Role of syndecan-1 in tumor cell proliferation and epithelial-mesenchymal plasticity

Abstract: Syndecan-1 (SDC1) is a heparan sulfate proteoglycan (HSPG) intercalated in the cell membrane but also translocated to the cell nucleus in a regulated manner. SDC1 is involved in several malignancy-associated processes such as proliferation and migration. Altered SDC1 expression can induce changes along the epithelial-mesenchymal axis and it may influence the prognosis of cancer. Transforming growth factor-ß (TGF-ß) plays a pivotal role in many cellular functions, including epithelial-mesenchymal transition (EMT). In the early stages of tumorigenesis TGF-ß inhibits cell growth and induces cell apoptosis, while in the later stages it promotes tumor growth. The overall aim of this thesis was to study the role of SDC1 in mesenchymal tumors and functions related to the presence or absence of SDC1 in the nucleus. Understanding the role of cell surface and nuclear SDC1 and its interactions could be of importance for the understanding of tumor growth, proliferation, differentiation, and migration in these tumors. In paper I we used fibrosarcoma cell sub-lines to study the functions of SDC1, especially the molecular targets and signaling pathways regulated by its nuclear translocation. The TGF-ß pathway was activated by nuclear SDC1, and three genes were altered with the deletion of nuclear localization signal: EGR-1, NEK11 and DOCK8. The study shows the importance of the localization of SDC1 for its effect on tumor cells. The aim of paper II was to further study the role of nuclear SDC1 through characterizing its nuclear interactome, using a mesothelioma cell line. SDC1 was immunoprecipitated to identify co-precipitating interacting proteins. The results indicate a previously unknown role for SDC1 in RNA biogenesis. In Paper III we investigated if SDC1 plays a role in regulating TGF-ß-induced EMT. The knockdown of SDC1 in a carcinoma cell line resulted in decreased expression of E-cadherin, and increased expression of N-cadherin. In fibrosarcoma cells, with its low basic SDC1 levels, overexpression of SDC1 was sufficient to repress N-cadherin and vimentin. The results indicate that SDC1 regulates epithelial-mesenchymal plasticity in tumor cells. Together, these studies provide new insights into the role of SDC1 in tumors and the functional importance of the transport of SDC1 to the nucleus, as well as the connection between SDC1 and TGF-ß.