Estrogen maintains trabecular bone volume in rats not only by suppression of bone resorption but also by stimulation of bone formation.

Estrogen is generally considered to maintain bone mass through suppression of bone resorption. We have previously demonstrated that administration of pharmacologic doses of estrogen increases bone formation in ovary-intact rats. To assess the effects of physiological concentrations of estrogen on bone formation, estrogen was administered to ovariectomized rats in which bone resorption was suppressed by the bisphosphonate 3-amino-1-hydroxypropylidene-1-bisphosphonate (AHPrBP). Animals receiving exogenous 17 beta-estradiol (E2) (1, 10, and 100 micrograms/kg daily for 17 d) showed a dose-dependent increase in trabecular bone volume of 1.9, 25.8, and 43.6%, respectively, compared with those rats treated with AHPrBP alone. The increase in bone volume was associated with an increase in bone formation in E2-treated animals, in which bone resorption had been almost completely suppressed by AHPrBP. Neither ovariectomy, AHPrBP, nor E2 treatment had a significant effect on the volume or rate of formation of cortical bone. Thus, the increased bone resorption, which is a consequence of estrogen-deficiency, entrains increased bone formation, which masks a simultaneous reduction in estrogen-dependent bone formation. Therefore, in addition to the nonspecific effect of estrogen to depress formation via coupling, we have identified a specific effect of estrogen to increase formation independent of coupling. Thus it appears that estrogen maintains bone volume not only through inhibition of bone resorption, but also through stimulation of bone formation.

Potentiation by vitamin D analogs of TNFalpha and ceramide-induced apoptosis in MCF-7 cells is associated with activation of cytosolic phospholipase A2.

Synthetic analogs of vitamin D induce apoptosis in cultured breast cancer cells and cause regression of experimentally-induced rat mammary tumors. To further elucidate the mechanisms involved, we have examined interactions between two vitamin D analogs (CB1093 and EB1089) and known mediators of apoptosis, TNFalpha and ceramide. Pretreatment of MCF-7 breast cancer cells with CB1093 and EB1089 substantially potentiated cytotoxic effects of TNFalpha as assessed by cell viability assay, DNA fragmentation and videomicroscopy. No significant changes in the levels of TNFalpha or TNF-RI transcripts were detected. CB1093 primed cells demonstrated enhanced responsiveness to cell permeable C2-ceramide in terms of increased DNA fragmentation and loss of cell viability. Activation of cytosolic phospholipase A2 (cPLA2) has been implicated in TNFalpha-mediated apoptosis. As assessed by [3H]-arachidonic acid release, cells primed for 48 h with CB1093 (50 nM) showed enhanced cPLA2 activation in response to TNFalpha or ceramide. CB1093 treatment alone led to cPLA2 activation and loss of cell viability which was inhibited by the specific inhibitor AACOCF3. These results suggest that TNFalpha and vitamin D analogs share a common pathway leading to apoptosis involving cPLA2 activation and/or ceramide generation.

Interaction of vitamin D derivatives and granulocyte-macrophage colony-stimulating factor in leukaemic cell differentiation.

The ability of the physiologically active form of vitamin D, 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) and two novel vitamin D analogues, EB1089 and KH1060 to induce the differentiation of the U937 and HL-60 leukaemic cell lines was evaluated, alone or in combination with granulocyte-macrophage colony-stimulating factor (GM-CSF). Studies revealed that following 96 h treatment, the vitamin D derivatives inhibited the proliferation, and induced the differentiation of U937 and HL-60 cells in a dose-dependent manner, as determined by cell counts and nitroblue tetrazolium (NBT) reduction assays, respectively. EB1089 and KH1060 were found to be more effective than 1,25(OH)2D3 in exhibiting their antiproliferative and differentiative effects. In contrast, induction of leukaemic cell differentiation with 1 ng/ml GM-CSF after 96 h was less effective when compared with the vitamin D derivatives used individually. Fluorescence activated cell scanning (FACS) analyses indicated that the vitamin D derivatives readily induced the expression of the monocyte-associated cell surface antigen, CD14, and also the beta2-integrins, CD11b and CD18 in both cell lines after 48 h and 96 h treatment. The ability of EB1089 and KH1060 to induce these antigens was achieved with greater efficacy relative to the native hormone. When U937 and HL-60 cell cultures were cotreated for 48 h with the vitamin D compounds and GM-CSF and analysed by FACS, enhanced effects on CD14 and CD11b induction were observed compared to those of the compounds alone. These co-operative effects may occur as a consequence of molecular events which involve the transcription by vitamin D receptors (VDR) of genes required for the responsiveness of immature cells to factors such as GM-CSF, and place these and other related vitamin D analogues as potential therapeutic agents in the treatment of leukaemia.

Interaction of androgen and 1,25-dihydroxyvitamin D3: effects on normal rat bone cells.

We studied the actions of testosterone (T) and 5 alpha-dihydrotestosterone (DHT) in combination with 1,25-dihydroxyvitamin D3 [1,25-(OH)2D3] on primary rat bone cells. The actions of androgens were generally anabolic, although response patterns varied considerably between cultures. For example, DHT caused striking dose- and time-dependent increases in [3H]thymidine incorporation into calvarial cells over the range 1-100 nM, with maximal stimulation of 2.5-fold after 9 days in culture. Testosterone (50 nM) also stimulated [3H]thymidine incorporation into long bone-derived cells. 1,25-(OH)2D3 generally blunted or abolished the proliferative action of androgens but was not itself always inhibitory; in some experiments, stimulation of [3H]thymidine incorporation occurred. Collagen production, as assessed by [3H]proline incorporation into pepsin-resistant protein secreted by calvarial cells, was also stimulated by DHT. In some cultures, androgen responses were absent, although striking inhibitory responses to 1,25-(OH)2D3 were observed. These results illustrate complex patterns of responses to androgens and 1,25-(OH)2D3 in cells derived from rat bone.

Estrogen receptors and human bone cells: immunocytochemical studies.

In this immunocytochemical study we have probed a number of human bone cell types and bone preparations for the presence of the estrogen receptor (ER) with two distinct monoclonal antibodies. Using a well-validated antibody (H222) that recognizes human ER and standard peroxidase-antiperoxidase methodology, we were unable to demonstrate nuclear staining for ER in cultured primary or transformed human bone-derived cells or in fetal bone sections. Attempts to visualize ER in osteosarcoma cell lines (TE85C and HTB96) using a silver enhancement procedure were also unsuccessful. Additionally, we failed to detect immunocytochemical staining for the progesterone receptor (using monoclonal antibody mPR1) in control or estrogen-treated human bone cell cultures. Estrogen and progesterone receptor staining was readily detectable in MCF7 human breast cancer cells. In contrast, with a monoclonal antibody that recognizes a 29 kDa cytoplasmic component (p29) closely related to human ER, we observed specific staining in all the osteoblastlike cells studied. Cytoplasmic staining for this p29 antigen was most intense in primary cultures of human bone-derived cells. It is possible that the relatively abundant but as yet undefined p29 antigen may act as a sensitive marker for the presence of ER in cells at levels below the detection limit of the anti-ER monoclonal antibody. If so, our results are consistent with the presence of ER in osteoblastlike cells at very low concentrations.

Mechanisms implicated in the growth regulatory effects of vitamin D in breast cancer.

It is now well established that, in addition to its central role in the maintenance of extracellular calcium levels and bone mineralization, 1,25-dihydroxyvitamin D(3) (1,25(OH)(2)D(3)), the active form of vitamin D, also acts as a modulator of cell growth and differentiation in a number of cell types, including breast cancer cells. The anti-proliferative effects of 1,25(OH)(2)D(3) have been linked to suppression of growth stimulatory signals and potentiation of growth inhibitory signals, which lead to changes in cell cycle regulators such as p21(WAF-1/CIP1) and p27(kip1), cyclins and retinoblastoma protein as well as induction of apoptosis. Such studies have led to interest in the potential use of 1,25(OH)(2)D(3) in the treatment or prevention of certain cancers. Since this approach is limited by the tendency of 1,25(OH)(2)D(3) to cause hypercalcaemia, synthetic vitamin D analogues have been developed which display separation of the growth regulating effects from calcium mobilizing actions. This review examines mechanisms by which 1,25(OH)(2)D(3) and its active analogues exert both anti-proliferative and pro-apoptotic effects and describes some of the synthetic analogues that have been shown to be of particular interest in relation to breast cancer.

High concentrations of 17 beta-estradiol stimulate trabecular bone formation in adult female rats.

Although the effects of low concentrations of 17 beta-estradiol (E2) on bone formation and resorption are well described, little is known of the effects of E2 on bone at concentrations that circulate during pregnancy. We, therefore, investigated the effects of administration of high dose E2 to 3-month-old female Wistar rats on biochemical and histomorphometric indices of bone formation and resorption. Animals receiving exogenous E2 (4 mg/kg.day for 17 days; n = 9) showed a mean serum E2 concentration of 17.5 +/- 2.9 nM, compared with 0.6 +/- 0.2 nM in those receiving vehicle alone (n = 10). The bone formation rate, measured at the proximal tibial metaphysis after the administration of double fluorochrome labels, was greatly increased in the E2 group (13.6 +/- 2.0 x 10(-2) microns3/microns2.day) compared to controls (3.9 +/- 0.9), as was serum alkaline phosphatase (E2, 133.6 +/- 10.1 IU; controls, 87.5 +/- 5.5). This increase in the rate of bone formation was associated with a significant increase in trabecular bone volume. E2 treatment did not affect urinary hydroxyproline excretion or histomorphometric indices of bone resorption. These findings suggest that high concentrations of E2 strongly stimulate the formation of trabecular bone. This may represent an important mechanism by which calcium stores are accumulated during pregnancy in rats, in anticipation of the mineral requirements of lactation.

Demonstration of a 1,25-dihydroxycholecalciferol cytoplasmic receptor-like binder in mouse kidney.

Isolated mouse renal tubule cells have been employed to demonstrate the presence of a specific high affinity cytoplasmic binding protein for 1,25-dihydroxycholecalciferol (1,25(OH)2D3) in kidney. This receptor-like macromolecule sedimented at 3.2 S in hypertonic sucrose density gradients. Scatchard analysis of [3H]1,25-(OH)2D3 binding at O C revealed an apparent Kd of 0.2 nM and a concentration of binding sites of 50 fmol/mg cytosol protein. In competition experiments, the binder exhibited a low affinity for other vitamin D3 metabolites; the order of potency was 1,25(OH)2D3 greater than 250HD3 greater than u alpha OHD3 greater than 24R,25(OH)2D3. The sedimentation properties, binding affinity, and specificity of this 1,25(OH)2D3 binding protein are strikingly similar to the receptors in rat intestine, mouse bone, and human intestine. The demonstration of a renal receptor-like binder adds further support to the concept that the kidney is a 1,25(OH)2D3 target organ.

Vitamin D analogues suppress IGF-I signalling and promote apoptosis in breast cancer cells.

Survival factors are known to promote cell viability, and factor deprivation can be a potent apoptotic signal. Insulin-like growth factors are potent mitogens and inhibitors of apoptosis for many normal and neoplastic cells with insulin-like growth factor-I (IGF-I) being the most effective in many breast cancer cell lines. 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) and its analogues inhibit IGF-I-stimulated growth of MCF-7 human breast cancer cells. The aim of this study was to determine the relationship between inhibition of IGF-I responsiveness and induction of apoptosis by vitamin D analogues in breast cancer cells. Vitamin D analogues EB1089 and CB1093 inhibited autonomous and IGF-I-stimulated growth of MCF-7 and T47D cells and autonomous growth of IGF-I-insensitive Hs578T cells. In MCF-7 cells, IGF-I alone (4 nM) protected against apoptosis mediated by serum deprivation. Co-treatment with vitamin D analogues prevented the anti-apoptotic effects of IGF-I. In T47D cells, IGF-I treatment provided only partial protection against apoptosis induced by serum deprivation and co-incubation of serum-deprived cells with 100 nM CB1093 and IGF-I abrogated this partial protection. In Hs578T cells, addition of IGF-I did not prevent apoptosis induced by serum deprivation. However, treatment with CB1093 attenuated the protective effect of the serum in these cells. Our findings suggest that vitamin D analogues inhibit IGF-I signalling pathways to promote apoptosis in breast cancer cells.

Vitamin D derivatives in combination with 9-cis retinoic acid promote active cell death in breast cancer cells.

The effects of the novel vitamin D analogue, EB1089 alone, or in combination with the retinoid, 9-cis retinoic acid (9-cis RA) on indices of apoptosis in MCF-7 breast cancer cells have been examined. EB1089 was capable of reducing bcl-2 protein, a suppressor of apoptosis, and increasing p53 protein levels in MCF-7 cell cultures following 96h treatment. In the presence of 9-cis RA, EB1089 acted to further enhance the down-regulation and up-regulation of bcl-2 and p53 respectively. Furthermore, EB1089 induces DNA fragmentation in MCF-7 cells, a key feature of apoptosis, alone and in combination with 9-cis RA in situ. The observation that EB1089 and 9-cis RA act in a cooperative manner to enhance induction of apoptosis in these cells may have therapeutic implications.

Growth inhibition of both MCF-7 and Hs578T human breast cancer cell lines by vitamin D analogues is associated with increased expression of insulin-like growth factor binding protein-3.

The effects of two vitamin D analogues, EB1089 and CB1093, on insulin-like growth factor binding protein (IGFBP) expression have been examined in MCF-7 and Hs578T human breast cancer cell lines. Both vitamin D analogues inhibited IGF-1 stimulated growth of MCF-7 cells and enhanced the production of IGFBP-3 as determined by Western-ligand blotting. Recombinant human IGFBP-3 inhibited the growth of MCF-7 cells over the concentration range 1-235 ng/ml. Hs578T cells were unresponsive to the mitogenic effects of IGF-1 but growth was inhibited by the two vitamin D analogues. Treatment of Hs578T cells with EB1089 and CB1093 (10 nM) as well as 100 nM 9-cis retinoic acid (9-cis RA) or all-trans retinoic acid (ATRA) was associated with increased accumulation of IGFBP-3 in conditioned medium. Furthermore, cotreatment of Hs578T cells with EB1089 and 9-cis RA led to augmented effects on both inhibition of cell growth and IGFBP-3 accumulation in conditioned medium as assessed by Western ligand blotting and radioimmunoassay. These findings suggest a role for IGFBP-3 in the growth inhibitory effects of vitamin D analogues.

EB1089, a synthetic analogue of vitamin D, induces apoptosis in breast cancer cells in vivo and in vitro.

1. Effects of the synthetic vitamin D analogue EB1089 on indices of apoptosis in cultured human breast cancer cells and in nitrosomethylurea-induced rat mammary tumours in vivo were investigated. 2. At a dose of 0.5 microg kg(-1) body weight, EB1089 caused significant inhibition of tumour progression over the 28 day treatment period in the absence of a significant increase in serum calcium concentration. Higher doses of EB1089 (1 and 2.5 microg kg(-1)) produced substantial regression of the experimental tumours which was accompanied by a striking change in the histological appearance of tumours consistent with induction of tumour cell death. 3. Fragmentation of genomic DNA is a characteristic feature of apoptosis. With the terminal transferase (TdT) assay, 3' DNA breaks indicative of DNA fragmentation were detected histochemically in mammary tumour cells from animals treated with EB1089 (2.5 microg kg(-1)) for 14 days. 4. Effects of the vitamin D analogue on induction of apoptosis were examined in vitro using the MCF-7 human breast cancer cell line. Using the TUNEL method, positive nuclear staining indicative of DNA fragmentation was detected in cells treated for 4 days with 10 nM EB1089. Apoptosis was also quantitated using a cell death ELISA which revealed a time and dose dependent induction of apoptosis by EB1089. 5. The effects of EB1089 on the expression of two oncoproteins which may regulate apoptosis, bcl-2 and bax were examined by Western analysis. In MCF-7 cell cultures treated with 1,25(OH)2D3 or EB1089 (1 x 10(-8) M), bcl-2 protein levels were decreased in a time-dependent manner relative to control levels. In contrast bax protein was not markedly regulated by these compounds. Densitometric analyses indicate that the vitamin D compounds lower the bcl-2/bax ratio favouring increased susceptibility of MCF-7 cells to undergo apoptosis. 6. These results suggest that the synthetic vitamin D analogue EB1089 may promote tumour regression by inducing active cell death.

Effects of a new synthetic vitamin D analogue, EB1089, on the oestrogen-responsive growth of human breast cancer cells.

The anti-proliferative effects of the novel vitamin D analogue, EB1089, were assessed in the hormone-dependent breast cancer cell line, MCF-7, in vitro. In the present study, EB1089 was shown to be at least an order of magnitude more potent at inhibiting MCF-7 cell proliferation than the native hormone, 1 alpha,25-dihydroxyvitamin D3 (1,25(OH)2D3). Treatment of MCF-7 cell cultures with combinations of oestradiol and EB1089 ranging from 5 x 10(-11) M to 5 x 10(-9) M revealed the ability of EB1089 to suppress the mitogenic effects of oestradiol in these cells dose-dependently, as determined by [3H]thymidine incorporation and cell counts. EB1089 also exhibited a significant time- and dose-dependent decrease in MCF-7 oestrogen receptor (ER) concentration, as assessed by ligand binding assay. A fourfold reduction of ER levels by 5 x 10(-9) M EB1089 relative to control ER levels was observed, whilst 5 x 10(-9) M 1,25(OH)2D3 produced a significant but less dramatic decrease in ER levels. In addition, reduction of ER protein in EB1089-treated cell cultures was also demonstrated using an oestrogen receptor enzyme immunoassay. The interaction of EB1089 and anti-oestrogens on the oestradiol-stimulated growth of MCF-7 cells was investigated. The treatment of cell cultures with 5 x 10(-10) M EB1089 in combination with the pure anti-oestrogen, ICI 182,780 (5 x 10(-8) M), and in the presence of between 5 x 10(-10) M and 5 x 10(-9) M oestradiol, produced an augmented inhibition of MCF-7 cell proliferation compared with the actions of either compound alone.(ABSTRACT TRUNCATED AT 250 WORDS)

Vitamin D derivatives inhibit the mitogenic effects of IGF-I on MCF-7 human breast cancer cells.

The effects of 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) and four novel synthetic analogues (EB1089, KH1060, KH1230 and CB1093) on IGF-I-stimulated growth of MCF-7 human breast cancer cells have been determined. A significant time- and dose-dependent inhibition of IGF-I-stimulated cell growth was seen with EB1089, such that after 7 days of treatment with 10(-8) M EB1089, the mitogenic effect of IGF-I (30 ng/ml) was negated. Comparison with 1,25(OH)2D3 showed the synthetic analogues to be more potent. The anti-oestrogen ICI 182,780 similarly inhibited IGF-I-stimulated growth of these cells and in combination with EB1089 exerted additional inhibitory effects. Retinoids (all-trans-retinoic acid or the isomer 9-cis-retinoic acid) were less effective in limiting MCF-7 cell responsiveness to IGF-I but, in combination with EB1089, a co-operative effect was achieved. Using radioligand-binding techniques, we observed that 1,25(OH)2D3 and EB1089 down-regulated the levels of 125I-IGF-I binding to MCF-7 cell membranes. Scatchard analysis showed that EB1089 decreased maximal binding approximately 2-fold. Vitamin D derivatives were also demonstrated to reduce IGF-I receptor expression in MCF-7 cells by Western analysis. Our findings demonstrate that vitamin D derivatives limit responsiveness of MCF-7 cells to the mitogenic effects of IGF-I, which may be mediated by reduction of IGF-I receptor expression.

EB1089: a new vitamin D analogue that inhibits the growth of breast cancer cells in vivo and in vitro.

EB1089 is a novel vitamin D analogue which has been tested for its effects on breast cancer cell growth in vitro, using the established human breast cancer cell line MCF-7, and in vivo on the growth of established rat mammary tumours. Both EB1089 and 1,25-dihydroxyvitamin D3 (1,25-(OH)2D3) inhibited MCF-7 cell proliferation with the synthetic analogue being at least an order of magnitude more potent than the native hormone. In vivo anti-tumour effects were investigated using the N-methyl-nitrosourea-induced rat mammary tumour model. Oral treatment with EB1089 was tested at three doses. With the lower dose, significant inhibition of tumour growth was seen in the absence of a rise in serum calcium. The same dose of 1,25-(OH)2D3 had no effect on tumour growth but caused hypercalcaemia. With the higher dose of EB1089, striking tumour regression was seen although serum calcium rose. This report demonstrates that EB1089 possess enhanced anti-tumour activity coupled with reduced calcaemic effects relative to 1,25-(OH)2D3 and thus may have therapeutic potential as an anti-tumour agent.

The role of vitamin D derivatives and retinoids in the differentiation of human leukaemia cells.

The capabilities of 1alpha, 25-dihydroxyvitamin D3 (1,25(OH)2D3), and two novel vitamin D analogues, EB1089 and KH1060, to induce the differentiation of two established leukaemia cell lines, U937 and HL-60, were assessed alone or in combination with the retinoid compounds, 9-cis retinoic acid (9-cis RA) and all-trans retinoic acid (ATRA). The vitamin D derivatives acted to increase the differentiation of U937 and HL-60 cell cultures in a dose-dependent manner, as determined by nitroblue tetrazolium (NBT) reduction, with EB1089 and KH1060 being more effective than the native hormone. As an additional index of leukaemic cell differentiation, induction of expression of the phenotypic cell surface antigen, CD14, and the beta2-integrins, CD11b and CD18 by the vitamin D and retinoid compounds were monitored using fluorescence activated cell sorting (FACS) analyses. Following 96-hr treatment of U937 and HL-60 cells with 5 x 10(-10) M of the vitamin D derivatives, a striking increase in CD14 antigen expression was apparent, indicating the promotion by these compounds of a monocyte/macrophage lineage of cells. CD11b and CD18 antigen expression were also raised above control levels. In contrast, both retinoid compounds used at the higher concentration of 1 x 10(-8) M were not effective inducers of CD14 antigen expression. However, CD11b and CD18 were both readily increased in U937 and HL-60 cell cultures. Treatment of U937 cell cultures with the vitamin D compounds and the retinoids resulted in cooperative effects on induction of differentiation, with correlation by both NBT reduction and FACS analyses of CD14 antigen expression. The presence of 9-cis RA or ATRA appeared to contribute to the further increase of CD14 in these cells. HL-60 cell cotreatment with these compounds also displayed enhanced cooperative effects in phagocytic function by NBT reduction. However, analysis of CD14 revealed a dramatic diminution in HL-60 cells treated with the combinations of the vitamin D derivatives and the retinoids. Assessment of HL-60 cell morphology treated with these combinations demonstrated the presence of a mixed population of monocytes and granulocytes. CD11b and CD18 antigen expression was also enhanced in both cell lines with cotreatment. The ability of EB1089 and KH1060 to induce leukaemic cell differentiation may provide an additional option for therapeutic use alone or together with other differentiation agents such as 9-cis RA or ATRA.

Effects of synthetic vitamin D analogues on breast cancer cell proliferation in vivo and in vitro.

Calcipotriol (MC903) is a novel vitamin D analogue which effects cellular differentiation and proliferation in vitro and has reduced effects on calcium metabolism in vivo. In the present study its in vitro activity was evaluated using the MCF-7 breast cancer cell line, and its effects on calcium metabolism and mammary tumour growth were measured in vivo in adult female rats. Calcipotriol was compared to the natural metabolite of vitamin D3, 1 alpha,25-dihydroxycholecalciferol [1,25(OH)2D3] and its synthetic analogue 1 alpha hydroxycholecalciferol [1 alpha(OH)D3]. Both calcipotriol and 1,25(OH)2D3 produced significant inhibition of MCF-7 cell proliferation at a concentration of 5 x 10(-11) M. Intraperitoneal administration of calcipotriol to normal female rats showed that the analogue was 100-200 times less active than 1,25(OH)2D3 in raising serum calcium concentration and urinary calcium excretion. Anti-tumour activity of the vitamin D analogues was investigated in vivo using the nitrosomethylurea-induced rat mammary tumor model. Rats, maintained on a low calcium diet, were treated with 1 alpha(OH)D3 (0.25 and 1.25 micrograms/kg). Both doses produced a response rate of 25% but hypercalcaemia developed. Treatment with calcipotriol (50 micrograms/kg) of rats maintained on a normal laboratory diet caused inhibition of tumour progression (response rate 17%) without the development of severe hypercalcaemia. This study supports the concept that vitamin D derivatives may inhibit breast cancer cell proliferation in vivo.

Interactions of vitamin D analogue CB1093, TNFalpha and ceramide on breast cancer cell apoptosis.

Mechanisms by which vitamin D analogues promote apoptosis in tumour cells are unclear. In this study we have examined possible interactions between the synthetic vitamin D analogue CB1093 and two other known mediators of apoptosis, TNFalpha and ceramide, in MCF-7, T47D and Hs578T breast cancer cells. These studies indicated that cytosolic phospholipase A(2) (cPLA(2)) is involved in CB1093 as well as TNFalpha-mediated cell death. CB1093 promoted both TNFalpha and ceramide-induced c-PLA(2) activation, which was inversely related to loss of cell viability in MCF-7 and Hs578T cells. TNFalpha alone (5-20 ng/ml) failed to induce cytotoxicity and activation of cPLA(2) in T47D cells. However, pretreatment of these cells with CB1093 potentiated C(2)-ceramide-induced cPLA(2) activation and cell death. Treatment with CB1093 alone induced loss of cell viability and DNA fragmentation in all three cell lines by 5 days and these effects were accompanied by activation of cPLA(2). Furthermore, co-treatment with the cPLA(2) inhibitor AACOCF(3) led to partial protection against loss of cell viability induced by CB1093 in Hs578T and T47D cells as well as MCF-7 cells. The broad-spectrum caspase inhibitor z-VAD-fmk prevented TNFalpha but not C(2)-ceramide and CB1093-mediated release of arachidonic acid and cell death in MCF-7 cells. These results indicate that CB1093 potentiates responsiveness of breast cancer cells to TNFalpha and suggest that ceramide and/or cPLA(2) might be involved as downstream effectors in vitamin D-mediated caspase-independent cell death.

Effect of prolactin on vitamin D metabolism.

The effect of ovine prolactin on the renal 25-hydroxycholecalciferol-1-hydroxylase was studied in the chick. Prolactin was found to increase the activity of this enzyme in both long-term and short-term experiments. In the long term, 7 days treatment with prolactin caused a marked stimulation of the 1-hydroxylase activity, however this effect was only seen when the enzyme was assayed 2-3 hours after the final injection of prolactin. A single subcutaneous injection of prolactin was also effective in increasing the 1-hydroxylase activity, this effect was maximal at one hour and had largely disappeared 3 hours after prolactin administration.

Mammary gland 1,25-dihydroxyvitamin D3 receptor content during pregnancy and lactation.

The purpose of this study was to establish the time course and magnitude of changes in 1,25-dihydroxy-vitamin D receptor activity in rat mammary gland during pregnancy and lactation and to correlate these changes with casein production and alkaline phosphatase activity. Marked increases in both 1,25-dihydroxyvitamin D3 receptor and alkaline phosphatase activities were seen towards the end of pregnancy but the time course of these changes was not synchronous. Receptor activity was first detectable at 11 days of pregnancy with a marked rise in receptor levels at 3 days post-partum. Changes in alkaline phosphatase activity more closely correlated with casein production and peak activity was observed at the time of parturition. We conclude that 1,25-dihydroxyvitamin D3 receptor content increases during pregnancy and lactation and may be involved in maintaining milk calcium concentration.