Mutationally activated BRAF V600E (BRAF VE) is detected in ∼6% of human malignancies and promotes sustained MEK1/2-ERK1/2 pathway activation. We have designed BRaf CA mice to express normal BRaf prior to Cre-mediated recombination after which BRaf VE is expressed at physiological levels. BRaf CA mice infected with an Adenovirus expressing Cre recombinase developed benign lung tumors that only rarely progressed to adenocarcinoma. Moreover, BRaf VE -induced lung tumors were prevented by pharmacological inhibition of MEK1/2. BRaf VE expression initially induced proliferation that was followed by growth arrest bearing certain hallmarks of senescence. Consistent with Ink4a/Arf and TP53 tumor suppressor function, BRaf VE expression combined with mutation of either locus led to cancer progression.Supplemental material is available at http://www.genesdev.org.

The four tissue inhibitors of metalloproteinases (TIMPs) are endogenous inhibitors that regulate the activity of matrix metalloproteinases (MMPs) and certain disintegrin and metalloproteinase (ADAM) family proteases in mammals. The protease inhibitory activity is present in the N-terminal domains of TIMPs (N-TIMPs). In this work, the N-terminal inhibitory domain of the only TIMP produced by Drosophila (dN-TIMP) was expressed in Escherichia coli and folded in vitro. The purified recombinant protein is a potent inhibitor of human MMPs, including membrane-type 1-MMP, although it lacks a disulfide bond that is conserved in all other known N-TIMPs. Titration with the catalytic domain of human MMP-3 [MMP-3(DeltaC)] showed that dN-TIMP prepared by this method is correctly folded and fully active. dN-TIMP also inhibits, in vitro, the activity of the only two MMPs of Drosophila, dm1- and dm2-MMPs, indicating that the Drosophila TIMP is an endogenous inhibitor of the Drosophila MMPs. dN-TIMP resembles mammalian N-TIMP-3 in strongly inhibiting human tumor necrosis factor-alpha-converting enzyme (TACE/ADAM17) but is a weak inhibitor of human ADAM10. Models of the structures of dN-TIMP and N-TIMP-3 are strikingly similar in surface charge distribution, which may explain their functional similarity. Although the gene duplication events that led to the evolutionary development of the four mammalian TIMPs might be expected to be associated with functional specialization, Timp-3 appears to have conserved most of the functions of the ancestral TIMP gene.