely reversed the 17alpha stereospecificity of the enzyme. The crystal structure of the Y224D mutant at 2.3 A resolution revealed that the mutation resulted in a change in the conformation of the flexible loop B, including the V-shaped groove, which is a unique feature of the active-site architecture of wild-type AKR1C21 and is formed by the side chains of Tyr224 and Trp227. Furthermore, mutations (Y224F and Q222N) of residues involved in forming the safety belt for binding of the coenzyme showed similar alterations in kinetic constants for 3alpha-hydroxy/3-ketosteroids and 17-hydroxy/ketosteroids compared with the wild type.

Factorizing the role of a critical leucine residue in the binding of substrate to human 20alpha-hydroxysteroid dehydrogenase (AKR1C1): molecular modeling and kinetic studies of the Leu308Val mutant enzyme.

A comparison of the structures and kinetic properties of human 20alpha-hydroxysteroid dehydrogenase (AKR1C1) and its mutant enzymes (Leu308Val and Leu308Ala) indicates that Leu308 is a selectivity determinant for substrate binding. While the Leu308Val mutation improved the catalytic efficiency (k(cat)/K(m)) of AKR1C1 towards the two substrates 5alpha-pregnane-3alpha,20alpha-diol (PregA) and 5beta-pregnan-3alpha-ol-20-one (PregB), the Leu308Ala mutation rendered the enzyme inactive. In the docked model of PregA the conformation of the steroid molecule was similar to that of 20alpha-hydroxyprogesterone in the crystal structure of the AKR1C1 complex where the steroid did not interact with the catalytic residues Tyr55 and His117. In the case of PregB the steroid interacted with the catalytic residue His117 and formed close contacts with Leu308, suggesting that the binding mechanism of 3alpha-hydroxysteroids in the active site of AKR1C1 is different from that of 20alpha-hydroxysteroids.

Structure of the G225P/G226P mutant of mouse 3(17)alpha-hydroxysteroid dehydrogenase (AKR1C21) ternary complex: implications for the binding of inhibitor and substrate.

3(17)alpha-Hydroxysteroid dehydrogenase (AKR1C21) is a unique member of the aldo-keto reductase (AKR) superfamily owing to its ability to reduce 17-ketosteroids to 17alpha-hydroxysteroids, as opposed to other members of the AKR family, which can only produce 17beta-hydroxysteroids. In this paper, the crystal structure of a double mutant (G225P/G226P) of AKR1C21 in complex with the coenzyme NADP(+) and the inhibitor hexoestrol refined at 2.1 A resolution is presented. Kinetic analysis and molecular-modelling studies of 17alpha- and 17beta-hydroxysteroid substrates in the active site of AKR1C21 suggested that Gly225 and Gly226 play an important role in determining the substrate stereospecificity of the enzyme. Additionally, the G225P/G226P mutation of the enzyme reduced the affinity (K(m)) for both 3alpha- and 17alpha-hydroxysteroid substrates by up to 160-fold, indicating that these residues are critical for the binding of substrates.

Molecular determinants for the stereospecific reduction of 3-ketosteroids and reactivity towards all-trans-retinal of a short-chain dehydrogenase/reductase (DHRS4).

DHRS4, a member of the short-chain dehydrogenase/reductase superfamily, reduces all-trans-retinal and xenobiotic carbonyl compounds. Human DHRS4 differs from other animal enzymes in kinetic constants for the substrates, particularly in its low reactivity to retinoids. We have found that pig, rabbit and dog DHRS4s reduce benzil and 3-ketosteroids into S-benzoin and 3alpha-hydroxysteroids, respectively, in contrast to the stereoselectivity of human DHRS4 which produces R-benzoin and 3beta-hydroxysteroids. Among substrate-binding residues predicted from the crystal structure of pig DHRS4, F158 and L161 in the animal DHRS4 are serine and phenylalanine, respectively, in the human enzyme. Double mutation (F158S/L161F) of pig DHRS4 led to an effective switch of its substrate affinity and stereochemistry into those similar to human DHRS4. The roles of the two residues in determining the stereospecificity in 3-ketosteroid reduction were confirmed by reverse mutation (S158F/F161L) in the human enzyme. The stereochemical control was evaluated by comparison of the 3D models of pig wild-type and mutant DHRS4s with the modeled substrates. Additional mutation of T177N into the human S158F/F161L mutant resulted in almost complete kinetic conversion into a pig DHRS4-type form, suggesting a role of N177 in forming the substrate-binding cavity through an intersubunit interaction in pig and other animal DHRS4s, and explaining why the human enzyme shows low reactivity towards retinoids.

Author Correction: EPO does not promote interaction between the erythropoietin and beta-common receptors.

A correction to this article has been published and is linked from the HTML and PDF versions of this paper. The error has not been fixed in the paper.

Inhibition of 3(17)alpha-hydroxysteroid dehydrogenase (AKR1C21) by aldose reductase inhibitors.

Mouse 3(17)alpha-hydroxysteroid dehydrogenase (AKR1C21) is a member of the aldo-keto reductase superfamily that catalyses the oxido-reduction of steroid hormones such as estrogens, androgens and neurosteroids. Inhibitors of aldose reductase (AR), a member of the same superfamily, were evaluated against AKR1C21. Models of the enzyme-inhibitor complexes suggest that Tyr118 and Phe311 are important residues for inhibitor recognition and orientation in the active site of AKR1C21.

EPO does not promote interaction between the erythropoietin and beta-common receptors.

A direct interaction between the erythropoietin (EPOR) and the beta-common (ßc) receptors to form an Innate Repair Receptor (IRR) is controversial. On one hand, studies have shown a functional link between EPOR and ßc receptor in tissue protection while others have shown no involvement of the ßc receptor in tissue repair. To date there is no biophysical evidence to confirm a direct association of the two receptors either in vitro or in vivo. We investigated the existence of an interaction between the extracellular regions of EPOR and the ßc receptor in silico and in vitro (either in the presence or absence of EPO or EPO-derived peptide ARA290). Although a possible interaction between EPOR and ßc was suggested by our computational and genomic studies, our in vitro biophysical analysis demonstrates that the extracellular regions of the two receptors do not specifically associate. We also explored the involvement of the ßc receptor gene (Csf2rb) under anaemic stress conditions and found no requirement for the ßc receptor in mice. In light of these studies, we conclude that the extracellular regions of the EPOR and the ßc receptor do not directly interact and that the IRR is not involved in anaemic stress.

The mechanism of GM-CSF inhibition by human GM-CSF auto-antibodies suggests novel therapeutic opportunities.

Granulocyte-macrophage colony-stimulating factor (GM-CSF) is a hematopoietic growth factor that can stimulate a variety of cells, but its overexpression leads to excessive production and activation of granulocytes and macrophages with many pathogenic effects. This cytokine is a therapeutic target in inflammatory diseases, and several anti-GM-CSF antibodies have advanced to Phase 2 clinical trials in patients with such diseases, e.g., rheumatoid arthritis. GM-CSF is also an essential factor in preventing pulmonary alveolar proteinosis (PAP), a disease associated with GM-CSF malfunction arising most typically through the presence of GM-CSF neutralizing auto-antibodies. Understanding the mechanism of action for neutralizing antibodies that target GM-CSF is important for improving their specificity and affinity as therapeutics and, conversely, in devising strategies to reduce the effects of GM-CSF auto-antibodies in PAP. We have solved the crystal structures of human GM-CSF bound to antigen-binding fragments of two neutralizing antibodies, the human auto-antibody F1 and the mouse monoclonal antibody 4D4. Coordinates and structure factors of the crystal structures of the GM-CSF:F1 Fab and the GM-CSF:4D4 Fab complexes have been deposited in the RCSB Protein Data Bank under the accession numbers 6BFQ and 6BFS, respectively. The structures show that these antibodies bind to mutually exclusive epitopes on GM-CSF; however, both prevent the cytokine from interacting with its alpha receptor subunit and hence prevent receptor activation. Importantly, identification of the F1 epitope together with functional analyses highlighted modifications to GM-CSF that would abolish auto-antibody recognition whilst retaining GM-CSF function. These results provide a framework for developing novel GM-CSF molecules for PAP treatment and for optimizing current anti-GM-CSF antibodies for use in treating inflammatory disorders.

A dual role for the N-terminal domain of the IL-3 receptor in cell signalling.

The interleukin-3 (IL-3) receptor is a cell-surface heterodimer that links the haemopoietic, vascular and immune systems and is overexpressed in acute and chronic myeloid leukaemia progenitor cells. It belongs to the type I cytokine receptor family in which the α-subunits consist of two fibronectin III-like domains that bind cytokine, and a third, evolutionarily unrelated and topologically conserved, N-terminal domain (NTD) with unknown function. Here we show by crystallography that, while the NTD of IL3Rα is highly mobile in the presence of IL-3, it becomes surprisingly rigid in the presence of IL-3 K116W. Mutagenesis, biochemical and functional studies show that the NTD of IL3Rα regulates IL-3 binding and signalling and reveal an unexpected role in preventing spontaneous receptor dimerisation. Our work identifies a dual role for the NTD in this cytokine receptor family, protecting against inappropriate signalling and dynamically regulating cytokine receptor binding and function.

Role of the ß Common (ßc) Family of Cytokines in Health and Disease.

The ß common ([ßc]/CD131) family of cytokines comprises granulocyte macrophage colony-stimulating factor (GM-CSF), interleukin (IL)-3, and IL-5, all of which use ßc as their key signaling receptor subunit. This is a prototypic signaling subunit-sharing cytokine family that has unveiled many biological paradigms and structural principles applicable to the IL-2, IL-4, and IL-6 receptor families, all of which also share one or more signaling subunits. Originally identified for their functions in the hematopoietic system, the ßc cytokines are now known to be truly pleiotropic, impacting on multiple cell types, organs, and biological systems, and thereby controlling the balance between health and disease. This review will focus on the emerging biological roles for the ßc cytokines, our progress toward understanding the mechanisms of receptor assembly and signaling, and the application of this knowledge to develop exciting new therapeutic approaches against human disease.

Structure of 3(17)alpha-hydroxysteroid dehydrogenase (AKR1C21) holoenzyme from an orthorhombic crystal form: an insight into the bifunctionality of the enzyme.

Mouse 3(17)alpha-hydroxysteroid dehydrogenase (AKR1C21) is a bifunctional enzyme that catalyses the oxidoreduction of the 3- and 17-hydroxy/keto groups of steroid substrates such as oestrogens, androgens and neurosteroids. The structure of the AKR1C21-NADPH binary complex was determined from an orthorhombic crystal belonging to space group P2(1)2(1)2(1) at a resolution of 1.8 A. In order to identify the factors responsible for the bifunctionality of AKR1C21, three steroid substrates including a 17-keto steroid, a 3-keto steroid and a 3alpha-hydroxysteroid were docked into the substrate-binding cavity. Models of the enzyme-coenzyme-substrate complexes suggest that Lys31, Gly225 and Gly226 are important for ligand recognition and orientation in the active site.

A salicylic acid-based analogue discovered from virtual screening as a potent inhibitor of human 20alpha-hydroxysteroid dehydrogenase.

20alpha-hydroxysteroid dehydrogenase (AKR1C1) plays a key role in the metabolism of progesterone and other steroid hormones, thereby regulating their action at the pre-receptor level. AKR1C1 is implicated in neurological and psychiatric conditions such as catamenial epilepsy and depressive disorders. Increased activity of AKR1C1 is associated with termination of pregnancy and the development of breast cancer, endometriosis and endometrial cancer. Inhibition of the undesired activity of AKR1C1 will help reduce risks of premature birth, neurological disorders and the development of cancer. In order to identify potential leads for new inhibitors of AKR1C1 we adopted a virtual screening-based approach using the automated DOCK program. Approximately 250,000 compounds from the NCI database were screened for potential ligands based on their chemical complementarity and steric fit within the active site of AKR1C1. Kinetic analysis revealed 3,5-diiodosalicylic acid, an analogue of salicylic acid, as a potent competitive inhibitor with respect to the substrate 5beta-pregnane-3alpha,20alpha-diol with a K(i) of 9 nM. Aspirin, which is a well known salicylic acid-based drug, was also found to inhibit AKR1C1 activity. This is the first report to show aspirin (IC(50)=21 microM) and its metabolite salicylic acid (IC(50)=7.8 microM) as inhibitors of AKR1C1.

Conformational Changes in the GM-CSF Receptor Suggest a Molecular Mechanism for Affinity Conversion and Receptor Signaling.

The GM-CSF, IL-3, and IL-5 receptors constitute the ßc family, playing important roles in inflammation, autoimmunity, and cancer. Typical of heterodimeric type I cytokine receptors, signaling requires recruitment of the shared subunit to the initial cytokine:α subunit binary complex through an affinity conversion mechanism. This critical process is poorly understood due to the paucity of crystal structures of both binary and ternary receptor complexes for the same cytokine. We have now solved the structure of the binary GM-CSF:GMRα complex at 2.8-Å resolution and compared it with the structure of the ternary complex, revealing distinct conformational changes. Guided by these differences we performed mutational and functional studies that, importantly, show GMRα interactions playing a major role in receptor signaling while ßc interactions control high-affinity binding. These results support the notion that conformational changes underlie the mechanism of GM-CSF receptor activation and also suggest how related type I cytokine receptors signal.

CSL311, a novel, potent, therapeutic monoclonal antibody for the treatment of diseases mediated by the common ß chain of the IL-3, GM-CSF and IL-5 receptors.

The ß common-signaling cytokines interleukin (IL)-3, granulocyte-macrophage colony stimulating factor (GM-CSF) and IL-5 stimulate pro-inflammatory activities of haematopoietic cells via a receptor complex incorporating cytokine-specific α and shared ß common (ßc, CD131) receptor. Evidence from animal models and recent clinical trials demonstrate that these cytokines are critical mediators of the pathogenesis of inflammatory airway disease such as asthma. However, no therapeutic agents, other than steroids, that specifically and effectively target inflammation mediated by all 3 of these cytokines exist. We employed phage display technology to identify and optimize a novel, human monoclonal antibody (CSL311) that binds to a unique epitope that is specific to the cytokine-binding site of the human ßc receptor. The binding epitope of CSL311 on the ßc receptor was defined by X-ray crystallography and site-directed mutagenesis. CSL311 has picomolar binding affinity for the human ßc receptor, and at therapeutic concentrations is a highly potent antagonist of the combined activities of IL-3, GM-CSF and IL-5 on primary eosinophil survival in vitro. Importantly, CSL311 inhibited the survival of inflammatory cells present in induced sputum from human allergic asthmatic subjects undergoing allergen bronchoprovocation. Due to its high potency and ability to simultaneously suppress the activity of all 3 ß common cytokines, CSL311 may provide a new strategy for the treatment of chronic inflammatory diseases where the human ßc receptor is central to pathogenesis. The coordinates for the ßc/CSL311 Fab complex structure have been deposited with the RCSB Protein Data Bank (PDB 5DWU).