Background/Purpose: Interleukin-1β (IL-1β) and IL-1α are cytokines of IL-1 family that orchestrate acute and chronic inflammatory diseases. However, their distinct role or the extent of overlap in the inflammatory processes remains poorly understood. This prompted us to explore the role of IL-1α in IL-1β-activated signaling pathways causing synovial inflammation in rheumatoid arthritis (RA).

Methods: Normal and RA synovial tissues were obtained from total joint replacement surgery or synovectomy under an Institutional Review Board approved protocol in compliance with the Helsinki Declaration. RNA isolated from these tissues were subjected to RNA sequencing using Illumina platform. Synovial fibroblasts from RA tissues (RASFs) were treated with various IL-1β, tumor necrosis factor-α (TNF-α), or lipopolysaccharide (LPS) to study the expression of IL-1α using Western blotting, immunofluorescence (IF), and qRT-PCR methods. Native immunoprecipitation (IP) and chromatin IP (ChIP) methods were used to study protein-protein interactions and chromatin remodeling function of IL-1α. Small interfering RNA (siRNA) method was used to study the role of IL-1α in IL-1β signaling pathway. Molecular dynamics (MD) simulation were performed to assess binding patterns of IL-1α or IL-1β as ligand on IL-1 receptor (IL-1R) crystal structure to identify functional differences in activating phosphorylation of key IL-1 signaling proteins.

Results: Results: IL-1β selectively induced the expression of IL-1α transcript (p<0.01; n=4), which reached to >1,000-fold within 6 hours stimulation. Evaluation of the signaling inhibitors showed that IL-1β significantly stimulated IL-1α expression, which was selectively inhibited by blocking NF-κB pathway, whereas further exacerbated by p38-MAPK inhibition. Interestingly, knockdown of IL-1α using siRNA abolished IL-1β-induced pro-IL-1α and pro-IL-1β expression and suppressed inflammation (p<0.05; n=3). IF results showed that IL-1α primarily resides in nuclear and chromatin bound fractions of RASF and interacts with p65 subunit of NF-κB protein upon IL-1β stimulation. Native IP and ChIP studies showed that IL-1α cooperates in NF-κBp65 binding to the distal region of IL-1α promoter and to the proximal region of IL-1β promoter upstream of the transcription start site to stabilize their gene transcription. Furthermore, MD simulation of IL-1α or IL-1β binding to IL-1R crystal structure showed distinct interaction sites for each cytokine, where more π-π interactions were observed with IL-1β over IL-1α with IL-1R indicating stable complex formation. These results corroborate with the ability of IL-1α to differentially activate phosphorylation of signaling proteins compared to IL-1β, where phosphorylation of proteins such as IRAK1^Ser387, IRAK4^Ser345, TAK1^{Thr184/187/412} and the degradation of IRAK1 were more rapid in IL-1β stimulated RASFs compared to IL-1α stimulation.

Conclusion: These results suggest that IL-1β relies on IL-1α to stabilize its expression and propagate inflammation. It may be postulated that IL-1α targeted therapies may have some benefit over IL-1R antagonists in regulating inflammation.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/critical-role-of-interleukin-1%ce%b1-il-1%ce%b1-in-il-1%ce%b2-induced-inflammatory-responses-cooperation-with-nf-%ce%babp65-in-transcriptional-regulation/