Diminished WNT → β-catenin → c-MYC signaling is a barrier for malignant progression of BRAF^V600E-induced lung tumors

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 BRAF^V600E 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 BRAF^V600E-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 BRAF^V600E-initiated lung tumorigenesis. Conversely, sustained activity of β-catenin or c-MYC significantly enhanced BRAF^V600E-induced lung tumorigenesis and rescued the anti-tumor effects of Porcupine blockade. These data indicate that early stage BRAF^V600E-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 BRAF^V600E-initiated lung tumors. Moreover, these data further suggest that the trigger for OIS in initiated BRAF^V600E-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.