The key events responsible for mouse liver tumors induced by a pesticide (viz., pronamide) were investigated in a series of studies employing molecular, biochemical, cellular, and apical endpoints. Based on these studies, it was demonstrated that the liver tumors were mediated by a mode of action (MoA) involving nuclear receptors (NRs) through the following key events: (1) CAR and PPAR-α receptor activation, (2) increased hepatocellular proliferation, eventually leading to (3) hepatocellular tumors. Specifically, gene expression analysis indicated robust, simultaneous coactivation of the CAR and PPAR-α NRs, as indicated by the induction of hepatic Cyp2b10 and Cyp4a10 transcripts, in response to dietary administration of pronamide to mice. The presence of hepatocellular hypertrophy and peroxisome proliferation was indicative of the activation of these two NRs at carcinogenic dose levels. Demonstrated induction of Cyp2b10 gene and protein, however, was not accompanied by enhancement of the corresponding enzyme activity (7-pentoxyresorufin-O-dealkylase (PROD)), suggesting that pronamide administration resulted in mechanism-based (suicide) inhibition of the enzyme in vivo. This was confirmed with an in vitro assay for suicide inhibition, where pronamide and/or its metabolites irreversibly inhibited Cyp2b10-mediated PROD activity. Analysis of hepatocellular proliferation via BrdU incorporation indicated a clear dose- and duration-related induction of S-phase DNA synthesis only in animals treated at and above the carcinogenic dose level. The available MoA data were evaluated for weight-of-evidence based upon the Bradford Hill criteria, followed by a human relevance framework. The conclusion from this evaluation is that pronamide-induced mouse liver tumors occur via an NR-mediated MoA involving CAR and PPAR-α activation and this MoA is not relevant to humans based on qualitative/quantitative differences between mice and humans.
Keywords: CAR; HRF; MoA; PPAR-α; constitutive androstane receptor; hepatocellular proliferation; mode of action; peroxisome proliferator-activated receptor alpha; pesticide; plant protection product.
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