306 and function of keratins in the nucleus

It is well known that the intracellular iron content in the epidermis is highest in the basal layer and declines towards the stratum corneum and that most of iron in epidermal keratinocytes is not lost by desquamation. These observations suggest that the epidermis is equipped with some mechanism by which intracellular iron in differentiated keratinocytes is excreted to extracellular space before leaching the stratum corneum. In this study, we first demonstrated a distinctive expression pattern of iron metabolism molecules in which molecules participating in iron import and storage are expressed in the lower epidermis while those used for iron release from heme or iron transport are expressed in the upper epidermis by immunohistochemistry and mRNA quantification of the lower and upper epidermis separately obtained by a laser capture microdissection. This expression pattern of iron metabolism molecules was confirmed by normal human epidermal keratinocytes (NHEKs) during in vitro differentiation. Consistent with their expression, keratinocyte differentiation induced by high calcium medium reduces the intracellular iron content. Next, to clarify whether these iron metabolism molecules are really functioning in the epidermis, we demonstrated reducing ferroportin expression in vitro by ferroportin-specific siRNAs and hepcidin significantly increased the intracellular iron content. Finally, we demonstrated that iron content of the epidermis and squames in the dorsal skin was significantly greater in Fpn Epi-KO mice than that in control mice and that Fpn Epi-KO decreased blood Hb concentration more rapidly than control mice on an iron low diet. These studies build upon the evidence that iron is absorbed in the lower epidermis and extruded in the upper epidermis by ferroportin before iron is lost as a part of shedding keratinocytes, consequently preventing iron being lost from the skin at the level of affecting the systemic iron homeostasis.