Colcemid inhibits the rejoining of the nucleotide excision repair of UVC-induced DNA damages in Chinese hamster ovary cells

APOBEC3B reporter myeloma cell lines identify DNA damage response pathways leading to APOBEC3B expression

Apolipoprotein B mRNA-editing enzyme catalytic polypeptide-like (APOBEC) DNA cytosine deaminase 3B (A3B) is a DNA editing enzyme which induces genomic DNA mutations in multiple myeloma and in various other cancers. APOBEC family proteins are highly homologous so it is especially difficult to investigate the biology of specifically A3B in cancer cells. To easily and comprehensively investigate A3B function in myeloma cells, we used CRISPR/Cas9 to generate A3B reporter cells that contain 3×FLAG tag and IRES-EGFP sequences integrated at the end of the A3B gene. These reporter cells stably express 3xFLAG tagged A3B and the reporter EGFP and this expression is enhanced by known stimuli, such as PMA. Conversely, shRNA knockdown of A3B decreased EGFP fluorescence and 3xFLAG tagged A3B protein levels. We screened a series of anticancer treatments using these cell lines and identified that most conventional therapies, such as antimetabolites or radiation, exacerbated endogenous A3B expression, but recent molecular targeted therapeutics, including bortezomib, lenalidomide and elotuzumab, did not. Furthermore, chemical inhibition of ATM, ATR and DNA-PK suppressed EGFP expression upon treatment with antimetabolites. These results suggest that DNA damage triggers A3B expression through ATM, ATR and DNA-PK signaling.

Overexpression of PCNA Attenuates Oxidative Stress-Caused Delay of Gap-Filling during Repair of UV-Induced DNA Damage

UVC irradiation-caused DNA lesions are repaired in mammalian cells solely by nucleotide excision repair (NER), which consists of sequential events including initial damage recognition, dual incision of damage site, gap-filling, and ligation. We have previously shown that gap-filling during the repair of UV-induced DNA lesions may be delayed by a subsequent treatment of oxidants or prooxidants such as hydrogen peroxide, flavonoids, and colcemid. We considered the delay as a result of competition for limiting protein/enzyme factor(s) during repair synthesis between NER and base excision repair (BER) induced by the oxidative chemicals. In this report, using colcemid as oxidative stress inducer, we showed that colcemid-caused delay of gap-filling during the repair of UV-induced DNA lesions was attenuated by overexpression of PCNA but not ligase-I. PCNA knockdown, as expected, delayed the gap-filling of NER but also impaired the repair of oxidative DNA damage. Fen-1 knockdown, however, did not affect the repair of oxidative DNA damage, suggesting repair of oxidative DNA damage is not of long patch BER. Furthermore, overexpression of XRCC1 delayed the gap-filling, and presumably increase of XRCC1 pulls PCNA away from gap-filling of NER for BER, consistent with our hypothesis that delay of gap-filling of NER attributes the competition between NER and BER.

Antibiotic amoxicillin induces DNA lesions in mammalian cells possibly via the reactive oxygen species

Amoxicillin is a commonly prescribed drug for anti- bacterial infection. In this study, we are interested in the effect of the drug on the cellular DNA integrity. Amoxicillin was added to the human or hamster cells in culture, and the DNA lesions induced by the drug were assessed by a comet assay with nuclear extract incubation (Wang et al., 2005 Anal Biochem 337: 70-75). Amoxicillin at 5mM rapidly induced DNA lesions in human AGS cells. The level of DNA lesions attained a maximum at about 1h, and then declined steadily and reached almost the basal level at 6h following the drug treatment. Similar induction pattern of DNA lesions was found with amoxicillin-related antibiotics such as ampicillin but not with the unrelated antibiotics such as kanamycin. For studying the repair kinetics, the cells were treated with amoxicillin for only 1h and continued culture in the absence of the drug for a certain period of time before subsequent analysis. Repair of the amoxicillin-induced DNA lesions was essentially completed within 4h. Such repair may not involve nucleotide excision repair (NER) pathway because the repair was completed with similar kinetics in both NER proficient Chinese hamster CHO-K1 cells and its isogenic NER deficient UV24 cells. Instead, the repair may involve base excision repair (BER) pathway because immunodepletion of OGG1/2, glycosylases involved in BER rendered the nuclear extract unable to excise DNA lesions induced by amoxicillin in the modified comet assay. Furthermore, amoxicillin induced intracellular reactive oxygen species (ROS) at the tempo similar to that of DNA lesions induction. Thus, we hypothesize that amoxicillin causes oxidative DNA damage in mammalian cells via ROS.