title: Homologous desensitization of thromboxane A2 receptor in 1321N1 human astrocytoma cells.
abstract: The desensitization mechanisms that regulate the response to thromboxane A2 (TXA2) were investigated in 1321N1 human astrocytoma cells. Exposure of the cells to 9,11-epithio-11, 12-methanothromboxane A2 (STA2), a stable TXA2 receptor agonist, at a concentration of 1 microM for 30 to 45 min resulted in about a 50% decrease in subsequent STA2-stimulated phosphoinositide hydrolysis and Ca2+ mobilization. However, exposure to STA2 for 0 to 5 hr did not change the binding of [3H]SQ29548, a TXA2 receptor antagonist. Because STA2-induced GTPase activation decreased and the GTP sensitivity in inhibition of [3H]SQ29548 binding by STA2 disappeared after the cells had been exposed to STA2 for 30 min, the TXA2 receptor desensitization during the short-term might result from G-protein-receptor uncoupling. STA2-induced desensitization was specific for the TXA2 receptor and homologous, because SQ29548 suppressed the desensitization and STA2 pretreatment did not affect the response to carbachol, a muscarinic cholinergic receptor agonist. Exposure to STA2 for 24 hr decreased [3H]SQ29548 binding sites to 20 to 30% of control and abolished STA2-stimulated phosphoinositide hydrolysis, indicating that long-term desensitization might induce down-regulation of the TXA2 receptor. However, exposure to STA2 for 1 to 24 hr did not change the level of TXA2 receptor mRNA. These results show that homologous desensitization of the TXA2 receptor in human astrocytoma cells can be divided into two stages; the early stage involves uncoupling of receptors from G-proteins and the late stage involves a loss of receptors in cells. The mRNA levels may not be controlled by stimulation of the TXA2 receptor.
authors.affiliation: Department of Pharmaceutical Molecular Biology, Faculty of Pharmaceutical Sciences, Tohoku University, Sendai, Japan.
authors.forename: K
authors.initials: K
authors.lastname: Sakai
authors.forename: N
authors.initials: N
authors.lastname: Nakahata
authors.forename: H
authors.initials: H
authors.lastname: Ono
authors.forename: T
authors.initials: T
authors.lastname: Yamamoto
authors.forename: Y
authors.initials: Y
authors.lastname: Ohizumi
mesh_terms: D001254:Astrocytoma / Q000737:chemistry; D015152:Blotting, Northern; D002118:Calcium / Q000378:metabolism; D002478:Cells, Cultured; D019204:GTP-Binding Proteins / Q000032:analysis; D006801:Humans; D007295:Inositol Phosphates / Q000378:metabolism; D012333:RNA, Messenger / Q000032:analysis; D017482:Receptors, Thromboxane / Q000032:analysis / Q000187:drug effects* / Q000235:genetics; D013928:Thromboxane A2 / Q000031:analogs & derivatives / Q000494:pharmacology
chemical_list: D007295:Inositol Phosphates; D012333:RNA, Messenger; D017482:Receptors, Thromboxane; D013928:Thromboxane A2; C037866:STA 2; D019204:GTP-Binding Proteins; D002118:Calcium