Protein Kinase Cζ Is Up-regulated in Osteoarthritic Cartilage and Is Required for Activation of NF-κB by Tumor Necrosis Factor and Interleukin-1 in Articular Chondrocytes

Protein kinase Cζ (PKCζ) is an intracellular serine/threonine protein kinase that has been implicated in the signaling pathways for certain inflammatory cytokines, including interleukin-1 (IL-1) and tumor necrosis factor α (TNF-α), in some cell types. A study of gene expression in articular chondrocytes from osteoarthritis (OA) patients revealed that PKCζ is transcriptionally up-regulated in human OA articular cartilage clinical samples. This finding led to the hypothesis that PKCζ may be an important signaling component of cytokine-mediated cartilage matrix destruction in articular chondrocytes, believed to be an underlying factor in the pathophysiology of OA. IL-1 treatment of chondrocytes in culture resulted in rapidly increased phosphorylation of PKCζ, implicating PKCζ activation in the signaling pathway. Chondrocyte cell-based assays were used to evaluate the contribution of PKCζ activity in NF-κB activation and extracellular matrix degradation mediated by IL-1, TNF, or sphingomyelinase. In primary chondrocytes, IL-1 and TNF-α caused an increase in NF-κB activity resulting in induction of aggrecanase-1 and aggrecanase-2 expression, with consequent increased proteoglycan degradation. This effect was blocked by the pan-specific PKC inhibitors RO 31-8220 and bisindolylmaleimide I, partially blocked by Gö 6976, and was unaffected by the PKCζ-sparing inhibitor calphostin C. A cell-permeable PKCζ pseudosubstrate peptide inhibitor was capable of blocking TNFand IL-1-mediated NF-κB activation and proteoglycan degradation in chondrocyte pellet cultures. In addition, overexpression of a dominant negative PKCζ protein effectively prevented cytokine-mediated NF-κB activation in primary chondrocytes. These data implicate PKCζ as a necessary component of the IL-1 and TNF signaling pathways in chondrocytes that result in catabolic destruction of extracellular matrix proteins in osteoarthritic cartilage.