Studies on syndecan-1 in mesenchymal tumors

Abstract

The syndecans are a four-member family of type I transmembrane proteoglycans. Their core proteins have a short cytoplasmic domain, a transmembrane domain and a large N-terminal extracellular domain possessing glycosaminoglycan chains. All the protein domains and the glycosaminoglycan chains can contribute to the functions of syndecans. Syndecans are involved in many cellular processes including cell differentiation, proliferation, adhesion and migration. Syndecan-1 is the prototype and a hybrid proteoglycan containing both heparan sulfate and chondroitin sulfate chains.

Human malignant mesothelioma and fibrosarcoma are aggressive mesenchymal tumors; their low endogenous expression of syndecan-1 may correlate to a more malignant phonotype. In this thesis, we aimed to better understand the role of syndecan-1 in mesenchymal tumors, focusing on its nuclear translocation and structure-function relationship.

We stably transfected mesenchymal tumor cells with a full-length syndecan-1 construct and three truncated variants, namely 78 lacking the extracellular domain with the exception of the juxtamembrane DRKE sequence proposed to be essential for oligomerization; 77 lacking the whole extracellular domain; and RMKKK being a potential nuclear localization signal within the cytoplasmic domain.

Syndecan-1 and basic fibroblast growth factor share the tubulin-mediated transport route and co-localize with heparanase in the nucleus. The cytoplasmic RMKKK sequence of syndecan-1 is sufficient for its nuclear translocation and thus serves as a nuclear localization signal, with the arginine residue being vital for this function.

Overexpression of syndecan-1 influences the expression profile of the other syndecan family members, in particular downregulating syndecan-2. Both full-length and truncated syndecan-1 constructs decrease the proliferation of mesenchymal tumor cells in two ways: the full-length syndecan-1 prolongs the S phase of the cell cycle, whereas the extracellular truncated variants 77 and RMKKK prolong the G0/G1 phase.

Overexpression of syndecan-1 decreases mesenchymal tumor cell migration and motility, but enhances cell adhesion. Distinct protein domains have differential effects; the extracellular domain is more important for promoting cell adhesion, while the transmembrane and cytoplasmic domains are sufficient for inhibition of cell migration.

Reduction of endogenous chondroitin sulfate levels inhibits fibrosarcoma cell adhesion, motility and migration, whereas addition of exogenous chondroitin sulfate chains increases cell motility and migration through JNK and tyrosine kinase signaling pathways, but decreases cell adhesion.

Taken together, these results address the importance of nuclear translocation, and the functional protein domains and glycosaminoglycan chains, thereby providing new insights into the role of syndecan-1 in tumor progression.