The oncogenic transcription factor c-Myc (Myc) is frequently overexpressed in human cancers. Myc is known to induce or repress a large set of genes involved in cell growth and proliferation, explaining the selection for mutations in cancer that deregulate Myc expression. Inhibition of ornithine decarboxylase, an enzyme of the polyamine biosynthetic pathway and a Myc target, has been shown to be chemopreventive. In the present study, we have dissected the role of another enzyme in the polyamine biosynthetic pathway, spermidine synthase (Srm), in Myc-induced cancer. We find that Srm is encoded by a Myc target gene containing perfect E-boxes and that it is induced by Myc in a direct manner. RNA interference against Srm shows that it is important for Myc-induced proliferation of mouse fibroblasts but to a lesser extent for transformation. Using the compound trans-4-methylcyclohexylamine, we show that Srm inhibition can delay the onset of B-cell lymphoma development in λ-Myc transgenic mice. We therefore suggest that inhibition of Srm is an additional chemopreventive strategy that warrants further consideration.
Myc oncogenes encode transcription factors that bind to E-box sequences in DNA, driving the expression of a large number of target genes and are deregulated in approximately 70% of human cancers. Deregulated Myc expression cause enhanced proliferation (which is counteracted by apoptosis), angiogenesis and cancer. Though Myc’s importance in induction of S phase has been established, less is known about its functions in the G2 and M phases of the cell cycle. Paper I addresses the targeting of the Myc targets Aurora kinase A and B that have roles in G2/M transition and provide evidence that pharmaceutical Aurora kinase inhibition causes cell cycle arrest and apoptosis in a Myc-selective manner and is useful in treating Myc-induced lymphomas in vivo.
The assumption that the important target genes responsible for the biological effects of Myc overexpression were those encoding components of the cell cycle machinery lead to little interest in other potentially important groups of target genes. However, recent work challenged this view by indicating that Myc target genes encoding metabolic enzymes may be critical for Myc-induced tumorigenesis. Importantly, the targeting of Myc target genes encoding metabolic enzymes has the potential of providing a new treatment strategy of Myc-induced cancers. Paper II covers the pharmaceutical targeting of the Myc-induced spermidine synthase (Srm) that shows promise as a tool for chemoprevention by affecting proliferation, but not for the treatment of established tumors.
Paper III focuses on the negligible effect an Ldha mutation has on Myc- induced lymphomagenesis. Ldha has long been known to be a Myc target gene and in vitro experiments have recently indicated it to be important for transformation. It seems the negligible effect of the Ldh mutation can be explained by the high frequency of loss of either Arf or p53 in this mouse model, since enforced Ras-Myc oncogenic cooperation in soft agar assays of Ldh mutant MEFs effectively inhibits colony formation, and λ-Myc;Ldh mutant bone marrow infected with oncogenic Ras does not give rise to tumors when transplanted into wild-type mice. A role for Ldh in the ability of tumors to evade the immune system was also indicated in this study. The combined experiences and very different outcome of the three studies included in this thesis draw attention to the value of in vivo assessment of Myc downstream targets in Myc-induced lymphomagenesis.
The Myc oncogenes are dysregulated in 70% of human cancers. They encode transcription factors that bind to E-box sequences in DNA, driving the expression of a vast amount of target genes. The biological outcome is enhanced proliferation (which is counteracted by apoptosis), angiogenesis and cancer. Based on the biological effects of Myc overexpression it was originally assumed that the important Myc target genes are those encoding components of the cell cycle machinery. Recent work has challenged this notion and indicates that Myc target genes encoding metabolic enzymes deserve attention, as they may be critical arbiters of Myc in cancer. Thus targeting metabolic enzymes encoded by Myc-target genes may provide a new means to treat cancer that have arisen in response to deregulated Myc oncogenes.