SLPI is associated with AR-regulated epithelial adaptation and enzalutamide sensitivity in castration-resistant prostate cancer.

BACKGROUND: Castration-resistant prostate cancer (CRPC) develops through tumor-cell plasticity, androgen receptor (AR) pathway rewiring, and immune/stromal changes under therapeutic pressure. Because metastatic CRPC often responds incompletely to immunotherapy and AR-directed treatment, cell-resolved analyses may help place putative drug-response factors within their tissue context. We examined whether secretory leukocyte protease inhibitor (SLPI), an epithelial secreted factor with immunoregulatory functions, is related to epithelial-state changes and enzalutamide response. METHODS: Public single-cell RNA-sequencing data from localized prostate cancer and metastatic CRPC (GSE274229) were analyzed using Seurat-based preprocessing, cell-type annotation, UCell pathway/state scoring, CellChat ligand-receptor inference, CopyKAT malignant-cell inference, Monocle2 trajectory analysis, hdWGCNA network analysis, and TCGA-PRAD clinical association analysis. SLPI was evaluated in 22Rv1 and C4-2 prostate cancer models using stable overexpression, qRT-PCR, proliferation, colony formation, migration, AR-pathway modulation, enzalutamide dose-response, and Annexin V/PI apoptosis assays. RESULTS: Progression to mCRPC was accompanied by altered epithelial-immune/stromal communication and shifts in inflammatory, extracellular-matrix, growth-factor, and immune-regulatory signaling. Putative malignant epithelial populations showed reduced AR-dependency and luminal-identity scores together with increased stress-adaptation and plasticity programs. Network and clinical association analyses highlighted SLPI as a progression-related epithelial factor with outcome relevance. In vitro, SLPI expression was lower in prostate cancer cells than in RWPE-1 cells, and SLPI overexpression suppressed proliferation, migration, and clonogenic growth. AR-pathway modulation showed reciprocal changes in SLPI expression: androgen deprivation and AR knockdown increased SLPI expression, whereas DHT-mediated AR reactivation reduced SLPI expression. SLPI overexpression enhanced enzalutamide-induced growth inhibition and apoptosis in the tested cell-line models. CONCLUSION: These findings place SLPI at the intersection of AR-pathway state, epithelial adaptation, immune-context changes, and enzalutamide sensitivity in CRPC. SLPI may represent a therapy-response-associated epithelial factor in prostate cancer, but further protein-level, loss-of-function, in vivo, and clinically annotated validation will be required.