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The IL‐1 receptor signaling pathway

Interleukin‐1 (IL‐1) exerts pleiotropic effects on a variety of tissues through binding to its receptor. Two distinct types of receptors for IL‐1 have been characterized in mouse and human. Most of the IL‐1 signal has been shown to be transmitted through type I IL‐1R (80 kDa) in T lymphocytes as well as B lymphocytes and monocytes. Type II receptor may act as a suppressor of IL‐1 biological activities by competing in binding with type I receptors on the cell surface. Functional studies of the type I IL‐1R demonstrated that the cytoplasmic segments, possessing a sequence similarity with the Drosophila Toll gene product or IL‐6R β chain, gpl30, are important for transmitting activity that induces cytokine genes. In the past three years, several groups reported that IL‐1 and tumor necrosis factor (TNF) rapidly induce sphingomyelin turnover in various types of cells, producing ceramide, which may act as a second messenger molecule in an intracellular signaling cascade. Activation of both acid and neutral sphingomyelinases (SMases) has been suggested, and Schutz et al. proposed that the phosphatidylcholine–phospholipase C/acid SMase pathway is involved in TNF‐induced NF‐xB activation. However, our recent study showed that the NF‐xB activation is induced by IL‐1/TNF in fibroblasts from patients with type A Niemann‐Pick disease, with acid SMase deficiency. This finding implies that acid SMase activity is not essential for the activation of NF‐xB by IL‐1/TNF at least in fibroblasts. Other signaling pathways including neutral SMase and unidentified protein kinases may be important for NF‐xB–mediated cytokine gene activation. J. Leukoc. Biol. 56: 542–547; 1994.