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

Potential agnostic role of BRCA alterations in patients with several solid tumors: One for all, all for one?

Germline BRCA1/2 alterations in the Homologous Recombination (HR) pathway are considered as main susceptibility biomarkers to Hereditary Breast and Ovarian Cancers (HBOC). The modern molecular biology technologies allowed to characterize germline and somatic BRCA1/2 alterations in several malignancies, broadening the landscape of BRCA1/2-alterated tumors. In the last years, BRCA genetic testing, beyond the preventive value, also assumed a predictive and prognostic significance for patient management. The approval of molecules with agnostic indication is leading to a new clinical model, defined "mutational". Among these drugs, the Poly (ADP)-Ribose Polymerase inhibitors (PARPi) for BRCA1/2-deficient tumors were widely studied leading to increasing therapeutic implications. In this Review we provided an overview of the main clinical studies describing the association between BRCA-mutated tumors and PARPi response, focusing on the controversial evidence about the potential agnostic indication based on BRCA1/2 alterations in several solid tumors.

Functions of lncRNA DUXAP8 in non-small cell lung cancer

Non-small cell lung cancer (NSCLC) poses a serious threat to public health due to its significant morbidity and mortality rates. The processes of NSCLC formation and development are quite complex and involve numerous regulatory biomolecules. Long non-coding RNAs (lncRNAs) have attracted attention since they have been found to play critical roles in the tumorigenesis of various human malignancies. Recently, double homeobox A pseudogene 8 (DUXAP8) was identified as an oncogenic lncRNA that is overexpressed in different tumor types. In NSCLC, high expression of DUXAP8 is associated with poor prognosis in patients. The regulatory mechanism underlying the oncogenic effects of DUXAP8 can be divided into transcriptional level and post-transcriptional level. DUXAP8 promotes proliferation, epithelial-mesenchymal transition, and aerobic glycolysis in NSCLC cells. Moreover, DUXAP8 shows potential for the diagnosis and treatment of NSCLC. Herein, we review the molecular mechanisms underlying the DUXAP8-mediated phenotypes of NSCLC as well as its potential clinical applications.

Targeting BRCA and DNA Damage Repair Genes in GI Cancers: Pathophysiology and Clinical Perspectives

Mutated germline alleles in the DNA damage repair (DDR) genes “breast cancer gene 1” (BRCA1) and BRCA2 have originally been identified as major susceptibility genes in breast and ovarian cancers. With the establishment and approval of more cost-effective gene sequencing methods, germline and somatic BRCA mutations have been detected in several cancers. Since the approval of poly (ADP)-ribose polymerase inhibitors (PARPi) for BRCA-mutated cancers, BRCA mutations gained rising therapeutic implications. The impact and significance of BRCA mutations have been evaluated extensively in the last decades. Moreover, other genes involved in the DDR pathway, such as ATM, ATR, or CHK1, have emerged as potential new treatment targets, as inhibitors of these proteins are currently under clinical investigation. This review gives a concise overview on the emerging clinical implications of mutations in the DDR genes in gastrointestinal cancers with a focus on BRCA mutations.

PARP inhibitors and radiation potentiate liver cell death in vitro. Do hepatocellular carcinomas have an achilles' heel?

BACKGROUND

A promising avenue for cancer treatment is exacerbating the deregulation of the DNA repair machinery that would normally protect the genome. To address the applicability of poly(ADP-ribose) polymerase (PARP) inhibitors (PARPi) combined with radiotherapy for the treatment of hepatocellular carcinoma (HCC) two approaches were used: firstly, the in vitro sensitivity to the PARPi Veliparib and Talazoparib +/- radiation exposure was determined in liver cell lines and the impact of the HBV X protein (HBx) that deregulates cellular DNA damage repair via SMC5/6 degradation was investigated. Secondly, PARP expression profiles and DNA damage levels using the surrogate marker gammaH2AX were assessed in a panel of control liver vs HCC tissues.

METHODS

Cell cytotoxicity was measured by clonogenic survival or relative cell growth and the DNA damage response using immunological-based techniques in Hep3B, PLC/PRF/5, HepG2- and HepaRG-derived models. Transcriptome changes due to HBx expression vs SMC6 loss were assessed by RNA sequencing in HepaRG-derived models. PARP and PARG transcripts (qPCR) and PARP1, H2AX and gammaH2AX protein levels (RPPA) were compared in control liver vs HBV-, HCV-, alcohol- and non-alcoholic steatohepatitis-associated HCC (tumor/peritumor) tissues.

RESULTS

PARPi cytotoxicity was significantly enhanced when combined with X-rays (2Gy) with Talazoparib having a greater impact than Veliparib in most in vitro models. HBx expression significantly lowered survival, probably driven by SMC5/6 loss based on the transcriptome analysis and higher DNA damage levels. PARP1 and PARP2 transcript levels were significantly higher in tumor than peritumor and control tissues. The HBV/HCV/alcohol-associated tumor tissues studied had reduced H2AX but higher gammaH2AX protein levels compared to peritumor and control tissues providing evidence of increased DNA damage during liver disease progression.

CONCLUSIONS

These proof-of-concept experiments support PARPi alone or combined with radiotherapy for HCC treatment, particularly for HBV-associated tumors, that warrant further investigation.

lncRNA DUXAP8 Facilitates Multiple Malignant Phenotypes and Resistance to PARP Inhibitor in HCC via Upregulating FOXM1

In this study, we examined the clinical significance and molecular mechanisms of a long non-coding RNA (lncRNA), double homeobox A pseudogene 8 (DUXAP8) in hepatocellular carcinoma (HCC). DUXAP8 expression was compared using quantitative real-time PCR in HCC versus adjacent tissues and in HCC cell lines versus normal hepatic epithelial cells. The correlations between DUXAP8 level and clinicopathological features were analyzed. Assays including MTT, colony-forming analysis, Transwell assay, western blot, xenograft formation, experimental metastasis, luciferase assay, RNA pull-down, and RNA immunoprecipitation were used to examine DUXAP8-induced malignant phenotypes, its regulation on forkhead box protein M1 (FOXM1), and the importance of FOXM1 in mediating DUXAP8 phenotypes. Our results showed that DUXAP8 was significantly upregulated in HCC tissues or cell lines associated with tumors of advanced grades, tumors that were positive for lymph node metastasis, and patients with poor overall survival. DUAXP8 was essential in maintaining multiple malignant phenotypes (including resistance to olaparib) both in vitro and in vivo. Mechanistically, DUXAP8 upregulated FOXM1 expression by sponging miR-485-5p and interacting with the RNA-binding protein Fused in Sarcoma (FUS). Functionally, FOXM1 essentially mediated the oncogenic phenotypes of DUXAP8. Collectively, DUXAP8 acts through two distinct mechanisms to upregulate FOXM1 and becomes a pleotropic oncogenic lncRNA in HCC.

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.

MTBP inhibits migration and metastasis of hepatocellular carcinoma

Hepatocellular carcinoma (HCC) is one of the most common cancers worldwide with increasing incidence. Despite curative surgical resection and advanced chemotherapy, its survival rate remains low. The presence of microvascular invasion and occult metastasis is one of the major causes for this poor outcome. MDM2 Binding Protein (MTBP) has been implicated in the suppression of cell migration and cancer metastasis. However, clinical significance of MTBP, particularly in human cancer, is poorly understood. Specifically, clinical relevance of MTBP in human HCC has never been investigated. Here we demonstrated that expression of MTBP was significantly reduced in human HCC tissues compared to adjacent non-tumor tissues. MTBP expression was negatively correlated with capsular/vascular invasion and lymph node metastasis. Overexpression of MTBP resulted in the suppression of the migratory and metastatic potential of HCC cells, while its downregulation increased the migration. Consistent with the previous report, MTBP endogenously bound to alpha-actinin 4 (ACTN4) and suppressed ACTN4-mediated cell migration in multiple HCC cell lines. However, MTBP also inhibited migratory potential of PLC/PRF/5 HCC cells whose migration was not altered by manipulation of ACTN4 expression. These results suggest that mechanisms behind MTBP-mediated migration suppression may not be limited to the pathway involving ACTN4 in certain cellular contexts. Additionally, as a potential mechanism for reduced MTBP expression in tumors, we found that MTBP expression was increased following the treatment with histone deacetylase inhibitors (HDIs). Our study, for the first time, provides clinical relevance of MTBP in the suppression of HCC metastasis.

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.