Positive modulation of intracellular Ca2+ levels by adenosine A2b receptors, prostacyclin, and prostaglandin E1 via a cholera toxin-sensitive mechanism in human erythroleukemia cells
Feoktistov, I.; Murray, J.J.; Biaggioni, I.
Molecular Pharmacology 45(6): 1160-1167
1994
ISSN/ISBN: 0026-895X PMID: 8022409 Document Number: 430960
Human erythroleukemia (HEL) cells express megakaryocyte/platelet membrane markers and thus have been used as a model for studying platelet membrane receptors and their coupling to cell signaling pathways. Our previous studies, however, indicated that platelets and HEL cells possess different subtypes of adenosine A-2 receptors. Furthermore, we now report that, whereas adenosine inhibits intracellular Ca-2+ increases in platelets, it potentiates the rise in intracellular Ca-2+ produced by thrombin, prostaglandin E-1, thapsigargin, and the calcium ionophore A23187 in HEL cells. Stable adenosine analogs potentiated intracellular Ca-2+ increases with a rank order of potencies of 5'-N-ethylcarboxamidoadenosine (NECA) gt (R)-(-)-N-6-(2-phenylisopropyl)adenosine (R-PIA) mchgt CGS 21680, suggesting that this effect is mediated by A-2b receptors. EC-50 values for NECA and R-PIA were 0.8 and 42 mu-M, respectively. NECA (100 mu-M) potentiated by 2-3-fold the increase in intracellular Ca-2+ produced by 0.3 unit/ml thrombin. This effect was mimicked by cholera toxin and was shared by other G-s-coupled receptors, such as those activated by the prostacyclin analog iloprost and prostaglandin E-1, indicating the involvement of G-s proteins. Adenosine analogs also increased intracellular cAMP with the same rank order of potencies. The membrane-permeable analog 8-bromo-cAMP, however, had no effect on intracellular Ca-2+ levels, indicating that the potentiation of intracellular Ca-2+ increases and the activation of adenylate cyclase are parallel but independent events. The increase in intracellular Ca-2+ produced by adenosine is due not to an increase in phosphoinositide hydrolysis but, rather, to an increase in calcium influx, and it is lost if cells are studied in the absence of extracellular Ca-2+. We conclude, therefore, that adenosine A-2b receptors in HEL cells are coupled to G-s proteins and their activation leads to stimulation of adenylate cyclase and, independently, to potentiation of the rise in intracellular Ca-2+. We speculate that A-2b receptors in HEL cells activate a calcium channel through a cholera toxin-sensitive mechanism that requires an initial increase in intracellular Ca-2+.