Syndecan-1 and Syndecan-4 Capture Epidermal Growth Factor Receptor Family Members and the α3β1 Integrin Via Binding Sites in Their Ectodomains NOVEL SYNSTATINS PREVENT KINASE CAPTURE AND INHIBIT α6β4-INTEGRIN-DEPENDENT EPITHELIAL CELL MOTILITY

The alpha 6 beta 4 integrin is known to associate with receptor tyrosine kinases when engaged in epithelial wound healing and in carcinoma invasion and survival. Prior work has shown that HER2 associates with alpha 6 beta 4 integrin and syndecan-1 (Sdc1), in which Sdc1 engages the cytoplasmic domain of the beta 4 integrin subunit allowing HER2-dependent motility and carcinoma cell survival. In contrast, EGFR associates with Sdc4 and the alpha 6 beta 4 integrin, and EGFR-dependent motility depends on cytoplasmic engagement of beta 4 integrin with Sdc4. However, how HER2 and EGFR assimilate into a complex with the syndecans and integrin, and why kinase capture is syndecan-specific has remained unknown. In the present study, we demonstrate that HER2 is captured via a site, comprised of amino acids 210-240, in the extracellular domain of human Sdc1, and EGFR is captured via an extracellular site comprised of amino acids 87-131 in human Sdc4. Binding assays using purified recombinant proteins demonstrate that the interaction between the EGFR family members and the syndecans is direct. The alpha 3 beta 1 integrin, which is responsible for the motility of the cells, is captured at these sites as well. Peptides based on the interaction motifs in Sdc1 and Sdc4, called synstatins (SSTN210-240 and SSTN87-131) competitively displace the receptor tyrosine kinase and alpha 3 beta 1 integrin from the syndecan with an IC50 of 100-300 nM. The syndecans remain anchored to the alpha 6 beta 4 integrin via its cytoplasmic domain, but the activation of cell motility is disrupted. These novel SSTN peptides are potential therapeutics for carcinomas that depend on these HER2-and EGFR-coupled mechanisms for their invasion and survival.