SALL4-targeted therapeutic peptide PEN-FFW suppresses PD-L1 and enhances CD8⁺ T cell cytotoxicity via regulating PI3K/AKT signaling in breast cancer.

SALL4 is aberrantly reactivated in multiple malignancies, including breast cancer (BC), where it promotes tumor progression and therapy resistance. However, its therapeutic targeting remains underexplored. This study investigates the antitumor efficacy of a novel SALL4-inhibitory peptide, PEN-FFW, and its regulatory impact on the PI3K/AKT/PD-L1 axis and CD8⁺ T cell-mediated cytotoxicity in BC. SALL4 expression in BC was assessed using public databases and validated in cell lines by RT-qPCR and western blot. The interaction between SALL4 and the NuRD complex was evaluated by co-immunoprecipitation assay. Functional assays were conducted to assess the effects of PEN-FFW in vitro. Co-culture systems were used to evaluate CD8⁺ T cell-mediated cytotoxicity. Mechanistic studies investigated the involvement of the PTEN/PI3K/AKT/mTOR signaling axis. In vivo efficacy was tested in allograft mouse models, including combination therapy with anti-PD-L1 antibody. SALL4 was significantly upregulated in BC and associated with poor prognosis. PEN-FFW disrupted the SALL4-NuRD interaction, restored PTEN expression, and suppressed PI3K/AKT/mTOR signaling. This led to a reduction in PD-L1 expression and increased apoptosis, while inhibiting the proliferation and migration of BC cells. PEN-FFW also enhanced CD8⁺ T cell cytotoxicity by reducing PD-L1-mediated immune evasion. Furthermore, combination treatment with PEN-FFW and anti-PD-L1 antibody showed superior tumor suppression and increased CD8⁺ T cell infiltration compared to either treatment alone. PEN-FFW is a potent SALL4-inhibitory peptide that suppresses BC progression by downregulating PD-L1 through PI3K/AKT pathway inactivation and promoting CD8⁺ T cell-mediated tumor killing. These findings highlight a promising strategy for enhancing immunotherapy in SALL4-positive BC.