The wild-type p53 is a key tumor suppressor protein that eliminates genetically unstable cells by inducing either cell cycle arrest or apoptosis through transcriptional regulation or direct interaction with apoptotic proteins.1, 2
Functional inactivation of p53, a frequent event in cancer cells, occurs by three main mechanisms: (i) mutations, (ii) post-translational modifications and (iii) cytoplasmic sequestration3, 4, 5
by its binding proteins. Although several cellular proteins are shown to interact with p53, the mechanism of its inactivation still remains unclear. Furthermore, interaction of p53 with its binding partners is context dependent, and influenced by both intracellular and extracellular environment. Cellular stress response (intrinsic and extrinsic) has been shown to evoke p53 signaling6
through its modifications, including phosphorylation (at serine and/or threonine),7
Furthermore, it has been shown that p53 protein interacts with several stress proteins, including Hsp40, Hsp70, Hsp84, Hsp90, DnaK, DnaJ and GrpE in vivo
,13, 14, 15
that potentially modulate p53 activities. However, their roles in development and progression of cancer, physiologically a stressed condition, remain unclear.
Mortalin/mthsp70/GRP75/PBP74, a member of the heat shock protein (Hsp) 70 family, is enriched in human cancer cells.16, 17, 18
Overexpression of mortalin was sufficient to increase the malignancy of breast cancer cells in both in vitro
and in vivo
models. The underlying mechanism was shown to be the sequestration of wild-type p53 in the cytoplasm, leading to inhibition of its transcriptional activation and control of centrosome duplication functions.19, 20, 21, 22
A cationic inhibitor (MKT-077) of mortalin that releases p53 from mortalin–p53 complex was shown to cause activation of p53 and growth arrest of cancer cells.23
Similarly, mortalin-binding p53 peptides caused nuclear translocation and activation of p53.19
Mortalin was identified as a marker for hepatocellular carcinoma (HCC) metastasis and recurrence by proteomics analysis of matched tumor and non-tumor tissues,18
suggesting that it contributes to HCC development and recurrence.
In the present study, we found that unlike most cancer cells, HepG2 hepatoma lacked mortalin–p53 interaction. Using HepG2 as a model, we demonstrate that the mortalin–p53 interaction that causes inactivation of p53-mediated apoptosis depends on the cellular stress level. Unstressed or weakly stressed cancer and normal cells lack mortalin–p53 interaction. Furthermore, mortalin small hairpin RNA (shRNA) could reactivate p53-mediated apoptosis selectively in cancer cells. These findings open the possibilities for clinical application of mortalin shRNA, mortalin–p53 binding antagonists or their combination with chemotherapeutic drugs in the HCC treatment.