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Arch Hellen Med, 21(6), November-December 2004, 528-545


Increased cartilage loading triggers differentiation/maturation of articular surface chondroblasts
via activation of the MAPK–AP-1 signal transduction pathway

1Department of Pathology, School of Medicine, University of Ioannina, Ioannina
2Department of Biochemistry, School of Medicine, University of Patras, Patras
3Department of Orthodontics, School of Dentistry, Aristοtle University of Thessaloniki, Thessaloniki, Greece

OBJECTIVE Mechanical loading is an important regulator of chondro-osteogenesis and a determinant of skeletal morphology. The molecular mechanisms underlying the chondroblastic response to mechanical cues remain elusive. Mitogen-activated protein/stress activated protein (MAP/SAP) kinase (MAPK) pathways and the activator protein-1 (AP-1) transcription complex have been shown to be involved in mechanically-stimulated osteoblast differentiation. Also, mechano-induced extracellular signal-regulated kinase (ERK) MAPKs activate the bone-specific transcriptional modulator core binding factor 1 (Cbfa1) Runx2, which is engaged in osteoblast/ chondroblast (patho)biology. The present study was undertaken to explore the effect of mechanical load on c-Jun N-terminal kinase (JNK)–AP-1 and ERK signal transduction pathways activation and its implication in articular cartilage biology.

METHOD One hundred rats were assigned to two groups: the first group was fed hard diet, causing increased joint loading, while the second was fed soft diet, simulating normal mastication. Biopsies from the temporomandibular joints of both groups were obtained at 2 and 24 hours post-experiment initiation, and the expression levels of a range of protein profiles were assessed immunohistochemically, specifically: pc-Jun (phosphorylated-active form of c-Jun), JunB and JunD (members of the Jun family of proteins), c-Fos (the major pc-Jun partner in forming the AP-1 transcription complex), Fra-1 and Fra-2 (members of the Fos family of proteins), JNK2 (principal c-Jun kinase), p-JNK (phosphorylated-active form of JNK), p-ERK (phosphorylated-active ERK species) and Cbfa1/Runx2. The above protein profiles were subjected to statistical correlation.

RESULTS The protein levels of pc-Jun, c-Fos, p-ERK and Cbfa1/Runx2, but not those of JNK2, p- JNK, JunB, JunD, Fra-1 and Fra-2 were significantly higher in rats fed hard diet, at 24 but not at 2 hours after the experiment initiation. The immunoexpression of pc-Jun, JunB, JunD, c-Fos, Fra-1, Fra-2, p-JNK, p-ERK and Cbfa1/Runx2 was significantly enhanced at 24 compared to 2 hours, especially in animals fed hard diet. CONCLUSIONS Increased cartilage loading induces the MAPK signaling pathway and it appears that the mechano-potentiated JNK and ERK cascades “converge” on AP-1 and Cbfa1/Runx2 transcription factors to regulate the chondroblastic differentiation/maturation process.

Key words: AP-1, Cbfa1/Runx2, Chondrocytes, MAP kinase, Mechanical load.

© Archives of Hellenic Medicine