Role of C-Jun and AP-1 signaling in skin development and turmorigenesis

Research project P 14680 AP-1 signaling in skin development and tumorigenesis Erwin F. WAGNER 27.11.2000 Skin epidermis is a stratified epithelium composed primarily of keratinocytes, whose proliferation and differentiation must be tightly regulated and coordinated in order to maintain skin homeostasis. Epidermal differentiation leads to the formation of an organized tissue in which morphologically distinguishable cells are arranged in discrete layers of basal, spinous, granular, and cornified cells. Skin differentiation is the consequence of changes in gene expression. importantly, genes involved in keratinocyte differentiation (e.g. keratin 5, keratin 1, filaggrin, loricrin, and involucrin) are regulated by the transcription factor AP- I suggesting a prominent role for AP- I components in skin biology. The transcription factor AP-1 is generated by a series of dimers of products of the Fos, Jun, and CREB/ATF protein families, as well as other bZip proteins. Combinatorial association can involve three Jun genes (c-jun, junB, junD), four Fos genes (c-fos, fosB, fra-1, fra-2) and several CREB/ATF genes. The members of the AP-1 family are engaged in the control of cell proliferation as well as various types of differentiation and in neural function and stress responses. Several subunits of AP-1 are transforming proteins and required for transformation by other oncogenes. A functional role for AP- I components, particularly for c-jun, has been suggested for differentiation, carcinogenesis, UV-response, photoaging, and wound repair in the skin. The major goal of this project is to define the specific roles of AP-1 signaling components in skin biology. Therefore, skin-specific c-jun knock-out mice as well as mice carrying the c-jun phosphoacceptor mutant allele c- junAA (Ser 73 and 63 mutated to Ala thereby preventing the activation by JNKs), and knock-out mice of the Jun- N-terminal kinase 1 and 2 (JNK1 and JNK2) will be analyzed in terms of skin development, carcinogenesis, UV- response, and wound repair both in vivo and in vitro.

The major goal of this project was to define the specific roles of AP-l signaling components in skin biology using the mouse as a model organism. Skin-specific AP-1 knock-out mice were analyzed in terms of skin development. tumorigenesis. UV-response. and wound repair. Using the loxP/Cre-system we deleted one (c-jun, junB, c-fos, fra- 1 and fra-2) or two genes (c-jun and junB, c-jun and c-fos. and c-jun andfra-]) of the AP-I transcription family specifically in the epidermis. Mice lacking c-jun in keratinocytes (c-jun ep ) develop normal skin but express reduced levels of EGER in the eyelids, leading to open eyes at birth, as observed in EGFR null mice. Primary keratinocytes from c-jun ep mice proliferate poorly, show increased differentiation, and form prominent cortical actin bundles. most likely because of decreased expression of EGFR and its ligand HB-EGF, In the absence of c-Jun tumor-prone K5-SOS-F transgenic mice develop smaller papillomas. with reduced expression of EGFR in basal keratinocytes. Thus, using three experimental systems we show that EGFR and HB-EGF are regulated by c-Jun, which controls eyelid development, keratinocyte proliferation, and skin tumor formation. To analyze the epidermal function of c-jun and junB in adult mice we applied an inducible system to delete both genes ad libitum. Using the K5-Cre-ER transgenic mice we deleted both genes by daily tamoxifen injection for 5 days in 8 weeks old mice. Interestingly, these mice developed skin disorders resembling human psoriasis within 2 weeks. Primarily hairless skin like ear, tail and feet are affected. ilistological examination revealed abnormally thickened epidermis, parakeratosis (nucleated keratinocytes in the cornified layer), thickened keratinized upper layer (hyperkeratosis), and fingerlike epidermal projections into the dermis (rete ridges). Furthermore, epidermal microabscesses and the typical inflammatory cell infiltrates are seen: intraepidermal Tcells, increased numbers of neutrophils in the epidermis, and macrophages in the dermis. H&E staining from affected mouse finger demonstrated granulocytic infiltrates into the joint region, an early stage of psoriatic arthritis which is seen in 40% of human psoriasis patients. We performed RNAse protection assays (RPA) for cytokines and chemokines which are thought to be involved in human psoriasis. Basically all of them were heavily deregulated in the skin of the mutant mice. Currently we are investigating different time points to identify the molecular chronology of this important disease and analyze human psoriasis samples to confirm the relevance of our findings for this human disease.