Recent over-expression and pharmacologic studies suggest that Ras, operating via the conserved Ras/Raf/MEK/ERK signaling cascade, controls epidermal growth and differentiation and promotes human epidermal neoplasia. Obtaining conclusive genetic evidence for this role, however, has been hindered by redundancy at the levels of Ras and Raf. The current proposal aims to alter function of this pathway in epidermis where it narrows to only two homologous kinases, MEK1 and MEK2. First, we will generate epidermis with impaired MEK function to examine the role of MEK1/2 in homeostasis. MEK2 -/- mice are viable and display normal epidermal homeostasis, however, MEKI/mice die at E10.5. Therefore, we will conditionally ablate epidermal MEK1 expression in MEK2 -/- as well as MEK2 +/+ mice. Effects on epidermal growth and differentiation will be analyzed in double and single knockout mouse skin tissue. In parallel, we will regenerate human epidermis genetically engineered for MEK1/2 hypo-function to provide cross-species comparison and validation. Aim I is based on the hypothesis that ablating Ras/Raf/MEK/ERK function will characterize this pathway's role in epidermal growth and differentiation. Second, we will define the role of MEK1/2 in Ras-driven epidermal oncogenic changes. In epidermis, we have recently shown that oncogenic Ras promotes proliferation, augments integrin expression and inhibits differentiation. We believe that MEK1/2 may help mediate these oncogenic Ras impacts in epidermis, however, Ras also activates other effectors such as RalGEFs and PI3Ks. We will thus characterize the effects of oncogenic Ras in murine and human epidermis in the context of MEK1/2 deficiency. To do this, we will inducibly activate oncogenic Ras in both single and double MEK1/2 knock-out murine epidermis as well as in human epidermis expressing dominant-negative MEK1/2 mutants. In addition, we will test the sufficiency of MEK1/2 induction to mediate oncogenic Ras effects in murine and human epidermis by generating epidermis expressing regulated MEK1 and MEK2 proteins. Aim II is based on the hypothesis that MEK1/2 mediates a substantial portion of oncogenic Ras effects in epidermis. At the end of proposed funding, we plan to have defined MEK1/2 action in epidermal homeostasis and Ras-driven oncogenesis as a basis for new therapies for diseases of epidermal growth and differentiation.