Induction of NUPR1 and AP‑1 contributes to the carcinogenic potential of nickel

Nickel (Ni) is carcinogenic to humans, and causes cancers of the lung, nasal cavity, and paranasal sinuses. The primary mechanisms of Ni‑mediated carcinogenesis involve the epigenetic reprogramming of cells and the ability for Ni to mimic hypoxia. However, the exact mechanisms of carcinogenesis related to Ni are obscure. Nuclear protein 1 (NUPR1) is a stress‑response gene overexpressed in cancers, and is capable of conferring chemotherapeutic resistance. Likewise, activator protein 1 (AP‑1) is highly responsive to environmental signals, and has been associated with cancer development. In this study, NUPR1 was found to be rapidly and highly induced in human bronchial epithelial (BEAS‑2B) cells exposed to Ni, and was overexpressed in Ni‑transformed BEAS‑2B cells. Similarly, AP‑1 subunits, JUN and FOS, were induced in BEAS‑2B cells following Ni exposure. Knockdown of JUN or FOS was found to significantly suppress NUPR1 induction following Ni exposure, demonstrating their importance in NUPR1 transactivation. Reactive oxygen species (ROS) are known to induce AP‑1, and Ni has been shown to produce ROS. Treatment of BEAS‑2B cells with antioxidants was unable to prevent NUPR1 induction by Ni, suggesting that NUPR1 induction by Ni relies on mechanisms other than oxidative stress. To determine how NUPR1 is transcriptionally regulated following Ni exposure, the NUPR1 promoter was cloned and inserted into a luciferase gene reporter vector. Multiple JUN binding sites reside within the NUPR1 promoter, and upon deleting a JUN binding site in the upstream most region within the NUPR1 promoter using site‑directed mutagenesis, NUPR1 promoter activity was significantly reduced. This suggests that AP‑1 transcriptionally regulates NUPR1. Moreover, knockdown of NUPR1 significantly reduced colony formation and anchorage‑independent growth in Ni‑transformed BEAS‑2B cells. Therefore, these results collectively demonstrate a novel mechanism of NUPR1 induction following Ni exposure, and provide a molecular basis by which NUPR1 may contribute to lung carcinogenesis.

COM1, a factor of alternative non-homologous end joining, lagging behind the classic non-homologous end joining pathway in rice somatic cells

The role of rice (Oryza sativa) COM1 in meiotic homologous recombination (HR) is well understood, but its part in somatic double-stranded break (DSB) repair remains unclear. Here, we show that for rice plants COM1 conferred tolerance against DNA damage caused by the chemicals bleomycin and mitomycin C, while the COM1 mutation did not compromise HR efficiencies and HR factor (RAD51 and RAD51 paralogues) localization to irradiation-induced DSBs. Similar retarded growth at the post-germination stage was observed in the com1-2 mre11 double mutant and the mre11 single mutant, while combined mutations in COM1 with the HR pathway gene (RAD51C) or classic non-homologous end joining (NHEJ) pathway genes (KU70, KU80, and LIG4) caused more phenotypic defects. In response to γ-irradiation, COM1 was loaded normally onto DSBs in the ku70 mutant, but could not be properly loaded in the MRE11^RNAi plant and in the wortmannin-treated wild-type plant. Under non-irradiated conditions, more DSB sites were occupied by factors (MRE11, COM1, and LIG4) than RAD51 paralogues (RAD51B, RAD51C, and XRCC3) in the nucleus of wild-type; protein loading of COM1 and XRCC3 was increased in the ku70 mutant. Therefore, quite differently to its role for HR in meiocytes, rice COM1 specifically acts in an alternative NHEJ pathway in somatic cells, based on the Mre11-Rad50-Nbs1 (MRN) complex and facilitated by PI3K-like kinases. NHEJ factors, not HR factors, preferentially load onto endogenous DSBs, with KU70 restricting DSB localization of COM1 and XRCC3 in plant somatic cells.

ZmCom1 Is Required for Both Mitotic and Meiotic Recombination in Maize

CtIP/Ctp1/Sae2/Com1, a highly conserved protein from yeast to higher eukaryotes, is required for DNA double-strand break repair through homologous recombination (HR). In this study, we identified and characterized the COM1 homolog in maize. The ZmCom1 gene is abundantly expressed in reproductive tissues at meiosis stages. In ZmCom1-deficient plants, meiotic chromosomes are constantly entangled as a formation of multivalents and accompanied with chromosome fragmentation at anaphase I. In addition, the formation of telomere bouquet, homologous pairing and synapsis were disturbed. The immunostaining assay showed that the localization of ASY1 and DSY2 was normal, while ZYP1 signals were severely disrupted in Zmcom1 meiocytes, indicating that ZmCom1 is critically required for the proper SC assembly. Moreover, RAD51 signals were almost completely absent in Zmcom1 meiocytes, implying that COM1 is required for RAD51 loading. Surprisingly, in contrast to the Atcom1 and Oscom1 mutants, Zmcom1 mutant plants exhibited a number of vegetative phenotypes under normal growth condition, which may be partly attributed to mitotic aberrations including chromosomal fragmentation and anaphase bridges. Taken together, our results suggest that although the roles of COM1 in HR process seem to be primarily conserved, the COM1 dysfunction can result in the marked dissimilarity in mitotic and meiotic outcomes in maize compared to Arabidopsis and rice. We suggest that this character may be related to the discrete genome context.