CPM is a part of the molecular signature of many cancers. There is good evidence that it is useful for the discrimination and stratification of cancer types, possibly in combination with other markers such as EGFR and MDM2. Whether it is also a drug target remains to be determined. Lung, kidney, brain, and the reproductive system contain relatively high levels of CPM, but its functions in those tissues are largely unknown. CPM is expressed on tumor-associated macrophages. To facilitate the investigation of CPM in tumor-associated inflammation and in the other aspects of tumor biology, it is necessary to develop potent and selective CPM inhibitors.

Carboxypeptidase M (CPM) targets the basic amino acids arginine and lysine present at the C -terminus of peptides or proteins. CPM is thought to be involved in inflammatory processes. This is corroborated by CPM-mediated trimming and modulation of inflammatory factors, and expression of the protease in inflammatory environments. Since the function of CPM in and beyond inflammation remains mainly undefined, the identification of natural substrates can aid in discovering the (patho)physiological role of CPM. CCL1/I-309, with its three C -terminal basic amino acids, forms a potential natural substrate for CPM. CCL1 plays a role not only in inflammation but also in apoptosis, angiogenesis and tumor biology. Enzymatic processing differently impacts the biological activity of chemokines thereby contributing to the complex regulation of the chemokine system. The aim of the present study was to investigate whether (i) CCL1/I-309 is prone to trimming by CPM, and (ii) the biological activity of CCL1 is altered after C -terminal proteolytic processing. CCL1 was identified as a novel substrate for CPM in vitro using mass spectrometry. C -terminal clipping of CCL1 augmented intracellular calcium release mediated by CCR8 but reduced the binding of CCL1 to CCR8. In line with the higher intracellular calcium release, a pronounced increase of the anti-apoptotic activity of CCL1 was observed in the BW5147 cellular model. CCR8 signaling, CCR8 binding and anti-apoptotic activity were unaffected when CPM was exposed to the carboxypeptidase inhibitor DL-2-mercaptomethyl-3-guanidino-ethylthiopropanoic acid. The results of this study suggest that CPM is a likely candidate for the regulation of biological processes relying on the CCL1-CCR8 system.

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Although the kidney generally has been regarded as an excellent source of carboxypeptidase M (CPM), little is known about its renal-specific expression level and distribution. This study provides a detailed localization of CPM in healthy and diseased human kidneys. The results indicate a broad distribution of CPM along the renal tubular structures in the healthy kidney. CPM was identified at the parietal epithelium beneath the Bowman’s basement membrane and in glomerular mesangial cells. Capillaries, podocytes, and most interstitial cells were CPM negative. Tumor cells of renal cell carcinoma subtypes lose CPM expression upon dedifferentiation. Tissue microarray analysis demonstrated a correlation between low CPM expression and tumor cell type. CPM staining was intense on phagocytotic tumor-associated macrophages. Immunoreactive CPM was also detected in the tumor-associated vasculature. The absence of CPM in normal renal blood vessels points toward a role for CPM in angiogenesis. Coexistence of CPM and the epidermal growth factor receptor (EGFR) was detected in papillary renal cell carcinoma. However, the different subcellular localization of CPM and EGFR argues against an interaction between these h proteins. The description of the distribution of CPM in human kidney forms the foundation for further study of the (patho)physiological activities of CPM in the kidney.