Skin keratinocytes are subject to frequent chemical and physical injury and compensate through several cell survival mechanisms. Among these, the Akt/protein kinase B (PKB) pathway protects keratinocytes from the toxic effects of ultraviolet light (UV). In contrast, the protein kinase C (PKC) family is involved in several keratinocyte death pathways. The insulin-like growth factor (IGF-1) activates both PKC and Akt signaling pathways in cultured primary mouse keratinocytes. Inhibition of PKC increases Akt kinase activity and enhances IGF-1 protection from UVC-induced apoptosis while activation of PKC inhibits Akt activity and blocks IGF-1 protection from UVC-induced apoptosis. The inhibition of Akt by PKC appears to be through the PKC dependent activation of protein phosphatase 2 that binds to activated Akt and dephosphorylates the molecule at essential residues. Selective PKC inhibitors and transduction studies with specific isoforms suggest that PKCdelta and epsilon provide negative regulation for Akt kinase activity and modulate cell survival pathways in response to external stimuli.Skin keratinocytes are major mediators of host immune responses. The skin is also a target for inflammation in many pathological states. Activation of PKC can induce cutaneous inflammation, but the contribution of each of 6 PKC isoforms (alpha, delta, epsilon, eta, zeta and mu) that regulate normal skin homeostasis or contribute to skin pathology has not been clarified. We generated transgenic mice that overexpress PKCalpha in the basal layer of the epidermis and the outer root sheath of hair follicles under the regulation of the bovine keratin 5 promoter. K5-PKCalpha transgenic mice exhibit severe intraepidermal neutrophilic inflammation, systemic neutrophilia and apoptosis of the epidermis and upper hair follicles when treated topically with 12-0-tetradecanoylphorbol-13-acetate (TPA). Inhibiting AP-1 prevents apoptosis but not intraepidermal inflammation. NFkappaB inhibition reduces inflammation but not apoptosis. Apoptosis and inflammation are independent of TNFalpha. Gene array studies together with inhibitors of AP-1 or NFkappaB implicate specific cytokines, chemokines and signaling pathways that contribute to the apoptotic and inflammatory responses. In particular, ligands for CXCR2 are essential for the migration of neutrophils into the epidermis and G-CSF is essential for the systemic neutrophilia. Thus, PKCalpha inhibition may have therapeutic benefit in some human inflammatory skin disorders. K5-PKCalpha mice are very sensitive to skin tumor induction with TPA promotion. The sensitivity is independent of the inflammatory response but associated with reduced ability to suppress initiated keratinocytes and enhanced growth of neoplastic clones. Ingenol 3-angelate (Ing3A) is extracted from Euphorbia peplus, a plant used for topical treatment of skin conditions. Ing3A activates PKC in vitro, and causes inflammation on mouse ears and promotes tumor formation after DMBA initiation. In two keratinocyte derived tumor cell lines, 308 and PAM212, Ing3A is a potent inducer of PKC and MAPK phosphorylation. Ing3A exposure downregulates PKCalpha, delta and epsilon, but not PKCeta. Ing3A inhibits PAM212, but not 308, proliferation and causes irreversible rounding and detachment associated with alterations in actin filaments. PKC inhibitors and cytochalasian D prevent these changes. Thus, distinct differences exist between Ing3A and TPA for activation of PKC and cell killing in mouse keratinocytes. The calcium-binding proteins of the human S100A7/A15 subfamily are differentially expressed in normal and pathological epidermis. The human S100A7 (psoriasin) and S100A15 reside in a chromosomal cluster of highly similar paralogs. To exploit the power of mouse models for determining functions of gene products, the corresponding S100A7/A15 orthologs were cloned and examined in murine skin.
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