Exploring the Utility of ALDH1 as a Marker for the Cancer Stem Cell Population in OCCC Cell Lines

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.