Histone posttranslational modifications represent a versatile set of epigenetic marks involved not only in dynamic cellular processes, such as transcription and DNA repair, but also in the stable maintenance of repressive chromatin. In this article, we review many of the key and newly identified histone modifications known to be deregulated in cancer and how this impacts function. The latter part of the article addresses the challenges and current status of the epigenetic drug development process as it applies to cancer therapeutics.

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Histone Modifications and Cancer.

Platinum (Pt) compounds entered the clinic as anticancer agents when cisplatin was approved in 1978. More than 40 years later, even in the era of precision medicine and immunotherapy, Pt drugs remain among the most widely used anticancer drugs. As Pt drugs mainly target DNA, it is not surprising that recent insights into alterations of DNA repair mechanisms provide a useful explanation for their success. Many cancers have defective DNA repair, a feature that also sheds new light on the mechanisms of secondary drug resistance, such as the restoration of DNA repair pathways. In addition, genome-wide functional screening approaches have revealed interesting insights into Pt drug uptake. About half of cisplatin and carboplatin but not oxaliplatin may enter cells through the widely expressed volume-regulated anion channel (VRAC). The analysis of this heteromeric channel in tumour biopsies may therefore be a useful biomarker to stratify patients for initial Pt treatments. Moreover, Pt-based approaches may be improved in the future by the optimization of combinations with immunotherapy, management of side effects and use of nanodelivery devices. Hence, Pt drugs may still be part of the standard of care for several cancers in the coming years.

The rediscovery of platinum-based cancer therapy

Polycomb repressive complex 2 (PRC2) is the epigenetic regulator that induces histone H3 lysine 27 methylation (H3K27me3) and silences specific gene transcription. Enhancer of zeste homolog 2 (EZH2) is an enzymatic subunit of PRC2, and evidence shows that EZH2 plays an essential role in cancer initiation, development, progression, metastasis, and drug resistance. EZH2 expression is indeed regulated by various oncogenic transcription factors, tumor suppressor miRNAs, and cancer-associated non-coding RNA. EZH2 activity is also controlled by post-translational modifications, which are deregulated in cancer. The canonical role of EZH2 is gene silencing through H3K27me3, but accumulating evidence shows that EZH2 methlyates substrates other than histone and has methylase-independent functions. These non-canonical functions of EZH2 are shown to play a role in cancer progression. In this review, we summarize current information on the regulation and roles of EZH2 in cancer. We also discuss various therapeutic approaches to targeting EZH2.

/pdf/regulation-and-role-of-ezh2-in-cancer-2tk2wwhxm3.pdf

Regulation and Role of EZH2 in Cancer

The transcription factor nuclear factor kappa B (NF-κB) and the long non-coding RNA (lncRNA) HOTAIR (HOX transcript antisense RNA) have diverse functional roles in cancer. In this study, we show that upregulation of HOTAIR induced platinum resistance in ovarian cancer, and increased HOTAIR levels were observed in recurrent platinum-resistant ovarian tumors vs primary ovarian tumors. To investigate the role of HOTAIR during DNA damage induced by platinum, we monitored double-strand breaks and show that HOTAIR expression results in sustained activation of DNA damage response (DDR) after platinum treatment. We demonstrate that ectopic expression of HOTAIR induces NF-κB activation during DDR and interleukin-6 and interleukin-6 expression, both key NF-κB target genes. We show that HOTAIR regulates activation of NF-κB by decreasing Iκ-Bα (NF-κB inhibitor) and establish that by inducing prolonged NF-κB activation and expression of NF-κB target genes during DNA damage, HOTAIR has a critical role in cellular senescence and platinum sensitivity. Our findings suggest that an NF-κB-HOTAIR axis drives a positive-feedback loop cascade during DDR and contributes to cellular senescence and chemotherapy resistance in ovarian and other cancers.

NF-κB-HOTAIR Axis Links DNA Damage Response, Chemoresistance and Cellular Senescence in Ovarian Cancer

Platinum-based chemotherapy, with cytoreductive surgery, is the cornerstone of treatment of advanced ovarian cancer; however, acquired drug resistance is a major clinical obstacle. It has been proposed that subpopulations of tumor cells with stem cell-like properties, such as so-called side populations (SP) that overexpress ABC drug transporters, can sustain the growth of drug-resistant tumor cells, leading to tumor recurrence following chemotherapy. The histone methyltransferase EZH2 is a key component of the polycomb-repressive complex 2 required for maintenance of a stem cell state, and overexpression has been implicated in drug resistance and shorter survival of ovarian cancer patients. We observed higher percentage SP in ascites from patients that have relapsed following chemotherapy compared with chemonaive patients, consistent with selection for this subpopulation during platinum-based chemotherapy. Furthermore, ABCB1 (P-glycoprotein) and EZH2 are consistently overexpressed in SP compared with non-SP from patients' tumor cells. The siRNA knockdown of EZH2 leads to loss of SP in ovarian tumor models, reduced anchorage-independent growth, and reduced tumor growth in vivo. Together, these data support a key role for EZH2 in the maintenance of a drug-resistant, tumor-sustaining subpopulation of cells in ovarian cancers undergoing chemotherapy. As such, EZH2 is an important target for anticancer drug development.

https://mct.aacrjournals.org/content/molcanther/10/2/325.full.pdf

Ovarian Cancer Stem Cell–Like Side Populations Are Enriched Following Chemotherapy and Overexpress EZH2

https://www.gynecologiconcology-online.net/article/S0090-8258(10)00431-2/pdf

MiR-27a modulates MDR1/P-glycoprotein expression by targeting HIPK2 in human ovarian cancer cells

Enhancer of Zeste Homologue 2 (EZH2), a specific histone 3 lysine 27 (H3K27) methyltransferase, plays a critical role in tumorigenesis and cancer progression through epigenetic gene silencing and chromatin remodeling. However, the role of EZH2 in chemotherapy resistance is unknown. In this study, we found that EZH2 was overexpressed in cisplatin-resistant ovarian cancer cells compared with cisplatin-sensitive cells. Knockdown of EZH2 by RNA interference (RNAi) resensitized drug-resistant ovarian cancer A2780/DDP cells to cisplatin and decreased the level of H3K27 trimethylation (H3K27me3). Moreover, EZH2 downregulation suppressed cell proliferation and caused G2/M cell cycle arrest in A2780/DDP cells. Loss of EZH2 also enhanced sensibility of tumor xenografts to cisplatin and inhibited tumor growth in vivo. Our results indicate that EZH2 is essential for chemotherapy resistance in cisplatin-resistant cancer cells in vitro and in vivo, which is probably through H3K27 methylation as well as regulation of cell proliferation. EZH2 could be a potential novel epigenetic target to overcome drug resistance.

Overexpression of EZH2 contributes to acquired cisplatin resistance in ovarian cancer cells in vitro and in vivo.

To investigate the relationship between p38 mitogen-activated protein kinase (p38MAPK) and cell apoptosis during the paclitaxel resistance of ovarian carcinoma cell lines, flow cytometry (FCM) and PI staining were employed to determine the effect of p38MAPK inhibitor SB203580 on the apoptosis of A2780/Taxol cells, a drug-resistant human ovarian carcinoma cell line. p38MAPK protein expression in SB203580-treated cells was immunochemically measured. The 50% inhibition concentration (IC50) of paclitaxel on A2780/Taxol cells was determined by MTT assay. MDR-1 mRNA, and expression of p38MAPK and phospho-p53 protein were detected by RT-PCR and Western blotting, respectively. The apoptosis rate of A2780/Taxol cells was (19.7±1.04)% 24 h after SB203580 treatment. A significant difference in apoptosis rate was found among experiment group, control group and untreated group (P<0.05). The relative reversal rate of A2780/Taxol cells to paclitaxel was (57.18±2.01)%. As compared with the control group and the untreated group, p38MAPK protein and MDR-1 mRNA in SB203580-treated cells was substantially decreased. The expression of p53 protein was significantly increased. It is concluded that p38MAPK pathway is related to paclitaxel resistance of ovarian carcinoma, and blockade of this pathway can promote the apoptosis of the drug-resistant cells and reverse the drug-resistance. Moreover, p38MAPK-mediated apoptosis in paclitaxel-resistant ovarian carcinoma cells depends on the activation of p53.

The relationship between p38MAPK and apoptosis during paclitaxel resistance of ovarian cancer cells

The expression of N-myc down-regulated gene 1 (NDRG1) has previously been reported to be involved in the proliferation, differentiation, invasion and metastasis of cancer cells, but its role in cervical cancer is still unclear. This study aimed to investigate the expression of NDRG1gene in human cervical cancer and its effect on aggressive tumor behaviors. The NDRG1 expression in cervical tissues and cells was detected by RT-PCR. Specific expression plasmid pEGFP-N1-NDRG1-GFP was used to enhance the expression of NDRG1 in human cervical cancer cell lines. The mRNA and protein level of NDRG1 was assessed by RT-PCR and Western blotting, respectively. Its effects on cell proliferation, migration, invasion, cell cycle and apoptosis were detected by MTT, transwell migration assay and flow cytometry (FCM), respectively. The results showed that the expression of NDRG1 in cervical cancer tissues and cells was significantly lower than in normal cervical tissues (P<0.001). After transfection with pEGFP-N1-NDRG1-GFP, the mRNA and protein expression of NDRG1 was up-regulated in Siha cells, which suppressed cell proliferation (P<0.001), induced cell cycle arrest (P<0.05), reduced invasion and migration of Siha cells (P<0.05), but caused no cell apoptosis. Moreover, vascular endothelial growth factor (VEGF), a tumor-induced angiogenesis factor, was markedly reduced and E-cadherin, a cell adhesion molecule, was increased in the cells transfected with pEGFP-N1-NDRG1-GFP. It was concluded that up-regulated NDRG1 may play a role in the suppression of malignant cell growth, invasion and metastasis of human cervical cancer.

Expression and biological function of N-myc down-regulated gene 1 in human cervical cancer

Objective To investigate the expression of microRNA 27a (miR-27a) and relationship with drug resistance in human ovarian cancer A2780/Taxol cells.Methods A stem-loop-mediated real-time PCR was used to detect miR-27a expression in A2780 and A2780/Taxol cells.The cells were transfected with the mimics or inhibitors of miR-27a or negative control RNA ( NC) by lipofectamine 2000.The expressions of MDR1 gene,P-glycoprotein (P-gp) and homeodomain-interacting protein kinase 2 (HIPK2) protein levels were measured by real-time PCR and western blot respectively.Methyl thiazolyl tetrazolium (MTT) assay was used to analyze drug sensitivity.Apoptosis analysis was measured by fluorescence activated cell sorter ( FACS).Results (1) miR-27a was an average of 2.2-fold higher expression level in A2780/Taxol cells than that in A2780 cells,with a significant difference between the two groups (P <0.05).(2) A2780/Taxol cells transfection with inhibitors of miR-27a showed that the levels of MDR1 mRNA was decreased by 39%,P-gp protein level[(26 ±5)%]decreased than that in the NC group[(43 ±7)%],HIPK2 protein level[(30 ±6)%]increased than that in the NC group[(19 ±4)%],the 50% inhibitionconcentration (0.5 (μmol/L) was less than that in the NC group (6.8 μmol/L),apoptosis rate[(32.5 ± 3.6) %]was higher than that in the NC group[(5.6 ±2.1) %],and there were significant differences between two groups (all P < 0.05 ).( 3 ) Transfection of A2780 cells with mimics of miR-27a led to increase MDR1 mRNA expression by 121% as compared with one transfection with NC (P<0.05).Conclusion The expression of miR-27a is upregulated in A2780/Taxol cells,which may regulate MDR1 and P-gp expression by targeting HIPK2.

Key words: 
Ovarian neoplasms;  Paclitaxel;  MicroRNAs;  Drug resistance,neoplasm

Zhimin Li

Papers

MiR-27a modulates MDR1/P-glycoprotein expression by targeting HIPK2 in human ovarian cancer cells

Overexpression of EZH2 contributes to acquired cisplatin resistance in ovarian cancer cells in vitro and in vivo.

The relationship between p38MAPK and apoptosis during paclitaxel resistance of ovarian cancer cells

Expression and biological function of N-myc down-regulated gene 1 in human cervical cancer

[Expression of microRNA 27a and its correlation with drug resistance in human ovarian cancer A2780/Taxol cells].