Abstract B31: B-raf deletion and pharmacologic inhibition enhance K-ras-driven tumorigenesis

The Ras GTPase family controls numerous downstream signaling cascades in response to signals that regulate cellular processes including proliferation and survival. While Ras is one of the most prevalent targets for gain-of-function mutations in human tumors, questions remain regarding how the Ras effector pathway functions in mutant K-ras-driven tumorigenesis. Since an important function of K-ras involves B-Raf activation within the canonical MAPK signaling pathway, we initiated a study to determine B-raf's role in the context of mutant K-ras-driven tumor promotion and maintenance. We began by delivering adenovirus expressing the Cre recombinase to the lungs of genetically engineered mice possessing a conditional K-rasG12D allele (K-rasLSL-G12D) and either 0, 1 or both copies of the B-raf gene flanked by LoxP sites (B-rafCKO). This procedure results in expression of mutant K-rasG12D in the presence or absence of one or both B-raf alleles deleted within the mouse lung. Surprisingly, we observe that B-raf deletion significantly enhances lung tumor number and burden and decreases overall survival. When we used a highly specific small-molecule inhibitor that targets B-raf in a murine non-small cell lung carcinoma line harboring the K-rasG12D mutation, we observed an increase in cell proliferation and soft agar colony formation. Further investigation revealed that treating K-rasG12D expressing cells with the B-raf inhibitor enhanced MEK and Erk phosphorylation. Therefore, our data suggests that while B-raf deletion does not inhibit K-ras-driven tumor initiation and disease progression, its presence may play a pivotal role in establishing negative feedback regulation of constitutive mutant K-ras activity. Citation Information: Cancer Res 2009;69(23 Suppl):B31.