Inhibition of extracellular release of proinflammatory secretory phospholipase A2 (sPLA2) by sulfasalazine: a novel mechanism of anti-inflammatory activity.

Sulfasalazine is widely used in rheumatoid arthritis and inflammatory bowel diseases. The mechanisms of its activity have not been elucidated. In leukocytes, sulfasalazine and its analogue, CL 42A, inhibited the formation of leukotrienes and possibly of the second messenger compounds at the level of phospholipase C. Partial inhibition of interleukin-lbeta (IL-1beta), IL-6 and tumor necrosis factor-alpha (TNF-alpha) was also found. Since the synthesis of eicosanoids is induced by phospholipase A2 and since secretory phospholipase A2 (sPLA2) is proinflammatory, we investigated the impact of sulfasalazine and related compounds on mRNA, protein synthesis, and release of sPLA2 from osteoblasts. Sulfasalazine and CL 42A markedly inhibited extracellular release of sPLA2. The impact of sulfasalazine was evident at 50 microM (P < 0.001) and maximal at 400 microM, and that of CL 42A at 10 microM (P < 0.001) and 200 microM, respectively. Split products of sulfasalazine, 5-aminosalicylic acid (400 microM) and sulfapyridine (400 microM), had no impact. The effect of sulfasalazine and CL 42A was evident regardless of whether the cells were stimulated with IL-1beta/TNF-alpha, lipopolysaccharide/forskolin, or dibutyryl-cAMP. Sulfasalazine and CL 42A did not alter the level of sPLA2 mRNA. Exposure of stimulated fetal rat calvaria osteoblasts (FRCO) to sulfasalazine did not show accumulation of the intracellular sPLA2 protein as tested by western blot; however, enzymatic activity of PLA2 in disrupted cells was definitely increased. Thus, the impact is on the post-transcriptional release of sPLA2 rather than on the synthesis. There was also an increase in the extracellular release of prostaglandin E2 from FRCO exposed to sulfasalazine or to CL 42A. In contrast, sulfasalazine had no effect on the extracellular release of gelatinase from the cells or on mRNA of cytosolic PLA2 or cyclooxygenase 2. We conclude that the anti-inflammatory activity of sulfasalazine may be related, in part, to the selective inhibition of the extracellular release of proinflammatory sPLA2.