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

Mario Mikula, Martin Schreiber¹, Zvenislava Husak, Lucia Kucerova, Jochen Rüth, Rotraud Wieser², Kurt Zatloukal³, Hartmut Beug⁴, Erwin F. Wagner⁴, and Manuela Baccarini⁵

Department of Cell- and Microbiology, Institute of Microbiology and Genetics and ⁴Research Institute of Molecular Pathology, Vienna Biocenter, 1030 Vienna and ³Department of Pathology, University of Graz, A-8036 Graz, Austria ¹Present address: Department of Obstetrics and Gynecology, University of Vienna, Währinger Gürtel 18–20, A-1090 Vienna, Austria ²Present address: Department of Medical Biology, University of Vienna, Währinger Straße 10, A-1090 Vienna, Austria ⁵Corresponding author e-mail: manuela{at}gem.univie.ac.at

Abstract: 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-. 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.

Key Words: Keywords: apoptosis/development/gene inactivation/MAP kinase/proliferation