Molecular mechanisms of ultraviolet-B induced activator protein 1 activation in human keratinocytes

Abstract

Ultraviolet-B (UVB) irradiation is a major cause of human non-melanoma skin cancer. Transcription factor activator protein-1 (AP-1) plays an important role in tumor promotion. The mechanism of UVB induced tumor promotion may be through AP-1 activation. In this dissertation, the molecular mechanisms of UVB induced AP-1 activation were pursued in a human keratinocyte cell line, HaCaT. UVB induced AP-1 activation was observed by gel shift assays and transactivation assays. c-Fos and Jun D were the main components of the bound AP-1 complexes. Inhibition of RNA and protein synthesis significantly abrogated UVB induced AP-1 DNA binding. Further investigation revealed a correlation between increased AP-1 activity and c-fos gene expression after UVB. These results suggested that increased c-fos expression may play an important role in UVB induced AP-1 activation. To further examine the mechanism of UVB induced c-fos expression, the UVB signaling pathways were studied. Two members of mitogen-activated protein kinase (MAPK) family, p38 and extracellular signal-regulated protein kinase (ERK), were activated after UVB. Blocking p38 significantly abrogated UVB induced c-fos expression. Inhibiting ERK partially abrogated UVB induced c-fos expression. These data suggested that both p38 and ERK were required in UVB induced c-fos expression in human keratinocytes. Furthermore, p38 appeared to play a more important role than ERK in this process. Since c-fos expression may play an important role in UVB induced AP-1 activation, and AP-1 activation is known to play a role in tumor promotion, both p38 and ERK could be potential targets for chemoprevention of skin cancer. (-)-Epigallocatechin gallate (EGCG), the major polyphenol isolated from green tea, has been shown to inhibit UVB induced tumorigenesis. However, the mechanisms that underline the inhibitory effects of EGCG are not well understood. In this study, EGCG was found to inhibit UVB induced AP-1 transactivation and c-fos expression. Furthermore, EGCG inhibited UVB induced p38 activation. Thus, suppression of p38 signaling cascade was the mechanism of EGCG's inhibitory effect on UVB induced c-fos expression, which may further explain its inhibitory effect on UVB induced AP-1 activation.