Homodimerization Is Essential for the Receptor for Advanced Glycation End Products (RAGE)-mediated Signal Transduction

The receptor for advanced glycation end products (RAGE) is a pattern-recognition receptor that binds to diverse ligands and initiates a downstream proinflammatory signaling cascade. RAGE activation has been linked to diabetic complications, Alzheimer disease, infections, and cancers. RAGE is known to mediate cell signaling and downstream proinflammatory gene transcription activation, although the precise mechanism surrounding receptor-ligand interactions is still being elucidated. Recent fluorescence resonance energy transfer evidence indicates that RAGE may form oligomers on the cell surface and that this could be related to signal transduction. To investigate whether RAGE forms oligomers, protein-protein interaction assays were carried out. Here, we demonstrate the interaction between RAGE molecules via their N-terminal V domain, which is an important region involved in ligand recognition. By protein cross-linking using water-soluble and membrane-impermeable cross-linker bis(sulfosuccinimidyl) suberate and nondenaturing gels, we show that RAGE forms homodimers at the plasma membrane, a process potentiated by S100B and advanced glycation end products. Soluble RAGE, the RAGE inhibitor, is also capable of binding to RAGE, similar to V peptide, as shown by surface plasmon resonance. Incubation of cells with soluble RAGE or RAGE V domain peptide inhibits RAGE dimerization, subsequent phosphorylation of intracellular MAPK proteins, and activation of NF-kappa B pathways. Thus, the data indicate that dimerization of RAGE represents an important component of RAGE-mediated cell signaling.

Role of Delta Np63 gamma in Epithelial to Mesenchymal Transition

Although members of the p63 family of transcription factors are known for their role in the development and differentiation of epithelial surfaces, their function in cancer is less clear. Here, we show that depletion of the Delta Np63 alpha and beta isoforms, leaving only Delta Np63 gamma, results in epithelial to mesenchymal transition (EMT) in the normal breast cell line MCF10A. EMT can be rescued by the expression of the Delta Np63 alpha isoform. We also show that Delta Np63 gamma expressed in a background where all the other Delta Np63 are knocked down causes EMT with an increase in TGF beta-1, -2, and -3 and downstream effectors Smads2/3/4. In addition, a p63 binding site in intron 1 of TGF beta was identified. Inhibition of the TGF beta response with a specific inhibitor results in reversion of EMT in Delta Np63 alpha- and beta-depleted cells. In summary, we show that p63 is involved in inhibiting EMT and reduction of certain p63 isoforms may be important in the development of epithelial cancers.

DNA-dependent Protein Kinase-mediated Phosphorylation of Protein Kinase B Requires a Specific Recognition Sequence in the C-terminal Hydrophobic Motif

DNA-dependent protein kinase (DNA-PK) has been implicated in a variety of nuclear processes including DNA double strand break repair, V(D)J recombination, and transcription. A recent study showed that DNA-PK is responsible for Ser-473 phosphorylation in the hydrophobic motif of protein kinase B (PKB/Akt) in genotoxic-stressed cells, suggesting a novel role for DNA-PK in cell signaling. Here, we report that DNA-PK activity toward PKB peptides is impaired in DNA-PK knock-out mouse embryonic fibroblast cells when compared with wild type. In addition, human glioblastoma cells expressing a mutant form of DNA-PK (M059J) displayed a lower DNA-PK activity when compared with glioblastoma cells expressing wild-type DNA- PK (M059K) when PKB peptide substrates were tested. DNA- PK preferentially phosphorylated PKB on Ser-473 when compared with its known in vitro substrate, p53. A consensus hydrophobic amino acid surrounding the Ser-473 phospho-acceptor site in PKB containing amino acids Phe at position +1 and +4 and Tyr at position -1 are critical for DNA- PK activity. Thus, these data define the specificity of DNA- PK action as a Ser-473 kinase for PKB in DNA repair signaling.

Structural basis for the platelet-collagen interaction - The smallest motif within collagen that recognizes and activates platelet glycoprotein VI contains two glycine-proline-hydroxyproline triplets

Collagen-related peptide is a selective agonist for the platelet collagen receptor Glycoprotein VI. The triple helical peptide contains ten GPO triplets/strand (single letter amino acid nomenclature, where O is hydroxyproline) and so over-represents GPO compared with native collagen sequence. To investigate the ability of Glycoprotein VI to recognize GPO triplets in a setting more representative of the collagens, we synthesized a set of triple helical peptides containing fewer GPO triplets, varying their number and spacing within an inert (GPP)(n) backbone. The adhesion of recombinant human Glycoprotein VI ectodomain, like that of human platelets, to these peptides increased with their GPO content, and platelet adhesion was abolished by the specific anti-Glycoprotein VI-blocking antibody, 10B12. Platelet aggregation and protein tyrosine phosphorylation were induced only by cross-linked peptides and only those that contained two or more GPO triplets. Such peptides were less potent than cross-linked collagen-related peptide. Our data suggest that both the sequences GPOGPO and GPO center dot center dot center dot center dot center dot center dot center dot center dot center dot GPO represent functional Glycoprotein VI recognition motifs within collagen. Furthermore, we propose that the (GPO)(4) motif can support simultaneous binding of two glycoprotein VI molecules, in either a parallel or anti-parallel stacking arrangement, which could play an important role in activation of signaling.

Use of synthetic peptides to locate novel integrin alpha(2)beta(1)-binding motifs in human collagen III

A set of 57 synthetic peptides encompassing the entire triple-helical domain of human collagen III was used to locate binding sites for the collagen-binding integrin alpha(2)beta(1). The capacity of the peptides to support Mg2+-dependent binding of several integrin preparations was examined. Wild-type integrins (recombinant alpha(2) I-domain, alpha(2)beta(1) purified from platelet membranes, and recombinant soluble alpha(2)beta(1) expressed as an alpha(2)-Fos/beta(1)-Jun heterodimer) bound well to only three peptides, two containing GXX'GER motifs (GROGER and GMOGER, where O is hydroxyproline) and one containing two adjacent GXX'GEN motifs (GLKGEN and GLOGEN). Two mutant alpha(2) I-domains were tested: the inactive T221A mutant, which recognized no peptides, and the constitutively active E318W mutant, which bound a larger subset of peptides. Adhesion of activated human platelets to GER-containing peptides was greater than that of resting platelets, and HT1080 cells bound well to more of the peptides compared with platelets. Binding of cells and recombinant proteins was abolished by anti-alpha(2) monoclonal antibody 6F1 and by chelation of Mg2+. We describe two novel high affinity integrin-binding motifs in human collagen III (GROGER and GLOGEN) and a third motif (GLKGEN) that displays intermediate activity. Each motif was verified using shorter synthetic peptides.

Altered Toll-like Receptor 2-mediated Endotoxin Tolerance Is Related to Diminished Interferon β Production

Induction of endotoxin tolerance leads to a reduced inflammatory response after repeated challenge by LPS and is important for resolution of inflammation and prevention of tissue damage. Enterobacterial LPS is recognized by the TLR4 signaling complex, whereas LPS of some non-enterobacterial organisms is capable of signaling independently of TLR4 utilizing TLR2-mediated signal transduction instead. In this study we report that Porphyromonas gingivalis LPS, a TLR2 agonist, fails to induce a fully endotoxin tolerant state in a human monocytic cell line (THP-1) and mouse bone marrow-derived macrophages. In contrast to significantly decreased production of human IL-8 and TNF-alpha and, in mice, keratinocyte-derived cytokine (KC), macrophage inflammatory protein-2 (MIP-2), and TNF-alpha after repeated challenge with Escherichia coli LPS, cells repeatedly exposed to P. gingivalis LPS responded by producing less TNF-alpha but sustained elevated secretion of IL-8, KC, and MIP-2. Furthermore, in endotoxin-tolerant cells, production of IL-8 is controlled at the signaling level and correlates well with NF-kappa B activation, whereas TNF-alpha expression is blocked at the gene transcription level. Interferon beta plays an important role in attenuation of chemokine expression in endotoxin-tolerized cells as shown in interferon regulatory factor-3 knock-out mice. In addition, human gingival fibroblasts, commonly known not to display LPS tolerance, were found to be tolerant to repeated challenge by LPS if pretreated with interferon beta. The data suggest that the inability of the LPS-TLR2 complex to induce full endotoxin tolerance in monocytes/macrophages is related to diminished production of interferon beta and may partly explain the involvement of these LPS isoforms in the pathogenesis of chronic inflammatory diseases.

Regulation of transient receptor potential melastatin 8 (TRPM8) channel activity by its short isoforms.

One important mechanism of membrane ion channels regulation involves their non-functional isoforms generated by alternative splicing. However, knowledge of such isoforms for the members of transient receptor potential (TRP) superfamily of ion channels remains quite limited. This study focuses on TRPM member, TRPM8, which functions as a cold receptor in sensory neurons, but is also expressed in tissues not exposed to ambient temperatures, as well as in cancer tissues. We report the cloning from prostate cancer cells of new short-splice variants of TRPM8, termed short TRPM8a (sM8a) and short TRPM8ß (sM8ß). Our results show that both variants are in a closed configuration with the C-terminal tail of the full-size TRPM8 chan-nel, resulting in stabilization of its closed state and thus reducing both its cold sensitivity and its activity. Our findings, therefore, uncover a new mode of the regulation of TRPM8 channel by its splice variants.

Short Isoforms of the Cold Receptor TRPM8 Inhibit Channel Gating by Mimicking Heat Action Rather than Chemical Inhibitors

Transient receptor potential (TRP) channels couple various environmental factors to changes in membrane potential, calcium influx, and cell signaling. They also integrate multiple stimuli through their typically polymodal activation. Thus, although the TRPM8 channel has been extensively investigated as the major neuronal cold sensor, it is also regulated by various chemicals, as well as by several short channel isoforms. Mechanistic understanding of such complex regulation is facilitated by quantitative single-channel analysis. We have recently proposed a single-channel mechanism of TRPM8 regulation by voltage and temperature. Using this gating mechanism, we now investigate TRPM8 inhibition in cell-attached patches using HEK293 cells expressing TRPM8 alone or coexpressed with its short sM8-6 isoform. This is compared with inhibition by the chemicals N-(4-tert-butylphenyl)-4-(3-chloropyridin-2-yl)piperazine-1-carboxamide (BCTC) and clotrimazole or by elevated temperature. We found that within the seven-state single-channel gating mechanism, inhibition of TRPM8 by short sM8-6 isoforms closely resembles inhibition by increased temperature. In contrast, inhibition by BCTC and that by clotrimazole share a different set of common features. © 2012 by The American Society for Biochemistry and Molecular Biology, Inc.

Identification of ML204, a novel potent antagonist that selectively modulates native TRPC4/C5 ion channels.

Transient receptor potential canonical (TRPC) channels are Ca(2+)-permeable nonselective cation channels implicated in diverse physiological functions, including smooth muscle contractility and synaptic transmission. However, lack of potent selective pharmacological inhibitors for TRPC channels has limited delineation of the roles of these channels in physiological systems. Here we report the identification and characterization of ML204 as a novel, potent, and selective TRPC4 channel inhibitor. A high throughput fluorescent screen of 305,000 compounds of the Molecular Libraries Small Molecule Repository was performed for inhibitors that blocked intracellular Ca(2+) rise in response to stimulation of mouse TRPC4ß by µ-opioid receptors. ML204 inhibited TRPC4ß-mediated intracellular Ca(2+) rise with an IC(50) value of 0.96 µm and exhibited 19-fold selectivity against muscarinic receptor-coupled TRPC6 channel activation. In whole-cell patch clamp recordings, ML204 blocked TRPC4ß currents activated through either µ-opioid receptor stimulation or intracellular dialysis of guanosine 5'-3-O-(thio)triphosphate (GTP?S), suggesting a direct interaction of ML204 with TRPC4 channels rather than any interference with the signal transduction pathways. Selectivity studies showed no appreciable block by 10-20 µm ML204 of TRPV1, TRPV3, TRPA1, and TRPM8, as well as KCNQ2 and native voltage-gated sodium, potassium, and calcium channels in mouse dorsal root ganglion neurons. In isolated guinea pig ileal myocytes, ML204 blocked muscarinic cation currents activated by bath application of carbachol or intracellular infusion of GTP?S, demonstrating its effectiveness on native TRPC4 currents. Therefore, ML204 represents an excellent novel tool for investigation of TRPC4 channel function and may facilitate the development of therapeutics targeted to TRPC4.

Helminth Cysteine Proteases Inhibit TRIF-dependent Activation of Macrophages via Degradation of TLR3

Helminth pathogens prepare a Th2 type immunological environment in their hosts to ensure their longevity. They achieve this by secreting molecules that not only actively drive type 2 responses but also suppress type 1 responses. Here, we show that the major cysteine proteases secreted from the helminth pathogens Fasciola hepatica (FheCL1) and Schistosoma mansoni (SmCB1) protect mice from the lethal effects of lipopolysaccharide by preventing the release of inflammatory mediators, nitric oxide, interleukin-6, tumor necrosis factor alpha, and interleukin-12, from macrophages. The proteases specifically block the MyD88-independent TRIF-dependent signaling pathway of Toll-like receptor (TLR) 4 and TLR3. Microscopical and flow cytometric studies, however, show that alteration of macrophage function by cysteine protease is not mediated by cleavage of components of the TLR4 complex on the cell surface but occurs by degradation of TLR3 within the endosome. This is the first study to describe a parasite molecule that degrades this receptor and pinpoints a novel mechanism by which helminth parasites modulate the innate immune responses of their hosts to suppress the development of Th1 responses.

Investigating the effects of physiological bile acids on GLP-1 secretion and glucose tolerance in normal and GLP-1R(-/-) mice

Physiological secretion of bile acids has previously been linked to the regulation of blood glucose. GLP-1 is an intestinal peptide hormone with important glucose-lowering actions, such as stimulation of insulin secretion and inhibition of glucagon secretion. In this investigation, we assessed the ability of several bile acid compounds to secrete GLP-1 in vitro in STC-1 cells. Bile acids stimulated GLP-1 secretion from 3.3- to 6.2-fold but some were associated with cytolytic effects. Glycocholic and taurocholic acids were selected for in vivo studies in normal and GLP-1R(-/-) mice. Oral glucose tolerance tests revealed that glycocholic acid did not affect glucose excursions. However, taurocholic acid reduced glucose excursions by 40% in normal mice and by 27% in GLP-1R(-/-) mice, and plasma GLP-1 concentrations were significantly elevated 30 min post-gavage. Additional studies used incretin receptor antagonists to probe involvement of GLP-1 and GIP in taurocholic acid-induced glucose lowering. The findings suggest that bile acids partially aid glucose regulation by physiologically enhancing nutrient-induced GLP-1 secretion. However, GLP-1 secretion appears to be only part of the glucose-lowering mechanism and our studies indicate that the other major incretin GIP is not involved.