Objective: To examine the regulation of the intercellular adhesion molecule 1 (ICAM-1) gene in cultured human synovial fibroblasts in response to tumor necrosis factor alpha (TNF alpha), and investigate its modulation by the synthetic glucocorticoid, dexamethasone.
Methods: Cell surface expression of ICAM-1 was determined by flow cytometry, enzyme immunoassay, and immunoprecipitation. ICAM-1 messenger RNA (mRNA) levels were monitored by Northern blot. ICAM-1 function was determined by measuring the adhesion of monocytes to synovial fibroblasts.
Results: ICAM-1 expression on unstimulated cells was weak but was rapidly enhanced in both a time- and dose-dependent manner following exposure to TNF alpha. Treatment of the cells with TNF alpha also resulted in both a time- and dose-dependent increase in steady-state ICAM-1 mRNA levels, as determined by Northern blot. The increased expression of ICAM-1 was inhibited by cycloheximide and actinomycin D. Cultured synovial fibroblasts from patients with rheumatoid and nonrheumatoid arthropathies responded similarly to TNF alpha. Adhesion studies demonstrated that ICAM-1 is involved in the adherence of peripheral blood monocytes to TNF alpha-activated synovial fibroblasts. In addition, dexamethasone inhibited TNF alpha-induced surface expression of ICAM-1, accumulation of ICAM-1 mRNA, and adhesion of monocytes to TNF alpha-activated synovial fibroblasts.
Conclusion: These combined results provide further evidence of an important role of ICAM-1 in inflammatory synovitis, as well as a potentially novel site of antiinflammatory action of glucocorticoids.