Signal transduction by IL1

The role of internalization in signal transduction remains unclear. After IL-1 binding, IL-1 type I receptor is rapidly internalized. This ligand/receptor complex is relatively resistant to biochemical degra dation and can subsequently be found associated with the nucleus. The identification of the intracellular signal transduction pathways and second messenger systems for the biological effects of II.-1 remain to be completely determined. Different types of responding cells may utilize diverse signal transduction pathways for IL-1. IL-1 induces the phosphorylation of a number of cellular proteins such as epidermal growth factor receptor and heat shock proteins. Various kinases including protein kinase C (PKC), tyrosine kinase, mitogen-activated protein kinase (MAP kinase), and ceramide-activated kinase are activated by IL-1. Phosphorylation of phospholipase A, (PLA,)-activating protein activates PLA2, resulting in the release of arachidonic acid from membrane phospholipids. This leads to enhanced cyclooxygenase activity and the production of prostanoids. In T cells, IL-1 stimulates phosphatidylcho-line-specific phospholipase C to generate 1,2-diacyl-glycerol, which is a potent activator of protein kinase C. IL-1 also activates the transcription factors AP-1 and NFkB which bind to the promoter regions of certain genes (IL-6, IL-8, II.-2 receptor, endothelial cell adhesion molecule). IL-1 enhances the transcription of c-fun by activating two novel nuclear factors. Recent studies by Mathias and colleagues suggest that IL-1 binding to receptors activates a neutral sphingomyelinase, hydrolyzing sphingomyelin to cer-amide. Ceramide then stimulates a ceramide-acti-vated protein kinase that phoshorylates a specific subset of cellular proteins, thereby altering their function.

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