Intracellular IL-1Ra type 1 inhibits IL-1-induced IL-6 and IL-8 production in Caco-2 intestinal epithelial cells through inhibition of p38 mitogen-activated protein kinase and NF-kappaB pathways.

Interleukin-1 (IL-1) plays a pivotal role in the pathogenesis of inflammatory bowel disease (IBD). IL-1 action is regulated in part by its naturally occurring inhibitor, the IL-1 receptor antagonist (IL-1Ra). Four splice variants of IL-1Ra gene product have been described, one secreted (sIL-1Ra) and three intracellular (icIL-1Ra1, 2, 3). Although sIL-1Ra and icIL-1Ra1 bind to type I IL-1 receptor with equal affinity, icIL-1Ra1 may carry out unique functions inside cells. The goal of this study was to determine the role of icIL-1Ra1 in regulation of cytokine-induced IL-6 and IL-8 production in Caco-2 intestinal epithelial cells. icIL-1Ra1 inhibited IL-1-induced IL-6 and IL-8 production. IL-1 activated all three mitogen-activated protein (MAP) kinase family members: p38 MAP kinase, extracellular-regulated kinases (ERK), and c-Jun amino-terminal kinases (JNK). Specific inhibitors of each MAP kinase pathway decreased IL-1-induced IL-6 and IL-8 production. Overexpression of icIL-1Ra1 inhibited p38 MAP kinase phosphorylation, but had no effect on ERK and JNK phosphorylation. In addition, icIL-1Ra1 inhibited nuclear translocation of NF-kappaB after IL-1 stimulation. In conclusion, these data indicate that icIL-1Ra1, acting in the cytoplasm of Caco-2 cells, decreased IL-1-induced IL-6 and IL-8 production. This intracellular anti-inflammatory activity of icIL-1Ra1 was mediated through inhibition of p38 MAP kinase and NF-kappaB signal transduction pathways.