Characterization of interleukin 1 induced rabbit chondrocyte phospholipase A2

Stimulation of rabbit articular chondrocytes with interleukin 1 (IL-1) results in the activation of intracellular phospholipase A2 (PLA2) and the subsequent secretion of this enzyme into the extracellular milieu. Pretreatment of chondrocytes with actinomycin D or cycloheximide significantly inhibited IL-1 induced PLA2 activation and secretion, suggesting that the enzyme induction process is RNA and protein synthesis dependent. Chondrocyte PLA2 is highly calcium dependent with a 1 mM optimum CA++ concentration for hydrolytic activity; little or no hydrolysis is observed in the absence of calcium and the hydrolytic activity is abolished in the presence of 10 mM ethylenediamine tetraacetic acid. The enzyme is also pH sensitive with optimal PLA2 hydrolytic activity observed at pH 6.5-7. Further, chondrocyte PLA2 was sensitive to inhibition by mepacrine, a compound with PLA2 inhibitory activity. The IL-1 induced chondrocyte PLA2 has a molecular weight of approximately 10 kDa, as determined by molecular sieve G75 column chromatography. The apparent molecular weight and CA++, pH and drug sensitivity of the extracellular and intracellular forms of the IL-1 induced chondrocyte PLA2 are indistinguishable. Since this IL-1 induced enzyme has similar biochemical characteristics to PLA2 enzymes isolated from human rheumatoid and osteoarthritic synovial fluid, we suggest that the chondrocyte may be an important cellular source for this PLA2 enzymatic activity in inflamed joints.