PROJECT 4: Targeting Methionine Adenosyltransferases in Liver Metastasis ABSTRACT A key central hypothesis for this Program is that the normal liver has defense mechanisms to suppress metastatic growth. Project 4 is focused on the role of methionine adenosyltransferase (MAT) proteins, with MAT1A defending against liver metastasis, while MAT2A and MAT2B break down this defense. MAT catalyzes the formation of S-adenosylmethionine (SAMe), the methyl donor. The MAT1A and MAT2A genes encode MAT α1 and α2 catalytic subunits, respectively; a third gene, MAT2B, encodes the β subunit that regulates the MAT2A- encoded enzyme. MAT1A, expressed in normal liver, is downregulated in liver diseases, whereas MAT2A and MAT2B, expressed in non-hepatic tissues, are overexpressed in multiple human cancers. Preliminary data show that Mat1a knockout (KO) mice are highly sensitized to develop liver metastasis and release extracellular vesicles that contain oncoproteins; whereas MAT2A/MAT2B have a reciprocal inhibition interplay with metastasis-inhibiting tumor suppressor miRNAs miR-34a and miR-34b, but have a positive feed-forward loop with the metastasis-promoting oncogene Forkhead box M1 (FOXM1). In contrast, MAT1A and FOXM1 negatively regulate each other. Project 4 examines the effect of three molecules that target these interplays to inhibit liver metastasis: SAMe; its metabolite methylthioadenosine (MTA); and the FOXM1 inhibitor FDI-6. This project tests the hypothesis that high MAT2A/2B expression in cancer cells enhances liver metastasis by lowering hepatic MAT1A, and that SAMe, MTA, and FDI-6 inhibit liver metastasis in part by targeting MAT proteins. Importantly, human livers with metastatic cancers have lower MAT1A expression and the GEO database shows metastatic colon, pancreatic and prostate cancers have higher MAT2A/2B expression. Core B will provide human specimens and animal models to examine these MAT proteins in liver metastases from colon, pancreas, and prostate cancers over three specific aims: Aim 1. Examine the role of hepatic MAT1A expression in liver metastasis. This aim will test the hypothesis that loss of MAT1A alters extracellular vesicle content released by the liver to enhance metastatic growth. Aim 2. Determine if MAT proteins expressed by the cancer influence liver metastasis. This aim will elucidate the mechanisms responsible for MAT2A/MAT2B-mediated regulation of miR-34a/b and FOXM1, and examine whether higher MAT2A/2B expression in cancer cells enhance liver metastasis. Aim 3. Examine mechanisms and therapeutic potential of SAMe, MTA, and FDI-6 in liver metastasis. This aim will assess the effects of these three molecules (individually and combined) on cancer cell migration and invasion in vitro. The best therapy will be verified in vivo using mouse models and orthotopic patient-derived xenografts established using tissues from patients with liver metastasis. Project 4 synergizes with: Project 1, as endoglin/BMP signaling activates FOX...