FKH domain-mediated nuclear FOXP3 suppresses gastric cancer malignancy via c-MYC/CDKN1A regulation, EMT inhibition, and PI3K/AKT signaling modulation.

Gastric cancer (GC) remains a primary contributor to cancer-associated deaths worldwide, especially in East Asia. We investigated the function of nuclear-localized full-length FOXP3 (FOXP3FL) as an oncosuppressive factor in GC. Total FOXP3 was markedly reduced in GC tissues versus adjacent normal mucosa, with distinct cytoplasmic and nuclear patterns. Functional assays revealed that nuclear overexpression of FOXP3FL suppresses proliferation, migration, and invasion of GC cells in vitro and in vivo and promotes cellular senescence in vivo. Mechanistically, FOXP3FL directly represses MYC transcription and induces CDKN1A, thereby restraining proliferation and metastasis. These transcriptional changes, together with concomitant PTEN upregulation and PI3K-P110α downregulation, collectively attenuate PI3K/AKT signaling and impair epithelial-mesenchymal transition (EMT) in FOXP3FL-expressing GC cells. The forkhead (FKH) domain is essential: its deletion yields cytoplasmic FOXP3ΔFKH, which loses transcriptional control and antitumor activity. Our findings underscore nuclear-localized FOXP3FL as a tumor suppressor whose FKH domain is integral to its suppressive function.