Examining the role of metabolism in Myc-driven tumorigenesis

Sammanfattning: Myc transcriptionally regulates genes involved in processes such as cell proliferation, metabolism, differentiation, and angiogenesis.  MYC expression is deregulated in many types of human cancer; therefore discovering the mechanisms behind MYCs role in tumorigenesis is essential.  In this dissertation, I have focused on several Myc target genes, Spermidine synthase (Srm); Lactate dehydrogenase (Ldh); 3-phosphoglycerate dehydrogenase (Phgdh); Serine hydroxymethyltransferase (SHMT) 1 and 2; and Pim-3 (a member of the Pim family of serine/threonine kinases).  These enzymes play a role in various functions: Spermidine synthase (polyamine synthesis); Lactate dehydrogenase (glycolysis); Phgdh and Shmt (serine metabolism); and Pim-3 (cell signaling).  In order to elucidate the impact Myc over-expression has on metabolism in tumorigenesis, we use human cell lines, and transgenic mice as well as cell lines and tissues derived from these mice.  The impact of inhibition of these target genes on Myc-driven tumorigenesis was done by genetically inhibiting the target gene (using RNAi or mouse models) or inhibiting the protein with a chemical inhibitor.  Investigating these Myc target genes will help determine if inhibition of Myc target genes is a viable approach for chemotherapeutics, and under what conditions this inhibition may be the most valuable.  In paper I, we examine SRM; a highly expressed enzyme in the polyamine synthesis pathway that converts putrescine to spermidine, and is important for actively growing cells.  Genetic inhibition via RNAi against Srm, or chemical inhibition of Srm, resulted in decreased proliferation of B-cell tumor lines from transgenic mice in vitro.  In vivo treatment of ?-Myc transgenic mice with a chemical SRM inhibitor exhibited a significant chemopreventative effect on tumor formation. These results support previous findings that inhibition of polyamine synthesis pathway enzymes has a place in cancer therapy.  Many Myc target genes have been suggested as attractive targets in battling Myc-driven tumorigenesis.  Surprisingly in paper II, when we analyzed the inhibition of other Myc target genes, such as Ldh, Shmt, and Phgdh, we found that inhibition of these genes did not inhibit Myc-driven tumorigenesis to any significant degree. However, inhibition of Ldh, Phgdh and Shmt2 had a notable effect on in vitro Ras-driven transformation.  These findings suggest that chemotherapeutic inhibition of metabolic genes such as Ldh, Phgdh and Shmt2 may be effective in genetically defined settings, keeping in mind the oncogenic lesion behind the tumor.  The Pim kinase family consists of three serine/threonine kinases, Pim1-3.  In paper III, we found that Pim-3 is a direct Myc target gene and that Pim-3 expression is high in Burkitt Lymphoma samples taken from human patients, as well as spontaneously arising lymphomas from Myc transgenic mice. We also found that inhibition of Pim-3 using a pan-Pim kinase inhibitor, Pimi, in these spontaneously arising Myc lymphomas resulted in caspase independent cell death.  These results indicate that Pim kinase inhibition may be a potential chemotherapeutic strategy in human lymphomas that rely on Pim-3 kinase expression.