Interleukin 6 (IL-6) is a central alarm hormone of the mammalian body. During acute and chronic inflammations, it induces acute phase plasma protein synthesis by liver hepatocytes, modulates the immune response and participates in the regulation of body temperature (fever). In addition, it is a growth factor for certain tumor cells, such as myeloma cells. The details of the IL-6 signal transduction mechanism are unknown. We have contributed to this problem at 2 levels: (a), we have mapped an IL-6 response element (IL-6-RE) in the 5′ flanking region of the α2-macroglobulin gene (α2-M), a prototype rat liver acute phase gene. This element, CTGGGA, serves as a binding site for nuclear factors that facilitate hormone induced transcription. We have begun to characterize these factors from hepatic cells and demonstrated that they undergo characteristic IL-6-induced changes. Similar factors were also discovered in human Burkitt tumor derived cell lines (B cells). These bound at the IL-6-RE of the rat α1M gene and formed indistinguishable protein DNA complexes, as the corresponding hepatic factors. Thus, common elements probably operate in the IL-6 signal transduction cascade in liver cells and B cells; (b), we have cloned the rat liver IL-6 receptor (IL-6-R) and derived its amino acid sequence. It was 53% identical to the human leukocyte IL-6 R and all functional domains were highly conserved. Therefore, the cell-type specific responses to IL-6 in liver cells and lymphocytes were probably not due to cell-type specific formsof the receptor, but to other so far unknown elements of the signal transduction cascade. Two mRNA species (cDNA clones) for the rat liver IL-6-R were isolated. Both coded for an identical protein and differed in their 3′ untranslated regions (3′ UTR), due to alternative polyadenylation. The 3′ UTR of the longer mRNA species carried sequence elements associated with regulation of mRNA stability and translation efficiency. Upon transfection into hepatic cells and Jurkat cells (leukocytes), the shorter mRNA species selectively functioned in the Jurkat cells and only the longer mRNA species in hepatic cells. IL-6-R mRNA levels were upregulated 4-fold during an acute phase response. The receptor mRNA was inducible by glucocorticoids and under negative regulation by IL-6. IL-6-R mRNA was further inducible in HL60 promyelocytic cells upon induction of granulocytic differentiation.
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