Bradykinin-stimulated release of arachidonate from phosphatidyl inositol in mouse fibrosarcoma cells.

We recently proposed a new pathway by which arachidonate is released from platelet phosphatidyl inositol after stimulation by either thrombin or calcium ionophore A23187. The initial step in arachidonate liberation involves hydrolysis of phosphatidyl inositol to form 1,2-diacyglycerol which is subsequently hydrolyzed by a diacyglycerol lipase to liberate arachidonate for the prostaglandin and lipoxygenase pathways. Whether this pathway is unique to platelets or accounts for arachidonate release from other tissues has not been previously studied. Thus we have now investigated arachidonate metabolism in mouse fibrosarcoma cells (HSDM1C1) grown in culture. These cells contain approximately 7.6% of their total phospholipid as phosphatidyl inositol in the resting state (range 6.5-8.3%). When bradykinin (12 microM) is added to the fibrosarcoma cells, there is a rapid depletion of membrane phosphatidyl inositol reaching 62 +/- 8% S.D. of baseline values by 15 seconds, falling to 36 +/- 6% by 15 minutes. The drop in membrane phosphatidyl inositol is accompanied by release of arachidonate and PGE2 into the culture medium. The time course of phosphatidyl inositol breakdown and PGE2 formation supports the idea that phosphatidyl inositol breakdown provides he arachidonate for prostaglandin synthesis in mouse fibrosarcoma cells. Crude extracts of HSDM1C1 cells contained sufficient phosphatidyl inositol-specific phospholipase C activity and diacylglycerol lipase activity to account for arachidonate release in these cells.