1986 Volume 17 Issue 2 Pages 161-163
The effects of tumor promoter (PMA, phorbol-12-myristate-13-acetate) on thrombin-stimulated human platelet responses and membrane phospholipid metabolism were investigated. Thrombin (0.1U/ml)-induced serotonin release was inhibited by pretreatment with PMA (50μg/ml) for 5min. This inhibition was associated with a suppression of polyphosphoinositide breakdown and increase of cytoplasmic free Ca2+ ([Ca2+] i). PMA was observed to increase the incorporation of [32P]phosphate into phosphatidylinositol 4, 5-bisphosphate (PIP2) and phosphatidylinositol 4-phosphate (PIP). This indicates that treatment with PMA suppresses the hydrolysis of PIP2 by phospholipase C, resulting in the insufficient supply of a Ca2+ releaser, IP3. A considerably high radioactivity in PIP would be due to the enhanced conversion of phosphatidylinositol (PI) to PIP via PI kinase rather than the inhibited PIP breakdown, since there was no indication of PIP hydrolysis during stimulation with thrombin alone. These observations provide evidence that protein kinase C subserves a negative feedback role in a receptor-mediated cell activation.
