Role for Lyn tyrosine kinase as a regulator of stress-activated protein kinase activity in response to DNA damage.

The cellular response to DNA damage includes activation of the nuclear Lyn protein tyrosine kinase. Using cells deficient in Lyn expression, the present studies demonstrate that Lyn is required in part for induction of the stress-activated protein kinase (SAPK) in the response to 1-beta-D-arabinofuranosylcytosine (ara-C) and other genotoxic agents. By contrast, exposure of Lyn-deficient cells to ara-C, ionizing radiation, or cisplatin had no effect on activation of extracellular signal-regulated protein kinase or p38 mitogen-activated protein kinase. Similar findings were obtained in cells stably expressing a kinase-inactive, dominant-negative Lyn(K-R) mutant. Coexpression studies demonstrate that Lyn, but not Lyn(K-R), induces SAPK activity. In addition, the results demonstrate that Lyn activates SAPK by an MKK7-dependent, SEK1-independent mechanism. As MEKK1 functions upstream to MKK7 and SAPK, the finding that a dominant-negative MEKK1(K-M) mutant blocks Lyn-induced SAPK activity supports involvement of the MEKK1-->MKK7 pathway. The results also demonstrate that inhibition of Lyn-induced SAPK activity abrogates the apoptotic response of cells to genotoxic stress. These findings indicate that activation of SAPK by DNA damage is mediated in part by Lyn and that the Lyn-->MEKK1-->MKK7-->SAPK pathway is functional in the induction of apoptosis by genotoxic agents.