The dark side of activation-induced cytidine deaminase: relationship with leukemia and beyond

Mutagenic Activity of AID/APOBEC Deaminases in Antiviral Defense and Carcinogenesis

Proteins of the AID/APOBEC family are capable of cytidine deamination in nucleic acids forming uracil. These enzymes are involved in mRNA editing, protection against viruses, the introduction of point mutations into DNA during somatic hypermutation, and antibody isotype switching. Since these deaminases, especially AID, are potent mutagens, their expression, activity, and specificity are regulated by several intracellular mechanisms. In this review, we discuss the mechanisms of impaired expression and activation of AID/APOBEC proteins in human tumors and their role in carcinogenesis and tumor progression. Also, the diagnostic and potential therapeutic value of increased expression of AID/APOBEC in different types of tumors is analyzed. We assume that in the case of solid tumors, increased expression of endogenous deaminases can serve as a marker of response to immunotherapy since multiple point mutations in host DNA could lead to amino acid substitutions in tumor proteins and thereby increase the frequency of neoepitopes.

Risk of carcinogenesis in the biliary epithelium of children with congenital biliary dilatation through epigenetic and genetic regulation

PURPOSES

Congenital biliary dilatation (CBD), defined as pancreaticobiliary maljunction (PBM) with biliary dilatation, is a high risk factor for biliary tract cancer (BTC). KRAS and p53 mutations reportedly affect this process, but the mechanisms are unclear, as is the likelihood of BTC later in life in children with CBD. We investigated potential carcinogenetic pathways in children with CBD compared with adults.

METHODS

The subjects of this study were nine children with CBD and 13 adults with PBM (10 dilated, 3 non-dilated) without BTC who underwent extrahepatic bile duct resections, as well as four control patients who underwent pancreaticoduodenectomy for non-biliary cancer. We evaluated expressions of Ki-67, KRAS, p53, histone deacetylase (HDAC) and activation-induced cytidine deaminase (AID) in the biliary tract epithelium immunohistochemically.

RESULTS

The Ki-67 labeling index (LI) and expressions of KRAS, p53, HDAC, and AID in the gallbladder epithelium were significantly higher or tended to be higher in both the children with CBD and the adults with PBM than in the controls.

CONCLUSIONS

BTC may develop later in children with CBD and in adults with PBM, via HDAC and AID expression and through epigenetic and genetic regulation.

Stepwise generation of AID knock-in and conditional knockout mice from a single gene-targeting event

Activation-induced cytidine deaminase (AID) encoded by the Aicda gene initiates class-switch recombination and somatic hypermutation of immunoglobulin genes. In addition to this function, AID is also implicated in the epigenetic regulation in pluripotent stem cells and in the oncogenesis of lymphoid and non-lymphoid origins. To examine AID's role in specific cell types, we developed mouse strains of conditional knockout (Aicda-FL) and knock-in with a red fluorescent protein gene (RFP) inserted into the Aicda locus (Aicda-RFP). These two strains were obtained from a single targeting event in embryonic stem cells by a three-loxP or tri-lox strategy. Partial and complete recombination among the three loxP sites in the Aicda-RFP locus gave rise to Aicda-FL and AID-deficient loci (Aicda-KO), respectively, after mating Aicda-RFP mice with Cre-expressing mice driven by tissue-non-specific alkaline phosphate promoter. We confirmed RFP expression in B cells of germinal centers of intestine-associated lymphoid tissue. Mice homozygous for each allele were obtained and were checked for AID activity by class-switch and hypermutation assays. AID activity was normal for Aicda-FL but partially and completely absent for Aicda-RFP and Aicda-KO, respectively. Aicda-FL and Aicda-RFP mice would be useful for studying AID function in subpopulations of B cells and in non-lymphoid cells.

Chronic lung injury by constitutive expression of activation-induced cytidine deaminase leads to focal mucous cell metaplasia and cancer

Activation-induced cytidine deaminase (AID) is an enzyme required for antibody diversification, and it causes DNA mutations and strand breaks. Constitutive AID expression in mice invariably caused lung lesions morphologically similar to human atypical adenomatous hyperplasia (AAH), which can be a precursor of bronchioloalveolar carcinoma. Similar to AAH, mouse AAH-like lesion (MALL) exhibited signs of alveolar differentiation, judging from the expression of alveolar type II (AT2) cell marker surfactant protein C (SP-C). However, electron microscopy indicated that MALL, which possessed certain features of a mucous cell, is distinct from an AAH or AT2 cell. Although MALL developed in all individuals within 30 weeks after birth, lung tumors occurred in only 10%; this suggests that the vast majority of MALLs fail to grow into visible tumors. MALL expressed several recently described markers of lung alveolar regeneration such as p63, keratin 5, keratin 14, leucine-rich repeat containing G protein-coupled receptor 5 (Lgr5), and Lgr6. Increased cell death was observed in the lungs of AID transgenic mice compared with wild-type mice. Based on these observations, we speculate that MALL is a regenerating tissue compensating for cellular loss caused by AID cytotoxicity. AID expression in such regenerating tissue should predispose cells to malignant transformation via its mutagenic activity.

Digging deep into "dirty" drugs - modulation of the methylation machinery

DNA methylation and histone modification are epigenetic mechanisms that result in altered gene expression and cellular phenotype. The exact role of methylation in myelodysplastic syndromes (MDS) and acute myeloid leukemia (AML) remains unclear. However, aberrations (e.g. loss-/gain-of-function or up-/down-regulation) in components of epigenetic transcriptional regulation in general, and of the methylation machinery in particular, have been implicated in the pathogenesis of these diseases. In addition, many of these components have been identified as therapeutic targets for patients with MDS/AML, and are also being assessed as potential biomarkers of response or resistance to hypomethylating agents (HMAs). The HMAs 5-azacitidine (AZA) and 2'-deoxy-5-azacitidine (decitabine, DAC) inhibit DNA methylation and have shown significant clinical benefits in patients with myeloid malignancies. Despite being viewed as mechanistically similar drugs, AZA and DAC have differing mechanisms of action. DAC is incorporated 100% into DNA, whereas AZA is incorporated into RNA (80-90%) as well as DNA (10-20%). As such, both drugs inhibit DNA methyltransferases (DNMTs; dependently or independently of DNA replication) resulting in the re-expression of tumor-suppressor genes; however, AZA also has an impact on mRNA and protein metabolism via its inhibition of ribonucleotide reductase, resulting in apoptosis. Herein, we first give an overview of transcriptional regulation, including DNA methylation, post-translational histone-tail modifications, the role of micro-RNA and long-range epigenetic gene silencing. We place special emphasis on epigenetic transcriptional regulation and discuss the implication of various components in the pathogenesis of MDS/AML, their potential as therapeutic targets, and their therapeutic modulation by HMAs and other substances (if known). The main focus of this review is laid on dissecting the rapidly evolving knowledge of AZA and DAC with a special focus on their differing mechanisms of action, and the effect of HMAs on transcriptional regulation.

AID downregulation is a novel function of the DNMT inhibitor 5-aza-deoxycytidine

Activation-induced cytidine deaminase (AID) was originally identified as an inducer of somatic hypermutation (SHM) and class switch recombination (CSR) in immunoglobulin genes. However, AID can also cause mutations in host genes and contribute to cancer progression and drug resistance. In this study, molecular docking showed the interaction of free 5-aza-CdR and Zebularine (Zeb) with AID. However, only 5-aza-CdR-incorporated ssDNA bound to the active site of AID and inhibited AID expression through proteasomal degradation. 5-aza-CdR demonstrated cytotoxicity against AID-positive and -negative hematopoietic cancer cells. In contrast, Zeb exhibited a cytotoxic effect only in AID-negative cells due to its inability to inhibit AID expression. This differential effect might be due to the DNMT1 stabilization induced by AID, thus restricting the ability of Zeb to deplete DNMT1 and induce tumor suppressor genes (TSGs), such as p21, in AID-positive cells. Moreover, the in vivo anticancer effect of 5-aza-CdR but not Zeb in AID-positive hematopoietic cancer cells was demonstrated. The study not only displays the association of AID and DNMT1 and identifies a novel biological function of AID, but also provides novel information regarding the use of DNMT inhibitors to treat AID-positive hematopoietic cancers.

Role of activation-induced cytidine deaminase in the development of oral squamous cell carcinoma

Purpose

In humans, activation-induced cytidine deaminase (AID) expression results due to inflammation and this deaminase activity is also involved in carcinogenesis. The aim of this study is to investigate the correlation between AID expression and the clinical classification of oral cancer tissues.

Experimental Design

The current study investigated the correlation between AID expression and the clinical classification of oral cancer tissues from 27 patients who underwent surgical resection using immunohistochemistry. Specific AID expression and its induction by cytokine stimulation were investigated in cultured HSC oral cancer cell lines by reverse transcriptase PCR.

Results

AID expression was detected in 10 of 27 specimens (37.0%). AID expression was more frequently detected in early-stage cancer, especially in early stage T, than in late-stage cancer (T1/T2 vs. T3/4; P = 0.0493, N0 vs. N1/2/3; P = 0.0793). HSC-2, a nonmetastatic oral cancer cell line, abundantly expressed endogenous AID, whereas no such expression was observed in HSC-3, a metastatic oral cancer cell line. Moreover, AID expression was substantially induced in HSC-2 cells by stimulation of an inflammation-related cytokine, TNF-α.

Conclusions

Aberrant AID expression in the oral epithelium would contribute to the initiation of oral squamous cell carcinoma. Avoiding persistent AID inducible condition such as frequent cleaning of oral cavity would play an important role for the prevention of developing oral cancer.

Bile acid-induced expression of activation-induced cytidine deaminase during the development of Barrett's oesophageal adenocarcinoma

Activation-induced cytidine deaminase (AID) induces somatic mutations in various host genes of non-lymphoid tissues, thereby contributing to carcinogenesis. We recently demonstrated that Helicobacter pylori infection and/or proinflammatory cytokine stimulation triggers aberrant AID expression in gastric epithelial cells, causing mutations in the tumour-suppressor TP53 gene. The findings of the present study provide evidence of ectopic AID expression in Barrett's oesophagus and Barrett's oesophageal adenocarcinoma, a cancer that develops under chronic inflammatory conditions. Immunoreactivity for endogenous AID was observed in 24 of 28 (85.7%) specimens of the columnar cell-lined Barrett's oesophagus and in 20 of 22 (90.9%) of Barrett's adenocarcinoma, whereas weak or no AID protein expression was detectable in normal squamous epithelial cells of the oesophagus. We validated these results by analysing tissue specimens from another cohort comprising 16 cases with Barrett's oesophagus and four cases with Barrett's adenocarcinoma. In vitro treatment of human non-neoplastic oesophageal squamous-derived cells with sodium salt deoxycholic acid induced ectopic AID expression via the nuclear factor-kappaB activation pathway. These findings suggest that aberrant AID expression occurs in a substantial proportion of Barrett's epithelium, at least in part due to bile acid stimulation. Considering the genotoxic activity of AID, our current findings suggest that aberrant AID expression might enhance the susceptibility to genetic alterations in Barrett's columnar-lined epithelial cells, leading to cancer development.

Aberrant activation-induced cytidine deaminase expression is associated with mucosal intestinalization in the early stage of gastric cancer

Although Helicobacter pylori is a risk factor for gastric cancer (GC), its detailed carcinogenesis remains unclear. Recently, aberrant expression of activation-induced cytidine deaminase (AID) was demonstrated in gastric epithelium with H. pylori infection and seems to cause the accumulation of mutation. This investigation aims to elucidate whether or not AID expression plays an important role in the carcinogenesis of early GC. We examined the correlation between immunohistochemical AID expression and histological characteristics, including pre-existing chronic gastritis and cellular mucin phenotype in 138 cases of intramucosal GC. Furthermore, we investigated the relationship between AID, p53 protein, and β-catenin. The low degree of polymorphonuclear neutrophil activity, and the high degree of glandular atrophy and intestinal metaplasia were significantly correlated with the high levels of AID expression in non-neoplastic mucosa (P = 0.007, P ≤ 0.001, and P = 0.003). With regard to mucin phenotype of carcinoma, the intestinal phenotype tended to have the higher AID expression levels (P = 0.052). AID showed close correlations with Cdx2 and nuclear staining of β-catenin (P = 0.003, P = 0.034). As for p53 protein, no correlation was found with AID expression. Our findings suggest that aberrant AID expression is correlated with persistent inflammatory condition induced by H. pylori infection and may contribute to the development of GC through an inflammatory condition and intestinalization.

Role of activation-induced cytidine deaminase in inflammation-associated cancer development

Human cancer is a genetic disease resulting from the stepwise accumulation of genetic alterations in various tumor-related genes. Normal mutation rates, however, cannot account for the abundant genetic changes accumulated in tumor cells, suggesting that certain molecular mechanisms underlie such a large number of genetic alterations. Activation-induced cytidine deaminase (AID), a nucleotide-editing enzyme that triggers DNA alterations and double-strand DNA breaks in the immunoglobulin gene, has been identified in activated B lymphocytes. Recent studies revealed that AID-mediated genotoxic effects target not only immunoglobulin genes but also a variety of other genes in both B lymphocytes and non-lymphoid cells. Consistent with the finding that several transcription factors including nuclear factor-κB (NF-κB) mediate AID expression in B cells, proinflammatory cytokine stimulation of several types of gastrointestinal epithelial cells, such as gastric, colonic, hepatic, and biliary epithelium, induces aberrant AID expression through the NF-κB signaling pathway. In vivo studies revealed that constitutive AID expression promotes the tumorigenic pathway by enhancing the susceptibility to mutagenesis in a variety of epithelial organs. The activity of AID as a genome mutator provides a new avenue for studies aimed at understanding mutagenesis mechanisms during carcinogenesis.

Involvement of activation-induced cytidine deaminase in the development of colitis-associated colorectal cancers

Chronic inflammatory bowel disease (IBD) is an important etiologic factor in the development of colorectal cancer. However, the mechanism underlying the development of colorectal cancers through chronic inflammation is not known. Activation-induced cytidine deaminase (AID) was originally identified as an inducer of somatic hypermutation in the immunoglobulin gene. We recently found that the mutagenic activity of AID expression links inflammation to the development of cancer. Aberrant AID expression is triggered by hepatitis C virus infection in human hepatocytes or Helicobacter pylori infection in human gastric epithelial cells, and leads to the generation of somatic mutations in various tumor-related genes. Here, we review our findings relating to how AID contributes to the development of colitis-associated colorectal cancers (CACs). Immunohistochemistry revealed the enhanced expression of endogenous AID protein in not only in the inflamed colonic mucosa of ulcerative colitis patients but also CAC tumor lesions. Pro-inflammatory cytokine TNF-α induced strong aberrant expression of AID via IκB kinase-dependent NF-κB-signaling pathways in human colonic epithelial cells. Furthermore, AID expression was also elicited in response to the T helper cell-2-driven cytokines IL-4 and IL-13, which are activated in human IBD. Aberrant activation of AID in colonic cells preferentially evoked genetic mutations in the TP53 gene, whereas there were no nucleotide alterations of the APC gene. These findings suggested that pro-inflammatory cytokine-mediated aberrant expression of AID in colonic epithelial cells plays a role as a genotoxic factor that enhances genetic instability during chronic colonic inflammation, leading to CAC development.

Up-regulation of activation-induced cytidine deaminase causes genetic aberrations at the CDKN2b-CDKN2a in gastric cancer

BACKGROUND & AIMS

The DNA/RNA editing enzyme activation-induced cytidine deaminase (AID) is mutagenic and has been implicated in human tumorigenesis. Helicobacter pylori infection of gastric epithelial cells leads to aberrant expression of AID and somatic gene mutations. We investigated whether AID induces genetic aberrations at specific chromosomal loci that encode tumor-related proteins in gastric epithelial cells.

METHODS

Human gastric epithelial cell lines that express activated AID and gastric cells from AID transgenic mice were examined for DNA copy number changes and nucleotide alterations. Copy number aberrations in stomach cells of H pylori-infected mice and gastric tissues (normal and tumor) from H pylori-positive patients were also analyzed.

RESULTS

In human gastric cells, aberrant AID activity induced copy number changes at various chromosomal loci. In AID-expressing cells and gastric mucosa of AID transgenic mice, point mutations and reductions in copy number were observed frequently in the tumor suppressor genes CDKN2A and CDKN2B. Oral infection of wild-type mice with H pylori reduced the copy number of the Cdkn2b-Cdkn2a locus, whereas no such changes were observed in the gastric mucosa of H pylori-infected AID-deficient mice. In human samples, the relative copy numbers of CDKN2A and CDKN2B were reduced in a subset of gastric cancer tissues compared with the surrounding noncancerous region.

CONCLUSIONS

H pylori infection leads to aberrant expression of AID and might be a mechanism of the accumulation of submicroscopic deletions and somatic mutations in gastric epithelial cells. AID-mediated genotoxic effects appear to occur frequently at the CDKN2b-CDKN2a locus and contribute to malignant transformation of the gastric mucosa.

Helicobacter pylori-induced activation-induced cytidine deaminase expression and carcinogenesis

Tumorigenesis is a multistep process in which the accumulation of genetic alterations drives the transformation of normal cells into malignant derivatives. Activation-induced cytidine deaminase (AID) contributes to immune system diversity by inducing somatic hypermutations and class-switch recombinations of human immunoglobulin genes. The mutagenic activity of AID, however, can also induce genetic changes in various genes and may lead to the development of cancer. Helicobacter pylori, a class 1 carcinogen for human gastric cancer, affects AID expression by two different mechanisms, introduction of bacterial virulence factors into host cells and induction of inflammatory responses, thereby contributing to the accumulation of mutations in tumor-related genes. Aberrant AID activity may therefore be a novel link between infection and carcinogenesis.

Role of microRNAs in HTLV-1 infection and transformation

Human T-cell leukemia virus type 1 (HTLV-1), a retrovirus that infects more than 20 million people worldwide, is the etiological agent of ATLL (adult T-cell leukemia/lymphoma), an aggressive leukemia of CD4+ T lymphocytes which arises in a small percentage of infected individuals after a long clinical latency. Tumor emergence is attributed primarily to the oncogenic activity of the viral protein Tax, which drives the expression of viral transcripts and controls the expression and function of a broad variety of host-cell genes involved in proliferation, genetic stability and apoptosis. Nevertheless, many aspects of HTLV-1 replication, persistence and pathogenesis remain to be understood. The emerging role of microRNAs in tumor development and viral infection has prompted investigations on the interactions between HTLV-1 and the microRNA regulatory network. In the present review we discuss recent data demonstrating changes in cellular microRNA expression in HTLV-1-infected cell lines and ATLL cells, and the functional impact of a subset microRNAs deregulated by HTLV-1 on cellular gene expression and signal transduction pathways. Mechanisms through which the viral proteins may influence microRNA expression are discussed. Results of searches for potential cellular microRNAs that target viral transcripts and for microRNAs produced by HTLV-1 are described. Observations along with regarding the expression of tRNA-derived small regulatory RNAs in HTLV-1-infected cells are presented.

Organ-specific profiles of genetic changes in cancers caused by activation-induced cytidine deaminase expression

Various molecular changes characterizing organ-specific carcinogenesis have been identified in human tumors; however, the molecular mechanisms of the genomic changes specific for each cancer are not well defined. A transgenic (Tg) mouse model with constitutive expression of the nucleotide-editing enzyme, activation-induced cytidine deaminase (AID), develops tumors in various organs as a result of the mutagenic activities of AID. This phenotypic character of AID Tg mice allowed us to analyze the organ-specific genetic changes in tumor-related genes commonly triggered by AID-mediated mutagenesis. Among the 80 AID Tg mice analyzed, 11 mice developed hepatocellular carcinomas, and 7 developed lung cancers. In addition, 1 developed the gastric cancer and 3 developed gastric adenomas. Organ-specific preferences for nucleotide changes were observed in some of the tumor-related genes in each epithelial tissue of the AID Tg mice. Of note, the c-myc and K-ras genes were the preferential targets of the mutagenic activity of AID in lung and stomach cancers, respectively, whereas mutations in the p53 and beta-catenin genes were commonly observed in all 3 organs. Quantitative RT-PCR analyses revealed that alpha-fetoprotein, insulin-like growth factor-2 and cyclin D1 genes were specifically upregulated in HCC, whereas upregulation of the matrix metalloproteinase-7 gene was more marked in lung cancer. Our findings suggest that AID, a DNA mutator that plays a critical role linking inflammation to human cancers, might be involved in the generation of organ-specific genetic diversity in oncogenic pathways during cancer development.

A novel mouse model of hepatocarcinogenesis triggered by AID causing deleterious p53 mutations

Activation-induced cytidine deaminase (AID), the only enzyme that is known to be able to induce mutations in the human genome, is required for somatic hypermutation and class-switch recombination in B lymphocytes. Recently, we showed that AID is implicated in the pathogenesis of human cancers including hepatitis C virus (HCV)-induced human hepatocellular carcinoma (HCC). In this study, we established a new AID transgenic mouse model (TNAP-AID) in which AID is expressed in cells producing tissue-nonspecific alkaline phosphatase (TNAP), which is a marker of primordial germ cells and immature stem cells, including ES cells. High expression of TNAP was found in the liver of the embryos and adults of TNAP-AID mice. HCC developed in 27% of these mice at the age of approximately 90 weeks. The HCC that developed in TNAP-AID mice expressed alpha-fetoprotein and had deleterious mutations in the tumour suppressor gene Trp53, some of which corresponded to those found in human cancer. In conclusion, TNAP-AID is a mouse model that spontaneously develops HCC, sharing genetic and phenotypic features with human HCC, which develops in the inflamed liver as a result of the accumulation of genetic changes.

Activation-induced cytidine deaminase links between inflammation and the development of colitis-associated colorectal cancers

BACKGROUND & AIMS

Activation-induced cytidine deaminase (AID) was originally identified as an inducer of somatic hypermutations in the immunoglobulin gene. We recently revealed that ectopic AID expression serves as a link between the cellular editing machinery and high mutation frequencies, leading to human cancer development. In the current study, we investigated whether AID might contribute to the development of colitis-associated colorectal cancers.

METHODS

The expression and regulation of AID in association with proinflammatory cytokine stimulation were investigated in cultured colonic cells. Genotoxic activity of AID in colonic cells was analyzed using retroviral system. Immunohistochemistry for AID was carried out on various human colonic tissues specimens.

RESULTS

Tumor necrosis factor-alpha induced aberrant AID expression via IkappaB kinase-dependent nuclear factor (NF)-kappaB-signaling pathways in human colonic epithelial cells. Moreover, AID expression was also induced in response to the T helper cell 2-driven cytokines interleukin-4 and interleukin-13, which are activated in human inflammatory bowel disease. Aberrant activation of AID in colonic cells preferentially induced genetic mutations in the TP53 gene, whereas there were no nucleotide alterations of the APC gene. Immunohistochemistry revealed enhanced expression of endogenous AID protein not only in the inflamed colonic mucosa of ulcerative colitis patients but also in tumor lesions of colitis-associated colorectal cancers.

CONCLUSIONS

Our findings indicate that proinflammatory cytokine-mediated aberrant expression of AID in colonic epithelial cells is a genotoxic factor linking inflammation, somatic mutations, and colorectal cancer development.

Activation-induced cytidine deaminase links bile duct inflammation to human cholangiocarcinoma

Chronic inflammation plays a critical role in oncogenesis in various human organs. Epidemiological studies have demonstrated that patients with primary sclerosing cholangitis have a predisposition to develop cholangiocarcinoma (CC). However, the molecular mechanisms that account for the development of bile duct carcinomas are not well defined. We recently provided evidence that activation-induced cytidine deaminase (AID), a member of the DNA/RNA editing enzyme family, is implicated in human tumorigenesis via its mutagenic activity. We found here that ectopic AID production is induced in response to tumor necrosis factor-alpha (TNF-alpha) stimulation via the IkappaB kinase-dependent nuclear factor-kappaB (NF-kappaB) activation pathway in human cholangiocarcinoma-derived cells. Aberrant expression of AID in biliary cells resulted in the generation of somatic mutations in tumor-related genes, including p53, c-myc, and the promoter region of the INK4A/p16 sequences. In human tissue specimens, real-time reverse transcription polymerase chain reaction (RT-PCR) analyses revealed that AID was increased significantly in 28 of 30 CC tissues (93%), whereas only trace amounts of AID were detected in the normal liver. Immunohistochemistry showed that all of the CC tissue samples examined showed overproduction of endogenous AID protein in cancer cells. Moreover, immunostaining for AID was detectable in 16 of 20 bile epithelia in the tissues underlying primary sclerosing cholangitis.

CONCLUSION

The proinflammatory cytokine-induced aberrant production of AID might link bile duct inflammation to an enhanced genetic susceptibility to mutagenesis, leading to cholangiocarcinogenesis.

Expression of activation-induced cytidine deaminase in human hepatocytes via NF-kappaB signaling

Activation-induced cytidine deaminase (AID) is involved in somatic DNA alterations of the immunoglobulin gene for amplification of immune diversity. The fact that constitutive expression of AID in mice causes tumors in various organs, including lymphoid tissues and lungs, suggests the important role of the aberrant editing activity of AID on various tumor-related genes for carcinogenesis. AID expression, however, is restricted to activated B cells under physiological conditions. We demonstrate here that ectopic AID expression is induced in response to tumor necrosis factor-alpha stimulation in cultured human hepatocytes. The proinflammatory cytokine-mediated expression of AID is achieved by IkappaB kinase-dependent nuclear factor (NF)-kappaB signaling pathways. Hepatitis C virus, one of the leading causes of hepatocellular carcinoma (HCC), enhanced AID expression via NF-kappaB activation through expression of viral core protein. The aberrant expression of AID in hepatoma-derived cells resulted in accumulation of genetic alterations in the c-myc and pim1 genes, suggesting that inappropriate expression of AID acts as a DNA mutator that enhances the genetic susceptibility to mutagenesis in human hepatocytes. Our current findings indicate that the inappropriate expression of AID is induced by proinflammatory cytokine stimulation and may provide the link between hepatic inflammation and the development of HCC.

Helicobacter pylori infection triggers aberrant expression of activation-induced cytidine deaminase in gastric epithelium

Infection with Helicobacter pylori (H. pylori) is a risk factor for the development of gastric cancer. Here we show that infection of gastric epithelial cells with 'cag' pathogenicity island (cagPAI)-positive H. pylori induced aberrant expression of activation-induced cytidine deaminase (AID), a member of the cytidine-deaminase family that acts as a DNA- and RNA-editing enzyme, via the IkappaB kinase-dependent nuclear factor-kappaB activation pathway. H. pylori-mediated upregulation of AID resulted in the accumulation of nucleotide alterations in the TP53 tumor suppressor gene in gastric cells in vitro. Our findings provide evidence that aberrant AID expression caused by H. pylori infection might be a mechanism of mutation accumulation in the gastric mucosa during H. pylori-associated gastric carcinogenesis.