Ultraviolet B radiation (UVB) can cause DNA damage, induce gene mutations and modulate intracellular signal transduction contributing to the development of skin cancer. Signal transducer and activator of transcription 3 (Stat3) modulates various physiological functions including apoptosis, cell cycle regulation, and tumor angiogenesis through regulation of gene expression, and its constitutive activation is associated with human epithelial cancers. Recent studies indicated that Stat3 was rapidly dephosphorylated in keratinocytes after UVB irradiation. Vanadate treatment desensitized keratinocytes to UVB-induced apoptosis with the recovery of phosphorylated Stat3 protein levels, implying that a tyrosine phosphatase is involved in this mechanism. In the current work, the mechanism whereby UVB exposure leads to rapid dephosphorylation of Stat3 was examined. Western blot analysis showed that the nuclear form of TC-PTP, which is known to dephosphorylate various signaling molecules, was constitutively expressed in cultured mouse keratinocytes. After UVB exposure, activated stat3 was rapidly reduced, whereas the level of TC-PTP was not changed. Further immunofluorescence and western blot analyses revealed that TC-PTP translocates from cytosol to nucleus in response to UVB irradiation. Dephosphorylation of Stat3 by TC-PTP was rapidly increased after UVB irradiation of cultured keratinocytes with nuclear accumulation of TC-PTP. Stat3 target genes, such as cyclin D1 and c-Myc, were also down-regulated early following UVB exposure. Consistent with these observations, RNAi studies showed that the level of phosphorylated Stat3 in keratinocytes transfected with TC-PTP siRNA was higher compared with control keratinoctyes upon UVB irradiation. In addition, Inhibition of importin β1 expression by siRNA knockdown of importin β1, which is known to transport TC-PTP in to the nucleus, did not prevent its translocation and dephosphorylatin of Stat3 in nucleus, indicating that UVB-induced transport of TC-PTP is independent of importin β1. In mouse epidermis in vivo, the level of phosphorylated Stat3 was initially decreased, following by a significant increase at later time points after UVB exposure. The expression patterns of cyclin D1 and c-Myc followed the changes in activated Stat3. Consistently, dephosphorylation of Stat3 by TC-PTP was initially increased, and then decreased in vivo after UVB exposure. These results suggest that TC-PTP is responsible for the rapid dephosphorylation of Stat3 following UVB and may serve as part of an initial protective mechanisms against UV skin carcinogenesis.
98th AACR Annual Meeting-- Apr 14-18, 2007; Los Angeles, CA