Syndecan-1 Core Protein Signaling in Mammary Carcinoma

The syndecans are type I transmembrane proteins. Their cytoplasmic domains are short and contain two conserved regions (C1 and C2) and an intervening variable (V) region. These link the syndecans to cytoskeletal scaffolding proteins and signaling pathways. The V regions are distinct for each of the four syndecans and appear to endow each syndecan with specific signaling abilities. The transmembrane domains are also highly conserved and target the receptors to specialized lipid domains in the plasma membrane. Our recent work shows that the transmembrane domain of syndecan-1 targets the receptor to specialized lipid domains in Raji-S1 cells and is also necessary for signaling leading to rapid cell spreading and polarization.

The extracellular domain (ectodomain) of syndecan-1 has several important roles. First, it bears several heparan sulfate glycosaminoglycan chains that are critical for binding and regulating the activity of “heparin-binding” growth factors. The heparan sulfate chains also mediate anchorage of the syndecan to extracellular matrix ligands, thus participating in its adhesion role.

The heparan sulfate chains on the syndecans bind several families of “heparin-binding” growth factors, such as the FGFs, and enhance the binding of the growth factors to their receptors. In the case of the FGFs, the heparan sulfate chain also binds to the FGF receptor tyrosine kinase and assists in the formation of a ternary complex consisting of the FGF, the receptor, and the heparan sulfate chain. Complex events in the Golgi apparatus during synthesis of the heparan sulfate chains generate highly specific sulfated domains in the chain that allow it to assemble with specific FGF/receptor pairs. In this manner FGF signaling is regulated by the specific type of heparan sulfate chains expressed by a cell.

The ectodomain of syndecan-1 is also an essential regulator of integrin signaling. The syndecan-1 protein contains an active site that regulates the activity of the αvß3 integrin and αvß5 integrins, two matrix receptors that have important roles in disease. In tumor-induced angiogenesis, these two integrins are critical for the response of endothelial cells to the angiogenic factors FGF and VEGF, respectively. In mammary carcinoma, these integrins recognize bone matrix proteins and appear responsible for tumor cell metastasis to this site.

We also find that syndecan-1 regulates other integrins (matrix receptors) on mammary carcinoma cells. In one example it causes an inhibition of signaling downstream of ß1 integrins. This activity also traces to the syndecan-1 ectodomain. In another example, it specifically alters the activation of the small GTPase Rac downstream of the α3ß1 integrin, and localizes PI-3-kinase to cell-cell junctions, reverting invasive mammary carcinoma cells to a normal, polarized phenotype. This activity traces to its transmembrane and cytoplasmic domains.

We are currently using recombinant DNA technologies to map the active sites in the syndecan-1 core protein, RNAi technology to “silence” the endogenous syndecan-1 and biological assays to monitor the role of this receptor in cell invasion during tumor metastasis and tumor-mediated angiogenesis.

References:

• McFall, A. and Rapraeger, A.C. (1998). Characterization of the high affinity cell binding domain in the cell surface proteoglycan syndecan-4. J. Biol. Chem., 273: 28270-28276.
• Rapraeger, A.C. and Ott, V. (1998). Molecular interactions of the syndecan core proteins. Curr. Opin. Cell Biol. 10: 620-628.
• Rapraeger, A.C. (2000). Syndecan regulated receptor signaling. J. Cell Biol., 149: 995-997.
• Lebakken, C., McQuade, K. J. and Rapraeger, A.C. (2000). Syndecan-1 signals independently of ß1 integrins during Raji cell spreading. Exp. Cell Res., 259: 315-325.
• Beauvais, D. and Rapraeger, A.C. (2003). Syndecan-1 mediated cell spreading requires signaling by αvß3 integrins in human mammary carcinoma cells. Exp. Cell Res. 286: 219-232.
• McQuade, K.J. and Rapraeger, A.C. (2003). Syndecan transmembrane and extracellular domains have unique and distinct roles in cell spreading. J. Biol. Chem., in press.
• Beauvais, D.B. and Rapraeger, A.C. (2003). Syndecans in tumor cell adhesion and signaling. Reproductive Biology and Medicine, in press.