DNA-dependent protein kinase catalytic subunit (DNA-PKcs) is a member of the phosphatidylinositol 3-kinase related kinase family, which can phosphorylate more than 700 substrates. As the core enzyme, DNA-PKcs forms the active DNA-PK holoenzyme with the Ku80/Ku70 heterodimer to play crucial roles in cellular DNA damage response (DDR). Once DNA double strand breaks (DSBs) occur in the cells, DNA-PKcs is promptly recruited into damage sites and activated. DNA-PKcs is auto-phosphorylated and phosphorylated by Ataxia-Telangiectasia Mutated at multiple sites, and phosphorylates other targets, participating in a series of DDR and repair processes, which determine the cells’ fates: DSBs NHEJ repair and pathway choice, replication stress response, cell cycle checkpoints, telomeres length maintenance, senescence, autophagy, etc. Due to the special and multi-faceted roles of DNA-PKcs in the cellular responses to DNA damage, it is important to precisely regulate the formation and dynamic of its functional complex and activities for guarding genomic stability. On the other hand, targeting DNA-PKcs has been considered as a promising strategy of exploring novel radiosensitizers and killing agents of cancer cells. Combining DNA-PKcs inhibitors with radiotherapy can effectively enhance the efficacy of radiotherapy, offering more possibilities for cancer therapy.
Background An increasing number of studies have recently reported that microRNAs packaged in exosomes contribute to multiple biological processes such as cancer progression; however, little is known about their role in the development of radiation-induced bystander effects. Methods The exosomes were isolated from the culture medium of BEP2D cells with or without γ-ray irradiation by ultracentrifugation. To monitor DNA damage and repair efficiency, the DNA double-strand break biomarker 53BP1 foci, comet, micronuclei, expression of DNA repair genes and NHEJ repair activity were detected. The miR-1246 targeting sequence of the DNA ligase 4 ( LIG4 ) mRNA 3′UTR was assessed by luciferase reporter vectors. Results miR-1246 was increased in exosomes secreted from 2 Gy-irradiated BEP2D cells and inhibited the proliferation of nonirradiated cells. The miR-1246 mimic, exosomes from irradiated cells, and radiation-conditioned cell culture medium increased the yields of 53BP1 foci, comet tail and micronuclei in nonirradiated cells, and decreased NHEJ efficiency. miR-1246 downregulated LIG4 expression by directly targeting its 3′UTR. Conclusions Our findings demonstrate that miR-1246 packaged in exosomes could act as a transfer messenger and contribute to DNA damage by directly repressing the LIG4 gene. Exosomal miR-1246 may be a critical predictor of and player in radiation-induced bystander DNA damage.
End resection of DNA double-strand breaks (DSBs) to form 3′ single-strand DNA (ssDNA) is critical to initiate the homologous recombination (HR) pathway of DSB repair. HR pathway is strictly limited in the G1-phase cells because of lack of homologous DNA as the templates. Exonuclease 1 (EXO1) is the key molecule responsible for 3′ ssDNA formation of DSB end resection. We revealed that EXO1 is inactivated in G1-phase cells via ubiquitination-mediated degradation, resulting from an elevated expression level of RING-box protein 1 (RBX1) in G1 phase. The increased RBX1 significantly prompted the neddylation of Cullin1 and contributed to the G1 phase-specific degradation of EXO1. Knockdown of RBX1 remarkedly attenuated the degradation of EXO1 and increased the end resection and HR activity in γ-irradiated G1-phase cells, as demonstrated by the increased formation of RPA32, BrdU, and RAD51 foci. And EXO1 depletion mitigated DNA repair defects due to RBX1 reduction. Moreover, increased autophosphorylation of DNA-PKcs at S2056 was found to be responsible for the higher expression level of the RBX1 in the G1 phase. Inactivation of DNA-PKcs decreased RBX1 expression, and simultaneously increased EXO1 expression and DSB end resection in G1-phase cells. This study demonstrates a new mechanism for restraining the HR pathway of DNA DSB repair in G1 phase via RBX1-prompted inactivation of EXO1.
Abstract. Colon cancer stem cells (CSCs), which are highly capable of self-renewal and proliferation, are involved in colon tumorigenesis and response to therapy. CD133 is considered the most robust surface marker for colorectal cancer stem cells. Although the TP53 gene is frequently mutated in colon cancer, it remains not fully understood whether and how tumor protein p53 (p53) is associated with CD133 expression in colon cancer cells. In the present study, the expression of the CSC biomarker CD133 was investigated in terms of p53 status in colorectal carcinoma HCT116 cells. p53 wild-type HCT116 (HCT116 p53 IntroductionColorectal carcinoma (CRC), which is prone to metastasis and recurrence, is a cancer with a high lethality rate worldwide. More than 50% of patients will develop metastasis and recurrence (1,2). Multiple factors account for metastasis and recurrence, including tumor stage, cancer subtypes and cancer stem cells (CSCs). CSCs are a small population of tumor-initiating cells that have the ability to self-renew and differentiate in tumors in vivo. CSCs are involved in various processes during tumor formation, progression, and angiogenesis and are considered an important target for novel cancer treatment strategies (3). Several specific surface markers are expressed on CSCs, including CD133, CD44 and aldehyde dehydrogenase 1 (ALDH1) (4). CD133 (also known as prominin-1) is a 5-transmembrane glycoprotein of 865 amino acids with a total molecular weight of 120 kDa. CD133 is a CSC marker in colon carcinoma. CD133-positive cells correlate strongly with poor prognosis and synchronous liver metastasis (5). Cancer cell populations with high expression of CD133 are much more aggressive in terms of metastases compared with those with low expression of CD133, suggesting that CD133 may be a marker of increased tumorigenesis ability. CD133-positive cells from clinical biopsy-derived cultures have been demonstrated to possess multilineage differentiation potential and are capable of tumor initiation in vivo. CD133-positive CRC cells are more resistant to chemoradiotherapy, and CD133 expression is associated with poor prognosis (6,7). However, the regulation of CD133 expression has not been fully elucidated.The present study demonstrated that CD133 expression was associated with the tumor protein p53 (p53) expression in an HCT116 p53 +/+ cell line. Of note, CD133-negative cells were detected in the colon cancer cell line HCT116 in a previous report by Kai et al (8
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