Proteomic identification of galectin-3 binding ligands and characterization of galectin-3 proteolytic cleavage in human prostasomes.

Galectin-3 is a multifunctional carbohydrate-binding protein that was previously characterized as a proteolytic substrate for prostate-specific antigen (PSA) and was shown to be associated with prostasomes in human semen. Prostasomes are exosome-like vesicles that are secreted by the prostatic epithelium and have multiple proposed functions in normal reproduction and prostate cancer. In the current study, galectin-3 binding ligands in human prostasomes were identified and characterized with the goal to investigate galectin-3 function in prostasomes. Galectin-3 binding proteins were isolated by affinity column chromatography. Candidate ligands identified by MS/MS were PSA, prostatic acid phosphatase (PAP), zinc alpha-2-glycoprotein (ZAG), dipeptidyl peptidase-4 (CD26), aminopeptidase N (CD13), neprilysin, clusterin, antibacterial protein (FALL-39) and alpha-1-acid glycoprotein (ORM1). Biochemical methods were used to characterize the ability of galectin-3 to bind to selected ligands, and galectin-3 cleavage assays were utilized to investigate the protease(s) in prostasomes that cleaves galectin-3. CD26, CD13, PSA, PAP and ZAG immunoreactivity were detected in extracts of purified prostasomes. One-dimensional electroblot analysis of prostasomes demonstrated that CD26, PAP and CD13 immunoreactivity co-migrated with galectin-3-reactive protein bands. PSA and ZAG were found to be associated with the surface of prostasomes. Both intact and cleaved galectin-3 were detected in prostate and prostasome extracts. Cleavage and inhibition assays indicated that PSA in prostasomes proteolytically cleaves galectin-3. The identification of these glycoproteins as galectin-3 ligands lays the groundwork for future studies of galectin-3 and prostasome function in reproduction and prostate cancer.