p21^CIP1 attenuates Ras- and c-Myc-dependent breast tumor epithelial mesenchymal transition and cancer stem cell-like gene expression in vivo

p21^CIP1/WAF1 is a downstream effector of tumor suppressors and functions as a cyclin-dependent kinase inhibitor to block cellular proliferation. Breast tumors may derive from self-renewing tumor-initiating cells (BT-ICs), which contribute to tumor progression, recurrence, and therapy resistance. The role of p21^CIP1 in regulating features of tumor stem cells in vivo is unknown. Herein, deletion of p21^CIP1, which enhanced the rate of tumorigenesis induced by mammary-targeted Ha-Ras or c-Myc, enhanced gene expression profiles and immunohistochemical features of epithelial mesenchymal transition (EMT) and putative cancer stem cells in vivo. Silencing of p21^CIP1 enhanced, and expression of p21^CIP1 repressed, features of EMT in transformed immortal human MEC lines. p21^CIP1 attenuated oncogene-induced BT-IC and mammosphere formation. Thus, the in vitro cell culture assays reflect the changes observed in vivo in transgenic mice. These findings establish a link between the loss of p21^CIP1 and the acquisition of breast cancer EMT and stem cell properties in vivo.