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

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Abstract
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.

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