Jae Gyu Kim†, Sung Joong Lee‡ and Martin F. Kagnoff*
Laboratory of Mucosal Immunology, University of California at San Diego, La Jolla, California 92093-0623
The transcription factor NF-κB in human intestinal epithelial cells plays a central role in regulating genes that govern the onset of mucosal inflammatory responses following intestinal microbial infection. Nod1 is a cytosolic pattern recognition receptor in mammalian cells that senses components of microbial peptidoglycans and signals the activation of NF-κB. The aim of these studies was to assess the functional importance of Nod1 in activating NF-κB and NF-κB proinflammatory target genes in human intestinal epithelium. Human colon epithelial cells that constitutively express Nod1 were used as a model intestinal epithelium. These cells do not signal through Toll-like receptor 4 (TLR4) or respond to bacterial lipopolysaccharide, but they express functional TLR5 and interleukin 1 (IL-1) receptors that signal the activation of NF-κB in response to bacterial flagellin or IL-1 stimulation. Stable expression of dominant negative (DN) Nod1 in colon epithelial cells prevented IκB kinase and NF-κB activation in response to infection with enteroinvasive Escherichia coli. In contrast, DN Nod1 did not eliminate IL-1 or flagellin-stimulated NF-κB activation. Inhibition of NF-κB was accompanied by inhibition of NF-κB target genes that provide signals for the mucosal influx of neutrophils during intestinal infection. We conclude that signaling through Nod1 is required for activating NF-κB in human intestinal epithelial cells infected with gram-negative enteric bacteria that can bypass TLR activation. Signaling through Nod1 provides the intestinal epithelium with a backup mechanism for rapidly activating innate immunity during infection with a group of highly invasive pathogenic gram-negative bacteria.
*Corresponding author. Mailing address: Laboratory of Mucosal Immunology, Departments of Medicine and Pediatrics, University of California at San Diego, Mail Code 0623D, La Jolla, CA 92093-0623.
† Present address: Department of Medicine, Chung-Ang University College of Medicine, Jung-Ku, Seoul, 100-272, Korea.
‡ Present address: Department of Structure and Functional Biology, College of Dentistry, Seoul National University, Chongno-Ku, Seoul, 110-749, Korea.