Tumor necrosis factor α acceleration of inflammatory responses by down‐regulating heme oxygenase 1 in human peripheral monocytes

Objective To examine the interaction between heme oxygenase 1 (HO-1), a stress-induced antiinflammatory protein, and tumor necrosis factor α (TNFα) in human peripheral blood monocytes. Methods Peripheral blood mononuclear cells (PBMCs) were obtained from healthy donors or from patients with rheumatoid arthritis (RA) receiving the anti–tumor necrosis factor α (anti-TNFα) monoclonal antibody infliximab. CD14+ cells were isolated by magnetic cell sorting, cultured with TNFα or auranofin, and transfected with a plasmid encoding HO-1 or an HO-1–specific small interfering RNA vector. Protein and messenger RNA (mRNA) levels were examined by immunoblotting and real-time polymerase chain reaction. Cytokine levels in culture supernatants were measured by enzyme-linked immunosorbent assay. HO-1 gene transcription was evaluated using a luciferase reporter gene assay. Actinomycin D and cycloheximide were used to monitor the stability of mRNA and protein. Results HO-1 is constitutively expressed by CD14+ PBMCs from healthy donors. TNFα suppressed HO-1 expression by accelerating the decay of mRNA without affecting gene transcription or protein stability. Forced expression or selective knock-down of the HO-1 gene expression resulted in down-regulation or up-regulation, respectively, of proinflammatory cytokine synthesis by monocytes. Treatment with infliximab significantly increased HO-1 mRNA levels and reduced TNFα synthesis by PBMCs from RA patients. Conclusion TNFα accelerated inflammatory responses by down-regulating HO-1 expression in human monocytes. TNF antagonists may block this TNF-dependent suppression of HO-1 expression, resulting in an amelioration of inflammation.