Molecular mechanisms that mediate UVB-inducedc-Fos expression in a human keratinocyte cell line

Abstract

The UVB portion (280--320 nm) of the ultraviolet spectrum contributes to the development of non-melanoma skin cancer (NMSC) in humans. UVB irradiation causes epigenetic alterations in target keratinocytes, including the upregulation of Activator Protein-1, a transcription factor complex that alters normal cellular gene expression. c-Fos expression is induced in a manner that correlates with the UVB-induced activation of AP-1. This suggests that c-Fos functions as a major regulatory component in the UVB-induced transactivation of AP-1. The purpose of this dissertation is to characterize UVB-induced regulation of c-Fos expression. Transcriptional regulation of c-fos is investigated by evaluating the role of each of four cis elements within the c-fos promoter. While mutation of each of these four cis elements results in significantly lower levels of UVB-induced promoter activity, the CRE and FAP1 elements mediate most of the UVB transactivation of c-fos. In addition, UVB irradiation induces homodimers of phosphorylated CREB to bind to the CRE and FAP1 elements. To identify cellular signal transduction pathways that are induced by UVB-irradiation to regulate c-Fos expression, a UVB-inducible enzyme, phosphatidylinositol 3-kinase (PI 3-kinase), is studied. Inhibition of PI 3-kinase reduces c-Fos expression in UVB-irradiated cells. Akt and GSK-3beta, constituents of the PI 3-kinase signaling pathway, are also found to be part of this UVB-induced signaling pathway. To identify potential molecular targets for the development of skin cancer chemoprevention strategies, the polyphenolic compound nordihydroguaiaretic acid is tested and found to prevent UVB-induced c-Fos expression and AP-1 transactivation by inhibiting the PI 3-kinase signal transduction pathway. Thus, phospho-CREB binding to the CRE and FAP1 cis elements and PI 3-kinase signaling are both identified as molecular mechanisms and potential molecular targets that are involved in UVB-induced c-Fos expression and AP-1 transactivation.