Fig shows a possible scheme of the generation of AP

Fig. 8 shows a possible scheme of the generation of AP(+)-exosomes based on this study. In response to inflammatory stimuli, ERAP1 is secreted from ER into the extracellular milieu through the conventional secretion pathway. On the other hand, exosomes are derived from the endosomal pathway, and are formed within multivesicular bodies (MVB). They are released into the extracellular milieu upon fusion with the plasma membrane. After association with ERAP1 in the milieu, exosomes play important roles in host defense through the activation of macrophages.
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Acknowledgement
This work was supported in part by Grants-in-Aid from the Ministry of Education, Science, Sports and Culture of Japan (Project 16K08630).
Introduction The activity of the different enzymes in tissues is a determinant of the physiological state of cells. The altered activity of several proteases is associated with several disorders and diseases [1]. The measurement of enzyme proteolytic activity levels is particularly important in studies of the development, growth and metastasis of several types of cancer [1]. Aminopeptidase expression is crucial for the normal activity of healthy cells [2]. Aminopeptidases are responsible for the limited proteolysis of proteins and peptides, and they function as posttranslational modifiers of enzymes, inactivators of signal peptides and hormones, and digestive enzymes [2]. For example, methionine aminopeptidases catalyze the first step of protein maturation, and endoplasmic reticulum aminopeptidase (ERAP) trims Pasireotide to the proper length for accommodation in the MHC class I (MHCI) complex, while aminopeptidase N (APN) has multiple functions and is known as a “moonlighting enzyme” [3], [4], [5]. Despite intense efforts to understand the roles of individual enzymes, the activity and regulation of aminopeptidases in cells remains unclear. Aminopeptidases are enzymes that are also recognized as markers of different types of diseases [6], [7], [8]. Most attention has focused on the activity of aminopeptidase N (APN or CD13), which is a highly expressed enzyme that is present in several tissues and is strongly correlated with several types of tumors, including lung, thyroid, skin, ovary, kidney and bone cancers [9]. Additionally, inflammatory diseases such as rheumatoid arthritis and lupus are correlated with APN deregulation in lymphocytes [10]. Other aminopeptidases, such as leucyl aminopeptidase (LAP), aminopeptidase A (APA), aminopeptidase B (APB), ERAP, and insulin-regulated aminopeptidase (IRAP), are also involved in different diseases [11], [12], [13], [14], [15]. Several studies have analyzed the activity of aminopeptidases in both healthy and pathologically altered tissues. However, determining aminopeptidase activity in the kidney seems most promising [16], [17]. Aminopeptidases, especially APN, are considered biomarkers of kidney pathologies such as acute kidney injury or renal cancer [14]. During kidney injury, membrane-bound or cellular aminopeptidases are released into the urine [17]. Their early detection could improve the sensitivity of kidney failure diagnoses. Studies on aminopeptidase activity in tissues have largely been based on single chromogenic substrates specific for each aminopeptidase [18]. Aminoacyl-β-naphthylamides have been commonly used since the late 1950s [8], [19]. This approach has several advantages, including the ability to measure the natural activity of enzymes in tissues, because this method accounts for the influence of modulators of enzyme activity. Expression, maturation, activation or inhibition by factors present in cells can alter the activity of aminopeptidases, which are strictly regulated enzymes. Substrates can be applied to limit the detection to active forms of the enzymes. Single chromogenic or fluorogenic substrates can be convenient for studies; however, they can be potentially misleading because of the overlapping activity of aminopeptidases [5], [20], [21]. Some enzymes have wide specificity and are able to hydrolyze several different amino acids in a peptidic substrate. For example, APN can recognize and efficiently hydrolyze leucine-based substrates, which are normally used for leucine aminopeptidase (LAP) activity determination [22]. Studies on LAP activity in tissues using this method are hindered by high APN expression and activity.