Activation of MEK kinase 1 by the c-Abl protein tyrosine kinase in response to DNA damage

The c-Abl protein tyrosine kinase is activated by certain DNA-damaging agen ts and regulates induction of the stress-activated c-Jun N-terminal protein kinase (SAPK). Here we show that nuclear c-Abl associates with MEK kinase 1 (MEKK-1), an upstream effector of the SEK1-->SAPK pathway, in the respons e of cells to genotoxic stress. The results demonstrate that the nuclear c- Abl binds to MEKK-1 and that c-Abl phosphorylates MEKK-1 in vitro and in vi vo. Transient-transfection studies with wild-type and kinase-inactive c-Abl demonstrate c-Abl kinase-dependent activation of MEKK-1. Moreover, c-Abl a ctivates MEKK-1 in vitro and in response to DNA damage. The results also de monstrate that c-AbI induces MEKK-1-mediated phosphorylation and activation of SEK1-SAPK in coupled kinase assays. These findings indicate that c-Abl functions upstream of MEKK-1-dependent activation of SAPK in the response t o genotoxic stress.