B-Raf inhibits programmed cell death downstream of cytochrome c release from mitochondria by activating the MEK/Erk pathway.
Growth factor-dependent kinases, such as phosphatidylinositol 3-kinase (PI 3-kinase) and Raf kinases, have been implicated in the suppression of apoptosis. We have recently established Rat-1 fibroblast cell lines overexpressing B-Raf, leading to activation of the MEK/Erk mitogen-activated protein kinase pathway. Overexpression of B-Raf confers resistance to apoptosis induced by growth factor withdrawal or PI 3-kinase inhibition. This is accompanied by constitutive activation of Erk without effects on the PI 3-kinase/Akt pathway. The activity of MEK is essential for cell survival mediated by B-Raf overexpression, since either treatment with the specific MEK inhibitor PD98059 or expression of a dominant inhibitory MEK mutant blocks the antiapoptotic activity of B-Raf. Activation of MEK is not only necessary but also sufficient for cell survival because overexpression of constitutively activated MEK, Ras, or Raf-1, like B-Raf, prevents apoptosis after growth factor deprivation. Overexpression of B-Raf did not interfere with the release of cytochrome c from mitochondria after growth factor deprivation. However, the addition of cytochrome c to cytosols of cells overexpressing B-Raf failed to induce caspase activation. It thus appears that the B-Raf/MEK/Erk pathway confers protection against apoptosis at the level of cytosolic caspase activation, downstream of the release of cytochrome c from mitochondria.
Department of Biology, Boston University, Boston, Massachusetts 02215, USA.
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Erhardt
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Schremser
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GM
Cooper
D000818:Animals; D017209:Apoptosis / Q000502:physiology*; D017871:Calcium-Calmodulin-Dependent Protein Kinases / Q000502:physiology*; D020169:Caspases / Q000502:physiology; D002460:Cell Line; D003574:Cytochrome c Group / Q000378:metabolism*; D003600:Cytosol / Q000201:enzymology; D004789:Enzyme Activation; D004791:Enzyme Inhibitors / Q000494:pharmacology; D005347:Fibroblasts / Q000378:metabolism; D005419:Flavonoids / Q000494:pharmacology; D008928:Mitochondria / Q000378:metabolism*; D019950:Mitogen-Activated Protein Kinase 1; D048052:Mitogen-Activated Protein Kinase 3; D020928:Mitogen-Activated Protein Kinases; D019869:Phosphatidylinositol 3-Kinases / Q000378:metabolism; D017346:Protein Serine-Threonine Kinases / Q000037:antagonists & inhibitors / Q000502:physiology*; D011518:Proto-Oncogene Proteins / Q000378:metabolism; D051057:Proto-Oncogene Proteins c-akt; D019908:Proto-Oncogene Proteins c-raf / Q000235:genetics / Q000502:physiology*; D051381:Rats; D011993:Recombinant Fusion Proteins / Q000502:physiology; D015398:Signal Transduction / Q000502:physiology*; D014162:Transfection
D017871:Calcium-Calmodulin-Dependent Protein Kinases; D020169:Caspases; D003574:Cytochrome c Group; D004791:Enzyme Inhibitors; D005419:Flavonoids; D019950:Mitogen-Activated Protein Kinase 1; D048052:Mitogen-Activated Protein Kinase 3; D020928:Mitogen-Activated Protein Kinases; D019869:Phosphatidylinositol 3-Kinases; D017346:Protein Serine-Threonine Kinases; D011518:Proto-Oncogene Proteins; D051057:Proto-Oncogene Proteins c-akt; D019908:Proto-Oncogene Proteins c-raf; D011993:Recombinant Fusion Proteins; C494920:Akt1 protein, rat; C093973:2-(2-amino-3-methoxyphenyl)-4H-1-benzopyran-4-one