Abstract 3210: pRb, but not p107 or p130, is required for full induction of p16INK4a in human mammary epithelial cells

Pocket family proteins pRb, p107 and p130 are important cell cycle regulators in mammalian cells. In addition to controlling G1 to S transition, they also participate in differentiation and senescence. While all three pocket proteins have similar structures and apparently overlapping functions, defects in only one, pRb, are associated with human cancers. In breast cancer, pRb loss is associated with triple negative (ER-, PR-, HER2-) tumors, which tend to be more aggressive. To explore possible functions of pRb that may be unique, we investigated relative contributions of pocket proteins to cellular senescence initiated by high levels of the cyclin-dependent kinase inhibitor p16INK4a. For this purpose, we silenced pRb, p107 and p130 expression individually with shRNA constructs and exposed MCF7 and MDA-MB-231 breast cancer cell lines to tet-inducible p16. In both malignant lines, knockdown of any single pocket protein was insufficient to prevent p16-induced senescence, indicating the likely existence of compensatory activities of the three pocket proteins in mediating p16-induced senescence. However, our finding that the introduction of HPV E7 - which inactivates all three pocket proteins, prevented p16-induced senescence, is consistent with the hypothesis that as a group, pRb, p107 and p130 constitute essential mediators of p16-induced senescence. To examine the relative contributions of the pocket proteins to cellular senescence initiated by endogenous p16, we used cultured primary human mammary epithelial cells (HMEC) from non-malignant breast tissues. These cells spontaneously induce endogenous p16 and undergo senescence. When we examined the relative contributions of pocket proteins to senescence associated with increased endogenous p16 levels in HMEC, we found that suppressing pRb alone abrogated p16-induced senescence, whereas suppressing p107 or p130 did not. Remarkably, HMEC in which pRb alone was knocked down showed reduced p16 expression and longer proliferative life spans than controls or HMEC in which p107 or p130 were knocked down. These results suggest that pRb, but not p107 or p130, is required for full induction of endogenous p16 in breast cells, and provide a possible explanation for the special tumor-suppressor role of pRb. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 3210.