Endoplasmic reticulum stress induces the expression of COX-2 through activation of eIF2α, p38-MAPK and NF-κB in advanced glycation end products stimulated human chondrocytes.

INTRODUCTION: During aging, advanced glycation end products (AGEs) accumulate in articular cartilage. In this study we determined whether AGEs induce endoplasmic reticulum (ER) stress and studied the ER stress-activated pathways that stimulate cyclooxygenase-2 (COX-2) expression in human chondrocytes.
METHODS: Chondrocytes were stimulated with AGE-BSA. Gene expression was determined by quantitative PCR and protein expression was studied by immunoblotting. Studies to elucidate involved pathways were executed using siRNAs and specific inhibitors of eukaryotic initiation factor-2α (eIF2α), MAPKs and NF-κB.
RESULTS: AGE-BSA induced expression of GRP78 with concomitant increase in COX-2 expression was observed in human chondrocytes. In addition, expression of Bag-1, an ER stress marker was also increased by AGE-BSA. RAGE knockdown inhibited AGE-BSA-induced expression of GRP78 and COX-2. Treatment with eIF2α inhibitor or eIF2α knockdown inhibited AGE-BSA-induced expression of GRP78 and COX-2 with decreased PGE(2) production. Treatment with SB202190 inhibited AGE-BSA-induced expression of GRP78 and COX-2, while treatment with PD98051 inhibited AGE-BSA-induced GRP78 protein expression but had no effect on COX-2 protein expression. SP600125 had no effect on either GRP78 or COX-2 protein expression. Bay 11-7082 suppressed AGE-BSA-induced GRP78 and COX-2 expression. AGE-BSA-induced activation of NF-κB was inhibited by treatment with SB202190 and by eIF2α knockdown, but was not inhibited when chondrocytes were treated with SP600125 or PD98059.
CONCLUSION: This study demonstrates that AGEs induce ER stress and stimulate the expression of COX-2 through eIF2α, p38-MAPK and NF-κB pathways in human chondrocytes. Our results provide important insights into cartilage degradation in osteoarthritis associated with latent ER stress.