CA2+ CALMODULIN-DEPENDENT AND CA2+/CALMODULIN-INDEPENDENT DOWN-REGULATION OF C-MYB MESSENGER-RNA LEVELS IN ERYTHROPOIETIN-RESPONSIVE MURINEERYTHROLEUKEMIA-CELLS - THE ROLE OF CALCINEURIN/

Down-regulation of c-myb mRNA levels by [Ca2+](i)-increasing agents (A 23187, thapsigargin, cyclopiazonic acid) and erythropoietin-responsive murine erythroleukemia cell line, ELM-I-1. The Ca2+-induced suppressi on of c-myb trifluoperazine and calmidazolium, as well as by cyclospor in A, an inhibitor of the Ca2+/calmodulin-dependent protein phosphatas e 2B (calcineurin). KN-62, an inhibitor of Ca2+/calmodulin-dependent p rotein kinases, did not antagonize the Ca2+-mediated decrease in c-myb mRNA. In cyclosporin A-treated ELM-I-1, cells, a close correlation co uld by demonstrated between the antagonization of the calcineurin phop hatase activity. On the other hand, FK506, which did not inhibit calci neurin activity in ELM-I-1 cells, failed to prevent the Ca2+-mediated decrease in c-myb mRNA. The erythropoietin-induced down-regulation of c-myb mRNA levels could be demonstrated also in the presence of EGTA a nd was resistant to calmodulin antagonists and cyclosporin A. In addit ion, no increase in [Ca2+](i), was observed in ELM-I-1 cells in respon se to erythropoietin. Cyclosporin A inhibited the Ca2+-induced hemoglo bin production, while the erythropoietin-mediated increase in hemoglob in synthesis was not affected. The results indicate that the Ca2+-indu ced decrease in c-myb mRNA and increase in hemoglobin synthesis is med iated by calcineurin, while these effects of erythropoietin occur inde pendently of Ca2+ in ELM-I-1 cells. Calcineurin may be involved in the regulation of c-myb expression in erythroid precursor cells and Ca2signals via calcineurin may positively modulate the differentiation in ducing action of erythropoietin.