| Abstract |
Extremely aggressive transplantable leukemias were derived in mice
inoculated with a Friend virus isolate which was molecularly cloned,
replication-competent, and free of spleen focus-forming activity.
Assessment of the hormone-sensitivity of the leukemic spleen cells led to
the isolation of several continuous lines which survive only in the
presence of erythropoietin (EPO). These lines have been frozen, thawed and
passaged for more than one year without loss of the EPO requirement. Five
sources of EPO, including human (recombinant and urinary), sheep, and
murine EPO produced a dose-related stimulation of proliferation as
assessed by tritiated thymidine uptake. In contrast, interleukin-1 (IL-1),
IL-2, or IL-3 did not stimulate thymidine uptake. By light and electron-
microscopy the majority of the cells exhibited early eyrthroid morphology
although there was considerable heterogeneity with respect to cell size.
Approximately 15-30% of the cells wore similar in size to late
erythroblasts. Nevertheless, the cells were hemoglobin negative by
benzidine staining although low levels of globin mRNA could be detected by
hybridisation. Ten subclones derived from single cells were all EPO-
dependent and provided a sensitive, convenient, biological assay for the
hormone. The assay could reliably quantitate between 0.3 and 30 milliunits
of EPO with a half maximal response at 3 milliunits. In addition to their
practical implications, these observations raise the possibility that EPO
may not directly induce the synthesis of hemoglobin and other erythroid
proteins but rather may act as a viability agent to keep erythroid
progenitors alive while they differentiate. The results are discussed in
relation to an alternate model of development which suggests that
production of committed progenitors is continuous and that expansion of a
lineage is due to selection rather than induction.
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