Sanchita Basu

Execution of macrophage apoptosis by PE_PGRS33 of Mycobacterium tuberculosis is mediated by toll-like receptor 2-dependent release of tumor necrosis factor-α

Combating tuberculosis requires a detailed understanding of how mycobacterial effectors modulate the host immune response. The role of the multigene PE family of proteins unique to mycobacteria in the pathogenesis of tuberculosis is still poorly understood, although certain PE_PGRS genes have been linked to virulence. Tumor necrosis factor-α (TNF-α) is essential for successfully combating tuberculosis. In this study we provide evidence that PE_PGRS33, a surface exposed protein, elicits TNF-α release from macrophages in a TLR2 (Toll-like receptor 2)-dependent manner. ASK1 (apoptosis signal-regulating kinase 1) is activated downstream of TLR2. ASK1 activates the MAPKs p38 and JNK. PE_PGRS33-induced signaling leads to enhanced expression of TNF-α and TNF receptor I (TNFRI) genes. Mycobacterium smegmatis expressing PE_ PGRS33 elicits the same effects as purified PE_PGRS33. TNF-α release occurs even when internalization of the bacteria is blocked by cytochalasin D, suggesting that interaction of PE_ PGRS33 with TLR2 is sufficient to trigger the effects described. Release of TNF-α plays the determining role in triggering apoptosis in macrophages challenged with PE_PGRS33. The death receptor-dependent signals are amplified through classical caspase 8-dependent mitochondrial release of cytochrome c, leading to the activation of caspases 9 and 3. An important aspect of our findings is that deletions within the PGRS domain (simulating those occurring in clinical strains) attenuate the TNF-α-inducing ability of PE_PGRS33. These results provide the first evidence that variations in the polymorphic repeats of the PGRS domain modulate the innate immune response.

Mycobacterium tuberculosis lipoarabinomannan-mediated IRAK-M induction negatively regulates Toll-like receptor-dependent interleukin-12 p40 production in macrophages.

Mannose-capped lipoarabinomannans (Man-LAMs) are members of the repertoire of Mycobacterium tuberculosis modulins that the bacillus uses to subvert the host innate immune response. Interleukin-12 (IL-12) production is critical for mounting an effective immune response by the host against M. tuberculosis. We demonstrate that Man-LAM inhibits IL-12 p40 production mediated by subsequent challenge with lipopolysaccharide (LPS). Man-LAM inhibits LPS-induced IL-12 p40 expression in an IL-10-independent manner. It attenuates LPS-induced NF-κB-driven luciferase gene expression, suggesting that its effects are likely directly related to inhibition of NF-κB. This is probably because of dampening of the Toll-like receptor signaling. Man-LAM inhibits IL-1 receptor-associated kinase (IRAK)-TRAF6 interaction as well as IκB-α phosphorylation. It directly attenuates nuclear translocation and DNA binding of c-Rel and p50. Man-LAM exerts these effects by inducing the expression of Irak-M, a negative regulator of TLR signaling. Knockdown of Irak-M expression by RNA interference reinstates LPS-induced IL-12 production in Man-LAM-pretreated cells. The fact that Irak-M expression could be elicited by yeast mannan suggested that ligation of the mannose receptor by the mannooligosaccharide caps of LAM was the probable trigger for IRAK-M induction.

TLR4-Dependent NF-κB Activation and Mitogen- and Stress-Activated Protein Kinase 1-Triggered Phosphorylation Events Are Central to Helicobacter pylori Peptidyl Prolyl cis-, trans-Isomerase (HP0175)-Mediated Induction of IL-6 Release from Macrophages

Helicobacter pylori infection is associated with the local production of chemokines and cytokines, of which IL-6 is overexpressed at the margin of gastric ulcer in H. pylori-positive gastritis. Cells of the monocytic lineage are the major sources of IL-6, and mononuclear cell infiltration in the lamina propria is characteristic of H. pylori-induced chronic infection. Our study shows for the first time that a secreted peptidyl prolyl cis-, trans-isomerase, HP0175 elicits IL-6 gene expression and IL-6 release from macrophages. An isogenic strain inactivated in the HP0175 gene (knockout) was attenuated in its IL-6-inducing ability, which was restored after complementation with the HP0175 gene. The specificity of the HP0175-induced effect was confirmed by the fact that rHP0175 purified from HEK293 cells could also induce IL-6 release, ruling out the possibility that the observed effect was due to bacterial contaminants. HP0175 was capable of interacting directly with the extracellular domain of TLR4. HP0175-induced IL-6 gene expression was critically dependent on TLR4-dependent NF-κB and MAPK activation. TLR4/PI3K-dependent ERK1/2 and p38 MAPK signaling converged upon activation of mitogen- and stress-activated protein kinase 1 (MSK1). The central role of MSK1 was borne out by the fact that silencing of MSK1 expression abrogated HP0175-mediated NF-κB-dependent IL-6 gene transcription. MSK1 regulated the recruitment of p65 and phopho-Ser10-histone H3 to the IL-6 promoter. HP0175 therefore regulated IL-6 gene transcription through chromatin modification at the IL-6 promoter.

A TNF- and c-Cbl-dependent FLIP S -degradation pathway and its function in Mycobacterium tuberculosis –induced macrophage apoptosis

Apoptosis is central to the interaction between pathogenic mycobacteria and host macrophages. Caspase-8-dependent apoptosis of infected macrophages, which requires activation of the mitogen-activated protein (MAP) kinase p38, lowers the spread of mycobacteria. Here we establish a link between the release of tumor necrosis factor (TNF) and mycobacteria-mediated macrophage apoptosis. TNF activated a pathway involving the kinases ASK1, p38 and c-Abl. This pathway led to phosphorylation of FLIPS, which facilitated its interaction with the E3 ubiquitin ligase c-Cbl. This interaction triggered proteasomal degradation of FLIPS, which promoted activation of caspase-8 and apoptosis. Our findings identify a previously unappreciated signaling pathway needed for Mycobacterium tuberculosis–triggered macrophage cell death.

Helicobacter pylori Protein HP0175 Transactivates Epidermal Growth Factor Receptor through TLR4 in Gastric Epithelial Cells

The pathophysiology of Helicobacter pylori-associated gastroduodenal diseases, ulcerogenesis, and carcinogenesis is intimately linked to activation of epidermal growth factor receptor (EGFR) and production of vascular endothelial growth factor (VEGF). Extracellular virulence factors, such as CagA and VacA, have been proposed to regulate EGFR activation and VEGF production in gastric epithelial cells. We demonstrate that the H. pylori secretory protein, HP0175, by virtue of its ability to bind TLR4, transactivates EGFR and stimulates EGFR-dependent VEGF production in the gastric cancer cell line AGS. Knock-out of the hp0175 gene attenuates the ability of the resultant H. pylori strain to activate EGFR or to induce VEGF production. HP0175-induced activation of EGFR is preceded by translocation of TLR4 into lipid rafts. In lipid rafts, the Src kinase family member Lyn interacts with TLR4, leading to tyrosine phosphorylation of TLR4. Knockdown of Lyn prevents HP0175-induced activation of EGFR and VEGF production. Tyrosine-phosphorylated TLR4 interacts with EGFR. This interaction is necessary for the activation of EGFR. Disruption of lipid rafts with methyl β-cyclodextrin prevents HP0175-induced tyrosine phosphorylation of TLR4 and activation of EGFR. This mechanism of transactivation of EGFR is novel and distinct from that of metalloprotease-dependent shedding of EGF-like ligands, leading to autocrine activation of EGFR. It provides new insight into our understanding of the receptor cross-talk network.

Mycobacterium avium-induced matrix metalloproteinase-9 expression occurs in a cyclooxygenase-2-dependent manner and involves phosphorylation- and acetylation-dependent chromatin modification

Matrix metalloproteinases (MMPs) contribute to the matrix‐degrading phenotype of mycobacterial diseases. Considering that MMPs could contribute to the mutual exacerbation of both Mycobacterium avium and HIV in coinfections, it is of importance to understand the mechanisms of M. avium‐induced MMP induction. Focusing on MMP‐9, our work demonstrates that a cyclooxygenase‐2 (COX‐2)‐dependent signalling loop is critical for activation of MMP‐9 transcription in RAW264.7 cells and murine bone marrow‐derived macrophages. M. avium‐stimulated MMP‐9 induction involves the p65 and p50 subunits of NF‐κB and the c‐Fos and c‐jun subunits of AP‐1. The c‐Fos gene is upregulated in a MEK1‐dependent manner in M. avium‐challenged macrophages. M. avium‐induced MMP‐9 gene induction requires the histone acetyltransferase p300 and chromatin modifications involving phosphorylation of p65 at serine 276 and its acetylation at lysines 221 and 310. At the same time, histone H3 modified by mitogen and stress‐activated protein kinase 1 (MSK1)‐dependent phosphorylation on serine 10 and by acetylation on lysine 14, typical signatures linked to transcriptional activation, also associates with the MMP‐9 promoter following M. avium challenge. Taken together, our results show that co‐ordinated post‐translational modifications of p65 and histone H3 involving phosphorylation and acetylation drive COX‐2‐dependent transcriptional activation of the MMP‐9 gene in response to challenge of macrophages with M. avium.