Platinum-based drugs, and in particular cisplatin, are widely used for the treatment of many kinds of solid malignancies, including cervical cancer [2
]. Cisplatin often leads to an initial therapeutic success associated with partial responses or disease stabilization. However, some patients developed resistance to cisplatin before and even after exposing to this chemotherapeutic agent. It is widely accepted that the high incidence of chemoresistance is the main reason for cisplatin treatment failure. Although the pre-target, on-target, post-target and off-target mechanisms of cisplatin-resistance have been proposed recently [25
], the molecular mechanisms underlying cisplatin-resistance are still far from understood.
In the present study, we provide evidence that NDRG2 is involved in the regulation of cisplatin-resistance of cervical cancer cells. Firstly, it was shown that cisplatin treatment induced up-regulation of NDRG2 in a time-dependent manner. Secondly, knock-down of NDRG2 by siRNA increased the suppressive effects of cisplatin on colony-forming ability of Hela cells. Thirdly, inhibition of NDRG2 significantly lowered the IC50 values of cisplatin for Hela cells, indicating down-regulation of NDRG2 increased the sensitivity of Hela cells to cisplatin. Finally, down-regulation of NDRG2 resulted in decreased expression level of Bcl-2, one of important regulators of apoptosis and drug resistance.
NDRG2, similar to NDRG1, has been proposed to be involved in cell stress [6
]. Previous studies demonstrated that NDGR2 expression in cancer cell lines could be enhanced by adriamycin [19
], hypoxia [20
] and radiation [18
], and that NDRG2 was implicated in regulation of adriamycin-induced apoptosis [19
] and radioresistance [18
]. All those data support a role of NDRG2 in cell stress. It is well-characterized that the mode of action of cisplatin involves the DNA-damage response and mitochondrial apoptosis [26
]. From this view, it is reasonable that NDRG2 can be induced by cisplatin. In line with previous reports, the present study indicates modulation of NDRG2 expression occurs at transcriptional level. Although NDRG2 transcription can be suppressed by Myc [28
], several binding sites for hypoxia-inducible factor 1 (HIF-1) have been found in the promoter region of NDRG2 gene and HIF-1 is responsible for NDRG2 up-regulation induced by hypoxia and radiation [18
]. It has been reported that HIF-1 can be activated by vincristine in gastric cancer cells under normoxic condition [29
]. Cisplatin, as a chemotherapeutic drug like adriamycin and vincristine, might induce NDRG2 expression through activation of HIF-1. However, this speculation needs to be validated in future study.
It is well known that Bcl-2 and Bax play critical roles in mitochondria apoptosis. As discussed in a recently published review [25
], there are increasing evidences that overexpression of Bcl-2 confers multidrug resistance and that clinical data have linked Bcl-2 expression level with cisplatin resistance and recurrent disease. The present study revealed that suppression of NDRG2 significantly inhibited Bcl-2 expression, which depictes how does NDRG2 influence cisplatin-induced apoptosis of Hela cells. Interestingly, NDRG2 regulates Bcl-2 expression at post-transcriptional level. It has been well defined that Bcl-2 can be targeted and regulated by microRNAs such as miR-15b and miR-16 in gastric cancer cells [21
]. These microRNAs bind to the 3’ untranslational region of Bcl-2 mRNA and inhibit translation of Bcl-2 protein without changing mRNA level. Similarly, the present study indicated that down-regulation of NDRG2 resulted in increased level of miR-15b and miR-16, which might in turn reduce Bcl-2 protein expression. However, the study does not exclude the possibility that NDRG2 influences stabilization of Bcl-2 protein.
It has been well documented that hypoxia- and radiation-induced NDRG2 promotes resistance of Hela cells to radiation [18
]. The present study demonstrated that cisplatin-induced NDRG2 increases chemoresistance of Hela cells. Considering cisplatin is often used in combination with radiotherapy for advanced cervical cancer [2
], NDRG2 may represent a key regulator of therapy-resistance in cervical cancer cells. It should be noted that NDRG2 has been proposed as a potential tumor suppressor. An increasing number of reports showed that the level of NDRG2 was reduced in many kinds of malignant tumor comparing to the normal counterpart and that NDRG2 level was an independent prognostic factor for cancer patients [7
]. It was also reported that restoration of NDRG2 could inhibit proliferation of cancer cells [6
]. Recently, the crystal structure of human NDRG2 protein was resolved and NDRG2 was proposed to suppress TCF/β-catenin signaling in the tumorigenesis of human colorectal cancer via a molecular interaction [30
]. Moreover, a role for NDRG2 in the oncogenic properties of renal cell carcinoma has been suggested [31
]. However, these data cannot exclude the possibility of NDRG2 to promote drug and radiation resistance. We proposed NDRG2 as a multifunctional protein. As a tumor suppressor, NDRG2 is inhibited to facilitate tumor development in the process of tumorigenesis. As a resistance regulator, NDRG2 may be re-activated or up-regulated to promote cancer cell survival during or after chemotherapy/radiotherapy. In an early study, it was shown inhibition of NDRG2 resulted in slightly increased proliferation and cisplatin resistance as well as decreased Fas expression and Fas-mediated cell death in gastric cancer cells [9
]. The discrepancy of NDRG2 in cisplatin resistance in cervical cancer and gastric cancer cells may due to tissue specificity. However, our speculation needs supporting data from further studies. It will be helpful to validate the role of NDRG2 in cisplatin resistance in multiple cell lines other than Hela and to explore the clinical relevance of NDRG2 to cisplatin resistance in patients with cervical cancer.