Induction of the fms proto-oncogene product in HL-60 cells by vitamin D: a flow cytometric analysis

Vitamin D(3) and analogues modulate the expression of CSF-1 and its receptor in human dendritic cells

The active vitamin D(3)-metabolite 1,25(OH)(2)D(3) inhibits the interleukin 4/granulocyte-macrophage colony-stimulating factor (IL-4/GM-CSF)-induced differentiation of human monocytes into dendritic cells without altering survival. Colony-stimulating factor 1 (CSF-1) is an important survival factor for cells of the monocytic lineage. We therefore investigated whether the inhibitory activity of 1,25(OH)(2)D(3) is paralleled by a regulation of CSF-1 and its receptor. Purified human monocytes were cultured together with IL-4/GM-CSF in the presence of 1,25(OH)(2)D(3), its analogue tacalcitol, the low-affinity vitamin D receptor ligand 24,25(OH)(2)D(3), or the solvent ethanol for up to 5 days. Expression of CSF-1, CSF-1R, and GM-CSF mRNA was measured by RT-PCR. Protein secretion for CSF-1 was measured by ELISA, expression of CSF-1R by flow cytometry. The results showed that 1,25(OH)(2)D(3) and tacalcitol significantly up-regulated CSF-1 mRNA-expression and protein secretion in a dose-dependent manner. The effect of 1,25(OH)(2)D(3) occurred already after 1h of pre-treatment. In contrast, CSF-1R mRNA- and cell surface-expression was down-regulated simultaneously. The solvent ethanol and 24,25(OH)(2)D(3) were without effect. GM-CSF mRNA expression was not modulated in 1,25(OH)(2)D(3)-treated cells. These data point towards a distinct and specific regulation of CSF-1 and its receptor by 1,25(OH)(2)D(3) and its analogue tacalcitol in human monocytes which parallels the inhibition of differentiation into dendritic cells without altering survival.

All-trans and 9-cis retinoic acid enhance 1,25-dihydroxyvitamin D3-induced monocytic differentiation of U937 cells

Retinoic acid (RA) and 1,25-dihydroxyvitamin D3 (D3) are well known for inducing differentiation in many leukemic cell lines. The nuclear signalling pathways of RA and D3 are mediated through their cognate receptors, the retinoic acid receptor (RAR) and vitamin D3 receptor (VDR), respectively. Retinoid X receptor (RXR) is an auxiliary factor that forms a heterodimer with RAR and VDR, enabling their efficient transcriptional activation. 9-cis RA, a high-affinity ligand for RXR, greatly enhanced D3-induced CD14 expression in U937 cells, while RA alone did not induce CD14 expression. 9-cis RA also resulted in morphological changes of U937 cells to macrophage-like cells when combined with D3, while RA alone resulted in granulocyte-like cells. RA and D3 together enhanced c-fms expression, phagocytic activity, and acted synergistically to promote nitroblue tetrazolium reduction activity and inhibit proliferation. Northern analysis showed that U937 cells constitutively expressed RAR-alpha, VDR and RXR-alpha mRNAs. RA or D3 alone or in combination did not affect RAR-alpha and VDR expression, while 9-cis RA and 9-cis RA plus all-trans RA significantly reduced RXR-alpha expression. Interestingly, D3 could restore the down-regulation of RXR-alpha mRNA by 9-cis RA. These findings suggest that there is crossover of the nuclear signalling pathways of RA and D3. This may have clinical implications in that RA and D3 may be used in combination for differentiation-inducing therapy in acute myelogenous leukemia and myelodysplastic syndrome.

C-FMS dependent HL-60 cell differentiation and regulation of RB gene expression

The dependence of induced myelomonocytic cell differentiation, and regulation of the RB tumor suppressor gene during this process, on the c-fms gene product, the CSF-1 lymphokine receptor, was determined in HL-60 promyelocytic leukemia cells. Adding a monoclonal antibody with specificity for the c-fms gene product to cells treated with various inducers of myelomonocytic or macrophage differentiation, including retinoic acid and 1,25-dihydroxy vitamin D3, inhibited the rate of differentiation. During the period of inducer treatment usually preceding onset of differentiation, longer periods of antibody exposure caused greater inhibition of differentiation. In a stable HL-60 transfectant overexpressing the CSF-1 receptor at the cell surface due to a constitutively driven c-fms trans gene, the rate of differentiation was enhanced compared to the wild type cell, consistent with a positive regulatory role for the CSF-1 receptor. The anti-fms antibody caused much less inhibition of differentiation in the transfectants than in wild type cells, consistent with a larger number of receptors causing reduced sensitivity. During the induced metabolic cascade leading to differentiation, the typical early down regulation of RB gene expression was inhibited by the antibody. The antibody itself caused an increase in RB expression per cell, which offset the decrease normally caused by differentiation inducers (1,25-dihydroxy vitamin D3 and retinoic acid). The changes in RB expression preceded changes in the RB protein to the hypophosphorylated state. Most of the RB protein in proliferating cells was phosphorylated and no significant accumulation of hypophosphorylated RB protein occurred until after onset of G0 arrest. Thus the metabolic cascade leading to myelomonocytic differentiation of HL-60 cells appears to be driven by a function of the c-fms protein. Inhibiting that process by attacking this receptor impedes differentiation and also compromises the early down regulation of RB tumor suppressor gene expression which normally precedes differentiation. These findings provide additional support for a potential role for down regulating RB expression in promoting cell differentiation, and suggest the possibility that RB may be either a target or intermediate mediator of CSF-1 actions.

Modulation of cell proliferation and cell cycle, and inhibition of cytokinesis by 1,25-dihydroxyvitamin D3 in C3H/10T1/2 fibroblasts

The growth-modulating effects of 1,25-dihydroxyvitamin D3 [1,25-(OH)2D3] were studied on three mouse embryo fibroblast cell lines. Concentrations ranging from 0.1 to 100 nM inhibited dose-dependently proliferation in the non-tumorigenic C3H/10T1/2 Cl 8 (10T1/2) and the chemically transformed C3H/10T1/2 Cl 16 (Cl 16) cells. The hormone had a biphasic effect on the transformed cell line C3H/10T1/2 TPA 482 (TPA 482) in which growth was stimulated by low concentrations. Exposure to 10 nM 1,25-(OH)2D3 for 5 days resulted in a 90% growth inhibition of 10T1/2 cells, and the hormone was 10 and 100 times less potent in Cl 16 and TPA 482 cells, respectively. The inhibition of cell replication was fully reversible on removal of the hormone. Treatment of 10T1/2 and Cl 16 cells with 10 nM 1,25-(OH)2D3 reduced the saturation density to 30 and 37% that of controls, respectively, suggesting an enhancement of cell-cell contact mediated growth inhibition. 1,25-(OH)2D3 inhibited cytokinesis in 10T1/2 cells, inducing the formation of binucleated cells. Flow cytometric studies showed that 1,25-(OH)2D3-treated cells accumulated in the Go/G1 phase while the number of cells in S phase decreased. This in vitro model system seems to be useful for studies of the molecular mechanisms of the growth modulating effect of 1,25-(OH)2D3.