Rat, but not human, sulfotransferase activates a tamoxifen metabolite to produce DNA adducts and gene mutations in bacteria and mammalian cells in culture

Abstract

Tamoxifen increases the risk of human endometrial cancer and is a potent carcinogen in rat liver, in which it produces DNA adducts and cytogenetic damage. Nevertheless its prophylactic use against breast cancer in healthy women is under investigation in several large trials. To investigate whether rat hepatocarcinogenicity predicts human hepatocarcinogenicity we used genetically engineered bacterial and mammalian target cells to investigate how α-hydroxytamoxifen, a major phase I metabolite of tamoxifen, is further metabolised by rat and human phase II enzymes, sulfotransferases, to mutagenic and DNA-adduct-forming species. We expressed rat hydroxysteroid sulfotransferase a, a liver-specific enzyme, and corresponding human sulfotransferase in bacteria (Salmonella typhimurium) and in a mammalian cell line (Chinese hamster V79 cells) and tested α-hydroxytamoxifen for DNA adduct formation and mutagenicity in these systems, using unmodified cells as controls. In cells that expressed rat hydroxysteroid sulfotransferase, α-hydroxytamoxifen was mutagenic and formed the same pattern of DNA adducts as that found in the liver of tamoxifen-treated rats. α-Hydroxytamoxifen was not activated, or was at least 20 times less active in cells expressing human hydroxysteroid sulfotransferase. All the other six known human xenobiotic-metabolising sulfotransferases were also expressed in S.typhimurium. None activated α-hydroxytamoxifen to a mutagen. These results suggest that the risk of DNA adduct formation, and cancer, in the human liver is low and explain why tamoxifen is a powerful carcinogen to the rat liver, and why standard short-term tests fail to detect its mutagenicity.

Tamoxifen Forms DNA Adducts in Human Colon after Administration of a Single [14C]-Labeled Therapeutic Dose

“…The high levels of DNA binding in rat liver is due to the sulfation of a-hydroxytamoxifen by an isoform of hydroxysteroid sulfotransferase, ST2A2 (36). Evidence suggests that a-hydroxytamoxifen is a poor substrate for the human form of the enzyme (SULT2A1) that produces in the region of 3-fold lower to undetectable levels of a-sulfate tamoxifen compared with rat hydroxysteroid sulfotransferase (37,38). Furthermore, SULT2A1 activity in human colon, measured using dehydroepiandrosterone, is almost nonexistent relative to liver tissue (39).…”

Multiple Mechanisms Are Involved in the Biliary Excretion of Acetaminophen Sulfate in the Rat: Role of Mrp2 and Bcrp1

“…Xenobiotic inactivation and/or detoxification may occur by direct sulfation of the parent compound (e.g., acetaminophen) or may follow phase I oxidation (e.g., phenobarbital). However, hepatic xenobiotic sulfation also can lead to activation of prodrugs [e.g., minoxidil (Buhl et al, 1990)] and hepatotoxins [e.g., covalent microsomal protein binding of phenacetin sulfate (Mulder et al, 1977); formation of DNA adducts with tamoxifen sulfate (Glatt et al, 1998); potent bile salt export pump inhibition by troglitazone sulfate leading to cholestasis (Funk et al, 2001)]. Considering that sulfated xenobiotics may exhibit pharmacologic or toxicologic activity, perturbations in transport mechanisms responsible for excretion of sulfate conjugates may have important therapeutic and toxic implications.…”

FORMATION OF TAMOXIFEN-DNA ADDUCTS VIA O-SULFONATION, NOT O-ACETYLATION, OF α-HYDROXYTAMOXIFEN IN RAT AND HUMAN LIVERS

“…The increased formation of TAM-DNA adducts was observed when cells expressing rat STa were exposed to ␣-OHTAM, but not with cells expressing human HST; the authors concluded that ␣-OHTAM is a substrate for STa, but not for human SULT 2A1 (Glatt et al, 1998). In contrast, ␣-OHTAM can be O-sulfonated by both rat STa and human SULT 2A1 and react with DNA, forming dG-N 2 -TAM adducts (Shibutani et al, 1998a,b).…”