TNF and rheumatoid arthritis: TNFα is the main proinflammatory cytokine in RA and promotes bone erosion by triggering osteoclast on through the RANK-RANKL system and by suppressing osteoblast bone formation through the overexpression of Dickkopf-1 (Dkk1), a potent inhibitor of the anabolic Wnt bone signaling pathway [5, 6].