Aberrant expression of cystatin C in prostate cancer is associated with neuroendocrine differentiation

OBJECTIVE To investigate the expression of cystatin C and the relationship with neuroendocrine differentiation and proliferation in benign and malignant prostatic tissues, as cystatin C, the most important inhibitor of human lysosomal cysteine proteases, is considered to be a major regulator of pathological protein degradation in inflammatory and neoplastic diseases. MATERIALS AND METHODS Immunoreactivity for cystatin C, prostate-specific antigen, Ki-67 and the neuroendocrine marker chromogranin A was examined in whole-mount radical prostatectomy specimens and using tissue microarrays. Cystatin C in tissue homogenates was analysed by Western blotting and enzyme-linked immunosorbent assay (ELISA). The expression and relative levels of cystatin C mRNA were assessed by in situ hybridization and quantitative real-time polymerase chain reaction (QRT-PCR). RESULTS The intensity of cystatin C immunostaining in Gleason grade 2 and 3 prostate cancer was significantly higher than in benign prostatic tissues, but decreased significantly with increasing Gleason grades. There was strong expression of cystatin C in neuroendocrine-like cells, which increased significantly with increasing Gleason grades. The Ki-67 immunoreactivity also increased significantly during de-differentiation. In situ hybridization showed staining patterns in concordance with the immunohistochemical results. ELISA showed high concentrations of cystatin C in benign and malignant tissue extracts and QRT-PCR further corroborated that the cystatin C gene is highly expressed in both benign and malignant prostatic tissues. CONCLUSIONS There was a significant decrease in the immunohistochemical expression of cystatin C in non-neuroendocrine prostate cancer cells, concomitant with increasing Gleason grades. That there were more strongly cystatin C-positive neuroendocrine-like cells in prostate cancer than in benign prostatic tissue suggests a connection between cystatin C and neuroendocrine differentiation in prostate cancer progression.