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)) a re secreted into inflammatory synovial fluid and they are believed to induce the synthesis of lipid mediators by articular chondrocytes. Pre liminary experiments showed that insulin-like growth factor-I, which c ounteracts cartilage degradation in arthritis, inhibits interleukin-1 beta-induced type II sPLA(2) gene expression in rabbit articular chond rocytes (Berenbaum F., G. Thomas, S. Poiraudeau, G. Bereziat, M.T. Cor vol, and J. Masliah. 1994. FEES Lett. 340: 51-55). The present study s howed that IL-1 beta induced the sustained synthesis of prostaglandin E-2 and a parallel increase in type II sPLA(2) gene expression (assess ed 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 chondr ocytes. 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 st imulated the transcription rate of type II sPLA(2) gene, giving rise t o long-lived mRNA in cells treated with actinomycin D. IGF-I did not a ffect transcription rate, suggesting that it acts as a post-transcript ional 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 th e polysomal pool or on its distribution into the various polysomal com plexes, suggesting that IGF-I does not act on the translation of the m RNA. Lastly, IGF-I strongly decreased the half-life of IL-1 beta-induc ed type II sPLA(2) mRNA (from 92 to 12 h), suggesting that IGF-I desta bilizes mRNA. These data demonstrate that IL-1 beta stimulates the tra nscription rate of the type II sPLA(2) gene and gives rise to a very s table mRNA. In contrast, IGF-I decreases the half-life of the type II sPLA(2) message.