Feline XLF accumulates at DNA damage sites in a Ku-dependent manner

Molecular cloning, subcellular localization, and rapid recruitment to DNA damage sites of chicken Ku70

Ku70 is a multifunctional protein with pivotal roles in DNA repair via non-homologous end-joining, V(D)J recombination, telomere maintenance, and neuronal apoptosis control. Nonetheless, its regulatory mechanisms remain elusive. Chicken Ku70 (GdKu70) cDNA has been previously cloned, and DT40 cells expressing it have significantly contributed to critical biological discoveries. GdKu70 features an additional 18 amino acids at its N-terminus compared to mammalian Ku70, the biological significance of which remains uncertain. Here, we show that the 5' flanking sequence of GdKu70 cDNA is not nearly encoded in the chicken genome. Notably, these 18 amino acids result from fusion events involving the NFE2L1 gene on chromosome 27 and the Ku70 gene on chromosome 1. Through experiments using newly cloned chicken Ku70 cDNA and specific antibodies, we demonstrated that Ku70 localizes within the cell nucleus as a heterodimer with Ku80 and promptly accumulates at DNA damage sites following injury. This suggests that the functions and spatiotemporal regulatory mechanisms of Ku70 in chickens closely resemble those in mammals. The insights and resources acquired will contribute to elucidate the various mechanisms by which Ku functions. Meanwhile, caution is advised when interpreting the previous numerous key studies that relied on GdKu70 cDNA and its expressing cells.

Feline XRCC4 undergoes rapid Ku-dependent recruitment to DNA damage sites

Radiation and chemotherapy resistance remain some of the greatest challenges in human and veterinary cancer therapies. XRCC4, an essential molecule for nonhomologous end joining repair, is a promising target for radiosensitizers. Genetic variants and mutations of XRCC4 contribute to cancer susceptibility, and XRCC4 is also the causative gene of microcephalic primordial dwarfism (MPD) in humans. The development of clinically effective molecular‐targeted drugs requires accurate understanding of the functions and regulatory mechanisms of XRCC4. In this study, we cloned and sequenced the cDNA of feline XRCC4. Comparative analysis indicated that sequences and post‐translational modification sites that are predicted to be involved in regulating the localization of human XRCC4, including the nuclear localization signal, are mostly conserved in feline XRCC4. All examined target amino acids responsible for human MPD are completely conserved in feline XRCC4. Furthermore, we found that the localization of feline XRCC4 dynamically changes during the cell cycle. Soon after irradiation, feline XRCC4 accumulated at laser‐induced DNA double‐strand break (DSB) sites in both the interphase and mitotic phase, and this accumulation was dependent on the presence of Ku. Additionally, XRCC4 superfamily proteins XLF and PAXX accumulated at the DSB sites. Collectively, these findings suggest that mechanisms regulating the spatiotemporal localization of XRCC4 are crucial for XRCC4 function in humans and cats. Our findings contribute to elucidating the functions of XRCC4 and the role of abnormal XRCC4 in diseases, including cancers and MPD, and may help in developing XRCC4‐targeted drugs, such as radiosensitizers, for humans and cats.

Inhibition of Crandell-Rees Feline Kidney cell proliferation by X-ray-induced senescence

Radioresistance and radiotoxicity have been reported following cancer treatments in felines. Optimizing radiation doses to induce cytotoxic effects to only cancer cells and not normal cells is critical in achieving effective radiation therapy; however, the mechanisms of radiation resistance, radiotoxicity, and DNA damage response (DDR) in feline cells have not yet been elucidated. A DNA double-strand break (DSB) is the most toxic type of DNA damage induced by X-rays and heavy ion beams used in treating cancers. Crandell-Rees Feline Kidney (CRFK) cells is one of the most widely used cat cells in life science research. Here, we report that DSB-triggered senescence induced by X-rays is important in inhibiting the proliferation of CRFK cells. We demonstrated through cell proliferation assay that X-rays at doses 2 Gy and 10 Gy are toxic to CRFK cells that irradiating CRFK cells inhibits their proliferation. In X-irradiated CRFK cells, a dose-dependent increase in DSB-triggered senescence was detected according to morphological changes and using senescence-associated β galactosidase staining assay. Moreover, our data indicated that in CRFK cells, the major DDR pathway, which involves the phosphorylation of H2AX at Ser139, was normally activated by ATM kinases. Our findings are useful in the understanding of X-rays-induced cellular senescence and in elucidating biological effects of radiation, e.g., toxicity, in feline cells. Furthermore, our findings suggest that the CRFK cell line is an excellent matrix for elucidating radioresistance and radiotoxicity in cat cells.