Squamous cell carcinoma (SCC) constitutes approximately 20% of all nonmelanoma skin cancer, which is the most common type of human malignancy. SCC has a relatively high propensity for invasion and metastasis although the biological basis for the aggressive behavior of cutaneous SCCs is poorly understood. One characteristic of epithelial tumors such as SCC with a high risk of tumor progression is inappropriate alpha6beta4 integrin expression. However, the impact of aberrant alpha6beta4 expression in otherwise normal epithelium on the initiation and course of the disease is relatively unknown. Suprabasal expression of alpha6beta4 predisposes the epidermis to form chemically-induced tumors and perturbs transforming growth factor beta (TGFbeta) signaling in basal cells via a mechanism that requires E-cadherin-mediated intercellular adhesion and PI 3-kinase activity. The goal of this proposal is to define the molecular events by which suprabasal alpha6beta4 expression can enhance epidermal tumor formation and disrupt TGFbeta signaling. Proteins of the Rho family of small GTPases mediate actin cytoskeleton reorganization and are implicated in epithelial tumor progression. Preliminary findings indicate that suprabasal alpha6beta4 is linked to the actin cytoskeleton and co-localizes with Rac1 and that Rac activation is altered in transgenic murine epidermis and human SCCs that exhibit suprabasal alpha6beta4 expression. To envision a way that alpha6beta4 integrin, E-cadherin and PI 3-kinase could all act in concert to manipulate TGFbeta signaling and epidermal tumor formation, the aims of this proposal will focus on the role of the Rho family of small GTPases. We will characterize changes in Rho GTPase expression and activity in transgenic murine epidermis and human SCCs exhibiting suprabasal alpha6beta4 expression. We will manipulate Rho GTPase and PI 3-kinase signaling in cells overexpressing alpha6beta4 and determine its impact on TGFbeta-mediated growth inhibition. To define the effect of Rho GTPase activity on epidermal tumor formation, we will generate transgenic murine epidermis that is deficient in Rho GTPase activity and exhibits suprabasal alpha6beta4 expression. In this model we will measure the effect of ablated Rho GTPase signaling on basal cell proliferation and SCC induction. Overall, we plan to ascertain how changes in the way epidermal skin cells communicate with one another can impact on the development of potentially fatal human skin cancers. These studies will illustrate how aberrations in a cellular system essential for normal function can strongly influence the susceptibility for neoplasia, and therefore will have broad implications for the entire field of epithelial cancer.
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