Nrf transcription factors in keratinocytes are essential for skin tumor prevention but not for wound healing.

The Nrf2 transcription factor is a key player in the cellular stress response through its regulation of cytoprotective genes. In this study we determined the role of Nrf2-mediated gene expression in keratinocytes for skin development, wound repair, and skin carcinogenesis. To overcome compensation by the related Nrf1 and Nrf3 proteins, we expressed a dominant-negative Nrf2 mutant (dnNrf2) in the epidermis of transgenic mice. The functionality of the transgene product was verified in vivo using mice doubly transgenic for dnNrf2 and an Nrf2-responsive reporter gene. Surprisingly, no abnormalities of the epidermis were observed in dnNrf2-transgenic mice, and even full-thickness skin wounds healed normally. However, the onset, incidence, and multiplicity of chemically induced skin papillomas were strikingly enhanced, whereas the progression to squamous cell carcinomas was unaltered. We provide evidence that the enhanced tumorigenesis results from reduced basal expression of cytoprotective Nrf target genes, leading to accumulation of oxidative damage and reduced carcinogen detoxification. Our results reveal a crucial role of Nrf-mediated gene expression in keratinocytes in the prevention of skin tumors and suggest that activation of Nrf2 in keratinocytes is a promising strategy to prevent carcinogenesis of this highly exposed organ.

Biological roles of APP in the epidermis.

The amyloid precursor protein (APP) was initially detected in cells of the central nervous system where it is considered to be involved in the pathogenesis of Alzheimer's disease. However, APP is also found in peripheral organs with exceptionally strong expression in the mammalian epidermis where it fulfils a variety of distinct biological roles. Full length APP appears to facilitate keratinocyte adhesion due to its ability to interact with the extracellular matrix. The C-terminus of APP also serves as adapter protein for binding the motor protein kinesin thereby mediating the centripetal transport of melanosomes in epidermal melanocytes. By the action of alpha-secretase sAPPalpha, the soluble N-terminal portion of APP, is released. sAPPalpha has been shown to be a potent epidermal growth factor thus stimulating proliferation and migration of keratinocytes as well as the exocytic release of melanin by melanocytes. The release of sAPPalpha can be almost completely blocked by inhibiting alpha-secretase with hydroxamic acid-based zinc metalloproteinase inhibitors. In hyperproliferative keratinocytes from psoriatic skin this inhibition results in normalized growth.

Cytoprotective function of sAppalpha in human keratinocytes.

sAPPalpha, the soluble form of the beta-amyloid precursor protein, has been shown to act as a potent epidermal growth factor by stimulating keratinocyte proliferation and migration. In this report we provide evidence for a cytoprotective role of sAPPalpha. As a model we used HaCaT cells and normal human keratinocytes (NHK) cultured in the absence of fetal calf serum and bovine pituitary extract. Under these conditions keratinocytes began to undergo apoptosis at increasing rates after 96 h of culture. Surprisingly, keratinocytes were protected from apoptosis by the addition of 50 nM recombinant sAPPalpha. Subsequent experiments were performed to elucidate the regulatory basis of the cytoprotective role of sAPPalpha. We found that recombinant sAPPalpha facilitated the substrate adhesion of keratinocytes in the first 30 minutes after seeding. The basis for this adhesion-promoting function was shown by the ability of recombinant sAPPalpha to continuously coat the culture dish thereby promoting the ability to bind keratinocytes. A second mechanism explaining the cytoprotective role was found in the significant inhibition of apoptosis by recombinant sAPPalpha. In HaCaT cells moderate UV-B irradiation was sufficient to induce apoptosis. In contrast, induction of apoptosis in NHK required additionally the depletion of endogenous sAPPalpha suggesting that sAPPalpha mediates protection against UV-B irradiation. Staurosporine-induced apoptosis rates were significantly reduced by about 59% after addition of recombinant sAPPalpha. These results show that sAPPalpha exerts a pronounced cytoprotective effect and that this effect is mediated by facilitated cell adhesion and by the antiapoptotic function of sAPPalpha.

Normalized proliferation of normal and psoriatic keratinocytes by suppression of sAPPalpha-release.

The soluble form of the beta-amyloid precursor protein (sAPPalpha) is known to function in the autocrine regulation of epidermal growth and repair. Here we show that its proteolytic release by alpha-secretase in normal human keratinocytes is susceptible to hydroxamic-acid-based zinc metalloproteinase inhibitors and suppressed by these inhibitors by 80%-90%. As various other growth factors participate in regulating epidermal growth we investigated whether the inhibitor-induced sAPPalpha-deficiency would affect keratinocyte proliferation. At optimal inhibitor concentrations the suppression of sAPPalpha-release was followed by a decline in proliferation by 50%-60%, indicating that sAPPalpha is a major growth factor that cannot be compensated for by other growth factors. This finding was the basis for the treatment of human lesional psoriatic keratinocytes with these inhibitors, which resulted in the normalization of their increased proliferation rates. The reversibility of these effects and the lack of toxicity underline the value of these inhibitors and suggest their therapeutic application in psoriatic skin diseases.

Keratinocytes from APP/APLP2-deficient mice are impaired in proliferation, adhesion and migration in vitro.

Growing evidence shows that the soluble N-terminal form (sAPPalpha) of the amyloid precursor protein (APP) represents an epidermal growth factor fostering keratinocyte proliferation, migration and adhesion. APP is a member of a protein family including the two mammalian amyloid precursor-like proteins APLP1 and APLP2. In the mammalian epidermis, only APP and APLP2 are expressed. APP and APLP2-deficient mice die shortly after birth but do not display a specific epidermal phenotype. In this report, we investigated the epidermis of APP and/or APLP2 knockout mice. Basal keratinocytes showed reduced proliferation in vivo by about 40%. Likewise, isolated keratinocytes exhibited reduced proliferation rates in vitro, which could be completely rescued by either exogenously added recombinant sAPPalpha, or by co-culture with dermal fibroblasts derived from APP knockout mice. Moreover, APP-knockout keratinocytes revealed reduced migration velocity resulting from severely compromised cell substrate adhesion. Keratinocytes from double knockout mice died within the first week of culture, indicating essential functions of APP-family members for survival in vitro. Our data indicate that sAPPalpha has to be considered as an essential epidermal growth factor which, however, in vivo can be functionally compensated to a certain extent by other growth factors, e.g., factors released from dermal fibroblasts.

The influence of L-arginine on the regulation of epidermal arginase.

Topical preparations containing urea are firmly established in dermatological therapy and nursing care therapy. In addition, urea is frequently used as an inactive ingredient with disinfecting, keratoplastic and penetration-promoting action in topical preparations. Despite good tolerance and ensured action, particularly the irritating effect on erosive, exudative or strongly inflammatory skin restricts its application. Endogenic urea is synthesised from L-arginine by an extra-hepatic arginase in keratinocytes. This enzymatic reaction is subject to regulation by manganese ions and the intracellular L-arginine concentration. L-Arginine is predominantly transported into the cell by a membrane transport system that is dependent on the membrane potential. By incubating keratinocyte cultures in different concentrations of L-arginine and manganese chloride, it could be shown that the keratinocytic urea synthesis can be increased. In relevant concentrations, L-arginine and manganese chloride do not exhibit any proliferation-inhibiting action, do not trigger any apoptosis or necrosis, and are stable. An increase in the expression of arginase cannot be detected using L-arginine. The application of L-arginine alone or in combination with manganese chloride increases the endogenous intrakeratinocytic urea synthesis and thus offers an option for topical therapeutic application in cases of dry skin conditions.