Protein Kinase Inhibitor Alpha Drives Vincristine Resistance in Ewing Sarcoma via cAMP–EPAC Signaling Reprogramming

Ewing sarcoma (ES) is an aggressive bone malignancy with poor outcomes for chemotherapy‐resistant patients, yet the mechanisms underlying vincristine resistance remain unclear. Here, we identify protein kinase inhibitor alpha (PKIA) as a critical driver of chemoresistance through cAMP–EPAC signaling reprogramming. Transcriptomic analysis of vincristine‐resistant ES cells revealed PKIA upregulation, which correlated with poor survival in clinical cohorts (HR = 2.14, p < 0.001). Mechanistically, PKIA overexpression elevated intracellular cAMP levels but suppressed PKA activity, instead activating the noncanonical EPAC–Rap1–ERK pathway to promote drug efflux and survival. Pharmacological inhibition of EPAC with ESI‐09 reversed resistance (IC~50~ reduction: 52%, p < 0.01), while PKIA knockdown restored vincristine sensitivity in xenografts. Strikingly, PKIA exhibited a dual role, with low expression in primary ES (potentially tumor‐suppressive) and high expression in resistant/metastatic tumors (prosurvival), mirroring observations in prostate and hepatocellular cancers. Our work establishes PKIA as a therapeutic vulnerability and supports targeting the cAMP–EPAC axis to overcome chemoresistance in high‐risk ES.