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ABSTRACT: The Raf/MEK/ERK cascade is a highly conserved signal transduction module whose activation reportedly results in a plethora of physiological outcomes. Depending on the cell type or the stimulus used, the pathway has been implicated in proliferation, differentiation, survival, and migration. Their wide range of activities renders the component of the Raf/MEK/ERK pathway prime candidates for molecule-targeted therapies, in particular, but not exclusively, in the context of cancer. Ras, Raf and MEK inhibitors have been developed, and some of them are in advanced clinical trials. Somewhat surprising in view of all this interest, our understanding of the fundamental biology of the ERK pathway in vivo is still scanty. Its investigation has been hampered by the fact that conventional targeting of many of these genes results in embryonic lethality. Recently, we and others have generated mouse strains that allow the conditional ablation of the genes coding for Raf-1, B-Raf and MEK-1. We are using these tools to identify the essential biological functions of these kinases, and to understand how the ERK pathway is wired in vivo. Here, we discuss some of the surprises yielded by the analysis of the role of B-Raf and Raf-1 and of their downstream effectors.
Cell cycle (Georgetown, Tex.) 08/2006; 5(14):1514-8. · 5.36 Impact Factor
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ABSTRACT: Normal erythropoiesis critically depends on the balance between the renewal of precursor cells and their differentiation. If the renewal phase is shortened, the decrease in the precursor pool results in anemia; conversely, impaired differentiation increases the number of proliferating progenitors and the potential risk of leukemic transformation. Using gene ablation, we have discovered 2 self-sustaining signal transduction loops that antagonize each other and regulate erythroid progenitor proliferation and differentiation, respectively. We identify Raf-1 as the main activator of the MEK/ERK cascade and as the key molecule in maintaining progenitor proliferation. Differentiation, in contrast, is mediated by Fas via the activation of both the ASK1/JNK/p38 module and the caspase cascade. The point of convergence between the 2 cascades is activated ERK, which positively feeds back on the proliferation pathway by maintaining the expression of Raf-1, while inhibiting the expression of Fas and therefore differentiation. In turn, Fas, once expressed, antagonizes proliferation by exerting a negative feedback on ERK activation and Raf-1 expression. Simultaneously, Fas-mediated caspase activation precipitates differentiation. These results identify Raf-1 and Fas as the key molecules whose expression finely tunes erythropoiesis and the extent of ERK activation as the switch that tips the balance between them.
Blood 08/2006; 108(1):152-9. · 9.90 Impact Factor
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ABSTRACT: Ablation of the Raf-1 protein causes fetal liver apoptosis, embryonic lethality, and selective hypersensitivity to Fas-induced cell death. Furthermore, Raf-1-deficient cells show defective migration as a result of the deregulation of the Rho effector kinase Rok-alpha. In this study, we show that the kinase-independent modulation of Rok-alpha signaling is also the basis of the antiapoptotic function of Raf-1. Fas activation stimulates the formation of Raf-1-Rok-alpha complexes, and Rok-alpha signaling is up-regulated in Raf-1-deficient cells. This leads to increased clustering and membrane expression of Fas, which is rescued both by kinase-dead Raf-1 and by interfering with Rok-alpha or its substrate ezrin. Increased Fas clustering and membrane expression are also evident in the livers of Raf-1-deficient embryos, and genetically reducing Fas expression counteracts fetal liver apoptosis, embryonic lethality, and the apoptotic defects of embryonic fibroblasts. Thus, Raf-1 has an essential function in regulating Fas expression and setting the threshold of Fas sensitivity during embryonic life.
The Journal of Cell Biology 01/2006; 171(6):1013-22. · 10.26 Impact Factor
