Potential role of HOXD9 in synoviocyte proliferation

To investigate the role of HOXD9 in the proliferation activity of cultured synoviocytes as well as the mechanisms that regulate HOXD9 transcription.

Synoviocytes from patients with rheumatoid arthritis (RA) and osteoarthritis (OA) were transfected with HOXD9 complementary DNA to establish stable transformants that overexpressed HOXD9. HOXD9 expression was detected by Western blotting with anti-HOXD9 antibody. The growth properties of the transformants were investigated by proliferation and colony formation assays. The expression of basic fibroblast growth factor (bFGF), tumor necrosis factor alpha (TNFalpha), interleukin-1beta, c-Fos, and c-Myc was examined by Western blotting. Transcriptional regulation of HOXD9 was examined by transient cotransfection.

HOXD9 protein was highly expressed in RA synoviocytes, but there was no expression in OA synoviocytes. HOXD9 transfection induced stable HOXD9 protein expression in synoviocytes and showed an increased proliferation rate under both normal and serum-starved conditions, as well as an enhanced capacity to proliferate anchorage independently to form colonies in soft agar cultures, compared with control transfectants. Higher levels of bFGF and c-Fos were detected in HOXD9 transformants than in controls. Transient cotransfection assays of NIH3T3 fibroblasts and synoviocytes showed that HOXD9 activated the luciferase reporter construct containing the highly conserved region (HCR), an autoregulatory element of HOXD9 promoter. This activation was significantly increased by bFGF, suppressed by TNFalpha, and unchanged by transforming growth factor beta in synoviocytes. Human T lymphotropic virus type I tax also activated the luciferase reporter construct containing the HCR and had a synergistic effect with HOXD9 on HCR promoter activation.

Our data suggest that HOXD9 plays a potential role in synovial proliferation. In addition, they suggest that the involvement of HOXD9 in the regulation of cellular growth might be mediated, at least in part, by up-regulation of growth-related factors such as bFGF and c-Fos and/or might result from increased transcription activity by its regulators.