1,25-Dihydroxyvitamin D3 enhances phorbol ester-stimulated differentiation and protein kinase C-dependent substrate phosphorylation activity in the U937 human monoblastoid cell

In the U937 human monoblastoid cell line, 1,25-dihydroxyvitamin D3[1,25(OH)2D3] through a specific interaction with the 1,25(OH)2D3 receptor promotes differentiation toward a more mature phenotype. In addition to this direct effect, 1,25(OH)2D3 also potentiates differentiation in response to lymphokines and (Bu)2cAMP. We examined the effect of 1,25(OH)2D3 on phorbol ester-stimulated differentiation. Either preincubation with or simultaneous exposure to 1,25(OH)2D3 enhanced phorbol ester-stimulated differentiation. Over a 72-h period, the increase in phorbol ester responsiveness was dependent on the duration of 1,25(OH)2D3 exposure. Enhancement of phorbol ester-induced differentiation was observed with 1,25(OH)2D3 concentrations ranging from 0.1-10 nM. The 1,25(OH)2D3 vitamin D metabolite was more potent than the 24,25-dihydroxyvitamin D3 and 25-hydroxyvitamin D3 metabolites in potentiating phorbol ester-induced differentiation. Phorbol esters can exert at least a portion of their effects on cellular function by activating protein kinase C. Thus, one mechanism by which 1,25(OH)2D3 could amplify signal transduction leading to potentiation of phorbol ester-stimulated differentiation would be by enhancing phorbol ester-stimulated phosphorylation. To examine this possibility, we measured protein kinase C-dependent substrate phosphorylation in extracts derived from cells pretreated with 1,25(OH)2D3. In extracts derived from cells treated with 1,25(OH)2D3, the protein kinase C-dependent phosphorylation of endogenous U937 substrates stimulated by calcium, phosphatidyl serine, and diolein was increased compared to that observed in vehicle-treated cells. The conditions required for 1,25(OH)2D3 to increase protein kinase C-dependent phosphorylation of endogenous substrates (concentration, duration of exposure, and metabolite specificity) were similar to those required to enhance phorbol ester-stimulated differentiation. Possibly mediating this enhanced phosphorylation was an increase in protein kinase C activity observed in extracts derived from 1,25(OH)2D3-treated cells.     

Cooperative effects of gamma interferon and 1-alpha,25-dihydroxyvitamin D3 in inducing differentiation of human promyelocytic leukemia (HL-60) cells

Various agents induce differentiation of human leukemia cells in vitro. Most of these agents cause myeloid differentiation, but phorbol diesters, 1-alpha,25-dihydroxyvitamin D3 (1,25[OH2]D3), and certain lymphokines cause differentiation to monocyte-like cells. The purpose of this study was to determine the cooperative effects of 1,25(OH2)D3 and the lymphokine gamma interferon (IFN-gamma) on HL-60 cell differentiation. The recombinant human IFN-gamma or 1,25(OH2)D3 caused a slight reduction in the proliferation of the HL-60 cells (30%-40% reduction at doses of 100-200 U/ml [0.25-0.50 nM] IFN-gamma, or 5-25 nM 1,25[OH2]D3). HL-60 cells treated with 100 U/ml IFN-G had an eightfold increase in expression of nonspecific esterase (NSE) and a twofold increase in H2O2 production in response to phorbol myristate acetate (PMA). 1,25(OH2)D3 enhanced NSE expression eight- to 30-fold and H2O2 secretion twofold in response to PMA. There was also enhanced expression of HLA-DR and the receptor for C3bi. The 1,25(OH2)D3- and IFN-gamma-differentiating effects appeared to be additive or synergistic. Populations of IFN-gamma-treated HL-60 cells (but not the 1,25[OH2]D3-treated cells) had multinucleated giant cells. The polykaryons had NSE activity and had some properties of macrophage polykaryons or osteoclasts. 1,25(OH2)D3 did not augment the IFN-gamma-induced polykaryon formation.     

Combinations of interferon-gamma and retinoic acid or 1 alpha, 25-dihydroxycholecalciferol induce differentiation of the human monoblast leukemia cell line U-937

The monoblastlike leukemia cell line, U-937, is induced to differentiate into monocytelike cells by incubation with 200-500 U/ml of recombinant human immune interferon (IFN-gamma) judging from capacity to reduce nitroblue tetrazolium. At least an additive differentiation-inducing effect was found between IFN-gamma and 1-100 nM retinoic acid (RA). A marked synergistic differentiation-inducing effect was found between IFN-gamma and 0.1-1.0 nM 1 alpha,25-dihydroxycholecalciferol (1,25[OH]2D3). It is also shown that U-937 can be primed for differentiation by treatment for approximately one day with 1,25(OH)2D3 followed by exposure to IFN-gamma. Priming of these cells does not depend on the normal rate of RNA synthesis, as it occurs even better in the presence of cordycepin, suggesting that a decrease in RNA synthesis favors IFN-induced differentiation. Actually, the addition of cordycepin during initial incubation with IFN increased the subsequent response to IFN-gamma (and also to RA and 1,25[OH]2D3). These results, indicating that combinations of IFN-gamma and either RA or 1,25(OH)2D3 induce differentiation of U-937, may be of importance in combination biotherapy of leukemia.     

17 Beta-estradiol inhibits the oxidative metabolism of U937 cells indirectly via lymphocytes

The effect of 17 beta-estradiol (E) on the oxidative metabolism of U937 cells was studied. E had no direct effect on the proliferation, surface adherence, or luminol-enhanced luminescence (LEL) of the U937 cells. Exposure of U937 cells to lymphocyte-conditioned medium (LCM) and 1,25-dihydroxyvitamin D3 allowed a maximal LEL response by cells stimulated with phorbol myristate acetate. In contrast, LCM from lymphocytes exposed to E (LCM-E) did not stimulate LEL to the same extent as did an equal volume of LCM. Increasing the E concentration in the lymphocyte medium was associated with a dose-dependent reduction in the LEL response of the U937 cells. Mixing equal quantities of LCM and LCM-E significantly decreased LEL levels. LEL stimulated by LCM, gamma-interferon, or differentiation-inducing factor was reduced by the presence of LCM-E. The inhibitory action of LCM-E was reversible and metabolite specific. 17 alpha-E produced an effect that was only one tenth the magnitude of the E effect. These findings indicate that E can modulate the differentiation of phagocytes indirectly by altering the synthesis and/or secretion of lymphokines.     

Synergistic effects of alpha interferon and 1,25 dihydroxyvitamin D3: preliminary evidence suggesting that interferon induces expression of the vitamin receptor.

BACKGROUND. To active metabolite of vitamin D3-1,25(OH)2D3-is a well-known differentiation inducer. The addition of this metabolite to sensitive cell cultures inhibits proliferation and induces monocytic-macrophagic differentiation. Alpha interferon may also inhibit proliferation and increase the expression of some surface antigens in some neoplastic cells. In the present report, we describe the synergistic activity of these two drugs on U-937 and on cultured cells from a leukemic patient. METHODS. Proliferation was studied by 3H-thymidine incorporation; differentiation markers were evaluated immunologically by monoclonal antibodies and by cytochemical tests. Phagocytosis and NBT reduction test were also performed in order to confirm the differentiating properties of these drugs. Finally, the expression of the 1,25(OH)2D3 receptor was evaluated by immunochemical methods. RESULTS. After culturing these cells for 72 hours in the presence of 1,25(OH)2D3, cell proliferation was reduced and the expression of some phenotypic and functional markers suggested monocytic-macrophagic differentiation. Alpha interferon and 1,25(OH)2D3 synergistically inhibit the proliferation of U-937 cells. Alpha interferon increased the expression of the 1,25(OH)2D3 receptor in U-937 cells. CONCLUSIONS. The reported results confirm the synergistic activity of INF and 1,25(OH)2D3 on cell proliferation in monoblastic cells. The possible role of the increased expression of the vitamin receptor in cells cultured in the presence of INF is discussed.     

1 alpha,25-Dihydroxyvitamin D3 inhibits gamma-interferon synthesis by normal human peripheral blood lymphocytes.

1 alpha,25-Dihydroxyvitamin D3 [1,25-(OH)2D3], the biologically active metabolite of vitamin D3, inhibited synthesis of gamma-interferon (IFN-gamma) by phytohemagglutinin-activated peripheral blood lymphocytes (PBLs). A significant reduction of IFN-gamma protein levels in PBL culture medium was achieved with a physiologic 1,25-(OH)2D3 concentration (0.1 nM). 1,25-(OH)2D3 also inhibited accumulation of IFN-gamma mRNA in activated PBLs in a dose-dependent fashion. The ability of 1,25-(OH)2D3 to modulate IFN-gamma protein synthesis was unaltered in the presence of high concentrations of recombinant human interleukin 2. The suppression of IFN-gamma synthesis by PBLs was specific for 1,25-(OH)2D3; the potencies of other vitamin D3 metabolites were correlated with their affinities for the cellular 1,25-(OH)2D3 receptor. The time course of 1,25-(OH)2D3 receptor expression in phytohemagglutinin-activated PBLs was correlated with the time course of 1,25-(OH)2D3-mediated inhibition of IFN-gamma synthesis. In selected experiments, T-lymphocyte-enriched cell preparations were utilized. In these experiments, 1,25-(OH)2D3 was equally active as in PBL preparations. Finally, we examined the effects of 1,25-(OH)2D3 on the constitutive IFN-gamma production by two human T-lymphocyte lines transformed by human T-lymphotropic virus type I. The cell lines were established from a normal donor (cell line S-LB1) and from a patient with vitamin D-dependent rickets type 2 (cell line Ab-VDR). IFN-gamma synthesis by S-LB1 cells was inhibited in a dose-dependent fashion by 1,25-(OH)2D3, whereas IFN-gamma synthesis by Ab-VDR cells was not altered by 1,25-(OH)2D3. The data presented in this study provide further evidence for a role of 1,25-(OH)2D3 in immunoregulation.     

1,25-Dihydroxyvitamin D3 increases the toxicity of hydrogen peroxide: the role of calcium and heat shock

1,25-Dihydroxyvitamin D3 [1,25-(OH)2D3] increases synthesis of heat shock proteins in monocytes and U937 cells and protects these cells from thermal injury. We therefore examined whether 1,25-(OH)2D3 would also modulate the susceptibility to H2O2-induced oxidative stress. Prior incubation for 24 h with 1,25-(OH)2D3 (25 pM or higher) produced unexpected increased H2O2 toxicity. Since cellular Ca2+ may be a mediator of cell injury, we investigated the effects of altering extracellular Ca2+ ([Ca2+]e) on 1,25-(OH)2D3-enhanced H2O2 toxicity, as well as the effects of 1,25-(OH)2D3 and H2O2 on cytosolic-free Ca2+ concentration ([Ca2+]f). Basal [Ca2+]f in medium containing 1.5 mM Ca2+ as determined by fura-2 fluorescence was higher in 1,25-(OH)2D3-pretreated cells than control cells (137 versus 112 nM, p less than 0.005). H2O2 induced a rapid increase in [Ca2+]f (to greater than 300 nM) in both 1,25-(OH)2D3-treated and control cells, which was prevented by a reduction in [Ca2+]e to less than basal [Ca2+]f. The 1,25-(OH)2D3-induced increase in H2O2 toxicity was also prevented by preincubation with 1,25-(OH)2D3 in Ca2(+)-free medium or by exposing the cells to H2O2 in the presence of EGTA. Preexposure of cells to 45 degrees C for 20 min, 4 h earlier, partially prevented the toxic effects of H2O2 particularly in 1,25-(OH)2D3-treated cells, even in the presence of physiological levels of [Ca2+]e. Thus, 1,25-(OH)2D3 potentiates H2O2-induced injury probably by increasing cellular Ca2+ stores. The protective effects of heat shock are probably exerted at a site distal to the toxic effects of Ca2+. The 1,25-(OH)2D3-induced amplification of the heat shock response likely represents a mechanism for counteracting the Ca2(+)-associated enhanced susceptibility of oxidative injury due to 1,25-(OH)2D3.     

Differential responsiveness of intestinal epithelial cells to 1,25-dihydroxyvitamin D3--role of protein kinase C

Non-transformed rat intestinal epithelial cell (IEC) lines were used to study the action of 1,25-dihydroxyvitamin D(3) (1,25(OH)2D) in the intestine. The capacity of 1,25(OH)2D to increase the expression of the cytochrome P450 component of the vitamin D 24-hydroxylase (CYP24) was determined in IEC-6 and IEC-18 cell lines. In IEC-6 cells, which are derived from crypt cells isolated from the whole small intestine, 1,25(OH)2D markedly increased expression of CYP24 protein and mRNA within 12 h. In contrast, in IEC-18 cells, which are derived from crypt cells from the ileum only, 1,25(OH)2D did not increase expression of CYP24 until 24-48 h. The maximal levels of CYP24 mRNA seen in the IEC-18 cells were only 31% of the maximal levels seen in the IEC-6 cells. In the presence of 1,25(OH)2D, phorbol esters rapidly increased CYP24 mRNA levels in IEC-18 cells from almost undetectable to levels seen in IEC-6 cells. Protein kinase inhibitors abolished the stimulation by 1,25(OH)2D and by phorbol esters in both cell lines. Stimulation of mRNA levels by phorbol esters required new protein synthesis but stimulation by 1,25(OH)2D did not. These studies demonstrated that the rapid action of 1,25(OH)2D in IEC-6 cells is related to the activation of protein kinase C, an event which is missing in the IEC-18 cells. This differential response to 1,25(OH)2D probably takes place at a post-receptor site, since the number of vitamin D receptors in each cell line was found to be similar.     

Expression of C5a anaphylatoxin receptor in monoblastic cells involves facilitation of an adenosine 3',5'-monophosphate-dependent process

We have previously reported a synergistic effect of 1,25-dihydroxyvitamin D [1,25-(OH)2D] and agents that elevate intracellular cAMP to induce the expression of the C5a complement receptor in U937 cells. In this report we examine the mechanism of this synergy, considering the hypothesis that the steroid hormone works by facilitating what is ultimately a cAMP-dependent process. We show that U937 cells cultured with 1,25-(OH)2D alone before culture with prostaglandin (PGE2) alone will express C5a receptor (an average of 55 +/- 4% of the receptors expressed with continuous exposure of cells to both agents; P less than 0.05). The reverse, PGE2 followed by 1,25-(OH)2D, causes very little receptor induction. This demonstrates the ability of 1,25-(OH)2D to induce changes in the state of the cell, such that activation of cAMP-dependent protein kinase has effects that are otherwise not seen, in other words 1,25-(OH)2D can prime the cell for the subsequent action of the cAMP messenger system. Furthermore, we are able to substitute, during the priming period, the protein synthesis inhibitor cycloheximide (CHX) for 1,25-(OH)2D. Cells cultured for 24 h with CHX will express C5a receptor when cultured for a second 2-day period with PGE2 at about 77 +/- 7% of the amount obtained with simultaneous exposure to 1,25-(OH)2D and PGE2. The CHX effect is time dependent and visible after 2 h. CHX is not synergistic with 1,25-(OH)2D. Other agents that can also substitute for 1,25-(OH)2D, but not for cAMP, in facilitating C5a receptor expression include retinoic acid and ionomycin, but with less potency. The 1,25-(OH)2D and PGE2 synergy is sensitive to the presence of isobutylmethylxanthine, implicating its dependence on the maintained elevation of intracellular cAMP levels. The synergy does not appear to be sensitive to changes in extracellular or intracellular calcium. We conclude from these results that 1,25-(OH)2D may promote the expression of C5a receptor in these cells in a fashion similar to that by which CHX potentiates other genes, i.e. that 1,25-(OH)2D increases levels of the mRNA encoding the C5a receptor. The mechanism of cAMP's subsequent and necessary action in the induction of C5a receptor expression is not yet clear.     

1,25-Dihydroxyvitamin D3 stimulates the secretion of insulin-like growth factor binding protein 3 (IGFBP-3) by cultured human osteosarcoma cells.

Several types of specific insulin-like growth factor binding proteins have been reported. These binding proteins are produced by peripheral tissue-derived cells and they modulate the functions of insulin-like growth factors. In this study, we investigated both the secretion of insulin-like growth factor binding protein 3 (IGFBP-3) from a human osteosarcoma cell line MG63, and the effects of 1,25-dihydroxyvitamin D3 (1,25-(OH)2D3) on the production of this binding protein. The beta subunit of IGFBP-3 was detected in perinuclear cytoplasm of MG63 cells by immunocytochemical study. Immunoblotting and SDS-PAGE analysis revealed that both 150KD MW entire molecules and 40-60KD MW beta subunit molecules of IGFBP-3 were present in cell-conditioned media. 1,25-(OH)2D3 stimulated the production of the IGFBP-3 molecule by MG63 cells. The concentration of IGFBP-3 in conditioned media began to rise at 12 hours after the addition of 10(-8) M of 1,25-(OH)2D3 and reached peak level at 48 hours. Dose-dependent effects of 1,25-(OH)2D3 were demonstrated. The its maximum effect was observed at 10(-10) M. The concentration of IGFBP-3 in cytosol also increased at a 10(-10) M concentration of 1,25-(OH)2D3. We conclude from these results that human osteosarcoma cells MG63 produce the IGFBP-3 molecule and that 1,25-(OH)2D3 stimulates the production of this protein. These data suggests that the synergistic effects of 1,25-(OH)2D3 on the action of IGF-I on osteoblastic cells, which we reported previously, may be modulated by locally produced IGFBP-3.     

Early regulation of c-myc mRNA by 1,25-dihydroxyvitamin D3 in human myelomonocytic U937 cells

The effects of 1,25-dihydroxyvitamin D3 (1,25-(OH)2D3; 10 nmol/l) on the human monomyelocytic cell line U937 were investigated. Addition of 1,25-(OH)2D3 led to a decrease in cell proliferation which fell at 72 h to 67.8 +/- 4.3% (mean +/- S.E.M.) of control values. The presence of CD14, a surface marker found on mature monocytes/macrophages but not on U937 cells, was detectable as early as 18 h and peaked at 48 h, when 63.6 +/- 4.2% of the cells were positive. However, changes in c-myc mRNA levels were detected earlier, starting within 4 h of exposure to the hormone and being reduced to 38 +/- 8.2% of control values of 24 h. These effects were reversible after removal of the hormone, with the same sequence of events seen following addition of the hormone. There was first an increase in c-myc mRNA levels, starting within 2 h and reaching control values by 24 h. These changes were followed by loss of CD14 which became undetectable after 72 h. Proliferation recovered slowly and incompletely, since it was 81.7 +/- 0.7% of control after 72 h. A constant reciprocal relationship between c-myc mRNA and CD14 levels was found both in the presence and after removal of 1,25-(OH)2D3. Regulation of U937 cell proliferation and maturation by 1,25-(OH)2D3 is thus preceded by early modulation of c-myc mRNA.     

Calcium requirement for the lymphokine and vitamin D mediated differentiation of monoblastic U937 cells

The monocyte-osteoclast hypothesis states that osteoclasts are derived from the differentiation of monocytes. Recently we reported that the U937 cell line, a monoblastic cell of human origin, differentiated to an osteoclast precursor when cultured in the presence of 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) and lymphokines. Using this model of monocytic differentiation we have tested the role of calcium (Ca) in the culture medium by the addition of CaCl2 in the presence or absence of EGTA, a chelating agent, on the growth and differentiation of the U937 cells. Increasing the Ca concentration ([Ca]) to 2.0 mM inhibited the differentiation of resting or 1,25(OH)2D3 exposed cells, but had no effect on the cells exposed to lymphokines or lymphokines and 1,25 (OH)2D3. Reducing the [Ca] to 10(-7) M inhibited differentiation but also reduced cell viability. Addition of CaCl2 to medium containing EGTA to produce [Ca] in the range of 10(-5) M to 10(-3) M restored the cellular growth and viability. Inhibition of differentiation associated with the reduced calcium concentration over the range of 10(-5) M to 10(-3) M was not due to growth inhibition. These findings show the importance of the extracellular [Ca] in an in vitro model of monocytic differentiation induced by vitamin D and lymphokines.     

Inhibition of interleukin-1 production by 1,25-dihydroxyvitamin D3

The hormonal form of vitamin D, 1,25-dihydroxyvitamin D3 [1,25-(OH)2D3], inhibits the proliferation of T lymphocytes and production of growth-promoting factors (including interleukin-2) (IL2) in CTLL2 murine cells. In this study, we investigated the role of monocytes in this hormone-mediated inhibitory effect, by testing the effects of 1,25-(OH)2D3 on the ability of the mitogenic lectin phytohemagglutinin (PHA) to induce T cell activation in either a monocyte-dependent or phorbol myristate acetate (PMA)-driven (monocyte-independent) system. The results indicate that proliferation of T cells and production of growth-promoting factors are inhibited by 1,25-(OH)2D3 only in the monocyte-dependent system. Preincubation of monocytes with 1,25-(OH)2D3 for various periods of time and subsequent removal of the hormone resulted in inhibition of the PHA-driven proliferation of T cells. Preincubation for 2 h resulted in 20% inhibition, while preincubation for 36 h reduced proliferation to 50% of the control value [no 1,25-(OH)2D3 exposure]. These data suggested that monocytes are important participants in 1,25-(OH)2D3-mediated events. Therefore, we tested the effects of the hormone on the production of IL1, a monocyte-derived product thought to be involved in the induction of IL2 release and the subsequent development of the T cell proliferative response. 1,25-(OH)2D3 inhibited the production of both extracellular and cell-associated immunoreactive IL1 alpha and IL1 beta. Indomethacin, a prostaglandin synthetase inhibitor, did not alter the inhibitory properties of 1,25-(OH)2D3, suggesting that prostaglandins are not responsible for the inhibitory phenomenon. We conclude that part of the ability of 1,25-(OH)2D3 to inhibit T cell proliferation may be due to direct effects on monocytes by down-regulating IL-1 production. However, it is unlikely that the immunoregulatory properties of 1,25-(OH)2D3 on T cells are mediated solely through monocytes, and it is possible that the hormone also exerts its influence directly on T cells.     

Synergistic effect of granulocyte-macrophage colony-stimulating factor and 1,25-dihydroxyvitamin D3 on the differentiation of the human monocytic cell line U937

The human monoblastlike cell line U937 can be induced to differentiate by a variety of agents including gamma-interferon, phorbol esters, retinoic acid, and 1,25-dihydroxyvitamin D3 (VD3). Incubation of U937 with 1 to 1,000 units of recombinant human granulocyte-macrophage colony-stimulating factor (GM-CSF) did not induce macrophage differentiation. A synergistic effect on macrophage differentiation was observed, however, when U937 was cocultured with 10(-8) mol/L VD3 plus 50 U/mL GM-CSF. GM-CSF-plus VD3-treated cells demonstrated significant increases in OKM1 antigen expression, increased chemokinesis and chemotaxis, and increased Fc receptor-mediated erythrophagocytosis. Human peripheral blood monocyte cultures also demonstrated increased OKM1 antigen expression and chemotaxis when incubated with 50 to 500 U/mL of GM-CSF for 48 to 72 hours. VD3, however, was not necessary for the increases in effector function observed for GM-CSF-stimulated monocyte cultures. In distinction to the synergistic effect of GM-CSF on VD3-induced differentiation of U937, recombinant human granulocyte colony-stimulating factor (G-CSF) at comparable concentrations had no augmenting effect over that observed for VD3 alone. These results suggest that GM-CSF, in the presence of other physiological stimuli, can induce significant phenotypic changes in GM-CSF-nonresponsive cells of the monocytic lineage and can increase the effector functions of GM- CSF-responsive peripheral blood monocyte cultures.     

Calcium antagonizes 1,25-dihydroxyvitamin D3 inhibition of breast cancer cell proliferation

The hormone 1,25-dihydroxyvitamin D3 [1,25-(OH)2D3] inhibited proliferation of two breast cancer cell lines (HT-39 and MCF-7) in a dose-dependent manner. We showed that 10 nM 1,25-(OH)2D3 inhibited HT-39 cell growth by 26.3 +/- 3.2% and MCF-7 proliferation by 19.5 +/- 8.6% (mean +/- SE). When both cell lines were cultured in the presence of medium containing varying concentrations of calcium, analysis of cell growth revealed that lowering the medium Ca2+ concentration from 1.3 to 0.1 mM inhibited cell proliferation of HT-39 cells by 20.0 +/- 2.3% and that of MCF-7 cells by 13.4 +/- 3.4% (mean +/- SE). Raising medium Ca2+ from 1.3 to 2.6 mM stimulated cell proliferation of HT-39 cultures by 51.8 +/- 2.3% and that of MCF-7 cells by 13.3 +/- 3.3% (mean +/- SE). When HT-39 cells were grown in medium containing different calcium levels and dosed with 10 nM 1,25-(OH)2D3, we found that the inhibitory action of 1,25-(OH)2D3 was modified. HT-39 cells in medium containing 0.1 mM calcium were more potently inhibited by 10 nM 1,25-(OH)2D3 (increased by 74% relative to that in cells in 1.3 mM calcium). Moreover, the inhibitory action of 1,25-(OH)2D3 (10 nM) on HT-39 cells grown in 2.6 mM calcium media was decreased by 68% relative to that in cells in 1.3 mM extracellular calcium. The phorbol ester 4 beta-phorbol 12,13-dibutyrate (PDB) stimulated HT-39 proliferation by 30.1 +/- 4.1%. Cells treated with PDB showed increased cell mitotic index, but unlike extracellular calcium, PDB had no effect on the dose response of 1,25-(OH)2D3 to inhibit HT-39 cell growth. We show that 1,25-(OH)2D3 action on malignant cell proliferation is dependent on the extracellular calcium concentration. The data suggest that the effect of calcium to antagonize 1,25-(OH)2D3 on HT-39 cells is not shared by all agents (e.g. PDB) that increase HT-39 cell proliferation.     

Action of 1,25-(OH)2D3 in nude mice bearing transplantable human myelogenous leukemic cell lines

After cyclophosphamide priming, subcutaneously (s.c.) transplanted cells from established human leukemia cell lines U937, K562, or HL-60 consistently yielded single, nonmetastatic tumors. Tumorigenesis with KG-1 cells was inconstant. Within each cell line, cytologic, electron-microscopic, cytogenetic, isoenzyme, immunochemical, and enzyme cytochemical studies confirmed identity of cultured and tumor cells. Adenosine triphosphatase reactivity was limited to leukemic cells in vivo. Isoenzyme electrophoretic patterns, distinct for each cell line, provided a reliable criterion to establish clonality and to verify tumor cell origin. Antitumor activity of the active vitamin-D3 metabolite 1,25-(OH)2D3 was assessed in vivo against U937, K562, and HL-60 cells by cell transplantation and concurrent s.c. contralateral implantation of miniosmotic pumps containing the 1,25-(OH)2D3 in a propylene glycol vehicle. Tumors developed in all treated U937 mice, 50% with K562 and 25% bearing HL-60 transplants. All transplants proliferated in mice either with pumps containing only vehicle or no pumps. Coincidence of tumor and vehicle decreased survival time. No differences in cytoreactivities or morphology were apparent between cultured cells and tumor cells in treated or untreated mice. This nude mouse system is useful for in vivo studies of human myelogenous leukemia cells. Implanted miniosmotic pumps provide controlled delivery of antineoplastic agents and their vehicles for in vivo studies. 1,25-(OH)2D3 may be a valuable adjunctive therapeutic for control of human myelogenous leukemias.