C-MYC EXPRESSION AFFECTS PROLIFERATION BUT NOT TERMINAL DIFFERENTIATION OR SURVIVAL OF EXPLANTED ERYTHROID PROGENITOR CELLS

The expression of c-myc was analyzed in murine and human erythroblasts throughout their differentiation in vitro into reticulocytes. The mur ine cells were splenic erythroblasts from animals infected with the an emia strain of Friend virus (FVA cells). In FVA cells cultured without EPO, the c-myc mRNA and protein levels decrease sharply within 3 to 4 h, showing that continual EPO stimulation is required to maintain c-m yc expression. When cultured with EPO, the c-myc mRNA level of FVA cel ls is raised within 30 min of exposure. The c-myc mRNA and protein rea ch maxima at 1 to 3 h, then decline slowly to very low levels by 18 h. In contrast, c-fos and c-jun mRNA levels are not regulated by EPO in FVA cells. The human cells analyzed were colony-forming units-erythroi d, CFU-E, derived in vitro by the culture of peripheral blood burst-fo rming units-erythroid (BFU-E). When grown in EPO and insulin-like grow th factor 1 (IGF-1) these cells differentiate into reticulocytes over 6 days rather than the 2 days required for murine cells, but the c-myc mRNA kinetics and response to EPO parallel those of mouse cells at si milar stages of differentiation. Both IGF-1 and c-kit ligand (SCF) cau se an additive increase in c-myc mRNA in human CFU-E in conjunction wi th EPO. These additive effects suggest that EPO, IGF-1, and SCF affect c-myc mRNA accumulation by distinct mechanisms. Addition of an antise nse oligonucleotide to c-myc in cultures of human CFU-E specifically i nhibited cell proliferation but did not affect erythroid cell differen tiation or apoptosis. When human cells were grown in high SCF concentr ations, an environment which enhances proliferation and retards differ entiation, antisense oligonucleotide to c-myc strongly inhibited proli feration, but such inhibition did not induce differentiation: This lat ter result indicates that differentiation requires signals other than depression of c-Myc and resultant depression of proliferation. (C) 199 6 Wiley-Liss, Inc.*