TNF and rheumatoid arthritis: It has also been documented that the expression of hypoxia‐inducible factor (HIF)‐1α, a cardinal transcription factor of VEGF 4, was elevated in RA FLSs with the stimulation of interleukin (IL)‐1 or tumor necrosis factor (TNF)‐α 5, indicating the unique role of FLSs in generating and maintaining the pannus in RA.