Abstract 4: Vitamin D blocks platinum-induced ovarian cancer stem cell plasticity

Most women diagnosed with late-stage high grade serous ovarian cancer (HGSOC) develop recurrent, platinum-resistant tumors. Ovarian cancer stem cells (OCSCs) are hypothesized to contribute to the emergence of these tumors. CSCs reside in a plastic state, with the ability to convert between non-CSC to CSC. This process of dedifferentiation is thought to continue during tumor development and chemotherapeutic agents such as platinum can exaggerate CSC plasticity. However, the underlying mechanism has not been established. Furthermore, how platinum transforms non-OCSCs into OCSCs to contribute to this subpopulation of cells remains unclear. To examine OCSC plasticity, aldehyde dehydrogenase (ALDH; functional marker) and fluorescence activated cell sorting were used to isolate OCSCs (ALDH+) and non-OCSCs (ALDH-) from HGSOC cell lines, OVCAR5 and OVCAR3. To determine if platinum induced conversion of non-OCSC to OCSC, ALDH- cells were cultured for 3-10 days after sorting. At timepoints examined, ALDH- cells remained ALDH-, (>99% were ALDH- after 10 days). ALDH- cells were then treated with cisplatin (OVCAR5: 12μM,16h, OVCAR3: 15μM 16h), and the percent of ALDH+ cells was measured using flow cytometry. Treatment of cisplatin resulted in conversion of approximately 4% and 10% of ALDH- cells into ALDH+ cells (p<0.05) in OVCAR5 and OVCAR3 cells, respectively. Furthermore, converted OCSCs displayed increased (p<0.05) expression of stemness genes (BMI1, NANOG, OCT4, SOX2) and spheroid formation compared to parental ALDH+, demonstrating that platinum induced the observed differences in the stemness phenotype. With the goal of targeting key genes and pathways to inhibit platinum-induced OCSC conversion, RNA-sequencing was performed on ALDH- and ALDH+ cells before and after cisplatin treatment. Vitamin D receptor (VDR)/retinoid x receptor (RXR) activation was a major pathway in the platinum-induced converted OCSC transcriptome, which was of interest because VDR signaling plays a role in differentiation, and its key binding partner, RXR and retinoic acid receptor (RAR) are linked to stemness. To determine the effect of vitamin D on OCSC conversion, ALDH- cells were treated with vitamin D for 24h (OVCAR5: 34nM, OVCAR3: 56nM) and/or cisplatin and the ALDH activity was measured with flow cytometry. Vitamin D treatment in combination with cisplatin significantly decreased the platinum-induced percent of converting cells (p<0.05). Additionally, vitamin D treatment decreased expression of stemness genes and spheroid formation in both whole cell and sorted cell populations. Currently, we are further investigating the relationship between cisplatin, RAR/RXR signaling and stemness. Collectively, these results demonstrate a role for VDR signaling in platinum-induced OCSC conversionproviding further support for the development targeted therapies to block the persistence of OCSCs and ultimately reduce patient mortality. Citation Format: Tara X. Metcalfe, Sophie Xanders, Shu Zhang, Christiane Hassel, Heather M. O'Hagan, Kenneth P. Nephew. Vitamin D blocks platinum-induced ovarian cancer stem cell plasticity [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2025; Part 1 (Regular Abstracts); 2025 Apr 25-30; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2025;85(8_Suppl_1):Abstract nr 4.