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Feng Xu, Yali Ling, Jingjing Yuan, Qin Zeng, Lusha Li, Dexing Dai, Xuedi Xia, Ruoman Sun, Ran Zhang, and Zhongjian Xie

Differentiated thyroid carcinoma (DTC) is the most common endocrine malignancy and highly expresses the receptor for 1,25-dihydroxyvitamin D (1,25(OH)2D). However, it is unclear whether 1,25(OH)2D regulates DTC proliferation and differentiation. Here, we found that 1,25(OH)2D3 inhibited proliferation but not differentiation of the DTC cells. Notably, CYP27B1was elevated in DTC cells and 25-hydroxyvitamin D3 (25(OH)D3) reduced DTC cell proliferation. Knockdown of VDR did not affect the anti-proliferative effects of 1,25(OH)2D3. However, knockdown of CCAAT enhancer-binding protein β (C/EBPβ)abolished 1,25(OH)2D3-suppressed DTC cell proliferation. In addition, 1,25(OH)2D3 induced phosphorylation and translocation of C/EBPβto the nucleus from the cytoplasm. However, inhibition of p38 mitogen-activated protein kinases (MAPK) abrogated 1,25(OH)2D3-induced phosphorylation and nuclear translocation of C/EBPβas well as 1,25(OH)2D3-suppressed DTC cell proliferation. Knockdown of C/EBPβreduced the expression of Notch3. Knockdown of Notch3 blocked 1,25(OH)2D3-suppressed DTC cell proliferation. In the DTC cell-derived xenograft SCID mouse, knockdown of C/EBPβmarkedly increased tumor growth and proliferation and decreased apoptosis. In DTC patients, C/EBPβwas predominantly located in the cytoplasm of DTC cells in the tumor tissue when compared with adjacent non-cancerous tissue in which C/EBPβis located in the nucleus. In conclusion, C/EBPβstimulated Notch3signaling via the p38 MAPK-dependent pathway mediates the inhibitory effect of 1,25(OH)2D on DTC cell proliferation.