hK2 and PSA : Functions and Targets for Treatment of Prostate Cancer

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Title: hK2 and PSA : Functions and Targets for Treatment of Prostate Cancer
Author: Hekim, Can
Contributor organization: University of Helsinki, Faculty of Medicine, Institute of Clinical Medicine, Department of Clinical Chemistry
Helsingin yliopisto, lääketieteellinen tiedekunta, kliininen laitos
Helsingfors universitet, medicinska fakulteten, institutionen för klinisk medicin
Publisher: Helsingin yliopisto
Date: 2012-02-24
Language: eng
Thesis level: Doctoral dissertation (article-based)
Abstract: Prostate cancer is the most common cancer in males and a major cause of cancer death in industrial- ized countries. It is generally a very slowly growing cancer and potentially curable at early stages by radical prostatectomy or radiotherapy. However, radical therapy is associated with side effects. Therefore, there is need for novel treatments for advanced prostate cancer, and for curing or slowing down the growth of the tumors. Proteases play important roles in the progression of prostate and other cancers. Prostate produces high levels of two kallikreins, human kallikrein 2 (hK2, kallikrein-related peptidase 2, KLK2) and prostate specific antigen (PSA, KLK3). These proteases are secreted into seminal fluid and mediate liquefaction of the seminal clot that forms after ejaculation. Furthermore, enzymatically active PSA has been shown to reduce angiogenesis in vitro and in vivo, which may contribute to the slow growth of prostate cancer. PSA expression is lower in malignant than in normal prostatic epithelium. It is further reduced in poorly differentiated tumors, in which the expression of hK2 is increased. hK2 may mediate tumor growth and invasion by participating in proteolytic cascades degrading extracelullar matrix and thereby promoting tumor spread. Both PSA and hK2 degrade insulin-like growth factor- binding protein-3 (IGFBP-3) in vitro. IGFBP-3 is an important regulator of cell proliferation and survival via IGF-system, and independently. hK2 is more potent than PSA in degrading IGFBP-3. Because its expression is increased in prostate cancer, degradation of IGFBP-3 by hK2 locally in the prostate may promote prostate cancer growth. By using phage display technology we developed biologically active peptides, which specifically inhibit the enzymatic activity of hK2. The peptides were characterized and the motifs required for their inhibitory activity were determined. These may be used to target hK2 for treatment and their binding property be utilized in tumor imaging. However, the peptides were degraded in plasma within minutes and thus, have limited use in vivo. By head-to-tail cyclization we were able to improve plasma stability of the original linear hK2-inhibiting peptide, while the activity towards hK2 was retained. When secreted from the prostate, most of PSA is free and enzymatically active. In circulation major portion of PSA occurs in complexes with protease inhibitors and, thus, is inactive. To justify the use of mouse models for evaluation of the function of PSA and for studies in therapeutic modalities based on modulation of PSA activity it is important to know whether PSA complexation is similar in mouse and man. We characterized the circulating forms of PSA in mouse, by directly adding to mouse serum or using subcutaneous PSA-producing human prostate cancer cell xenograft tumor models. When added to mouse serum, over 70% of PSA forms complexes within 30 min. The complexes contained α2-macroglobulin and serpins of the α1-antitrypsin (AAT) family. Thus, in mouse serum, PSA forms complexes similar to those in man, but the major immunoreactive complex contains AAT rather than α1-antichymotrypsin (ACT). In mice bearing LNCaP xenograft tumors 70% of immunore- active PSA occurs in complex with AAT. Hence, the metabolism of PSA produced by xenograft tumor models in mice is similar to that of human prostate tumors with respect to the fate of released PSA and would allow the evaluation of treatment modalities based on PSA activity. We studied the degradation of IGFBP-3 by hK2 and identified the cleavage sites by mass spectrometry. Furthermore, we showed that hK2-inhibiting peptides inhibit hK2 activity towards natural protein substrates, including IGFBP-3. As degradation of IGFBP-3 leads to release of IGF-I, which may stimulate cancer growth, these peptides may be useful for treatment of prostate cancer and other diseases associated with increased hK2 activity. The peptides identified in this study have potential therapeutic value for treatment of prostate cancer. They can also be used as lead molecules for the development of peptide analogues suitable for imaging and treatment of prostate cancer.Ei saatavilla
Subject: lääketiede
Rights: Julkaisu on tekijänoikeussäännösten alainen. Teosta voi lukea ja tulostaa henkilökohtaista käyttöä varten. Käyttö kaupallisiin tarkoituksiin on kielletty.

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