Embryonic lethality and fetal liver apoptosis in mice lacking the c-raf-1 gene

The Raf kinases play a key role in relaying signals elicited by mitogens or oncogenes. Here, we report that c‐ raf‐1 −/− embryos are growth retarded and die at midgestation with anomalies in the placenta and in the fetal liver. Although hepatoblast proliferation does not appear to be impaired, c‐ raf‐1 −/− fetal livers are hypocellular and contain numerous apoptotic cells. Similarly, the poor proliferation of Raf‐1 −/− fibroblasts and hematopoietic cells cultivated in vitro is due to an increase in the apoptotic index of these cultures rather than to a cell cycle defect. Furthermore, Raf‐1‐ deficient fibroblasts are more sensitive than wild‐ type cells to specific apoptotic stimuli, such as actinomycin D or Fas activation, but not to tumor necrosis factor‐α. MEK/ERK activation is normal in Raf‐1‐deficient cells and embryos, and is probably mediated by B‐Raf. These results indicate that the essential function of Raf‐1 is to counteract apoptosis rather than to promote proliferation, and that effectors distinct from the MEK/ERK cascade must mediate the anti‐apoptotic function of Raf‐1.