ALTERATION OF THE HORMONAL BIOACTIVITY OF PARATHYROID HORMONE-RELATEDPROTEIN (PTHRP) AS A RESULT OF LIMITED PROTEOLYSIS BY PROSTATE-SPECIFIC ANTIGEN

Objectives, To discover whether the proteolytic activity of prostate-s pecific antigen (PSA) affects the structure and function of parathyroi d hormone-related protein (PTHrP), as both are abundant components of human seminal plasma. Methods. The ability of PTHrP to act as a substr ate was studied by incubating a synthetic polypeptide, consisting of 3 4 amino acid residues of the amino-terminal domain of PTHrP, with puri fied PSA. The incubate was then analyzed by sodium dodecyl sulfate-pol yacrylamide gel electrophoresis, high-pressure liquid chromatography s eparation, amino-terminal peptide sequencing, and mass spectrometry. T he physiologic effect of the proteolytic activity of PSA on PTHrP was studied by measuring any alteration in PTHrP (1-34)-induced elevation of cyclic adenosine monophosphate (cAMP) production by UMR-106 rat ost eosarcoma cells in culture. All cell culture experiments were performe d with PSA and PTHrP (1-34) at physiologic concentrations. Results. Ou r data show that PSA proteolytically cleaves PTHrP (1-34) after either residue 22 or 23, generating three peptide fragments. Both cleavages occur carboxy terminally of a phenylalanine residue. The cAMP producti on in rat osteosarcoma cells, induced by the amino-terminal portion of PTHrP (1-54), as a result of its structural similarity with parathyro id hormone (PTH), was abated by PSA in a dose- and time-dependent fash ion. In contrast, heat-inactivated PSA had no effect on cAMP productio n. Conclusions. Our study demonstrates that PTHrP is a substrate for P SA. The cleavage of the amino-terminal portion of PTHrP completely dis rupts its ability to interact with the PTH/PTHrP receptor and thus inh ibits its PTH-like activity. The proteolytic processing of PTHrP by PS A may play an important role in the post-translational/post-secretiona l regulation of prostatic PTHrP activities, which are believed to incl ude regulation of prostate growth and differentiation.