1,25-Dihydroxyvitamin D-3 or 1,25(OH)(2)D-3 is known to play an important role in the differentiation of human myeloid cells. However, the molecular mechanism underlying the 1,25(OH)(2)D-3-mediated differentiation of human myeloid cells is incompletely understood. Here, we report that 1,25(OH)(2)D-3 induces differentiation of human myeloid cell lines such as U937 and THP-1 cells via the mammalian target of rapamycin (mTOR) signaling pathway. Both the expression of the differentiation marker CD14 and activation of the mTOR signaling pathway were induced by 1,25(OH)(2)D-3 in phorbol 12-myristate 13-acetate (PMA)-differentiated U937 and THP-1 cells. The 1,25(OH)(2)D-3 -induced expression of CD14 in PMA-differentiated U937 and THP-1 cells was prevented by mTOR inhibitors, PP242 and Torinl. The 1,25(OH)(2)D-3-induced morphological changes as characteristics of differentiated myeloid cells were also reversed after PP242 and Torinl treatment. Silencing of either regulatory-associated protein of mTOR (Raptor) or rapamycin-insensitive companion of mTOR (Rictor) in PMA-differentiated THP-1 cells with small-interfering RNA resulted in the inhibition of CD14 expression and morphological changes induced by 1,25(OH)(2)D-3, indicating that both mTORC1 and mTORC2 were important for the differentiation of myeloid THP-1 cells. Previous studies have shown that phosphatidic acid (PA) maintains the stability of the mTOR complex. Here we found that the attenuation of PA production with 1-butanol or a PLD inhibitor prevented the 1,25(OH)(2)D-3 -induced upregulation of CD14. Taken together, our results show that 1,25(OH)(2)D-3 enhances the differentiation of human myeloid cells through the mTOR signaling pathway. (C) 2019 Elsevier Inc. All rights reserved.