V-RAF SUPPRESSES APOPTOSIS AND PROMOTES GROWTH OF INTERLEUKIN-3-DEPENDENT MYELOID CELLS

Interleukin-3 (IL-3) is required for the proliferation, survival and d ifferentiation of myeloid progenitors. In the absence of IL-3, murine myeloid 32D.3 cells accumulate in the G1 phase of the cell cycle and s ubsequently undergo programmed cell, death, or apoptosis. Here we demo nstrate that enforced expression of the v-raf oncogene suppresses apop tosis of myeloid 32D.3 cells following the withdrawal of IL-3. Surpris ingly, steady state levels of Bcl-2, an oncogene known to suppress apo ptosis, were not dependent upon IL-3 in 32D.3 cells and its levels wer e not augmented in v-raf clones. This suggests that ability of v-raf t o suppress apoptosis in the absence of ligand is either Bcl-2 independ ent or that v-raf kinase promotes Bcl-2 function. v-raf also promoted growth of these cells in the presence of IL-3. v-raf clones proliferat ed at an increased rate due to a shortened G1 phase and had decreased requirements for IL-3 for growth. Therefore, transformation of myeloid cells by v-raf involves signaling pathways which promote both cell cy cle progression and cell survival.