Heme Oxygenase-1 Is Upregulated during Differentiation of Keratinocytes but Its Expression Is Dispensable for Cornification of Murine Epidermis

Abstract

The epidermal barrier of mammals is initially formed during embryonic development and continuously regenerated by the differentiation and cornification of keratinocytes in postnatal life. Cornification is associated with the breakdown of organelles and other cell components by mechanisms which are only incompletely understood. Here, we investigated whether heme oxygenase 1 (HO-1), which converts heme into biliverdin, ferrous iron and carbon monoxide, is required for normal cornification of epidermal keratinocytes. We show that HO-1 is transcriptionally upregulated during the terminal differentiation of human keratinocytes in vitro and in vivo. Immunohistochemistry demonstrated expression of HO-1 in the granular layer of the epidermis where keratinocytes undergo cornification. Next, we deleted the Hmox1 gene, which encodes HO-1, by crossing Hmox1-floxed and K14-Cre mice. The epidermis and isolated keratinocytes of the resulting Hmox1f/f K14-Cre mice lacked HO-1 expression. The genetic inactivation of HO-1 did not impair the expression of keratinocyte differentiation markers, loricrin and filaggrin. Likewise, the transglutaminase activity and formation of the stratum corneum were not altered in Hmox1f/f K14-Cre mice, suggesting that HO-1 is dispensable for epidermal cornification. The genetically modified mice generated in this study may be useful for future investigations of the potential roles of epidermal HO-1 in iron metabolism and responses to oxidative stress.

Iron Metabolism of the Skin: Recycling versus Release

“…Many steps of the iron turnover are conserved in all cell types and have been summarized in recent excellent review articles [ 5 , 6 , 7 ]. However, there are several specific features, such as the regulated expression of regulators of iron metabolism and transport, that point to unique adaptations of the iron metabolism in the epidermis [ 8 , 9 , 10 ].…”

“…The expression of HMOX1 is induced by Nrf2 in many cell types, including proliferating keratinocytes, under conditions of oxidative stress, and Nrf2-dependent HO-1 expression has also been reported for the inflammatory skin disease psoriasis [ 50 , 51 , 52 ]. In contrast to the inducible expression of HMOX1 , differentiated keratinocytes of the upper epidermis contain constitutively high levels of HO-1 [ 9 , 10 , 53 ]. Both mRNA and protein levels of HO-1 increase during the differentiation of epidermal keratinocytes in vitro [ 10 ].…”

“…In contrast to the inducible expression of HMOX1 , differentiated keratinocytes of the upper epidermis contain constitutively high levels of HO-1 [ 9 , 10 , 53 ]. Both mRNA and protein levels of HO-1 increase during the differentiation of epidermal keratinocytes in vitro [ 10 ]. Likewise, the transcriptional activity is induced by the HMOX1 promoter and the HO-1 protein is present in the suprabasal epidermis of unstressed skin [ 9 , 10 , 54 ].…”

“…Both mRNA and protein levels of HO-1 increase during the differentiation of epidermal keratinocytes in vitro [ 10 ]. Likewise, the transcriptional activity is induced by the HMOX1 promoter and the HO-1 protein is present in the suprabasal epidermis of unstressed skin [ 9 , 10 , 54 ]. The regulation of HMOX1 gene expression remains to be investigated, for example, by comparing wildtype and Nrf2 knockout mice.…”

“…To exert its enzymatic activity, HO-1 depends on its substrate, heme. It was proposed that heme is released from hemoproteins in the course of the intracellular remodeling prior to cornification [ 10 ]. Multiple hemoproteins, such as CYP450 protein and cytochrome c, are present in epidermal keratinocytes.…”