Catechol-O-methyltransferase: Factors Relating to the Carcinogenic Potential of Catecholestrogens

Polymorphisms of COMT and XPD and risk of esophageal squamous cell carcinoma in a population of Yili Prefecture, in Xinjiang, China

OBJECTIVE

To investigate polymorphisms of COMT (Rs4680) and XPD (Rs13181) and risk of esophageal squamous cell carcinoma (ESCC) in a population from Yili Prefecture, Xinjiang, China.

METHODS

A hospital-based case-control study was designed. Genotypes were determined by polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP). Odds ratios (OR) and 95% confidence intervals (CI) were analysed using unconditional logistic regression.

RESULTS

An increased risk of ESCC was discovered with COMT in relation to the frequency of the presence of the A allele (Rs4680; OR 1.30, 95% CI 1.00-1.68). An individual with combined COMT 158 (Val/Met or Met/Met) and XPD 751 (Lys/Gln or Gln/Gln) genotype had an increased ESCC risk.

CONCLUSIONS

Polymorphic variation in COMT Val158Met and XPD Lys751Gln may be important for ESCC susceptibility.

Characterization of human soluble high and low activity catechol-O-methyltransferase catalyzed catechol estrogen methylation

The major detoxification pathway of the carcinogenic catechol estrogens is methylation by catechol- -methyltransferase (COMT). It has been hypothesized that the enzyme encoded by the low-activity allele (COMT(L) ) has a lower catalytic activity for catechol estrogen methylation than that encoded by the high activity allele (COMT(H) ). We expressed and purified human soluble (S)-COMT(H) and S-COMT(L) in and characterized the methylation of 2- and 4-hydroxyestradiol (2- and 4-OH-E2). There were no differences between the kinetic parameters for COMT(H) and COMT(L). The kinetic parameters for S-adenosylmethionine (SAM), the methyl donor in these reactions, also did not differ for COMT(H) and COMT(L). S-adenosylhomocysteine, the demethylated SAM metabolite, inhibited methylation of the catechol estrogens in a non-competitive manner similarly for COMT(H) and COMT(L). Each COMT substrate tested inhibited the methylation of other substrates in a mixed competitive and non-competitive fashion similarly for COMT(H) and COMT(L). Furthermore, in cytosolic fractions of COMT(HH)(MCF-10A and ZR-75-1) and COMT(LL)(MCF-7 and T47D) human breast epithelial cell lines, no differences were detected between the kinetic parameters of COMT with respect to 2- and 4-OH-E2 methylation; nor were COMT protein levels associated with the COMT genotype. These data suggest that the decreased COMT enzymatic activity that has been detected in human tissue in association with the COMT(L) allele is not reflected by differences in the affinity or capacity of COMT(H) and COMT(L) for catechol estrogen methylation. These results raise the question of what accounts for the difference in COMT activity associated with the COMT(HH) and COMT(LL) genotypes in human tissue.

Interactions of (-)-ilimaquinone with methylation enzymes: implications for vesicular-mediated secretion

BACKGROUND

The marine sponge metabolite (-)-ilimaquinone has antimicrobial, anti-HIV, anti-inflammatory and antimitotic activities, inhibits the cytotoxicity of ricin and diptheria toxin, and selectively fragments the Golgi apparatus. The range of activities demonstrated by this natural product provides a unique opportunity for studying these cellular processes.

RESULTS

Affinity chromatography experiments show that (-)-ilimaquinone interacts with enzymes of the activated methyl cycle: S-adenosylmethionine synthetase, S-adenosylhomocysteinase and methyl transferases. Known inhibitors of these enzymes were found to block vesicle-mediated secretion in a manner similar to (-)-ilimaquinone. Moreover, the antisecretory effects of (-)-ilimaquinone and inhibitors of methylation chemistry, but not brefeldin A, could be reversed in the presence of the cellular methylating agent S-adenosylmethionine. Of the enzymes examined in the activated methyl cycle, S-adenosylhomocysteinase was specifically inhibited by (-)-ilimaquinone. Consistent with these observations, (-)-ilimaquinone was shown to obstruct new methylation events in adrenocorticotrophic hormone (ACTH)-secreting pituitary cells.

CONCLUSIONS

(-)-ilimaquinone inhibits cellular methylations through its interactions with S-adenosylhomocysteinase. Furthermore, these studies indicate that the inhibition of secretion by ilimaquinone is the result of the natural product's antimethylation activity. It is likely that the ability to fragment the Golgi apparatus, as well as other activities, are also related to ilimaquinone's influence on methylation chemistry.

Characterization and implications of estrogenic down-regulation of human catechol-O-methyltransferase gene transcription

Catechol-O-methyltransferase (COMT, EC 2.1.1.6) is a ubiquitous enzyme that is crucial to the metabolism of carcinogenic catechols and catecholamines. Regulation of human COMT gene expression may be important in the pathophysiology of various human disorders including estrogen-induced cancers, Parkinson's disease, depression, and hypertension. The gender difference in human COMT activity and variations in rat COMT activity during the estrous cycle led us to explore whether estrogen can regulate human COMT gene transcription. Our Northern analyses showed that physiological concentrations of 17-beta-estradiol (10(-9)-10(-7) M) could decrease human 1. 3-kilobase COMT mRNA levels in MCF-7 cells in a time- and dose-dependent manner through an estrogen receptor-dependent mechanism. Two DNA fragments immediately 5' to the published human COMT gene proximal and distal promoters were cloned. Sequence analyses revealed several half-palindromic estrogen response elements and CCAAT/enhancer binding protein sites. By cotransfecting COMT promoter-chloramphenicol acetyltransferase reporter genes with human estrogen receptor cDNA and pSV-beta-galactosidase plasmids into COS-7 cells, we showed that 17-beta-estradiol could down-regulate chloramphenicol acetyltransferase activities, and COMT promoter activities dose-dependently. Functional deletion analyses of COMT promoters also showed that this estrogenic effect was mediated by a 280 base pair fragment with two putative half-palindromic estrogen response elements in the proximal promoter and a 323-base pair fragment with two putative CCAAT/enhancer binding protein sites in the distal promoter. Our findings provide the first evidence and molecular mechanism for estrogen to inhibit COMT gene transcription, which may shed new insight into the role of estrogen in the pathophysiology of different human disorders.