title: Multiple Toll-like receptor ligands induce an IL-6 transcriptional response in skeletal myocytes.
abstract: Toll-like receptors (TLRs) comprise a critical sentinel that monitors body compartments for the presence of pathogens. Skeletal muscle expresses TLRs and responds to pathogen-associated molecular patterns (PAMPs), such as lipopolysaccharide (LPS), by mounting an innate immune response. In the present study, we used C2C12 myocytes as a model system for skeletal muscle during infection. C2C12 cells responded to LPS in a time frame and with a pattern of gene expression that faithfully mimicked the response of skeletal muscle to LPS in vivo. LPS from a variety of Escherichia coli serotypes stimulated IL-6 synthesis. C2C12 cells expressed TLR1-7, but not TLR8 or TLR9, mRNA by RT-PCR. A synthetic tripalmitoylated cysteine-, serine-, and lysine-containing peptide (Pam) and LPS from Porphyromonas gingivalis, two TLR2 ligands, also stimulated IL-6 expression. LPS and Pam stimulated luciferase activity driven from NF-kappaB and IL-6 promoter-containing plasmids, and this response was blunted when the NF-kappaB binding site was mutated. LPS- and Pam-stimulated IL-6 expression was inhibited by the proteasome inhibitor MG-132 and the IkappaB kinase-2 (IKK2) inhibitor 2-[(aminocarbonyl)amino]-5-(4-fluorophenyl)-3-thiophenecarboxamide (TPCA-1). Pam-stimulated NF-kappaB and IL-6 promoter activities were disrupted by a dominant-negative form of TLR2, but not TLR4. Local injection of LPS or Pam into the gastrocnemius muscle stimulated IL-6 mRNA expression in the injected, but not the contralateral, muscle. The LPS- but not Pam-stimulated expression of IL-6 mRNA was blunted in skeletal muscle of mice carrying an inactivating mutation in TLR4. The data suggest that skeletal muscle and muscle cells recognize pathogen-associated molecules with specific TLRs to initiate an IL-6 transcriptional response.
authors.affiliation: Department of Cellular and Molecular Physiology (H166), Pennsylvania State University College of Medicine, 500 University Dr., Hershey, Pennsylvania 17033, USA. rfrost@psu.edu
authors.forename: Robert A
authors.initials: RA
authors.lastname: Frost
authors.forename: Gerald J
authors.initials: GJ
authors.lastname: Nystrom
authors.forename: Charles H
authors.initials: CH
authors.lastname: Lang
mesh_terms: D000818:Animals; D001426:Bacterial Proteins / Q000008:administration & dosage*; D002352:Carrier Proteins / Q000008:administration & dosage*; D002460:Cell Line; D004305:Dose-Response Relationship, Drug; D015850:Interleukin-6 / Q000276:immunology*; D008024:Ligands; D008070:Lipopolysaccharides / Q000008:administration & dosage*; D051379:Mice; D018485:Muscle Fibers, Skeletal / Q000187:drug effects / Q000276:immunology*; D051193:Toll-Like Receptors / Q000276:immunology*; D015533:Transcriptional Activation / Q000187:drug effects / Q000276:immunology*
chemical_list: D001426:Bacterial Proteins; D002352:Carrier Proteins; D015850:Interleukin-6; D008024:Ligands; D008070:Lipopolysaccharides; D051193:Toll-Like Receptors; C084529:plasminogen-binding protein, bacteria