C-Jun and JunD suppress maturation of chondrocytes

To analyze the function of AP-1 in cartilage formation, two types of primary chondrocytes, LS and US cells, were prepared from caudal (lower) and cephalic (upper) regions of chicken sterna, respectively. All the known components of chicken AP-1 (c-Fos, Fra-2, c-Jun, and JunD) were detected in both cell types, but the expression level of c-Jun was much higher in LS cells, which are rich in less mature chondrocytes than US cells. In the sterna, the expression level of c-Jun was also lower in the maturating or hypertropic chondrocytes than in proliferating chondrocytes. When US cells were treated with parathyroid hormone (PTH), which prevented maturation as judged from the maturation-associated markers such as alkaline phosphatase and type X collagen, the expression levels of c-Jun and JunD were constitutively elevated. To analyze the possible relationship between differentiation status and expression levels of Jun family proteins, they were exogenously introduced into the entire population of US cells within 2 days by using high titer, replication-competent retroviral vectors. Maturation-associated markers in US cells were specifically lowered by exogenous expression of c-Jun or JunD to similar levels to those of LS cells or US cells treated with PTH. When US cells were infected with the virus encoding a dominant negative mutant of AP-1 (supJunD-1), maturation markers were moderately increased 10 days after infection. The potent induction of alkaline phosphatase activity in US cells by all-trans retinoic acid was annulled by exogenous expression of either c-Jun or JunD. These results suggest that Jun family proteins negatively regulate the maturation process of chondrocytes.