AJP - Cell Physiology, Vol 262, Issue 4 C991-C999, Copyright © 1992 by American Physiological Society
Regulation of response to thromboxane A2 in CHRF-288 megakaryocytic cells
G. W. Dorn 2nd
Department of Medicine, University of Cincinnati College of Medicine, Ohio 45267-0542.
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.
