Foxm1 in Lung Tumors Figure 7. Foxm1 is essential for lung tumor growth induced by MCA/BHT. Experimental epFoxm12/2 (epFoxm1fl/fl+Dox) and control Foxm1fl/fl (Foxm1fl/fl+Dox) mice were given Dox in water to activate Cre expression either 4 weeks prior to or 1 week after tumor induction/promotion with MCA/ BHT as described in Materials and Methods. The epFoxm1fl/fl (epFoxm1fl/fl – Dox) mice given regular water were used as controls for possible Dox- independent recombination. All three groups of mice were injected with a single dose of MCA, followed by six weekly injections of BHT to induce lung tumors. Mice were sacrificed at 25 weeks after MCA injection and examined for lung tumors using a dissecting microscope. A. Experimental design for conditional deletion of Foxm1fl/fl prior to tumor induction/promotion (left panel). epFoxm12/2 mice are resistant to lung tumorigenesis after MCA/BHT treatment compared to control mice (right panel). Mean number of tumors per lung (6SD) was calculated from n = 10 mouse lungs per group. B. Experimental design for conditional deletion of Foxm1fl/fl after tumor induction/promotion. epFoxm12/2 mice are resistant to lung tumorigenesis after MCA/BHT treatment compared to control mice (right panel). Mean number of tumors per lung (6SD) was calculated from n = 10 mouse lungs per group. C. Decreased levels of Foxm1, TOPO-2a, PPARc and cyclin B1 mRNAs and increased levels of PPARa in epFoxm12/2 (epFoxm1fl/fl+Dox) lung. qRT-PCR was performed using total RNA from control Foxm1fl/fl and epFoxm12/2 lungs. Three mice per group were used. Asterisks **indicate P values,0.001 calculated by Student T Test. doi:10.1371/journal.pone.0006609.g007 and sizes of lung tumors induced with MCA/BHT in transgenic mice [21]. Thus, previous gain-of-function or loss-of-function studies with ubiquitous changes in Foxm1 expression had identified Foxm1 as an important transcription factor during lung tumorigenesis. However, lung cancer lesions contain a heteroge- neous population of cells, that includes epithelial, inflammatory (macrophages, granulocytes) and stromal cells, that express increased levels of Foxm1 [22] and may influence tumorigenesis. Although our previous studies emphasized the essential role of Foxm1 in lung tumorigenesis, its specific role in lung epithelial cells, the precursors of lung adenoma or adenocarcinoma cells, was not addressed. In the present study, deletion of Foxm1 from respiratory epithelial cells was sufficient to significantly decrease lung tumor formation, demonstrating the cell autonomous role of Foxm1 in the progression of pulmonary tumors. Two different models of lung cancer in mice were used. In the first model, urethane acts as a complete carcinogen leading to DNA damage with K-ras mutations and providing both initiation PLoS ONE | www.plosone.org 8 August 2009 | Volume 4 | Issue 8 | e6609