Comment on Kamata et al, page 833
Both B-Raf and Raf-1 have overlapping functions and B-Raf–specific functions are unknown. Leavitt and colleagues provide evidence of a role for B-Raf in normal hematopoiesis and megakaryocyte differentiation.
B-Raf has recently been identified as a novel target of mutagenesis in human cancer, through its activation of the mitogen-activated protein (MAP) kinase cascade. The biochemical actions of B-Raf are thought to overlap with Raf-1, a ubiquitously expressed isoform of the Raf family of kinases, and the specific requirements of B-Raf in normal cellular processes are not known. Kamata and colleagues provide the first evidence of specific roles for B-Raf in normal hematopoiesis, and identify a critical function for B-Raf in megakaryocyte differentiation.
The MAP kinase or ERK (extracellular signal–regulated kinase) cascade is intimately involved in cellular growth and differentiation of a wide variety of cell types. The canonical ERK pathway couples extracellular signals to the nucleus via the small G protein Ras. Ras, a target of activating mutations in a wide range of hematological malignancies, activates the proximal member of the ERK cascade, Raf-1, initiating a cascade of phosphorylation/activation of MEK and ERK.
The Raf/MEK/ERK cascade, along with the JAK/signal transducer and activator of transcription (STAT) and phosphatidylinositol 3-kinase/protein kinase B (PI3K/Akt) signaling pathways, are essential for the cytokine-dependent survival expansion and differentiation of pluripotent hematopoietic cells. While Raf-1 is ubiquitously expressed, a second Raf isoform, B-Raf, is expressed in a subset of tissues and cells, including hematopoeitic progenitor cells, myeloid cells, and megakaryocytes,1 and, like Raf-1, is a major activator of ERK signaling. B-Raf has received recent attention for its role as a target of oncogenic mutation in human melanoma, thyroid and colon cancers, and less frequently in leukemia/lymphoma. The specific function of B-Raf in normal cell growth is not well understood. However, the generation of embryonic null mice deficient in B-Raf has allowed Leavitt and colleagues to test the requirement of B-Raf in hematopoiesis.
The elegant genetic studies by Kamata and colleagues identify 2 distinct requirements for B-Raf during myelopoiesis. One is the development of myeloid progenitors. The second occurs during megakaryopoiesis. Fetal livers of B-Raf–/– mice were small and exhibited a reduced number of hematopoietic cells that was not caused by altered proliferation or apoptosis. Indeed, the authors show that the remaining B-Raf–/– hematopoietic cells were competent to generate the full range of myeloid lineages. The failure to establish myeloid precursors in the B-Raf–null mice may reflect a defect in the hematopoietic precursors themselves or a defect in the environment supporting these precursors. In this regard, the known endothelial defects apparent in the B-Raf–/– embryo may be significant given the possible role of specific endothelial populations in supporting expansion of hematopoietic stem cells.
The authors have also identified a requirement for B-Raf during thrombopoeitin (TPO)–induced megakaryopoiesis. TPO, through the Mpl receptor (c-Mpl, or gene transduced by the myeloproliferative leukemia virus), activates the ERK cascade, and this activation is required for megakaryocytic differentiation. B-Raf was selective for TPO-induced megakaryopoiesis; EPO-induced erythroid development was B-Raf independent.
The nonredundant functions of B-Raf in hematopoiesis may reflect unique mechanisms of activation or unique actions of B-Raf that are not shared by Raf-1. The suggestion that B-Raf might be required for megakaryocyte differentiation comes from model systems showing that both B-Raf and sustained ERK activation were required for TPO-induced megakaryocytic differentiation.2 B-Raf, unlike Raf-1, can be activated by the small G protein Rap1,3 and Rap1/B-Raf signaling to ERKs has been implicated in this sustained activation of ERKs and differentiation by TPO.2,4
These studies establish that B-Raf and Raf-1 appear to have lineage-specific functions. Recent studies demonstrate that Raf-1 is required for erythropoiesis,5 but not megakaryopoiesis.6 It will be important to determine whether the relative expression levels of B-Raf and Raf-1 participate in the lineage choices made by hematopoietic progenitor cells. Given the hematological abnormalities identified in this paper, these findings have important implications for the use of Raf inhibitors currently in clinical trial as cancer therapies. ▪
