SDF-1 infiltrated cartilage, decreased cartilage proteoglycan content, and increased both glycosaminoglycan and MMP-13 activity levels in the explants culture medium. Immunostaining showed that SDF-1 penetrated cartilage explants partially by day 1 (top toward bottom) and completely by day 2 of SDF-1 incubation (250 ng/ml) (A). Frozen sections were used to detect CXCR4 expression with immunohistochemistry. CXCR4 expression was upregulated in osteoarthritic chondrocytes (right panel, B) compared with normal chondrocytes (left panel, B). Isotype Ig antibody was used as the negative control (middle panel, B). Safranin-O staining of cartilage explants demonstrated proteoglycan depletion with SDF-1 treatment (C, panels 2 and 3), which was attenuated by pretreatment with anti-CXCR4 antibody (C, panels 4 and 5). Proteoglycan loss in the SDF-1-treated explants began in the pericellular matrix and expanded into the territorial matrix. The absence of SDF-1 incubation resulted in no loss of proteoglycan (panel 1, C). Spectrophotometry analysis demonstrated increased levels of GAG in the cultured media of explants treated with SDF-1 and no increase in levels when anti-CXCR4 antibody was preadministered (n = 3). *P < 0.05 compared with no SDF-1 control. ^P < 0.05 compared with SDF-1-treated group (D). ELISA assay showed increased MMP-13 activity in the cultured media of explants treated with SDF-1 for 2 days, whereas pretreatment with anti-CXCR4 antibody before SDF-1 blocked this increase in activity (n = 3) (E). *P < 0.05 compared with the control. ^P < 0.05 compared with SDF-1-treated group.
Wei et al. Arthritis Research & Therapy 2012 14:R177 doi:10.1186/ar3930