Inhibition by 1α,25-Dihydroxyvitamin D3of Activin A-Induced Differentiation of Murine Erythroleukemic F5-5 Cells

1α,25-Dihydroxyvitamin D3(1α,25-(OH)2D3) and other vitamin D3(VD3) analogs enhanced the inhibitory effect of Activin A on murine erythroleukemia (MEL) cell proliferation and differentiation in a dose-dependent manner. 1α,25-(OH)2D3inhibited differentiation more potently than proliferation by one order of magnitude. The VD3analog study demonstrated either effect of VD3on MEL cells via vitamin D receptor (VDR), as evidenced from the close relationship with the reported affinities for VDR. The effects of 1α,25-(OH)2D3were preceded by the suppression of ornithine decarboxylase (ODC) activity, a rate-limiting enzyme in polyamine metabolism. Difluoromethylornithine (DFMO), an inhibitor of ODC, inhibited MEL cell proliferation, which was reversed by the simultaneous addition of putrescine, a product of ODC, but did not affect differentiation. 1α,25-(OH)2D3inhibited cell differentiation during the phenotype-expression stage as reflected by the inhibition of β-globin gene expression, while it inhibited proliferation in the commitment stage. Furthermore, it seems unlikely that the different effects of VD3on proliferation and differentiation may be a result of up-regulation of VDR or nongenomic action. In summary, it was suggested that 1α,25-(OH)2D3inhibited Activin A-induced MEL cell proliferation and differentiation by distinct mechanisms and inhibited the proliferation by inhibiting ODC activity. We demonstrated the presence of 1α,25-(OH)2D3action on leukemic cells at physiological concentration, which was distinct from the pharmacological effect of VD3reported thus far.