A Novel TLR4-Mediated Signaling Pathway Leading to IL-6 Responses in Human Bladder Epithelial Cells

Abstract

The vigorous cytokine response of immune cells to Gram-negative bacteria is primarily mediated by a recognition molecule, Toll-like receptor 4 (TLR4), which recognizes lipopolysaccharide (LPS) and initiates a series of intracellular NF-κB–associated signaling events. Recently, bladder epithelial cells (BECs) were reported to express TLR4 and to evoke a vigorous cytokine response upon exposure to LPS. We examined intracellular signaling events in human BECs leading to the production of IL-6, a major urinary cytokine, following activation by Escherichia coli and isolated LPS. We observed that in addition to the classical NF-κB–associated pathway, TLR4 triggers a distinct and more rapid signaling response involving, sequentially, Ca2+, adenylyl cyclase 3–generated cAMP, and a transcriptional factor, cAMP response element–binding protein. This capacity of BECs to mobilize secondary messengers and evoke a more rapid IL-6 response might be critical in their role as first responders to microbial challenge in the urinary tract.

Similar inflammatory DC maturation signatures induced by TNF or Trypanosoma brucei antigens instruct default Th2‐cell responses

“…Our data show that the T. brucei antigen MiTat1.5 sVSG-conditioned DCs to produce IL-6 and IL-1b, which is dependent on TLR4 and the adaptor molecule MyD88. A novel TLR4-mediated signaling pathway was identified in which TLR4 stimuli trigger a rapid increase in intracellular cAMP followed by translocation of the transcription factor CREB and IL-6 production [47]. Further investigation is needed to address whether MiTat1.5 sVSG activation of DCs is accompanied with an intracellular cAMP rise and CREB transcription factor translocation.…”

Urinary Tract Immunology

“…Regardless of the fimbria involved in stimulation, all pathways involving these adaptor molecules result in activation of NF-κB and proinflammatory gene expression. Song and colleagues identified an accompanying proinflammatory bladder cell signaling pathway that is also dependent on TLR4 but results in a spike in intracellular calcium levels (39). This calcium spike leads to adenylyl cyclase 3 (AC3)-mediated increase in cAMP, protein kinase A (PKA) activation, phosphorylation of the cAMP response element-binding protein transcription factor (CREB), and proinflammatory gene expression such as transcription of IL-6 and IL-8 (39).…”

“…This calcium spike leads to adenylyl cyclase 3 (AC3)-mediated increase in cAMP, protein kinase A (PKA) activation, phosphorylation of the cAMP response element-binding protein transcription factor (CREB), and proinflammatory gene expression such as transcription of IL-6 and IL-8 (39). Using selective blockade of these signaling cascades, Song et al determined that activation of epithelial IL-6 secretion by E. coli might even be faster via the CREB pathway than the canonical pathway and can also be activated by TLR2 and TLR3 ligands (39).…”

Transcriptional Regulation of Tlr11 Gene Expression in Epithelial Cells

“…cDNA was prepared by oligo(dT) [12][13][14][15][16][17][18] and reverse transcriptase SuperScript II from Invitrogen with 2 g of DNase I-treated total RNA. One l of cDNA was amplified using primers shown in Table 1.…”

“…It is clear that epithelial cells express functional TLRs; however, these epithelial TLRs display different sensitivity to their cognate ligands from the TLRs expressed on DCs and macrophages (15)(16)(17). This difference in sensitivity is poorly understood (18 -20).…”