Human hepatocellular carcinoma (HCC) is the fifth most common cancer, and the third leading cause of cancer death worldwide. All conventional treatments for liver cancer are fraught with side effects and limited efficiency. Hence, it is important to identify novel drugs that target the underlying molecular events of HCC growth and interfere with HCC development by blocking cell division and selectively inducing tumor cell apoptosis. One particular gene that has gained attention in recent years due to its importance in a number of cancers including HCC is Forkhead Box M1 (FoxM1). FoxM1 is a transcription factor that regulates the expression of a number of genes that are involved in cell cycle regulation. Interestingly, it was found that FoxM1 is essential for the development of HCC in mice, suggesting that inhibition of FoxM1 might be a promising strategy for treating HCC. In our preliminary studies, we identified the antibiotic thiazole compounds Siomycin A and thiostrepton as potent inhibitors of FoxM1. The goal of this proposal is to investigate the molecular mechanisms of action of Siomycin A/thiostrepton and to test their efficacy in vivo in mouse models of liver cancer. In the first specific aim we will determine how Siomycin A/thiostrepton inhibit the transcriptional activity of FoxM1. Next, we will test the hypothesis that downregulation of FoxM1 is a key event that leads to Siomycin A/thiostrepton-induced apoptosis. To evaluate the efficacy of Siomycin A/thiostrepton as anticancer agents in vivo, we will use 3 human liver cancer cell lines that are sensitive to thiazole antibiotics in vitro to induce xenograft tumors in athymic mice. The effect of Siomycin A/thiostrepton treatment on FoxM1 expression and growth of xenograft tumors will be examined in these mice. Furthermore, to test the anticancer properties of the thiazole antibiotics we will use a Diethylnitrosamine (DEN)/Phenobarbital (PB) liver tumor induction protocol in male mice and a new ARF-/- Rosa-26 FoxM1b TG mouse model of aggressive metastatic liver cancer. We will test if Siomycin A/thiostrepton can inhibit HCC growth and metastasis in these mice. In addition, we will test whether the small fragment of thiostrepton representing the dehydropiperidine core also inhibits FoxM1 and exhibits anticancer properties against liver cancer. These data may not only help to develop new drugs against liver cancer, but also will help to better understand the etiology of HCC. If the thiazole antibiotics have low toxicity and lead to liver tumor regression in mice, we will conclude that these compounds may have a potential for further clinical development against liver cancer. Human hepatocellular carcinoma (HCC) is the fifth most common cancer, and the third leading cause of cancer death worldwide. All conventional treatments for liver cancer are fraught with side effects and limited efficiency. Hence, it is important to identify novel drugs that target the underlying molecular events of HCC growth and interfere with HCC development. It was shown that oncogene FoxM1 is essential for the development of HCC, suggesting that inhibition of FoxM1 might be a promising strategy for treating HCC. Thiazole antibiotics Siomycin A and thiostrepton were identified in our laboratory as potent inhibitors of FoxM1 and inducers of apoptosis in liver cancer cells. The goal of this proposal is to investigate the molecular mechanisms of action of Siomycin A/thiostrepton and to test their efficacy in vivo in mouse models of liver cancer. Thiazole antibiotics could be promising drugs against HCC because they induce cell death in cancer, but not in normal cells. Completion of our proposal will enable us to determine if thiazole antibiotics are suitable for further clinical development.