Dynamic regulation of CD24 and the invasive, CD44(pos)CD24(neg )phenotype in breast cancer cell lines

INTRODUCTION: The invasive, mesenchymal phenotype of CD44(pos)CD24(neg )breast cancer cells has made them a promising target for eliminating the metastatic capacity of primary tumors. It has been previously demonstrated that CD44(neg/low)CD24(pos )breast cancer cells lack the ability to give rise to their invasive CD44(pos)CD24(neg )counterpart. Here we demonstrate that noninvasive, epithelial-like CD44(pos)CD24(pos )cells readily give rise to invasive, mesenchymal CD44(pos)CD24(neg )progeny in vivo and in vitro. This interconversion was found to be dependent upon Activin/Nodal signaling. METHODS: Breast cancer cell lines were sorted into CD44(pos)CD24(pos )and CD44(pos)CD24(neg )populations to evaluate their progeny for the expression of CD44, CD24, and markers of a mesenchymal phenotype. The populations, separated by fluorescence activated cell sorting (FACS) were injected into immunocompromised mice to evaluate their tumorigenicity and invasiveness of the resulting xenografts. RESULTS: CD24 expression was dynamically regulated in vitro in all evaluated breast cancer cell lines. Furthermore, a single noninvasive, epithelial-like CD44(pos)CD24(pos )cell had the ability to give rise to invasive, mesenchymal CD44(pos)CD24(neg )progeny. Importantly, this interconversion occurred in vivo as CD44(pos)CD24(pos )cells gave rise to xenografts with locally invasive borders as seen in xenografts initiated with CD44(pos)CD24(neg )cells. Lastly, the ability of CD44(pos)CD24(pos )cells to give rise to mesenchymal progeny, and vice versa, was blocked upon ablation of Activin/Nodal signaling. CONCLUSIONS: Our data demonstrate that the invasive, mesenchymal CD44(pos)CD24(neg )phenotype is under dynamic control in breast cancer cell lines both in vitro and in vivo. Furthermore, our observations suggest that therapies targeting CD44(pos)CD24(neg )tumor cells may have limited success in preventing primary tumor metastasis unless Activin/Nodal signaling is arrested.