Functional inhibition of RECQL5 helicase elicits non-homologous end joining response and sensitivity of breast cancers to PARP inhibitor

Poly (ADPRibose) Polymerase inhibitor (PARPi) are clinically approved for the treatment of BRCA-mutated hereditary breast and ovarian cancers with homologous recombination (HR) deficiency, based on synthetic lethality concept. However, ∼90% of breast cancers are BRCA-wild type; they repair PARPi mediated damage through HR, leading to intrinsic de novo resistance. Hence, there is an unmet need of exploring novel targets in HR-proficient aggressive breast cancers for PARPi treatment. RECQL5 physically interacts and disrupts RAD51 from pre-synaptic filaments, aiding HR resolution, replication fork protection and preventing illegitimate recombination. In the current investigation, we show that targeted inhibition of HR by stabilization of RAD51-RECQL5 complex by a pharmacological inhibitor of RECQL5 (4a; 1,3,4-oxadiazole derivative) in the presence of PARPi [talazoparib (BMN673)] leads to abolition of functional HR with uncontrolled activation of NHEJ repair. This was assessed by GFP based NHEJ reporter assay, KU80 recruitment and in vitro NHEJ based plasmid ligation assay. Concomitant treatment with talazoparib and 4a generates copious amounts of replication stress, prolonged cell cycle arrest, extensive double strand breaks (DSBs) and mitotic catastrophe, leading to sensitization of HR-proficient breast cancers. Suppression of NHEJ activity abolishes 4a-mediated sensitization of breast cancers to PARPi treatment. Imperatively, 4a was ineffective against normal mammary epithelial cells, which expresses low RECQL5 vis-à-vis breast cancer cells. Moreover, functional inhibition of RECQL5 suppresses metastatic potential of breast cancer cells in response to PARPi. Together, we identified RECQL5 as a novel pharmacological target for expanding PARPi based treatment horizon for HR-proficient cancers.

Design, synthesis and development of a dual inhibitor of Topoisomerase 1 and poly (ADP-ribose) polymerase 1 for efficient killing of cancer cells

Combinatorial inhibition of Topoisomerase 1 (TOP1) and Poly (ADP-ribose) polymerase 1 (PARP1) is an attractive therapeutic strategy which is under active investigation to address chemoresistance to TOP1 inhibitors. However, this combinatorial regimen suffers from severe dose limiting toxicities. Dual inhibitors often offer significant advantages over combinatorial therapies involving individual agents by minimizing toxicity and providing conducive pharmacokinetic profiles. In this study, we have designed, synthesized and evaluated a library of 11 candidate conjugated dual inhibitors for PARP1 and TOP1, named as DiPT-1 to DiPT-11. Our extensive screening showed that one of the hits i.e.DiPT-4 has promising cytotoxicity profile against multiple cancers with limited toxicities towards normal cells. DiPT-4 induces extensive DNA double stand breaks (DSBs), cell cycle arrest and apoptosis in cancer cells. Mechanistically, DiPT-4 has the propensity to bind catalytic pockets of TOP1 and PARP1, leading to significant inhibition of both TOP1 and PARP1 at in vitro and cellular level. Interestingly, DiPT-4 causes extensive stabilization of TOP1-DNA covalent complex (TOP1cc), a key lethal intermediate associated with induction of DSBs and cell death. Moreover, DiPT-4 inhibited poly (ADP-ribosylation) i.e. PARylation of TOP1cc, leading to long lived TOP1cc with a slower kinetics of degradation. This is one of the important molecular processes which helps in overcoming resistance in cancer in response to TOP1 inhibitors. Together, our investigation showed DiPT-4 as a promising dual inhibitor of TOP1 and PARP1, which may have the potential to offer significant advantages over combinatorial therapy in clinical settings.

Non-enzymatic function of WRN RECQL helicase regulates removal of topoisomerase-I-DNA covalent complexes and triggers NF-κB signaling in cancer

Mutation in Werner (WRN) RECQL helicase is associated with premature aging syndrome (Werner syndrome, WS) and predisposition to multiple cancers. In patients with solid cancers, deficiency of the WRN RECQL helicase is paradoxically associated with enhanced overall survival in response to treatment with TOP1 inhibitors, which stabilize pathological TOP1‐DNA‐covalent‐complexes (TOP1cc) on the genome. However, the underlying mechanism of WRN in development of chemoresistance to TOP1 inhibitors is not yet explored. Our whole‐genome transcriptomic analysis for ~25,000 genes showed robust activation of NF‐κB‐dependent prosurvival genes in response to TOP1cc. CRISPR‐Cas9 knockout, shRNA silencing, and under‐expression of WRN confer high‐sensitivity of multiple cancers to TOP1 inhibitor. We demonstrated that WRN orchestrates TOP1cc repair through proteasome‐dependent and proteasome‐independent process, unleashing robust ssDNA generation. This in turn ensues signal transduction for CHK1 mediated NF‐κB‐activation through IκBα‐degradation and nuclear localization of p65 protein. Intriguingly, our site‐directed mutagenesis and rescue experiments revealed that neither RECQL‐helicase nor DNA‐exonuclease enzyme activity of WRN (WRN^E84A, WRN^K577M, and WRN^E84A‐K577M) were required for TOP1cc removal, ssDNA generation and signaling for NF‐κB activation. In correlation with patient data and above results, the TOP1 inhibitor‐based targeted therapy showed that WRN‐deficient melanoma tumors were highly sensitive to TOP1 inhibition in preclinical in vivo mouse model. Collectively, our findings identify hitherto unknown non‐enzymatic role of WRN RECQL helicase in pathological mechanisms underlying TOP1cc processing and subsequent NF‐κB‐activation, offering a potential targeted therapy for WRN‐deficient cancer patients.

Pharmacological targeting of differential DNA repair, radio-sensitizes WRN-deficient cancer cells in vitro and in vivo

Werner (WRN) expression is epigenetically downregulated in various tumors. It is imperative to understand differential repair process in WRN-proficient and WRN-deficient cancers to find pharmacological targets for radio-sensitization of WRN-deficient cancer. In the current investigation, we showed that pharmacological inhibition of CHK1 mediated homologous recombination repair (HRR), but not non-homologous end joining (NHEJ) repair, can causes hyper-radiosensitization of WRN-deficient cancers. This was confirmed in cancer cell lines of different tissue origin (osteosarcoma, colon adenocarcinoma and melanoma) with WRN silencing and overexpression. We established that WRN-depleted cells are dependent on a critical but compromised CHK1-mediated HRR-pathway for repairing ionizing radiation (IR) induced DSBs for their survival. Mechanistically, we unraveled a new finding that the MRE11, CTIP and WRN proteins are largely responsible for resections of late and persistent DSBs. In response to IR-treatment, MRE11 and CTIP-positively and WRN-negatively regulate p38-MAPK reactivation in a CHK1-dependent manner. A degradation resistant WRN protein, mutated at serine 1141, abrogates p38-MAPK activation. We also showed that CHK1-p38-MAPK axis plays important role in RAD51 mediated HRR in WRN-silenced cells. Like CHK1 inhibition, pharmacological-inhibition of p38-MAPK also hyper-radiosensitizes WRN-depleted cells by targeting HR-pathway. Combination treatment of CHK1-inhibitor (currently under various clinical trials) and IR exhibited a strong synergy against WRN-deficient melanoma tumor in vivo. Taken together, our findings suggest that pharmacological targeting of CHK1-p38-MAPK mediated HRR is an attractive strategy for enhancing therapeutic response of radiation treatment of cancer.

Thiol antioxidants sensitize malabaricone C induced cancer cell death via reprogramming redox sensitive p53 and NF-κB proteins in vitro and in vivo

Specific focus on "redox cancer therapy" by targeting drugs to redox homeostasis of the cancer cells is growing rapidly. Recent clinical studies showed that N-acetyl cysteine (NAC) treatment significantly decreased the metabolic heterogeneity and reduced Ki67 (a proliferation marker) with simultaneous enhancement in apoptosis of tumor cells in patients. However, it is not yet precisely known how thiol antioxidants enhance killing of cancer cells in a context dependent manner. To this end, we showed that a dietary compound, malabaricone C (mal C) generated copious amounts of reactive oxygen species (ROS) and also reduced GSH level in lung cancer cells. Paradoxically, although antioxidants supplementation reduced mal C-induced ROS, thiol-antioxidants (NAC/GSH) restored intracellular GSH level but enhanced DNA DSBs and apoptotic cell death induced by mal C. Our results unraveled two tightly coupled biochemical mechanisms attributing this sensitization process by thiol antioxidants. Firstly, thiol antioxidants enable the "catechol-quinone redox cycle" of mal C and ameliorate ROS generation and bio-molecular damage (DNA and protein). Secondly, thiol antioxidants cause rapid glutathionylation of transcription factors [p53, p65 (NF-κB) etc.], oxidized by mal C, and abrogates their nuclear sequestration and transcription of the anti-apoptotic genes. Furthermore, analyses of the mitochondrial fractions of p53 expressing and silenced cells revealed that cytoplasmic accumulation of glutathionylated p53 (p53-SSG) triggers a robust mitochondrial death process. Interestingly, mutation of redox sensitive cysteine residues at 124, 141 and 182 position in p53 significantly reduces mal C plus NAC mediated sensitization of cancer cells. The preclinical results, in two different tumor models in mice, provides further support our conclusion that NAC is able to sensitize mal C induced suppression of tumor growth in vivo.

Salinomycin reduces growth, proliferation and metastasis of cisplatin resistant breast cancer cells via NF-kB deregulation

Cisplatin (cis-diamminedichloro-platinum, CDDP), is a widely used platinum compound for various solid tumors including breast cancer as first line of therapy. However, its positive effects are limited due to acquired drug resistance and severe side effects in non-malignant tissue, especially due to dose-dependent nephro- and/or neuro-toxicity. Salinomycin is an antibiotic with coccidiostat effect and has shown anticancer efficacy against various cancer cells with selectivity in targeting cancer stem cells. In the present study, anticancer efficacy and mechanism of action of salinomycin in CDDP-resistant human breast cancer (MCF7^DDP) cells has been examined. Initially, we generated CDDP-resistant cells by a new protocol followed by checking the anticancer efficacy of salinomycin through MTT, clonogenic, annexin-V/PI and sub-G1 assay. Our results demonstrated that salinomycin diminished both cell proliferation and metastatic migration of MCF7^DDP cells. Salinomycin also induced mitochondrial dysfunction in CDDP-resistant breast cancer cells. The analysis of nuclear translocation of pro-survival transcription factors by western blotting showed a distinct role of p65 (NF-κB) in CDDP-mediated resistance in breast cancer. Salinomycin abrogated nuclear translocation of NF-κB proteins and also caused a concurrent reduction in NF-κB regulated expression of pro-survival proteins e.g., survivin, XIAP and BCL-2 in CDDP-resistant cells. These results suggest that a follow up treatment of salinomycin may be promising strategy against CDDP resistant breast cancer cells and metastasis and help in reducing CDDP-induced side effects.

Mechanism of coralyne-mediated DNA photo-nicking process

Previously, we reported that coralyne and UVA combination sensitized a wide range of human carcinoma cells regardless of their p53 status. The coralyne induced photosensitization of cancer cells may be clinically attractive, as mutation in the p53 gene is prevalent in many types of tumors. Coralyne mediated photosensitization of cancer cells is attributable to its ability to cause extensive DNA single strand breaks (SSB). However, the precise mechanism of coralyne induced DNA photo-damage is not yet known. The present study was aimed to understand the hitherto unknown mechanism of the coralyne-induced DNA photo-cleavage process. To this end, we compared the DNA photo-nicking properties of berberine, jatrorrhizine and coralyne, and deciphered involvement of the photochemical processes in the photo-nuclease action of coralyne using absorption and electron spin resonance spectroscopy, high performance liquid chromatography and mass spectroscopy (MS) techniques in conjunction with relevant in vitro studies with plasmid DNA. In association with UVA, coralyne, but not berberine and jatrorrhizine induced significant nicking of plasmid DNA via an O₂-independent photo-chemical process. The Job's plot of our spectrophotometric data suggested that one coralyne molecule remains intercalated with two DNA base pairs (i. e., 1:2) and starts forming aggregates beyond this molar ratio. The DNA photo-nicking by the combination of coralyne and UVA (designated as CUVA) was primarily caused by the coralyne aggregates without any significant contribution from the DNA-intercalated coralyne monomer.

Resveratrol analogue, trans-4,4'-dihydroxystilbene (DHS), inhibits melanoma tumor growth and suppresses its metastatic colonization in lungs

The prevalence of melanoma and the lack of effective therapy for metastatic melanoma warrant extensive and systematic evaluations of small molecules in cellular and pre-clinical models. We investigated, herein, the antitumor and anti-metastatic effects of trans-4,4'-dihydroxystilbene (DHS), a natural product present in bark of Yucca periculosa, using in vitro and in vivo melanoma murine models. DHS showed potent melanoma cytotoxicity, as determined by MTT and clonogenic assay. Further, DHS induced cytotoxicity was mediated through apoptosis, which was assessed by annexin V-FITC/PI, sub-G1 and caspase activation assays. In addition, DHS inhibited cell proliferation by inducing robust cell cycle arrest in G1-phase. Imperatively, these inhibitory effects led to a significant reduction of melanoma tumor in pre-clinical murine model. DHS also inhibited cell migration and invasion of melanoma cells, which were examined using wound healing and Transwell migration/invasion assays. Mechanistically, DHS modulated the expressions of several key metastasis regulating proteins e.g., MMP-2/9, N-cadherin, E-cadherin and survivin. We also showed the anti-metastatic effect of DHS in a melanoma mediated lung metastasis model in vivo. DHS significantly reduced large melanoma nodule formation in the parenchyma of lungs. Therefore, DHS may represent a promising natural drug in the repertoire of treatment against melanoma tumor growth and metastasis.

Coralyne, a protoberberine alkaloid, causes robust photosenstization of cancer cells through ATR-p38 MAPK-BAX and JAK2-STAT1-BAX pathways

Photodynamic therapy (PDT) provides an effective cancer treatment option but it requires sufficient cellular oxygen concentration to exert its photosensitizing effects. Due to hypoxic nature of most tumors, widespread clinical application of PDT is restricted and warrants development of photosensitizers which can kill cancer cells in ROS independent manner. Previously, we reported significant enhancement of the anti-cancer property of coralyne in presence of ultraviolet-A (UVA) light exposure against several human carcinoma cell lines. This study aimed at unravelling molecular cascades of events in CUVA treatment (coralyne and UVA light)-mediated photosensitization of human skin cancer. The CUVA-treatment caused robust apoptosis of A431 cancer cells, primarily through mitochondrial and lysosomal dysfunctions. Silencing of BAX conferred a significant protection against CUVA-induced apoptosis. Both lysosomal proteases and caspase-8 activation contributed to BID cleavage. Further, our results revealed that a dual signaling axis e.g., ATR-p38 MAPK and JAK2-STAT1 pathways functioned upstream of BAX activation in apoptosis response. Moreover, transient silencing of ATR and pharmacological inhibition of p38-MAPK or JAK2 significantly abolished the effect of CUVA treatment induced BAX expression and cell death, linking the extrinsic and intrinsic pathways with the observed cell death. Our data suggest that coralyne, which is known topoisomerase-I inhibitor, may be an attractive agent for photo-chemotherapeutic treatment of human skin cancers.

Spice-derived phenolic, malabaricone B induces mitochondrial damage in lung cancer cellsviaa p53-independent pathway

The spice-derived phenolic, malabaricone B induces mitochondrial cell death and reduces lung tumor growthin vivo.