CD44v6, a member of the CD44 family of transmembrane glycoproteins is a co-receptor for two receptor tyrosine kinases (RTKs), Met and VEGFR-2 (vascular endothelial growth factor receptor 2). CD44v6 is not only required for the activation of these RTKs but also for signalling. In order to understand the role of CD44v6 in Met and VEGFR-2 activation and signalling we tested whether CD44v6 binds to their ligands, HGF (hepatocyte growth factor) and VEGF (vascular endothelial growth factor), respectively. FACS analysis and cellular ELISA showed binding of HGF and VEGF only to cells expressing CD44v6. Direct binding of CD44v6 to HGF and VEGF was demonstrated in pull-down assays and the binding affinities were determined using MicroScale Thermophoresis, fluorescence correlation spectroscopy and fluorescence anisotropy. The binding affinity of CD44v6 to HGF is in the micromolar range in contrast with the high-affinity binding measured in the case of VEGF and CD44v6, which is in the nanomolar range. These data reveal a heparan sulfate-independent direct binding of CD44v6 to the ligands of Met and VEGFR-2 and suggest different roles of CD44v6 for these RTKs.

Within the family of RTKs (receptor tyrosine kinases), PDGFR (platelet-derived growth factor receptor) has been implicated in carcinogenesis and tumour development. miRNAs (microRNAs), which can target the mRNAs (messenger RNAs) of cancer-associated genes, are abnormally expressed in various cancers. In this study, our aim was to identify the miRNAs that target PDGFR-α/β and to study the functions of these miRNAs. miR-34a was predicted to target PDGFR, and luciferase reporter assays showed that miR-34a could directly target PDGFR. Meanwhile, we found that miR-34a was down-regulated in gastric cancer tissues and was associated with metastasis. Our findings showed that miR-34a could inhibit gastric cancer cell migration, invasion and proliferation, but these tumourigenic properties were only partially restored when PDGFR-α/β was overexpressed. In subsequent experiments, we found that the overexpression of both PDGFR and MET could completely restore the gastric cancer tumourigenic properties. Moreover, the cancer-associated cell signalling pathway was studied, and we found that miR-34a could inhibit Akt [PKB (protein kinase B)] phosphorylation, which was restored by the overexpression of both PDGFR and MET. In conclusion, miR-34a may act as a potential tumour suppressor in gastric cancer and is associated with the mechanisms of gastric cancer metastasis; miR-34a can inhibit gastric cancer tumourigenesis by targeting PDGFR and MET through the PI3K (phosphoinositide 3-kinase)/Akt pathway.