ABSTRACT
Protein tyrosine kinases (PTKs) play crucial roles in a wide variety of cellular responses, including cellular activation, proliferation, and dif ferentiation ( 1 ). It has been also reported that extracellular stresses, ion izing radiation, UV irradiation, H20 2, or genotoxic agents, activate a set of nonreceptor PTKs, the Src-and/or Syk/ZAP-70-family PTKs (2-6), although the exact roles of these nonreceptor PTKs in stress signalings remain elusive. Although the participation of PTKs in stress signaling has been indicated (7), the identities and exact roles of such PTKs are largely unknown. Interestingly, recent studies reveal that c-Abl kinase, a nonreceptor PTK localized in both cytoplasm and nucleus (8,9), posi tively regulates the activities of members of the MAPK superfamily, JNKs, in response to extracellular stresses that selectively induce DNA damage (10,11).
