Stress signals elicit a wide variety of cellular responses, many of which 
converge on the phosphorylation of JNK and p38 kinases, the activation of which 
has been well-characterized. How these kinases are switched off by 
dephosphorylation is not well understood. Here we describe how diverse cellular 
stresses affect differently the stability and activity of a JNK-inactivating 
dual-specificity threonine-tyrosine phosphatase M3/6. Both anisomycin and 
arsenite activate the JNK pathway and, in addition, inactivate the M3/6 
phosphatase. However, while anisomycin treatment of cells leads to M3/6 protein 
degradation, arsenite appears to inactivate M3/6 directly. These results might 
have implications for the mechanism of tumour promotion by arsenic.