Abstract The long-term goals of this project are to advance sphingosine kinase 1 (SK1), an important enzyme in bioactive sphingolipid metabolism and signaling, as a novel and critical therapeutic target for p53 null and mutant cancers. SK1, one of two SK enzymes, is a highly regulated enzyme that plays a critical role in the production of the bioactive sphingolipid sphingosine-1-phosphate (S1P), and in the clearance of sphingosine and ceramide in the `exit pathway' of sphingolipid breakdown. S1P has emerged as a tumor-promoting lipid with actions on cell growth, angiogenesis and protection from apoptosis whereas ceramide has emerged as a growth suppressor lipid. Thus, SK1, by regulating the interconversion of these critical bioactive lipids, assumes a central role in tumor biology. Our work over the last 15 years has led to i) discovering that SK1 is overexpressed in many cancers both at the message and protein level, ii) that SK1 gets degraded by DNA damaging agents in a caspase 2 and p53-dependent manner, iii) that null p53 cells and animal tissues have increased expression of SK1, and that iv) SK1 KO mice are significantly protected from p53 null thymic lymphoma as well as several other types of tumors in p53 heterozygous mice. Moreover, our very recent unpublished data demonstrate that SK1 is degraded upon serine/glycine (Ser/Gly) deprivation. We have exploited this to show that serine deprivation results in death of p53 null and mutant cells in a mechanism involving loss of SK1. These and related findings have led us to propose the hypothesis that SK1 is a therapeutic target for p53 null and mutant cancers, and that Ser/Gly deprivation can be harnessed therapeutically to degrade SK1 and lead to cancer cell death. This hypothesis will be investigated by pursuing the following specific aims: Specific Aim 1. To establish that SK1 is a therapeutic target for p53 null and mutant thymic lymphomas in vivo by: A. Establishing that SK1 KO protects from tumor development in a humanized p53 mutant mouse model; and B. Determining if removal of SK1 after established thymic lymphomas are formed in p53 null and mutant mice can lead to tumor regression. Specific Aim 2. To advance SK1 as a novel therapeutic target for p53 null and mutant tumors in mice by: A. Determine if Ser/Gly deprivation in vivo will lead to tumor regression or inhibit tumor formation in the above models. B. Determining if inhibiting SK1 leads to the tumor regression by utilizing pharmacologic inhibitors for SK1 (SKI, PF543). Together, these studies will significantly advance our understanding of the roles of SK1 in cancers with mutated p53. They will also define specific therapeutic contexts for targeting SK1 in human cancer.