Prostate-specific antigen and other prostate-derived proteases cleave IGFBP-3, but prostate cancer is not associated with proteolytically cleaved circulating IGFBP-3.

BACKGROUND: Proteolysis of insulin-like growth factor-binding proteins (IGFBPs) may increase IGF-mediated growth stimulation and development of cancer in the organs producing large amounts of proteases, such as the prostate.
METHODS: We studied proteolysis of IGFBP-3 by three prostate-derived proteases, namely prostate specific antigen (PSA), human kallikrein 2 (hK2), and trypsin, and also by native seminal plasma. Cleavage of 125I-IGFBP-3 was studied by SDS-PAGE and autoradiography. We also used two different sandwich-type IGFBP-3 immunoassays, called "intact" and "total" IGFBP-3 assays. These assays differ in their capacity to recognize proteolytically degraded IGFBP-3.
RESULTS: HK2, PSA, and trypsin all cleaved IGFBP-3 at the concentrations normally present in seminal plasma. The IGFBP-3 cleavage by seminal plasma was inhibited by ZnCl2, which strongly inhibits hK2 and PSA, but not by a specific trypsin inhibitor. The IGFBP-3 fragments resulting from proteolytic cleavage by PSA, hK2, or trypsin were undetectable in the "intact IGFBP-3 assay," whereas the "total IGFBP-3 assay" also detected the proteolytic fragments. No increased fragmentation of IGFBP-3 was found in serum of 659 men with elevated PSA concentrations, of whom 178 had a proven prostate cancer. Furthermore, the IGFBP-3 levels were not associated with the PSA concentrations.
CONCLUSIONS: These results show that, while seminal plasma and prostate-derived proteases can cleave IGFBP-3, in patients with prostate cancer the circulating IGFBP-3 is not significantly proteolyzed by either PSA, hK2, or trypsin. Copyright 2002 Wiley-Liss, Inc.