Cell surface nucleolin interacts with CXCR4 receptor via the 212 c-terminal portion.

Previously, we reported that CXCR4 receptor interacted with cell surface nucleolin, and the synergy of CXCR4 and nucleolin plays an essential role in malignant transformation. Here, we continued to conduct a structure-function analysis of nucleolin to identify which portion can efficaciously bind to CXCR4. In the present study, the expression of CXCR4 and nucleolin in 100 cases of papillary thyroid cancer (PTC) samples was investigated through immunohistochemistry (IHC). Subsequently, using nucleolin mutants and pull-down assay, we investigated precise interactions between CXCR4 and nucleolin in HEK-293 cells. A previous study demonstrated CXCR4 and nucleolin co-expressed in cell lines, and the present study further identified that CXCR4 and nucleolin co-expressed in PTC tissues, instead of normal tissues. The nucleolin mutant analysis revealed that nucleolin can efficaciously bind CXCR4 to activate CXCR4 signaling by 212 C-terminal domain. Conversely, N-terminal, RBD and GAR mutants of nucleolin showed no sign of activation of CXCR4 signaling, and differences were statistically insignificant (p > 0.05). In conclusion, these results suggested nucleolin is essential to activate CXCR4 signaling via 212 C-terminal domain, which is required for cell growth, migration, and invasiveness. Furthermore, nucleolin may interact with more G protein-coupled receptors, at least chemokine receptor. Our study will lay a new foundation for cancer therapy by antagonizing nucleolin and CXCR4.