TY - JOUR
T1 - Thromboxane-insensitive dog platelets have impaired activation of phospholipase C due to receptor-linked G protein dysfunction
AU - Johnson, G. J.
AU - Leis, L. A.
AU - Dunlop, P. C.
PY - 1993
Y1 - 1993
N2 - Human platelet thromboxane A2/prostaglandin H2 (TXA2/PGH2) receptors are linked to phosphoinositide-specific phospholipase C (PI-PLC) via a G protein tentatively identified as a member of the G(q) class. In contrast, platelet thrombin receptors appear to activate PI-PLC via other unidentified G proteins. Platelets from most dogs are TXA2 insensitive (TXA2-); i.e., they do not aggregate irreversibly or secrete although they bind TXA2, but they respond normally to thrombin. In contrast, a minority of dogs have TXA2-sensitive (TXA2+) platelets that are responsive to TXA2. To determine the mechanism responsible for TXA2- platelets, we evaluated receptor activation of PI-PLC. Equilibrium binding of TXA2/PGH2 receptor agonists, [125I]BOP and [3H]U46619, and antagonist, [3H]SQ29,548, revealed comparable high-affinity binding to TXA2-, TXA2+, and human platelets. U46619-induced PI-PLC activation was impaired in TXA2- platelets as evidenced by reduced (a) phosphorylation of the 47-kD substrate of protein kinase C, (b) phosphatidic acid (PA) formation, (c) rise in cytosolic calcium concentration, and (d) inositol 1,4,5 trisphosphate (IP3) formation, while thrombin-induced PI-PLC activation was not impaired. GTPase activity stimulated by U46619, but not by thrombin, was markedly reduced in TXA2- platelets. Antisera to G(q) class α subunits abolished U46619-induced GTPase activity in TXA2-, TXA2+, and human platelets. Direct G protein stimulation by GTPγS yielded significantly less PA and IP3 in TXA2- platelets. Immunotransfer blotting revealed comparable quantities of G(q) class α- subunits in all three platelet types. Thus, TXA2- dog platelets have impaired PI-PLC activation in response to TXA2/PGH2 receptor agonists secondary to G protein dysfunction, presumably involving a member of the G(q) class.
AB - Human platelet thromboxane A2/prostaglandin H2 (TXA2/PGH2) receptors are linked to phosphoinositide-specific phospholipase C (PI-PLC) via a G protein tentatively identified as a member of the G(q) class. In contrast, platelet thrombin receptors appear to activate PI-PLC via other unidentified G proteins. Platelets from most dogs are TXA2 insensitive (TXA2-); i.e., they do not aggregate irreversibly or secrete although they bind TXA2, but they respond normally to thrombin. In contrast, a minority of dogs have TXA2-sensitive (TXA2+) platelets that are responsive to TXA2. To determine the mechanism responsible for TXA2- platelets, we evaluated receptor activation of PI-PLC. Equilibrium binding of TXA2/PGH2 receptor agonists, [125I]BOP and [3H]U46619, and antagonist, [3H]SQ29,548, revealed comparable high-affinity binding to TXA2-, TXA2+, and human platelets. U46619-induced PI-PLC activation was impaired in TXA2- platelets as evidenced by reduced (a) phosphorylation of the 47-kD substrate of protein kinase C, (b) phosphatidic acid (PA) formation, (c) rise in cytosolic calcium concentration, and (d) inositol 1,4,5 trisphosphate (IP3) formation, while thrombin-induced PI-PLC activation was not impaired. GTPase activity stimulated by U46619, but not by thrombin, was markedly reduced in TXA2- platelets. Antisera to G(q) class α subunits abolished U46619-induced GTPase activity in TXA2-, TXA2+, and human platelets. Direct G protein stimulation by GTPγS yielded significantly less PA and IP3 in TXA2- platelets. Immunotransfer blotting revealed comparable quantities of G(q) class α- subunits in all three platelet types. Thus, TXA2- dog platelets have impaired PI-PLC activation in response to TXA2/PGH2 receptor agonists secondary to G protein dysfunction, presumably involving a member of the G(q) class.
KW - G proteins
KW - guanosine triphosphate phosphohydrolase
KW - phospholipase C
KW - platelet activation
KW - receptors
KW - thromboxane
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U2 - 10.1172/JCI116855
DO - 10.1172/JCI116855
M3 - Article
C2 - 8227362
AN - SCOPUS:0027489597
SN - 0021-9738
VL - 92
SP - 2469
EP - 2479
JO - Journal of Clinical Investigation
JF - Journal of Clinical Investigation
IS - 5
ER -