The role of diacylglycerol o-acyltransferases1 and 2 in the obesity reinforced lipid droplet-mediated growth and maintenance of colon cancer stem cells
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Description
With obesity being a worldwide epidemic associated with an increase in the risk, progression, and recurrence of colon cancer and colon cancer being the third leading cause of cancer related deaths, it is urgent that we understand the mechanisms behind obesity-mediated colon cancer progression. Emerging studies have also implicated heterogenic colon cancer stem cells in colonic tumorigenic disease processes. Further, recent studies have revealed that lipid droplets, intracellular organelles responsible for lipid storage, are increased in tissues (both fat-storing and non-fat-storing) in obese individuals to potentially support tumorigenesis; however, the mechanism behind this remains unknown. In this study, we aimed to determine the role of diacylglycerol acyltransferases (DGATs), responsible for the initiation and growth of lipid droplets, in colon cancer stem cells, especially when reinforced by obesity. Initially, we found that inhibiting DGATs in colonospheres, which enriched in colon cancer stem cells, significantly decreased the size of LD-accumulated fluorescently labeled fatty acid analogue carbon-12, along with decreasing the size of colonospheres. Next, colonospheres (derived from HT29 and HCT116 cells) grown in presence of the obesity mediator oleic acid (OA), were treated with DGATs inhibitors. We found that DGATs inhibition significantly attenuated colonospheres size, which was quantified via surface area. Further, targeting DGATs in colonospheres substantially decreased the levels of colon cancer stem cell markers CD133 and EpCam. OA-stimulated colonosphere formation was significantly attenuated by the inhibition of DGATs. Moreover, we determined systemic transcriptional changes (RNAseq) in colonospheres mediated by DGATs inhibition. Transcriptomic analysis of DGATs inhibited colonospheres revealed 2218 differentially expressed genes (DEGs) that are related to pathways associated with tumorigenesis such as those related to cancer, cell cycle, DNA replication, cellular movement, and tumor morphology as well as those related to metabolic reprogramming, such as lipid metabolism and metabolic diseases. When these DEGs were compared to transcriptomes of human crypt epithelial cells from colon cancer patients, which are enriched in colon cancer stem cells, we found targeted pathways related to cell cycle control, glycolysis, and tumor growth. Additionally, pathways analysis of DGATs inhibited colonospheres relative to DGATs inhibited adherent cells show that, while there are some similar pathways, colonospheres contain many altered pathways that differ from adherent cells. We validated select DEGs that are targeted by DGATs inhibition in colonospheres derived from HCT116 and HT29 cells. Among these DEGs are those that are implicated in established tumorigenic processes, such as glucose metabolism (ALDOC), ECM maintenance (KLK6), and cytokine signaling (STX3 and CERT1), in addition to novel genes whose roles in tumorigenesis are still not well understood (MMAB, SQLE, and ACLY). Our findings establish the critical role of DGATs in the growth of colon cancers stem cells and in tumorigenic processes, especially when reinforced by obesity and reveals DGATs as a potential target for precision therapies in obesity-mediated colonic tumorigenesis.