PROJECT SUMMARY Hepatocellular carcinoma (HCC) is one of the deadliest cancers worldwide. Often the disease is diagnosed at a late stage of high-grade cancer progression that is resistant to the available therapies. HCC is also a significant problem among the Veterans. According to VA Research Currents (Mike Richman, 2017, VA website) the outlook for hepatocellular carcinoma among the US Veterans is bleak, as it is increasing at a rate about five times greater than that of general US population. A 2015 study indicated that HCC among US Veterans is going peak in the year of 2021. Clearly, a greater understanding of the molecular mechanisms underlying HCC will be important for development of efficacious therapeutic strategies. This proposal focuses on the fork-head box M1 (FoxM1) gene because it is essential for HCC development and progression. Also, overexpression of FoxM1 in HCC predicts poor prognosis. FoxM1 has been extensively studied as an activator of genes involved in proliferation, pluripotency, and metastasis. Recently, we discovered a repression function of FoxM1. In this proposal we will investigate the hypothesis is that the newly discovered repression function of FoxM1 plays critical roles in the development and progression of HCC. We are using a Ras-transgenic model for HCC, mainly because the Ras-pathway is ubiquitously activated in human HCC through epigenetic silencing of its regulators. Using that model, we showed that conditional deletion of FoxM1 after HCC development causes preferential loss of the liver cancer stem cells (LCSCs). Interestingly, we observed that FoxM1 represses the liver differentiation gene FoxA2. Repression of FoxA2 by FoxM1 involves interactions of FoxM1 with the Rb-family proteins, and it occurs in G1 phase of the cell cycle. We will investigate the hypothesis that G1 phase repression of FoxM1 is important for the mechanism by which FoxM1 supports the liver cancer stem cells. We have developed a novel knock-in mouse strain that expresses a mutant FoxM1 deficient in the repression function, but retains its ability to activate transcription. That strain will be used to investigate the significance of the repression function in liver cancer development and liver cancer stem cells. In addition, using that knock-in mouse strain, we will determine whether the repression function of FoxM1 is important for recovery from chronic liver injury. The specific aims are: 1. Investigate the G1 phase repression function of FoxM1 in maintenance of the liver cancer stem cells. 2. Investigate the repression function of FoxM1 in HCC development and progression. 3. Investigate whether the repression function of FoxM1 is important for recovery from chronic liver injury.