Pilot: Co-Targeting SMS and OXPHOS as a Novel Combination Therapy for Colorectal Cancer

Abstract Colorectal cancer (CRC) is the second leading cause of cancer-related deaths in the U.S. and often develops resistance to existing therapies, highlighting the urgent need for novel therapeutic targets and more effective treatments. Dysregulated polyamine metabolism has been implicated in CRC development. Spermine synthase (SMS), a polyamine biosynthetic enzyme that converts spermidine to spermine, is frequently overexpressed and linked to poor prognosis in several cancers, including CRC, as we recently reported. Our work demonstrated that SMS overexpression is critical for balancing cellular spermidine levels to facilitate CRC tumorigenesis, underscoring its potential as a therapeutic target. However, SMS inhibition alone has shown only modest anti-CRC effects. The overall objective of this project is to identify novel mechanisms that can enhance the efficacy of SMS-targeted therapy in CRC. Through unbiased transcriptomic profiling and gene set enrichment analyses, we identified mitochondrial translation as one of the most significantly enriched and upregulated biological processes following SMS inhibition in CRC cells. Specifically, SMS inhibition increases the expression of key mitochondrial translational regulators and electron transport chain proteins essential for oxidative phosphorylation (OXPHOS), consistent with the observed elevation in mitochondrial OXPHOS activity in SMSinhibited CRC cells. Notably, our preliminary data further show that combined SMS and OXPHOS inhibition induces a synergistic antitumor effect in SMS-overexpressing CRCs. Based on these compelling findings, we hypothesize that SMS inhibition upregulates mitochondrial translation, leading to increased OXPHOS activity, which sustains CRC cell survival and tumorigenesis while rendering CRC cells vulnerable to OXPHOS inhibition. Thus, the combined inhibition of SMS and OXPHOS could synergistically suppress tumor growth and progression. To test this hypothesis, we propose two specific aims. Aim 1 will elucidate the molecular mechanisms by which SMS inhibition enhances mitochondrial translation to support OXPHOS and CRC cell survival. Aim 2 will determine how targeting OXPHOS sensitizes CRC cells to SMS inhibition and evaluate the synergistic antitumor efficacy of combined SMS and OXPHOS inhibition using cell line- and patient-derived xenograft (PDX) models of SMS-overexpressing CRC. Successful completion of this project will reveal a novel synthetic lethal interaction between SMS and OXPHOS inhibition, providing a promising combination therapy to improve CRC treatment, particularly in high-SMS-expressing subsets. The data generated will serve as a strong foundation and rationale for an NCI R01 application, which we will refine and submit over the next year.