Protease/PAR-2 Signaling in Atopic Dermatitis and at least one aspartate protease, cathepsin D.3-7 These proteases are tightly regulated by specific protease inhibi- tors and mediate various cellular responses in the skin, such as inflammation and immune responses, host defense, chemotaxis, cytokine expression, vascular function, tissue repair, and apoptosis.8 In addition to endogenous proteases, exogenous proteases from allergens such as house dust mites, cockroaches, certain bacteria, and fungi can also signal the epidermis. A number of biological activities of proteases are mediated, at least in part, by the activation of its receptors, protease-activated receptors (PARs).9 Recent works have indicated that PAR-2, as a sensor for endoge- nous as well as exogenous proteases, plays numerous physiological and pathophysiological roles in the skin.8,10 In addition, there is increasing evidence that protease and PAR-2 play an important role in the maintenance of epi- dermal permeability barrier homeostasis.7,11,12 Moreover, abnormal expression or activity of serine proteases and PAR-2 has been associated with several inflammatory skin disorders involving barrier abnormalities, including atopic dermatitis, netherton syndrome (NS), psoriasis, and peeling skin syndrome.13-18 In this review, we will discuss the role of protease/PAR-2 signaling in epidermal permea- bility barrier homeostasis, as well as its contribution to the pathogenesis of atopic dermatitis (AD). SERINE PROTEASES AND THEIR INHIBITORS IN SKIN Human tissue kallikreins (KLKs) are the largest family of trypsin- or chymotrypsin-like secreted serine proteases.13 Eight KLKs, including KLK5, -6, -7, -8, -10, -11, -13, and -14 are known to be expressed in the epidermis and skin appendages, such as sebaceous gland.19 Among these, only KLK7 exhibits chymotrypsin-like substrate specificity; the other KLKs exhibit trypsin-like serine proteases. It is well known that KLK5 and KLK7 are the major active KLKs in the SC, regulating the desquamation process through corneodesmosomal cleavage and lipid barrier formation by degrading lipid processing enzymes.19-21 In addition, KLK5 and KLK7 have been shown to control the enzymatic processing of cathelicidin, thereby affecting its antimicro- bial activity and inflammatory responses.22 KLK14 has also been detected in its active form in the SC and is thought to be a candidate protease involved in the process of desqua- mation, however, its precise role has not been elucidat- ed.23,24 KLK5 and KLK7 are stored in the form of pro- enzyme in the lamellar bodies (LBs) along with a substrate of KLK7, corneodesmosin and their inhibitor, lymphoe- pithelial Kazal-type-related inhibitor (LEKTI).25 Upon release into the stratum granulosum (SG)-SC interface, these pro-KLK zymogens are activated through a KLK proteolytic activation cascade.26 KLK5 can be activated by itself or by KLK14 and then activate several other pro- KLKs; therefore KLK5 is thought to be the key protease for the initiation of the KLK cascade.24 Significantly, KLK5 and KLK14 are known to activate PAR-2, thereby modulat- ing epidermal permeability barrier homeostasis, immune and inflammatory responses, skin pigmentation, itching sensations, as well as tumor surveillance.10,27 KLK8 has been reported to be localized in the LBs with KLK5, KLK7 and their inhibitors and secreted in the SG-SC interface, regulating desquamation and epidermal proliferation.28,29 Recent studies have investigated factors regulating the expression of KLKs in the skin and reported that epider- mal calcium ions, vitamin D3, and retinoic acid indepen- dently regulate the expression of KLK5 and KLK7 in normal human epidermal keratinocytes.30 An increase in extracellular calcium induced KLK5 and KLK7 expres- sion with induction of differentiation markers, suggesting that the expression of KLKs is induced during epidermal differentiation. In contrast, both 9-cis retinoic acid and 13- cis retinoic acid increased KLK5 and KLK7 expression, independently of keratinocyte differentiation. 1,25 (OH)2 vitamin D3, which is well known to induce cathelicidin expression, was also demonstrated to stimulate the expres- sion of KLK5 and KLK7, which are co-localized with cathelicidin. In addition to KLKs, the two transmembrane serine protease, matriptase (MT-SP1) and prostasin have been identified in the uppermost epidermal layers and postulated to be involved in epidermal barrier formation and SC hydra- tion via epithelial sodium channel-induced intracellular calcium influx and consequent activation of calcium- dependent proteases and transglutaminases (TGMs).31 Moreover, the observation that mice deleted for matriptase showed impaired desquamation suggests that matriptase is also involved in the process of desquamation.32 A number of serine protease inhibitors are present in the skin, and regulate proteolytic activity in order to prevent excessive serine protease cascade, while also maintaining permeability barrier homeostasis. Among several serine protease inhibitors observed in the SC, such as secretory leucocyte protease inhibitor (SLPI), elafin skin-dericed antileukoproteinase (SKALP), and plasminogen activator inhibitor type 2, which are normally cross-linked to the cornified envelope, LEKTI-1, a secreted serine protease in- hibitor, is thought to be the major player in the SC. LEKTI is encoded by SPINK5 (serine protease inhibitor Kazal- type 5) and its multidomains exhibit diverse inhibitory effects toward trypsin, plasmin, subtilisin A, cathepsin G, and human neutrophil elastase.33 In addition, previous in vitro studies demonstrated that LEKTI fragments could Yonsei Med J http://www.eymj.org Volume 51 Number 6 November 2010 809