EPHA2-dependent Ephrin-B1 signaling supports self-renewal ability and recurrence of oral cancer
Eph-Ephrin pathway that drives a bidirectional signaling regulates a plethora of biological activities with its varied level of complexity. While the canonical trans-interaction of the ligand and receptor on neighboring cells initiates forward signaling that brings about biological activities, their cis-interaction on the same cell attenuates the forward signaling, modulating the biological effects. Yet, non-canonical cis-interactions with heterotypic surface proteins and kinases regulate certain biological effects. In cancer, the canonical signaling is believed to be tumor suppressive, while the ligand-independent non-canonical signaling drives tumor progression and poor prognosis. Self-renewal ability of cancer cells is a major underlying cause of recurrence and poor prognosis of cancer. Using SILAC-based proteomics, we identified Ephrin-B1 signaling as a crucial regulator of oral cancer self-renewal. Though Ephrin-B1 is known to regulate normal stem cells, its role in cancer remains underexplored. Our biochemical analyses show that Ephrin-B1 binds to a nonconventional receptor EPHA2, which is known to regulate cancer stem cells (CSCs), though the mechanism is less explored. Contrary to the belief that the cis-interaction of receptors and ligands is a means to block functional signaling, our immunoprecipitation, FRET facilitated photoswitching analysis, proximity ligation assay, and in vitro kinase assay provide evidence for the Ephrin-B1-EPHA2 cis-interaction leading to the phosphorylation of EphrinB1 at Y324/329 and Y317. Extreme limiting dilution assay in vitro and in vivo confirmed that this cis-interaction promotes CSC enrichment. Substantiating our in vitro results, mouse orthotopic models showed that Ephrin-B1/EPHA2 interaction regulates prognosis. The clinical relevance of the finding was validated using a TCGA data set and immunohistochemical analysis of tissue microarray using samples from oral cancer patients with recurrence in comparison to patients, who showed disease-free survival. Given that Ephrin-B interacts with EphB for normal stem cell homeostasis, this unconventional EPHA2/Ephrin-B1 cis-interaction, specifically manifested in CSC niches, might serve as an attractive target for therapy, warranting further validation.