Senescence of chondrocytes in aging articular cartilage: GADD45β mediates p21 expression in association with C/EBPβ in senescence-accelerated mice.

Growth arrest and DNA damage-inducible protein 45β (GADD45β) is expressed in normal and early osteoarthritic articular cartilage. We recently reported that GADD45β enhances CCAAT/enhancer binding protein β (C/EBPβ) activation in vitro. This study was undertaken in order to determine whether GADD45β is expressed with C/EBPβ in aging articular cartilage. We also investigated whether the synergistic expression of GADD45β and C/EBPβ may be involved in the mechanism of chondrocyte senescence. Senescence-accelerated mice (SAMP1) were used as a model of aging. GADD45β, C/EBPβ, and p21 were analyzed by immunohistochemistry. A luciferase reporter assay using ATDC5 cells was performed in order to examine p21 as a target gene of the GADD45β/C/EBPβ cascade. GADD45β exhibited increased expression in the aging articular cartilage of SAMP1 mice compared to that in control mice. The co-localization of GADD45β and C/EBPβ was confirmed by double immunostaining. The synergistic mechanisms of GADD45β and C/EBPβ on the gene regulation of p21, a molecule related to cellular senescence, were verified by a p21-luciferase reporter assay. Co-expression of C/EBPβ and p21 was confirmed. These observations suggest that the synergism between GADD45β and C/EBPβ may play an important role in cellular senescence in the aging articular cartilage.