COOPERATION OF SPI-1 PU.1 WITH AN ACTIVATED ERYTHROPOIETIN RECEPTOR INHIBITS APOPTOSIS AND EPO-DEPENDENT DIFFERENTIATION IN PRIMARY ERYTHROBLASTS AND INDUCES THEIR KIT LIGAND-DEPENDENT PROLIFERATION/

Spi-1/PU.1 is a myeloid-and B-cell specific transcription factor which is also involved in Friend virus-induced murine erythroleukemia. The pre-leukemic phase of Friend erythroleukemia results from activation o f the erythropoietin receptor (EpoR) by the spleen focus forming virus (SFFV) envelope glycoprotein, followed by the emergence of leukemic c lones characterized by overexpression of Spi-1 and mutation of the p53 tumor suppressor gene, We developed a heterologous system to analyze the contribution of these alterations to the induction of primary eryt hroblast transformation, Avian erythroblasts expressing the activated mouse EpoR(R129C) differentiated into erythrocytes in response to hEpo . Expression of Spi-1 in these cells inhibited this ability to differe ntiate and rescued the cells from the apoptotic cell death program nor mally induced upon hEpo withdrawal, Although devoid of any effect by i tself, a mutant p53 cooperated with Spi-1 and EpoR(R129C) to reinforce both phenotypes. Analysis of erythroblasts co-expressing Spi-1 and th e wild-type mouse EpoR showed that differentiation arrest and inhibiti on of apoptosis depended on specific cooperation between Spi-1 and Epo R(R129C), This cooperation was also required to induce the sustained p roliferation of differentiation-blocked erythroblasts in response to l igand activation of the endogenous tyrosine kinase receptor c-Kit Thes e results show that Spi-1/PU.1 requires signals emanating from specifi c cytokine and growth factor receptors to affect the survival, prolife ration and differentiation control of primary erythroblasts. They also suggest that the function of Spi-1/PU.1 in the late phase of Friend l eukemia requires specific signaling from the gp55-modified EpoR genera ted during the early phase of the disease.