1,25-Dihydroxyvitamin D induces human myeloid cell differentiation via the mTOR signaling pathway.

1,25-Dihydroxyvitamin D or 1,25(OH)D is known to play an important role in the differentiation of human myeloid cells. However, the molecular mechanism underlying the 1,25(OH)D-mediated differentiation of human myeloid cells is incompletely understood. Here, we report that 1,25(OH)D 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)D in phorbol 12-myristate 13-acetate (PMA)-differentiated U937 and THP-1 cells. The 1,25(OH)D-induced expression of CD14 in PMA-differentiated U937 and THP-1 cells was prevented by mTOR inhibitors, PP242 and Torin1. The 1,25(OH)D-induced morphological changes as characteristics of differentiated myeloid cells were also reversed after PP242 and Torin1 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)D, 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)D-induced upregulation of CD14. Taken together, our results show that 1,25(OH)D enhances the differentiation of human myeloid cells through the mTOR signaling pathway.