Mechanistic options of erythropoietin-stimulated erythropoiesis.

The in vivo mechanism of hematopoietic growth factor-induced cell multiplication is in debate. Several options can be examined: 1) growth factors can reduce the cycling time of their dividing target cells, 2) growth factors can add extra cell divisions within the differentiation pathway, 3) the combination of the first two possibilities, and 4) growth factors can prevent premature cell death (apoptosis) from occurring in the absence of the stimulating factor. We studied these options in vitro and in vivo in the murine erythroid pathway. Results from in vitro cultures of purified splenic colony-forming units-erythroid (CFU-E), with and without erythropoietin (Epo), and in vivo Epo treatments of thiamphenicol (TAP)-pretreated mice showed neither reduction in cycle times nor addition of extra cell divisions in the differentiating erythroid lineage. The phenomenon of apoptosis was demonstrated as time- and Epo-dependent in vitro with electrophoretic (DNA-ladder), flow-cytometric (subdiploid cells), and morphologic (fragmented nuclei) methods applied on CFU-E. A high dose of Epo administered to mice caused a rapid transient rise in the number of CFU-E to 350% of normal. Early erythroblasts also increased, whereas burst-forming unit-erythroid (BFU-E) numbers did not change. Our results favor a mechanism in which Epo acts as a survival factor for early erythroid cells (CFU-E and early erythroblasts) in vitro, as well as in vivo, preventing apoptosis.