Abstract A112: Mechanisms underlying platinum-induced ovarian cancer stem cell plasticity

Abstract 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 resistant tumors. CSCs have been postulated to reside in a plastic state, which can allow for the conversion of non-CSC to CSC. This process of dedifferentiation continues during tumor development and chemotherapeutic agents like platinum can exaggerate CSC plasticity. We have previously demonstrated that acute platinum treatment enriched for OCSCs. However, whether platinum transforms non-OCSCs into OCSCs to contribute to this subpopulation of cells remains unclear, and the underlying mechanism remains incompletely understood. 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 the stability of the non-OCSC phenotype, ALDH- cells were cultured for 3 and 5 days and ALDH activity was measured using flow cytometry. At both timepoints examined, ALDH- cells remained ALDH-, with approximately less than 1% being ALDH+ (p0. 05). To determine if platinum can induce conversion of non-OCSC to OCSC, ALDH- cells were treated with cisplatin (12µM or 15 µM for 16h), and the percent of ALDH+ cells was measured using flow cytometry. Treatment of ALDH- cells with cisplatin resulted in conversion of approximately 4% and 10% of ALDH- cells into ALDH+ cells (p0. 05) in OVCAR5 and OVCAR3 cells, respectively. Furthermore, both cell lines displayed increased expression (p0. 05) of stemness genes BMI1, NANOG, OCT4, and SOX2 and spheroid formation was observed in converted OCSCs compared to parental ALDH+ and whole cell populations treated with cisplatin, suggesting 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 using converted OCSCs. Activated pathways in converted OCSCs were identified, including protein kinase signaling, inhibition of matrix metalloproteases, SPINK1 pathway, and VDR/RXR activation. We are further investigating if these pathways contribute to the mechanism involved in OCSC plasticity and whether targeting these pathways can reduce the OCSC population, which is critical to blocking the persistence of OCSCs and ultimately reduce mortality in patients. Citation Format: Tara X. Metcalfe, Shu Zhang, Christiane Hassel, Heather M. O'Hagan, Kenneth P. Nephew. Mechanisms underlying platinum-induced ovarian cancer stem cell plasticity abstract. In: Proceedings of the AACR Special Conference on Ovarian Cancer; 2023 Oct 5-7; Boston, Massachusetts. Philadelphia (PA): AACR; Cancer Res 2024;84 (5 Suppl₂): Abstract nr A112.