Tissue-Specific Chk1 Activation Determines Apoptosis by Regulating the Balance of p53 and p21

Preliminary Study on 53BP1-Mediated DNA Double-Strand Break Response in Spermatogonial Stem Cells

53BP1 mediates DNA repair process in somatic cells; however, the function of 53BP1 in germline stem cells still remains unclear. In the present study, animals and cells DNA damage repair (DDR) model was established by irradiation and HU treatment; immunofluorescence staining and laser confocal microscopy were used to detect the expression of 53BP1, p-CHK2, and p-P53 in the DDR process of mSSCs. 53BP1 knockdown expression mSSCs cell line conducted by Trp53bp1-shRNA was established and EdU staining was adopted to analyze cell cycle and cell proliferation. Moreover, NHEJ reporter vector was applied to detect the repair efficacy after Trp53bp1 knocked-down (KD) expression. Results showed that 53BP1 could form foci signals in mSSCs during DDR process both in vivo and in vitro, which was independent of γH2AX. 53BP1 downstream protein, p-P53, and p-CHK2 were involved and dynamically expressed in DDR response. Knocking down of Trp53bp1 expression in mSSCs could not dramatically inhibit cell proliferation, but may increase cell sensitivity to HU. The NHEJ repair efficacy was sharply decreased in Trp53bp-KD SSCs via flow cytometry analysis. We revealed the specific mechanism of 53BP1 in SSCs DDR process, which is expected to provide a new theoretical basis and insights for the diagnosis and treatment of male infertility.

Role of Dietary Antioxidants in p53-Mediated Cancer Chemoprevention and Tumor Suppression

Cancer arises through a complex interplay between genetic, behavioral, metabolic, and environmental factors that combined trigger cellular changes that over time promote malignancy. In terms of cancer prevention, behavioral interventions such as diet can promote genetic programs that may facilitate tumor suppression; and one of the key tumor suppressors responsible for initiating such programs is p53. The p53 protein is activated by various cellular events such as DNA damage, hypoxia, heat shock, and overexpression of oncogenes. Due to its role in cell fate decisions after DNA damage, regulatory pathways controlled by p53 help to maintain genome stability and thus “guard the genome” against mutations that cause cancer. Dietary intake of flavonoids, a C₁₅ group of polyphenols, is known to inhibit cancer progression and assist DNA repair through p53-mediated mechanisms in human cells via their antioxidant activities. For example, quercetin arrests human cervical cancer cell growth by blocking the G₂/M phase cell cycle and inducing mitochondrial apoptosis through a p53-dependent mechanism. Other polyphenols such as resveratrol upregulate p53 expression in several cancer cell lines by promoting p53 stability, which in colon cancer cells results in the activation of p53-mediated apoptosis. Finally, among vitamins, folic acid seems to play an important role in the chemoprevention of gastric carcinogenesis by enhancing gastric epithelial apoptosis in patients with premalignant lesions by significantly increased expression of p53. In this review, we discuss the role of these and other dietary antioxidants in p53-mediated cell signaling in relation to cancer chemoprevention and tumor suppression in normal and cancer cells.

The Role of Cell Cycle Regulators in Cell Survival-Dual Functions of Cyclin-Dependent Kinase 20 and p21Cip1/Waf1

The mammalian cell cycle is important in controlling normal cell proliferation and the development of various diseases. Cell cycle checkpoints are well regulated by both activators and inhibitors to avoid cell growth disorder and cancerogenesis. Cyclin dependent kinase 20 (CDK20) and p21^Cip1/Waf1 are widely recognized as key regulators of cell cycle checkpoints controlling cell proliferation/growth and involving in developing multiple cancers. Emerging evidence demonstrates that these two cell cycle regulators also play an essential role in promoting cell survival independent of the cell cycle, particularly in those cells with a limited capability of proliferation, such as cardiomyocytes. These findings bring new insights into understanding cytoprotection in these tissues. Here, we summarize the new progress of the studies on these two molecules in regulating cell cycle/growth, and their new roles in cell survival by inhibiting various cell death mechanisms. We also outline their potential implications in cancerogenesis and protection in heart diseases. This information renews the knowledge in molecular natures and cellular functions of these regulators, leading to a better understanding of the pathogenesis of the associated diseases and the discovery of new therapeutic strategies.

The CHK1 inhibitor MU380 significantly increases the sensitivity of human docetaxel-resistant prostate cancer cells to gemcitabine through the induction of mitotic catastrophe

As treatment options for patients with incurable metastatic castration-resistant prostate cancer (mCRPC) are considerably limited, novel effective therapeutic options are needed. Checkpoint kinase 1 (CHK1) is a highly conserved protein kinase implicated in the DNA damage response (DDR) pathway that prevents the accumulation of DNA damage and controls regular genome duplication. CHK1 has been associated with prostate cancer (PCa) induction, progression, and lethality; hence, CHK1 inhibitors SCH900776 (also known as MK-8776) and the more effective SCH900776 analog MU380 may have clinical applications in the therapy of PCa. Synergistic induction of DNA damage with CHK1 inhibition represents a promising therapeutic approach that has been tested in many types of malignancies, but not in chemoresistant mCRPC. Here, we report that such therapeutic approach may be exploited using the synergistic action of the antimetabolite gemcitabine (GEM) and CHK1 inhibitors SCH900776 and MU380 in docetaxel-resistant (DR) mCRPC. Given the results, both CHK1 inhibitors significantly potentiated the sensitivity to GEM in a panel of chemo-naïve and matched DR PCa cell lines under 2D conditions. MU380 exhibited a stronger synergistic effect with GEM than clinical candidate SCH900776. MU380 alone or in combination with GEM significantly reduced spheroid size and increased apoptosis in all patient-derived xenograft 3D cultures, with a higher impact in DR models. Combined treatment induced premature mitosis from G1 phase resulting in the mitotic catastrophe as a prestage of apoptosis. Finally, treatment by MU380 alone, or in combination with GEM, significantly inhibited tumor growth of both PC339-DOC and PC346C-DOC xenograft models in mice. Taken together, our data suggest that metabolically robust and selective CHK1 inhibitor MU380 can bypass docetaxel resistance and improve the effectiveness of GEM in DR mCRPC models. This approach might allow for dose reduction of GEM and thereby minimize undesired toxicity and may represent a therapeutic option for patients with incurable DR mCRPC.

Role of poly (ADP-ribose) polymerase-1 in cadmium-induced cellular DNA damage and cell cycle arrest in rat renal tubular epithelial cell line NRK-52E

With the development of modern industry, the problem of cadmium (Cd) pollution cannot be ignored and its toxicity has caused great personal injury to humans. Poly (ADP-ribose) polymerase 1 (PARP-1) protein is a research hotspot in recent years, the research we have published shows that 5 μM of Cd-treated NRK-52E cells activated PARP-1, but the specific effects of PARP-1 on DNA damage and cell cycle is unclear. Therefore, the purpose of this study is to reveal the effect of Cd on DNA damage and cell cycle arrest in NRK-52E cells, in addition, to investigate the role of PARP-1 in mediating this effect. Western blotting, comet assay, QRT-PCR, immunofluorescence, and co-immunoprecipitation were used to detect DNA damage and cell cycle-associated protein expression. Flow cytometry was used to assess cell cycle distribution and the apoptosis rates. Results showed that after the increase in treatment time and Cd concentration, the degree of DNA damage was significantly increased, and a transition from G0/G1 to S phase arrest was observed. In addition, inhibition of PARP-1 expression exacerbated cell damage and cell cycle arrest when DNA damage was low, but attenuated cell damage and even cell cycle arrest when DNA damage was severe. These findings in this study indicate that Cd causes DNA damage in NRK-52E cells, leading to cell cycle arrest at different phases depending on the degree of DNA damage. Moreover, PARP-1 plays an important role in mediating this effect, when DNA damage is low, it functions in DNA repair, however, when DNA damage is severe, it aggravates cell damage and induces cell death.

Cell-type-specific role of CHK2 in mediating DNA damage-induced G2 cell cycle arrest

Cancer is a life-threatening disease that affects one in three people. Although most cases are sporadic, cancer risk can be increased by genetic factors. It remains unknown why certain genes predispose for specific forms of cancer only, such as checkpoint protein 2 (CHK2), in which gene mutations convey up to twofold higher risk for breast cancer but do not increase lung cancer risk. We have investigated the role of CHK2 and the related kinase checkpoint protein 1 (CHK1) in cell cycle regulation in primary breast and lung primary epithelial cells. At the molecular level, CHK1 activity was higher in lung cells, whereas CHK2 was more active in breast cells. Inhibition of CHK1 profoundly disrupted the cell cycle profile in both lung and breast cells, whereas breast cells were more sensitive toward inhibition of CHK2. Finally, we provide evidence that breast cells require CHK2 to induce a G2–M cell cycle arrest in response of DNA damage, whereas lung cells can partially compensate for the loss of CHK2. Our results provide an explanation as to why CHK2 germline mutations predispose for breast cancer but not for lung cancer.

Aneuploidy and a deregulated DNA damage response suggest haploinsufficiency in breast tissues of BRCA2 mutation carriers

Women harboring heterozygous germline mutations of BRCA2 have a 50 to 80% risk of developing breast cancer, yet the pathogenesis of these cancers is poorly understood. To reveal early steps in BRCA2-associated carcinogenesis, we analyzed sorted cell populations from freshly-isolated, non-cancerous breast tissues of BRCA2 mutation carriers and matched controls. Single-cell whole-genome sequencing demonstrates that >25% of BRCA2 carrier (BRCA2^mut/+ ) luminal progenitor (LP) cells exhibit sub-chromosomal copy number variations, which are rarely observed in non-carriers. Correspondingly, primary BRCA2^mut/+ breast epithelia exhibit DNA damage together with attenuated replication checkpoint and apoptotic responses, and an age-associated expansion of the LP compartment. We provide evidence that these phenotypes do not require loss of the wild-type BRCA2 allele. Collectively, our findings suggest that BRCA2 haploinsufficiency and associated DNA damage precede histologic abnormalities in vivo. Using these hallmarks of cancer predisposition will yield unanticipated opportunities for improved risk assessment and prevention strategies in high-risk patients.