Lysosomal proteases may be involved in facilitating cancer invasion and metastatic spread by degradation of basement membranes and intercellular matrix. Overexpression of cathepsin D, a lysosomal aspartyl protease, has been reported in different tumours and seems to constitute a prognostic factor for survival in patients with breast cancer. The current study investigates immunohistochemical staining using anti-cathepsin D monoclonal antobodies (M1G8) in prostate cancer specimens and tissue from patients with benign prostatic hyperplasia (BPH). Among 41 tumours expression of cathepsin D was observed in 14 of 26 (54%) low stage and grade tumours (T-1-2/G(1-2)) and in 12 of 15 (86%) high stage and grade tumours (T-3, G(3)). Cathepsin D positivity was found within the cytoplasm and at the surface of tumour cells localized in glandular structures and in single cells invading the prostatic stroma, while no staining was observed in normal prostatic tissue and in mesenchymal cells. Two of ten specimens from patients with benign prostatic hyperplasia showed a weakly positive staining reaction within glandular structures. The clinical course of localized prostate cancer appears to be highly variable and the different treatment strategies (radical prostatectomy, radiation therapy or surveillance) have come under debate. For the determination of the biological aggressiveness of prostate cancer in the individual patient easily available biological prognostic factors are needed. This report demonstrates overexpression of cathepsin D in prostate cancer specimens with increasing frequency in patients with tumours of high grade and stage. The usefulness of cathepsin D immunohistochemistry as a prognostic factor should be prospectively evaluated.

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