Polyamines are small cationic molecules that play important roles in most vital cellular processes including cell growth and proliferation, regulation of chromatin structure, translation and programmed cell death. Cellular polyamine pools are maintained by a balance between biosynthesis and transport (export and import). Increased polyamine biosynthesis activity and an active transport system are characteristics of many cancer cell lines, and polyamine depletion has been shown to be a viable anticancer strategy. Polyamine levels can be depleted by α-difluoromethylornithine (DFMO), an inhibitor of the key polyamine biosynthesis enzyme ornithine decarboxylase. However, malignant cells often circumvent DFMO therapy by up-regulating polyamine import; therefore, there is a need to develop compounds that inhibit polyamine transport. Collectively, DFMO and polyamine transport inhibitors provide the basis for a combination therapy leading to effective intracellular polyamine depletion. Using a Drosophila leg imaginal disc model for polyamine transport, I studied three candidate transport inhibitors (Ant444, Trimer44 and Triamide44) for their ability to inhibit transport in the Drosophila model. Ant444 and Trimer44 effectively inhibited the uptake of the toxic polyamine analog Ant44 that gains entry to cells via the polyamine transport system. Ant444 and Trimer44 were also able to inhibit the import of exogenous polyamines into DFMO-treated imaginal discs. Triamide44 was an ineffective inhibitor, however a structurally redesigned compound, Triamide444, showed a 50-fold increase in transport inhibition and was comparable to Ant444 and Trimer44. Ant444 and Trimer44 showed differences in their relative abilities to block import of specific polyamines, and I therefore asked if a cocktail of these inhibitors would be more effective than either alone. My data show that a cocktail of polyamine transport inhibitors is more effective than single inhibitors when used in combination with DFMO, and suggests the existence of multiple polyamine transport systems. To further the development of effective transport inhibitors it is important to identify components of the transport system. The mechanism of polyamine transport in multicellular organisms including mammals is still unknown. Our laboratory has developed a simple assay to detect components of the transport system using RNAi knockdown and over-expression of candidate genes. However, the assay requires that animals live until the pupal stage of development. Pleiotropic effects of individual gene products following over-expression or knockdown may result in early developmental lethality for reasons unrelated to polyamine transport. Our assay is based on the GAL4/UAS system and involves the use of enhancers driving GAL4 expression (GAL4 driver). GAL4 in turn determines the expression level of UAS-candidate gene constructs (UAS responder). I reasoned that in some cases it might be possible to bypass early lethality by judicious choice of drivers that reduce responder expression, thus permitting survival to the pupal phase. To this end, I used five imaginal disc drivers (30A, 71B, 32B, 69B, and T80) as well as a ubiquitously expressed control driver to over-express and knockdown EGFR and components of the Rho signaling pathway. The relative strength of each driver was ranked, and I was able to demonstrate in principle that animals could survive to later stages of development in a manner that correlated with the relative strength of the driver. The approach I developed is broadly applicable to other studies of Drosophila development. To identify new components of the polyamine transport system I studied the role of proteoglycans in this process. The proteoglycan glypican-1 has been previously implicated in mammalian polyamine transport. In particular, the heparin sulfate side chains of glypican-1 appear to play an important role. In order to extend our knowledge of the role of proteoglycans in polyamine transport, I examined the role of the core proteoglycans perlecan and syndecan as well as genes encoding enzymes in the heparin sulfate and chondroitin sulfate biosynthetic pathways. I was able to confirm a role for glypican-1 in polyamine transport in imaginal discs but not in whole animals. This may indicate that glypican-1 is not required for polyamine uptake through the gut. Studies of genes encoding perlecan, syndecan and enzymes in the heparin sulfate and chondroitin sulfate biosynthetic pathways did not reveal a role for these genes in polyamine transport. These studies were conducted in whole animals and my data may reflect tissue-specific differences between the imaginal disc and gut transport systems where transport in imaginal discs is proteoglycan dependent and transport in the gut is not.
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Von Kalm, Laurence
Doctor of Philosophy (Ph.D.)
College of Sciences
Length of Campus-only Access
Doctoral Dissertation (Open Access)
Wang, Minpei, "A Chemical and Genetic Approach to Study the Polyamine Transport System in Drosophila" (2017). Electronic Theses and Dissertations, 2004-2019. 6029.