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Novel intracellular function of IL-1 receptor antagonist type 1 in endothelial cells
Arthritis Res Thervolume 5, Article number: 72 (2003)
IL-1 receptor antagonist (IL-1Ra) is a member of the IL-1 family that competitively inhibits the binding of IL-1 to its cell surface receptors, and thus acts as a natural IL-1 inhibitor. Four different IL-1Ra peptides are produced from the same gene by the use of alternative first exons, mRNA splicing, and alternative translation. One isoform is secreted (sIL-1Ra), while the three other isoforms remain intracellular (icIL-1Ra1, icIL-1Ra2, icIL-1Ra3). In a previous study, we observed that icIL-1Ra1 is produced in high amounts in the joints of mice with collagen-induced arthritis (CIA) and that the expression of icIL-1Ra1 coincided with the resolution of articular inflammation. Recently, we showed that mice transgenic for icIL-1Ra1 were protected from CIA in a similar manner to sIL-1Ra transgenic mice. However, as icIL-1Ra1 was also detected in the circulation, it was impossible to determine whether icIL-1Ra1 exerted its anti-inflammatory effect inside cells or through its interaction with cell surface receptors. To address this question, we examined the intracellular effects of icIL-1Ra1 on endothelial cell migration. IL-1a is produced as a 31 kDa protein (pre-IL-1a) that is cleaved to generate C-terminal mature IL-1a. The N-terminal region of pre-IL-1a contains a nuclear localization domain, and both pre-IL-1a and the 16kDa N-terminal IL-1a are able to migrate into the nuclei and to modulate endothelial cell migration. The migration of the endothelial cell line ECV was significantly increased in cells transfected with pre-IL-1a or N-terminal IL-1a propiece. In contrast, the addition of mature IL-1a in culture medium was devoid of any effect on cell migration, indicating that the effect of pre-IL-1a and 16kDa N-terminal IL-1a was independent of their interaction with cell surface receptors. Most interestingly, expression of icIL-1Ra1 in ECV cells completely reversed the effects of pre-IL-1a and N-terminal IL-1a. Immunofluorescence studies indicated that the N-terminal IL-1a was located in the nuclei. icIL-1Ra1 was located both in the cytoplasm and in the nuclei, and co-expression of icIL-1Ra1 modified the intracellular localization of the N-terminal IL-1a. In conclusion, this study demonstrates for the first time that icIL-1Ra1 may carry out important intracellular regulatory functions in endothelial cells.