In this study, we investigated the specific function of PTEN in UVB damage responses and skin tumorigenesis. Using low suberythemal UVB radiation, we demonstrated that mice with a targeted PTEN down-regulation in their epidermis are predisposed to skin tumorigenesis. These findings support the conclusion that a 50% reduction of the PTEN level increases susceptibility to skin tumorigenesis following UVB radiation. In human skin malignancies PTEN is significantly down-regulated in association with sun damage. Our findings further support the conclusion that the PTEN dose is critical for tumor suppression under genotoxic stress.
Our findings clearly indicate that PTEN positively regulates GG-NER by promoting XPC transcription in keratinocytes. Down-regulation of XPC transcription caused by PTEN down-regulation provides a previously unrecognized mechanism in inhibiting XPC function in addition to deletion and mutations in the XPC
gene in human SCCs (12
). Considering that failure to repair CPD is the principal cause of skin cancer, the role of PTEN in GG-NER through XPC is critical for the tumor-suppressing action of PTEN for the skin.
Our current studies identified the AKT/p38 pathways as important actors in the regulation of XPC by PTEN. PTEN is known as a negative regulator of the PI3K/AKT pathway (13
). PTEN down-regulation increases AKT activation and enhances cell survival after UVB damage (17
). Indeed, the regulation of the p38 pathway by AKT and its impact on cell survival have been reported in several other models, including endothelial cells (35
), β-cell (36
), and E1A-induced apoptosis (37
). Although the role of the p38 pathway in UVB responses is complex (1
), in human fibroblasts and HeLa cells p38 has recently been shown to promote GG-NER by stabilizing DDB2 (32
). In line with these findings, our studies have demonstrated that the reduction in the p38 pathway through increased AKT activation upon PTEN inhibition can be mimicked by the effect of biochemical inhibition of p38 in regulating XPC levels and GG-NER, suggesting that in PTEN-down-regulated keratinocytes, suppression of p38 by AKT signaling reduces XPC levels and thus impairs GG-NER. In SIRT1-inhibited cells, however, PTEN inhibition and AKT activation due to increased PTEN acetylation suppress XPC expression through increased nuclear translocation of a transcription repressor p130 (38
), but not the p38 pathway (data not shown). Further investigation is needed to reveal these different molecular mechanisms depending on SIRT1 status. Taken together, these results imply that the PTEN/AKT/p38 axis is critical for GG-NER and regulates tumor susceptibility.
Interestingly we found that, in contrast to the response to ionizing or UVC radiation (23
), PTEN loss increases UVB-induced Chk1 activation in keratinocytes in an AKT-independent manner. It seems that AKT plays differential roles in DNA repair and checkpoint response. Therefore the effect of PTEN on checkpoint pathways may depend on the type of DNA damage, the repair efficiency, the waveband-specific signaling pathways between UVB and UVC (31
), and/or the cell-type specific response to PTEN inhibition, similar to the different findings on the regulation of Rad51 expression (40
). Much more investigation is needed to elucidate these important differences at the molecular level. Nevertheless, our findings indicate that normal PTEN expression and AKT activation are required for proper XPC expression and thus better DNA repair. These data also suggest that, in keratinocyte response to UVB-induced DNA damage, the checkpoint response is passive, associated with the levels of unrepaired DNA damage, and depends on the PTEN levels but not AKT activation, further underscoring the importance of constitutive PTEN levels in reducing susceptibility to UVB tumorigenesis.
In summary, we demonstrated that PTEN down-regulation is a predisposing factor for UVB-induced skin carcinogenesis in vivo, and negatively regulates UVB-induced DNA damage repair through limiting XPC expression. Taken together, our findings suggest that the interaction between PTEN levels and UVB is critical for early tumorigenesis and tumor progression.