Base excision repair (BER) has evolved to preserve the integrity of DNA following cellular oxidative stress and in response to exogenous insults. The pathway is a coordinated, sequential process involving 30 proteins or more in which single strand breaks are generated as intermediates during the repair process. While deficiencies in BER activity can lead to high mutation rates and tumorigenesis, cancer cells often rely on increased BER activity to tolerate oxidative stress. Targeting BER has been an attractive strategy to overwhelm cancer cells with DNA damage, improve the efficacy of radiotherapy and/or chemotherapy, or form part of a lethal combination with a cancer specific mutation/loss of function. We provide an update on the progress of inhibitors to enzymes involved in BER, and some of the challenges faced with targeting the BER pathway.

/pdf/base-excision-repair-and-its-implications-to-cancer-therapy-4aio4qwdw3.pdf

Base excision repair and its implications to cancer therapy

Prostate cancer is one of the most commonly diagnosed cancers and a pressing health challenge in men worldwide Radiation therapy (RT) is widely considered a standard therapy for advanced as well as localized prostate cancer Although this primary therapy is associated with high cancer control rates, up to one-third of patients undergoing radiation therapy becomes radio-resistant and/or has tumor-relapse/recurrence Therefore, focus on new molecular targets and pathways is essential to develop novel radio-sensitizing agents for the effective and safe treatment of prostate cancer Here, we describe functional studies that were performed to investigate the role of structural maintenance of chromosome-1 (SMC1A) in radioresistance of metastatic prostate cancer cells Short hairpin RNA (shRNA) was used to suppress SMC1A in metastatic castration-resistant prostate cancer cells, DU145 and PC3 Clonogenic survival assays, Western blot, RT-PCR, and γ-H2AX staining were used to assess the effect of SMC1A knockdown on radiation sensitivity of these prostate cancer cells We demonstrate that SMC1A is overexpressed in human prostate tumors compared to the normal adjacent tissue SMC1A knockdown limits the clonogenic potential, epithelial-mesenchymal transition (EMT), and cancer stem-like cell (CSC) properties of DU145 and PC3 cells and enhanced efficacy of RT in these cells Targeted inhibition of SMC1A not only plays a critical role in overcoming radio-resistance in prostate cancer cells, but also suppresses self-renewal and the tumor-propagating potential of x-irradiated cancer cells We propose that SMC1A could be a potential molecular target for the development of novel radio-sensitizing therapeutic agents for management of radio-resistant metastatic prostate cancer

https://onlinelibrary.wiley.com/doi/pdf/10.1002/mc.22913

SMC1A is associated with radioresistance in prostate cancer and acts by regulating epithelial-mesenchymal transition and cancer stem-like properties.

Oxidative DNA damage is recognized by 8-oxoguanine (8-oxoG) DNA glycosylase 1 (OGG1), which excises 8-oxoG, leaving a substrate for apurinic endonuclease 1 (APE1) and initiating repair. Here, we describe a small molecule (TH10785) that interacts with the phenylalanine-319 and glycine-42 amino acids of OGG1, increases the enzyme activity 10-fold, and generates a previously undescribed β,δ-lyase enzymatic function. TH10785 controls the catalytic activity mediated by a nitrogen base within its molecular structure. In cells, TH10785 increases OGG1 recruitment to and repair of oxidative DNA damage. This alters the repair process, which no longer requires APE1 but instead is dependent on polynucleotide kinase phosphatase (PNKP1) activity. The increased repair of oxidative DNA lesions with a small molecule may have therapeutic applications in various diseases and aging. Description Teaching an enzyme a new trick Small molecules that activate enzymes commonly operate using allosteric control. Michel et al. describe the mode of action of a small-molecule activator that binds to the active site of 8-oxo guanine DNA glycosylase 1 (OGG1) and catalyzes a biochemical reaction not found in the naïve protein. The molecule enables the protein to completely cleave the damaged DNA strand, which results in an overall increased repair of oxidative DNA damage. The underlying creation of a new repair pathway has potential medical application in diseases related to oxidative DNA damage. —DJ A small-molecule activator of a DNA glycolase enzyme generates a new enzymatic function and increases oxidative DNA damage repair.

/pdf/small-molecule-activation-of-ogg1-increases-oxidative-dna-1zlg9if8.pdf

Small-molecule activation of OGG1 increases oxidative DNA damage repair by gaining a new function

Cancer cells are defective in DNA repair, so they experience increased DNA strand breaks, genome instability, gene mutagenesis, and tumorigenicity; however, multiple classic DNA repair genes and pathways are strongly activated in malignant tumor cells to compensate for the DNA repair deficiency and gain an apoptosis resistance. The mechanisms underlying this phenomenon in cancer are unclear. We speculate that a key DNA repair gene or signaling pathway in cancer has not yet been recognized. Here, we show that the lipogenic liver X receptor (LXR)-sterol response element binding factor-1 (SREBF1) axis controls the transcription of a key DNA repair gene polynucleotide kinase/phosphatase (PNKP), thereby governing cancer cell DNA repair and apoptosis. Notably, the PNKP levels were significantly reduced in 95% of human pancreatic cancer (PC) patients, particularly deep reduction for sixfold in all of the advanced-stage PC cases. PNKP is also deficient in three other types of cancer that we examined. In addition, the expression of LXRs and SREBF1 was significantly reduced in the tumor tissues from human PC patients compared with the adjacent normal tissues. The newly identified LXR-SREBF1-PNKP signaling pathway is deficient in PC, and the defect in the pathway contributes to the DNA repair deficiency in the cancer. Strikingly, further diminution of the vulnerable LXR-SREBF1-PNKP signaling pathway using a small molecule triptonide, a new LXR antagonist identified in this investigation, at a concentration of 8 nM robustly activated tumor-suppressor p53 and readily elevated cancer cell DNA strand breaks over an apoptotic threshold, and selectively induced PC cell apoptosis, resulting in almost complete elimination of tumors in xenograft mice without obvious complications. Our findings provide new insight into DNA repair and apoptosis in cancer, and offer a new platform for developing novel anticancer therapeutics.

/pdf/the-lipogenic-lxr-srebf1-signaling-pathway-controls-cancer-1no46xnc9j.pdf

The lipogenic LXR-SREBF1 signaling pathway controls cancer cell DNA repair and apoptosis and is a vulnerable point of malignant tumors for cancer therapy.

High-intensity focused ultrasound (HIFU/FUS) has expanded as a noninvasive quantifiable option for hyperthermia (HT). HT in a temperature range of 40–47 °C (thermal dose CEM43 ≥ 25) could work as a sensitizer to radiation therapy (RT). Here, we attempted to understand the tumor radiosensitization effect at the cellular level after a combination treatment of FUS+RT. An in vitro FUS system was developed to induce HT at frequencies of 1.147 and 1.467 MHz. Human head and neck cancer (FaDU), glioblastoma (T98G), and prostate cancer (PC-3) cells were exposed to FUS in ultrasound-penetrable 96-well plates followed by single-dose X‑ray irradiation (10 Gy). Radiosensitizing effects of FUS were investigated by cell metabolic activity (WST‑1 assay), apoptosis (annexin V assay, sub-G1 assay), cell cycle phases (propidium iodide staining), and DNA double-strand breaks (γH2A.X assay). The FUS intensities of 213 (1.147 MHz) and 225 W/cm2 (1.467 MHz) induced HT for 30 min at mean temperatures of 45.20 ± 2.29 °C (CEM43 = 436 ± 88) and 45.59 ± 1.65 °C (CEM43 = 447 ± 79), respectively. FUS improves the effect of RT significantly by reducing metabolic activity in T98G cells 48 h (RT: 96.47 ± 8.29%; FUS+RT: 79.38 ± 14.93%; p = 0.012) and in PC-3 cells 72 h (54.20 ± 10.85%; 41.01 ± 11.17%; p = 0.016) after therapy, but not in FaDu cells. Mechanistically, FUS+RT leads to increased apoptosis and enhancement of DNA double-strand breaks compared to RT alone in T98G and PC-3 cells. Our in vitro findings demonstrate that FUS has good potential to sensitize glioblastoma and prostate cancer cells to RT by mainly enhancing DNA damage.

/pdf/focused-ultrasound-radiosensitizes-human-cancer-cells-by-3gczffv54d.pdf

Focused ultrasound radiosensitizes human cancer cells by enhancement of DNA damage

High linear energy transfer (LET) radiation or heavy ion such as carbon ion radiation is used as a method for advanced radiotherapy in the treatment of cancer. It has many advantages over the conventional photon based radiotherapy using Co-60 gamma or high energy X-rays from a Linear Accelerator. However, charged particle therapy is very costly. One way to reduce the cost as well as irradiation effects on normal cells is to reduce the dose of radiation by enhancing the radiation sensitivity through the use of a radiomodulator. PNKP (polynucleotide kinase/phosphatase) is an enzyme which plays important role in the non-homologous end joining (NHEJ) DNA repair pathway. It is expected that inhibition of PNKP activity may enhance the efficacy of the charged particle irradiation in the radioresistant prostate cancer cell line PC-3. To test this hypothesis, we investigated cellular radiosensitivity by clonogenic cell survival assay in PC-3 cells.12Carbon ion beam of62 MeVenergy (equivalent 5.16 MeV/nucleon) and with an entrance LET of 287 kev/μm was used for the present study. Apoptotic parameters such as nuclear fragmentation and caspase-3 activity were measured by DAPI staining, nuclear ladder assay and colorimetric caspase-3method. Cell cycle arrest was determined by FACS analysis. Cell death was enhanced when carbon ion irradiation is combined with PNKPi (PNKP inhibitor) to treat cells as compared to that seen for PNKPi untreated cells. A low concentration (10μM) of PNKPi effectively radiosensitized the PC-3 cells in terms of reduction of dose in achieving the same survival fraction. PC-3 cells underwent significant apoptosis and cell cycle arrest too was enhanced at G2/M phase when carbon ion irradiation was combined with PNKPi treatment. Our findings suggest that combined treatment of carbon ion irradiation and PNKP inhibition could enhance cellular radiosensitivity in a radioresistant prostate cancer cell line PC-3. The synergistic effect of PNKPi and carbon ion irradiation could be used as a promising method for carbon-ion therapy in radioresistant cells.

/pdf/targeting-dna-repair-with-pnkp-inhibition-sensitizes-312izzcs52.pdf

Pallavi Srivastava

Papers

Targeting DNA repair with PNKP inhibition sensitizes radioresistant prostate cancer cells to high LET radiation