Doxycycline-Regulated p16 Expression Suppresses the Anchorage-Independence and Tumorigenicity of Breast Cancer Cell Lines that Lack Endogenous p16.

The RB pathway controls the critical transition from G1 into S phase of the mammalian cell cycle. Deregulation of the RB pathway by means of RB or p16 inactivation has been implicated in the development of virtually all human cancers. Such findings have led to the view that the loss of RB-mediated regulation at the G1/S checkpoint is a precondition for human malignancy. Our analysis of the RB-positive MCF-7 and ZR75.1 breast cancer cell lines revealed a lack of endogenous p16 protein expression as a result of the homozygous deletion and methylation of the p16 gene at the CDKN2A locus, respectively. We employed the TET-OFF inducible expression system to investigate the effects of non-growth inhibitory levels of functional p16 protein upon the in vitro and in vivo transformed properties of the MCF-7 and ZR75.1 cell lines. Stable transfectants of MCF-7 and ZR75.1 cells were isolated that expressed different levels of p16 protein in the absence of doxycycline (DOX) but continued to proliferate in culture. Transfectants that expressed modest levels of p16 (relative to SV40 T antigen-transformed HBL-100 breast epithelial cells) demonstrated a marked suppression of anchorage-independent growth in soft agar. Further, the induction of moderate and high levels of p16 (relative to HBL-100) resulted in the suppression of tumorigenicity of both MCF-7 and ZR75.1 cells as assayed by injection into nude mice. From these data, we concluded that RB pathway restoration by non-growth inhibitory levels of p16 protein was sufficient to revert breast cancer cells to a non-transformed and non-tumorigenic state.