Abstract
Filaggrin is completely degraded in the uppermost layer of the stratum corneum, affording a mixture of free and modified hygroscopic amino acids known as natural moisturizing factor (NMF),which serves to maintain skin hydration. Recently, we purified a novel aminopeptidase based on its citrulline-cleavage activity and identified it as a neutral cysteine protease, bleomycin hydrolase (BH). BH generated free amino acids from filaggrin-derived peptides but not from filaggrin monomer itself. For NMF synthesis, it is necessary that filaggrin monomer be degraded into smaller peptides. We tested caspase-14, an up-stream enzyme in the NMF generation pathway. Caspase-14 showed limited hydrolysis on filaggrin monomer, liberating three peptide fragments. We examined whether BH is involved in NMF generation in human epidermis. We also examined age-related changes of BH in women in their forties and sixties. We found that BH activity was significantly down-regulated in corneocyte extract from the skin of women in their sixties, compared to women in their forties. The content of free amino acids in the extract was also significantly decreased. Skin of subjects self-reported to be hypersensitive showed markedly decreased BH activity. Lastly, we examined BH expression in patients with atopic dermatitis (AD). BH activity and expression were markedly decreased in lesional skin, suggesting a defect of the filaggrin degradation pathway. Our results support the idea that BH plays a major role in NMF generation. BH may be a novel target for treatment of dry skin as well as skin diseases, such as AD.
