We recently obtained evidence that treatment of human colon cancer cells wi
th exisulind (sulindac sulfone) and related compounds induces apoptosis by
activation of protein kinase G (PKG) and c-Jun kinase (JNK1), The present s
tudy further explores this mechanism, We demonstrate that in NIH3T3 cells a
constitutively active mutant of PKG causes a dose-dependent activation of
JNK1 and thereby transactivates c-Jun and stimulates transcription from the
AP-I enhancer element. The activation of JNK1 and the transactivation of c
-Jun by this mutant of PKG were inhibited by a dominant negative MEKK1. In
vitro assays showed that a purified PKG directly phosphorylated the N-termi
nal domain of MEKK1. PKG also directly phosphorylated a full-length MEKK1,
and this was associated with enhanced MEKK1 phosphorylation. Thus, it appea
rs that PKG activates JNK1 through a novel PKG-MEKK1-SEK1-JNK1 pathway, by
directly phosphorylating and activating MEKK1.