Hyperplasia of synovial lining cells is one of the main features of rheumatoid arthritis (RA). We previously reported that ErbB2 is highly expressed in RA synovial cells, and that they play an important role in their hyperproliferative growth. Recent findings have suggested that poly(ADP-ribose) polymerase 1 (PARP1) is involved in the transactivation of NF-B-dependent genes such as ErbB2. Here, we investigate the role of PARP1 in ErbB2 transcription in RA synovial cells. The expression level of PARP1 is significantly high in synovial cells derived from all of 3 independent patients with RA, as compared with 3 patients with osteoarthritis (OA). Luciferase assays revealed that PARP1 augments the transcription of the erbB2 gene, and that a region between -404 and -368 is responsible for this activation. A protein with an apparent molecular mass 115 kDa was isolated mainly from nuclear extracts of RA synovial cells with an affinity matrix harboring DNA fragment identical to the above region. Mass spectrometric analysis demonstrated this protein to be PARP1. Southwestern blotting analysis showed that PARP1 binds to this region, but not to adjacent regions. PARP1 associates directly with NF-B, and a chromatin immunoprecipitation assay indicated that these proteins interact with this enhancer region in the erbB2 gene. Treatment of RA synovial cells with PARP1 siRNA attenuated their ErbB2 expression, while an inhibitor of the polymerase activity of PARP1 had no effect. PARP1 DNA binding is not required for transcriptional activation. These findings suggest that PARP1 is involved in the expression of ErbB2 in concert with NF-B, which might be associated with the proliferation of RA synovial cells.