Increased erythropoietin sensitivity after in vitro transformation of hematopoietic precursors by RNA tumor viruses.

Two strains of Friend virus differ in their in vivo actions in that one strain induces anemia (FVA), while the other induces polycythemia (FVP). This study characterizes differences in the in vitro effects of these viruses on hematopoietic cells of (BALB/c x DBA/2)F1 mice. Both variants induced erythroid bursts that proliferated and differentiated without added erythropoietin (EPO). However, while the bursts induced by FVP were well "hemoglobinized" (i.e., most cells contained hemoglobin), the cells of FVA-induced bursts contained little or no hemoglobin. The nonhemoglobinized bursts, induced by FVA, were established to be erythroid by cytochemistry, electron microscopy, and hormone sensitivity. FVA-induced cells appeared to be hypersensitive to EPO, since small concentrations of the hormone produced marked increases in hemoglobin production--even when the hormone was added to the culture 3 days post infection. Time-lapse photography documented that EPO stimulated hemoglobin synthesis in virally transformed cells rather than uninfected erythroid precursors. This observation of FVA-induced hypersensitivity prompted the reexamination of the hormone requirements of FVP-induced bursts--previously considered to be EPO-independent. Reduction of the serum in the cultures allowed the demonstration that FVP-induced erythroid cells also were hypersensitive to EPO. Thus FVA and FVP can be readily distinguished in vitro by the relative EPO sensitivity of virus-induced bursts. From these findings, a hypothesis is drawn: i.e., oncogenic transformation may result from increased sensitivity of progenitor cells for natural, physiologic regulators, and transformation is not necessarily accompanied by a block in differentiation. In addition, since hypersensitive virus-induced bursts could be recognized and picked from the cultures, these studies provide a method for obtaining highly purified erythroid precursors for the study of the regulation of terminal differentiation by EPO and other regulatory factors.