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J. Biol. Chem., Vol. 282, Issue 2, 1161-1169, January 12, 2007

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Srcasm Corrects Fyn-induced Epidermal Hyperplasia by Kinase Down-regulation^*

From the Department of Dermatology, University of Pennsylvania Medical School, Philadelphia, Pennsylvania 19104

Src family tyrosine kinases (SFKs) are important regulators of epithelial cell growth and differentiation. Characterization of cellular mechanisms that regulate SFK activity will provide insights into the pathogenesis of diseases associated with increased SFK activity. Keratin 14-Fyn (K14) transgenic mice were derived to characterize the effect of Fyn on epidermal growth and differentiation in vivo. The epidermis of K14-Fyn mice is thickened, manifests prominent scale, and exhibits features consistent with hyperproliferation. Increased epidermal Fyn levels correlate with activation of p44/42 MAP kinases, STAT-3, and PDK-1, key signaling molecules that promote epithelial cell growth. The Src-activating and signaling molecule (Srcasm) is a substrate of SFKs that becomes tyrosine-phosphorylated downstream of the EGF receptor. In vitro, increased Srcasm levels promote activation of endogenous Fyn and keratinocyte differentiation. To study the in vivo effect of Srcasm upon Fyn, double transgenic lines were derived. K14-Fyn/Srcasm transgenic mice did not manifest the hyperproliferative phenotype. In contrast, K14-Fyn/Srcasm-P transgenic mice, which express a nonphosphorylatable Srcasm mutant, maintained the hyperproliferative phenotype. Resolution of the hyperproliferative phenotype correlated with reduced Fyn levels in vivo in three experimental systems: transgenic mice, primary keratinocytes, and cell lines. Biochemical studies revealed that Srcasm-dependent Fyn down-regulation requires Fyn kinase activity, phosphorylation of Srcasm, and the Srcasm GAT domain. Therefore, Srcasm is a novel regulator of Fyn promoting kinase down-regulation in a phosphorylation-dependent manner. Srcasm may act as a molecular "rheostat" for activated SFKs, and cellular levels of Srcasm may be important for regulating epithelial hyperproliferation associated with increased SFK activity.

Received for publication, July 11, 2006 , and in revised form, October 6, 2006.

^* This work was supported by NIAMS/National Institutes of Health Grants KO8-AR47597 and RO1-AR051380 (to J. T. S.) and The Department of Dermatology, University of Pennsylvania Medical School. The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

¹ To whom correspondence should be addressed: Depts. of Dermatology and Pathology, Cell and Molecular Biology Graduate Group, University of Pennsylvania Medical School, 235a Clinical Research Bldg., 415 Curie Blvd., Philadelphia, PA 19104. Tel.: 215-898-0170; Fax: 215-573-2143; E-mail: john.seykora{at}uphs.upenn.edu.

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