We previously reported that desmosomes play a key role in the adhesion of corneocytes, and their digestion by two types of serine proteases leads to desquamation. Patients with recessive X-linked ichthyosis show hyperkeratosis attributable to desmosomes, associated with an increased content of cholesterol sulfate (CS) and an increased thickness of stratum corneum. In this study, therefore, we examined the possibility that CS provokes the abnormal desquamation, acting as a protease inhibitor. Scaling was induced on mice after topical application of chymostatin and leupeptin. Visible scale was also observed on mice after topical application of CS. We found that the stratum corneum thickness of CS-treated mice was increased in comparison with that of vehicle-treated mice. The thickness of the epidermis and the labeling index with proliferating cell nuclear antigen from CS-treated mice was almost the same as that from vehicle-treated mice. Moreover, in the stratum corneum of CS-treated mice, the content of desmosomes was higher than that in vehicle-treated mice. CS also inhibited the protease-induced cell dissociation of human stratum corneum sheets. In vitro, CS competitively inhibited both types of serine protease: the Ki for trypsin was 5.5 x 10(-6) M and that for chymotrypsin was 2.1 x 10(-6) M. These results indicate that CS retards desquamation by acting as a protease inhibitor. Thus, accumulation of stratum corneum in recessive X-linked ichthyosis may be a result of the inhibition by excessive CS of proteases involved in the dissolution of desmosomes, required for desquamation of the stratum corneum.

Protection from ultraviolet (UV) irradiation is a fundamental issue for living organisms. Although melanin's critical role in the protection of basal keratinocytes is well understood, other factors remain essentially unknown. We demonstrate that up-regulation of squamous cell carcinoma antigen-1 (SCCA1) suppresses c-Jun NH2-terminal kinase-1 (JNK1) and thus blocks UV-induced keratinocyte apoptosis. We found that serpin SCCA1 is markedly elevated in the top layers of sun-exposed or UV-irradiated epidermis. UV-induced apoptosis was significantly decreased when SCCA was overexpressed in 3T3/J2 cells. It was significantly increased when SCCA was down-regulated with small interfering RNA in HaCaT keratinocytes. A search for SCCA-interacting molecules showed specific binding with phosphorylated JNK. Interestingly, SCCA1 specifically suppressed the kinase activity of JNK1. Upon exposure of keratinocytes to UV, SCCA1 was bound to JNK1 and transferred to the nucleus. Involucrin promoter–driven SCCA1 transgenic mice showed remarkable resistance against UV irradiation. These findings reveal an unexpected serpin function and define a novel UV protection mechanism in human skin.

Notch signalling has an important role in skin homeostasis, promoting keratinocyte differentiation and suppressing tumorigenesis. Here we show that this pathway also has an essential anti‐apoptotic function in the keratinocyte UVB response. Notch1 expression and activity are significantly induced, in a p53‐dependent manner, by UVB exposure of primary keratinocytes as well as intact epidermis of both mouse and human origin. The apoptotic response to UVB is increased by deletion of the Notch1 gene or down‐modulation of Notch signalling by pharmacological inhibition or genetic suppression of ‘canonical’ Notch/CSL/MAML1‐dependent transcription. Conversely, Notch activation protects keratinocytes against apoptosis through a mechanism that is not linked to Notch‐induced cell cycle withdrawal or NF‐κB activation. Rather, transcription of FoxO3a, a key pro‐apoptotic gene, is under direct negative control of Notch/HERP transcription in keratinocytes, and upregulation of this gene accounts for the increased susceptibility to UVB of cells with suppressed Notch signalling. Thus, the canonical Notch/HERP pathway functions as a protective anti‐apoptotic mechanism in keratinocytes through negative control of FoxO3a expression.

Woolly hair (WH) is an abnormal variant of tightly curled hair, which is frequently associated with hypotrichosis. Non-syndromic forms of WH can show either autosomal-dominant WH (ADWH) or autosomal-recessive WH (ARWH) inheritance patterns. ARWH has recently been shown to be caused by mutations in either the lysophosphatidic acid receptor 6 (LPAR6) or lipase H (LIPH) gene. More recently, a mutation in the keratin K74 (KRT74) gene has been reported to underlie ADWH. Importantly, all of these genes are abundantly expressed in the inner root sheath (IRS) of human hair follicles. Besides these findings, the molecular mechanisms underlying hereditary WH have not been fully disclosed. In this study, we identified a Japanese family with ADWH and associated hypotrichosis. After exclusion of known causative genes, we discovered the heterozygous mutation c.422T>G (p.Phe141Cys) within the helix initiation motif of the IRS-specific keratin K71 (KRT71) gene in affected family members. We demonstrated that the mutant K71 protein led to disruption of keratin intermediate filament formation in cultured cells. To our knowledge, it is previously unreported that the KRT71 mutation is associated with a hereditary hair disorder in humans. Our findings further underscore the crucial role of the IRS-specific keratins in hair follicle development and hair growth in humans.

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