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RECEPTORS AND SIGNAL TRANSDUCTION

Transient membrane recruitment of IRAK-1 in response to LPS and IL-1β requires TNF R1

Departments of ¹Biochemistry and Molecular Biology and ²Cellular and Integrative Physiology, the Walther Oncology Center, Indiana University School of Medicine and the Walther Cancer Institute, Indianapolis, Indiana

Submitted 19 October 2007 ; accepted in final form 12 June 2008

The transcription factor NF-B is an essential regulator of the innate immune response that functions as the first line of defense against infections. Activation of the innate immune response by bacterial lipopolysaccharide (LPS) triggers production of tumor necrosis factor- (TNF-) followed by interleukin-1 (IL-1). The IL-1 receptor associated kinase-1 (IRAK-1) is an integral component of the LPS, TNF-, and IL-1 signaling pathways that regulate NF-B. Thus we hypothesized that IRAK-1 coordinates cellular NF-B responses to LPS, TNF-, and IL-1. In contrast to TNF- where IRAK-1 subcellular localization does not change, treatment with LPS or IL-1 leads to a loss in cytoplasmic IRAK-1 with a coordinate increase in plasma membrane associated modified IRAK-1. In fibroblasts lacking the type 1 TNF- receptor (TNF R1), IRAK-1 turnover is altered and modification of IRAK-1 in the plasma membrane is decreased in response to LPS and IL-1, respectively. When NF-B controlled gene expression is measured, fibroblasts lacking TNF R1 are hyperresponsive to LPS, whereas a more variable response to IL-1 is seen. Further analysis of the LPS response revealed that plasma membrane-associated IRAK-1 is found in Toll 4, IL-1, and TNF R1-containing complexes. The data presented herein suggest a model whereby the TNF R1-IRAK-1 interaction integrates the cellular response to LPS, TNF-, and IL-1, culminating in a cell poised to activate TNF--dependent NF-B controlled gene expression. In the absence of TNF R1-dependent events, exposure to LPS or IL-1 leads to hyperactivation of the inflammatory response.

TNF-; cytokines; ubiquitinylation; cytoplasmic; phosphorylation