Galectin-3 Is a Substrate for Prostate Specific Antigen (PSA) in Human Seminal Plasma

BACKGROUND. Galectin-3 is a multivalent carbohydrate-binding protein involved in cell adhesion, cell cycle control, immunomodulation, and cancer progression, including prostate cancer. Galectin-3 function is regulated by proteolytic cleavage that destroys galectin-3 multivalency while preserving carbohydrate-binding activity. In human semen, galectin-3 is present in seminal plasma and is also associated with prostasomes, exosome-like vesicles secreted by the prostate. In the current study, we characterized the proteolytic activity that cleaves galectin-3 in human seminal plasma. METHODS. An in vitro assay was developed to investigate galectin-3 cleavage in seminal plasma. The effect of protease inhibitors, divalent ion chelators, and Zn2+ on the cleavage activity was determined. Proteases enriched from seminal plasma were tested for their ability to cleave galectin-3. Affinity purification and microsequence analysis were used to identify the cleavage site in galectin-3. RESULTS. Galectin-3 was identified in human seminal plasma in an intact and truncated form. Gelatinases enriched from seminal plasma did not cleave galectin-3. Inhibitor studies indicated that the galectin-3 cleavage activity in seminal plasma is a Zn2+ sensitive, serine protease. Prostate specific antigen (PSA) was demonstrated to cleave galectin-3 between tyrosine(107)-glycine(108) and produce a functionally active, monovalent lectin. CONCLUSIONS. PSA is a chymotrypsin-like serine protease secreted by the prostatic epithelium and normally functions in liquefaction of semen following ejaculation. Furthermore, PSA is implicated in the promotion of localized prostate tumors and bone metastases by its roles in immunomodulation, invasion, and apoptosis. Our results indicate that PSA regulates galectin-3 in human semen and may regulate galectin-3 function during prostate cancer progression. Prostate 71: 197-208, 2011. (C) 2010 Wiley-Liss, Inc.