Thrombin-induced thromboxane synthesis by human platelets. Properties of anion binding exosite I-independent receptor.
These studies have examined the effects of thrombin-related agonists in stimulating thromboxane production by human platelets. The results presented show that (1) the maximal response elicited by thrombin receptor agonist peptide (TRAP) stimulation was 40% to 50% of that seen with thrombin or the thrombin mutant Thrombin Quick I; (2) pretreatment of platelets with prostaglandin E1 or genistein resulted in differential inhibition of thromboxane production in response to TRAP compared with either enzyme agonist; (3) an antibody to the thrombin receptor cleavage site that inhibits increases in intracellular [Ca2+] only partially reduced thromboxane production in response to 5 nmol+L thrombin and 15 nmol/L Thrombin Quick I; (4) preincubation with 20 mumol/L TRAP resulted in desensitization to further stimulation by 100 mumol/L TRAP, but not by 100 nmol/L thrombin; and (5) the response to thrombin after TRAP desensitization was completely inhibited by the tyrosine kinase inhibitor genistein and was independent of an intracellular [Ca2+] flux, The cumulative results may be explained by the existence of two proteolytically activated receptors that result in thromboxane production in response to thrombin. One is the thrombin receptor/substrate, PAR-1. Stimulation through the second receptor/substrate depends on a genistein-sensitive step, is independent of an intracellular Ca2+ flux, and is initiated by a thrombin-activated receptor that does not depend on interaction with anion-binding exosite I, as previously indicated by the relative activity of Thrombin Quick I in stimulating platelet aggregation and thromboxane production. The proposed second thrombin receptor on platelets represents an additional member of the class of proteolytically activated receptors.
Henriksen
R A
RA
Department of Medicine, East Carolina University, Greenville, NC 27858-4354, USA. rhenriksen@brody.med.ecu.edu
Samokhin
G P
GP
Tracy
P B
PB
D000328:Adult; D001792:Blood Platelets / Q000378:metabolism*; D004305:Dose-Response Relationship, Drug; D004791:Enzyme Inhibitors / Q000494:pharmacology; D019833:Genistein / Q000494:pharmacology; D006801:Humans; D010446:Peptide Fragments / Q000494:pharmacology*; D011505:Protein-Tyrosine Kinases / Q000037:antagonists & inhibitors; D018179:Receptors, Thrombin / Q000502:physiology*; D015398:Signal Transduction; D013917:Thrombin / Q000494:pharmacology*; D013931:Thromboxanes / Q000096:biosynthesis*
D004791:Enzyme Inhibitors; D010446:Peptide Fragments; D018179:Receptors, Thrombin; D013931:Thromboxanes; C072032:thrombin receptor peptide (42-55); D019833:Genistein; D011505:Protein-Tyrosine Kinases; D013917:Thrombin