Role for transcription factor AP-2alpha in protecting keratinocytes from UVA irradiation.

摘要

AP-2alpha is a key transcription factor regulating the growth and differentiation of keratinocytes. The expression of AP-2alpha has been shown to be responsive to UVA irradiation through the second messenger molecule ceramide. In the present study, we found that H2O2 induced by UVA irradiation was, at least in part, involved in the upregulation of AP-2alpha expression. Decreasing the intracellular H2O2 level by forced expression of GPx-1 abolished the UVA induced increase of ceramide as well as the AP-2alpha upregulation. Increased AP-2alpha had the capacity to spare HaCaT keratinocytes from 5 J/cm2 UVA irradiation. The increased growth was due to an increase of clonogenic survival instead of change of cell cycle distribution.; The molecular mechanism by which AP-2alpha protects keratinocytes from UVA radiation is likely through a protein-protein interaction with c-MYC. c-MYC is one of the early genes induced by UVA radiation. Forced expression of c-MYC in HaCaT cells caused massive apoptosis. Overexpression of AP-2alpha decreased the apoptosis induced by c-MYC. A transactivation domain truncated AP-2 also showed a similar effect, suggesting that the transactivating activity of AP-2alpha was not required to counteract c-MYC. Furthermore, we found that increasing the level of AP-2alpha inhibited the endogenous c-MYC expression through direct binding of AP-2 at the c-myc promoter region. The protein-protein interaction between AP-2alpha and c-MYC showed a reciprocal effect on AP-2alpha proteins. c-MYC binds to the DNA binding domain of AP-2alpha, thus blocking the DNA binding activity of AP-2alpha. In vivo chromatic immunoprecipitation assays revealed that the decrease of AP-2alpha DNA binding by c-MYC could relieve the inhibition of c-myc transcription. There are three identified pathways through which c-MYC can cause apoptosis: inducing p53, Fas/FasL or disrupting the mitochondrial outer membrane. The mechanism by which c-MYC causes apoptosis in HaCaT cells appears to be through the latter pathway. Overexpression of c-MYC induced a 30 fold increase of reactive oxygen species. Co-overexpression of AP-2alpha partially blocked that increase of ROS as well as c-MYC induced apoptosis. Taken together, these data describe a relationship between AP-2alpha and c-MYC and provide an example of how a proto-oncogene manages to escape the regulation from its inhibitor.