Fatty acids and their receptors-mediated tumor metastasis and progression

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Overcoming metastasis and resistance to chemotherapy remains a major unmet need for colorectal cancer (CRC), despite significant progress made in the molecular characterization and understanding of metastatic CRC. RAS and BRAF mutations (occur in 50% and 10% of CRC patients, respectively) are known to have worse overall prognosis and/or clinical outcome. However, no inhibitors, including those against RAS pathway and BRAF mutation, are able to overcome CRC progression. Increased adipocytes and lipids associated with obesity/diet that accumulate at tumor sites as in CRC are recognized as critical for cancer metastasis and resistance to therapy. Although CD36 is well-documented as a lipid transporter that plays an important role in initiating metastasis and therapy resistance, our preliminary studies indicate that CD36 is only partially responsible for lipid uptake in CRC tumor cells. Moreover, MDR1, known for its function in pumping out drugs, co-expresses with CD36 and plays a role in lipid uptake into metastatic initiating/chemo-resistant CRC cells. The nature of lipid species transported by CD36 and MDR1 critical for initiating metastasis and resistance to apoptosis is unknown. We therefore propose to (1) determine the specific roles of MDR1 and CD36 as fatty acid receptors and elucidate the oncogenic lipids they transport in mediating tumor metastasis in CRC; (2) investigate how excess lipids fail to cause lipotoxicity while promoting metastasis and resistance to apoptosis of CRC cells with RAS and BRAF mutations. Our published data demonstrate that STAT3 upregulates fatty acid oxidation (FAO), leading to increased cancer cell stemness, which is important for metastasis and drug resistance. We further demonstrated in preliminary data that extra acetyl-CoAs, generated by increased FAO, activate STAT3 by acetylation, which in turn upregulates Acyl-CoA synthetases (ACSL), supporting lipid catabolism and phospholipid biogenesis. We will test the hypothesis that the fatty acid receptors, CD36 and MDR1, mitigate lipotoxicity and resist apoptosis through enhanced phospholipid biosynthesis and heightened mitochondrial integrity. Our preliminary data are indicative that acetylated STAT3 is critical for metabolizing excess lipids and for fatty acids-mediated resistance of CD36+MDR1+ CRC cells to apoptosis by increasing mitochondrial membrane potential, which will be further validated in Aim 2. We therefore propose to (3) validate acetylated-STAT3 as a target for controlling CRC metastasis and chemo-resistance in highly metastatic CRC xenografts and in metastatic patient-derived xenografts (contain either RAS or BRAF mutations). To this effect we will utilize our newly developed cell-penetrating acetylated-STAT3 decoy peptide that effectively and specifically targets activated (acetylated) STAT3. Our proposed studies will provide mechanistic insights into tumor progression mediated by increased lipids in the tumor microenvironment. They may also lead to more effective therapeutic interventions for CRC with various genetic mutations.