Abstract
NF-E2-related factor 2 (NRF2) is a transcription factor that controls the expression of a variety of antioxidant and detoxification genes. Accumulating evidence strongly suggests that NRF2 mediates cancer cell proliferation and drug resistance, as well. Single nucleotide polymorphism (SNP) -617C > A in the anti-oxidant response element-like loci of the human NRF2 gene play a pivotal role in the positive feedback loop of transcriptional activation of the NRF2 gene. Since the SNP (-617A) reportedly decreases the binding affinity to the transcription factors of NRF2/small multiple alignment format (MafK), the homozygous -617A/A allele may attenuate the positive feedback loop of transcriptional activation of the NRF2 gene and reduce the NRF2 protein level. As the consequence, cancer cells are considered to become more sensitive to therapy and less aggressive than cancer cells harboring the -617C (WT) allele. Indeed, Japanese lung cancer patients carrying SNP homozygous alleles (c. -617A/A) exhibited remarkable survival over 1,700 days after surgical operation (log-rank p = 0.021). The genetic polymorphism in the human NRF2 gene is considered as one of prognosis markers for cancer therapy.
INTRODUCTION
HISTORICAL BACKGROUND
In the field of cancer chemotherapy, it has been well documented that glutathione (GSH) plays a pivotal role in conferring cancer cells resistance to anti-tumor drugs, such as cisplatin and alkylating agents. Several lines of evidence suggest that certain multi-drug efflux pumps encoded by ATP-binding cassette (ABC) transporter genes are up-regulated by oxidative stress and/or chemotherapeutic agents to contribute to multi-drug resistance of cancer cells. About two decades ago, Ishikawa and Kuo first reported that many cytotoxic agents induced the expression of both γ-glutamylcysteine synthetase (γ-GCS) and ABCC1 (MRP1) genes (; , ; ; Yamane et al., 1998). Coordinated up-regulation of both γ-GCS and ABCC1 genes was found in human malignant tissues. Among 32 cases of human colorectal cancer biopsies, 78% of the cases exhibited co-elevated expression of ABCC1 and γ-GCS genes in tumor samples as compared with their corresponding adjacent naïve normal samples (). At that time, it was speculated that a common transcriptional regulator might exist for the coordinated expression of both γ-GCS and ABCC1 genes ().
TRANSCRIPTION FACTOR NRF2 AS A MASTER SWITCH IN GENE EXPRESSION
During the past two decades, evidence has accumulated to show that one transcription factor named NF-E2-related factor 2 (NRF2) is a common redox regulator to control cellular adaptation/protection to external stimuli by inducing antioxidant and detoxification genes (; ; ). In fact, NRF2 is a major player in the transcriptional upregulation of many target genes in phase II drug metabolizing enzymes and certain phase III ABC transporters (ABCC2, ABCC3, and ABCG2; ). The 5′-flanking region of many of phase II xenobiotic detoxifying genes (e.g., γ-GCS) contains an antioxidant response element (ARE). NRF2 directly binds to the ARE sequence in those target genes (Shen and Kong, 2009; Singh et al., 2010). Furthermore, it has recently been reported NRF2 mediates cancer cell proliferation and drug resistance (; ; ; Taguchi et al., 2011; Sporn and Liby, 2012; Yamadori et al., 2012; Shelton and Jaiswal, 2013).
NF-E2-related factor 2 is a “cap‘n’collar” basic region-leucine zipper (CNC-bZip) transcription factor involved in the induction of ARE-regulated genes (; ; ; ; ; ; ; Taguchi et al., 2011; Sporn and Liby, 2012; Yamadori et al., 2012). Under non-stressed conditions, NRF2 protein is associated with Kelch-like ECH associating protein 1 (KEAP1; ) that negatively regulate NRF2 by retrieving the NRF2 protein in the cytoplasmic compartment. However, oxidative stress and/or electrophilic attack modifies the KEAP1 protein, which leads to dissociation of NRF2 from KEAP1. The NRF2 protein, thus released, is subsequently translocated into the nucleus. Coupling with small multiple alignment format (MAF) sequences, NRF2 binds to ARE sequences (). Many genes encoding detoxifying and antioxidant enzymes have been found to be regulated by the NRF2 protein in this manner (; ; ; ; ; ; ). Recent studies, on the other hand, have shown that NRF2 contributes to cancer cell proliferation, drug resistance, and metabolic re-programming, as well (; ; ; ; Taguchi et al., 2011; ; Sporn and Liby, 2012; Yamadori et al., 2012). In this context, the NRF2 gene is regarded as a “double-edged sword,” namely, protection of normal cells and progression of cancer malignancy.
GENETIC POLYMORPHYSMS IN THE NRF2 GENE
Yamamoto et al. (2004) first reported three single nucleotide polymorphisms (SNPs; -653A > G, -651G > A, and -617C > A) and one triplet repeat polymorphism in the regulatory region of the human NRF2 gene. The physiological significance of these SNPs was not known at that time. Three years later, reported the impact of those SNPs on the regulation of NRF2 gene expression. In fact, the -617C > A SNP significantly affected basal NRF2 protein levels in vitro (). Moreover, the SNP -617C > A was found to be associated with a higher risk of oxidant-induced acute lung injury in humans (). These findings suggest that the SNP (-617C > A) in the ARE-like loci of the human NRF2 gene is important for self-induction of the NRF2 gene (); refer to schematic illustrations in Figure 1.
FIGURE 1
SNP (–617C > A) IN THE NRF2 GENE AS A BIOMARKER FOR PROGNOSIS OF LUNG CANCER
NF-E2-related factor 2 plays a pivotal role in protecting normal cells from external toxic challenges and oxidative stress, whereas it can modulate the cancer phenotype (Figure 1A) and also endow cancer cells resistance to anticancer drugs (Figure 1B). NRF2 activation appears to be associated with the emergence of cancer resistance to various anticancer drugs by transcriptionally activating a battery of self-defense genes. Indeed, NRF2 can induction the expression of γ-GCS and ABCC1 genes involved cancer cell resistance to cisplatin and alkylating agents (; ). In addition, ABCG2 is known to mediate the efflux of gefitinib (Iressa) from cancer cells (), and its expression is regulated by NRF2 (Singh et al., 2010) and the EGFR-tyrosine kinase cascade (; ; Figure 1B).
Single nucleotide polymorphism -617C > A could affect the positive feedback loop of transcriptional activation of the NRF2 gene, and thereby it can regulate the NRF2 protein level. It is proposed that the homozygote -617A/A significantly attenuates the positive feedback loop of transcriptional activation of the NRF2 gene. Interestingly, Asians, including Japanese, have higher frequencies of the -617A allele as compared with African–Americans and Caucasians (). As demonstrated in Figure 2A, Japanese lung cancer patients carrying SNP homozygous alleles (c.-617A/A) exhibited remarkable survival over 1,700 days after surgical operation (log-rank p = 0.021). This SNP is considered as a new biomarker for prognosis of lung cancer in Japanese population, and a hypothetical molecular mechanism has been proposed ().
FIGURE 2
SOMATIC MUTATIONS IN NRF2 AND KEAP1 GENES
The genetic polymorphisms are the “intrinsic” mechanism, whereas the mutations are the “acquired” mechanism in cancer cells. Hitherto, several mutations in the NRF2 and KEAP1 genes have been reported in carcinomas of the lung (Sporn and Liby, 2012), liver (Yoo et al., 2012), stomach (Yoo et al., 2012), and breast (Sjöblom et al., 2006). Abnormalities in NRF2 activity were correlated with poor prognosis in terms of either recurrence-free or overall 5-year survival. Increased expression of NRF2 protein and decreased expression of KEAP1 protein were often observed as common abnormalities in non-small cell lung cancer (NSCLC), being associated with poor prognosis (Solis et al., 2010).
FUTURE PERSPECTIVES
Identification and validation of biomarkers for personalized cancer therapy is one of the challenges in cancer management. To practically realize personalized medicine, development of cost-effective methods is required. Furthermore, genetic information in each patient’s record should be timely provided for individualized cancer treatment. In this regards, we have recently developed a rapid isothermal method to detect genetic polymorphisms in the NRF2 gene and correlated the genotyping data with the survival of patients who had primary lung cancer (). By means of the new method (), we could detect the SNP -617C > A in the NRF2 gene within 30 to 45 min without DNA isolation and PCR amplification (Figure 2B). Such genotyping methods would provide a simple and practical tool for personalized cancer therapy and assessment of prognosis.
Statements
Acknowledgments
The author’s study was supported by a research fund of Japan Science and Technology Agency (JST) project named “Development of the world’s fastest SNP detection system.” This means, except for the following governmental research fund, the author had no financial relationships with third parties.
Conflict of interest
The author declares that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
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Summary
Keywords
drug resistance, anti-oxidant response element (ARE), ABCG2, NRF2, single nucleotide polymorphism (SNP)
Citation
Ishikawa T (2014) Genetic polymorphism in the NRF2 gene as a prognosis marker for cancer chemotherapy. Front. Genet. 5:383. doi: 10.3389/fgene.2014.00383
Received
22 July 2014
Accepted
17 October 2014
Published
04 November 2014
Volume
5 - 2014
Edited by
Luis Abel Quiñones, University of Chile, Chile
Reviewed by
Lucia Taja-Chayeb, Instituto Nacional de Cancerologia, Mexico; Giuseppe Toffoli, Centro di Riferimento Oncologico – National Cancer Institute, Italy
Copyright
© 2014 Ishikawa.
This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) or licensor are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.
*Correspondence: Toshihisa Ishikawa, Personalized Medicine Research Institute, NGO Personalized Medicine and Healthcare, 4-17-30 Kirigaoka, Midori-ku, Yokohama 226-0016, Japan e-mail: toshihisa.ishikawa.r@gmail.com
This article was submitted to Pharmacogenetics and Pharmacogenomics, a section of the journal Frontiers in Genetics.
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