Synergistic inhibition of hepatocellular carcinoma growth by cotargeting chromatin modifying enzymes and poly (ADP-ribose) polymerases

MK-8776 and Olaparib Combination Acts Synergistically in Hepatocellular Carcinoma Cells, Demonstrating Lack of Adverse Effects on Liver Tissues in Ovarian Cancer PDX Model

Hepatocellular carcinoma (HCC) cells critically depend on PARP1 and CHK1 activation for survival. Combining the PARP inhibitor (PARPi) olaparib with a CHK1 inhibitor (MK-8776, CHK1i) produced a synergistic effect, reducing cell viability and inducing marked oxidative stress and DNA damage, particularly in the HepG2 cells. This dual treatment significantly increased apoptosis markers, including γH2AX and caspase-3/7 activity. Both HCC cell lines exhibited heightened sensitivity to the combined treatment. The effect of drugs on the expression of proliferation markers in an olaparib-resistant patient-derived xenograft (PDX) model of ovarian cancer was also investigated. Ovarian tumors displayed reduced tissue growth, as reflected by a drop in proliferation marker Ki-67 levels in response to PARPi combined with CHK1i. No changes were observed in corresponding liver tissues using Ki-67 and pCHK staining, which indicates the absence of metastases and a hepatotoxic effect. Thus, our results indicate that the dual inhibition of PARP and CHK1 may prove to be a promising therapeutic approach in the treatment of primary HCC as well as OC tumors without the risk of liver metastases, especially in patients with olaparib-resistant tumor profiles.

Histone methyltransferase KMT5C drives liver cancer progression and directs therapeutic response to PARP inhibitors

BACKGROUND AND AIMS

Epigenetic plasticity is a major challenge in cancer-targeted therapy. However, the molecular basis governing this process has not yet been clearly defined. Despite the considerable success of poly(ADP-ribose) polymerase inhibitors (PARPi) in cancer therapy, the limited response to PARPi, especially in HCC, has been a bottleneck in its clinical implications. Herein, we investigated the molecular basis of the histone methyltransferase KMT5C (lysine methyltransferase 5C) that governs PARPi sensitivity and explored a potential therapeutic strategy for enhancing PARPi efficacy.

APPROACH AND RESULTS

We identified KMT5C, a trimethyltransferase of H4K20, as a targetable epigenetic factor that promoted liver tumor growth in mouse de novo MYC/Trp53-/- and xenograft liver tumor models. Notably, induction of KMT5C by environmental stress was crucial for DNA repair and HCC cell survival. Mechanistically, KMT5C interacted with the pivotal component of homologous recombination repair, RAD51, and promoted RAD51/RAD54 complex formation, which was essential for the activation of dsDNA breaks repair. This effect depended on the methyltransferase activity of KMT5C. We further demonstrated that the function of KMT5C in promoting HCC progression was dependent on RAD51. Importantly, either a pharmacological inhibitor (A196) or genetic inhibition of KMT5C sensitized liver cancer cells to PARPi.

CONCLUSIONS

KMT5C played a vital role in promoting liver cancer progression by activating the DNA repair response. Our results revealed a novel therapeutic approach using the KMT5C inhibitor A196, concurrent with olaparib, as a potential HCC therapy.

KLF5 Promotes Tumor Progression and Parp Inhibitor Resistance in Ovarian Cancer

One major characteristic of tumor cells is the aberrant activation of epigenetic regulatory elements, which remodel the tumor transcriptome and ultimately promote cancer progression and drug resistance. However, the oncogenic functions and mechanisms of ovarian cancer (OC) remain elusive. Here, super-enhancer (SE) regulatory elements that are aberrantly activated in OC are identified and it is found that SEs drive the relative specific expression of the transcription factor KLF5 in OC patients and poly(ADP-ribose) polymerase inhibitor (PARPi)-resistant patients. KLF5 expression is associated with poor outcomes in OC patients and can drive tumor progression in vitro and in vivo. Mechanistically, KLF5 forms a transcriptional complex with EHF and ELF3 and binds to the promoter region of RAD51 to enhance its transcription, strengthening the homologous recombination repair (HRR) pathway. Notably, the combination of suberoylanilide hydroxamic acid (SAHA) and olaparib significantly inhibits tumor growth and metastasis of PARPi-resistant OC cells with high KLF5. In conclusion, it is discovered that SEs-driven KLF5 is a key regulatory factor in OC progression and PARPi resistance; and potential therapeutic strategies for OC patients with PARPi resistance and high KLF5 are identified.

Inhibiting Src-mediated PARP1 tyrosine phosphorylation confers synthetic lethality to PARP1 inhibition in HCC

Hepatocellular carcinoma (HCC), a heterogeneous cancer with high mortality, is resistant to single targeted therapy; thus, combination therapy based on synthetic lethality is a promising therapeutic strategy for HCC. Poly (adenosine diphosphate [ADP]-ribose) polymerase 1 (PARP1) is the most recognized target for synthetic lethality; however, the therapeutic effect of PARP1 inhibition on HCC is disappointing. Therefore, exploring new synthetic lethal partners for the efficient manipulation of HCC is urgently required. In this study, we identified Src and PARP1 as novel synthetic lethal partners, and the combination therapy produced significant anti-tumor effects without causing obvious side effects. Mechanistically, Src interacted with PARP1 and phosphorylated PARP1 at the Y992 residue, which further mediated resistance to PARP1 inhibition. Overall, this study revealed that Src-mediated PARP1 phosphorylation induced HCC resistance to PARP1 inhibitors and indicated a therapeutic window of the Y992 phosphorylation of PARP1 for HCC patients. Moreover, synthetic lethal therapy by co-targeting PARP1 and Src have the potential to broaden the strategies for HCC and might benefit HCC patients with high Src activation and resistance to PARP1 inhibitors alone.

The Persistence of Hepatitis C Virus Infection in Hepatocytes Promotes Hepatocellular Carcinoma Progression by Pro-Inflammatory Interluekin-8 Expression

BACKGROUND

A large amount of epidemiological evidence indicates that persistent HCV infection is the main risk factor for HCC. We aimed to study the effects of persistent HCV infection on the interaction of the virus and host cell to identify cancer gene profiles.

METHODS

Next-generation sequencing (NGS) was used to identify differentially expressed genes between uninfected Huh7.5.1 control cells, short-term HCV (S-HCV), early long-term HCV (eL-HCV), and long-term HCV (L-HCV) infections, which were analyzed using different dynamic bioinformatics and analytic tools. mRNA expression was validated and quantified using q-PCR. One hundred ninety-six serum samples of HCV patients with IFN/RBV treatment were used to study chemokine levels.

RESULTS

S-HCV activates an inflammatory response and drives cell death and apoptosis through cell cycle arrest via MAPK signaling. L-HCV promotes cell growth and alters cell adhesion and chemokine signaling via CXCL8-mediated-SRC regulation. A total of 196 serum samples from the HCV and HCV-HCC cohorts demonstrated significantly upregulated pro-inflammatory CXCL8 in non-SVR (persistent HCV infection) patients in the HCV-HCC group.

CONCLUSIONS

Persistent infection with HCV induced pro-inflammatory CXCL8 and the oncogene SRC, thereby triggering and promoting hepatocarcinogenesis. CXCL8 may be a potential biomarker for monitoring HCV-related HCC progression.

Functional and Clinical Characterization of Tumor-Infiltrating T Cell Subpopulations in Hepatocellular Carcinoma

Tumor-infiltrating T-lymphocytes are defined as T-lymphocytes that infiltrated into tumor tissues; however, their composition, clinical significance, and underlying mechanism in hepatocellular carcinoma (HCC) and adjacent non-tumor tissues are still not completely understood. Herein, we collected marker genes of T cell subpopulations from a previous study and estimated their relative infiltrating levels in HCC and adjacent non-tumor tissues. Specifically, the infiltrating levels of all the T cells were significantly reduced in HCC as compared with non-tumor tissues. Unsupervised clustering of the HCC samples by the T cell infiltrating levels revealed that the HCC samples could be clearly classified into two groups. The driver genes, including PTK2B, ATM, PIK3C2B, and KIT, and several CNAs were observed to be associated with reduced T cell infiltrating levels. Particularly, deletion of TP53 more frequently occurred in low T cell infiltration HCC samples and resulted in its downregulation and cell cycle progression, indicating that cell cycle progression was closely associated with reduced T cell infiltration. In contrast, for the samples with high infiltration T cells, its immune evasion might be regulated by the immune checkpoint regulators, such as PD-1/PD-L1 and CTLA4. Moreover, Olaparib, one of the PARP inhibitors, and immune checkpoint inhibitors might be therapeutic candidates for the samples from the two T cell infiltrating clusters. Clinically, the tumor-infiltrating levels of cytotoxic CD4 cell, Mucosal associated invariant T (MAIT) cell, and exhausted CD8⁺ T cell might be used as predictors for vascular invasion, recurrence, and overall survival. Collectively, we systematically evaluated the clinical significance and potential molecular mechanisms of tumor-infiltrating T cell subpopulations in hepatocellular carcinoma, which might broaden our insights into the immunological features of HCC and provide potential immunotherapeutic targets.

Targeting PARP and autophagy evoked synergistic lethality in hepatocellular carcinoma

Hepatocellular carcinoma (HCC), one of the most lethal malignancies worldwide, has limited efficient therapeutic options. Here, we first demonstrated that simultaneously targeting poly (ADP-ribose) polymerase (PARP) and autophagy could evoke striking synergistic lethality in HCC cells. Specifically, we found that the PARP inhibitor Niraparib induced cytotoxicity accompanied by significant autophagy formation and autophagic flux in HCC cells. Further experiments showed that Niraparib induced suppression of the Akt/mTOR pathway and activation of the Erk1/2 cascade, two typical signaling pathways related to autophagy. In addition, the accumulation of reactive oxygen species was triggered, which was involved in Niraparib-induced autophagy. Blocking autophagy by chloroquine (CQ) in combination with Niraparib further enhanced cytotoxicity, induced apoptosis and inhibited colony formation in HCC cells. Synergistic inhibition was also observed in Huh7 xenografts in vivo. Mechanistically, we showed that autophagy inhibition abrogated Niraparib-induced cell-cycle arrest and checkpoint activation. Cotreatment with CQ and Niraparib promoted the formation of γ-H2AX foci while inhibiting the recruitment of the homologous recombination repair protein RAD51 to double-strand break sites. Thus, the present study developed a novel promising strategy for the management of HCC in the clinic and highlighted a potential approach to expand the application of PARP inhibitors.

Synthetic lethality: A promising therapeutic strategy for hepatocellular carcinoma

Hepatocellular carcinoma (HCC), the main cause of liver cancer-related death, is one of the main cancers in terms of incidence and mortality. However, HCC is difficult to target and develops strong drug resistance. Therefore, a new treatment strategy is urgently needed. The clinical application of the concept of synthetic lethality in recent years provides a new therapeutic direction for the accurate treatment of HCC. Here, we introduce the concept of synthetic lethality, the screening used to study synthetic lethality, and the identified and potential genetic interactions that induce synthetic lethality in HCC. In addition, we propose opportunities and challenges for translating synthetic lethal interactions to the clinical treatment of HCC.

Targeting DNA-dependent protein kinase sensitizes hepatocellular carcinoma cells to proton beam irradiation through apoptosis induction

Recent studies have highlighted the implications of genetic variations in the relative biological effectiveness (RBE) of proton beam irradiation over conventional X-ray irradiation. Proton beam radiotherapy is a reasonable radiotherapy option for hepatocellular carcinoma (HCC), but the impact of genetic difference on the HCC RBE remains unknown. Here, we determined proton RBE in human HCC cells by exposing them to various doses of either 6-MV X-rays or 230-MeV proton beams. Clonogenic survival assay revealed variable radiosensitivity of human HCC cell lines with survival fraction at 2 Gy ranging from 0.38 to 0.83 and variable proton RBEs with 37% survival fraction ranging from 1.00 to 1.48. HCC cells appeared more sensitive to proton irradiation than X-rays, with more persistent activation of DNA damage repair proteins over time. Depletion of a DNA damage repair gene, DNA-PKcs, by siRNA dramatically increased the sensitivity of HCC cells to proton beams with a decrease in colony survival and an increase in apoptosis. Our findings suggest that there are large variations in proton RBE in HCC cells despite the use of a constant RBE of 1.1 in the clinic and targeting DNA-PKcs in combination with proton beam therapy may be a promising regimen for treating HCC.

Histone deacetylase inhibitors in hepatocellular carcinoma: A therapeutic perspective

Hepatocellular carcinoma (HCC) is a major contributor to the global cancer burden. Given the current limited options to treat advanced HCC, understanding the molecular basis of HCC carcinogenesis and pinpointing druggable targets will be important to identify future HCC treatments. Epigenetic modification by inhibiting histone deacetylases (HDAC) is an emerging approach with promising results in cancer treatment. In the preclinical setting, HDAC inhibitors such as valproic acid sodium, panobinostat, vorinostat, trichostatin A, sodium butyrate, belinostat and romidepsin have demonstrated antitumor efficacy via activation of classic and alternative cell death molecular cascades. Combination regimens with the tyrosine kinase inhibitor sorafenib, poly(ADP-ribose) polymerases, proteasome and mammalian target of rapamycin inhibitors have shown promise. Phase I/II clinical studies with belinostat monotherapy and the combination of resminostat with sorafenib have suggested response and survival benefits. The safety profile was favorable with manageable adverse events and a low incidence of grade 3/4 toxicity. We herein review the role and potential therapeutic impact of epigenetic regulation through histone deacetylase inhibitors (HDACi) in the treatment of HCC.

Poly(ADP-Ribose) Polymerase Inhibitor PJ34 Attenuated Hepatic Triglyceride Accumulation in Alcoholic Fatty Liver Disease in Mice

Poly(ADP-ribose) polymerase (PARP) is an NAD-consuming enzyme and its specific role in the pathogenesis of alcoholic fatty liver disease (AFLD) remains elusive. In this study, we applied PJ34 [N-(5,6-dihydro-6-oxo-2-phenanthridinyl)-2-acetamide hydrochloride] to inhibit hepatic PARP activity to examine the corresponding pathologic alteration in AFLD in mice and the underlying molecular mechanism. We found that PJ34 decreased the intracellular triglyceride (TG) content in hepatocytes. Moreover, PJ34 suppressed the gene expression of diglyceride acyltransferases DGAT1 and DGAT2 and elevated intracellular NAD+ levels in hepatocytes. These mechanistic observations were validated in alcohol-fed mice injected with PJ34 intraperitoneally. Our results indicate that the PJ34 injection attenuated hepatic TG accumulation in alcohol-fed mice. Furthermore, PJ34 injection lowered the gene expression of hepatic sterol regulatory element binding protein 1c, DGAT1, and DGAT2, whereas PJ34 injection augmented hepatic NAD+ levels in alcohol-fed mice. Finally, nicotinamide riboside supplementation alleviated hepatic TG accumulation in alcohol-fed mice. These data indicate that applying PARP-specific inhibitor PJ34 by intraperitoneal injection attenuated hepatic NAD+ depletion and TG accumulation in alcohol-fed mice and may be a potential candidate for use in AFLD therapy.

Modification of Epigenetic Histone Acetylation in Hepatocellular Carcinoma

Cells respond to various environmental factors such as nutrients, food intake, and drugs or toxins by undergoing dynamic epigenetic changes. An imbalance in dynamic epigenetic changes is one of the major causes of disease, oncogenic activities, and immunosuppressive effects. The aryl hydrocarbon receptor (AHR) is a unique cellular chemical sensor present in most organs, and its dysregulation has been demonstrated in multiple stages of tumor progression in humans and experimental models; however, the effects of the pathogenic mechanisms of AHR on epigenetic regulation remain unclear. Apart from proto-oncogene activation, epigenetic repressions of tumor suppressor genes are involved in tumor initiation, procession, and metastasis. Reverse epigenetic repression of the tumor suppressor genes by epigenetic enzyme activity inhibition and epigenetic enzyme level manipulation is a potential path for tumor therapy. Current evidence and our recent work on deacetylation of histones on tumor-suppressive genes suggest that histone deacetylase (HDAC) is involved in tumor formation and progression, and treating hepatocellular carcinoma with HDAC inhibitors can, at least partially, repress tumor proliferation and transformation by recusing the expression of tumor-suppressive genes such as TP53 and RB1.

Microhomology-mediated end joining: Good, bad and ugly

DNA double-strand breaks (DSBs) are induced by a variety of genotoxic agents, including ionizing radiation and chemotherapy drugs for treating cancers. The elimination of DSBs proceeds via distinctive error-free and error-prone pathways. Repair by homologous recombination (HR) is largely error-free and mediated by RAD51/BRCA2 gene products. Classical non-homologous end joining (C-NHEJ) requires the Ku heterodimer and can efficiently rejoin breaks, with occasional loss or gain of DNA information. Recently, evidence has unveiled another DNA end-joining mechanism that is independent of recombination factors and Ku proteins, termed alternative non-homologous end joining (A-NHEJ). While A-NHEJ-mediated repair does not require homology, in a subtype of A-NHEJ, DSB breaks are sealed by microhomology (MH)-mediated base-pairing of DNA single strands, followed by nucleolytic trimming of DNA flaps, DNA gap filling, and DNA ligation, yielding products that are always associated with DNA deletion. This highly error-prone DSB repair pathway is termed microhomology-mediated end joining (MMEJ). Dissecting the mechanisms of MMEJ is of great interest because of its potential to destabilize the genome through gene deletions and chromosomal rearrangements in cells deficient in canonical repair pathways, including HR and C-NHEJ. In addition, evidence now suggests that MMEJ plays a physiological role in normal cells.

Therapeutic Targeting of Poly(ADP-Ribose) Polymerase-1 (PARP1) in Cancer: Current Developments, Therapeutic Strategies, and Future Opportunities

Poly(ADP-ribose) polymerase-1 (PARP-1) plays a central role in numerous cellular processes including DNA repair, replication, and transcription. PARP interacts directly, indirectly or via PARylation with various oncogenic proteins and regulates several transcription factors thereby modulating carcinogenesis. Therapeutic inhibition of PARP is therefore perceived as a promising anticancer strategy and a number of PARP inhibitors (PARPi) are currently under development and clinical evaluation. PARPi inhibit the DNA repair pathway and thus form the concept of synthetic lethality in cancer therapeutics. Preclinical and clinical studies have shown the potential of PARPi as chemopotentiator, radiosensitizer, or as adjuvant therapeutic agents. Recent studies have shown that PARP-1 could be either oncogenic or tumor suppressive in different cancers. PARP inhibitor resistance is also a growing concern in the clinical setting. Recently, changes in the levels of PARP-1 activity or expression in cancer patients have provided the basis for consideration of PARP-1 regulatory proteins as potential biomarkers. This review focuses on the current developments related to the role of PARP in cancer progression, therapeutic strategies targeting PARP-associated oncogenic signaling, and future opportunities in use of PARPi in anticancer therapeutics.

Overcoming Resistance of Cancer Cells to PARP-1 Inhibitors with Three Different Drug Combinations

Inhibitors of poly[ADP-ribose] polymerase 1 (PARPis) show promise for treatment of cancers which lack capacity for homologous recombination repair (HRR). However, new therapeutic strategies are required in order to overcome innate and acquired resistance to these drugs and thus expand the array of cancers that could benefit from them. We show that human cancer cell lines which respond poorly to ABT-888 (a PARPi), become sensitive to it when co-treated with vorinostat (a histone deacetylase inhibitor (HDACi)). Vorinostat also sensitized PARPis insensitive cancer cell lines to 6-thioguanine (6-TG)–a drug that targets PARPis sensitive cells. The sensitizing effect of vorinostat was associated with increased phosphorylation of eukaryotic initiation factor (eIF) 2α which in and of itself increases the sensitivity of cancer cells to ABT-888. Importantly, these drug combinations did not affect survival of normal fibroblasts and breast cells, and significantly increased the inhibition of xenograft tumor growth relative to each drug alone, without affecting the mice weight or their liver and kidney function. Our results show that combination of vorinostat and ABT-888 could potentially prove useful for treatment of cancer with innate resistance to PARPis due to active HRR machinery, while the combination of vorinostat and 6-TG could potentially overcome innate or acquired resistance to PARPis due to secondary or reversal BRCA mutations, to decreased PARP-1 level or to increased expression of multiple drug resistant proteins. Importantly, drugs which increase phosphorylation of eIF2α may mimic the sensitizing effect of vorinostat on cellular response to PARPis or to 6-TG, without activating all of its downstream effectors.

Defeating EpCAM(+) liver cancer stem cells by targeting chromatin remodeling enzyme CHD4 in human hepatocellular carcinoma

BACKGROUND & AIMS

Hepatocellular carcinoma is composed of a subset of cells with enhanced tumorigenicity and chemoresistance that are called cancer stem (or stem-like) cells. We explored the role of chromodomain-helicase-DNA-binding protein 4, which is encoded by the CHD4 gene and is known to epigenetically control gene regulation and DNA damage responses in EpCAM(+) liver cancer stem cells.

METHODS

Gene and protein expression profiles were determined by microarray and immunohistochemistry in 245 and 144 hepatocellular carcinoma patients, respectively. The relationship between gene/protein expression and prognosis was examined. The functional role of CHD4 was evaluated in primary hepatocellular carcinoma cells and in cell lines in vitro and in vivo.

RESULTS

CHD4 was abundantly expressed in EpCAM(+) hepatocellular carcinoma with expression of hepatic stem cell markers and poor prognosis in two independent cohorts. In cell lines, CHD4 knockdown increased chemosensitivity and CHD4 overexpression induced epirubicin chemoresistance. To inhibit the functions of CHD4 that are mediated through histone deacetylase and poly (ADP-ribose) polymerase, we evaluated the effect of the histone deacetylase inhibitor suberohydroxamic acid and the poly (ADP-ribose) polymerase inhibitor AG-014699. Treatment with either suberohydroxamic acid or AG-014699 reduced the number of EpCAM(+) liver cancer stem cells in vitro, and suberohydroxamic acid and AG-014699 in combination successfully inhibited tumor growth in a mouse xenograft model.

CONCLUSIONS

CHD4 plays a pivotal role in chemoresistance and the maintenance of stemness in liver cancer stem cells and is therefore a good target for the eradication of hepatocellular carcinoma.

Differences in Expression of Key DNA Damage Repair Genes after Epigenetic-Induced BRCAness Dictate Synthetic Lethality with PARP1 Inhibition

The triple-negative breast cancer (TNBC) subtype represents a cancer that is highly aggressive with poor patient outcome. Current preclinical success has been gained through synthetic lethality, targeting genome instability with PARP inhibition in breast cancer cells that harbor silencing of the homologous recombination (HR) pathway. Histone deacetylase inhibitors (HDACi) are a class of drugs that mediate epigenetic changes in expression of HR pathway genes. Here, we compare the activity of the pan-HDAC inhibitor suberoylanilide hydroxamic acid (SAHA), the class I/IIa HDAC inhibitor valproic acid (VPA), and the HDAC1/2-specific inhibitor romidepsin (ROMI) for their capability to regulate DNA damage repair gene expression and in sensitizing TNBC to PARPi. We found that two of the HDACis tested, SAHA and ROMI, but not VPA, indeed inhibit HR repair and that RAD51, BARD1, and FANCD2 represent key proteins whose inhibition is required for HDACi-mediated therapy with PARP inhibition in TNBC. We also observed that restoration of BRCA1 function stabilizes the genome compared with mutant BRCA1 that results in enhanced polyploid population after combination treatment with HDACi and PARPi. Furthermore, we found that overexpression of the key HR protein RAD51 represents a mechanism for this resistance, promoting aberrant repair and the enhanced polyploidy observed. These findings highlight the key components of HR in guiding synthetic lethality with PARP inhibition and support the rationale for utilizing the novel combination of HDACi and PARPi against TNBC in the clinical setting.

Class I histone deacetylase inhibitors inhibit the retention of BRCA1 and 53BP1 at the site of DNA damage

BRCA1 and 53BP1 antagonistically regulate homology-directed repair (HDR) and non-homologous end-joining (NHEJ) of DNA double-strand breaks (DSB). The histone deacetylase (HDAC) inhibitor trichostatin A directly inhibits the retention of 53BP1 at DSB sites by acetylating histone H4 (H4ac), which interferes with 53BP1 binding to dimethylated histone H4 Lys20 (H4K20me2). Conversely, we recently found that the retention of the BRCA1/BARD1 complex is also affected by another methylated histone residue, H3K9me2, which can be suppressed by the histone lysine methyltransferase (HKMT) inhibitor UNC0638. Here, we investigate the effects of the class I HDAC inhibitors MS-275 and FK228 compared to UNC0638 on histone modifications and the DNA damage response. In addition to H4ac, the HDAC inhibitors induce H3K9ac and inhibit H3K9me2 at doses that do not affect the expression levels of DNA repair genes. By contrast, UNC0638 selectively inhibits H3K9me2 without affecting the levels of H3K9ac, H3K56ac or H4ac. Reflecting their effects on histone modifications, the HDAC inhibitors inhibit ionizing radiation-induced foci (IRIF) formation of BRCA1 and BARD1 as well as 53BP1 and RIF1, whereas UNC0638 suppresses IRIF formation of BRCA1 and BARD1 but not 53BP1 and RIF1. Although HDAC inhibitors suppressed HDR, they did not cooperate with the poly(ADP-ribose) polymerase inhibitor olaparib to block cancer cell growth, possibly due to simultaneous suppression of NHEJ pathway components. Collectively, these results suggest the mechanism by that HDAC inhibitors inhibit both the HDR and NHEJ pathways, whereas HKMT inhibitor inhibits only the HDR pathway; this finding may affect the chemosensitizing effects of the inhibitors.

Histone deacetylase inhibitor, suberoylanilide hydroxamic acid (SAHA), enhances anti-tumor effects of the poly (ADP-ribose) polymerase (PARP) inhibitor olaparib in triple-negative breast cancer cells

INTRODUCTION

Olaparib, a poly (ADP-ribose) polymerase (PARP) inhibitor, has been found to have therapeutic potential for treating cancers associated with impaired DNA repair capabilities, particularly those with deficiencies in the homologous recombination repair (HRR) pathway. Histone deacetylases (HDACs) are important for enabling functional HRR of DNA by regulating the expression of HRR-related genes and promoting the accurate assembly of HRR-directed sub-nuclear foci. Thus, HDAC inhibitors have recently emerged as a therapeutic agent for treating cancer by inhibiting DNA repair. Based on this, HDAC inhibition could be predicted to enhance the anti-tumor effect of PARP inhibitors in cancer cells by blocking the HRR pathway.

METHODS

We determined whether suberoylanilide hydroxamic acid (SAHA), a HDAC inhibitor, could enhance the anti-tumor effects of olaparib on breast cancer cell lines using a cytotoxic assay, cell cycle analysis, and Western blotting. We evaluated how exposure to SAHA affects the expression of HRR-associated genes. The accumulation of DNA double strand breaks (DSBs) induced by combination treatment was assessed. Induction of autophagy was monitored by imaging green fluorescent protein-tagged microtubule-associated protein 1A/1B-light chain 3 (LC3) expression following co-treatment with olaparib and SAHA. These in vitro data were validated in vivo using a human breast cancer xenograft model.

RESULTS

Triple-negative breast cancer cell (TNBC) lines showed heterogeneous responses to the PARP and HDAC inhibitors. Co-administration of olaparib and SAHA synergistically inhibited the growth of TNBC cells that expressed functional Phosphatase and tensin homolog (PTEN). This effect was associated with down-regulation of the proliferative signaling pathway, increased apoptotic and autophagic cell death, and accumulation of DNA damage. The combined anti-tumor effect of olaparib and SAHA was also observed in a xenograft model. These data suggest that PTEN expression in TNBC cells can sensitize the cell response to simultaneous inhibition of PARP and HDAC both in vitro and in vivo.

CONCLUSION

Our findings suggest that expression of functional PTEN may serve as a biomarker for selecting TNBC patients that would favorably respond to a combination of olaparib with SAHA. This provides a strong rationale for treating TNBC patients with PTEN expression with a combination therapy consisting of olaparib and SAHA.

Synergy between HDAC and PARP Inhibitors on Proliferation of a Human Anaplastic Thyroid Cancer-Derived Cell Line

Anaplastic thyroid carcinoma (ATC) is a very aggressive human malignancy, having a marked degree of invasiveness and no features of thyroid differentiation. It is known that either HDAC inhibitors or PARP inhibitors have antiproliferative effects on thyroid cancer cells. Therefore, in this study the possible synergy between the two types of compounds has been investigated. The ATC-derived cell line SW1736 has been treated with the HDAC inhibitor suberoylanilide hydroxamic acid (SAHA) and the PARP inhibitor PJ34, alone or in combination. In terms of cell viability, the combination index value was always lower than 1 at various tested dosages, indicating, therefore, synergy in a wide range of doses for both compounds. Synergy was also observed in induction of apoptosis. In terms of thyroid-specific gene expression, synergy was observed for TSHR mRNA levels but not for NIS, TTF1, TTF2, and PAX8 mRNA levels. Altogether, these data suggest that the combined use of HDAC and PARP inhibitors may be a useful strategy for treatment of ATC.

PARP inhibition and the radiosensitizing effects of the PARP inhibitor ABT-888 in in vitro hepatocellular carcinoma models

BACKGROUND

Hepatocellular carcinoma is the third cause of cancer related death for which new treatment strategies are needed. Targeting DNA repair pathways to sensitize tumor cells to chemo- or radiotherapy is under investigation for the treatment of several cancers with poly(ADP-ribose) polymerase (PARP) inhibitors showing great potential. The aim of this preclinical study was to evaluate the expression of PARP and PARG genes in a panel of liver cancer cell lines and primary human hepatocytes, their DNA repair capacity and assess the impact on cell survival of PARP inhibitors alone and in combination with radiotherapy.

METHODS

Quantitative PCR was used to measure PARP-1, -2, -3 and PARG mRNA levels and western blotting for PARP-1 protein expression and ADP-ribose polymer formation after exposure of cells to doxorubicin, a topoisomerase II poison. DNA repair capacity was assessed using an in vitro DNA lesion excision/synthesis assay and the effects on cell killing of the PARP inhibitor ABT-888 alone and in combination with ionizing radiation using clonogenic survival.

RESULTS

Although a wide range in expression of the PARPs and PARG was found correlations between PARP-1 and PARP-2 mRNA levels and PARP-1 mRNA and protein levels were noted. However these expression profiles were not predictive of PARP activity in the different cell lines that also showed variability in excision/synthesis repair capacity. 4 of the 7 lines were sensitive to ABT-888 alone and the two lines tested showed enhanced radiosensitivity in the presence of ABT-888.

CONCLUSIONS

PARP inhibitors combined with radiotherapy show potential as a therapeutic option for hepatocellular carcinoma.

Emerging signaling pathways in hepatocellular carcinoma

Signaling pathways have become a major source of targets for novel therapies in hepatocellular carcinoma (HCC). Survival benefits achieved with sorafenib, a multikinase inhibitor, are unprecedented and underscore the importance of improving our understanding of how signaling networks interact in transformed cells. Numerous signaling modules are de-regulated in HCC, including some related to growth factor signaling (e.g., IGF, EGF, PDGF, FGF, HGF), cell differentiation (WNT, Hedgehog, Notch), and angiogenesis (VEGF). Intracellular mediators such as RAS and AKT/MTOR may also play a role in HCC development and progression. Different molecular mechanisms have been shown to induce aberrant pathway activation. These include point mutations, chromosomal aberrations, and epigenetically driven down-regulation. The use of novel molecular technologies such as next-generation sequencing in HCC research has enabled the identification of novel pathways previously underexplored in the HCC field, such as chromatin remodeling and autophagy. Considering recent failures of molecular therapies in advanced clinical trials (e.g., sunitinib, brivanib), survey of these and other new pathways may provide alternative therapeutic targets.

Update on hepatocellular carcinoma breakthroughs: poly(ADP-ribose) polymerase inhibitors as a promising therapeutic strategy

Hepatocellular carcinoma is the most common form of primary liver cancer which is the fifth most common cancer in men and the seventh in women and the third most common cause of cancer-related death worldwide. Only 10-20% of patients are eligible for curative treatments that result in a 5-year survival rate of 40% to 70%. Therefore, the development of novel treatment options is necessary for the majority of patients and remains a considerable challenge. Conformal radiotherapy is used in certain circumstances and preliminary data obtained from phase 1/2 trials are showing promising curative effects. There is thus an interest in identifying drugs that can be exploited to enhance radiation sensitivity that could be used in therapy and might improve clinical outcome. Small molecules inhibitors of poly(ADP-ribose) polymerases (PARP) are an example of a radio- and chemo-sensitizing drug, as well as being an efficient single agent treatment in certain genetic backgrounds. In this review, we discuss the role of PARP-1 in hepatocellular carcinoma and present the results of preclinical studies that have assessed the potential of PARP inhibition as a single treatment or combined with chemotherapy or radiotherapy for the treatment of hepatocellular carcinoma.

Predictive biomarkers for cancer therapy with PARP inhibitors

Poly(ADP-ribose) polymerase (PARP) inhibitors have raised high expectations for the treatment of multiple malignancies. PARP inhibitors, which can be used as monotherapies or in combination with DNA-damaging agents, are particularly efficient against tumors with defects in DNA repair mechanisms, in particular the homologous recombination pathway, for instance due to BRCA mutations. Thus, deficient DNA repair provides a framework for the success of PARP inhibitors in medical oncology. Here, we review encouraging results obtained in recent clinical trials investigating the safety and efficacy of PARP inhibitors as anticancer agents. We discuss emerging mechanisms of regulation of homologous recombination and how inhibition of DNA repair might be used in cancer therapy. We surmise that the identification of patients that are likely to benefit from PARP inhibition will improve the clinical use of PARP inhibitors in a defined target population. Thus, we will place special emphasis on biomarker discovery.

Targeting poly (ADP-ribose) polymerase partially contributes to bufalin-induced cell death in multiple myeloma cells

Despite recent pharmaceutical advancements in therapeutic drugs, multiple myeloma (MM) remains an incurable disease. Recently, ploy(ADP-ribose) polymerase 1 (PARP1) has been shown as a potentially promising target for MM therapy. A previous report suggested bufalin, a component of traditional Chinese medicine ("Chan Su"), might target PARP1. However, this hypothesis has not been verified. We here showed that bufalin could inhibit PARP1 activity in vitro and reduce DNA-damage-induced poly(ADP-ribosyl)ation in MM cells. Molecular docking analysis revealed that the active site of bufalin interaction is within the catalytic domain of PAPR1. Thus, PARP1 is a putative target of bufalin. Furthermore, we showed, for the first time that the proliferation of MM cell lines (NCI-H929, U266, RPMI8226 and MM.1S) and primary CD138(+) MM cells could be inhibited by bufalin, mainly via apoptosis and G2-M phase cell cycle arrest. MM cell apoptosis was confirmed by apoptotic cell morphology, Annexin-V positive cells, and the caspase3 activation. We further evaluated the role of PARP1 in bufalin-induced apoptosis, discovering that PARP1 overexpression partially suppressed bufalin-induced cell death. Moreover, bufalin can act as chemosensitizer to enhance the cell growth-inhibitory effects of topotecan, camptothecin, etoposide and vorinostat in MM cells. Collectively, our data suggest that bufalin is a novel PARP1 inhibitor and a potentially promising therapeutic agent against MM alone or in combination with other drugs.

HDAC turnover, CtIP acetylation and dysregulated DNA damage signaling in colon cancer cells treated with sulforaphane and related dietary isothiocyanates

Histone deacetylases (HDACs) and acetyltransferases have important roles in the regulation of protein acetylation, chromatin dynamics and the DNA damage response. Here, we show in human colon cancer cells that dietary isothiocyanates (ITCs) inhibit HDAC activity and increase HDAC protein turnover with the potency proportional to alkyl chain length, i.e., AITC < sulforaphane (SFN) < 6-SFN < 9-SFN. Molecular docking studies provided insights into the interactions of ITC metabolites with HDAC3, implicating the allosteric site between HDAC3 and its co-repressor. ITCs induced DNA double-strand breaks and enhanced the phosphorylation of histone H2AX, ataxia telangiectasia and Rad3-related protein (ATR) and checkpoint kinase-2 (CHK2). Depending on the ITC and treatment conditions, phenotypic outcomes included cell growth arrest, autophagy and apoptosis. Coincident with the loss of HDAC3 and HDAC6, as well as SIRT6, ITCs enhanced the acetylation and subsequent degradation of critical repair proteins, such as CtIP, and this was recapitulated in HDAC knockdown experiments. Importantly, colon cancer cells were far more susceptible than non-cancer cells to ITC-induced DNA damage, which persisted in the former case but was scarcely detectable in non-cancer colonic epithelial cells under the same conditions. Future studies will address the mechanistic basis for dietary ITCs preferentially exploiting HDAC turnover mechanisms and faulty DNA repair pathways in colon cancer cells vs. normal cells.

Anticancer effects of sodium butyrate on hepatocellular carcinoma cells in vitro

In the present study, we investigated the anticancer effects of sodium butyrate (NaBu) on hepatocellular carcinoma (HCC) cells in vitro. As a histone deacetylase (HDAC) inhibitor, NaBu upregulated Ac-H3 and inhibited HDAC4 protein expression in a time- and dose-dependent manner. MTT assays showed that treatment with NaBu at high concentrations significantly inhibited the growth of various HCC cells. Exposure to NaBu for 24 h induced cell cycle arrest in the SMMC-7721 and HepG2 cells. NaBu also induced the apoptosis of SMMC‑7721 cells. The expression levels of cell cycle- and apoptosis-related proteins were further investigated by western blot analysis using specific antibodies. The results provided a possible mechanism responsible for the inhibitory effects of NaBu on the growth of HCC cells. To further analyze the role of NaBu in cell migration, wound healing and Transwell assays were performed, indicating that NaBu significantly inhibits cell migration/invasion in HCC cells. Transforming growth factor-β1 (TGF-β1)-induced epithelial to mesenchymal transition (EMT) has been associated with tumor cell migration and invasion. The EMT markers, E-cadherin, vimentin and N-cadherin, were regulated by TGF-β1, while NaBu inhibited this process in which HDAC4 and matrix metalloproteinase (MMP)7 may be involved. Based on our findings, we propose that NaBu may be useful as an anticancer drug for HCC therapy.

Cytotoxic isoferulic acidamide from Myricaria germanica (Tamaricaceae)

Tamgermanitin, a unique N-trans-Isoferuloyltyramine, together with the hitherto unknown polyphenolics, 2,4-di-O-galloyl-(α/β)-glucopyranose and kaempferide 3,7-disulphate have been isolated from the leaf aqueous ethanol extract of the false tamarisk, Myricaria germanica DESV. In addition, 18 known phenolics were also separated and characterized. All structures were elucidated on the basis of detailed analysis of 1D- (1)H and (13)C NMR, COSY, HSQC, HMBC and HRFTESIMS spectral data. The extract, its chromatographic column fractions and the isolated isoferuloyltyramine, tamgermanetin demonstrated potential cytotoxic effect against three different tumor cell lines, namely liver (Huh-7), breast (MCF-7) and prostate (PC-3). The IC 50''s were found to be substantially low with low-resistance possibility. DNA flow-cytometic analysis indicated that column fractions and tamgermanetin enhanced pre-G apoptotic fraction. Both materials showed inhibiting activity against PARP enzyme activity. In conclusion, we report the isolation and identification of a novel compound, tamgermanitin, from the aqueous ethanol extract of Myricaria germanica leaves. Further, different fractions of the extract and tamgermanitin exhibit potent cytotoxic activities which warrant further investigations.

Emerging signaling pathways in hepatocellular carcinoma

Signaling pathways have become a major source of targets for novel therapies in hepatocellular carcinoma (HCC). Survival benefits achieved with sorafenib, a multikinase inhibitor, are unprecedented and underscore the importance of improving our understanding of how signaling networks interact in transformed cells. Numerous signaling modules are de-regulated in HCC, including some related to growth factor signaling (e.g., IGF, EGF, PDGF, FGF, HGF), cell differentiation (WNT, Hedgehog, Notch), and angiogenesis (VEGF). Intracellular mediators such as RAS and AKT/MTOR may also play a role in HCC development and progression. Different molecular mechanisms have been shown to induce aberrant pathway activation. These include point mutations, chromosomal aberrations, and epigenetically driven down-regulation. The use of novel molecular technologies such as next-generation sequencing in HCC research has enabled the identification of novel pathways previously underexplored in the HCC field, such as chromatin remodeling and autophagy. Considering recent failures of molecular therapies in advanced clinical trials (e.g., sunitinib, brivanib), survey of these and other new pathways may provide alternative therapeutic targets.