Modulating the DNA Damage Response to Improve Treatment Response in Cervical Cancer

Stat1 confers sensitivity to radiation in cervical cancer cells by controlling Parp1 levels: a new perspective for Parp1 inhibition

Cervical cancer (CC) is the fourth most common cause of cancer-related death in women. According to international guidelines, a standard treatment for locally advanced cervical cancer (LACC) consists of exclusive concurrent chemoradiation treatment (CRT). However, chemoradioresistance and subsequent relapse and metastasis of cancer occur in many patients, and survival for these women has generally remained poor. Therefore, strategies to overcome resistance are urgently needed. We have recently reported a radiosensitizing effect of the signal transducer and activator of transcription 1 (STAT1) in CC, associated with the control of [Poly(ADP-ribose) polymerase −1] PARP1 levels, a key factor in cell response to DNA damage induced by radiation. Here, we sought to decipher the underlying mechanism of STAT1-mediated control of PARP1, elucidating its role as a radiosensitizer in CC. Functional and molecular biology studies demonstrated that STAT1 may act at both transcriptional and posttranscriptional levels to modulate PARP1 expression in CC cells. In light of these results, we tested the effect of Olaparib in sensitizing CC cells to radiation and investigated signaling pathways involved in the activity observed. Results showed that PARP1 inhibition, at clinically achievable doses, may indeed selectively improve the sensitivity of resistant CC cells to DNA-damaging treatment. The translational relevance of our findings was supported by preliminary results in a limited patient cohort, confirming that higher PARP1 levels are significantly associated with a radioresistant phenotype. Finally, bioinformatics analysis of GEPIA and TCGA databases, demonstrated that PARP1 mRNA is higher in CC than in normal tissues and that increased PARP1 mRNA expression levels are associated with poor prognosis of LACC patients. Overall, our data open new opportunities for the development of personalized treatments in women diagnosed with CC.

Exploiting DNA repair pathways for tumor sensitization, mitigation of resistance, and normal tissue protection in radiotherapy

More than half of cancer patients are treated with radiotherapy, which kills tumor cells by directly and indirectly inducing DNA damage, including cytotoxic DNA double-strand breaks (DSBs). Tumor cells respond to these threats by activating a complex signaling network termed the DNA damage response (DDR). The DDR arrests the cell cycle, upregulates DNA repair, and triggers apoptosis when damage is excessive. The DDR signaling and DNA repair pathways are fertile terrain for therapeutic intervention. This review highlights strategies to improve therapeutic gain by targeting DDR and DNA repair pathways to radiosensitize tumor cells, overcome intrinsic and acquired tumor radioresistance, and protect normal tissue. Many biological and environmental factors determine tumor and normal cell responses to ionizing radiation and genotoxic chemotherapeutics. These include cell type and cell cycle phase distribution; tissue/tumor microenvironment and oxygen levels; DNA damage load and quality; DNA repair capacity; and susceptibility to apoptosis or other active or passive cell death pathways. We provide an overview of radiobiological parameters associated with X-ray, proton, and carbon ion radiotherapy; DNA repair and DNA damage signaling pathways; and other factors that regulate tumor and normal cell responses to radiation. We then focus on recent studies exploiting DSB repair pathways to enhance radiotherapy therapeutic gain.

A combined ANXA2-NDRG1-STAT1 gene signature predicts response to chemoradiotherapy in cervical cancer

BACKGROUND

A better understanding of locally advanced cervical cancer (LACC) is mandatory for further improving the rates of disease control, since a significant proportion of patients still fail to respond or undergo relapse after concurrent chemoradiation treatment (CRT), and survival for these patients has generally remained poor.

METHODS

To identify specific markers of CRT response, we compared pretreatment biopsies from LACC patients with pathological complete response (sensitive) with those from patients showing macroscopic residual tumor (resistant) after neoadjuvant CRT, using a proteomic approach integrated with gene expression profiling. The study of the underpinning mechanisms of chemoradiation response was carried out through in vitro models of cervical cancer.

RESULTS

We identified annexin A2 (ANXA2), N-myc downstream regulated gene 1 (NDRG1) and signal transducer and activator of transcription 1 (STAT1) as biomarkers of LACC patients' responsiveness to CRT. The dataset collected through qPCR on these genes was used as training dataset to implement a Random Forest algorithm able to predict the response of new patients to this treatment. Mechanistic investigations demonstrated the key role of the identified genes in the balance between death and survival of tumor cells.

CONCLUSIONS

Our results define a predictive gene signature that can help in cervical cancer patient stratification, thus providing a useful tool towards more personalized treatment modalities.

Ubiquitin-specific protease 7 sustains DNA damage response and promotes cervical carcinogenesis

Central to the recognition, signaling, and repair of DNA double-strand breaks (DSBs) are the MRE11-RAD50-NBS1 (MRN) complex and mediator of DNA damage checkpoint protein 1 (MDC1), the interplay of which is essential for initiation and amplification of the DNA damage response (DDR). The intrinsic rule governing the regulation of the function of this molecular machinery remains to be investigated. We report here that the ubiquitin-specific protease USP7 was physically associated with the MRN-MDC1 complex and that the MRN-MDC1 complex acted as a platform for USP7 to efficiently deubiquitinate and stabilize MDC1, thereby sustaining the DDR. Accordingly, depletion of USP7 impaired the engagement of the MRN-MDC1 complex and the consequent recruitment of the downstream factors p53-binding protein 1 (53BP1) and breast cancer protein 1 (BRCA1) at DNA lesions. Significantly, USP7 was overexpressed in cervical cancer, and the level of its expression positively correlated with that of MDC1 and worse survival rates for patients with cervical cancer. We demonstrate that USP7-mediated MDC1 stabilization promoted cervical cancer cell survival and conferred cellular resistance to genotoxic insults. Together, our study reveals a role for USP7 in regulating the function of the MRN-MDC1 complex and activity of the DDR, supporting the pursuit of USP7 as a potential therapeutic target for MDC1-proficient cancers.

The Human Papillomavirus E6 PDZ Binding Motif Links DNA Damage Response Signaling to E6 Inhibition of p53 Transcriptional Activity

The presence of a PDZ binding motif (PBM) in the human papillomavirus (HPV) E6 oncoprotein appears to be a characteristic marker of high oncogenic potential and confers interaction with a number of different cellular PDZ domain-containing substrates. The E6 PBM is also subject to phosphorylation, resulting in inhibition of E6 PDZ binding activity and instead allowing E6 to associate with 14-3-3 proteins. In this study, we analyzed the conditions under which the E6 PBM is phosphorylated. We demonstrate that in normal cycling cells, the levels of E6 phosphorylation are very low. However, following exposure of cells to oxidative stress or the induction of DNA damage, there is a striking increase in the levels of E6 phosphorylation. Depending on the specific stimulus, this phosphorylation of E6 can involve the ATM/ATR pathway and is performed primarily through Chk1, although the Chk2 pathway is also involved indirectly through activation of protein kinase A (PKA). To understand the biological relevance of these phospho-modifications of E6, we analyzed their effects upon the ability of E6 to inhibit p53 transcriptional activity. We show that an intact E6 phospho-acceptor site plays an essential role in the ability of E6 to inhibit p53 transcriptional activity on a subset of p53-responsive promoters in a manner that is independent of E6's ability to direct p53 degradation. These results are, to our knowledge, the first example of a DNA damage response controlling PBM-PDZ recognition. This study also provides links between the DNA damage response, the regulation of E6 PBM function, and the inhibition of p53 activity and begins to explain how HPV-infected cells remain within the cell cycle, despite activation of DNA damage response pathways during productive virus infections.IMPORTANCE The cancer-causing HPV E6 oncoproteins all possess a PDZ binding motif at their extreme carboxy termini. Depending upon whether this motif is phosphorylated, E6 can recognize PDZ domain-containing proteins or members of the 14-3-3 family of proteins. We show here that DNA damage response pathways directly signal to the E6 PBM, resulting in Chk1- and Chk2-driven phosphorylation. This phosphorylation is particularly pronounced following treatment of cells with a variety of different chemotherapeutic drugs. A direct functional consequence of this signaling is to confer an enhanced ability upon E6 to inhibit p53 transcriptional activity in a proteasome-independent but phosphorylation-dependent manner. These results are the first example of DNA damage signaling pathways regulating PBM-PDZ interactions and provide the mechanistic link between E6 PBM function and perturbation of p53 activity.

Effect of HPV E6/E7 siRNA with Chemotherapeutic Agents on the Regulation of TP53/E2F Dynamic Behavior for Cell Fate Decisions

Toxicity and resistance remain major challenges for advanced or recurrent cervical cancer therapies, as treatment requires high doses of chemotherapeutic agents. Restoration of TP53 and hypophosphorylated-retinoblastoma (pRB) proteins by human papillomavirus (HPV) E6/E7 siRNA sensitizes HPV-positive cervical cancer cells toward chemotherapeutic agents. Here, we investigated the therapeutic effects of E6/E7 siRNA on the dynamic behavior of TP53 and RB/E2F signaling networks in deciding the cell fate. The synergistic effect of HPV E6/E7 siRNA pool (SP) with chemotherapeutic agents on TP53 and RB/E2F signaling, proliferation, and apoptosis was analyzed in vitro and in vivo. Compared to the E6/E7 SP alone, E6/E7 SP with cisplatin treatment effectively restored TP53 and RB/E2F signaling and contributes to differences in cell fate, such as apoptosis or cell cycle arrest. We also developed a cellular dynamics model that includes TP53-RB/E2F dynamics and cell proliferation profiles, and confirmed its utility for investigating E6/E7 siRNA-based combination regimens. Using a dual reporter system, we further confirmed the cross talk between TP53 and RB/E2F signaling mechanisms. Treatment of E6/E7 SP cationic liposome (i.v.) with cisplatin and paclitaxel (i.p.) potentially inhibited tumor growth in BALB/c-nude mice. Altogether, our findings suggest that stabilization of TP53 and the RB/E2F repressor complex by E6/E7 SP combined with low-dose chemotherapy can effectively suppress tumor growth.