Early differential responses elicited by BRAFV600E in adult mouse models.

ABSTRACT

The BRAF gene is frequently mutated in cancer. The most common genetic mutation is a single nucleotide transition which gives rise to a constitutively active BRAF kinase (BRAFV600E) which in turn sustains continuous cell proliferation. The study of BRAFV600E murine models has been mainly focused on the role of BRAFV600E in tumor development but little is known on the early molecular impact of BRAFV600E expression in vivo. Here, we study the immediate effects of acute ubiquitous BRAFV600E activation in vivo. We find that BRAFV600E elicits a rapid DNA damage response in the liver, spleen, lungs but not in thyroids. This DNA damage response does not occur at telomeres and is accompanied by activation of the senescence marker p21CIP1 only in lungs but not in liver or spleen. Moreover, in lungs, BRAFV600E provokes an acute inflammatory state with a tissue-specific recruitment of neutrophils in the alveolar parenchyma and macrophages in bronchi/bronchioles, as well as bronchial/bronchiolar epithelium transdifferentiation and development of adenomas. Furthermore, whereas in non-tumor alveolar type II (ATIIs) pneumocytes, acute BRAFV600E induction elicits rapid p53-independent p21CIP1 activation, adenoma ATIIs express p53 without resulting in p21CIP1 gene activation. Conversely, albeit in Club cells BRAFV600E-mediated proliferative cue is more exacerbated compared to that occurring in ATIIs, such oncogenic stimulus culminates with p21CIP1-mediated cell cycle arrest and apoptosis. Our findings indicate that acute BRAFV600E expression drives an immediate induction of DNA damage response in vivo. More importantly, it also results in rapid differential responses of cell cycle and senescence-associated proteins in lung epithelia, thus revealing the early molecular changes emerging in BRAFV600E-challenged cells during tumorigenesis in vivo.