No abstract

Oncogene-induced senescence (OIS) is proposed as a cellular defense mechanism that restrains malignant progression of oncogene-expressing, initiated tumor cells. Consistent with this, expression of BRAFV600E in the mouse lung epithelium elicits benign tumors that fail to progress to cancer due to an apparent senescence-like proliferative arrest. Here we demonstrate that nuclear β-catenin → c-MYC signaling is essential for early stage proliferation of BRAFV600E-induced lung tumors and is inactivated in the subsequent senescence-like state. Furthermore, either β-catenin silencing or pharmacological blockade of Porcupine, an acyl-transferase essential for WNT ligand secretion and activity, significantly inhibited BRAFV600E-initiated lung tumorigenesis. Conversely, sustained activity of β-catenin or c-MYC significantly enhanced BRAFV600E-induced lung tumorigenesis and rescued the anti-tumor effects of Porcupine blockade. These data indicate that early stage BRAFV600E-induced lung tumors are WNT-dependent and suggest that inactivation of WNT → β-catenin → c-MYC signaling is a trigger for the senescence-like proliferative arrest that constrains the expansion and malignant progression of BRAFV600E-initiated lung tumors. Moreover, these data further suggest that the trigger for OIS in initiated BRAFV600E-expressing lung tumor cells is not simply a surfeit of signals from oncogenic BRAF but an insufficiency of WNT → β-catenin → c-MYC signaling. These data have implications for understanding how genetic abnormalities cooperate to initiate and promote lung carcinogenesis.

Genetically engineered mouse (GEM) models of lung tumorigenesis allow careful evaluation of lung tumor initiation, progression, and response to therapy. Using GEM models of oncogene-induced lung cancer, we demonstrate the striking similarity of the earliest stages of tumorigenesis induced by KRASG12D or BRAFV600E. Cre-mediated expression of KRASG12D or BRAFV600E in the lung epithelium of adult mice initially elicited benign lung tumors comprising cuboidal epithelial cells expressing markers of alveolar pneumocytes. Strikingly, in a head-to-head comparison, oncogenic BRAFV600E elicited many more such benign tumors and did so more rapidly than KRASG12D. However, despite differences in the efficiency of benign tumor induction, only mice with lung epithelium expression of KRASG12D developed malignant non-small-cell lung adenocarcinomas. Pharmacologic inhibition of MEK1/2 combined with in vivo imaging demonstrated that initiation and maintenance of both BRAFV600E- or KRASG12D-induced lung tumors was dependent on MEK→ERK signaling. Although the tumors dramatically regressed in response to MEK1/2 inhibition, they re-grew following cessation of drug treatment. Together, our findings demonstrate that RAF→MEK→ERK signaling is both necessary and sufficient for KRASG12D-induced benign lung tumorigenesis in GEM models. The data also emphasize the ability of KRASG12D to promote malignant lung cancer progression compared with oncogenic BRAFV600E.

Due to a production error, a source of funding was omitted from the authors' acknowledgments. The grant information has been restored in the html and pdf versions of the paper, and the error remains only in print. The corrected acknowledgment paragraph appears below.

Lung carcinogenesis is a multistep process in which normal lung epithelial cells are converted to cancer cells through the sequential acquisition of multiple genetic or epigenetic events. Despite the utility of current genetically engineered mouse (GEM) models of lung cancer, most do not allow temporal dissociation of the cardinal events involved in lung tumor initiation and cancer progression. Here we describe a novel two-switch GEM model for BRAFV600E-induced lung carcinogenesis allowing temporal dissociation of these processes. In mice carrying a Flp recombinase-activated allele of Braf (BrafFA) in conjunction with Cre-regulated alleles of Trp53, Cdkn2a or c-MYC, we demonstrate that secondary genetic events can promote bypass of the senescence-like proliferative arrest displayed by BRAFV600E-induced lung adenomas leading to malignant progression. Moreover, restoring or activating TP53 in cultured BRAFV600E/TP53Null or BRAFV600E/INK4A-ARFNull lung cancer cells triggered a G1 cell cycle arrest regardless of p19ARF status. Perhaps surprisingly, neither senescence nor apoptosis was observed upon TP53 restoration. Our results establish a central function for the TP53 pathway in restricting lung cancer development, highlighting the mechanisms that limit malignant progression of BRAFV600E-initiated tumors.