Posttranscriptional effect of insulin-like growth factor-I on interleukin-1 beta-induced type II-secreted phospholipase A(2) gene expression in rabbit articular chondrocytes

Large amounts of type II-secreted phospholipase A, (type II sPLA(2)) are secreted into inflammatory synovial fluid and they are believed to induce the synthesis of lipid mediators by articular chondrocytes. Preliminary experiments showed that insulin-like growth factor-I, which counteracts cartilage degradation in arthritis, inhibits interleukin-1 beta-induced type II sPLA(2) gene expression in rabbit articular chondrocytes (Berenbaum F., G. Thomas, S. Poiraudeau, G. Bereziat, M.T. Corvol, and J. Masliah. 1994. FEES Lett. 340: 51-55). The present study showed that IL-1 beta induced the sustained synthesis of prostaglandin E-2 and a parallel increase in type II sPLA(2) gene expression (assessed by enzymatic activity and Northern blot analysis), but no increase in cytosolic PLA(2) gene expression (assessed by Northern and Western blot analysis) or cytosolic PLA(2) activity in rabbit articular chondrocytes. IGF-I inhibited both IL-1 beta-stimulated PGE(2) synthesis and type II sPLA, gene expression, but had no effect on cytosolic PLA(2) gene expression. Nuclear run-on experiments revealed that IL-1 beta stimulated the transcription rate of type II sPLA(2) gene, giving rise to long-lived mRNA in cells treated with actinomycin D. IGF-I did not affect transcription rate, suggesting that it acts as a post-transcriptional step. Sucrose density gradient analysis of the translation step showed no effect of IGF-I on the entry of type II sPLA(2) mRNA into the polysomal pool or on its distribution into the various polysomal complexes, suggesting that IGF-I does not act on the translation of the mRNA. Lastly, IGF-I strongly decreased the half-life of IL-1 beta-induced type II sPLA(2) mRNA (from 92 to 12 h), suggesting that IGF-I destabilizes mRNA. These data demonstrate that IL-1 beta stimulates the transcription rate of the type II sPLA(2) gene and gives rise to a very stable mRNA. In contrast, IGF-I decreases the half-life of the type II sPLA(2) message.