Temporal Dissection of K-ras^G12D Mutant <em>In Vitro</em> and <em>In Vivo</em> Using a Regulatable K-ras^G12D Mouse Allele

<div><p>Animal models which allow the temporal regulation of gene activities are valuable for dissecting gene function in tumorigenesis. Here we have constructed a conditional inducible estrogen receptor-<em>K-ras^G12D</em> (<em>ER-K-ras^G12D</em>) knock-in mice allele that allows us to temporally switch on or off the activity of K-ras oncogenic mutant through tamoxifen administration. <em>In vitro</em> studies using mice embryonic fibroblast (MEF) showed that a dose of tamoxifen at 0.05 µM works optimally for activation of <em>ER-K-ras^G12D</em> independent of the gender status. Furthermore, tamoxifen-inducible activation of <em>K-ras^G12D</em> promotes cell proliferation, anchor-independent growth, transformation as well as invasion, potentially via activation of downstream MAPK pathway and cell cycle progression. Continuous activation of <em>K-ras^G12D in vivo</em> by tamoxifen treatment is sufficient to drive the neoplastic transformation of normal lung epithelial cells in mice. Tamoxifen withdrawal after the tumor formation results in apoptosis and tumor regression in mouse lungs. Taken together, these data have convincingly demonstrated that K-ras mutant is essential for neoplastic transformation and this animal model may provide an ideal platform for further detailed characterization of the role of <em>K-ras</em> oncogenic mutant during different stages of lung tumorigenesis.</p>