Myeloid leukemias are characterized by aberrant blockade of differentiation due to expression of leukemic oncoproteins such as PML/RAR-alpha. Elucidation of the mechanisms responsible for normal myeloid differentiation will further our understanding of differentiation blockades in leukemias, and foster the development of novel differentiation therapies. Thus, our long-term objective is to define the intracellular signaling pathways that lead to myeloid differentiation, and the role these pathways play in leukemogenesis. In preliminary studies we have observed rapid and prolonged activation of the MEK/ERK signal transduction pathway during myeloid differentiation. Furthermore, MEK/ERK activation was required for differentiation in myeloid cell lines. This contrasts with known roles for MEK/ERK activation in proliferation and survival, and constitutive MEK/ERK hyperactivation in myeloid leukemias. Together, these studies suggest a dichotomy of roles for MEK/ERK activation: promoting differentiation under normal conditions, and transformation under conditions where differentiation is blocked. Therefore, we hypothesize that prolonged activation of the MEK/ERK pathway is critically important for cytokine-induced myeloid differentiation. We further hypothesize that activation of the MEK/ERK pathway cooperates with differentiation-inhibiting leukemic oncoproteins to promote leukemogenesis. To test this hypothesis, we will: 1) elucidate the importance of MEK/ERK activation during cytokine-induced myeloid differentiation of myeloid cell lines and primary cultures of normal murine myeloid progenitors;2) determine the functional consequences of MEK/ERK activation during cytokine-induced myeloid differentiation;3) examine whether MEK/ERK activation promotes myeloid differentiation in vivo through the generation of transgenic mice that inducibly express constitutively active MEK enzyme in myeloid lineage cells;and 4) determine whether MEK/ERK activation cooperates with expression of PML/RAR-alpha to promote myeloid leukemogenesis.