Induction of endogenous retroelements as a potential mechanism for mouse-specific drug-induced carcinogenicity

<div><p>A number of chemical compounds have been shown to induce liver tumors in mice but not in other species. While several mechanisms for this species-specific tumorigenicity have been proposed, no definitive mechanism has been established. We examined the effects of the nongenotoxic rodent hepatic carcinogen, WY-14,643, in male mice from a high liver tumor susceptible strain (C3H/HeJ), and from a low tumor susceptible strain (C57BL/6). WY-14,643, a PPARα activator induced widespread increases in the expression of some endogenous retroelements, namely members of LTR and LINE elements in both strains. The expression of a number of known retroviral defense genes was also elevated. We also demonstrated that basal immune-mediated viral defense was elevated in C57BL/6 mice (the resistant strain) and that WY-14,643 further activated those immuno-defense processes. We propose that the previously reported >100X activity of retroelements in mice drives mouse-specific tumorigenicity. We also propose that C57BL/6’s competent immune to retroviral activation allows it to remove cells before the activation of these elements can result in significant chromosomal insertions and mutation. Finally, we showed that WY-14,643 treatment induced gene signatures of DNA recombination in the sensitive C3H/HeJ strain.</p></div

Volcano plot of LINE and LTR elements.

<p>Panels A and B show fold-change versus -log10 of p-value of LINE elements which show significant change in the respective strains. Panels C and D show fold-change versus -log10 of p-value for the LTR elements. Panels A and C show elements significantly changed in the C3H/HeJ strain while panels B and C show elements from the C57BL/6 strain. Panels were clipped at plus/minus 50X fold-change for clarity resulting in 15, 14, 4, & 5 outliers not being shown on panels A, B, C, & D respectively. Unclipped figures are available in the supplementary files (<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0176768#pone.0176768.s001" target="_blank">S1 Excel File</a>).</p

Ingenuity biofunction analysis results for selected categories.

<p>The gray bars are proportional to the -log10 of the function p-values and the black line indicates the Activation z-scores. Results compare the functions predicted for WY-14,643 treatment of the C3H/HeJ animals with the treatment of the C57BL/6 animals. Panel A compares several of the most significant viral biofunctions identified by Ingenuity’s Bio & Tox Function analysis. Panel B compares the significant biofunctions related to recombination in the chromosomes.</p

Myeloperoxidase inhibition in mice alters atherosclerotic lesion composition.

Myeloperoxidase (MPO) is a highly abundant protein within the neutrophil that is associated with lipoprotein oxidation, and increased plasma MPO levels are correlated with poor prognosis after myocardial infarct. Thus, MPO inhibitors have been developed for the treatment of heart failure and acute coronary syndrome in humans. 2-(6-(5-Chloro-2-methoxyphenyl)-4-oxo-2-thioxo-3,4-dihydropyrimidin-1(2H)-yl)acetamide PF-06282999 is a recently described selective small molecule mechanism-based inactivator of MPO. Here, utilizing PF-06282999, we investigated the role of MPO to regulate atherosclerotic lesion formation and composition in the Ldlr-/- mouse model of atherosclerosis. Though MPO inhibition did not affect lesion area in Ldlr-/- mice fed a Western diet, reduced necrotic core area was observed in aortic root sections after MPO inhibitor treatment. MPO inhibition did not alter macrophage content in and leukocyte homing to atherosclerotic plaques. To assess non-invasive monitoring of plaque inflammation, [18F]-Fluoro-deoxy-glucose (FDG) was administered to Ldlr-/- mice with established atherosclerosis that had been treated with clinically relevant doses of PF-06282999, and reduced FDG signal was observed in animals treated with a dose of PF-06282999 that corresponded with reduced necrotic core area. These data suggest that MPO inhibition does not alter atherosclerotic plaque area or leukocyte homing, but rather alters the inflammatory tone of atherosclerotic lesions; thus, MPO inhibition could have utility to promote atherosclerotic lesion stabilization and prevent atherosclerotic plaque rupture

Expression of anti-retroviral TRIM genes.

<p>Panel A) Expression of Trim6 and Trim24 genes which show significant expression increases with WY-14,643 treatment. Panel B) Expression of Trim5, Trim12a, Trim30a and Trim30d genes in the C3H/HeJ and C57BL/6 treatment groups. Genes exhibit significant expression increases in C57BL/6 samples as compared to the C3H/HeJ.</p

Differentially expressed retroelements between treatment and controls.

<p>Differentially expressed retroelements between treatment and controls.</p

Graded histopathologic changes in individual animals.

<p>Graded histopathologic changes in individual animals.</p