Identification of ABCG2(+) cells in nasopharyngeal carcinoma cells

Tumor stem cells are a small subset of tumor cells with the ability of self-renewal and differentiation and are regarded as a cause of tumor growth and recurrence. Previously we have shown that stem-like label-retaining cells (LRCs) can be detected in nasopharynx, tongue, esophagus and xenograft tumors formed by nasopharyngeal carcinoma (NPC) cell lines (5–8F, 6–10B and TMNE). The present study aimed to identify ABCG2(+) cells in 5–8F NPC cells and compare their tumorigenic potential with ABCG2(−) cells, expecting that we can obtain insight into the mechanism of the differential phenotypes of ABCG2(+) and ABCG2(−) cells. By using magnetic cell sorting (MACS) method, we isolated ABCG2(+) cells and ABCG2(−) cells from 5–8F cells. Among these two subpopulations and unsorted 5–8F cells, the rate of ABCG2(+) cells at G1 phase was highest, while the rate of ABCG2(−) cells at S phase was highest, indicating that ABCG2(+) cells were mostly quiescent. However, ABCG2(+) cells showed lower cloning efficiency and tumorigenicity than ABCG2(−) cells. We also used Affymetrix U133 plus 2.0 human whole genome expression chip to identify the gene expression profile of ABCG2(+) and ABCG2(−) cells and found that both subpopulations expressed some stem cell associated genes, e.g., PSCA, ABCG2 and ALPI were expressed in ABCG2(+) cells, and K19, integrin α6, integrin β4, CD44 and K14 were expressed in ABCG2(−) cells, suggesting there were stem cells in both ABCG2(+) and ABCG2(−) cells. Our data demonstrated that there exist ABCG2(+) cells in NPC cells, but ABCG2 alone is not sufficient for isolating cancer stem cells in 5–8F NPC cells.