Retinoblastoma family proteins have distinct functions in pulmonary epithelial cells in vivo critical for suppressing cell growth and tumorigenesis

Lung cancer is the leading cause of cancer deaths, accounting for more deaths than breast, colon, and prostate cancer combined. The retinoblastoma (Rb)/p16 tumor suppressive pathway is deregulated in most cancers. Loss of p16 occurs more frequently than Rb loss, suggesting that p16 suppresses cancer by regulating Rb as well as the related proteins p107 and p130. However, direct evidence demonstrating that p130 or p107 cooperate with Rb to suppress epithelial cancers associated with p16 loss is currently lacking. Moreover, the roles of p130 and p107 in lung cancer are not clear. In the present studies, Rb ablation was targeted to the lung epithelium in wild-type, p107, or p130 null mice to determine unique and overlapping Rb family functions critical in tumor suppression. Rb ablation during development resulted in marked epithelial abnormalities despite p107 upregulation. In contrast, p130 and p107 were not required during development but had distinct functions in the Rb-deficient epithelium: p107 was required to suppress proliferation, whereas a novel proapoptotic function was identified for p130. Adult Rb-ablated lungs lacked the epithelial phenotype seen at birth and showed compensatory p107 upregulation and p16 induction in epithelial cell lineages that share phenotypic characteristics with human non-small cell lung cancers (NSCLC) that frequently show p16 loss. Importantly, Rb/p107-deficient, but not Rb/p130-deficient, lungs developed tumors resembling NSCLC. Taken together, these studies identify distinct Rb family functions critical in controlling epithelial cell growth, and provide direct evidence that p107 cooperates with Rb to protect against a common adult cancer.

Retinoblastoma function is essential for establishing lung epithelial quiescence after injury

The retinoblastoma gene product (RB) regulates cell cycle, quiescence, and survival in a cell type-dependent and environment-dependent manner. RB function is critical in the pulmonary epithelium, as evidenced by nearly universal RB inactivation in lung cancer and increased lung cancer risk in persons with germline RB gene mutations. Lung carcinomas occur in the context of epithelial remodeling induced by cytotoxic damage. Whereas the role of RB in development and normal organ homeostasis has been extensively studied, RB function in the context of cellular injury and repair has remained largely unexplored. In the current studies, the RB gene was selectively deleted in the respiratory epithelium of the mouse. Although RB was not required for establishing or maintaining quiescence during lung homeostasis, RB was essential for establishing quiescence during epithelial repair after injury. Notably, aberrant cell cycle progression was sustained for 9 months after injury in RB-deficient lungs. Prenatal and postnatal RB ablation had similar effects, providing evidence that timing of RB loss was not critical to the outcome and that the injury-induced phenotype was not secondary to compensatory alterations occurring during development. These data show that RB is essential for repair of the respiratory epithelium after cytotoxic damage and support a critical unique role for RB in the context of epithelial remodeling after injury. Because human cancers are associated with chronic cellular damage, these findings have important new implications for RB-mediated tumor suppression.