A Metabolomics Investigation of Non-genotoxic Carcinogenicity in the Rat

Non-genotoxic
carcinogens (NGCs) promote tumor growth by altering gene expression,
which ultimately leads to cancer without directly causing a change
in DNA sequence. As a result NGCs are not detected in mutagenesis
assays. While there are proposed biomarkers of carcinogenic potential,
the definitive identification of non-genotoxic carcinogens still rests
with the rat and mouse long-term bioassay. Such assays are expensive
and time-consuming and require a large number of animals, and their
relevance to human health risk assessments is debatable. Metabolomics
and lipidomics in combination with pathology and clinical chemistry
were used to profile perturbations produced by 10 compounds that represented
a range of rat non-genotoxic hepatocarcinogens (NGC), non-genotoxic
non-hepatocarcinogens (non-NGC), and a genotoxic hepatocarcinogen.
Each compound was administered at its maximum tolerated dose level
for 7, 28, and 91 days to male Fisher 344 rats. Changes in liver metabolite
concentration differentiated the treated groups across different time
points. The most significant differences were driven by pharmacological
mode of action, specifically by the peroxisome proliferator activated
receptor alpha (PPAR-α) agonists. Despite these dominant effects,
good predictions could be made when differentiating NGCs from non-NGCs.
Predictive ability measured by leave one out cross validation was
87% and 77% after 28 days of dosing for NGCs and non-NGCs, respectively.
Among the discriminatory metabolites we identified free fatty acids,
phospholipids, and triacylglycerols, as well as precursors of eicosanoid
and the products of reactive oxygen species linked to processes of
inflammation, proliferation, and oxidative stress. Thus, metabolic
profiling is able to identify changes due to the pharmacological mode
of action of xenobiotics and contribute to early screening for non-genotoxic
potential.