Two Distinct Interleukin-3-Mediated Signal Pathways, Ras-NFIL3 (E4BP4) and Bcl-x(L), Regulate the Survival of Murine Pro-B Lymphocytes

Hematopoietic cells require cytokine-initiated signals for survival as well as proliferation. The pathways that transduce these signals, ensuring timely regulation of cell fate genes, remain largely undefined. The NFIL3 (E4BP4) transcription factor, Bcl-x(L), and constitutively active mutants of components in Ras signal transduction pathways have been identified as key regulation proteins affecting murine interleukin-3 (IL-3)-dependent cell survival. Here we show that expression of NFIL3 is regulated by oncogenic Ras mutants through both the Raf–mitogen-activated protein kinase and phosphatidylinositol 3-kinase pathways. NFIL3 inhibits apoptosis without affecting Bcl-x(L) expression. By contrast, Bcl-x(L) levels are regulated through the membrane proximal portion in the cytoplasmic domain of the receptor (βc chain), which is shared by IL-3 and granulocyte-macrophage colony-stimulating factor. Activation of either pathway alone is insufficient to ensure cell survival, indicating that multiple independent signal transduction pathways mediate the survival of developing B-lymphoid cells.