The function and regulation of kynurenine pathway intermediates in breast cancer metabolism

Breast cancer (BC) is the second leading cause of cancer related deaths in Canadian women, making it an important area for research. Most cancer cells require an altered metabolism making them vulnerable to metabolism-based therapies. Among metabolic pathways, the kynurenine pathway (KP) of de-novo NAD+ biosynthesis contains several intermediates, which have been shown to have a bioactive role in immune, neuronal, and BC cells. In this work, we have investigated the KP across BC cell lines and tumour samples and observed highly divergent levels of several KP enzymes including indoleamine 2,3-dioxygenase (IDO1) and kynureninase (KYNU). In concurrent with metabolomics data, we identified high levels of KP metabolites, picolinic acid and kynurenic acid, in KYNU-high tumours. We investigated the effects of KP metabolites in BC cell lines and observed that, 3-hydroxyanthranilic acid (3-HAA) significantly decreases cell growth. Of the cell lines, 3-HAA had the greatest effect on cell growth in MCF-7 cells (p-value <0.001, log2 fold change of -4.8). To further investigate this effect, we treated three BC cell lines (with varying KYNU levels and 3-HAA effect on cell growth) with 3-HAA and performed proteomics analysis. The results of the proteomics and Gene Ontology (GO) enrichment analysis of significantly up-regulated proteins (p-value <0.05) in MCF-7 cells revealed molecular functions including leucine transporter, dehydrogenase, and monooxygenase activity. The mechanism behind these functions is not well-established but appears to involve AKR1C1, AKR1C3, SLC7A5, and SLC3A2. These proteins may reveal targetable vulnerabilities for high-KYNU and highly 3-HAA sensitive cell lines. The results of this research suggest a role for the kynurenine pathway in BC biology and understanding KP metabolite function and regulation in BC could offer essential information that may lead to the development of new targeted cancer therapies.