Pilot: Lipid Metabolism Reprograming upon Spermine Synthase Inhibition in Colorectal Cancer

Abstract: Colorectal cancer (CRC) is the second most common cause of cancer death in the United States for men and women combined. Patients with advanced stage of CRC are predominantly unresponsive to existing therapies mainly due to drug resistance. Thus, better understanding of CRC biology and new therapeutic approaches for effective treatment of this disease are critical unmet need. Dysregulation of polyamine metabolism has been linked to the development of CRC. Our recent work demonstrates that spermine synthase (SMS), a polyamine biosynthetic enzyme that converts spermidine to spermine, is overexpressed in CRC, which is required for balancing cellular spermidine levels to facilitate CRC tumorigenesis. Our findings highlight SMS as an attractive therapeutic target in CRC; yet, genetic depletion of SMS expression only shows a limited antitumor effect with no cell cycle arrest or induction of apoptosis. Using unbiased metabolomics and transcriptomics analyses, we identified a lipid metabolism reprograming as among the most impacted metabolic change by SMS depletion in CRC cells. Specifically, SMS depletion significantly altered long-chain fatty acid, triacylglycerol and phospholipid metabolism. Furthermore, targeted inhibition of SMS induced lipid droplet accumulation and oxidative phosphorylation in mitochondria, and increased expression of genes associated with mobilization of polyunsaturated fatty acids (PUFAs) and genes associated with lipid peroxidation for induction of ferroptosis. Importantly, we demonstrated that SMS inhibition-induced lipid metabolism reprograming elicits robust anti-CRC activity when lipid peroxidation is induced by blockade of the lipid peroxide scavenging system (system xc/GPX4). Based on these findings, the central hypothesis of the proposed study is that targeting SMS-induced lipid metabolism reprograming enables CRC cells to adapt and survival by hijacking metabolic processes, and that these vulnerabilities can be therapeutically exploited. To test this hypothesis, two specific aims are proposed. Aim 1 will determine how lipid metabolism reprograming maintains CRC cell growth upon SMS inhibition. We will determine the roles of lipid droplets and the identified lipid metabolic enzymes in mediating lipid metabolism reprograming and PUFA mobilization to contribute to mitochondria respiration and maintain CRC cell growth upon SMS inhibition. Aim 2 will establish the therapeutic effect of inducing ferroptosis in the setting of SMS inhibition in CRC. We will determine if coordinated induction of ferroptosis by combined inhibition of SMS and the lipid peroxide scavenging system will synergistically eradicate CRC cells in vivo and investigate the underlying anti-CRC effect. Our proposed study will conceptually elucidate an unexpected lipid metabolism reprograming by targeting SMS in CRC. The impact of this project will be significant because it will result in novel mechanism-guided therapeutic strategies to overcome cell death resistance and increase the effectiveness of CRC therapy. We anticipate that the results obtained from this study will provide a strong rationale for a NIH R01 application.