Identification of cancer initiating cells in K-Ras driven lung adenocarcinoma

Abstract

Ubiquitous expression of a resident K-RasG12V oncogene in adult mice revealed that most tissues are resistant to K-Ras oncogenic signals. Indeed, K-RasG12V expression only induced overt tumors in lungs. To identify these transformation-permissive cells, we induced K-RasG12V expression in a very limited number of adult lung cells (0.2%) and monitored their fate by X-Gal staining, a surrogate marker coexpressed with the K-RasG12V oncoprotein. Four weeks later, 30% of these cells had proliferated to form small clusters. However, only SPC+ alveolar type II (ATII) cells were able to form hyperplastic lesions, some of which progressed to adenomas and adenocarcinomas. In contrast, induction of K-RasG12V expression in lung cells by intratracheal infection with adenoviral-Cre particles generated hyperplasias in all regions except the proximal airways. Bronchiolar and bronchioalveolar duct junction hyperplasias were primarily made of CC10+ Clara cells. Some of them progressed to form benign adenomas. However, only alveolar hyperplasias, exclusively made up of SPC+ ATII cells, progressed to yield malignant adenocarcinomas. Adenoviral infection induced inflammatory infiltrates primarily made of T and B cells. This inflammatory response was essential for the development of K-RasG12V–driven bronchiolar hyperplasias and adenomas, but not for the generation of SPC+ ATII lesions. Finally, activation of K-RasG12V during embryonic development under the control of a Sca1 promoter yielded CC10+, but not SPC+, hyperplasias, and adenomas. These results, taken together, illustrate that different types of lung cells can generate benign lesions in response to K-Ras oncogenic signals. However, in adult mice, only SPC+ ATII cells were able to yield malignant adenocarcinomas.