A damaged DNA binding protein 2 mutation disrupting interaction with proliferating-cell nuclear antigen affects DNA repair and confers proliferation advantage

Epithelial-to-mesenchymal transition and NF-kB pathways are promoted by a mutant form of DDB2, unable to bind PCNA, in UV-damaged human cells

BACKGROUND

DNA-Damaged Binding protein 2 (DDB2) is a protein involved in the early step of Nucleotide Excision Repair. Recently, it has been reported that DDB2 is involved in epithelial-to-mesenchymal transition (EMT), key process in tumour invasiveness and metastasis formation. However, its role is not completely known.

METHODS

Boyden chamber and cell adhesion assays, and ICELLigence analysis were performed to detect HEK293 adhesion and invasion. Western blotting and gelatine zymography techniques were employed to assess the EMT protein levels and MMP enzymatic activity. Immunofluorescence analysis and pull-down assays facilitated the detection of NF-kB sub-cellular localization and interaction.

RESULTS

We have previously demonstrated that the loss of DDB2-PCNA binding favours genome instability, and increases cell proliferation and motility. Here, we have investigated the phenotypic and molecular EMT-like changes after UV DNA damage, in HEK293 clones stably expressing DDB2Wt protein or a mutant form unable to interact with PCNA (DDB2PCNA-), as well as in HeLa cells transiently expressing the same DDB2 constructs. Cells expressing DDB2PCNA- showed morphological modifications along with a reduced expression of E-cadherin, an increased activity of MMP-9 and an improved ability to migrate, in concomitance with a significant upregulation of EMT-associated Transcription Factors (TFs), whose expression has been reported to favour tumour invasion. We observed a higher expression of c-Myc oncogene, NF-kB, both regulating cell proliferation and metastatic process, as well as ZEB1, a TF significantly associated with tumorigenic potential and cell migratory ability. Interestingly, a novel interaction of DDB2 with NF-kB was detected and found to be increased in cells expressing the DDB2PCNA-, suggesting a direct modulation of NF-kB by DDB2.

CONCLUSION

These results highlight the role of DDB2-PCNA interaction in counteracting EMT since DDB2PCNA- protein induces in HEK293 transformed cells a gain of function contributing to the acquisition of a more aggressive phenotype.

Revisiting the Function of p21CDKN1A in DNA Repair: The Influence of Protein Interactions and Stability

The p21CDKN1A protein is an important player in the maintenance of genome stability through its function as a cyclin-dependent kinase inhibitor, leading to cell-cycle arrest after genotoxic damage. In the DNA damage response, p21 interacts with specific proteins to integrate cell-cycle arrest with processes such as transcription, apoptosis, DNA repair, and cell motility. By associating with Proliferating Cell Nuclear Antigen (PCNA), the master of DNA replication, p21 is able to inhibit DNA synthesis. However, to avoid conflicts with this process, p21 protein levels are finely regulated by pathways of proteasomal degradation during the S phase, and in all the phases of the cell cycle, after DNA damage. Several lines of evidence have indicated that p21 is required for the efficient repair of different types of genotoxic lesions and, more recently, that p21 regulates DNA replication fork speed. Therefore, whether p21 is an inhibitor, or rather a regulator, of DNA replication and repair needs to be re-evaluated in light of these findings. In this review, we will discuss the lines of evidence describing how p21 is involved in DNA repair and will focus on the influence of protein interactions and p21 stability on the efficiency of DNA repair mechanisms.

An Integrated Analysis of the Response of Colorectal Adenocarcinoma Caco-2 Cells to X-Ray Exposure

Colorectal cancer is among the three top cancer types for incidence and the second in terms of mortality, usually managed with surgery, chemotherapy and radiotherapy. In particular, radiotherapeutic concepts are crucial for the management of advanced rectal cancer, but patients’ survival remains poor, despite advances in treatment modalities. The use of well-characterized in vitro cell culture systems offers an important preclinical strategy to study mechanisms at the basis of cell response to therapeutic agents, including ionizing radiation, possibly leading to a better understanding of the in vivo response to the treatment. In this context, we present an integrated analysis of results obtained in an extensive measurement campaign of radiation effects on Caco-2 cells, derived from human colorectal adenocarcinoma. Cells were exposed to X-rays with doses up to 10 Gy from a radiotherapy accelerator. We measured a variety of endpoints at different post-irradiation times: clonogenic survival after ~ 2 weeks; cell cycle distribution, cell death, frequency of micronucleated cells and atypical mitoses, activation of matrix metalloproteases (MMPs) and of different proteins involved in DNA damage response and cell cycle regulation at earlier time points, up to 48 h post-exposure. Combined techniques of flow cytometry, immunofluorescence microscopy, gelatin zymography and western blotting were used. For selected endpoints, we also addressed the impact of the irradiation protocol, comparing results obtained when cells are plated before irradiation or first-irradiated and then re-plated. Caco-2 resistance to radiation, previously assessed up to 72 h post exposure in terms of cell viability, does not translate into a high clonogenic survival. Survival is not affected by the irradiation protocol, while endpoints measured on a shorter time frame are. Radiation mainly induces a G₂-phase arrest, confirmed by associated molecular markers. The activation of death pathways is dose- and time-dependent, and correlates with a dose-dependent inhibition of MMPs. Genomic aberrations are also found to be dose-dependent. The phosphorylated forms of several proteins involved in cell cycle regulation increase following exposure; the key regulator FoxM1 appears to be downregulated, also leading to inhibition of MMP-2. A unified molecular model of the chain of events initiated by radiation is proposed to interpret all experimental results.

A functional in vitro cell-free system for studying DNA repair in isolated nuclei

Assessment of DNA repair is an important endpoint measurement when studying the biochemical mechanisms of the DNA damage response and when investigating the efficacy of chemotherapy, which often uses DNA-damaging compounds. Numerous in vitro methods to biochemically characterize DNA repair mechanisms have been developed so far. However, such methods have some limitations, which are mainly due to the lack of chromatin organization in the DNA templates used. Here we describe a functional cell-free system to study DNA repair synthesis in vitro, using G1-phase nuclei isolated from human cells treated with different genotoxic agents. Upon incubation in the corresponding damage-activated cytosolic extracts, containing biotinylated dUTP, nuclei were able to initiate DNA repair synthesis. The use of specific DNA synthesis inhibitors markedly decreased biotinylated dUTP incorporation, indicating the specificity of the repair response. Exogenously added human recombinant PCNA protein, but not the sensors of UV-DNA damage DDB2 and DDB1, stimulated UVC-induced dUTP incorporation. In contrast, a DDB2PCNA- mutant protein, unable to associate with PCNA, interfered with DNA repair synthesis. Given its responsiveness to different types of DNA lesions, this system offers an additional tool to study DNA repair mechanisms.This article has an associated First Person interview with the first author of the paper.

Al-induced proteomics changes in tomato plants over-expressing a glyoxalase I gene

Glyoxalase I (Gly I) is the first enzyme in the glutathionine-dependent glyoxalase pathway for detoxification of methylglyoxal (MG) under stress conditions. Transgenic tomato 'Money Maker' plants overexpressing tomato SlGlyI gene (tomato unigene accession SGN-U582631/Solyc09g082120.3.1) were generated and homozygous lines were obtained after four generations of self-pollination. In this study, SlGlyI-overepxressing line (GlyI), wild type (WT, negative control) and plants transformed with empty vector (ECtr, positive control), were subjected to Al-treatment by growing in Magnavaca's nutrient solution (pH 4.5) supplemented with 20 µM Al3+ ion activity. After 30 days of treatments, the fresh and dry weight of shoots and roots of plants from Al-treated conditions decreased significantly compared to the non-treated conditions for all the three lines. When compared across the three lines, root fresh and dry weight of GlyI was significant higher than WT and ECtr, whereas there was no difference in shoot tissues. The basal 5 mm root-tips of GlyI plants expressed a significantly higher level of glyoxalase activity under both non-Al-treated and Al-treated conditions compared to the two control lines. Under Al-treated condition, there was a significant increase in MG content in ECtr and WT lines, but not in GlyI line. Quantitative proteomics analysis using tandem mass tags mass spectrometry identified 4080 quantifiable proteins and 201 Al-induced differentially expressed proteins (DEPs) in root-tip tissues from GlyI, and 4273 proteins and 230 DEPs from ECtr. The Al-down-regulated DEPs were classified into molecular pathways of gene transcription, RNA splicing and protein biosynthesis in both GlyI and ECtr lines. The Al-induced DEPs in GlyI associated with tolerance to Al3+ and MG toxicity are involved in callose degradation, cell wall components (xylan acetylation and pectin degradation), oxidative stress (antioxidants) and turnover of Al-damaged epidermal cells, repair of damaged DNA, epigenetics, gene transcription, and protein translation. A protein-protein association network was constructed to aid the selection of proteins in the same pathway but differentially regulated in GlyI or ECtr lines. Proteomics data are available via ProteomeXchange with identifiers PXD009456 under project title '25Dec2017_Suping_XSexp2_ITAG3.2' for SlGlyI-overexpressing tomato plants and PXD009848 under project title '25Dec2017_Suping_XSexp3_ITAG3.2' for positive control ECtr line transformed with empty vector.

Exploring new potential role of DDB2 by host cell reactivation assay in human tumorigenic cells

BACKGROUND

The Host Cell Reactivation assay (HCR) allows studying the DNA repair capability in different types of human cells. This assay was carried out to assess the ability in removing UV-lesions from DNA, thus verifying NER efficiency. Previously we have shown that DDB2, a protein involved in the Global Genome Repair, interacts directly with PCNA and, in human cells, the loss of this interaction affects DNA repair machinery. In addition, a mutant form unable to interact with PCNA (DDB2PCNA-), has shown a reduced ability to interact with a UV-damaged DNA plasmid in vitro.

METHODS

In this work, we have investigated whether DDB2 protein may influence the repair of a UV-damaged DNA plasmid into the cellular environment by applying the HCR method. To this end, human kidney 293 stable clones, expressing DDB2Wt or DDB2PCNA-, were co-transfected with pmRFP-N2 and UV-irradiated pEGFP-reported plasmids. Moreover, the co-localization between DDB2 proteins and different NER factors recruited at DNA damaged sites was analysed by immunofluorescence and confocal microscopy.

RESULTS

The results have shown that DDB2Wt recognize and repair the UV-induced lesions in plasmidic DNA transfected in the cells, whereas a delay in these processes were observed in the presence of DDB2PCNA-, as also confirmed by the different extent of co-localization of DDB2Wt and some NER proteins (such as XPG), vs the DDB2 mutant form.

CONCLUSION

The HCR confirms itself as a very helpful approach to assess in the cellular context the effect of expressing mutant vs Wt NER proteins on the DNA damage response. Loss of interaction of DDB2 and PCNA affects negatively DNA repair efficiency.