SLPI in Prostate Cancer

Secretory Leukocyte Protease Inhibitor (SLPI) is a conserved serine protease inhibitor expressed on mucosal surfaces, which has multiple functions including anti-protease, anti-microbial and anti-inflammatory properties. SLPI plays critical roles in tissue homeostasis and pathology. Through its anti-protease ability, SLPI safeguards tissues from excessive damage caused by proteolytic enzymes released during inflammation and contributes to extracellular matrix remodeling, thereby influencing the cellular and tumor microenvironment. Furthermore, SLPI expression is implicated in shaping the immune landscape that facilitates tumor progression, and in driving epithelial–mesenchymal transition (EMT). Consequently, it is not surprising that SLPI plays a complex and context-dependent role across various malignancies. It is overexpressed in most cancers such as colorectal, gastric, pancreatic, and breast carcinomas, and this overexpression often correlates with a more advanced and aggressive disease. Conversely, its levels are reduced in head and neck squamous cell carcinoma and hepatocellular carcinoma, where elevated expression may be associated with a more favorable prognosis. This diverse behavior underscores that SLPI function in cancer is tissue-specific and dependent on the functional or pathological state. In prostate cancer, SLPI expression exhibits a bimodal behavior: levels are reduced in the early stages of the disease compared to normal tissues but become significantly upregulated in more advanced and aggressive stages of disease, with significantly higher levels observed in patients with castration-resistant prostate cancer. Elevated SLPI levels in prostate cancer correlate with a reduced prostate-specific antigen (PSA) progression-free survival. In this review, we outline the current evidence regarding the multifaceted functions of SLPI and its expanding role in cancer, focusing primarily on the recently described molecular mechanisms and clinical significance of SLPI in prostate carcinoma.