Mothers against decapentaplegic homolog 3-activated FAT atypical cadherin 2 regulates programmed death ligand-1 to mediate immune evasion in lung squamous cell carcinoma.

BACKGROUND: Lung squamous cell carcinoma (LUSC) represents a distinct subtype of lung cancer, diverging significantly from lung adenocarcinoma. It demonstrates the ability to evade the immune system before the tumor has even begun to infiltrate. However, our understanding of these immune evasion mechanisms in LUSC is still in its infancy, calling for more intensive research.

METHODS: The expression levels of FAT atypical cadherin 2 (FAT2) and mothers against decapentaplegic homolog 3 (SMAD3) in LUSC were assessed by bioinformatics analysis, with the correlation between FAT2 and CD8+ T cell infiltration examined. The FAT2 expression and immune cell infiltration in clinical samples were analyzed via immunohistochemistry. FAT2, SMAD3, and programmed death ligand-1 (PD-L1) expressions were determined using quantitative reverse transcriptase polymerase chain reaction. The interplay between FAT2 and SMAD3 was elucidated through chromatin immunoprecipitation and dual-luciferase reporter gene assays. The mechanism by which FAT2 activates the Wnt/β-catenin pathway to induce PD-L1 expression was further validated by TOPFlash/FOPFlash dual-luciferase reporter assays and chromatin immunoprecipitation followed by quantitative PCR. After culturing tumor cells alongside CD8+ T cells, the effect of SMAD3-activated FAT2 on PD-L1-mediated immune escape was tested through Cell Counting Kit-8 and lactate dehydrogenase detection and evaluated by enzyme-linked immunosorbent assay. The rescue experiments were designed by introducing Wnt pathway inhibitors to determine whether FAT2 affected PD-L1 expression through the Wnt pathway. Based on a mouse transplant tumor model, we verified the effect of knockdown of FAT2 and combination therapy with anti-PD-L1 on tumor growth.

RESULTS: An upregulation of FAT2 in LUSC was observed, which showed a negative correlation with CD8+ T cell infiltration. High FAT2 expression was linked to increased PD-L1 levels, which suppressed the anti-tumor immune function of CD8+ T cells. SMAD3 was found to bind to the FAT2 promoter, stimulating its transcription. Rescue experiments illustrated that FAT2 overexpression could neutralize the positive impact of SMAD3 knockdown on the immune response of CD8+ T cells. In further research, FAT2 was found to mainly promote PD-L1 expression by activating the Wnt pathway. In addition, animal experiments demonstrated that the combination therapy of knocking down FAT2 and anti-PD-L1 significantly repressed LUSC growth.

CONCLUSION: In summary, this study elucidated the important role of SMAD3 activation of FAT2 expression in LUSC immune escape and proposed a combined immune strategy targeting the FAT2/PD-L1 axis. The study can provide a new direction for the clinical intervention of LUSC.