Rabbit synoviocyte inositol phospholipid metabolism is stimulated by hydroxyapatite crystals

Inhibition of prostaglandin E2 synthesis partially ameliorates some aspects of synovitis, but joint destruction still progresses. Other aspects of phospholipid metabolism may play a role in synovial tissue pathophysiology. Products of phosphatidylinositol metabolism can activate intracellular processes in response to extracellular stimuli. We asked whether this pathway is activated in synoviocytes in monolayer tissue culture by the addition of hydroxyapatite (HA) crystals in medium. These crystals are found in pathological human synovial fluid. These crystals are associated with the secretion of degradative enzymes and with a destructive arthritis in humans. Rabbit synoviocyte cultures, previously incubated with [3H]inositol to label inositol phospholipids, were stimulated with the addition of hydroxyapatite (180 micrograms/ml) to the cultures. There was enhanced intracellular accumulation of [3H]inositol monophosphate (30-100%) after 4 h. This indicated an increased phospholipase C activity. The radioactivity in [3H]inositol bis- and trisphosphates was too low to reliably measure. The use of [32P]Pi allowed detection of these compounds. In the presence of HA, incorporation of [32P]Pi into phosphatidylinositol, phosphatidylinositol monophosphate, and phosphatidylinositol bisphosphate was increased. In addition, cultures exposed to [32P]Pi during stimulation with HA had an increased content of [32P]inositol monophosphate, bisphosphate, and trisphosphate.