Novel Approaches for Chemosensitization of Breast Cancer Cells: The E1A Story

The Effect of PEI-Mediated E1A on the Radiosensitivity of Hepatic Carcinoma Cells

Objective:

The study was undertaken to investigate the effects of polyethyleneimine (PEI)-mediated adenovirus 5 early region 1A (E1A) on radiosensitivity of human hepatic carcinoma cell in vitro and to disclosure the underlying mechanism.

Materials and Methods:

Human hepatic carcinoma SMMC-7721 cell line was transfected with E1A gene using PEI vector. Untransfected cells (SMMC-7721 group), cells transfected with blank-vector (SMMC-7721-vect group), and cells transfected with E1A gene (SMMC-7721-E1A group) were treated with 6 MV X-ray irradiation at doses of 0, 1, 2, 4, 8 and Gy, respectively. Radiosensitivity was determined by MTT assay and quantified by calculating the cell survival rate. Cell-cycle distribution and apotosis rate were monitored by flow cytometry.

Results:

The survival rate of SMMC-7721-E1A was significantly lower than that of SMMC-7721 cell. Apoptosis rate of SMMC-7721-E1A group was significantly higher than that of SMMC-7721group (P<0.01).The ratio of S stage in cell cycle of SMMC-7721-E1A was significantly lower than that in SMMC-7721 cell. The ratio of G2/M stage in cell cycle of SMMC-7721-E1A was significantly higher than that in SMMC-7721 cell (P<0.01).

Conclusion:

PEI could transfect E1A gene into hepatic carcinoma cells PEI-mediated E1A could effectively enhance radiosensitivity of hepatic carcinoma cells which may be related to its effects on apoptosis promoting leading to S phase suppression and G2/M phase arrest.

Expression of Adenoviral E1A in Transformed Cells as an Additional Factor of HDACi-Dependent FoxO Regulation

The adenoviral early region 1A (E1A) protein has proapoptotic and angiogenic activity, along with its chemosensitizing effect, making it the focus of increased interest in the context of cancer therapy. It was previously shown that E1A-induced chemosensitization to different drugs, including histone deacetylases inhibitors (HDACi), appears to be mediated by Forkhead box O (FoxO) transcription factors. In this study, we explore the relationship between E1A expression and the modulation of FoxO activity with HDACi sodium butyrate (NaBut). We show here that the basal FoxO level is elevated in E1A-expressing cells. Prolonged NaBut treatment leads to the inhibition of the FoxO expression and activity in E1A-expressing cells. However, in E1A-negative cells, NaBut promotes the transactivation ability of FoxO over time. A more detailed investigation revealed that the NaBut-induced decrease of FoxO activity in E1A-expressing cells is due to the NaBut-dependent decrease in E1A expression. Therefore, NaBut-induced inhibition of FoxO in E1A-positive cells can be overcome under unregulated overexpression of E1A. Remarkably, the CBP/p300-binding domain of E1Aad5 is responsible for stabilization of the FoxO protein. Collectively, these data show that the expression of E1A increases the FoxO stability but makes the FoxO level more sensitive to HDACi treatment.

Polyethylenimine (PEI)-Mediated E1A Increases the Sensitivity of Hepatocellular Carcinoma Cells to Chemotherapy

BACKGROUND The aim of this study was to assess the ability of polyethylenimine (PEI) as an E1A plasmid vector to transfect hepatocellular carcinoma SMMC-7721 cells and to analyze the sensitization effect of E1A on various anti-tumor drugs. MATERIAL AND METHODS PEI-mediated recombinant plasmid psv-E1A with high expression of the E1A gene was introduced into hepatocellular carcinoma SMMC-7721 cells, and the effective transfection of E1A gene was determined by RT-PCR and Western blot analysis. The CCK8 method was used to detect the proliferation inhibition of docetaxel, epirubicin, gemcitabine, and 5-fluorouracil on SMMC-7721 cells before and after the transfection of the E1A gene. RESULTS RT-PCR and Western blot analysis showed that PEI could transfect plasmid psv-E1A with stable expression. After the transfection of E1A gene, the sensitivity of SMMC-7721 cells to docetaxel, epirubicin, gemcitabine, and 5-fluorouracil was increased (P<0.05), and the sensitivity to docetaxel was significantly improved (P<0.01). CONCLUSIONS PEI can transfect plasmid psv-E1A. The E1A gene can increase the sensitivity of hepatocellular carcinoma cells to chemotherapeutic drugs. The mechanism may be related to the increased ability of the E1A gene to inhibit proliferation of hepatocellular carcinoma cells and altering the cell cycle of hepatocellular carcinoma cells.

A p53-independent apoptotic mechanism of adenoviral mutant E1A was involved in its selective antitumor activity for human cancer

The conserved regions (CR) of adenoviral E1A had been shown to be necessary for disruption of pRb-E2F transcription factor complexes and induction of the S phase. Here we constructed a mutant adenoviral E1A with Rb-binding ability absent (E1A 30-60aa and 120-127aa deletion, mE1A) and investigated its antitumor capacities in vitro and in vivo. The mE1A suppressed the viability of tumor cells as efficiently as the wild type E1A, and there was no cytotoxic effect on normal cells. Although the mE1A arrested tumor cell cycle with the same manner as E1A, the former played a different role on cell cycle regulation compared with E1A in normal cells, which might contribute to its selective antitumor activity. E1A and mE1A had accumulated inactive p53, decreased the expression of mdm2, Cdkn1a (also named p21), increased p21's nuclear distribution and induced tumor cell apoptosis in a p53-indenpent manner. Further, E1A or mE1A significantly suppressed tumor growth in subcutaneous hepatocellular carcinoma xenograft models. Especially, tumor-bearing mice treated with mE1A had higher survival rate than those treated with E1A. Our data demonstrated that mutant adenoviral E1A significantly induced tumor cell apoptosis in a p53-indenpednt manner and had selective tumor suppressing ability. The observations of adenoviral E1A mutant had provided a novel mechanism for E1A's complex activities during infection.

The E1B19K-deleted oncolytic adenovirus mutant AdΔ19K sensitizes pancreatic cancer cells to drug-induced DNA-damage by down-regulating Claspin and Mre11

Adenovirus-mediated sensitization of cancer cells to cytotoxic drugs depends on simultaneous interactions of early viral genes with cell death and survival pathways. It is unclear what cellular factors mediate these interactions in the presence of DNA-damaging drugs. We found that adenovirus prevents Chk1-mediated checkpoint activation through inactivation of Mre11 and downregulation of the pChk1 adaptor-protein, Claspin, in cells with high levels of DNA-damage induced by the cytotoxic drugs gemcitabine and irinotecan. The mechanisms for Claspin downregulation involve decreased transcription and increased degradation, further attenuating pChk1-mediated signalling. Live cell imaging demonstrated that low doses of gemcitabine caused multiple mitotic aberrations including multipolar spindles, micro- and multi-nucleation and cytokinesis failure. A mutant virus with the anti-apoptotic E1B19K-gene deleted (AdΔ19K) further enhanced cell killing, Claspin downregulation, and potentiated drug-induced DNA damage and mitotic aberrations. Decreased Claspin expression and inactivation of Mre11 contributed to the enhanced cell killing in combination with DNA-damaging drugs. These results reveal novel mechanisms that are utilised by adenovirus to ensure completion of its life cycle in the presence of cellular DNA damage. Taken together, our findings reveal novel cellular targets that may be exploited when developing improved anti-cancer therapeutics.

Adenoviral vectors for prodrug activation-based gene therapy for cancer

Cancer cell heterogeneity is a common feature - both between patients diagnosed with the same cancer and within an individual patient's tumor - and leads to widely different response rates to cancer therapies and the potential for the emergence of drug resistance. Diverse therapeutic approaches have been developed to combat the complexity of cancer, including individual treatment modalities designed to target tumor heterogeneity. This review discusses adenoviral vectors and how they can be modified to replicate in a cancer-specific manner and deliver therapeutic genes under multi-tiered regulation to target tumor heterogeneity, including heterogeneity associated with cancer stem cell-like subpopulations. Strategies that allow for combination of prodrug-activation gene therapy with a novel replication-conditional, heterogeneous tumor-targeting adenovirus are discussed, as are the benefits of using adenoviral vectors as tumor-targeting oncolytic vectors. While the anticancer activity of many adenoviral vectors has been well established in preclinical studies, only limited successes have been achieved in the clinic, indicating a need for further improvements in activity, specificity, tumor cell delivery and avoidance of immunogenicity.

E1a promotes c-Myc-dependent replicative stress: implications in glioblastoma radiosensitization

The E1a gene from adenovirus is known to be a potent inducer of chemo/radiosensitivity in a wide range of tumors. However, the molecular bases of its radiosensitizer properties are still poorly understood. In an attempt to study this effect, U87MG cells, derived from a radio-resistant tumor as glioblastoma, where infected with lentivirus carrying E1a gene developing an acute sensitivity to ionizing radiation. The induction of radiosensitivity correlated with a marked G 2/M phase accumulation and a potent apoptotic response. Our findings demonstrate that c-Myc plays a pivotal role in E1a-associated radiosensitivity through the induction of a replicative stress situation, as our data support by genetic approaches, based in interference and overexpression in U87MG cells. In fact, we present evidence showing that Chk1 is a novel transcriptional target of E1a gene through the effect exerted by this adenoviral protein onto c-Myc. Moreover, c-Myc upregulation also explains the marked phosphorylation of H2AX associated to E1a expression in the absence of DNA damage. Indeed, all these observations were applicable to other experimental models, such as T98G, LN-405 and A172, rendering the same pattern in terms of radiosensitivity, cell cycle distribution, upregulation of Chk1, c-Myc, and phosphorylation pattern of H2AX. In summary, our data propose a novel mechanism to explain how E1a mediates radiosensitivity through the signaling axis E1a→c-Myc→ replicative stress situation. This novel mechanism of E1a-mediated radiosensitivity could be the key to open new possibilities in the current therapy of glioblastoma.

Impact of E1 and Cre on adenovirus vector amplification: developing MDCK CAV-2-E1 and E1-Cre transcomplementing cell lines

Adenovirus vectors have been extensively studied through the manipulation of viral genome. However, little attention is being paid to their producer cell-lines; cells are selected according to virus yields, neglecting the expression profile of transcomplementing gene products underlying cell performance. This work evaluates the impact of E1 (E1A and E1B) and Cre recombinase levels in the production of E1-deleted and helper-dependent canine adenovirus type 2 (CAV-2) vectors using MDCK cells. E1A and E1B gene expression and Cre activity were evaluated in different cell clones and compared with the corresponding cell productivity and susceptibility to oxidative stress injury. CAV-2 production was proportional to E1A expression (the highest levels of E1A corresponding to productivities of 3000–5000 I.P./cell), while E1B prolonged host cell viability after infection, conferring protection against apoptosis. Cre recombinase counteracted E1B anti-apoptotic properties, however viral production was maintained under high levels of Cre. Yet, Cre recombinase side effects can be reduced using cell lines with lower Cre-activities, without compromising the excision efficiency of helper vector packaging signal. These results highlight the influence of transcomplementing gene products on CAV-2 producer cell line performance, and the ability to express high levels of E1A and E1B as an important feature for cell line establishment and high adenovirus titers.

Adenovirus-mediated sensitization to the cytotoxic drugs docetaxel and mitoxantrone is dependent on regulatory domains in the E1ACR1 gene-region

Oncolytic adenoviruses have shown promising efficacy in clinical trials targeting prostate cancers that frequently develop resistance to all current therapies. The replication-selective mutants AdΔΔ and dl922–947, defective in pRb-binding, have been demonstrated to synergise with the current standard of care, mitoxantrone and docetaxel, in prostate cancer models. While expression of the early viral E1A gene is essential for the enhanced cell killing, the specific E1A-regions required for the effects are unknown. Here, we demonstrate that replicating mutants deleted in small E1A-domains, binding pRb (dl1108), p300/CBP (dl1104) and p400/TRRAP or p21 (dl1102) sensitize human prostate cancer cells (PC-3, DU145, 22Rv1) to mitoxantrone and docetaxel. Through generation of non-replicating mutants, we demonstrate that the small E1A12S protein is sufficient to potently sensitize all prostate cancer cells to the drugs even in the absence of viral replication and the E1A transactivating domain, conserved region (CR) 3. Furthermore, the p300/CBP-binding domain in E1ACR1 is essential for drug-sensitisation in the absence (AdE1A1104) but not in the presence of the E1ACR3 (dl1104) domain. AdE1A1104 also failed to increase apoptosis and accumulation of cells in G2/M. All E1AΔCR2 mutants (AdE1A1108, dl922–947) and AdE1A1102 or dl1102 enhance cell killing to the same degree as wild type virus. In PC-3 xenografts in vivo the dl1102 mutant significantly prolongs time to tumor progression that is further enhanced in combination with docetaxel. Neither dl1102 nor dl1104 replicates in normal human epithelial cells (NHBE). These findings suggest that additional E1A-deletions might be included when developing more potent replication-selective oncolytic viruses, such as the AdΔCR2-mutants, to further enhance potency through synergistic cell killing in combination with current chemotherapeutics.

FOXO3a-Dependent Mechanism of E1A-Induced Chemosensitization

Gene therapy trials in human breast, ovarian, and head and neck tumors indicate that adenovirus E1A can sensitize cancer cells to the cytotoxic effects of paclitaxel in vitro and in vivo. Resistance to paclitaxel has been reported to occur in cells expressing low levels of the Forkhead transcription factor FOXO3a. In this article, we report that FOXO3a is critical for E1A-mediated chemosensitization to paclitaxel. RNA interference-mediated knockdown of FOXO3a abolished E1A-induced sensitivity to paclitaxel. Mechanistic investigations indicated that E1A indirectly stabilized FOXO3a by acting at an intermediate step to inhibit a ubiquitin-dependent proteolysis pathway involving the E3 ligase βTrCP and the FOXO3a inhibitory kinase IKKβ. E1A derepressed this inhibitory pathway by stimulating expression of the protein phosphatase 2A (PP2A)/C protein phosphatases, which by binding to the TGF-β-activated kinase TAK1, inhibited its ability to activate IKKβ and, thereby, to suppress βTrCP-mediated degradation of FOXO3a. Thus, by stimulating PP2A/C expression, E1A triggers a signaling cascade that stabilizes FOXO3a and mediates chemosensitization. Our findings provide a leap forward in understanding paclitaxel chemosensitization by E1A, and offer a mechanistic rational to apply E1A gene therapy as an adjuvant for improving therapeutic outcomes in patients receiving paclitaxel treatment.

Combining conditionally replicating adenovirus-mediated gene therapy with chemotherapy: a novel antitumor approach

Despite significant improvements in diagnosis and innovations in the therapy of specific cancers, effective treatment of neoplastic diseases still presents major challenges. Recent studies have shown that conditionally replicating adenoviruses (CRAds) not only have the ability to destroy cancer cells but may also be potential vectors for the expression of therapeutic genes. Several studies in animal models have demonstrated that the combination of CRAds-mediated gene therapy and chemotherapy has greater therapeutic benefit than either treatment modality alone. In this review, an overview of specifications for a novel antitumor approach combining CRAd-gene therapy and chemotherapy is provided and recent progress in this field is discussed.

Dual E1A oncolytic adenovirus: targeting tumor heterogeneity with two independent cancer-specific promoter elements, DF3/MUC1 and hTERT

The therapeutic utility of oncolytic adenoviruses controlled by a single, tumor-specific regulatory element may be limited by the intra- and inter-tumoral heterogeneity that characterizes many cancers. To address this issue, we constructed an oncolytic adenovirus that uses two distinct tumor-specific promoters (DF3/Muc1 and hTERT) to drive separate E1A expression cassettes, in combination with deletion of the viral E1B region, which confers additional tumor selectivity and increased oncolytic activity. The resultant virus, Adeno-DF3-E1A/hTERT-E1A, induced higher levels of E1A oncoprotein, enhanced oncolysis, and an earlier and higher apoptotic index in infected tumor cells than following infection with Adeno-hTERT-E1A, which harbors a single hTERT promoter-driven E1A cassette. In isolated U251 human gliosarcoma cell holoclones (putative cancer stem cells), where DF3/Muc1 expression is substantially enriched and hTERT expression is decreased compared to the parental U251 cell population, E1A production and oncolysis were strongly decreased following infection with Adeno-hTERT-E1A but not Adeno-DF3-E1A/hTERT-E1A. The strong oncolytic activity of Adeno-DF3-E1A/hTERT-E1A translated into superior anti-tumor activity over Adeno-hTERT-E1A in vivo in a U251 solid tumor xenograft model, where hTERT levels were >90% suppressed and the DF3/Muc1 to hTERT expression ratio was substantially increased compared to cultured U251 cells. The enhanced anti-tumor activity of the dual-targeted Adeno-DF3-E1A/hTERT-E1A was achieved despite premature viral host cell death and decreased production of functional viral progeny, which limited tumor cell spread of the viral infection. These findings highlight the therapeutic benefit of targeting oncolytic viruses to heterogeneous tumor cell populations.

Adenovirus 5 E1A enhances histone deacetylase inhibitors-induced apoptosis through Egr-1-mediated Bim upregulation

Histone deacetylase inhibitors (HDACi) are potent anti-cancer agents for variety of cancer types. Suberoylanilide hydroxamic acid (SAHA) has been approved as a drug to treat cutaneous T cell lymphoma, and the combination of HDACi and other agents have been actively tested in many clinical trials. Adenovirus 5 early region 1A (E1A) has been shown to exhibit high tumor suppressor activity, and gene therapy using E1A has been tested in clinical trials. Here, we showed that proapoptotic activity of HDACi was robustly enhanced by E1A in multiple cancer cells, but not in normal cells. Moreover, we showed that combination of E1A gene therapy and SAHA showed high therapeutic efficacy with low toxicity in vivo ovarian and breast xenograft models. SAHA downregulated Bcl-XL and upregulated proapoptotic BH3-only protein Bim, whose expression was further enhanced by E1A in cancer cells. These alterations of Bcl-2 family proteins were critical for apoptosis induced by the combination in cancer cells. SAHA enhanced acetylation of histone H3 in Bim promoter region, while E1A upregulated Egr-1, which was directly involved in Bim transactivation. Together, our results provide not only a novel insight into the mechanisms underlying anti-tumor activity of E1A, but also a rationale for the combined HDACi and E1A gene therapy in future clinical trials.

A truncated minimal-E1a gene with potency to support adenoviral replication mediates antitumor activity by down-regulating Neu expression and preserving Rb function

Oncolytic adenovirus is capable of infecting, replicating in and lysing cancer cells. In adenovirus infection and replication, the wild type E1a gene (wE1a) mediates various genetic events to facilitate viral replication and exert antitumor effect. To enhance its antitumor efficacy and optimize its safety, we manipulated the wE1a gene and designed a 720-bp truncated minimal-E1a (mE1a) by deletions and mutations of amino acid residues. The mE1a gene was incorporated in an adenovirus under the control of hTERT promoter, giving the vector AdDC315-mE1a. A variety of cancer cell lines infected with the virus expressed the mE1a protein and showed considerable down-regulation in Neu protein expression as compared to normal cell lines. mE1a also had a lower binding affinity to the Rb protein, preserving the Rb tumor suppressive function. The mE1a expression allowed efficient adenovirus replication with high and stable replication ratios in cancer cells (about 125- to 8500-fold higher at 48 h and 180- to 10,900-fold higher at 96 h post-infection). Further, the mE1a-supported oncolytic adenovirus induced higher cancer cell apoptosis, stronger cell cycle arrest and more effective antitumor efficacy in hepatocarcinoma xenografts in nude mice. In conclusion, the truncated minimal mE1a can act as a tumor inhibitor gene, and may be used to construct oncolytic adenovirus vectors for use in gene therapy of a variety of cancers.

Adenoviral vector-based strategies for cancer therapy

Definitive treatment of cancer has eluded scientists for decades. Current therapeutic modalities like surgery, chemotherapy, radiotherapy and receptor-targeted antibodies have varied degree of success and generally have moderate to severe side effects. Gene therapy is one of the novel and promising approaches for therapeutic intervention of cancer. Viral vectors in general and adenoviral (Ad) vectors in particular are efficient natural gene delivery systems and are one of the obvious choices for cancer gene therapy. Clinical and preclinical findings with a wide variety of approaches like tumor suppressor and suicide gene therapy, oncolysis, immunotherapy, anti-angiogenesis and RNA interference using Ad vectors have been quite promising, but there are still many hurdles to overcome. Shortcomings like increased immunogenicity, prevalence of preexisting anti-Ad immunity in human population and lack of specific targeting limit the clinical usefulness of Ad vectors. In recent years, extensive research efforts have been made to overcome these limitations through a variety of approaches including the use of conditionally-replicating Ad and specific targeting of tumor cells. In this review, we discuss the potential strengths and limitations of Ad vectors for cancer therapy.

c-Myc-induced aberrant DNA synthesis and activation of DNA damage response in p300 knockdown cells

We previously showed that in quiescent cells, p300/CBP (CREB-binding protein)family coactivators repress c-myc and prevent premature induction of DNA synthesis. p300/CBP-depleted cells exit G(1) early and continue to accumulate in S phase but do not progress into G(2)/M, and eventually they die of apoptosis. Here, we show that the S-phase arrest in these cells is because of an intra-S-phase block. The inappropriate DNA synthesis that occurs as a result of forced expression of c-myc leads to the activation of the DNA damage response as evidenced by the phosphorylation of several checkpoint related proteins and the formation of foci containing gamma-H2AX. The activation of checkpoint response is related to the induction of c-myc, as the phosphorylation of checkpoint proteins can be reversed when cells are treated with a c-Myc inhibitor or when Myc synthesis is blocked by short hairpin RNA. Using the DNA fiber assay, we show that in p300-depleted cells initiation of replication occurs from multiple replication origins. Chromatin loading of the Cdc45 protein also indicates increased origin activity in p300 knockdown cells. Immunofluorescence experiments indicate that c-Myc colocalizes with replication foci, consistent with the recently reported direct role of c-Myc in the initiation of DNA synthesis. Thus, the inappropriate S-phase entry of p300 down-regulated cells is likely to be because of c-Myc-induced deregulated replication origin activity, which results in replicative stress, activation of a DNA damage response, and S-phase arrest. Our results point to an important role for p300 in maintaining genomic integrity by negatively regulating c-myc.