Keratin 5 marks cancer-propagating cells sustained by an osteopontin-producing niche in high-grade serous ovarian carcinoma

High-grade serous carcinoma (HGSC) is the most common and aggressive form of ovarian cancer. Advanced HGSCs exhibit pronounced cellular heterogeneity, including a subset of cancer-propagating cells (CPCs, also known as cancer stem cells) that are highly tumorigenic and display stem cell-associated properties such as self-renewal and chemoresistance. In contrast, a substantial fraction of HGSC cells is non-tumorigenic. The role of these non-cancer-propagating cells (non-CPCs) and their relationship to CPCs remain poorly understood. Here, we demonstrate that neoplastic cells expressing the intermediate filament protein keratin 5 (KRT5) represent bona fide CPCs. KRT5⁺ cells form cancer organoids over successive passages, are tumorigenic in serial dilution xenograft assays, and are resistant to the antineoplastic agents, doxorubicin and cisplatin. Single-cell lineage-tracing experiments show that KRT5⁺ CPCs give rise to KRT5^- cells. KRT5⁺ and KRT5^- populations exhibit distinct gene expression profiles, with KRT5^- cells characterized by expression of <i>SPP1</i>, which encodes the secreted factor osteopontin (OPN). Treatment with OPN enhances HGSC organoid growth and chemoresistance, whereas <i>SPP1</i> knockdown reverses these effects. Together, these findings support a model in which HGSC contains two hierarchically related cell populations: KRT5⁺, OPN-responsive CPCs and KRT5^-, non-tumorigenic cells that form a niche producing OPN. Targeting pathways that sustain both stem-like tumor cells and their supportive niche may enable reduced dosing of highly toxic chemotherapeutic agents while enhancing therapeutic efficacy in HGSC.