Terminating the Stress: Peripheral Peptidolysis of Proopiomelanocortin-Derived Regulatory Hormones by the Dermal Microvascular Endothelial Cell Extracellular Peptidases Neprilysin and Angiotensin-Converting Enzyme

The skin including the microvascular endothelium is an established peripheral source and target of the immunomodulatory proopiomelanocortin (POMC) peptides ACTH and α-MSH. Whereas intracellular POMC peptide generation is well characterized, less is known on their extracellular processing in peripheral tissues by the neuropeptide-specific zinc metalloproteases neprilysin (NEP) and angiotensin-converting enzyme (ACE). This may locally control POMC peptide bioavailability and activation of ACTH/α-MSH-specific melanocortin receptors (MCs). In a cell-free system, endothelial cell (EC) membranes prepared from ACEhigh/NEPlow-expressing primary human dermal microvascular ECs and the ACElow/NEPhigh expressing EC line HMEC-1 degraded ACTH1–39 over time, resulting in temporary increased α-MSH immunoreactivity. Matrix-assisted laser desorption/ionization time-of-flight mass spectroscopy peptide mapping and electrospray ionization-mass spectroscopy sequencing identified several stable fragments generated from ACTH1–39, ACTH1–24, and α-MSH by EC membranes or recombinant NEP and ACE. Whereas some fragments could be assigned to a cell-specific NEP or ACE activity, other degradation products require additional enzyme activity. Pharmacological NEP inhibition enhanced the ACTH and α-MSH-mediated activation of EC ectopically expressing MC1. Likewise, selected peptides such as α-MSH2–12 generated from ACTH1–39 and α-MSH by recombinant NEP displayed equipotent MC1-activating properties in vitro and antiinflammatory activity in murine allergic contact dermatitis in vivo as compared with the parental peptides. Thus, NEP and ACE significantly contribute to the EC processing of stress hormones (ACTH) and antiinflammatory peptides (α-MSH), which modulates MC1 activation but does not completely inactivate the peptide ligand. Because NEP and ACE are regulated by inflammatory mediators and UV light, this may be important for ACTH/MSH-modulated skin inflammation.