Cancer is responsible for many deaths and is a major source of healthcare expenditures. The identification of new, non-invasive
biomarkers might allow improvement of the direct diagnostic or prognostic ability of already available tools. Here, we took the innovative approach of interrogating the activity of
exopeptidases in the serum of
cancer patients with the aim of establishing a distinction based on enzymatic function, instead of simple
protein levels, as a means to
biomarker discovery. We first analyzed two well-characterized mouse models of
prostate cancer, each with a distinct genetic lesion, and established that broad
exopeptidase and targeted
aminopeptidase activity tests reveal proteolytic changes associated with
tumor development. We also describe new
peptide-based freeze-frame
reagents uniquely suited to probe the altered balance of selected
aminopeptidases, as opposed to the full array of
exopeptidases, and/or their modulators in patient serum or plasma. One particular proteolytic activity was impaired in animals with aggressive disease relative to
cancer-free littermates. We identified the
protease in question as
dipeptidyl peptidase 4 (DPP4) by analyzing selected knockout mice and evaluating the effect of specific inhibitors. DPP4 activity was also reduced in the sera of patients with metastatic
prostate cancer relative to patients with localized disease or healthy controls. However, no significant differences in DPP4 serum levels were observed, which established the loss of activity as the result of impaired enzymatic function. Biochemical analysis indicated that reduced activity was the result not of post-translational modifications or allosteric changes, but instead of a low-molecular-weight inhibitor. After we adjusted for age and total
prostate-specific antigen, reduced DPP4 activity remained a significant predictor of
cancer status. The results of this proof-of-principle study suggest that DPP4 activity might be a potential blood-based
indicator of the presence of metastatic
cancer of prostatic origin, either by itself or, more likely, as a means to improve the sensitivity and specificity of existing markers.