Regulation of expression of N-methylpurine DNA glycosylase in human mammary epithelial cells: role of transcription factor AP-2

Transcriptional dysregulation of base excision repair proteins in breast cancer

Base excision repair (BER) addresses the numerous base lesions and strand breaks induced by exogenous and endogenous stressors daily. The complexity and importance of BER requires careful regulation of basal levels of these proteins and inducible responses following DNA damage. Several reports have noted the dysregulation of BER proteins and defects in BER capacity in cancer. Modulated gene and protein expression of several BER proteins, including APE1, PARP1, POL β, and XRCC1, have been observed in breast cancer. Overexpression of these factors has been associated with chemoresistance and cancer aggressiveness, but the regulatory mechanisms that drive overexpression have not been defined. Here, we review the known transcriptional regulators of these key BER proteins and examine potential mechanisms that may drive overexpression in breast cancer.

ZEB1 promotes inflammation and progression towards inflammation-driven carcinoma through repression of the DNA repair glycosylase MPG in epithelial cells

OBJECTIVE

Chronic inflammation is a risk factor in colorectal cancer (CRC) and reactive oxygen species (ROS) released by the inflamed stroma elicit DNA damage in epithelial cells. We sought to identify new drivers of ulcerative colitis (UC) and inflammatory CRC.

DESIGN

The study uses samples from patients with UC, mouse models of colitis and CRC and mice deficient for the epithelial-to-mesenchymal transition factor ZEB1 and the DNA repair glycosylase N-methyl-purine glycosylase (MPG). Samples were analysed by immunostaining, qRT-PCR, chromatin immunoprecipitation assays, microbiota next-generation sequencing and ROS determination.

RESULTS

ZEB1 was induced in the colonic epithelium of UC and of mouse models of colitis. Compared with wild-type counterparts, Zeb1-deficient mice were partially protected from experimental colitis and, in a model of inflammatory CRC, they developed fewer tumours and exhibited lower levels of DNA damage (8-oxo-dG) and higher expression of MPG. Knockdown of ZEB1 in CRC cells inhibited 8-oxo-dG induction by oxidative stress (H₂O₂) and inflammatory cytokines (interleukin (IL)1β). ZEB1 bound directly to the MPG promoter whose expression inhibited. This molecular mechanism was validated at the genetic level and the crossing of Zeb1-deficient and Mpg-deficient mice reverted the reduced inflammation and tumourigenesis in the former. ZEB1 expression in CRC cells induced ROS and IL1β production by macrophages that, in turn, lowered MPG in CRC cells thus amplifying a positive loop between both cells to promote DNA damage and inhibit DNA repair.

CONCLUSIONS

ZEB1 promotes colitis and inflammatory CRC through the inhibition of MPG in epithelial cells, thus offering new therapeutic strategies to modulate inflammation and inflammatory cancer.

Transcriptional regulation of human DNA repair genes following genotoxic stress: trigger mechanisms, inducible responses and genotoxic adaptation

DNA repair is the first barrier in the defense against genotoxic stress. In recent years, mechanisms that recognize DNA damage and activate DNA repair functions through transcriptional upregulation and post-translational modification were the focus of intensive research. Most DNA repair pathways are complex, involving many proteins working in discrete consecutive steps. Therefore, their balanced expression is important for avoiding erroneous repair that might result from excessive base removal and DNA cleavage. Amelioration of DNA repair requires both a fine-tuned system of lesion recognition and transcription factors that regulate repair genes in a balanced way. Transcriptional upregulation of DNA repair genes by genotoxic stress is counteracted by DNA damage that blocks transcription. Therefore, induction of DNA repair resulting in an adaptive response is only visible through a narrow window of dose. Here, we review transcriptional regulation of DNA repair genes in normal and cancer cells and describe mechanisms of promoter activation following genotoxic exposures through environmental carcinogens and anticancer drugs. The data available to date indicate that 25 DNA repair genes are subject to regulation following genotoxic stress in rodent and human cells, but for only a few of them, the data are solid as to the mechanism, homeostatic regulation and involvement in an adaptive response to genotoxic stress.

The adaptive imbalance in base excision-repair enzymes generates microsatellite instability in chronic inflammation

Chronic infection and associated inflammation are key contributors to human carcinogenesis. Ulcerative colitis (UC) is an oxyradical overload disease and is characterized by free radical stress and colon cancer proneness. Here we examined tissues from noncancerous colons of ulcerative colitis patients to determine (a) the activity of two base excision-repair enzymes, AAG, the major 3-methyladenine DNA glycosylase, and APE1, the major apurinic site endonuclease; and (b) the prevalence of microsatellite instability (MSI). AAG and APE1 were significantly increased in UC colon epithelium undergoing elevated inflammation and MSI was positively correlated with their imbalanced enzymatic activities. These latter results were supported by mechanistic studies using yeast and human cell models in which overexpression of AAG and/or APE1 was associated with frameshift mutations and MSI. Our results are consistent with the hypothesis that the adaptive and imbalanced increase in AAG and APE1 is a novel mechanism contributing to MSI in patients with UC and may extend to chronic inflammatory or other diseases with MSI of unknown etiology.

The adaptive imbalance in base excision-repair enzymes generates microsatellite instability in chronic inflammation

Chronic infection and associated inflammation are key contributors to human carcinogenesis. Ulcerative colitis (UC) is an oxyradical overload disease and is characterized by free radical stress and colon cancer proneness. Here we examined tissues from noncancerous colons of ulcerative colitis patients to determine (a) the activity of two base excision-repair enzymes, AAG, the major 3-methyladenine DNA glycosylase, and APE1, the major apurinic site endonuclease; and (b) the prevalence of microsatellite instability (MSI). AAG and APE1 were significantly increased in UC colon epithelium undergoing elevated inflammation and MSI was positively correlated with their imbalanced enzymatic activities. These latter results were supported by mechanistic studies using yeast and human cell models in which overexpression of AAG and/or APE1 was associated with frameshift mutations and MSI. Our results are consistent with the hypothesis that the adaptive and imbalanced increase in AAG and APE1 is a novel mechanism contributing to MSI in patients with UC and may extend to chronic inflammatory or other diseases with MSI of unknown etiology.

Promoter structure and cell cycle dependent expression of the human methylpurine-DNA glycosylase gene

The methylpurine-DNA glycosylase (MPG) gene coding for human 3-methyladenine (3-meAde)-DNA glycosylase functions in the first step of base excision repair (BER) to remove numerous damaged bases including 3-meGua, ethenoadenine, and hypoxanthine (Hx) in addition to 3-meAde. In this report, we identify the length of the minimal MPG promoter region, demonstrate the involvement of several transcription factors in expression of the MPG gene, and determine the point at which transcription initiates. We also demonstrate that control of MPG expression is linked to MPG activity. To initiate studies on how the MPG functions with the ensemble of BER genes to effect repair, we have investigated the cell cycle control of MPG and other BER genes in normal human cells. Steady-state mRNA levels of MPG, human Nth homologue (NTH), and uracil-DNA glycosylase (UDG), DNA glycosylases, and human AP site-specific endonuclease (APE), an endonuclease incising DNA at abasic sites, are cell cycle dependent. In contrast, expression levels of genes coding for human 8-oxoguanine-DNA glycosylase (OGG1) and TDG DNA glycosylases, and omicron 6-methylguanine-DNA methyltransferase (MGMT) gene, and the RPA4 subunit gene do not vary with cell cycle. These observed cell cycle dependent differences might reflect distinct roles of individual BER proteins in mutation avoidance.