SUN-302 - Autocrine Surfactant Protein D Dependency in Obesity-Associated Breast Cancer.

Background: Obesity is a major modifiable risk factor for aggressive triple negative breast cancer (TNBC), yet the tumor-cell programs that enable growth under obese metabolic stress remain unclear. By mapping obesity-conditioned breast tumor dependencies, we identified secreted surfactant protein D (SFTPD) as required to preserve mitochondrial health in tumor cells adapting to obesity. We hypothesize that obesity induces a distinct mitochondrial stress that SFTPD counteracts. This project aims to define molecular links between obesity, mitochondrial function, and breast cancer progression.

Methods: EO771 cells were implanted orthotopically into lean or high-fat diet C57BL/6 mice and screened in vivo with a pooled CRISPRi kinase/phosphatase library. Hits were prioritized with clinical survival associations and validated in four TNBC cell lines using independent shRNAs and lean/obese mouse and patient sera, metabolomics, Seahorse/ radiotracer flux, lipidomics, and lipid-peroxidation (LPO)/oxidized LDL (ox-LDL) assays. SFTPD binding partners were identified by SFTPD co-immunoprecipitation/mass spectrometry and validated using AF647-labelled recombinant SFTPD binding and genetic epistasis. Genome-wide CRISPRi modifier screens were performed to identify intracellular communication pathways linking SFTPD to mitochondrial regulation.

Results: The screen identified SFTPD as a context-dependent essential gene: its depletion selectively suppressed orthotopic tumor growth in obese hosts (little impact in lean controls) and increased energy-stress/tumor-suppressor signaling in obesity. Across models, SFTPD loss induced mitochondrial dysfunction (reduced oxidative metabolism and membrane potential) and, in obese serum, increased triglyceride/lipid-droplets with elevated LPO. Obese serum had higher oxLDL; oxLDL alone was sufficient to phenocopy LPO and mitochondrial impairment. In obese serum, ferrostatin-1 (but not NAC or mitoTEMPO) rescued SFTPD-loss phenotypes, implicating oxidative lipid injury as the dominant stressor. Recombinant SFTPD partially restored mitochondrial function. MYO18A emerged as an SFTPD-associated interactor; MYO18A depletion reduced AF647–SFTPD binding by ~4-fold and prevented SFTPD-mediated mitochondrial rescue, placing MYO18A downstream of extracellular SFTPD. Modifier screens nominated MARCH5 as a shared suppressor of SFTPD/MYO18A-linked mitochondrial dysfunction, linking this axis to mitochondrial quality control (QC) circuitry and DRP1/MFN1 regulation.

Conclusion: Obesity-associated oxidized lipids impose mitochondrial stress that breast cancer cells buffer via a secreted SFTPD–MYO18A signaling axis coupled to mitochondrial QC. This obesity-conditioned extracellular vulnerability suggests metabolically stratified therapeutic opportunities targeting oxidized-lipid stress buffering.