IGPR-1 Is Phosphorylated on the Immunoreceptor Tyrosine-Based Motif, Stimulates the AKT Pathway and Supports Melanoma Growth.

Immunoglobulin (Ig) and Proline-rich Receptor-1 (IGPR-1, also called TMIGD2) is closely related to immune checkpoint CD28/PDL1 family receptors. It controls important cellular processes, including immune cell regulation, cell-cell adhesion, mechanosensing, and autophagy, and its activity is associated with multiple human malignancies. However, the role and signaling mechanism of IGPR-1 remains largely undefined. Here, we report that IGPR-1's pro-tumor signaling in melanoma cells is mediated by phosphorylation of the immunoreceptor tyrosine-based activation motif (ITAM) tyrosine (Y222). IGPR-1 is phosphorylated at Y222 in human melanoma tissues and cell lines. Phosphorylation of Y222 is context-dependent and catalyzed by EGFR and Src kinase. Pharmacological inhibition of EGFR using Erlotinib, as well as EGFR knockdown via shRNA, suppressed phosphorylation of Y222. In contrast, stimulation with EGF enhanced Y222 phosphorylation in vivo, and recombinant active EGFR directly phosphorylated Y222 in an in vitro kinase assay. In vivo coimmunoprecipitation and in vitro GST pull-down assays demonstrated that phospho-Y222 facilitates the binding of IGPR-1 with the SH2 domain-containing protein SHC1. IGPR-1 stimulates multiple key downstream signal transduction pathways relevant to tumorigenesis, including AKT, mTOR, and MAPK pathways. Mutation of Y222 blocked IGPR-1-mediated activation of AKT and MAPK, leading to the inhibition of tumor spheroid growth. This study uncovered new findings that have potential diagnostic and therapeutic implications in melanoma.