The c-Jun NH2-terminal kinase (JNK) is activated when cells are exposed to ultraviolet (UV) radiation. However, the functional consequence of JNK activation in UV-irradiated cells has not been established. It is shown here that JNK is required for UV-induced apoptosis in primary murine embryonic fibroblasts. Fibroblasts with simultaneous targeted disruptions of all the functional Jnk genes were protected against UV-stimulated apoptosis. The absence of JNK caused a defect in the mitochondrial death signaling pathway, including the failure to release cytochrome c. These data indicate that mitochondria are influenced by proapoptotic signal transduction through the JNK pathway.

Rac and Rho GTPases function as critical regulators of actin cytoskeleton remodelling during cell spreading and migration. Here we demonstrate that Rac-mediated reactive oxygen species (ROS) production results in the downregulation of Rho activity. The redox-dependent decrease in Rho activity is required for Rac-induced formation of membrane ruffles and integrin-mediated cell spreading. The pathway linking generation of ROS to downregulation of Rho involves inhibition of the low-molecular-weight protein tyrosine phosphatase (LMW-PTP) and then an increase in the tyrosine phosphorylation and activation of its target, p190Rho-GAP. Our findings define a novel mechanism for the coupling of changes in cellular redox state to the control of actin cytoskeleton rearrangements by Rho GTPases.

Targeted gene disruption studies have established that the c-Jun NH2-terminal kinase (JNK) signaling pathway is required for stress-induced release of mitochondrial cytochrome c and apoptosis. Here we demonstrate that activated JNK is sufficient to induce rapid cytochrome c release and apoptosis. However, activated JNK fails to cause death in cells deficient of members of the Bax subfamily of proapoptotic Bcl2-related proteins. Furthermore, exposure to stress fails to activate Bax, cause cytochrome c release, and induce death in JNK-deficient cells. These data demonstrate that proapoptotic members of the Bax protein subfamily are essential for JNK-dependent apoptosis