Insulin-like growth factor binding protein-3 (IGFBP-3), the major serum carrier protein for the IGFs, is absent from Western ligand blots of seminal plasma, but is detectable by RIA. IGFBP-3 protease activity has recently been described in pregnancy serum. We investigated the possibility that seminal plasma contains an IGFBP-3 protease, by incubating seminal plasma with 125I-labeled human IGFBP-3. Seminal plasma was found to have potent IGFBP-3 protease activity with a cleavage pattern different from that of pregnancy serum. Prostate-specific antigen (PSA) is a serine protease found in semen. Autoradiographs measuring IGFBP-3 protease activity demonstrated that purified PSA cleaved IGFBP-3, yielding a cleavage pattern identical to that of seminal plasma. IGFBP-2 and -4 in seminal plasma were not degraded by PSA. Cleavage of IGFBP-3 by PSA resulted in a marked reduction in the binding affinity of the fragments to IGF-I, but not IGF-II. We speculate that PSA may serve to modulate IGF function within the reproductive system or in prostate cancer by altering IGF-IGFBP-3 interactions.

Prostate specific antigen (PSA) is an insulin-like growth factor (IGF) binding protein-3 (IGFBP-3) protease found in seminal plasma and produced by prostatic epithelial cells (PC-E) in vivo. We examined the effects of PSA-proteolysis of IGFBP-3 on the affinity of IGFBP-3 fragments for IGFs and on the mitogenic action of IGFs on PC-E. Recombinant human IGFBP-3 was cleaved by PSA, then incubated with 125I-IGF-I or -II in the presence of varying concentrations of unlabelled peptides, and then cross-linking electrophoresis and densitometric analysis were performed. While the affinity of IGF-II for the PSA-generated IGFBP-3 fragments fell slightly compared to intact IGFBP-3, the affinity of the PSA-generated IGFBP-3 fragments for IGF-I fell by ten fold. The addition of IGF-I or -II to PC-E in serum-free culture conditions resulted in a two-fold stimulation of cell number compared to control. The presence of IGFBP-3 in the media blocked the IGF-induced stimulation, but had no independent effect in the absence of IGFs. When PSA was added to PC-E cultures to which both IGF-I or -II and IGFBP-3 were added, the inhibitory effects of IGFBP-3 on IGF mitogenesis were reversed. We conclude that PSA decreases the affinity of IGFBP-3 for IGF and can potentiate IGF action in the presence of inhibitory IGFBP-3. This phenomenon may contribute to normal and malignant prostate growth.

Despite the widely accepted use of prostate specific antigen (PSA) as a marker of prostate cancer, this molecule has not yet been completely characterized. Past studies have well established, however, using both amino acid and cDNA sequencing techniques, that PSA contains 237 amino acids, with a molecular mass of 26,079 Da for the peptide moiety of the molecule. The present study reports analysis of this protein by ion spray mass spectrometry (ISMS) and analysis of its carbohydrate moiety by NMR spectroscopy. The predominant PSA molecular species detected by ISMS was at relative molecular mass (M(r)) of 28,430, indicating that PSA contains a carbohydrate residue of M(r) 2,351, for a total percentage of carbohydrate of 8.3%. Analysis of PSA by SDS-PAGE, however, showed a M(r) of 32,000 to 33,000, suggesting an overestimation of the molecular weight by the latter technique. The complete primary structure of the PSA carbohydrate chain was determined by NMR spectroscopy in combination with carbohydrate composition analysis. The experimentally determined carbohydrate content of PSA confirms that only one N-glycosylation site is occupied in the protein. The proposed carbohydrate structure is a diantennary N-linked oligosaccharide of the N-acetyllactosamine type, with a sialic acid group at the end of each of the two branches. In addition, our data indicate that approximately 70% of the PSA molecules contain a fucose group in the core chitobiose moiety. The calculated molecular weight of this carbohydrate structure (M(r) 2,351.8) is in excellent agreement with the predicted molecular weight of the carbohydrate group, based on the M(r) 28,430 for PSA measured by ion spray mass spectrometry and M(r) 26,079 calculated from the consensus sequence for the peptide portion of the molecule. ISMS of PSA is thus proposed as a convenient and reliable method of quality control, an indispensible step towards international standardization of this very important tumor marker for detection and monitoring of prostatic diseases, especially prostate cancer.