Extracellular calcium is involved in the mechanism of differentiation of mouse myeloid leukemia cells (M1) induced by 1 alpha, 25-dihydroxyvitamin D3.

We have reported that 1 alpha, 25-dihydroxyvitamin D3 [1 alpha, 25(OH)2D3] suppresses proliferation and induces differentiation of murine myeloid leukemia cells (M1) into macrophages. In the current study, M1 cells were cultured either with 2.0 or 0.15 mM total calcium to examine the effect of calcium on the process of differentiation induced by the vitamin. The 0.15 mM calcium medium greatly enhanced 1 alpha, 25-dihydroxyvitamin D3 [1 alpha, 25(OH)2D3]-induced inhibition of cell growth and suppression of [3H]thymidine incorporation. Addition of Verapamil, a calcium antagonist, to the 2.0 mM calcium medium also elicited similar responses. The absolute number of cells with phagocytic activity induced by 1 alpha, 25(OH)2D3 was almost identical in media containing either concentration of calcium, and in cultures with or without Verapamil. Culture in the 0.15 mM calcium medium or addition of Verapamil to the 2.0 mM calcium medium did not suppress cell growth nor induce phagocytic activity in the absence of the vitamin. To confirm the preferential effect of calcium on cell growth, M1 cells were pretreated for 3 days with 1 alpha, 25(OH)2D3 in either the 2.0 or 0.15 mM calcium medium. Then the pretreated cells were washed and subcultured in the absence of 1 alpha, 25(OH)2D3 in either medium. The growth rate was inhibited much more effectively in the subculture with 0.15 mM calcium than with 2.0 mM calcium. These results suggest that the M1 cells' increased requirement of extracellular calcium, caused by the treatment with 1 alpha, 25(OH)2D3, is closely related to cell growth rather than differentiation.