Differential expression of GADD45β in normal and osteoarthritic cartilage -: Potential role in homeostasis of articular chondrocytes

Objective. Our previous study suggested that growth arrest and DNA damage-inducible protein 45 beta (GADD45 beta) prolonged the survival. of hypertrophic chondrocytes in the developing mouse embryo. This study was undertaken, therefore, to investigate whether GADD45 beta plays a role in adult articular cartilage. Methods. Gene expression profiles of cartilage from patients with late-stage osteoarthritis (OA) were compared with those from patients with early OA and normal controls in 2 separate microarray analyses. Histologic features of cartilage were graded using the Mankin scale, and GADD45 beta was localized by immunohistochemistry. Human chondrocytes were transduced with small interfering RNA (siRNA)-GADD45 beta or GADD45 beta-FLAG. GADD45 beta and COL2A1 messenger RNA (mRNA) levels were analyzed by real-time reverse transcriptase-polymerase chain reaction, and promoter activities were analyzed by transient transfection. Cell death was detected by Hoechst 33342 staining of condensed chromatin. Results. GADD45 beta was expressed at higher levels in cartilage from normal donors and patients with early OA than in cartilage from patients with late-stage OA. All chondrocyte nuclei in normal cartilage immunostained for GADD45 beta. In early OA cartilage, GADD45 beta was distributed variably in chondrocyte clusters, in middle and deep zone cells, and in osteophytes. In contrast, COL2A1, other collagen genes, and factors associated with skeletal development were up-regulated in late OA, compared with early OA or normal cartilage. In overexpression and knockdown experiments, GADD45 beta down-regulated COL2A1 mRNA and promoter activity. NF-kappa B overexpression increased GADD45 beta promoter activity, and siRNA-GADD45 beta decreased cell survival per se and enhanced tumor necrosis factor a-induced cell death in human articular chondrocytes. Conclusion. These observations suggest that GADD45 beta might play an important role in regulating chondrocyte homeostasis by modulating collagen gene expression and promoting cell survival in normal adult cartilage and in early OA.