Increased levels of macrophage-secreted Cathepsin S during Prostate Cancer progression in TRAMP mice and patients

• Background: Protein expression during prostate tumour progression in transgenic TRAMP mice was studied, with the aim of identifying proteins associated with tumour progression and castration resistant tumour growth. Materials and Methods: Protein expression was compared between normal mouse prostate, primary TRAMP tumours and peripheral metastases in long-term castrated TRAMP mice using 2-dimensional differential in-gel electrophoresis and MALDI TOF/TOF analysis. Results were verified with Western blot analysis and immunohisto-chemistry in the TRAMP model and samples from patients. Results: The active form of cathepsin S (Cat S) was identified as being significantly up-regulated in poorly differentiated TRAMP tumours and in castration-resistant metastases compared to normal mouse prostate and well-differentiated tumours. Increased Cat S levels were also found in high Gleason grade tumour areas in patients. Cat S was primarily expressed by tumour-infiltrating macrophages, as shown by double staining of Cat S and CD68 expressing cells. A significantly higher number of Cat S expressing macrophages was found in castration-resistant than in hormone naïve high grade tumours in patients. No relation was found between Cat S levels and suggested Cat S regulated, matrix-derived fragments of collagen IV or laminin 5 γ2. Conclusion: Macrophage-secreted Cat S levels increase during prostate cancer progression and could be an interesting target for therapy.

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