Apoptosis induced by Rac GTPase correlates with induction of FasL and ceramides production

Rho proteins, members of the Ras superfamily of GTPases, are critical elements in signal transduction pathways governing cell proliferation and cell death. Different members of the family of human Rho GTPases, including RhoA, RhoC, and Rad, participate in the regulation of apoptosis in response to cytokines and serum deprivation in different cell systems. Here, we have characterized the mechanism of apoptosis induced by Rad in NM 3T3 cells. It requires protein synthesis and caspase-3 activity, but it is independent of the release of cytochrome c from mitochondria. Moreover, an increase in mitochondria membrane potential and the production of reactive oxygen species was observed. Rad-induced apoptosis was related to the simultaneous increase in ceramide production and synthesis of Fast. Generation of Fast may be mediated by transcriptional regulation involving both c-Jun amino terminal kinase as well as nuclear factor-kappaB-dependent signals. None of these signals, ceramides or Fast, was sufficient to induce apoptosis in the parental cell line, NIH 3T3 cells. However, any of them was sufficient to induce apoptosis in the Rad-expressing cells. Finally, inhibition of Fast signaling drastically reduced apoptosis by Rad. Thus, Rad seems to induce apoptosis by a complex mechanism involving the generation of ceramides and the de novo synthesis of Fast. These results suggest that apoptosis mediated by Rad results from a signaling mechanism that involves biochemical and transcriptional events under control of Rad.