Pro-inflammatory cytokines such as IL-1, TNF-α, and IL-33 have previously been reported to increase the expression of HIF isoforms in synovial fibroblasts (Thornton et al., 2000; Hu et al., 2013; Ryu et al., 2014) and in animal models of RA (Xu et al., 2008; Hu et al., 2013; Ryu et al., 2014). Here, TNF is linked to rheumatoid arthritis.