Peptidoglycan Induces the Production of Interleukin-8 via Calcium Signaling in Human Gingival Epithelium

Transcriptional regulation of suppressors of cytokine signaling during infection with Mycobacterium tuberculosis in human THP-1-derived macrophages and in mice

Mycobacterium tuberculosis (Mtb) infection leads to upregulation of Suppressors of Cytokine signaling (SOCS) expression in host macrophages (Mϕ). SOCS proteins inhibit cytokine signaling by negatively regulating JAK/STAT. We investigated this host-pathogen dialectic at the level of transcription. We used phorbol-differentiated THP-1 Mϕ infected with Mtb to investigate preferential upregulation of some SOCS isoforms that are known to inhibit signaling by IFN-γ, IL-12, and IL-6. We examined time kinetics of likely transcription factors and signaling molecules upstream of SOCS transcription, and survival of intracellular Mtb following SOCS upregulation. Our results suggest a plausible mechanism that involves PGE2 secretion during infection to induce the PKA/CREB axis, culminating in nuclear translocation of C/EBPβ to induce expression of SOCS1. Mtb-infected Mϕ secreted IL-10, suggesting a mechanism of induction of STAT3, which may subsequently induce SOCS3. We provide evidence of temporal variation in SOCS isoform exspression and decay. Small-interfering RNA-mediated knockdown of SOCS1 and SOCS3 restored the pro-inflammatory milieu and reduced Mtb viability. In mice infected with Mtb, SOCS isoforms persisted across Days 28-85 post infection. Our results suggest that differential temporal regulation of SOCS isoforms by Mtb drives the host immune response towards a phenotype that facilitates the pathogen's survival.

Activation of the Endoplasmic Reticulum Stress Response Impacts the NOD1 Signaling Pathway

Nucleotide-binding oligomerization domain 1 (NOD1) is an intracellular pattern recognition receptor (PRR) responsible for sensing bacterial peptidoglycan fragments. Stimulation of NOD1 leads to a robust innate immune response via activation of the major transcription factor NF-κB. In addition to peptidoglycan sensing, NOD1 and the closely related PRR NOD2 have been linked to inflammation by responding to the endoplasmic reticulum (ER) stress-induced unfolded protein response (UPR).

Nucleotide-binding oligomerization domain 1 (NOD1) is an intracellular pattern recognition receptor (PRR) responsible for sensing bacterial peptidoglycan fragments. Stimulation of NOD1 leads to a robust innate immune response via activation of the major transcription factor NF-κB. In addition to peptidoglycan sensing, NOD1 and the closely related PRR NOD2 have been linked to inflammation by responding to the endoplasmic reticulum (ER) stress-induced unfolded protein response (UPR). Here we show that differential ER stress induction renders cells more susceptible to Salmonella enterica serovar Typhimurium infection in a NOD1-dependent manner, measured by increased NF-κB activation and cytokine expression. In HeLa57A cells stably transfected with an NF-κB::luciferase reporter, we show that cells undergoing ER stress induced by thapsigargin display a significant increase in NF-κB activation in response to NOD1 stimulation by C12-iE-DAP (acylated derivative of the iE-DAP dipeptide [gamma-d-glutamyl-meso-diaminopimelic acid]) and the S. Typhimurium effector protein SopE. Tunicamycin-induced ER stress had no effect on NOD1-stimulated NF-κB activation. We further show that the mouse intestinal epithelial cell line MODE-K and RAW264.7 macrophages are more responsive to Salmonella infection when treated with thapsigargin but not with tunicamycin. These profound differences between thapsigargin- and tunicamycin-treated cells upon inflammation suggest that different components downstream of the UPR contribute to NOD1 activation. We found that the NOD1-induced inflammatory response is dependent on protein kinase R (PKR)-like endoplasmic reticulum kinase (PERK) activation in conjunction with stimulation of the inositol triphosphate receptor (IP3R). Together, these results suggest that differential UPR activation makes cells more responsive to bacterial infections in a NOD1-dependent manner.

SOCS Proteins as Regulators of Inflammatory Responses Induced by Bacterial Infections: A Review

Severe bacterial infections can lead to both acute and chronic inflammatory conditions. Innate immunity is the first defense mechanism employed against invading bacterial pathogens through the recognition of conserved molecular patterns on bacteria by pattern recognition receptors (PRRs), especially the toll-like receptors (TLRs). TLRs recognize distinct pathogen-associated molecular patterns (PAMPs) that play a critical role in innate immune responses by inducing the expression of several inflammatory genes. Thus, activation of immune cells is regulated by cytokines that use the Janus kinase/signal transducers and activators of transcription (JAK/STAT) signaling pathway and microbial recognition by TLRs. This system is tightly controlled by various endogenous molecules to allow for an appropriately regulated and safe host immune response to infections. Suppressor of cytokine signaling (SOCS) family of proteins is one of the central regulators of microbial pathogen-induced signaling of cytokines, principally through the inhibition of the activation of JAK/STAT signaling cascades. This review provides recent knowledge regarding the role of SOCS proteins during bacterial infections, with an emphasis on the mechanisms involved in their induction and regulation of antibacterial immune responses. Furthermore, the implication of SOCS proteins in diverse processes of bacteria to escape host defenses and in the outcome of bacterial infections are discussed, as well as the possibilities offered by these proteins for future targeted antimicrobial therapies.

PLC-γ1 is involved in the inflammatory response induced by influenza A virus H1N1 infection

We have previously reported that phosphoinositide-specific phospholipase γ1 (PLC-γ1) signaling is activated by influenza virus H1N1 infection and mediates efficient viral entry in human epithelial cells. In this study, we show that H1N1 also activates PLCγ-1 signaling in human promonocytic cell line -derived macrophages. Surprisingly, the activated PLCγ-1 signaling is not important for viral replication in macrophages, but is involved in the virus-induced inflammatory responses. PLC-γ1-specific inhibitor U73122 strongly inhibits the H1N1 virus-induced NF-κB signaling, blocking the up-regulation of TNF-α, IL-6, MIP-1α, and reactive oxidative species. In a positive feedback loop, IL-1β and TNF-α activate the PLCγ-1 signaling in both epithelial and macrophage cell lines. In summary, we have shown for the first time that the PLCγ-1 signaling plays an important role in the H1N1-induced inflammatory responses. Our study suggests that targeting the PLCγ-1 signaling is a potential antiviral therapy against H1N1 by inhibiting both viral replication and excessive inflammation.

The Regulatory Role of Rolipram on Inflammatory Mediators and Cholinergic/Adrenergic Stimulation-Induced Signals in Isolated Primary Mouse Submandibular Gland Cells

Exposure to bacterial lipopolysaccharides (LPS) induces inflammatory signals in salivary glands. We investigated the regulatory role of phosphodiesterase 4 (PDE4) inhibitor rolipram on inflammatory mediators and cholinergic/adrenergic stimulation-induced intracellular Ca(2+) signaling in salivary acinar and ductal cells. Submandibular gland (SMG) expressed PDE4A through 4D mRNA and PDE4 was localized in the luminal membrane of SMG. LPS induced Ca(2+) signaling and ROS production in SMG. Treatment with rolipram blocked LPS-induced Ca(2+) increase and ROS production. The application of histamine evoked Ca(2+) signals and ROS production, which were attenuated by rolipram in SMG cells. Moreover, LPS-induced NLRP3 inflammasome and cleaved caspase-1 were inhibited by rolipram. The inhibitory role of rolipram in ROS-induced Ca(2+) signaling was mainly observed in acinar cells and not in ductal cells. Rolipram also protected SMG acinar but not ductal cells from LPS-induced cell membrane damage. In the case of cholinergic/adrenergic stimulation, carbachol/isoproterenol-induced Ca(2+) signals were upregulated by the treatment of rolipram in SMG. In the case of cAMP-dependent ductal bicarbonate secretion by rolipram, no effect was observed on the modulation of ductal chloride/bicarbonate exchange activity. Rolipram could suppress the inflammatory signals and could be a potential therapeutic strategy against LPS-induced inflammation to protect the salivary gland cells.

Anti-inflammatory activities of phospholipase C inhibitor U73122: Inhibition of monocyte-to-macrophage transformation and LPS-induced pro-inflammatory cytokine expression

A wide range of biological processes are controlled by phospholipase C (PLC)/Ca(2+) signaling, which could be blocked by PLC-specific inhibitor U73122. Whether inhibition of PLC with chemical inhibitor U73122 affects the inflammatory response in monocytes/macrophages is currently unknown. In this study, we demonstrated that U73122 inhibited PMA-induced in vitro differentiation of human promonocytic U937 cells into macrophages as reflected by the reduction of cell adherence and the decreased expression of macrophage specific marker CD163. It is possible that U73122 blocked PMA-induced adhesion of U937 cells partially by down regulation and inactivation of both Pyk2 and paxillin signaling. Furthermore, the expression of LPS-induced pro-inflammatory cytokines TNF-α and IL-1β was significantly blocked by U73122 in both dU937 cells and mouse primary peritoneal macrophages. These results suggest that PLC is involved in the sophisticated inflammatory response by monocytes/macrophages, and thereby chemical antagonists of PLC may be potential agents for the suppression of inflammatory response.

Bacterial PAMPs and Allergens Trigger Increase in [Ca(2+)]i-induced Cytokine Expression in Human PDL Fibroblasts

An oral environment is constantly exposed to environmental factors and microorganisms. The periodontal ligament (PDL) fibroblasts within this environment are subject to bacterial infection and allergic reaction. However, how these condition affect PDL fibroblasts has yet to be elucidated. PDL fibroblasts were isolated from healthy donors. We examined using reverse transcription-polymerase chain reaction and measuring the intracellular Ca²⁺ concentration ([Ca²⁺]_i). This study investigated the receptors activated by exogenous bacterial pathogens (Lipopolysaccharide and peptidoglycan) and allergens (German cockroach extract and house dust mite) as well as these pathogenic mediators-induced effects on the intracellular Ca²⁺ signaling in human PDL fibroblasts. Moreover, we evaluated the expression of pro-inflammatory cytokines (interleukin (IL)-1β, IL-6, and IL-8) and bone remodeling mediators (receptor activator of NF-κB ligand and osteoprotegerin) and intracellular Ca²⁺-involved effect. Bacterial pathogens and allergic mediators induced increased expression of pro-inflammatory cytokines, and these results are dependent on intracellular Ca²⁺. However, bacterial pathogens and allergic mediators did not lead to increased expression of bone remodeling mediators, except lipopolysaccharide-induced effect on receptor activator of NF-κB ligand expression. These experiments provide evidence that a pathogens and allergens-induced increase in [Ca²⁺]_i affects the inflammatory response in human PDL fibroblasts.