CROSSTALK BETWEEN THROMBIN AND ADENYLYL CYCLASE-STIMULATING AGONISTS IN PROLIFERATING HUMAN ERYTHROID PROGENITOR CELLS

Human erythroid progenitor cells grown in a suspension culture system were used to study possible interactions between different guanine nuc leotide-binding protein (G-protein)-coupled receptor-effector systems during normal cell differentiation. Agonist-stimulated adenylyl cyclas e was not inhibited by any one of a panel of ligands (ADP, UTP, platel et-activating factor, thrombin, alpha(2)-adrenoceptor agonists, interl eukin 8, lysophosphatidic acid) most of which are known, in other cell s, to reduce cAMP formation by a G(i)-mediated, pertussis toxin-sensit ive mechanism. ism. The first four of these ligands are also known to cause transient changes in intracellular [Ca2+] in erythroid cells. Ra ther than inhibiting, thrombin (but not ADP, UTP or PAF) specifically caused a fivefold increase in the maximum adenosine- or prostaglandin E(1)-stimulated cAMP formation, without any shift of the concentration /response curves. Thrombin did not enhance forskolin- and AlF4--stimul ated cyclase activity and had only a marginal effect on isoprenaline-d ependent stimulation. The effect of thrombin seemed to be unrelated to intracellular Ca2+ release but could be partially mimicked by phorbol ester (PMA)-induced stimulation of protein kinase C (PKC) and was inh ibited by staurosporin or by inactivation of PKC after long-term incub ation with PMA. The activity of thrombin was restricted to proliferati ng, colony-forming progenitor cells while proerythroblasts were comple tely unresponsive. Our results suggest that the interaction of thrombi n with G(s)-linked receptors requires phosphorylation of a target prot ein that is different from adenylyl cyclase, G(s) or G(i) but may be i nvolved in the regulation of receptor desensitization.