Expression of hornerin in skin lesions of atopic dermatitis and skin diseases

Abstract

Abstract Cornification is an essential process that confers the skin barrier function. Among the proteins associated with cornification, filaggrin (FLG) plays an important role, and mutations in the FLG gene were shown to be an important predisposing factor for atopic dermatitis (AD). We previously identified an FLG-like protein, hornerin (HRNR). The exact function remains unknown; however, it is hypothesized that HRNR is associated with cornification. Recently, there have been several reports regarding a relationship between HRNR and the pathogenesis of AD. In the present study, we examined the HRNR expression in the skin lesions of 7 unrelated AD patients to clarify the role of HRNR in the pathogenesis of AD. Three of 7 AD specimens were obtained from erythematous lesions, suggesting an acute phase; the others were obtained from lichenified or pruriginous eruptions, suggesting a chronic phase. HRNR was detected in chronic AD lesions, although no signals of HRNR were observed in acute AD lesions. HRNR was detected in the cytokeratin6-expressing area, while Ki67-positive keratinocytes were more abundant in HRNR-positive epidermis. These findings suggest that the expression of HRNR may be associated with the hyperproliferation of epidermis in chronic AD lesions. We next examined the HRNR expression in skin diseases associated with hyperkeratosis, including ichthyosis vulgaris, epidermolytic ichthyosis, Darier’s disease, lichen planus, pustulosis et plantaris, actinic keratosis, and seborrheic keratosis. The expression of HRNR was decreased in ichthyosis vulgaris and was increased in lichen planus and pustulosis et plantaris, in parallel with the expression of FLG. In contrast, HRNR signals were irregularly observed in different cells from those expressing FLG in epidermolytic ichthyosis or actinic keratosis. The abnormal epidermal differentiation in these diseases may affect the unique expression of HRNR. Accordingly, HRNR may play an overlapping role with FLG in cornification; however, this unique expression pattern of HRNR in some diseases supports that HRNR may have a distinct role in the molecular processes of cornification.