The fetal mouse liver (FL) is an organ of intense, but transient, hematopoietic activity during mid-gestation, with erythropoiesis being predominant during days 11 through 16. It therefore seemed reasonable to expect that hematopoietic cytokines, such as erythropoietin (epo), interleukin-3 (IL-3), and stem cell factor (SCF), may play important roles in maintaining a homeostatic balance of erythropoiesis and apoptosis in liver during ontogeny. First, we determined the effects of these growth factors on hematopoiesis by measuring colony formation and hemoglobin synthesis of cultured FLs. Secondly, we determined the protection from apoptosis afforded by these cytokines, using electrophoretic analysis of DNA and by flow cytometry of FL cells deprived in culture of epo, IL-3, and SCF. Erythropoietin was necessary and alone sufficient for hemoglobin synthesis in colony-forming units- erythroid colonies, but IL-3 was a required cofactor to obtain maximal development of burst-forming units-erythroid colonies. SCF alone caused little colony formation in methylcellulose cultures of FLs, but when combined with epo and IL-3, it had dramatic effects both on the number of colonies and their size. Secondly, indices of apoptosis were determined by measuring DNA fragmentation caused by endogenous nuclease activity in apoptotic cells. Liver cells from cultures without cytokines showed the extensive degradation of DNA to low molecular weight nucleosomal oligomers, which is characteristic of apoptosis. Protection from apoptosis afforded by epo directly corresponded to the level of erythropoiesis in FLs of different gestational age. Erythropoietin was by far the most critical cytokine in sparing FL cells from apoptosis. Analyses of agarose gels showed that SCF and IL-3 alone had no apparent effect in reducing the amount of DNA in fragments, and when combined with epo they had no more protective effect than that provided by epo alone. However, using the more sensitive flow cytometric determination of cells with subdiploid amounts of DNA, SCF, and IL-3 alone had measurable protective effects that were less than those caused by epo. Thus, we show that normal, untransformed cells of the developing hematopoietic system not only require cytokines for proliferation and differentiation, but they have an initial and absolute requirement of them for protection from apoptosis.

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