Mechanistic/mammalian target of rapamycin complex 1 (mTORC1) signaling is involved in phagocytosis activation during THP-1 cell differentiation

Mechanistic/mammalian target of rapamycin complex 1 (mTORC1) is a serine/threonine kinase that plays a major role in cell metabolism. Although mTORC1 inhibitors are known to exert immunosuppressive effects, their effects on immune cells are not fully understood. In the present study, we examined the involvement of mTORC1 in the differentiation and functions of macrophages using THP-1 cells, which are derived from human monocytic leukemia and differentiate into macrophage-like cells upon treatment with 12-O-tetradecanoylphorbol-13-acetate (TPA). We also examined the effects of two mTOR inhibitors, Torin 1 and rapamycin, on TPA-stimulated THP-1 cells. Although mTORC1 activation was observed upon TPA stimulation, mTOR inhibitors did not affect TPA-induced morphological changes or expression of the general macrophage marker, CD11b. In contrast, phagocytosis and fluid endocytosis were significantly impaired by the mTOR inhibitors. Endocytosis suppression was observed when mTOR inhibitors were added during differentiation, but not before or after differentiation, suggesting that endocytosis suppression altered the direction of differentiation. Furthermore, mTOR inhibitors altered the expression of M1/M2 polarization markers. These results suggest that the immunosuppressive effects of mTOR inhibitors may involve the suppression of macrophage endocytosis caused by abnormal cell differentiation.