Bcl-x(L) blocks activation of related adhesion focal tyrosine kinase proline-rich tyrosine kinase 2 and stress-activated protein kinase c-Jun N-terminal protein kinase in the cellular response to methylmethane sulfonate

The stress-activated protein kinase/c-Jun N-terminal protein kinase (JNK) i s induced in response to ionizing radiation and other DNA-damaging agents. Recent studies indicate that activation of JNK is necessary for induction o f apoptosis in response to diverse agents. Here we demonstrate that methyln ethane sulfonate (MMS)-induced activation of JNK is inhibited by overexpres sion of the anti-apoptotic protein Bcl-x(L), but not by caspase inhibitors CrmA and p35, By contrast, UV-induced JNK activity is insensitive to Bcl-x( L). The results demonstrate that treatment with MMS is associated with an i ncrease in tyrosine phosphorylation of related adhesion focal tyrosine kina se (RAFTK)/proline-rich tyrosine kinase 2 (PYK2), an upstream effector of J NK and that this phosphorylation is inhibited by overexpression of Bcl-x(L) . Furthermore, overexpression of a dominant-negative mutant of RAFTK (RAFTK K-M) inhibits MMS-induced JNK activation, The results indicate that inhibi tion of RAFTK phosphorylation by MMS in Bcl-x(L) cells is attributed to an increase in tyrosine phosphatase activity in these cells. Hence, treatment of Bcl-x(L) cells with sodium vanadate, a tyrosine phosphatase inhibitor, r estores MMS-induced activation of RAFTK and JNK. These findings indicate th at RAFTK-dependent induction of JNK in response to MMS is sensitive to Bcl- x(L), but not to CrmA. and p35, by a mechanism that inhibits tyrosine phosp horylation and thereby activation of RAFTK. Taken together, these findings support a novel role for Bcl-x(L) that is independent of the caspase cascad e.