Ovarian clear cell carcinoma (OCCC) is a rare subtype of gynecological cancer for which well-characterized and authenticated model systems are scarce. We provide an extensive characterization of ‘105C’, a cell line generated from an adenocarcinoma of the clear cell histotype using targeted next-generation sequencing, cytogenetic microarrays, along with analyses of AKT/mTOR signaling. We report that that the 105C cell line is a bona fide OCCC cell line, carrying PIK3CA, PTEN, and ARID1A gene mutations, consistent with OCCC, yet maintain a stable genome as reflected by low copy number variation. Unlike KOC-7c, TOV-21G, and RMG-V OCCC lines also mutated for the above genes, the 105C cells do not carry mutations in mismatch repair genes. Importantly, we show that 105C cells exhibit greater resistance to mTOR inhibition and carboplatin treatment compared to 9 other OCCC cell lines in 3D spheroid cultures. This resistance may be attributed to 105C cells remaining dormant in suspension culture which surprisingly, contrasts with several other OCCC lines which continue to proliferate in long-term suspension culture. 105C cells survive xenotransplantation but do not proliferate and metastasize. Collectively, we show that the 105C OCCC cell line exhibits unique properties useful for the pre-clinical investigation of OCCC pathobiology.

Epithelial ovarian cancer (EOC) exhibits a unique mode of metastasis, involving spheroid formation in the peritoneum. Our research on EOC spheroid cell biology has provided valuable insights into the signaling plasticity associated with metastasis. We speculate that EOC cells modify their biology between tumour and spheroid states during cancer dormancy, although the specific mechanisms underlying this transition remain unknown. Here, we present novel findings from direct comparisons between cultured EOC spheroids and organoids. Our results indicated that AMP-activated protein kinase (AMPK) activity was significantly upregulated and protein kinase B (Akt) was downregulated in EOC spheroids compared to organoids, suggesting a clear differential phenotype. Through RNA sequencing analysis, we further supported these phenotypic differences and highlighted the significance of cell cycle regulation in organoids. By inhibiting the G2/M checkpoint via kinase inhibitors, we confirmed that this pathway is essential for organoids. Interestingly, our results suggest that specifically targeting aurora kinase A (AURKA) may represent a promising therapeutic strategy since our cells were equally sensitive to Alisertib treatment as both spheroids and organoids. Our findings emphasize the importance of studying cellular adaptations of EOC cells, as there may be different therapeutic targets depending on the step of EOC disease progression.

Cancer cell spheroids autonomously form in the ascites fluid and are considered a conduit for epithelial ovarian cancer metastasis within the peritoneal cavity. Spheroids are homotypic, avascular 3D structures that acquire resistance to anoikis to remain viable after cellular detachment. We used in vitro spheroid model systems to interrogate pathways critical for spheroid cell proliferation, distinct from those driving monolayer cancer cell proliferation. Using the 105C and KOC-7c human ovarian clear cell carcinoma (OCCC) cell lines, which have distinct proliferative phenotypes as spheroids but the same prototypical OCCC gene mutation profile of constitutively activated AKT signaling with the loss of ARID1A, we revealed therapeutic targets that efficiently kill cells in spheroids. RNA-seq analyses compared the transcriptome of 3-day monolayer and spheroid cells from these lines and identified the characteristics of dormant spheroid cell survival, which included the G2/M checkpoint, autophagy, and other stress pathways induced in 105C spheroids, in sharp contrast to the proliferating spheroid cells of the KOC-7c cell line. Next, we assessed levels of various G2/M checkpoint regulators and found a consistent reduction in steady-state levels of checkpoint regulators in dormant spheroid cells, but not proliferative spheroids. Our studies showed that proliferative spheroid cells were sensitive to Wee1 inhibition by AZD1775, but the dormant spheroid cells showed a degree of resistance to AZD1775, both in terms of EC50 values and spheroid reattachment abilities. Thus, we identified biomarkers of dormant spheroids, including the G2/M checkpoint regulators Wee1, Cdc25c, and PLK1, and showed that, when compared to proliferating spheroid cells, the transcriptome of dormant OCCC spheroids is a source of therapeutic targets.

Multicellular cancer cell aggregates, termed spheroids, are anoikis-resistant, avascular, heterogeneous structures responsible for transcoelomic metastasis of ovarian clear cell carcinoma (OCCC). OCCC is a rare subtype of ovarian cancer with high ARID1A gene mutation rates, resulting in genome-wide changes to H3K27Ac levels and histone deacetylase (HDAC) function. Our study investigated the utility of HDAC inhibitor (HDACi) treatment and H3K27Ac dynamics in OCCC spheroids. By comparing KOC-7c and 105C OCCC cell lines, which have opposing abilities to proliferate as spheroids, we revealed that KOC-7c and 105C spheroids differentially regulated H3K27Ac levels, which correlated with the sensitivity of KOC-7c and the resistance of 105C spheroids to H3K27Ac-altering HDACi treatment. RNA-seq of Entinostat-treated versus vehicle-treated spheroids resulted in a dramatic change in the 105C spheroid transcriptome such that it more closely resembled the proliferative KOC-7c transcriptome over the short term. Comparative pathway analysis identified preferential de-repression of a G2/M checkpoint gene program in 105C spheroids upon Entinostat treatment when compared directly to the KOC-7c spheroids. Our results suggest that the utility of HDACi in OCCC is highly context-dependent.

Metastasis, chemoresistance, and tumour recurrence are facilitated by cancer stem cells (CSCs), a small subpopulation of cells capable of regenerating a primary tumour while maintaining the tumour’s genetic and phenotypic features. CSCs can be identified by the expression of specific markers; however, the CSC population in ovarian clear cell carcinoma (OCCC), a rare histotype of ovarian cancer, remains poorly defined. Given the well-established role that CSCs play in cancer progression and metastasis, it is critical to identify reliable markers of CSCs in OCCC. Here, we endeavoured to determine whether ALDH1 expression could be used to define OCCC stem cells in OCCC cell lines using a variety of methods including assessing ALDH1A1 expression in spheroids generated under distinct conditions. We also generated and used chemo-resistant cell lines to assess the enrichment of cancer stem cells. Human OCCC cell lines were enriched for CSCs using selective culture conditions and drug resistance methods. CSC-enriched spheroids demonstrated increased expression of stemness markers NANOG and SOX2, while ALDH1A1 expression was enriched only in drug-resistant cell lines, relative to parental cell lines. RNA-seq analyses of CSC-media-derived spheroids versus standard media spheroids provided novel data supporting CSC enrichment and identified transcription factors induced by CSC media. These findings highlight the ambiguous role of ALDH1A1 as a CSC marker in OCCC and demonstrates the utility of CSC enrichment methods for identifying CSC populations in OCCC cell lines.

Epithelial ovarian cancer (EOC) exhibits a unique mode of metastasis, involving spheroid formation in the peritoneum. Our research on EOC spheroid cell biology has provided valuable insights into the signalling plasticity associated with metastasis. We speculate that EOC cells modify their biology between tumor and spheroid states during cancer dormancy, although the specific mechanisms underlying this transition remain unknown. Here, we present novel findings from direct comparisons between cultured EOC spheroids and organoids. Our results indicated that AMP-activated protein kinase (AMPK) activity was significantly upregulated and protein kinase B (Akt) was down-regulated in iOvCa spheroids compared to organoids, suggesting a clear differential phenotype. Through a comprehensive RNA sequencing analysis, we further supported these phenotypic differences and highlighted the significance of cell cycle regulation in organoids. By inhibiting the G2/M checkpoint via kinase inhibitors, we confirmed that this pathway is essential for organoids. Interestingly, our results suggest that specifically targeting aurora kinase A (AURKA) may represent a promising therapeutic strategy since our cells were equally sensitive to Alisertib treatment as both spheroids and or-ganoids. Our findings emphasize the importance of studying cellular adaptations of EOC cells, as there may be different therapeutic targets depending on the step of EOC disease progression.