Acid-activated insulin-like growth factor binding protein protease activity of cathepsin D in normal and malignant prostatic epithelial cells and seminal plasma

In this study, we demonstrate insulin-like growth factor binding protein (IGFBP) acid proteolysis in conditioned media (CM) from normal and malignant primary cultures of prostatic epithelial cells, prostatic cell lines, and in seminal plasma. We further demonstrate the absence of such activity in CM from prostatic stromal cells. Radio-labeled IGFBPs (1-6) were incubated with various acidified CM and seminal plasma. None of these media showed IGFBP proteolytic activity at neutral pH, but all CM from prostatic epithelial cells (PC-E) demonstrated strong IGFBP proteolysis at acidic pH. No acid-activated proteolysis was observed in the CM from stromal cell cultures. In order to ascertain the role of cathepsin D, anti-cathepsin antibodies were used to immunodeplete the media of the selected enzymes prior to incubation with IGFBPs. Depletion of cathepsin D greatly reduced the proteolytic activity of the PC-E CM. Additionally, purified cathepsin D yielded a digestion pattern identical to that produced by prostatic cell CM and seminal plasma, following acidic incubation with IGFBP-3. Remarkably, the proteolytic pattern generated by seminal plasma, when incubated with IGFBP-3 at neutral pH, corresponded to that produced by prostate-specific antigen (PSA), demonstrating the interpolation of both neutral and acid proteases from prostate cells into seminal plasma. In conclusion, prostatic epithelial cells secrete acid-specific IGFBP protease(s) related to cathepsin D. Although no significant statistical difference was observed in the degree of acid-specific proteolysis in the media from normal versus malignant primary epithelial cell cultures, physiological characteristics of the malignant state might facilitate increased cathepsin D activity. We suspect this proteolysis may play a role in prostatic cell proliferation and invasive tumor growth.