Human Microsomal Epoxide Hydrolase (EPHX1): Genetic Polymorphism And Regulation By Chemopreventive Agents

Open Access
Author:
Yang, Xi
Graduate Program:
Genetics
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
June 02, 2009
Committee Members:
  • Curtis John Omiecinski, Dissertation Advisor
  • Curtis John Omiecinski, Committee Member
  • Robert Paulson, Committee Chair
  • Jeffrey Maurice Peters, Committee Member
  • Adam Bleier Glick, Committee Member
  • Joshua D Lambert, Committee Member
Keywords:
  • chemopreventive
  • Microsomal epoxide hydrolase
  • polymorphism
Abstract:
Microsomal epoxide hydrolase (EPHX1) is a critical catalytic determinant in the formation of the highly reactive electrophilic, and ultimately carcinogenic, epoxide metabolites of the polyaromatic hydrocarbons. A central hypothesis of our research is that genetic variability and differential regulation of EPHX1 in human tissue are likely important determinants of interindividual responsiveness, resulting toxicities, and carcinogenicity outcomes related to chemical exposures. Our laboratory has demonstrated that the expression of the human EPHX1 gene is driven by the use of alternative promoters. An alternative promoter region, termed the E1-b promoter, is localized ~ 18.5 kb 5’-upstream from the structural region of the EPHX1 gene. The E1-b promoter is used exclusively to drive expression of EPHX1 mRNA transcripts in most tissues, along with a more proximal and highly liver-specific E1 promoter. Results of quantitative Real Time-PCR analyses demonstrated that the E1-b variant transcript is preferentially and broadly expressed in most tissues, such that it accounts for the majority of total EPHX1 transcript in vivo. In the studies conducted within this thesis research, detailed analysis of the E1-b promoter region demonstrated that this upstream EPHX1 promoter is replete with transposable elements. Further, we identified that two specific Alu elements are polymorphic (i.e. some chromosomes carry the two Alu insertions, whereas other do not) in the genome structure of different individuals. Results of luciferase gene reporter assays conducted in several human cell lines with E1-b promoter constructs demonstrated that the inclusion of the Alu (+/+) insertion significantly decreases basal transcriptional activities. Although we identified a putative aryl hydrocarbon receptor (AhR) binding motif within the Alu element structure, expression of human EPHX1 in an in vitro system was only modestly responsive to AhR ligands and we conclude that AhR regulation does not associate with the presence/absence of Alu elements. Two non-synonomous genetic polymorphisms that alter the EPHX1 amino acid structure were identified in our laboratory’s previous research efforts and several epidemiologic investigations have now implicated these EPHX1 coding region polymorphisms as a risk factor for lung cancer. In this study, using haplotype block analyses, we determined that the E1-b polymorphic promoter region was not in linkage disequilibrium with two previously identified non-synonomous SNPs in the coding region or with functional SNPs previously identified in the proximal promoter region of the gene. Modulation of xenobiotic-metabolizing enzymes by chemopreventive agents is a promising strategy offering protection against toxicity mediated by certain chemical carcinogens. In this research, we discovered that in human cells, EPHX1 mRNA and protein expression were regulated tissue-specificaly by the potent chemopreventive agent, sulforaphane. Subsequent mechanistic studies revealed that Nrf2/ARE pathway plays a central role in sulforaphane’s modulation of EPHX1. In the HepG2 cell line, co-expression of Nrf2 was found to activate EPHX1 promoter activity. However, in BEAS-2B cells, the presence of Nrf2 suppresses the EPHX1 transcriptional expression. A novel E1-b’ transcript was further identified in this research and our results suggest that upstream open reading frames existing in the EPHX1 gene transcripts may function to regulate translational efficiency of EPHX1 expression. In summary, the investigations contributed by this research have substantially expanded our knowledge of the genetics and complex transcriptional regulation of EPHX1; and, given the significant association between EPHX1 genetic polymorphisms and lung cancer risk, highlight the chemopreventive potential of targeting EPHX1 as a defense against carcinogens.