EXT proteins: Role in heparan sulfate Assembly and in Tumor biology
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The exostosin family of proteins is necessary for heparan sulfate biosynthesis. Heparan sulfate polysaccharide biosynthesis is one of several different forms of proteinglycosylation processes that take place in the Golgi apparatus. Heparan sulfate modification of a few selected proteins results into the formation of heparan sulfate proteoglycans that are found on cell surfaces, inside the cell and in extracellular matrices. The heparan sulfate chains of the proteoglycans bind a wide range of molecules such as growth factors, serine protease inhibitors and extracellular matrix proteins, thereby influencing a number of cellular processes including cell-signaling and inter-cellular communication. The heparan sulfate chains are important in development, homeostasis and in pathogenesis of various diseases including cancer.
Whereas the activity of the exostosin family of proteins is deemed pivotal for functional heparan sulfate chains, the roles of each of the exostosin members and their mechanisms of action or interaction are not well known. Thus, in this thesis we aimed at generating more knowledge about the role of the exostosin proteins. We used gene expression profiling techniques to study the expression of exostosin genes in both normal and cancer cell-lines. We analyzed the effect of exostosin-1 deficiency in stromal fibroblasts on gene expression in co-cultured A549-carcinoma cells. Our data suggests a new potential role of the exostosin-1 protein in influencing tumor behavior through Tgf-β1. We also investigated the link between the expression levels of the exostosin family of genes and HS-structure in non-malignant and breast cancer epithelial cell-lines. The results of this investigation showed no direct correlation between gene expression of the exostosin family members and heparan sulfatestructure. In addition, we studied the effect of pH on the in vitro glycosyltransferase activity of Exostosin-1 and Exostosin-2 proteins. We observed decreased enzyme activities with reduced pH values.