p38/NF-kB-dependent expression of COX-2 during differentiation and inflammatory response of chondrocytes

Studying cartilage differentiation, we observed the emergence of inflammation‐related proteins suggesting that a common pathway was activated in cartilage differentiation and inflammation. In the present paper, we investigated the expression pathway of the inflammation‐related enzyme Cyclooxygenase‐2 (COX‐2) during differentiation and inflammatory response of the chondrocytic cell line MC615. Cells were cultured either as (i) proliferating prechondrogenic cells expressing type I collagen or (ii) differentiated hyperconfluent cells expressing Sox9 and type II collagen. The p38 and the NF‐kB pathways were investigated in standard conditions and after inflammatory agents treatment. NF‐kB was constitutively activated in differentiated cells. The activation level of NF‐kB in differentiated cells was comparable to the level in proliferating cells treated with the inflammatory agent LPS. In both cases, p65 was bound to the NF‐kB consensus sequence of COX‐2 promoter. p38, constitutively activated in differentiated cells, was activated in proliferating cells by treatment with LPS or IL‐1α. In stimulated proliferating cells the two pathways are connected since addition of the p38‐specific inhibitor SB203580 inhibited p38 activation, significantly reduced NF‐kB activation and repressed COX‐2 synthesis indicating that p38 is upstream NF‐kB activation and COX‐2 synthesis. In differentiated cells, the treatment with the inflammatory agent neither enhance NF‐kB activation, nor synthesis of COX‐2 while the addition of SB203580 neither repressed activation of p38, nor COX‐2 synthesis, suggesting a constitutive activation of a p38/NF‐kB/COX2 pathway. Our data indicate that in chondrocytes, COX‐2 is expressed via p38 activation/NF‐kB recruitment during both differentiation and inflammatory response. J. Cell. Biochem. 104: 1393–1406, 2008. © 2008 Wiley‐Liss, Inc.