Protease/PAR-2 Signaling in Atopic Dermatitis activity stimulates inflammation via a PAR-2-independent mechanism.99 The cockroach allergens, another type of major aeroallergen, are also reported to activate PAR-2 on keratinocytes. Jeong, et al.12 reported that the topical appli- cation of house dust mites and cockroach allergens to bar- rier-disrupted skin delayed barrier recovery and lamellar body secretion in murine and human skin via PAR-2 acti- vation. The topical application of cockroach allergens also showed an inhibitory effect on the epidermal calcium gra- dient change after barrier disruption, suggesting that the negative effects of cockroach allergens on barrier recovery may be due to the PAR-2 signal-induced epidermal calcium modulation. Recently, Staphylococcus aureus has also been reported to produce extracellular V8 protease, which exhi- bits a similar specificity of glutamate-specific cleavage and a similar sequence to exfoliative toxins and cause epidermal permeability barrier dysfunction in the skin of nude mice by directly degrading DSG1.100 These imply that proteoly- tically active allergens could break down the skin barrier via PAR-2-mediated inhibition of lamellar body secretion or PAR-2-non-mediated mechanisms, including degrada- tion of corneodesmosomal proteins and lipid processing enzymes, triggering further allergen penetration through the disrupted epithelial barrier to aggravate Th2-mediated inflammation and possibly switch the non-IgE-associated form of AD (atopiform AD) to the typical IgE-associated form of AD. Recently, several reports have suggested that proteolytically active allergens perturb not only the SC per- meability barrier but also the tight junction (TJ), another functional barrier of the skin.101-103 The exfoliative toxin- negative Staphylococcus strains were shown to decrease the expression of TJ and atypical protein kinase C, a key player in TJ assembly with decreased transepithelial resis- tance in the human keratinocyte cell line.101 Previous studies have reported that house dust mite allergen Der p 1 dis- rupts intercellular TJ via proteolytic cleavage of TJ adhesion protein, claudin-1 and occludin in airway epithelial cells.102 Pollens, which are known to be associated with allergic rhini- tis and conjunctivitis, also contain proteolytic enzymes on their surface and degrade the TJ proteins of human airway epithelial cells.103 These findings suggest that proteases deriv- ed from house dust mite, cockroaches, Staphylococcus aur- eus, and pollens contribute to the pathogenesis of AD by pro- pagating the vicious cycle of protease-mediated permeabi- lity barrier defect and increased allergen penetration (Fig. 4). THERAPEUTIC APPLICATION OF PAR-2 ANTAGONIZING TREATMENT Identification of the important role of PAR-2 in various AD symptoms, including inflammation, pruritus and skin barrier impairment, has suggested that PAR-2 antagoni- zing treatment may be a potential therapeutic strategy for AD. In addition, the crucial role of mast cells and mast cell-expressed PAR-2 in various inflammatory diseases also suggests that PAR-2 antagonizing treatment could be applied to several chronic inflammatory diseases, such as asthma,104 rheumatoid arthritis105 and inflammatory bowel diseases (IBD).106 Theoretically, several approaches could be used to antagonize PAR-2 activation, including down- regulation of protease activity, inhibition of PAR-2 expres- sion using siRNA technology, PAR-2 monoclonal antibody or PAR-2 specific antagonist; however, at this time, a practical application of PAR-2 antagonizing treatment has not yet reported. Down-regulation of protease activity to control PAR-2 activation in skin can be achieved by either topical protease inhibitors or pH controlling agents. We recently reported that the topical application of serine protease inhibitor (soy- bean tryptic inhibitor) significantly reduced the pruritus symptoms in end-stage renal dysfunction patients, where increased PAR-2 expression in skin was observed. Impro- vement of skin barrier functions was also observed after protease inhibitor application.107 While this is a relatively easier way to control PAR-2 activation, proteases are res- ponsible not only for PAR-2 activation, but also for other normal homeostatic processes such as desquamation. As a result, use of protease inhibitors may have adverse effects on the skin, which is a major drawback for practical appli- cation of protease inhibitors for topical use. Normal skin surface pH ranges from 5.0 to 5.5, which is relatively acidic compared to the normal physiologic pH, and it is well known that pH gradually increases across the SC. This “acid mantle” is very important for the maintain- ing skin’s homeostasis, including regulating protease activity. In AD, increase of skin surface pH is well reco- gnized and increased proteolytic activity is also observed. In addition to the PAR-2 activation, increased pH is also considered responsible for increased susceptibility to se- condary infections in AD patients. As a result, controlling skin surface pH within normal acidic range is a very im- portant therapeutic regimen for AD patients and use of soap or alkaline detergents is strongly discouraged. The inhibitory activity of the PAR-2 antibody on itching behavior in chronic dry skin itch in an animal model has also been recently reported. Intradermal injection of PAR- 2 antibody significantly reduced spontaneous scratching behavior, which alludes the important role of PAR-2 in itching sensation. Further evidence was provided by the in- creased scratching behavior after PAR-2 activator injec- tion.108 However, a therapeutic antibody against PAR-2 has not yet been reported, and practical application of PAR-2 antibody needs to be further developed. Yonsei Med J http://www.eymj.org Volume 51 Number 6 November 2010 817