This study aims to investigate the role of interleukin-21 (IL-21) in the mechanism of osteonecrosis of the femoral head. The serum content of IL-21 in patients with osteonecrosis of the femoral head (ONFH) and with femoral neck fracture (FNF) was examined by an enzyme-linked immunosorbent assay (ELISA). The cartilage specimens were stained with safranin-O. The expression of IL-21, tumor necrosis factor-α (TNF-α), cyclooxygenase (COX-2), a disintegrin and metalloproteinase with thrombospondin motifs 4 (ADAMTS-4), matrix metalloproteinase-13(MMP-13), and aggrecan in the cartilage was detected, by immunohistochemistry and western blot analysis. Moreover, chondrocytes were treated with IL-21 (100 ng/mL) or PBS to examine the mRNA content of TNF-α, COX-2, IL-1β and NOS-2 by RT-PCR, and the protein levels of ADAMTS-4, MMP-13, and aggrecan by western blot analysis. The activity of the JAK-STAT pathway was determined in vitro after stimulation with IL-21. IL-21 serum levels were obviously higher in the ONFH patients and positively correlated with the severity of ONFH. There were more cells positive for inflammatory cytokines, including IL-21, TNF-α, and COX-2, in the cartilage of the ONFH patients than the FNF patients. The level of certain relative biomarkers, such as ADAMTS-4 and MMP-13, was higher and aggrecan was lower in the ONFH patients. The mRNA contents of TNF-α, COX-2, IL-1β, and NOS-2, as well as the levels of ADAMTS-4, MMP-13, were enhanced, and aggrecan decreased after stimulation with IL-21. The protein content of p-STAT-1, as well as p-STAT-3, also increased after IL-21 stimulation, and the highest level appeared at 30 min. Furthermore, the protein level of ADAMTS-4 and MMP-13 and the mRNA level of TNF-α, COX-2, IL-1β, and NOS-2 significantly decreased after stimulation with AG-490. IL-21 enhances cartilage inflammation to promote cartilage degradation through the JAK-STAT signaling pathway in the cartilage of ONFH patients.
Keywords: IL-21; STAT; inflammation; osteonecrosis of the femoral head.