Effects of vitamin D3 on proliferation of cancer cells in vitro

The principal cause of death from most forms of cancer is metastatic disease. Cancer cells appear to grow quickly out of the control of the normal host regulatory mechanisms. Many factors contribute to this unrestrained proliferation, including increased metalloproteinase activity causing degradation of the extracellular matrix surrounding cancer cells, angiogenesis permitting easy access of the cells to the bloodstream and decrease or loss of programmed cell death, or apoptosis, an important mechanism for removal of abnormal or senescent cells. Treatment modalities targeted towards arresting cancer cell proliferation and spread are needed to improve the survival of patients with cancer. Vitamin D3, 1,25-dihydroxychole-calciferol D3, has been shown to induce apoptosis in the human breast cancer cell line, MCF-7. We have studied the effects of three concentrations of vitamin D3 on the human breast cancer cell line, MDA-MB-435, the human prostate cancer cell line, LNCaP, and a human osteosarcoma cell line, U20S. We report here that vitamin D3 strikingly inhibits cell proliferation and induces apoptosis in all three cell lines.

Allosteric interaction of the 1alpha,25-dihydroxyvitamin D3 receptor and the retinoid X receptor on DNA

Genomic actions of the hormone 1alpha,25-dihydroxy-vitamin D3(VD) are mediated by the transcription factor VDR, which is a member of the nuclear receptor superfamily. VDR acts in most cases as a heterodimeric complex with the retinoid X receptor (RXR) from specific DNA sequences in the promoter of VD target genes called VD response elements (VDREs). This study describes a mutation (K45A) of the VDR DNA binding domain that enhances the affinity and ligand responsiveness of VDR-RXR heterodimers on some VDREs. In analogy to a homologous mutation in the glucocorticoid receptor (K461A), this lysine residue appears to function as an allosteric 'lock'. Interestingly, overexpression of RXR was found to reduce the responsiveness and sensitivity of wild type VDR to VD, but enhance the response of VDRK45A. Moreover, the transactivation domains of both VDR and RXR were shown to be essential for obtaining responsiveness of the heterodimers to VD and 9- cis retinoic acid (the RXR ligand). This indicates that RXR is an active rather than silent partner of the VDR on the VDREs tested. Taken together, transactivation by VDR-RXR heterodimers can be triggered individually by all components of the protein-DNA complex, but full potency appears to be reached through allosteric interaction.

Sensitive induction of apoptosis in breast cancer cells by a novel 1,25-dihydroxyvitamin D3 analogue shows relation to promoter selectivity

The biologically active form of vitamin D3, the nuclear hormone 1 alpha,25-dihydroxyvitamin D3 (VD), is an important regulator of cellular growth, differentiation, and death. The hormone mediates its action through the activation of the transcription factor VDR, which is a member of the superfamily of nuclear receptors. In most cases the ligand-activated VDR is found in complex with the retinoid X receptor (RXR) and stimulates gene transcription mainly from VD response elements (VDREs) that are formed by two hexameric core binding motifs and are arranged either as a direct repeat spaced by three nucleotides (DR3) or as an inverted palindrome spaced by nine nucleotides (1P9). The two VD analogues CB1093 and EB1089 are both very potent inhibitors of the proliferation of MCF-7 cultured breast cancer cells displaying approximately 100-fold lower IC50 values (0.1 nM) than the natural hormone. In addition, CB1093 is even more potent in vivo than EB1089 in producing regression of experimental mammary tumors. Moreover, both VD analogues induce apoptosis in MCF-7 cells, but CB1093 is effective at concentrations approximately 10-fold lower than EB1089. In accordance, the reduction of Bcl-2 protein expression showed CB1093 to be more potent than EB1089. This suggests that the antiproliferative effect of CB1093 may be related mainly to its apoptosis inducing effect, whereas EB1089 may preferentially have effects on growth arrest. EB1089 is known to result in a selectivity for the activation of IP9-type VDREs, whereas CB1093 shows a preference for the activation of DR3-type VDREs. This promoter selectivity suggests that the effects of VD and its analogues on growth arrest and the induction of apoptosis may be mediated by different primary VD responding genes. In conclusion, CB1093 was found to be a potent inhibitor of rat mammary tumor growth in vivo. CB1093 also displayed a high potency in vitro in the induction of apoptosis, a process that may be linked to a promoter selectivity for DR3-type VDREs.

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.

Expression of integrin subunits and CD44 isoforms in psoriatic skin and effects of topical calcitriol application

Increasing evidence suggests involvement of integrins and CD44 isoforms in the pathogenesis of psoriasis, contributing to uncontrolled keratinocyte proliferation, neovascularization, and invasion of inflammatory cells. We have analyzed immunohistochemically in situ expression of integrins (CD29, CDw49b, CDw49c, CDw49e, CDw49f) and CD44 isoforms (CD44 standard, CD44 var/v6, CD44 v10) on frozen sections of normal and psoriatic skin (nonlesional skin, lesional skin before and along with topical calcitriol treatment). We did not observe visual changes of immunoreactivity in normal as compared to nonlesional psoriatic skin, while the staining pattern of CDw49c, CDw49f, and CD29 was severely altered in untreated lesional psoriatic skin. Most markedly, CDw49c, CDw49f, and CD29 were focally upregulated in suprapapillar epidermal compartments of lesional psoriatic skin, a staining pattern that is in accordance with the phenomenon that was described by Pinkus as "squirting papilla". Additionally, an increased proportion of inflammatory and endothelial cells revealed immunoreactivity for CD44(std.) in untreated lesional psoriatic as compared to nonlesional psoriatic or normal skin. After 8 weeks of topical calcitriol treatment (15 micrograms/g ointment), the staining pattern for CDw49c, CDw49f and CD29 was markedly changed in epidermis of lesional psoriatic skin, reverting to the staining pattern characteristic for the nonlesional psoriatic or normal human skin, although epidermal expression of CDw49f was still upregulated and CDw49e-, CDw49f-, CD29-, and CD44(std.)-immunoreactive inflammatory and endothelial cells were still to be found in the dermal compartment.

Conformational studies of human vitamin-D receptor by antipeptide antibodies, partial proteolytic digestion and ligand binding

We have studied conformational changes of human vitamin-D receptor by using antipeptide antibodies, partial proteolytic digestion and binding of the natural ligand calcitriol or its synthetic analogs. Before exposing either [35S]methionine-labelled in vitro translated human vitamin-D receptor or a recombinant human vitamin-D receptor produced either in Escherichia coli or in Sf9 insect cells to limited proteolysis by trypsin or chymotrypsin, the proteins were treated with calcitriol or its synthetic analogs. The digestion products were analyzed by SDS/PAGE, immunoblotting with polyclonal antipeptide antibodies targeted against different domains of the receptor, and Edman N-terminal sequencing. After limited proteolysis with trypsin, two fragments of Mr 21,000 and Mr 34,000 could be localized into N-terminus and C-terminus of the receptor, respectively, by antipeptide antibodies. We found that treatment with calcitriol or its synthetic analogs leads to differential resistance of the ligand-binding domain of the recombinant receptor to partial proteolysis in vitro. We suggest that this is due to distinct conformational changes in the domain induced by the different ligands. The short N-terminal region and the Zn-finger domain form, however, a protease-resistant structure which is independent on the presence or absence of the ligand. When the C-terminal fragment of Mr 34,000 was further analyzed by Edman N-terminal sequencing, the major cleavage site in the receptor between amino acids Arg173 and His174 was revealed.

1,25-Dihydroxyvitamin D3 induces the expression of vascular endothelial growth factor in osteoblastic cells

Angiogenesis is a fundamental process in skeletal development and repair, and previous studies indicate that vascular endothelial growth factor (VEGF), an endothelial cell-specific angiogenic factor, may be involved in bone formation and repair. Therefore, we studied the hormonal regulation of VEGF expression in SaOS-2 osteoblast-like cells, both at the protein level, and at the transcriptional level by transient transfection experiments. 1,25-Dihydroxyvitamin D3 [1,25-(OH)2D3], increased VEGF expression by approximately 3-fold, and the increase was dose dependent, with maximum stimulation between 1.0 and 10 nM of 1,25-(OH)2D3. Up-regulation of VEGF protein was detected already after 6 h of treatment. VEGF up-regulation was also observed in ROS-17/2.8 and OHS-4 osteoblast-like cells but not in MCF-7 and MDA-MB231 breast carcinoma cells. Dexamethasone (Dex) decreased VEGF expression to 40% of the control, but when added together with 1,25-(OH)2D3, had no effects on the up-regulation of VEGF by 1,25-(OH)2D3. PTH1-34 stimulated weakly VEGF expression, but combined with 1,25-(OH)2D3, resulted in a close to 5-fold stimulation. A 4-day pretreatment of the cells with Dex increased the vitamin D3 receptor expression and resulted in a stronger stimulation of VEGF by 1,25-(OH)2D3, alone or in combination with PTH1-34. The results show that the VEGF promoter is a target of 1,25-(OH)2D3 regulation in osteoblasts, despite the lack of classical vitamin D3 responsive elements. The up-regulation of VEGF in osteoblast-like cells by calciotropic hormones provides additional evidence of the involvement of VEGF in bone metabolism.

Expression of the vitamin D and the retinoid X receptors in Saccharomyces cerevisiae: alternative in vivo models for ligand-induced transactivation

The transcription factors of the nuclear hormone receptor family regulate gene expression via a complex network of macromolecular interactions. The ligand dependent activity of the vitamin D receptor is of particular interest because it modulates gene expression by the heterodimeric interaction with retinoid X receptors. We report here that individual functions of the vitamin D receptor including DNA-binding, homo- and heterodimerization and transactivation can be reconstituted in the yeast Saccharomyces cerevisiae. Interestingly, the simultaneous expression of the native vitamin D receptor and the retinoid X receptor beta resulted in a ligand independent transactivation of the lacZ reporter gene coupled to a mouse osteopontin vitamin D response element. However, homodimerization of the vitamin D receptor and heterodimerization were strongly enhanced upon ligand binding, when the receptors were expressed as fusion proteins with the Gal4 transcription factor in a yeast two-hybrid system. Furthermore, transactivating activity of a Gal4-fused vitamin D receptor was induced by vitamin D in a one-hybrid system devoid of retinoid X receptors. In addition, both Gal4-based systems behaved similar with regard to their dose-dependent response to vitamin D and related compounds when compared to the transcriptional activity of the vitamin D receptor in transiently transfected MCF-7 cells. Our results point out that specific ligands strongly enhanced receptor dimerization and induced transactivation in yeast and in MCF-7 cells. The constitutive transactivation by vitamin D receptor-retinoid X receptor heterodimers in yeast, depending on DNA binding of the receptors, strongly argues for the existence of cofactors, which are absent in yeast, but play a fundamental role in gene regulation in higher eukaryotic organisms.

Calcitriol is a positive effector of adipose differentiation in the OB 17 cell line: relationship with the adipogenic action of triiodothyronine

In a previous report, we showed that physiological concentrations of calcitriol (1 alpha,25-(OH)2 vitamin D3 or VD), markedly stimulated the terminal adipose differentiation of Ob 17 preadipocytes cultured under standard conditions with fetal calf serum (FCS), and increased the differentiating effect of triiodothyronine (T3) reported as a necessary adipogenic factor in these cells. Here, we demonstrate, for the first time, that VD is an intrinsic strong adipogenic factor for the Ob 17 preadipocytes cultured in thyroid hormone-deprived medium (adipogenic concentrations: 0.025-0.25 nM in the presence of stripped FCS, 1-10 pM under serum-free conditions). VD action was potentiated by the coaddition of either T3, or arachidonic acid, two agents which also bear proper adipogenic properties. The efficient concentration ranges of other vitamin D3 metabolites suggest a mediation through the VD nuclear receptor (VDR). An expression of the VDR gene is here demonstrated in the Ob 17 cells, and evidence is given that VDR mRNA level increased during the differentiation process and that this increase is moderately amplified under long term treatment with adipogenic concentrations of VD. Our results strongly suggest that adipose differentiation is under the control of different closely related nuclear receptors acting at an early preadipocyte step and probably in an interchangeable manner depending on the availability of their respective ligands. The existence of an interplay between these receptors in exerting their adipogenic action is suggested.

Functional characterization of a novel type of 1 alpha,25-dihydroxyvitamin D3 response element identified in the mouse c-fos promoter

The seco-steroid 1 alpha,25-dihydroxyvitamin D3 (VD) is known to inhibit cellular proliferation and to induce differentiation as well as programmed cell death (apoptosis). VD is the ligand of the transcription factor VDR, which is a member of the nuclear receptor superfamily. Primary VD responding genes contain a VD response element (VDRE), on which VDR binds as a dimeric complex. The main heterodimeric partner of VDR is the retinoid X receptor (RXR) and the majority of the known natural VDREs are formed by a direct repeat of hexameric core binding motifs spaced by 3 nucleotides. Most of the genes carrying DR3-type VDREs are associated with the hormone's classical function, which is the regulation of calcium homeostasis. Recently, it has been found that inverted palindromic arrangements spaced by 9 nucleotides also form functional VDREs. This paper reports the identification of a novel IP9-type VDRE in the mouse c-fos promoter. This elements is bound with high affinity by VDR-RXR heterodimers and responds at 10-fold lower concentrations to the potent anti-proliferative VD analogue EB1089 than to VD. This suggests that VD may be directly involved in the transcriptional regulation of the cell cycle via the activation of the c-fos gene.

1,25-dihydroxyvitamin D3 inhibits Osteocalcin expression in mouse through an indirect mechanism

1,25-Dihydroxyvitamin D3 (1,25-(OH)2D3), a key regulator of mineral metabolism, regulates the expression of several genes that are expressed in osteoblasts. In particular, in rat and human osteoblasts, 1,25-(OH)2D3 increases the expression of Osteocalcin by interacting, through a hormone-receptor complex, with a vitamin D-responsive element present in the promoter of the genes. Here we show that in mouse, 1,25-(OH)2D3 inhibits the expression of both osteocalcin genes, OG1 and OG2. This inhibition was observed in primary osteoblast cultures and in the whole animal. From sequence inspection, DNA transfection experiments, and DNA binding assays, we could not identify a functional vitamin D-responsive element in the promoter of OG2 or in the first 3.3 kilobases of the OG1 promoter. However, we show that 1,25-(OH)2D3 treatment of primary osteoblasts abolishes the binding of OSF2, an osteoblast-specific activator of transcription that binds to OSE2, a critical osteoblast-specific cis-acting element present in OG1 and OG2 promoters. Consistent with these DNA binding data, a mutation in OSE2 in the OG2 promoter abrogated the inhibitory effect of 1,25-(OH)2D3 treatment on this promoter activity. This study illustrates that 1,25-(OH)2D3 can play different roles in the expression of the same gene in various species and indicates that this regulation in mouse occurs through an indirect mechanism, 1,25-(OH)2D3 acting on a gene genetically located upstream of Osteocalcin.

Transcriptional synergism between vitamin D-responsive elements in the rat 25-hydroxyvitamin D3 24-hydroxylase (CYP24) promoter

Transcription of the CYP24 gene is induced by 1,25-(OH)2D3 through a vitamin D receptor-dependent process. The functional activities of three possible vitamin D response elements (VDREs), located on the antisense strand of the rat CYP24 promoter, were investigated by transient expression of native and mutant promoter constructs in COS-1, JTC-12, and ROS 17/2.8 cells. A putative VDRE with a half-site spacing of 6 base pairs at -249/-232 (VDRE-3) did not contribute to 1,25-(OH)2D3 induced expression in the native promoter, although activity has been reported when the element was fused to the heterologous thymidine kinase promoter. Two VDREs with half-site spacings of 3 base pairs at -150/-136 and -258/-244 (VDRE-1 and VDRE-2, respectively), showed transcriptional synergism in COS-1 cells when treated with 1,25-(OH)2D3 (10(-7) to 10(-11) M). The contribution of both VDREs was hormone-concentration dependent from 10(-10) to 10(-12) M, with VDRE-1 demonstrating greatest sensitivity to 1,25-(OH)2D3. Transactivation by VDRE-1 was always greater than VDRE-2, but the converse was observed for the binding of vitamin D receptor-retinoid X receptor complex by each VDRE in gel mobility shift assays. The synergy observed between VDRE-1 and VDRE-2 may have important implications in cellular responses to different circulating levels of 1,25-(OH)2D3.

The high affinity ligand binding conformation of the nuclear 1,25-dihydroxyvitamin D3 receptor is functionally linked to the transactivation domain 2 (AF-2)

The nuclear receptor for 1,25-dihydroxyvitamin D3 (VD), VDR, is a transcription factor that mediates all genomic actions of the hormone. The activation of VDR by ligand induces a conformational change within its ligand binding domain (LBD). Due to the lack of a crystal structure analysis, biochemical methods have to be applied in order to investigate the details of this receptor-ligand interaction. The limited protease digestion assay can be used as a tool for the determination of a functional dissociation constant (K(df)) of VDR with any potential ligand. This method provided with the natural hormone VD two protease-resistant fragments of the VDR LBD and with the 20-epi conformation of VD, known as MC1288, even an additional fragment of intermediate size. These fragments were interpreted as different receptor conformations and their decreasing size was found to be associated with decreasing ligand binding affinity. A critical amino acid for VDR's high ligand binding conformation has been identified by C-terminal receptor truncations and point mutations as phenylalanine 422. This amino acid appears to directly contact the ligand and belongs to the ligand-inducible activation function-2 (AF-2) domain. Moreover, functional assays supported the observation that high affinity ligand binding is directly linked to transactivation function.

Potent gene regulatory and antiproliferative activities of 20-methyl analogues of 1,25 dihydroxyvitamin D3

The biological active form of vitamin D3, 1,25-dihydroxyvitamin D3 (VD), regulates cellular growth and differentiation. This provides the hormone with an interesting therapeutic potential. However, hypercalcemia is a side effect, which is caused by VD's classical action, the regulation of calcium homeostasis. This made the need for VD analogues with selectively increased cell regulatory properties. Studies with 20-epi analogues pointed out the importance of the carbon-20 position and led to the development of 20-methyl derivatives of VD. In this report the biological properties of the compounds ZK161422 and ZK157202, which are 20-methyl- and 20-methyl-23-eneanalogues, respectively, have been analyzed in comparison with VD. Both compounds show about 2-fold lower affinity to the VD receptor (VDR) than VD. However, compared to VD, their antiproliferative effect is up to 30-fold higher on human peripheral blood mononuclear cells and even up to 300-fold higher on human breast cancer MCF-7 cells. Whereas the hypercalcemic effect for ZK157202 is also increased 10-fold, ZK161422 has the same calcium-mobilizing potency as VD. Moreover, ZK161422, but not ZK157202, showed preference for gene activation from a promoter carrying a VD response element with a palindromic arrangement of two hexameric receptor binding sites spaced by 9 nucleotides (IP9) rather than for activation from a response element formed by a direct repeat spaced by 3 nucleotides (DR3). This observation supports a model, in which promoter selectivity reflects the selectively increased antiproliferative effect of VD analogues.

The potent anti-proliferative effect of 20-epi analogues of 1,25 dihydroxyvitamin D3 in human breast-cancer MCF-7 cells is related to promoter selectivity

The hormone 1,25-dihydroxyvitamin D3 (VD) has a potential use as an anti-tumor agent, but its clinical applications are restricted by its strong calcemic activity. This has led to the development of VD analogues with selectively increased growth-inhibitory activity. One of the most potent analogues is KH1060, which is a 20-epi-22-oxa-derivative of VD. In human breast cancer MCF-7 cells, we studied the growth-inhibitory activities of a set of 8 analogues that cover conservative structural changes from 20-epi-VD (MC1288) to KH1060. In the same cellular system, we analyzed the potential of these 8 analogues to stimulate reporter gene activity driven by a recently discovered novel-type VD response element. We found that this VD response element is more appropriate than classical VD response elements to correlate anti-proliferative effects of VD analogues with their gene-regulatory properties.

High-affinity nuclear receptor binding of 20-epi analogues of 1,25-dihydroxyvitamin D3 correlates well with gene activation

The hormone 1,25-dihydroxyvitamin D3 (VD) has the potential for clinical use in several diseases, such as cancer, osteoporosis, and psoriasis. The action of VD is mediated by primary responding genes that contain in their promoter region a binding site for the transcription factor VDR. Most of the known VD response elements are formed by a direct repeat of two hexameric core binding motifs spaced by three nucleotides (DR3) bound by a heterodimer of VDR and the retinoid X receptor (RXR). Various VD analogues have been developed in order to optimize the therapeutic profile of VD. This report presents a novel experimental system that may help in the understanding of the structural basis for the high potency of a VD analogue like KH1060, which is a 20-epi-22-oxa-derivative of VD. In human breast cancer cells, MCF-7, the half-maximal gene activation values for KH1060 and seven of its structural precursors were determined on a DR3-type VD response element. These eight analogues cover conservative structural changes from 20-epi-VD (MC1288) to KH1060. With a modified version of the limited protease digestion assay the functional affinity of the analogues to VDR was measured. The functional receptor affinity of the eight analogues was found to be directly proportional to their potency in VDR-RXR-mediated gene activity.

Identification of natural monomeric response elements of the nuclear receptor RZR/ROR. They also bind COUP-TF homodimers

The receptor RZR/ROR is an important member of the nuclear receptor superfamily and has recently been shown to be the nuclear receptor for the pineal gland hormone melatonin. RZR/ROR binds as a monomer to DNA, and the human 5-lipoxygenase gene has been identified as the first RZR/ROR/melatonin-responding gene. Another prominent nuclear receptor is COUP-TF, which binds as a dimer to DNA. In this study, the sequences of known promoter regions of genes that may be involved in the physiological action of melatonin have been screened for putative monomeric RZR/ROR response elements. The binding of RZR/ROR and COUP-TF was compared and quantified on a set of 12 putative response elements. Interestingly, COUP-TF homodimers were found to bind with high affinity to some of the monomeric RZR/ROR response elements. Four RZR/ROR response elements, found in the genes of the mouse bifunctional enzyme, the rat bone sialoprotein, mouse Purkinje cell protein 2, and human p21(WAF1/CIP1), were shown to be inducible by melatonin under conditions of low constitutive activity. Surprisingly, the constitutive activity of COUP-TF was also stimulated by an unknown serum compound. The novel Purkinje cell protein 2 and p21(WAF1/CIP1) RZR/ROR/melatonin-responding genes may be the key for understanding the role of RZR/RORalpha in the mouse mutation staggerer and the antiproliferative action of melatonin, respectively.

Calcitriol and lexicalcitol (KH1060) inhibit the growth of human breast adenocarcinoma cells by enhancing transforming growth factor-beta production

The mechanisms involved in the antiproliferative action of calcitriol (1 alpha, 25(OH)2D3) were investigated using human breast carcinoma epithelial cells (the MCF-7 cell line). Calcitriol and KH1060, a synthetic analog, inhibited cell growth in a time-and dose-dependent way. The substances similarly stimulated total TGF-beta secretion after 24 hours, and Northern blot analyses showed that mRNA levels for TGF-beta 1 were increased, as well. When MCF-7 cells were co-incubated with calcitriol and a neutralizing anti TGF-beta 1, beta 2, beta 3 antibody, growth inhibition was completely abrogated. With KH1060, the antibody could only partly block growth inhibition. This study shows that TGF-beta is involved in the growth response to calcitriol and KH1060 in MCF-7 cells.

The vitamin D(3) receptor in the context of the nuclear receptor superfamily : The central role of the retinoid X receptor

The nuclear hormone 1 α,25-dihydroxyvitamin D(3) (VD) is an important regulator of calcium homeostasis and is also a modulator of the cell cycle. The genomic actions of the hormone are mediated by a single transcription factor, the vitamin D(3) receptor (VDR). On the majority of the known VD response elements, VDR binds as heterodimeric complex with the retinoid X receptor (RXR), which is a member of the nuclear receptor superfamily like VDR. RXR supports not only the DNA binding affinity and specificity of VDR, but allosterically also its transactivation properties. Moreover RXR is a partner in other hormone response systems, which supports the idea that the different nuclear hormone signaling pathways are functionally linked.

Identification of a vitamin D3 response element in the fibronectin gene that is bound by a vitamin D3 receptor homodimer

Fibronectin (FN) is an important adhesive noncollagenous glycoprotein involved in maintenance of the extracellular matrix and cell adhesiveness, loss of which has been implicated in the metastatic potential of cells. Regulation of FN occurs at the transcriptional level by the active metabolite of vitamin D3, 1,25-dihydroxyvitamin D3 (1,25-(OH)2D3). Transient transfection of homologous and heterologous promoter reporter constructs into ROS 17/2.8 (rat osteosarcoma), NIH 3T3 (mouse fibroblast), and MCF-7 (human mammary carcinoma) cell lines showed a consistent two- to threefold induction of transcription when stimulated with 1,25-(OH)2D3. These heterologous promoter transfection studies with gel shift analysis locate a third, natural DR6-type vitamin D responsive element (VDRE) at nucleotide positions -171 to -154 in the murine FN promoter. Interestingly, this VDRE is also present in rat and human FN promoters. This study shows that 1,25-(OH)2D3 induces FN transcription from an existing elevated basal transcriptional activity by acting through two putative hexameric core binding motifs which bind VDR homodimers. Furthermore, the FN VDRE is the first homodimer-type VDRE that is not overlaid by a DR3-type structure.