TAp73α enhances the cellular sensitivity to cisplatin in ovarian cancer cells via the JNK signaling pathway

Abstract

Ovarian cancer is the most lethal gynecological malignancy. Most of ovarian

cancer patients relapse and subsequently die due to the development of resistance

to chemotherapy. P73 belongs to the tumor suppressor p53 family. Like p53, the

transcriptionally active TAp73 can bind specifically to p53 responsive elements

and transactivates some of the p53 target genes, and finally leads to cell cycle

arrest and apoptosis. TAp73 can be induced by DNA damage to enhance cellular

sensitivity to anticancer agents in human cancer cells. However, the functions of

TAp73 in ovarian cancer cells and the role in the regulation of cellular response to

commonly used chemotherapeutic agents cisplatin are still poorly understood. The

aims of this study were to examine the functions of TAp73 in ovarian cancer cells

and its role in cellular response to cisplatin, as well as the relationship between

TAp73 and p53 in ovarian cancer cells.

Functional studies showed that over-expression of TAp73alpha (TAp73α)

inhibited cell proliferation, colony formation ability and anchorage-independent

growth of ovarian cancer cells, and this was irrespective of p53 expression status.

In addition, TAp73α inhibited cell growth by arresting cell cycle at G2/M phase

and up-regulating the expressions of G2/M regulators of p21, 14-3-3sigma and

GADD45α.

TAp73α enhanced the cellular sensitivity to cisplatin through the activation of

JNK signaling pathway, at least partially, in ovarian cancer cells. TAp73α

activated the JNK pathway through the up-regulation of its target gene GADD45α

and subsequent activation of MKK4, the JNK up-stream kinase. Inhibition of JNK

activity by a specific inhibitor (SP600125) or small interfering RNAs (siRNAs)

significantly abrogated TAp73-mediated apoptosis induced by cisplatin. Moreover,

the activations of MKK4, JNK and c-Jun were abolished when GADD45α was

knocked down by siRNAs, and the JNK-dependent apoptosis was not observed.

Collectively, these results supported that TAp73α was able to mediate apoptotic

response to cisplatin through the GADD45α/MKK4/JNK signaling pathway,

which was respective of p53 expression status.

Further investigation on the relationship between TAp73α and p53

demonstrated that TAp73α increased p53 protein, but not mRNA expression by

attenuating p53 protein degradation in wild-type p53 ovarian cancer cells.

TAp73α could directly interact with p53 protein, which might interfere with the

binding ability of MDM2 to p53, and consequently block the p53 protein

degradation. In addition, TAp73α inactivated the Akt and ERK pathways and

activated the p38 pathway in response to cisplatin in wild-type p53 OVCA433,

but not in null-p53 SKOV3 cells, suggesting that the effect of TAp73α on these

pathways might be p53-dependent. These results indicated that a functional

cooperation of TAp73α and p53, to some extent, existed in ovarian cancer cells.

In conclusion, this study demonstrated that TAp73α acted as a tumor

suppressor in ovarian carcinogenesis. It promoted the cellular sensitivity to

cisplatin via, at least partially, the activation of JNK signaling pathway. These

TAp73α functions were irrespective of p53 expression. In addition, TAp73α was

able to bind to p53 and increase p53 expression.