Sera Averbek

For double-strand breaks (DSBs) formed by radiation, the onset of 5′ to 3′ end resection is a deciding factor in repair pathway choice, favoring homologous recombination (HR) over non-homologous end-joining (NHEJ). Studying HR-proficient MCF7 breast cancer cells, we confirmed a role for PARP1 in promoting DSB repair and limiting resection stress and identified the hexosamine biosynthetic pathway (HBP)-dependent post-translational modification O-GlcNAcylation as an independent regulator. Using pharmacological and genetic perturbations of O-linked β-N-acetylglucosamine (O-GlcNAc) transferase (OGT) and O-GlcNAcase (OGA), we showed that O-GlcNAcylation can limit end resection as measured by BrdU and RPA staining, recruitment of HR proteins BRCA1 and RAD51, and accumulation of cytosolic DNA in S/G2-phase cells. These effects were independent of PARP1 but required the histone methyltransferase EZH2. Loss of OGT or EZH2 phenocopied PARP inhibition, leading to hyper-resection after irradiation. The OGA inhibitor PUGNAc suppressed hyper-resection due to PARP1 knockout whereas treatment with the PARP inhibitor veliparib exacerbated defects in OGT- or EZH2-deficient cells. In each case, increased resection correlated with cytosolic DNA accumulation, suggesting a link to inflammatory signaling. These findings implicate the Warburg effect, via the HBP and O-GlcNAcylation, in favoring NHEJ over HR and suggest that disrupting EZH2 may sensitize HR-proficient tumor cells to radiation via resection-dependent mechanisms. Our results highlight the potential of targeting cancer-associated metabolic reprogramming to overwhelm HR repair and drive resection stress. Combining PARP inhibition with blockade of O-GlcNAcylation or EZH2 might offer a strategy to radiosensitize proliferating HR-proficient cancers while sparing non-cycling normal tissues.

Background: Nonsteroidal anti-inflammatory drugs (NSAIDs), which are commonly used for their anti-inflammatory and analgesic properties, have also been found to prevent cancer. (±)(R,S) Etodolac is an NSAID that belongs to the class of cyclooxygenase-2 inhibitors. Various derivatives of etodolac are synthesized to boost its anti-proliferative action and lessen its potential negative effects. In our earlier studies, some novel derivatives of etodolac exhibited stronger cytotoxic effects on prostate cell lines and had similar effects on leukemia cells in pre-screening experiments. Objective: Using the K562 leukemia cell line as a model, we sought to investigate the anti-cancer properties of a hydrazide-hydrozone derivative (SGK-205) and a 4-thiazolidinone derivative of etodolac (SGK-216). Materials and Methods: In the current investigation, SGK-205 and SGK-216 compounds were administered to K562 cells for 24 and 48 hours at concentrations of 10, 25, 50, 75 and 100 μM. Cell viability was assessed using the MTT test, and apoptosis by Annexin V-PI staining and mitochondrial membrane potential assays, together with mRNA expressions of apoptotic proteins. The levels of the proteins, HER2 and COX2, were also examined to evaluate COX2 inhibitory capacity. Results: In K562 cells, there was a definite dose-dependent response to SGK-205 and SGK-216 compounds. Results from MTT viability tests, together with mitochondrial membrane potential measurements and Annexin V-PI staining, revealed that SGK-216 and SGK-205 significantly outperformed etodolac in terms of their apoptotic and anti-proliferative activities. The concentration range of 10-20 M for both chemicals was sufficient to start biological responses. Apoptosis was also investigated through the expressions of pro- and anti-apoptotic proteins. Additionally, gene expression research demonstrated SGK-205 to be a beneficial substitute to etodolac in lowering COX-2 and human epidermal growth factor receptor-2 (HER2) expression. Conclusion: Our data indicated both derivatives to have higher anti-proliferative and apoptotic effects compared to etodolac. An overall assessment highlighting apoptotic induction potential, acceptable toxicity levels, a consistent dose-response relationship, and COX2 inhibitory actions, in particular, indicated SGK-205 as a viable novel therapeutic.

Exposing cells to DNA damaging agents, such as ionizing radiation (IR) or cytotoxic chemicals, can cause DNA double-strand breaks (DSBs), which are crucial to repair to maintain genetic integrity. O-linked β-N-acetylglucosaminylation (O-GlcNAcylation) is a post-translational modification (PTM), which has been reported to be involved in the DNA damage response (DDR) and chromatin remodeling. Here, we investigated the impact of O-GlcNAcylation on the DDR, DSB repair and chromatin status in more detail. We also applied charged particle irradiation to analyze differences of O-GlcNAcylation and its impact on DSB repair in respect of spatial dose deposition and radiation quality. Various techniques were used, such as the γH2AX foci assay, live cell microscopy and Fluorescence Lifetime Microscopy (FLIM) to detect DSB rejoining, protein accumulation and chromatin states after treating the cells with O-GlcNAc transferase (OGT) or O-GlcNAcase (OGA) inhibitors. We confirmed that O-GlcNAcylation of MDC1 is increased upon irradiation and identified additional repair factors related to Homologous Recombination (HR), CtIP and BRCA1, which were increasingly O-GlcNAcyated upon irradiation. This is consistent with our findings that the function of HR is affected by OGT inhibition. Besides, we found that OGT and OGA activity modulate chromatin compaction states, providing a potential additional level of DNA-repair regulation.