CD13

CD13/aminopeptidase N (APN) is a widely expressed transmembrane ectoenzyme in endothelial, epithelial, fibroblast, and leukocyte populations^[1]. Mechanistically, CD13 regulates cytokine activity by N-terminal cleavage, participates in MHC class II peptide trimming, and also mediates signal transduction, receptor recycling, FcγR-mediated phagocytosis, and cytokine-receptor functions independent of enzymatic activity^[2]. In inflammatory models, IFN-γ regulated CD13/APN mRNA, membrane CD13, and enzyme activity in HL-60 myelo-monocytic cells, supporting cytokine-dependent control during inflammation^[3]. In cancer research, CD13 is a Zn²⁺-dependent membrane ectopeptidase associated with growth of different human cancers and evaluated as a target for inhibitors and APN-targeted carrier constructs^[4]. Compared with related aminopeptidase isoforms, CD13/APN is specifically framed as a \"moonlighting\" enzyme because its receptor and signaling roles do not always depend on catalytic activity^[5]. This distinction is experimentally important because lung cancer cell migration increased even with catalytically inactive CD13, whereas anti-CD13 antibodies reduced migration and invasion^[6]. For applications, natural and synthetic APN inhibitors help analyze peptide-response modulation, immune functions, proliferation, secretion, invasion, and angiogenesis^[1].