API5 protein impacts tumor progression, metastasis, and immune evasion

Apoptosis inhibitor 5 (API5) is an anti-apoptotic protein that has emerged as an important regulator of cancer cell survival, proliferation, therapeutic resistance, and tumor–immune interactions. API5 exerts its biological effects through interactions with several molecular partners, including Acinus, fibroblast growth factor 2 (FGF2), and Toll-like receptor 4 (TLR4), thereby influencing apoptosis, mRNA export, growth-factor signaling, and immune responses. In particular, API5 promotes FGF2/FGFR1-dependent activation of the PKCδ/ERK pathway, leading to degradation of the pro-apoptotic protein BIM and increased resistance of tumor cells to apoptosis and antigen-specific T-cell-mediated cytotoxicity. API5 has also been implicated in cancer stem cell-like phenotypes and the modulation of inflammatory and innate immune responses. Preclinical studies targeting API5 or its associated signaling pathways using RNA interference, peptides, and pharmacological approaches have demonstrated anti-tumor effects, including induction of apoptosis, inhibition of invasive and stem-like properties, and restoration of tumor-cell sensitivity to immune-mediated killing. Nevertheless, the therapeutic targeting of API5 remains largely preclinical, and further investigation is required to clarify its context-dependent functions and to develop specific, effective, and safe API5-directed strategies. This review summarizes the molecular functions, regulation, and cancer-associated activities of API5 and evaluates its potential as a therapeutic target in cancer.