Distinct cellular responses to replication stress leading to apoptosis or senescence

Biphasic DNA damage and non-canonical replication stress response govern radiation-induced senescence in glioblastoma

Therapy-induced senescence (TIS) in glioblastoma (GBM) residual disease and escape from TIS account for resistance and recurrence, but the mechanism of TIS manifestation remains obscure. Here, we demonstrate that replication stress (RS) is critical for the induction of TIS in residual cells by employing an in vitro GBM therapy-resistance cellular model. Interestingly, we found a 'biphasic' mode of DNA damage after radiation treatment and reveal that the second phase of DNA damage arises majorly in the S phase of residual cells due to RS. Mechanistically, we show that persistent phosphorylated ATR is a safeguard for radiation resilience, whereas the other canonical RS molecules remain unaltered during the second phase of DNA damage. Importantly, RS preceded the induction of senescence, and ATR inhibition resulted in TIS reduction, leading to apoptosis. Moreover, ATR inhibition sensitized PARP-1 inhibitor-induced enhanced TIS-mediated resistance, leading to cell death. Our study demonstrates the crucial role of RS in TIS induction and maintenance in GBM residual cells, and targeting ATR alone or in combination with a PARP-1 inhibitor will be an effective strategy to eliminate TIS for better treatment outcomes.

Achyrocline B (3,5 dihydroxy-6,7,8-trimethoxyflavone) synergizes with 5-fluorouracil allowing for dose reduction and reduced off-target toxicity in the treatment of colonic and pancreatic cancers

Surgically unresectable colorectal and pancreatic carcinomas have a high rate of mortality as current therapeutic options are limited. One common chemotherapeutic used to broadly treat both cancers is 5-flurouracil (5-Fu); however, treatment serves only to slow progression of the disease and comes with many side effects due to 5-Fu's intrinsic toxicity. Thus, strategies to decrease the dose of 5-Fu utilized therapeutically as well as reduce 5-Fu's off-target toxicity are paramount. Using cell models of colorectal and pancreatic cancers, we show that cotreatment with Achyrocline B (3,5 dihydroxy-6,7,8-trimethoxyflavone, AcB), a natural flavone from Achyrocline bogotensis, allows for four-fold reduction in 5-Fu dosage without loss of efficacy. We further show that the action of AcB is due to continued cell cycle progression despite 5-Fu pressure to synchronize at the G1/S threshold. In addition to AcB's effect on cancer cells, we found that AcB can directly reduce toxicity of 5-Fu in cells mimicking non-cancerous tissues. These in vitro results are then supported by xenograft modeling. AcB was shown to increase apoptosis in tumors leading to degeneration of the outer tumoral boundary. Furthermore, in 5-Fu treated animals it was found that AcB provided protection to the intestinal tract as indicated by preserved histological and immunohistochemical features. These results show promise for a new adjuvant therapy for colorectal and pancreatic carcinomas that not only reduces tumor progression, but more importantly has the potential to improve patient quality of life.

RETSAT associates with DDX39B to promote fork restarting and resistance to gemcitabine based chemotherapy in pancreatic ductal adenocarcinoma

Background

Severe hypoxia is a prominent character of pancreatic ductal adenocarcinoma (PDAC) microenvironment. In the process of gemcitabine based chemotherapy, PDAC cells are insulted from replication stresses co-induced by hypoxia and gemcitabine. However, PDAC cells get outstanding abilities to resist to such harsh conditions and keep proliferating, causing a major obstacle for current therapy. RETSAT (Retinol Saturase) is defined as a hypoxia convergent gene recently, with high expression in PDAC hypoxic sectors. This study aimed to explore the roles of RETSAT in replication stress resistance and hypoxia adaptation in PDAC cells, and decipher the underlying mechanism.

Methods

The expression of RETSAT was examined in TCGA (The Cancer Genome Atlas), human pancreatic cancer microarray, clinical specimens and cell lines. Functions of RETSAT were studied by means of DNA fiber assay and comet assay in monolayer cultured PDAC cell lines, three dimensional spheroids, patient derived organoids and cell derived xenograft mouse models. Mechanism was investigated by using iPOND (isolate proteins on nascent DNA) combined with mass spectrometry, immunoprecipitation and immunoblotting.

Results

First, we found the converse relationship of RETSAT expression and PDAC chemotherapy. That is, PDAC patients with high RETSAT expression correlated with poor survival, while ones holding low RETSAT expression were benefitted more in Gemcitabine based chemotherapy. Second, we identified RETSAT as a novel replication fork associated protein. HIF-1α signaling promotes RETSAT expression under hypoxia. Functionally, RETSAT promoted fork restarting under replication stress and maintained genomic stability. Third, we uncovered the interaction of RETSAT and R-loop unwinding helicase DDX39B. RETSAT detained DDX39B on forks to resolve R-loops, through which avoided fork damage and CHK1 initiated apoptosis. Targeting DDX39B using chemical CCT018159 sensitized PDAC cells and organoids to gemcitabine induced apoptosis, highlighting the synergetic application of CCT018159 and gemcitabine in PDAC chemotherapy.

Conclusions

This study identified RETSAT as a novel replication fork protein, which functions through interacting with DDX39B mediated R-loop clearance to promote fork restarting, leading to cellular resistance to replication stresses co-induced by tumor environmental hypoxia and gemcitabine in pancreatic ductal adenocarcinoma.

Sesquiterpene Lactones Potentiate Olaparib-Induced DNA Damage in p53 Wildtype Cancer Cells

Despite notable advances in utilising PARP inhibitor monotherapy, many cancers are not PARP inhibitor-sensitive or develop treatment resistance. In this work, we show that the two structurally-related sesquiterpene lactones, a 2-bromobenzyloxy derivative of dehydrosantonin (BdS) and alantolactone (ATL) sensitise p53 wildtype, homologous recombination-proficient cancer cells to low-dose treatment with the PARP inhibitor, olaparib. Exposure to combination treatments of olaparib with BdS or ATL induces cell-cycle changes, chromosomal instability, as well as considerable increases in nuclear area. Mechanistically, we uncover that mitotic errors likely depend on oxidative stress elicited by the electrophilic lactone warheads and olaparib-mediated PARP-trapping, culminating in replication stress. Combination treatments exhibit moderately synergistic effects on cell survival, probably attenuated by a p53-mediated, protective cell-cycle arrest in the G2 cell-cycle phase. Indeed, using a WEE1 inhibitor, AZD1775, to inhibit the G2/M cell-cycle checkpoint further decreased cell survival. Around half of all cancers diagnosed retain p53 functionality, and this proportion could be expected to increase with improved diagnostic approaches in the clinic. Utilising sublethal oxidative stress to sensitise p53 wildtype, homologous recombination-proficient cancer cells to low-dose PARP-trapping could therefore serve as the basis for future research into the treatment of cancers currently refractory to PARP inhibition.

Spatiotemporal Mislocalization of Nuclear Membrane-Associated Proteins in γ-Irradiation-Induced Senescent Cells

Cellular senescence, induced by genotoxic or replication stress, is accompanied by defects in nuclear morphology and nuclear membrane-heterochromatin disruption. In this work, we analyzed cytological and molecular changes in the linker of nucleoskeleton and cytoskeleton (LINC) complex proteins in senescence triggered by γ-irradiation. We used human mammary carcinoma and osteosarcoma cell lines, both original and shRNA knockdown clones targeting lamin B receptor (LBR) and leading to LBR and lamin B (LB1) reduction. The expression status and integrity of LINC complex proteins (nesprin-1, SUN1, SUN2), lamin A/C, and emerin were analyzed by immunodetection using confocal microscopy and Western blot. The results show frequent mislocalization of these proteins from the nuclear membrane to cytoplasm and micronuclei and, in some cases, their fragmentation and amplification. The timing of these changes clearly preceded the onset of senescence. The LBR deficiency triggered neither senescence nor changes in the LINC protein distribution before irradiation. However, the cytological changes following irradiation were more pronounced in shRNA knockdown cells compared to original cell lines. We conclude that mislocalization of LINC complex proteins is a significant characteristic of cellular senescence phenotypes and may influence complex events at the nuclear membrane, including trafficking and heterochromatin attachment.