Biomarkers of quercetin-mediated modulation of colon carcinogenesis

Abstract

Colorectal cancer (CRC) is hypothesized to be prevented by intake of fruits and vegetables that contain anti-carcinogenic compounds, including the<span class=SpellE>flavonoid</span><span class=SpellE>quercetin</span>that is found in apples and onions. In this thesis,<span class=SpellE>quercetin's</span>mechanisms of cancer-preventive action were studied both in vitro and in vivo . The in vitro experiments were performed using the human Caco-2 cell line as a model for CRC, and<span class=SpellE>quercetin</span>stabilized by<span class=SpellE>ascorbate</span>in the culture medium. Unexpectedly,<span class=SpellE>ascorbate</span>-stabilized<span class=SpellE>quercetin</span>showed enhancement of cellular processes involved in CRC-development, including stimulated cell proliferation, reduced cell differentiation and enhancement of pathways that stimulate cell survival. Furthermore,<span class=SpellE>transcriptomics</span>showed that<span class=SpellE>quercetin</span><span class=SpellE>downregulated</span>expression of genes involved in<span class=SpellE>tumor</span>suppression and phase II metabolism, and<span class=SpellE>upregulated</span><span class=SpellE>oncogenes</span>. Comparison with Caco-2 cells exposed to<span class=SpellE>quercetin</span>in the absence of<span class=SpellE>ascorbate</span>showed the opposite, i.e. anti-carcinogenic effects by this<span class=SpellE>flavonoid</span>. This led to the hypothesis that<span class=SpellE>quercetin</span>-induced reactive oxygen species that eradicate<span class=SpellE>tumor</span>cells were scavenged by vitamin C, causing<span class=SpellE>tumor</span>cell survival. Without<span class=SpellE>ascorbate</span>, these reactive oxygen species may be responsible for anti-carcinogenic effects, pointing to beneficial effects of supposed adverse reactive intermediates.<o:p></o:p></span>Subsequently, the CRC-modulating potency of<span class=SpellE>quercetin</span>and its conjugate<span class=SpellE>rutin</span>were investigated in a rat model for CRC.<span class=SpellE>Quercetin</span>, but not its conjugate<span class=SpellE>rutin</span>decreased the<span class=SpellE>tumor</span>incidence, which was associated with the blood plasma levels of this anti-oxidant, but not reflected by the putative<span class=SpellE>preneoplastic</span>biomarker lesions, designated aberrant crypt foci. The combination of<span class=SpellE>transcriptomics</span>and proteomics showed that<span class=SpellE>quercetin</span>inhibited the potentially<span class=SpellE>oncogenic</span><span class=SpellE>mitogen</span>-activated protein<span class=SpellE>kinase</span>(<span class=SpellE>Mapk</span>) pathway and enhanced expression of<span class=SpellE>tumor</span>suppressor genes, cell cycle inhibitors, and genes involved in<span class=SpellE>xenobiotic</span>metabolism. In addition,<span class=SpellE>quercetin</span>affected the energy production pathways, by increasing mitochondrial fatty acid degradation, and inhibiting<span class=SpellE>glycolysis</span>. This observation provided a new hypothesis pointing at another anti-carcinogenic mechanism for<span class=SpellE>quercetin</span>, based on an alteration in routes for energy metabolism, shifting them in<span class=SpellE>favor</span>of non-<span class=SpellE>tumor</span>like pathways like mitochondrial fatty acid degradation at the cost of the<span class=SpellE>tumor</span>-like<span class=SpellE>glycolytic</span>pathway for cellular energy supply.<o:p></o:p></span>Overall, the studies presented in the present thesis provided new hypotheses for the mode of action of<span class=SpellE>quercetin</span>as an anti-<span class=SpellE>tumor</span>agent, but it appeared that the actual dose needed to exert this beneficial effect amounted to about 60 - 100 times the already relatively high prescribed dose for<span class=SpellE>quercetin</span>supplements. Therefore, it is concluded that health claims on the use of<span class=SpellE>quercetin</span>as an anti-cancer agent need better scientific support