Lung cancer is the leading cause of cancer-related mortality in both men and women worldwide. To identify novel factors that contribute to lung cancer pathogenesis, we analyzed a lung cancer database from The Cancer Genome Atlas and found that Krüppel-like Factor 4 (KLF4) expression is significantly lower in patients' lung cancer tissue than in normal lung tissue. In addition, we identified seven missense mutations in the KLF4 gene. KLF4 is a transcription factor that regulates cell proliferation and differentiation as well as the self-renewal of stem cells. To understand the role of KLF4 in the lung, we generated a tamoxifeninduced Klf4 knockout mouse model. We found that KLF4 inhibits lung cancer cell growth and that depletion of Klf4 altered the differentiation pattern in the developing lung. To understand how KLF4 functions during lung tumorigenesis, we generated the K-ras LSL-G12D/+ ;Klf4 fl/fl mouse model, and we used adenovirus-expressed Cre to induce K-ras activation and Klf4 depletion in the lung. Although Klf4 deletion alone or K-ras mutation alone can trigger lung tumor formation, Klf4 deletion combined with K-ras mutation significantly enhanced lung tumor formation. We also found that Klf4 deletion in conjunction with K-ras activation caused lung inflammation. To understand the mechanism whereby KLF4 is regulated during lung tumorigenesis, we analyzed KLF4 promoter methylation and the profiles of epigenetic factors. We found that Class I histone deacetylases (HDACs) are overexpressed in lung cancer and that HDAC inhibitors induced expression of KLF4 and inhibited proliferation of lung cancer cells, suggesting that KLF4 is probably repressed by histone acetylation and that HDACs are valuable drug targets for lung cancer treatment.

Gastric cancer is one of the most common types of cancer in the world, particularly in underdeveloped countries. The mechanism of gastric cancer is less understood compared with other types of gastrointestinal (GI) cancers. Krüppel-like factor 4 (KLF4) is a zinc-finger transcription factor and is a potential tumor suppressor in GI cancers. In this study, we have generated two mouse models, Rosa-Cre;Klf4fl/fl and Lgr5-Cre;Klf4fl/fl. KLF4 was deleted by Rosa-Cre in the gastric epithelia cells or by Lgr5-Cre in the antral stem cells in the adult mice. KLF4 deletion resulted in increased proliferating cells and decreased pit mucous cells. Surprisingly, the intestinal goblet cell marker, MUC2, which is not expressed in normal gastric tissues, was strongly induced at the base of the KLF4-deleted antral glands. To understand the clinical relevance of these findings, we analyzed the expression of KLF4 and MUC2 in human gastric cancer. In a subset of human gastric cancer, the expression of KLF4 is negatively associated with MUC2 expression. In conclusion, KLF4 is essential for normal homeostasis of antral stem cells; loss of KLF4 and expression of MUC2 could be important markers for gastric cancer diagnosis.