p16INK4a Exerts an Anti-Inflammatory Effect through Accelerated IRAK1 Degradation in Macrophages

Abstract

Induction of cyclin-dependent kinase (CDK) inhibitor gene p16INK4a into the synovial tissues suppresses rheumatoid arthritis in animal models. In vitro studies have shown that the cell-cycle inhibitor p16INK4a also exerts anti-inflammatory effects on rheumatoid synovial fibroblasts (RSF) in CDK activity-dependent and -independent manners. The present study was conducted to discern how p16INK4a modulates macrophages, which are the major source of inflammatory cytokines in inflamed synovial tissues. We found that p16INK4a suppresses LPS-induced production of IL-6 but not of TNF-α from macrophages. This inhibition did not depend on CDK4/6 activity and was not observed in RSF. p16INK4a gene transfer accelerated LPS-triggered IL-1R–associated kinase 1 (IRAK1) degradation in macrophages but not in RSF. The degradation inhibited the AP-1 pathway without affecting the NF-κB pathway. Treatment with a proteosome inhibitor prevented the acceleration of IRAK1 degradation and downregulation of the AP-1 pathway. THP-1 macrophages with forced IRAK1 expression were resistant to the p16INK4a-induced IL-6 suppression. Senescent macrophages with physiological expression of p16INK4a upregulated IL-6 production when p16INK4a was targeted by specific small interfering RNA. These findings indicate that p16INK4a promotes ubiquitin-dependent IRAK1 degradation, impairs AP-1 activation, and suppresses IL-6 production. Thus, p16INK4a senescence gene upregulation inhibits inflammatory cytokine production in macrophages in a different way than in RSF.