Abstract
To investigate the type of phospholipase activated by agents that stimulate prostaglandin synthesis, we used transformed mouse cells whose phospholipids were doubly labeled with [14C]inositol and [3H]arachidonic acid. [14C]Inositol was incorporated mostly into the phosphatidylinositol and [3H]arachidonic acid was distributed into the various phospholipids. When these cells were incubated with bradykinin, a stimulator of prostaglandin synthesis, the release of 3H radioactivity from cellular phospholipids and the synthesis of prostaglandin were initiated within seconds and reached a maximum in 40 to 70 s. Analysis of the intracellular lipids revealed a concomitant increase of radioactivity associated with lysophosphatidylinositol, which was detectable within 5 s of incubation with bradykinin and reached a maximum between 40 and 70 s. Lysophosphatidylinositol which could be formed either from a phospholipase A1 or phospholipase A2 reaction, was identified by its chromatographic properties and conversion to glycerophosphorylinositol. We found that the 3H/14C ratio of purified lysophosphatidylinositol was 1/11 of that of phosphatidylinositol, which indicated that lysophosphatidylinositol formed in response to bradykinin is 1-acyl-sn-glycero-3-phosphorylinositol and most probably is formed from a phospholipase A2 deacylation of phosphatidylinositol (a phospholipase A1 deacylation would result in the formation of lysophosphatidylinositol of a 3H/14C ratio similar to phosphatidylinositol). Furthermore, we did not detect between control and stimulated cells any significant difference in the level of several phospholipase C metabolites including inositol phosphate, diglyceride, and phosphatidic acid. These results suggest that phospholipase C is probably not activated. The formation of lysophosphatidylinositol was also stimulated by thrombin and ionophore A23187, both activators of prostaglandin synthesis. Dexamethasone, a lipase inhibitor, inhibited the appearance of lysophosphatidylinositol, whereas aspirin and low concentrations of indomethacin, the cyclooxygenase inhibitor, did not inhibit. The results presented in ths paper provide evidence that a phospholipase A2-hydrolyzing phosphatidylinositol is activated when intact cells are stimulated for prostaglandin synthesis.
Highlights
The formation of lysophosphatidylinositol was stimulated by thrombin and ionophore A23187, both activators of prostaglandin synthesis
The results presented in this paper provide evidence that a phospholipase A2-hydrolyzing phosphatidylinositol is activated when intact cells are stimulated for prostaglandin synthesis
The results presented here provide direct evidence for the in vivo activation of phospholipase A2 acting on PI, when cells are stimulated for arachidonic acid release and prostaglandin synthesis
Summary
The formation of lysophosphatidylinositol was stimulated by thrombin and ionophore A23187, both activators of prostaglandin synthesis. A recent report by Bell et al [1] suggested that in human platelets phospholipase C is the key lipase involved in regulating the availability of free arachidonic acid for prostaglandin synthesis [1]. According to this view, free arachidonic acid is derived from PI' by a phospholipase C cleavage of PI to form inositol phosphate and diglyceride, followed by diglyceride lipase-catalyzed hydrolysis of diglyceride to release arachidonic acid. The results presented here provide direct evidence for the in vivo activation of phospholipase A2 acting on PI, when cells are stimulated for arachidonic acid release and prostaglandin synthesis
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
