Carboxypeptidase M in apoptosis, adipogenesis and cancer

Carboxypeptidase M (CPM), a metallo-carboxypeptidase, specifically cleaves off arginines and lysines from the C-terminus of peptides and proteins. Unlike carboxypeptidase U that has a well-defined antifibrinolytic activity, the proposed role for CPM in immunity and cellular differentiation still necessitates substantial scientific proof. From recent research evidence is emerging that CPM could represent a prognostic factor in tumoral processes and serve as a molecular biomarker for specific cancer types. The discovery of relevant natural substrates and modulation of their biological activities remains essential to uncover CPM's functions in vivo, as well as the knowledge of cell- and tissue-specific expression of the enzyme in (patho)physiological conditions. This research explored each of these strategies to contribute to a better insight into the functioning of CPM in human (patho)physiology. First, the chemokine CCL1/1-309 was identified as an excellent substrate for CPM in vitro. C-terminal clipping of CCL1 by CPM enhanced intracellular calcium release and anti-apoptotic/proliferative activity through CCR8, despite reducing the CCR8 binding affinity of CCL1. CPM, CCL1 and CCR8 appeared to strongly co-localize on plaque macrophages. This is strongly suggestive for an interplay of CPM with the CCL1/CCR8 system in these chronic inflammatory lesions, and the in vivo relevancy of our findings. Second, a vast upregulation of the CPM transcript and basic carboxypeptidase activity was observed during 3T3-L1 adipogenic differentiation. The CPM protein was detected in human and (probably also) mouse fat. CPM and/or basic carboxypeptidases did not appear to be involved in triglyceride accumulation during adipogenesis, nor in glucose transport via GLUT1/GLUT4 in mature adipocytes. Third, we confirmed the assumption present in literature that the human healthy kidney is indeed very rich in CPM. A detailed mapping study revealed a widespread distribution in renal tubular structures (especially apically), mesangial cells and the parietal epithelium beneath the Bowman's basement membrane. Vascular endothelium was CPM negative. Tumor cells of different renal cell carcinoma subtypes appeared to lose CPM expression (except for papillary renal cell carcinoma samples). Tumor-associated macrophages and neovascular endothelium showed important CPM positivity, pointing towards the implication of CPM in cancer-related inflammation and angiogenesis.