Early transient suppression of c-myb mRNA levels and induction of differentiation in Friend erythroleukemia cells by the [Ca2+]i-increasing agents cyclopiazonic acid and thapsigargin.

Cyclopiazonic acid and thapsigargin, inhibitors of the endoplasmic reticulum Ca2+ pump were shown to elevate [Ca2+]i in Friend erythroleukemia cells, line F4-6, at concentrations of 1-5 microM and 0.5-2 nM, respectively. At the same concentrations, these agents induced a strong suppression of c-myb mRNA levels within 3 h, whereas c-myc expression remained unaffected. The c-myb expression recovered and approached pretreatment levels at 9-12 h of incubation. The decrease in c-myb mRNA was prevented in Ca(2+)-free medium. Treatment of F4-6 cells with EGTA led to a transient increase in c-myb mRNA with the same kinetics as the Ca2+ pump inhibitor-induced suppression, indicating that c-myb expression is bidirectionally regulated by changes in [Ca2+]i. Studies on the differentiation status of F4-6 cells following cyclopiazonic acid or thapsigargin exposure demonstrated a marked increase in beta-globin mRNA synthesis at 60h and in hemoglobin production at 96 h. These results provide further evidence that a rise in the cytosolic Ca2+ concentration is capable, in Friend erythroleukemia cells, of inducing an early transient suppression of c-myb mRNA levels, which is followed by terminal erythroid differentiation.