Abstract 1093: p16 promotes Rb deficient pulmonary epithelial progenitor cell growth

Abstract

Abstract The retinoblastoma (Rb)/p16 pathway is deregulated in virtually all lung cancers, providing evidence that Rb and p16 are essential regulators in the lung epithelium. Although Rb is critical for suppressing epithelial proliferation and death during lung development, progenitor cells compensate for Rb loss after birth. Previous studies demonstrate that this compensation is associated with increased p16 expression. The purpose of this study was to 1) determine whether p16 repression in vivo was unique to Rb or represented a shared pocket protein function and 2) test the hypothesis that p16 has Rb-independent functions critical for controlling pulmonary epithelial progenitor cell growth. p16 was induced in Rb deficient lungs wherein Rb ablation was targeted to the lung epithelium using the Cre/LoxP system. In contrast, p16 expression was not induced in lungs from p107−/− or p130−/− mice. Despite robust p16 expression, Rb deficient lungs did not undergo senescence as assessed by senescence associated β-galactosidase activity. To determine the effects of altered Rb/p16 pathway function on epithelial progenitor cell growth, primary type II cell cultures were established from Rb ablated, combined Rb ablated/p16−/−, and control Rb/p16 proficient lungs. Rb loss resulted in increased cell growth over controls. Interestingly, combined Rb/p16 loss resulted in decreased growth as compared to Rb ablation alone, indicating that p16 enhances Rb deficient pulmonary progenitor cell growth. Flow cytometric analysis and BrdU labeling revealed that loss of Rb led to increased cell survival and that additional loss of p16 decreased cell growth by promoting cell death and suppressing proliferation. Taken together, these data demonstrate that p16 induction in Rb deficient pulmonary progenitor cells does not lead to senescence but rather enhances cell growth by promoting cell survival and proliferation. These studies identify fundamental differences between Rb/p16 pathway regulation of pulmonary epithelial progenitor cells and previously described functions in fibroblasts: 1) p16 repression is an Rb unique function in the lung in vivo, whereas all three Rb family proteins repress p16 in fibroblasts and 2) Rb and Rb/p16 loss have profound effects on epithelial progenitor cell growth, whereas growth of Rb−/− and p16−/− murine embryo fibroblasts is similar to wild type controls. The current data provide evidence that p16 enhances survival and proliferation of Rb deficient epithelial progenitor cells, thus identifying a novel Rb-independent p16 function. Greater than 80% of human adult malignancies are epithelial derived carcinomas. Therefore, identifying cell type specific Rb/p16 pathway functions critical for controlling epithelial cell growth is a vital step toward understanding cancer pathogenesis. 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 1093.