Background: CD44 is an important surface marker of breast cancer stem cells (BCSCs), but it is unclear whether it is involved in the stemness of BCSCs. This has limited the development of new therapeutic strategies for breast cancer. Previous studies have shown that many CD44 variants generated through alternative splicing are involved in the development of breast cancer, but their exact role in BCSCs remains unclear. Therefore, we analyzed the CD44 transcript variants in BCSCs derived from the MDA-MB-435 cell line, and aimed to investigate whether CD44s knockdown could affect the biological characteristics of BCSCs.Methods: CD44+/CD24- cells were isolated among the MDA-MB-435 cells by flow cytometry, and the CD44 transcript variants were detected by RT-PCR in CD44+/CD24- cells. Due to the high expression of CD44 standard splice isoform (CD44s) in CD44+/CD24- cells, CD44s knockdown was generated using small hairpin RNA (shRNA). The effects of CD44s knockdown on the biological characteristics of BCSCs was detected using cell proliferation assay, colony formation assay, cell cycle and apoptosis assay, tumor sphere formation assay, would-healing assay, and Matrigel invasion assay. Tumorigenesis of the CD44+/CD24- cells with CD44s knockdown was investigated in vivo with NOD/SCID mice. The expression of cancer stem cell stemness-related genes, such as Bcl-2, CCNE2, EGFR, MMP7, Muc1, and Myc was also detected by qPCR.Results: Our results revealed that the mRNA expression of CD44 transcript variants was heterogeneous, and CD44s is highly expressed in BCSCs. CD44s depletion inhibited the proliferation, made cell cycle stay in G0/G1 phase, promoted the apoptosis and necrosis of BCSCs, inhibited the ability of self-renewal and invasion along with the expression of cancer stem cell-related genes in BCSCs. Moreover, CD44s knockdown inhibited the tumorigenesis ability in vivo.Conclusion: Our findings revealed that CD44s is the predominant isoform expressed in BCSCs, and is an important molecule for maintaining the properties of BCSCs. Targeting CD44s in BCSCs may be a potential new direction for breast cancer treatment.
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