| Summary: | Increasing evidence indicates calcium-binding S100 protein involvement in inflammation and tumor progression. In this prospective study, we evaluated the mRNA levels of two members of this family, <i>S100A9</i> and <i>S100A12</i>, in peripheral blood mononuclear cells (PBMCs) in a cohort of 121 prostate cancer patients using RT-PCR. Furthermore, monocyte count was determined by flow cytometry. By stratifying patients into different risk groups, according to TNM stage, Gleason score and PSA concentration at diagnosis, expression of <i>S100A9</i> and <i>S100A12</i> was found to be significantly higher in patients with metastases compared to patients without clinically detectable metastases. In line with this, we observed that the protein levels of S100A9 and S100A12 in plasma were higher in patients with advanced disease. Importantly, in patients with metastases at diagnosis, high monocyte count and high levels of <i>S100A9</i> and <i>S100A12</i> were significantly associated with short progression free survival (PFS) after androgen deprivation therapy (ADT). High monocyte count and <i>S100A9</i> levels were also associated with short cancer-specific survival, with monocyte count providing independent prognostic information. These findings indicate that circulating levels of monocytes, as well as <i>S100A9</i> and <i>S100A12</i>, could be biomarkers for metastatic prostate cancer associated with particularly poor prognosis.
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