Immunofluorescence Analysis of γ-H2AX Foci in Mammalian Fibroblasts at Different Phases of the Cell Cycle

Phosphopantetheinyl transferase ClbA contributes to the virulence of avian pathogenic Escherichia coli in meningitis infection of mice

Avian pathogenic Escherichia coli (APEC), which has potential zoonotic risk, can cause severe systemic infections such as septicemia and meningitis in poultry. Colibactin is a hybrid non-ribosomal peptide/polyketide secondary metabolite produced by bacteria, which induces double-strand DNA breaks and chromosome instability in eukaryotic cells. ClbA is a 4’-phosphopantetheinyl transferase (PPTase) that is essential for colibactin and plays a role in siderophore synthesis. However, whether ClbA is associated with meningitis development in APEC is unclear. In this study, we abolished the clbA gene in the APEC XM strain, investigated the effect of clbA on colibactin synthesis and evaluated the pathogenic capacity of colibactin on meningitis development. Deletion of clbA reduced DNA damage to cells and hindered the normal synthesis of colibactin. Compared with the mice infected by wild-type APEC XM, the clbA deletion mutant infected mice had significant reduction in a series of characteristics associated with meningitis including clinical symptoms, bacterial loads of blood and brain, disruption of the blood brain barrier and the expression of inflammatory factors in the brain tissue. Complementation of ClbA recovered some APEC XM virulence. We conclude that ClbA is obligatory for the synthesis of colibactin and is responsible for the development of meningitis in mice infected by APEC.

Colibactin in avian pathogenic Escherichia coli contributes to the development of meningitis in a mouse model

Colibactin is synthesized by a 54-kb genomic island, leads to toxicity in eukaryotic cells, and plays a vital role in many diseases, including neonatal sepsis and meningitis. Avian pathogenic Escherichia coli (APEC) is speculated to be an armory of extraintestinal pathogenic Escherichia coli and can be a potential zoonotic bacterium that threatens human and animal health. In this study, the APEC XM meningitis mouse model was successfully established to investigate the effect of colibactin in in vivo infection. The clbH-deletion mutant strain induced lower γ-H2AX expression, no megalocytosis, and no cell cycle arrest in bEnd.3 cells, which showed that the deletion of clbH decreased the production of colibactin in the APEC XM strain. The deletion of clbH did not affect the APEC XM strain’s ability of adhering to and invading bEnd.3 cells. In vitro, the non-colibactin-producing strain displayed significantly lower serum resistance and it also induced a lower level of cytokine mRNA and few disruptions of tight junction proteins in infected bEnd.3 cells. Meningitis did not occur in APEC ΔclbH-infected mice in vivo, who showed fewer clinical symptoms and fewer lesions on radiological and histopathological analyses. Compared with the APEX XM strain, APEC ΔclbH induced lower bacterial colonization in tissues, lower mRNA expression of cytokines in brain tissues, and slight destruction of the brain blood barrier. These results indicate that clbH is a necessary component for the synthesis of genotoxic colibactin, and colibactin is related to the development of meningitis induced by APEC XM.

ClbG in Avian Pathogenic Escherichia coli Contributes to Meningitis Development in a Mouse Model

Colibactin is a complex secondary metabolite that leads to genotoxicity that interferes with the eukaryotic cell cycle. It plays an important role in many diseases, including neonatal mouse sepsis and meningitis. Avian pathogenic Escherichia coli (APEC) is responsible for several diseases in the poultry industry and may threaten human health due to its potential zoonosis. In this study, we confirmed that clbG was necessary for the APEC XM strain to produce colibactin. The deletion of clbG on APEC XM contributed to lowered γH2AX expression, no megalocytosis, and no cell cycle arrest in vitro. None of the 4-week Institute of Cancer Research mice infected with the APEC XM ΔclbG contracted meningitis or displayed weakened clinical symptoms. Fewer histopathological lesions were observed in the APEC XM ΔclbG group. The bacterial colonization of tissues and the relative expression of cytokines (IL-1β, IL-6, and TNF-α) in the brains decreased significantly in the APEC XM ΔclbG group compared to those in the APEC XM group. The tight junction proteins (claudin-5, occludin, and ZO-1) were not significantly destroyed in APEC XM ΔclbG group in vivo and in vitro. In conclusion, clbG is necessary for the synthesis of the genotoxin colibactin and affects the development of APEC meningitis in mice.

Nonhomologous end joining and homologous recombination involved in luteolin-induced DNA damage in DT40 cells

Luteolin (3',4',5,7-tetrahydroxyflavone), a naturally occurring flavonoid, has been shown to have anticancer activity in many types of cancer cell lines. The anticancer capacity of luteolin may be related to its ability to induce DNA double-strand breaks (DSBs). Here, we used DT40 cells to determine whether nonhomologous end joining (NHEJ) and homologous recombination (HR) are involved in the repair mechanism of luteolin-induced DNA damage. Cells defective in Ku70 (an enzyme associated with NHEJ) or Rad54 (an enzyme essential for HR) were hypersensitive and presented more apoptosis in response to luteolin. Moreover, the sensitivity and apoptosis of Ku70^-/- and Rad54^-/- cells were associated with increased DNA damage when the numbers of γ-H2AX foci and chromosomal aberrations (CAs) were compared with those from WT cells. Additionally, after treatment with luteolin, Ku70^-/- cells presented more Top2 covalent cleavage complexes (Top2cc). These results indicated that luteolin induced DSBs in DT40 cells and demonstrated that both NHEJ and HR participated in the repair of luteolin-induced DSBs, which might be related to the inhibition of topoisomerases. These results imply that simultaneous inhibition of NHEJ and HR with luteolin treatment would provide a powerful protocol in cancer chemotherapy.

Cadmium disrupts the DNA damage response by destabilizing RNF168

Cadmium (Cd) is a dispensable element for the human body and is usually considered a carcinogen. Occupational and environmental Cd exposure leads to sustained cellular proliferation in some tissues and tumorigenesis via an unclear mechanism. Here, we evaluated the role of Cd in the DNA damage response (DDR). We found that Cd exposure causes extensive DNA double-strand breaks (DSBs) and prevents accumulation of ubiquitination signals at these sites of DNA damage. Cd treatment compromises 53BP1 and BRCA1 recruitment to DSBs induced by itself or DNA damaging agents and partially inactivates the G2/M checkpoint. Mechanistically, Cd directly binds to the E3 ubiquitin ligase RNF168, induces the ubiquitin-proteasome pathway that mediates RNF168 degradation and suppresses RNF168 ubiquitin-ligase activity in vitro. Our study raises the possibility that Cd may target RNF168 to disrupt proper DSB signaling in cultured cells. This pathway may represent a novel mechanism for carcinogenesis induced by Cd.

Brca1 is involved in tolerance to adefovir dipivoxil‑induced DNA damage

Nucleos(t)ide analogues (NAs) are currently the most important anti‑viral treatment option for patients with chronic hepatitis B (CHB). Adefovir dipivoxil (ADV), a diester pro‑drug of adefovir, has been widely used for the clinical therapy of hepatitis B virus infection. It has been previously reported that adefovir induced chromosomal aberrations (CAs) in the in vitro human peripheral blood lymphocyte assay, while the genotoxic mechanism remains elusive. To evaluate the possible mechanisms, the genotoxic effects of ADV on the TK6 and DT40 cell lines, as well as DNA repair‑deficient variants of DT40 cells, were assessed in the present study. A karyotype assay revealed ADV‑induced CAs, particularly chromosomal breaks, in wild‑type DT40 and TK6 cells. A γ‑H2AX foci formation assay confirmed the presence of DNA damage following treatment with ADV. Furthermore, Brca1‑/‑ DT40 cells exhibited an increased sensitivity to ADV, while the knockdown of various other DNA damage‑associated genes did not markedly affect the sensitivity. These comprehensive genetic studies identified the genotoxic capacity of ADV and suggested that Brca1 may be involved in the tolerance of ADV‑induced DNA damage. These results may contribute to the development of novel drugs against CHB with higher therapeutic efficacy and less genotoxicity.