In vitro characterization of CD133(lo) cancer stem cells in Retinoblastoma Y79 cell line

BACKGROUND: Retinoblastoma (Rb), the most common childhood intraocular malignant tumor, is reported to have cancer stem cells (CSCs) similar to other tumors. Our previous investigation in primary tumors identified the small sized cells with low CD133 (Prominin-1) and high CD44 (Hyaluronic acid receptor) expression to be putative Rb CSCs using flow cytometry (FSC(lo)/SSC(lo)/CD133(lo)/CD44(hi)). With this preliminary data, we have now utilized a comprehensive approach of in vitro characterization of Y79 Rb cell line following CSC enrichment using CD133 surface marker and subsequent validation to confirm the functional properties of CSCs. METHODS: The cultured Rb Y79 cells were evaluated for surface markers by flow cytometry and CD133 sorted cells (CD133(lo)/CD133(hi)) were compared for CSC characteristics by size/percentage, cell cycle assay, colony formation assay, differentiation, Matrigel transwell invasion assay, cytotoxicity assay, gene expression using microarray and validation by semi-quantitative PCR. RESULTS: Rb Y79 cell line shared the profile (CD133, CD90, CXCR4 and ABCB1) of primary tumors except for CD44 expression. The CD133(lo) cells (16.1 ± 0.2%) were FSC(lo)/SSC(lo), predominantly within the G0/G1 phase, formed larger and higher number of colonies with ability to differentiate to CD133(hi) cells, exhibited increased invasive potential in a matrigel transwell assay (p < 0.05) and were resistant to Carboplatin treatment (p < 0.001) as compared to CD133(hi) cells. The CD133(lo) cells showed higher expression of several embryonic stem cell genes (HOXB2, HOXA9, SALL1, NANOG, OCT4, LEFTY), stem cells/progenitor genes (MSI2, BMI1, PROX1, ABCB1, ABCB5, ABCG2), and metastasis related gene- MACC1, when compared to the CD133(hi) cells. CONCLUSIONS: This study validates the observation from our earlier primary tumor study that CSC properties in Rb Y79 cell line are endowed within the CD133(lo) population, evident by their characteristics- i.e. small sized, dormant in nature, increased colony forming ability, differentiation to CD133(hi) cells, higher invasiveness potential, drug resistance and primitive gene expression pattern. These findings provide a proof of concept for methodological characterization of the retinoblastoma CSCs with future implications for improved diagnostic and treatment strategies.