Identification of inhibitors of PvdQ, an enzyme involved in the synthesis of the siderophore pyoverdine.

Pseudomonas aeruginosa produces the peptide siderophore pyoverdine, which is used to acquire essential Fe(3+) ions from the environment. PvdQ, an Ntn hydrolase, is required for the biosynthesis of pyoverdine. PvdQ knockout strains are not infectious in model systems, suggesting that disruption of siderophore production via PvdQ inhibition could be exploited as a target for novel antibacterial agents, by preventing cells from acquiring iron in the low iron environments of most biological settings. We have previously described a high-throughput screen to identify inhibitors of PvdQ that identified inhibitors with IC50 values of ∼100 µM. Here, we describe the discovery of ML318, a biaryl nitrile inhibitor of PvdQ acylase. ML318 inhibits PvdQ in vitro (IC50 = 20 nM) by binding in the acyl-binding site, as confirmed by the X-ray crystal structure of PvdQ bound to ML318. Additionally, the PvdQ inhibitor is active in a whole cell assay, preventing pyoverdine production and limiting the growth of P. aeruginosa under iron-limiting conditions.

Discovery of small-molecule enhancers of reactive oxygen species that are nontoxic or cause genotype-selective cell death.

Elevation of reactive oxygen species (ROS) levels has been observed in many cancer cells relative to nontransformed cells, and recent reports have suggested that small-molecule enhancers of ROS may selectively kill cancer cells in various in vitro and in vivo models. We used a high-throughput screening approach to identify several hundred small-molecule enhancers of ROS in a human osteosarcoma cell line. A minority of these compounds diminished the viability of cancer cell lines, indicating that ROS elevation by small molecules is insufficient to induce death of cancer cell lines. Three chemical probes (BRD5459, BRD56491, BRD9092) are highlighted that most strongly elevate markers of oxidative stress without causing cell death and may be of use in a variety of cellular settings. For example, combining nontoxic ROS-enhancing probes with nontoxic doses of L-buthionine sulfoximine, an inhibitor of glutathione synthesis previously studied in cancer patients, led to potent cell death in more than 20 cases, suggesting that even nontoxic ROS-enhancing treatments may warrant exploration in combination strategies. Additionally, a few ROS-enhancing compounds that contain sites of electrophilicity, including piperlongumine, show selective toxicity for transformed cells over nontransformed cells in an engineered cell-line model of tumorigenesis. These studies suggest that cancer cell lines are more resilient to chemically induced increases in ROS levels than previously thought and highlight electrophilicity as a property that may be more closely associated with cancer-selective cell death than ROS elevation.

Discovery of a new molecular probe ML228: an activator of the hypoxia inducible factor (HIF) pathway.

Hypoxia and ischemia are linked to several serious public health problems that affect most major organ systems. Specific examples include diseases of the cardiovascular, pulmonary, renal, neurologic, and musculoskeletal systems. The most significant pathway for cellular response to hypoxia is the hypoxia inducible factor (HIF) pathway. HIFs are transcription factors responsible for the activation of genes which encode proteins that mediate adaptive responses to reduced oxygen availability. A high-throughput cell-based HIF-mediated gene reporter screen was carried out using the NIH's Molecular Libraries Small Molecule Repository to identify activators of the HIF pathway. This communication describes the subsequent medicinal chemistry optimization of a triazine scaffold that led to the identification of the new molecular probe ML228. A discussion of HIF activation SAR within this chemotype as well as detailed in vitro characterization of the probe molecule is presented here.

Inhibition of the Unfolded Protein Response by metformin in renal proximal tubular epithelial cells.

Metformin (Met), an AMP-activated protein kinase (AMPK) inducer, is primarily transported by organic cation transporters expressed at the surface of renal proximal tubular epithelial cells. However, the implication of Met in renal function remains poorly understood. Interestingly, AICAR, another AMPK inducer, has been shown to inhibit the Unfolded Protein Response (UPR) generated by tunicamycin in cardiomyocytes in an AMPK-kinase dependent fashion suggesting metformin may also block the UPR. In this work, we have examined the effect of metformin on the expression of UPR-related markers (GRP94 and CHOP) induced by glucosamine (GlcN), 2-deoxyglucose (2-DOG) and tunicamycin (TUNI) in renal proximal tubular epithelial cells and in murine mesangial cells. Met attenuated GRP94 and CHOP expression induced by GlcN and 2-DOG, but not TUNI only in renal epithelial cells, even though the AMPK activation was observed in both renal epithelial and mesangial cells. Met did not require the contribution of its AMPK kinase inducing activity to block UPR markers expression. This report has identified a novel inhibitory function of metformin on UPR, which may have a beneficial impact on kidney homeostatic function.

Cell stress proteins: novel immunotherapeutics.

Heat shock proteins (HSPs) play important roles in the immune system as carriers of tumour antigens and inflammatory agents. The HSPs are abundantly expressed stress proteins intrinsic to all cellular life, permitting proteins to carry out essential enzymic, signalling and structural functions within the tightly crowded milieu of the cell. To carry out these tasks, HSPs are equipped with a domain that binds unstructured sequences in polypeptides and a N-terminal ATPase domain that controls the opening and closing of the peptide-binding domain. HSPs can, using these domains, capture antigens processed by partial proteolysis in the cytoplasm of cancer cells. HSP/peptide complexes formed in the cytoplasm can then be secreted to take part in immune surveillance. Extracellular Hsp70 interacts with receptors on antigen presenting cells (APCs) either during episodes of cell death and lysis in vivo or during vaccination. A number of candidate receptors for Hsp70 on APCs have been proposed to take part in the antitumour immune function including the alpha2 macroglobulin receptor CD91, Toll-like receptors, the signalling receptor CD40 and a number of scavenger receptors. Finally, Hsp70 complexes are able to deliver antigens to MHC class I and II molecules on the APC cell surface and lead to the presentation of tumour antigens to T lymphocytes. HSP-antigen complexes have proven effective in the treatment of rodent tumours in preclinical studies and are now undergoing clinical trials for treatment of human cancer.

Extracellular heat shock proteins in cell signaling.

Extracellular stress proteins including heat shock proteins (Hsp) and glucose regulated proteins (Grp) are emerging as important mediators of intercellular signaling and transport. Release of such proteins from cells is triggered by physical trauma and behavioral stress as well as exposure to immunological "danger signals". Stress protein release occurs both through physiological secretion mechanisms and during cell death by necrosis. After release into the extracellular fluid, Hsp or Grp may then bind to the surfaces of adjacent cells and initiate signal transduction cascades as well as the transport of cargo molecules such as antigenic peptides. In addition Hsp60 and hsp70 are able to enter the bloodstream and may possess the ability to act at distant sites in the body. Many of the effects of extracellular stress proteins are mediated through cell surface receptors. Such receptors include Toll Like Receptors 2 and 4, CD40, CD91, CCR5 and members of the scavenger receptor family such as LOX-1 and SREC-1. The possession of a wide range of receptors for the Hsp and Grp family permits binding to a diverse range of cells and the performance of complex multicellular functions particularly in immune cells and neurones.

Role of scavenger receptors in the binding and internalization of heat shock protein 70.

Extracellular heat shock protein 70 (Hsp70) exerts profound effects both in mediating tumor rejection by Hsp70-based vaccines and in autoimmunity. Further progress in this area, however, awaits the identification of the cell surface receptors for extracellular Hsp70 that mediate its immune functions. We have examined a wide range of candidate Hsp70 receptors and find significant binding through two main families of cell surface proteins, including 1) the scavenger receptor (SR) family and 2) C-type lectins of the NK family. In addition, given that the anticancer effects of Hsp70 vaccines have been shown to involve uptake of Ags by APC exposed to Hsp70-tumor Ag complexes, we have examined the ability of the receptors identified here to internalize Hsp70-peptide complexes. Our findings indicate that three members of the SR family (lectin-like oxidized low density lipoprotein receptor 1; fasciclin, epidermal growth factor-like, laminin-type epidermal growth factor-like, and link domain-containing scavenger receptor-1; and SR expressed by endothelial cells-1) are able to bind Hsp70-peptide complexes and mediate its efficient internalization. Indeed, each of the SR was able to mediate efficient uptake of Hsp70 when transfected into Chinese hamster ovary cells previously null for uptake. Curiously, Hsp70 internalization occurs independently of the intracellular domains of the SR, and Hsp70 uptake could be detected when the entire intracellular domain of lectin-like oxidized low density lipoprotein receptor 1 or SR expressed by endothelial cells-1 was truncated. The existence of a wide repertoire of cell surface Hsp70-binding structures may permit intracellular responses to extracellular Hsp70 that are cell specific and discriminate between Hsp70 family members.

How is the immune response affected by hyperthermia and heat shock proteins?

There is growing body of evidence linking the cellular response to heat stress with the response of the immune system to cancer. The anti-tumor immune response can be markedly enhanced by treatment with hyperthermia particularly in the fever range. In addition, the heat shock proteins (hsp) which are produced in abundant quantities in cells exposed to heat are potent immune modulators and can lead to stimulation of both the innate and adaptive immune responses to tumors. Immunostimulation by hyperthermia involves both direct effects of heat on the behavior of immune cells as well as indirect effects mediated through hsp release. In addition, the hsp can be deployed as components of antitumor vaccines in protocols that do not include hyperthermia. Understanding these process may permit the effective deployment of hyperthermia and hsp based vaccines in tumor treatment.

Message in a bottle: role of the 70-kDa heat shock protein family in anti-tumor immunity.

Extracellular heat shock protein 70 (HSP70) is a potent agent for tumor immunotherapy, which can break tolerance to tumor-associated antigens and cause specific tumor cell killing by cytotoxic CD8+ T cells. The pro-immune effects of extracellular HSP70 are, to some extent, extensions of its molecular properties as an intracellular stress protein. The HSP70 are characterized by massive inducibility after stress, preventing cell death by inhibiting aggregation of cell proteins and directly antagonizing multiple cell death pathways. HSP70 family members possess a domain in the C terminus that chaperones unfolded proteins and peptides, and a N-terminal ATPase domain that controls the opening and closing of the peptide binding domain. These properties not only enable intracellular HSP70 to inhibit tumor apoptosis, but also promote formation of stable complexes with cytoplasmic tumor antigens that can then escape intact from dying cells to interact with antigen-processing cells (APC) and stimulate anti-tumor immunity. HSP70 may be released from tumors undergoing therapy at high local extracellular concentrations, and send a danger signal to the host leading to APC activation. Extracellular HSP70 bind to high-affinity receptors on APC, leading to activation of maturation and re-presentation of the peptide antigen cargo of HSP70 by the APC. The ability of HSP70-peptide complexes (HSP70-PC) to break tolerance and cause tumor regression employs these dual properties as signaling ligand and antigen transporter. HSP70-PC thus coordinately activate innate immune responses and deliver antigens for re-presentation by MHC class I and II molecules on the APC cell surface, leading to specific anti-tumor immunity.

Structural instability caused by a mutation at a conserved arginine in the alpha-crystallin domain of Chinese hamster heat shock protein 27.

Mutations in the alpha-crystallin domain of 4 of the small heat shock proteins (sHsp) (Hsp27/HspB1, alphaA-crystallin/ HspB4, alphaB-crystallin/HspB5, and HspB8) are responsible for dominant inherited diseases in humans. One such mutation at a highly conserved arginine residue was shown to cause major conformational defects and intracellular aggregation of alphaA- and alphaB-crystallins and HspB8. Here, we studied the effect of this Arg mutation on the structure and function of Hsp27. Chinese hamster Hsp27 with Arg148 replaced by Gly (Hsp27R148G) formed dimers in vitro and in vivo, which contrasted with the 12- or 24-subunit oligomers formed by the wild-type protein (Hsp27WT). Despite these alterations, Hsp27R148G had a chaperone activity almost as high as Hsp27WT. The dimers of Hsp27R148G did not further deoligomerize on phosphorylation and like the dimers formed by phosphorylated Hsp27WT were not affected by the deletion of the N-terminal WD/EPF (single letter amino acid code) motif, suggesting that mutation of Arg148, deletion of the N-terminal WD/EPF motif, and phosphorylation of Ser90 may produce similar structural perturbations. Nevertheless, the structure of Hsp27R148G appeared unstable, and the mutated protein accumulated as aggregates in many cells. Both a lower basal level of phosphorylation of Hsp27R148G and the coexpression of Hsp27WT could reduce the frequency of formation of these aggregates, suggesting possible mechanisms regulating the onset of the sHsp-mediated inherited diseases.

Expression of heat shock proteins and heat shock protein messenger ribonucleic acid in human prostate carcinoma in vitro and in tumors in vivo.

Heat shock proteins (HSPs) are thought to play a role in the development of cancer and to modulate tumor response to cytotoxic therapy. In this study, we have examined the expression of hsf and HSP genes in normal human prostate epithelial cells and a range of prostate carcinoma cell lines derived from human tumors. We have observed elevated expressions of HSF1, HSP60, and HSP70 in the aggressively malignant cell lines PC-3, DU-145, and CA-HPV-10. Elevated HSP expression in cancer cell lines appeared to be regulated at the post-messenger ribonucleic acid (mRNA) levels, as indicated by gene chip microarray studies, which indicated little difference in heat shock factor (HSF) or HSP mRNA expression between the normal and malignant prostate cell lines. When we compared the expression patterns of constitutive HSP genes between PC-3 prostate carcinoma cells growing as monolayers in vitro and as tumor xenografts growing in nude mice in vivo, we found a marked reduction in expression of a wide spectrum of the HSPs in PC-3 tumors. This decreased HSP expression pattern in tumors may underlie the increased sensitivity to heat shock of PC-3 tumors. However, the induction by heat shock of HSP genes was not markedly altered by growth in the tumor microenvironment, and HSP40, HSP70, and HSP110 were expressed abundantly after stress in each growth condition. Our experiments indicate therefore that HSF and HSP levels are elevated in the more highly malignant prostate carcinoma cells and also show the dominant nature of the heat shock-induced gene expression, leading to abundant HSP induction in vitro or in vivo.

Extracellular HSP70 binding to surface receptors present on antigen presenting cells and endothelial/epithelial cells.

Extracellular HSP70 has been found to participate in both innate and adaptive immune responses. However, little is known about the molecular mechanisms that mediate this process. Previous reports suggest that HSP70 interacts with antigen presenting cells (APC) through a plethora of surface receptors. In this study, we have examined the relative binding of potential HSP70 receptors and found high affinity binding to LOX-1 but not other structures with a role in HSP70-APC interactions such as LRP/CD91, CD40, TLR2, TLR4 or another c-type lectin family member (DC-SIGN) closely related to LOX-1. In addition to APC, HSP70 can avidly bind to non-APC cell lines, especially those from epithelial or endothelial background.

Expression of a dominant negative heat shock factor-1 construct inhibits aneuploidy in prostate carcinoma cells.

Recent studies have implicated heat shock proteins (HSP) and heat shock transcription factor 1 (HSF1) in tumor progression. We have examined the role of HSF1 in the malignant phenotype of PC-3 prostate carcinoma cells. We have developed a dominant negative construct of HSF1 that antagonizes transcription from HSP promoters and results in the depletion of intracellular HSP 70. Our studies indicate that expression of DN-HSF1 dramatically alters the DNA content of PC-3 cells (derived from p53 null prostatic carcinoma) and inhibits aneuploidy in these cells. This effect is due to prolonged expression of DN-HSF1, and transient expression of the dominant negative factor from an inducible promoter failed to cause the effect. Inhibition of aneuploidy in p53 null PC-3 cells by DN-HSF1 expression was recapitulated by expression within the cells of wild type p53. Furthermore, cells expressing DN-HSF1 showed a profound inhibition in the development of aneuploidy when exposed to chemical agents that disrupt the mitotic spindle and prevent progression through metaphase. Inhibition of aneuploidy in PC-3 cells expressing DN-HSF1 was associated with delayed breakdown of cyclin B1 compared with controls, consistent with a role for wild type HSF1 in the regulation of cyclin B1 degradation, a key step in the control of mitosis. Our experiments therefore demonstrate that HSF1 plays a functional role in cancer cells under nonstress conditions and influences cell cycle behavior and progression through mitosis and promotes the development of the aneuploid state.

Essential role of the NH2-terminal WD/EPF motif in the phosphorylation-activated protective function of mammalian Hsp27.

Hsp27 is expressed at high levels after mild heat shock and contributes to making cells extremely resistant to subsequent treatments. The activity of the protein is regulated at the transcriptional level, but also by phosphorylation, which occurs rapidly during stress and is responsible for causing the dissociation of large 700-kDa Hsp27 oligomers into dimers. We investigated the mechanism by which phosphorylation and oligomerization modulate the protective activity of Chinese hamster Hsp27. In contrast to oligomer dissociation, which only required Ser90 phosphorylation, activation of Hsp27 thermoprotective activity required the phosphorylation of both Ser90 and Ser15. Replacement of Ser90 by Ala90, which prevented the dissociation of the oligomer upon stress, did cause a severe defect in the protective activity. Dissociation was, however, not a sufficient condition to activate the protein because replacement of Ser15 by Ala15, which caused little effect in the oligomeric organization of the protein, also yielded an inactive protein. Analyzes of mutants with short deletions in the NH2 terminus identified the Hsp27 WD/EPF or PF-rich domain as essential for protection, maintenance of the oligomeric structure, and in vitro chaperone activity of the protein. In light of a three-dimensional model of Hsp27 based on the crystallographic structure of wheat Hsp16.9, we propose that the conserved WD/EPF motif of mammalian Hsp27 mediates important intramolecular interactions with hydrophic surfaces of the alpha-crystallin domain of the protein. These interactions are destabilized by Ser90 phosphorylation, making the motif free to interact with heterologous molecular targets upon the additional phosphorylation of the nearby Ser15.

Distinct chaperone mechanisms can delay the formation of aggresomes by the myopathy-causing R120G alphaB-crystallin mutant.

A familial form of desmin-related myopathy (DRM) is associated with a missense mutation (R120G) in alphaB-crystallin (alphaB) and is characterized by intracellular desmin aggregation. Because alphaB is a molecular chaperone that participates in the assembly of desmin filaments, it has been suggested that the desmin aggregation might be due to the loss of alphaB function. We report here that alphaBR120G has indeed impaired in vivo function and structure as reflected by a highly reduced capacity to protect cells against heat shock and by an abnormal supramolecular organization even in cells not expressing desmin. In many cells, alphaBR120G accumulated in inclusion bodies that had characteristics of aggresomes concentrating around the centrosome following a microtubule-facilitated process. Three distinct chaperone mechanisms could reduce or even prevent the formation of the alphaBR120G aggresomes. Wild-type alphaB and Hsp27 prevented aggresome formation by co-oligomerizing with alphaBR120G. Hsp70 with its co-chaperone Hdj-1 or Chip-1 but not a mutant of Chip-1 lacking ubiquitin ligase activity, reduced the frequency of aggresome formation likely by targeting alphaBR120G for degradation. Finally, HspB8 interacted only transiently with alphaB but nonetheless rescued the alphaBR120G oligomeric organization, suggesting that it acted as a true chaperone assisting in the folding of the mutant protein. Hence, the formation of inclusion bodies in alphaBR120G-mediated DRM is probably due to the misfolding of alphaBR120G per se and can be delayed or prevented by expression of the wild type alphaB allele or other molecular chaperones, thereby explaining the adult onset of the disease.