The Mechanism of CIRP in Regulation of STAT3 Phosphorylation and Bag-1/S Expression Upon UVB Radiation.

Cold-inducible RNA binding protein (CIRP) is a stress-inducible protein, which could be activated by various cellular stresses, such as hypothermia, hypoxia and UV irradiation. Our previous study indicated that UVB radiation (3 mJ cm-2 ) induces CIRP expression, which promotes keratinocytes growth, survival and eventually transformation via activation of STAT3-Bag-1/S signaling cascade. However, the mechanism(s) of CIRP in regulating p-STAT3 activation and Bag-1/S expression have not been fully elucidated. In this study, we demonstrate that repeated exposure of UVB radiation (3 mJ cm-2 ) or overexpression of CIRP could lead to an elevation of the phosphorylation of Janus kinase (JAK) family proteins (JAK2 and JAK3) in HaCaT cells. The increased phosphorylation of the JAKs correlates to an increased phosphorylation of STAT3 (p-STAT3) in the cells; inhibiting JAKs using JAK inhibitor I lead to a reduction of STAT3 phosphorylation and Bag-1/S expression in wild type HaCaT and CIRP stably transfected HaCaT cells with or without UVB exposure. Furthermore, our data indicated that inhibiting the downstream factor of CIRP, NF-κB, using BAY 11-7085 could also decrease the p-STAT3. These results lead us to propose that CIRP mediates the activation of STAT3-Bag-1/S signaling cascade via activating the JAKs and NF-κB signaling pathways.