Regulatory role of heme oxygenase 1 in inflammation of rheumatoid arthritis

Objective To examine the expression and pathogenetic roles of heme oxygenase 1 (HO‐1), an inducible heme‐degrading enzyme with antiinflammatory properties, in rheumatoid arthritis (RA). Methods HO‐1 expression in synovial tissue from patients with RA, patients with osteoarthritis, and patients with noninflammatory joint diseases was determined by immunoblotting and immunohistochemistry. Effects of various agents, such as hemin (a chemical inducer of HO‐1), small interfering RNA (siRNA) specific for HO‐1, HO‐1 expression vector, and antirheumatic agents, on HO‐1 expression in RA synovial cell lines were analyzed by real‐time reverse transcription–polymerase chain reaction (PCR) and immunoblotting. Cytokine synthesis was evaluated by real‐time PCR and enzyme‐linked immunosorbent assay. Results HO‐1 was expressed more abundantly in the lesions of synovial tissue from patients with RA than in those from the other patient groups. Hemin, auranofin, and HO‐1 expression vector induced HO‐1 and reduced expression of tumor necrosis factor α (TNFα) messenger RNA, lipopolysaccharide (LPS)–induced secretion of interleukin‐6 (IL‐6) and IL‐8, and expression of cyclooxygenase 2 in the synovial cell lines. Treatment with HO‐1–specific siRNA augmented the synthesis of TNFα, IL‐6, and IL‐8 and canceled the suppressive effects of auranofin on TNFα secretion. When hemoglobin, as a scavenger of carbon monoxide, was added to auranofin‐treated synovial cell lines, LPS‐dependent production of IL‐6 and IL‐8 was increased. Conclusion Our data demonstrate that HO‐1 is expressed in RA synovial tissues and plays a regulatory role in the development of inflammation. The pharmacologic effects of auranofin depend, in part, on the levels of HO‐1, suggesting that HO‐1 induction is a novel therapeutic strategy for RA.