Immunohistochemical analysis of Omi/HtrA2 expression in prostate cancer and benign prostatic hyperplasia

The serine protease Omi/HtrA2 is released from mitochondria into the cytosol after apoptotic stimuli, inducing apoptosis in a caspase-independent manner through its protease activity and in a caspase-dependent manner by neutralizing the inhibition of inhibitor of apoptosis proteins (IAPs) on caspases. Alteration of apoptosis is essential for cancer development, and cancer cell death by radiation and chemotherapy is largely dependent upon apoptosis. Thus, analysis of the expression status of Omi/HtrA2, a regulator of apoptosis, in cancer tissues is needed for an understanding of cancer development. In the current study we analyzed the expression of Omi/HtrA2 in 65 prostate cancer, 40 benign prostatic hyperplasia and 10 normal prostate specimens by immunohistochemistry. Omi/HtrA2 mRNA levels of in vivo prostate cancer and benign prostatic hyperplasia samples were also assayed by semiquantitative reverse transcription-polymerase chain reaction. Immunopositivity (defined as > or =30%) was observed for Omi/HtrA2 in most of the prostate cancers, and the positive rate of Omi/HtrA2 was lower in the well-differentiated group than in the poorly and moderately differentiated groups (p<0.005). By contrast, the cells in the normal prostate and benign prostatic hyperplasia groups showed no or only weak expression of Omi/HtrA2. Meanwhile, the Omi/HtrA2 mRNA level of prostate cancer is much higher than that of benign prostatic hyperplasia (p<0.001). Taken together, these results suggest that prostate cancer cells in vivo may need Omi/HtrA2 expression for apoptosis, and that Omi/HtrA2 expression might be involved in prostate cancer development.