Although
transforming growth factor beta1 (TGF-beta1) acts via the Smad signaling pathway to initiate de novo gene transcription, the TGF-beta1-induced
MAPK kinase activation that is involved in the regulation of apoptosis is less well understood. Even though the
p38 MAP kinase and c-Jun NH(2)-terminal
kinases (JNKs) are involved in TGF-beta1-induced cell death in
hepatoma cells, the upstream mediators of these
kinases remain to be defined. We show here that the members of the mixed lineage
kinase (MLK) family (including MLK1, MLK2, MLK3, and
dual leucine zipper-bearing kinase (DLK)) are expressed in FaO rat
hepatoma cells and are likely to act between p38 and
TGF-beta receptor kinase in death signaling.
TGF-beta1 treatment leads to an increase in MLK3 activity. Overexpression of MLK3 enhances TGF-beta1-induced apoptotic death in FaO cells and Hep3B human
hepatoma cells, whereas expression of the dominant-negative forms of MLK3 suppresses cell death induced by
TGF-beta1. The dominant-negative forms of MLK1 and -2 also suppress TGF-beta1-induced cell death. In MLK3-overexpressing cells, ERK, JNKs, and p38 MAP
kinases were further activated in response to
TGF-beta1 compared with the control cells. In contrast, overexpression of the dominant-negative MLK3 resulted in suppression of TGF-beta1-induced MAP
kinase activation and TGF-beta1-induced
caspase-3 activation. We also show that only the inhibition of the p38 pathway suppressed TGF-beta1-induced apoptosis. These observations support a role for MLKs in the TGF-beta1-induced cell death mechanism.