Evidence for two distinct pathways in TNFα-induced membrane and soluble forms of ICAM-1 in human osteoblast-like cells isolated from osteoarthritic patients

SummaryObjectiveThe present study aimed to investigate the modulation of membrane-bound intercellular adhesion molecule-1 (mICAM-1) and soluble ICAM-1 (sICAM-1) expression by tumor necrosis factor-alpha (TNFα) in human osteoarthritic (OA) osteoblasts.MethodsCultured human primary osteoblasts were stimulated with increasing concentrations of human recombinant TNFα. Expression of mICAM-1 and sICAM-1 was evaluated by immunocytochemistry, enzyme-linked immunosorbent assay and semi-quantitative reverse transcriptase-polymerase chain reaction. In addition, we investigated the molecular mechanisms underlying ICAM-1 induction by TNFα, focusing on the activation of the mitogen-activated protein kinases (MAPKs) and nuclear factor-kappaB (NF-κB) pathways.ResultsOur data showed that TNFα dose-dependently increased mICAM-1 and sICAM-1 expression at the protein and mRNA levels in OA osteoblasts. The inhibitor of de novo mRNA synthesis, actinomycin D, suppressed TNFα-induced mICAM-1 and sICAM-1 expression. Upon examination of the signaling components, we found that TNFα was a potent activator of p38, p44/42, p54/46 MAPK, and IkappaBalpha (IκBα). The chemical inhibitors of p38, p44/42 MAPK, and NF-κB blocked TNFα-induced mICAM-1 expression but not that of sICAM-1. Transfection experiments revealed that p38 MAPK or IkappaB kinase alpha (IKKα) overexpression enhanced TNFα-induced mICAM-1 production. Furthermore, osteoblasts treatment with a chemical inhibitor of metalloproteinase-9 (MMP-9) activity, a proteolytic enzyme involved in ICAM-1 cleavage, evoked a significant 25% decrease of TNFα-induced sICAM-1 release.ConclusionTaken together, these findings illustrate the central role played by TNFα in the regulation of ICAM-1. We suggest that TNFα differentially regulates sICAM-1 and mICAM-1 expression and that sICAM-1 release involves, in part, the proteolytic cleavage of mICAM-1 by MMP-9. The capacity of the MMP-9 inhibitor to prevent sICAM-1 production may be useful for the development of novel therapeutic approaches relevant to OA.