title: Metabolic activation of the tumorigenic pyrrolizidine alkaloid, retrorsine, leading to DNA adduct formation in vivo.
abstract: Pyrrolizidine alkaloids are naturally occurring genotoxic chemicals produced by a large number of plants. The high toxicity of many pyrrolizidine alkaloids has caused considerable loss of free-ranging livestock due to liver and pulmonary lesions. Chronic exposure of toxic pyrrolizidine alkaloids to laboratory animals induces cancer. This investigation studies the metabolic activation of retrorsine, a representative naturally occurring tumorigenic pyrrolizidine alkaloid, and shows that a genotoxic mechanism is correlated to the tumorigenicity of retrorsine. Metabolism of retrorsine by liver microsomes of F344 female rats produced two metabolites, 6, 7-dihydro-7-hydroxy-1-hydroxymethyl-5H-pyrrolizine (DHP), at a rate of 4.8 +/- 0.1 nmol/mg/min, and retrorsine-N-oxide, at a rate of 17.6 +/- 0.5 nmol/mg/min. Metabolism was enhanced 1.7-fold by using liver microsomes prepared from dexamethasone-treated rats. DHP formation was inhibited 77% and retrorsine N-oxide formation was inhibited 29% by troleandomycin, a P450 3A enzyme inhibitor. Metabolism of retrorsine with lung, kidney, and spleen microsomes from dexamethasone-treated rats also generated DHP and the N-oxide derivative. When rat liver microsomal metabolism of retrorsine occurred in the presence of calf thymus DNA, a set of DHP-derived DNA adducts was formed; these adducts were detected and quantified by using a previously developed 32P-postlabeling/HPLC method. These same DNA adducts were also found in liver DNA of rats gavaged with retrorsine. Since DHP-derived DNA adducts are suggested to be potential biomarkers of riddelliine-induced tumorigenicity, our results indicate that (i) similar to the metabolic activation of riddelliine, the mechanism of retrorsine-induced carcinogenicity in rats is also through a genotoxic mechanism involving DHP; and (ii) the set of DHP-derived DNA adducts found in liver DNA of rats gavaged with retrorsine or riddelliine can serve as biomarkers for the tumorigenicity induced by retronecine-type pyrrolizidine alkaloids.
authors.affiliation: National Center for Toxicological Research, Jefferson, AR 72079, USA. ywang@nctr.fda.gov
authors.forename: Yu-Ping
authors.initials: YP
authors.lastname: Wang
authors.forename: Peter P
authors.initials: PP
authors.lastname: Fu
authors.forename: Ming W
authors.initials: MW
authors.lastname: Chou
mesh_terms: D000818:Animals; D001711:Biotransformation; D002478:Cells, Cultured; D004247:DNA / Q000378:metabolism; D018736:DNA Adducts / Q000032:analysis*; D005260:Female; D008099:Liver / Q000187:drug effects / Q000378:metabolism; D008861:Microsomes / Q000187:drug effects / Q000378:metabolism; D016686:Monocrotaline / Q000031:analogs & derivatives* / Q000378:metabolism; D011763:Pyrrolizidine Alkaloids / Q000378:metabolism / Q000493:pharmacokinetics* / Q000633:toxicity*; D051381:Rats; D011916:Rats, Inbred F344
chemical_list: D018736:DNA Adducts; D011763:Pyrrolizidine Alkaloids; C013577:isatidine; D016686:Monocrotaline; D004247:DNA; C427203:calf thymus DNA; C007241:dehydroretronecine; C003300:retrorsine