Claudin 6 (CLDN6), a member of
tight junction protein claudin (CLDN) family, inhibits proliferation and induces apoptosis of MCF-7
breast cancer cells. However, these molecular mechanisms of CLDN6-induced apoptosis remain largely elusive. We previously found that restoration of human CLDN6 gene expression was correlated with the expression level of apoptosis signal-regulating kinase 1 (ASK1) using
cDNA array and bioinformatics analysis. ASK1, a
mitogen-activated protein kinase kinase kinase, is involved in environmental stress-activation of the
c-jun N-terminal kinase (JNK) and p38 pathways, which contribute to apoptosis-associated
tumor cell death. In the present study, we show that the restoration of CLDN6 gene expression in MCF-7 cells marhedly decreased ASK1 phosphorylation at Ser967. Activated ASK1ser967 further induced the activation of downstream targets, JNK and p38
kinase. MCF-7/CLDN6 stable transfection cell clone treated with TRX1, an ASK1 inhibitor, showed suppressed JNK and p38 activation, and showed substantially increased survival and colony formation and reduced percent of apoptotic cells using TUNEL staining and
DNA ladder. Furthermore, TRX1 treatment increased Bcl-2/Bax ratio and reduced
caspase-3 cleavage in MCF-7/CLDN6 stable transfection cell clone. Therefore, these data show that CLDN6 mediates ASK1-p38/JNK apoptotic signaling in MCF-7 cells, and it is correlated with constitutive deregulation of the balance of Bcl-2 family
proteins and activation of
caspase-3.