Serine 897 Phosphorylation of EPHA2 Is Involved in Signaling of Oncogenic ERK1/2 Drivers in Thyroid Cancer Cells

Phosphorylation of the intracellular domain of the EPHA2 receptor tyrosine kinase (RTK) on serine 897 (S897) has been demonstrated to mediate EPHA2 oncogenic activity. Here, we show that in thyroid cancer cells harboring driver oncogenes that signal through the extracellular regulated kinase (ERK1/2) signaling pathway [rearranged RET RTK (RET/PTC), KRAS(G12R), or BRAF^V600E oncogenes], EPHA2 is robustly phosphorylated on S897. EPHA2 S897 is embedded in a consensus sequence for phosphorylation by the AGC family kinases, including p90RSK (ribosomal protein S6 kinase), a direct ERK1/2 target. We show that recombinant p90RSK phosphorylates in vitro EPHA2 S897 and that treatment with chemical inhibitors targeting p90RSK or other components of the ERK1/2 pathway blunts S897 phosphorylation. RNA interference-mediated knockdown combined with rescue experiments demonstrated that EPHA2 S897 phosphorylation mediates thyroid cancer cell proliferation and motility. These findings point to EPHA2 S897 as a crucial mediator of the oncogenic activity of the ERK1/2 signaling cascade in thyroid cancer.