Mammalian skin is a vital organ that serves to maintain fluid and electrolyte balance, exclude toxins and pathogens, and participate in immune surveillance. These functions are largely attributable to the epithelial layer of the skin (the epidermis). Derangements in epidermal function in diseases such as psoriasis, or in environmental injuries such as exposure to ultraviolet light, result from abnormalities in the precise balance of keratinocyte proliferation and differentiation that constitute the normal epidermal differentiation program. This balance is regulated within cells at the level of gene transcription by a variety of transcription factors whose identities and mechanisms of action are only now being appreciated. This proposal will test the hypothesis that a relatively new family of nuclear transcription factors, the CCAAT-enhancer binding proteins (C/EBPs), plays an important regulatory role in epidermal differentiation. Based upon preliminary data that demonstrate the presence of C/EBPS in keratinocytes of the skin, both in vivo and in vitro, the proposed experiments will establish, in a causal manner, how two members of the C/EBP family (C/EBPbeta and CHOP) may be critical for the proper and orderly progression of the epidermal differentiation program. Stable keratinocyte cell lines, expressing doxycyline-inducible transgenes that encode both activators and inhibitors of C/EBPbeta and CHOP (including antisense RNA, and inhibitory protein isoforms of C/EBPs) will be studied in a calcium-dependent differentiation model in vitro. Alterations in parameters of growth and differentiation will be assayed after induction of the transgenes with doxycycline. As an in vivo correlate to these studies, stable keratinocyte lines will be grafted onto the backs of nude mice, to test for doxycycline-inducible changes in the epidermis formed at the graft site. Results of these studies may enhance our understanding of mechanisms that govern the switch from proliferation to differentiation, and in later events such as apoptosis, in the epidermis. These studies will build a foundation for later studies on the significance of observed changes in C/EBPbeta and CHOP expression in psoriasis and in skin exposed to UV light. Thus, changes in C/EBPbeta and CHOP-regulated gene expression are probably involved not only in the normal balance proliferation and differentiation in the epidermis, but also in epidermal disease and in responses of the skin to environmental injury.