Death-associated protein kinase (DAPK) is a
serine/threonine kinase that participates in the modulation of apoptosis and
tumor suppression. Our previous study revealed high levels of DAPK
protein expression in differentiated endometrial
adenocarcinoma cells. To clarify the role of DAPK in human endometrial
adenocarcinomas, we down-regulated endogenous DAPK expression in HHUA cells, a well-differentiated endometrial
adenocarcinoma cell line, using specific small-interfering RNAs (siRNAs). The suppression of endogenous DAPK expression triggered apoptosis in HHUA cells, as evidenced by an increase in the sub-G1
DNA content in flow cytometric analyses. The apoptosis induced by the DAPK
siRNA transfections was
caspase-dependent, as characterized by the activations of
caspase-3, -8 and -9.
RNase protection assays detected higher levels of
tumor necrosis factor-related apoptosis-inducing
ligand (TRAIL), DR4 and DR5 transcripts in the DAPK
siRNA-transfected HHUA cells than in the control
siRNA-transfected cells. Consistent with these findings,
enzyme-linked
immunosorbent assays revealed that the DAPK
siRNA transfections significantly increased the secretion of
TRAIL protein from the cells. Treatment with recombinant human
TRAIL protein dose-dependently suppressed the cell viability of HHUA cells. The present findings reveal that down-regulation of endogenous DAPK expression in HHUA cells induces caspase-dependent apoptosis, possibly through increased TRAIL, DR4 and DR5 signaling, thereby suggesting that DAPK expression is essential for HHUA cell survival. Consequently, endogenous DAPK
mRNA may represent a potential candidate for molecularly targeted anticancer
therapies.