Regulation of response to thromboxane A2 in CHRF-288 megakaryocytic cells.
Thromboxane A2 (TxA2) is a potent platelet aggregating agent and a necessary intermediate for platelet stimulation by several other platelet agonists. A potentially important means whereby platelet responses to TxA2 could be modified in vivo is regulation of TxA2 receptors and/or effectors by platelet precursor megakaryocytes. We therefore investigated the mechanisms that regulate the response to TxA2 in CHRF-288, a cultured cell line derived from megakaryocytes. Incubation of CHRF-288 with the TxA2 agonist U-44069 resulted in a biexponential 75% decrease in subsequent TxA2 receptor-stimulated, but not thrombin-stimulated, Ca2+ release (half times of 1 and 4.7 h) and a monoexponential 57% decline in TxA2 receptor agonist binding sites (half time of 4.6 h). Desensitization with U-44069 for 24 h increased the proportion of internal TxA2 receptors from 23 to 82% of total receptors. Inhibition of endocytosis with phenylarsine oxide prevented U-44069-induced receptor downregulation but only partially inhibited desensitization of the Ca2+ response, indicating that internalization of receptors is not an immediate requirement for desensitization but that receptor internalization and degradation are both necessary for maximal desensitization. In summary, CHRF-288 megakaryocytic cells desensitize to TxA2 through incomplete functional uncoupling of receptors from Ca2+ signal transducers and delayed downregulation of TxA2 receptors via accelerated receptor internalization and degradation.
Dorn
G W
GW
Department of Medicine, University of Cincinnati College of Medicine, Ohio 45267-0542.
D001643:Bridged Bicyclo Compounds / Q000378:metabolism; D019086:Bridged Bicyclo Compounds, Heterocyclic; D002460:Cell Line; D005231:Fatty Acids, Unsaturated / Q000378:metabolism; D006801:Humans; D008533:Megakaryocytes / Q000187:drug effects* / Q000378:metabolism; D011982:Receptors, Prostaglandin / Q000037:antagonists & inhibitors / Q000378:metabolism; D017482:Receptors, Thromboxane; D013347:Subcellular Fractions / Q000378:metabolism; D013928:Thromboxane A2 / Q000494:pharmacology*
D001643:Bridged Bicyclo Compounds; D019086:Bridged Bicyclo Compounds, Heterocyclic; D005231:Fatty Acids, Unsaturated; D011982:Receptors, Prostaglandin; D017482:Receptors, Thromboxane; C061750:7-(3-(3-hydroxy-4-(4'-iodophenoxy)-1-butenyl)-7-oxabicyclo(2.2.1)heptan-2-yl)-5-heptenoic acid; D013928:Thromboxane A2