Elevation of cyclic AMP decreases phosphoinositide turnover and inhibits thrombin-induced secretion in human platelets.
Elevation of cyclic AMP (cAMP) in platelets inhibits agonist-induced, G protein-mediated responses and activation of polyphosphoinositide-specific phospholipase C (PLC) by ill-defined mechanism(s). Signal transduction steps downstream of PLC are inhibited by elevated cAMP, suggesting an inhibitory effect of cAMP, via protein kinase A, on PLC. In [32P]i-prelabeled platelets, forskolin increased intracellular cAMP (104 nmol/1011 cells at 10-5 M forskolin) and [32P]phosphatidylinositol 4-phosphate (Delta[32P]PIP) (30% at 10-7-10-6 M forskolin). The thrombin-induced (0.1 U/ml) increase in production of [32P]PA, 'overshoots' in [32P]PIP and [32P]PIP2 ([32P]phosphatidylinositol 4,5-bisphosphate), and the increase in [32P]PI and secretion of ADP+ATP were abolished by forskolin (10-7 M). Forskolin stimulated total [32P]Pi uptake in resting platelets (48%), increased 32P incorporation into PIP (110%), and inhibited 32P incorporation into PI (50%). The latter inhibition was most likely considerably greater due to the forskolin-induced stimulation of [32P]Pi uptake. The changes in radioactive PA, PIP and PIP2 are regarded as being proportional with their masses in the prelabeled platelets, while the increase in PI (phosphatidylinositol) is regarded as a change in specific radioactivity, and hence in its synthesis. The results suggest that cAMP elevation inhibits the flux in the polyphosphoinositide cycle through both inhibition of PIP 5-kinase and PI synthesis. The inverse relation between forskolin-produced DeltaPIP and [32P]PA production suggests that the PLC reaction is inhibited by elevated cAMP through reduction of substrate (PIP2) resynthesis, and not by inhibition of the PLC enzyme.
Ryningen
A
A
Department of Biochemistry and Molecular Biology, University of Bergen, Arstadveien 19, N-5009 Bergen, Norway. anita.ryningen@pki.uib.no
Olav Jensen
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B
Holmsen
H
H
D000225:Adenine / Q000097:blood; D000244:Adenosine Diphosphate / Q000097:blood; D000255:Adenosine Triphosphate / Q000097:blood; D001792:Blood Platelets / Q000187:drug effects / Q000378:metabolism*; D005576:Colforsin / Q000494:pharmacology; D000242:Cyclic AMP / Q000097:blood*; D017868:Cyclic AMP-Dependent Protein Kinases / Q000097:blood; D006801:Humans; D007700:Kinetics; D010710:Phosphates / Q000097:blood; D010712:Phosphatidic Acids / Q000097:blood; D019269:Phosphatidylinositol 4,5-Diphosphate / Q000097:blood; D043265:Phosphatidylinositol Diacylglycerol-Lyase; D018129:Phosphatidylinositol Phosphates / Q000097:blood; D010716:Phosphatidylinositols / Q000097:blood*; D010761:Phosphorus Radioisotopes; D015398:Signal Transduction; D013917:Thrombin / Q000494:pharmacology*; D010738:Type C Phospholipases / Q000097:blood
D010710:Phosphates; D010712:Phosphatidic Acids; D019269:Phosphatidylinositol 4,5-Diphosphate; D018129:Phosphatidylinositol Phosphates; D010716:Phosphatidylinositols; D010761:Phosphorus Radioisotopes; C037178:phosphatidylinositol 4-phosphate; D005576:Colforsin; D000244:Adenosine Diphosphate; D000255:Adenosine Triphosphate; D000242:Cyclic AMP; D017868:Cyclic AMP-Dependent Protein Kinases; D010738:Type C Phospholipases; D013917:Thrombin; D043265:Phosphatidylinositol Diacylglycerol-Lyase; D000225:Adenine