MECHANISTIC OPTIONS OF ERYTHROPOIETIN-STIMULATED ERYTHROPOIESIS

The in vivo mechanism of hematopoietic growth factor-induced cell mult iplication is in debate. Several options can be examined: 1) growth fa ctors can reduce the cycling time of their dividing target cells, 2) g rowth factors can add extra cell divisions within the differentiation pathway, 3) the combination of the first two possibilities, and 4) gro wth factors can prevent premature cell death (apoptosis) from occurrin g in the absence of the stimulating factor. We studied these options i n vitro and in vivo in the murine erythroid pathway. Results from in v itro 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 cyc le times nor addition of extra cell divisions in the differentiating e rythroid lineage. The phenomenon of apoptosis was demonstrated as time - and Epo-dependent in vitro with electrophoretic (DNA-ladder), flow-c ytometric (subdiploid cells), and morphologic (fragmented nuclei) meth ods applied an 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 (BF U-E) numbers did not change. Our results favor a mechanism in which Ep o acts as a survival factor for early erythroid cells (CFU-E and early erythroblasts) in vitro, as well as in vivo, preventing apoptosis.