Differential effects of 1,25-dihydroxyvitamin D3 and tetradecanoylphorbol acetate on cell cycle and apoptosis of MCF-7 cells and a vitamin D3-resistant variant

Effect of active vitamin D on proliferation, cell cycle and apoptosis in endometriotic stromal cells

RESEARCH QUESTION

What is the effect of vitamin D3 (1,25(OH)₂D₃) on proliferation, cell cycle and apoptosis of endometrial stromal cells (ESC) in endometriotic patients?

DESIGN

ESC isolated from 10 women with endometriosis and 10 healthy controls were treated with 1,25(OH)₂D₃. The proliferation of control endometrial stromal cells (CESC), eutopic endometrial stromal cells (EuESC) and ectopic endometrial stromal cells (EESC) was analysed 72 h after the treatment using methyl thiazolyl tetrazolium assay. Propidium iodide staining and flow cytometry were used to determine the cell cycle distribution in ESC. Annexin V/propidium iodide double staining was used to evaluate apoptosis in ESC.

RESULTS

In the presence of oestrogen, 1,25(OH)₂D₃ treatment inhibited the proliferation of ESC from all three origins (P = 0.009 for CESC, P = 0.005 for EuESC and P < 0.001 for EESC). The percentage of S phase cells in EESC was higher than in EuESC and CESC (P = 0.002 and P = 0.001, respectively). The percentage of S phase cells in EuESC was higher than in CESC (P = 0.005). The percentage of G1 phase cells in EESC was lower than that of EuESC and CESC (P = 0.003 and P = 0.002, respectively) and the percentage of G1 phase cells in EuESC was lower than that of CESC (P = 0.007). Moreover, 1,25(OH)₂D₃ inhibited cell cycle regardless of cell type (P = 0.002 in EESC, P = 0.001 in EuESC and P = 0.014 in CESC), but in the absence of oestrogen, inhibited cell cycle only in EuESC (P = 0.012).

CONCLUSIONS

Although 1,25(OH)₂D₃ increased apoptotic and necrotic cells and decreased live cells in the EuESC and EESC, it did not affect apoptosis in CESC and only increased necrotic cells. These findings indicate that 1,25(OH)₂D₃ potentially has a growth-inhibiting and pro-apoptotic effect on ESC from endometriotic patients.

Vitamin D in prostate cancer

Signaling through the vitamin D receptor has been shown to be biologically active and important in a number of preclinical studies in prostate and other cancers. Epidemiologic data also indicate that vitamin D signaling may be important in the cause and prognosis of prostate and other cancers. These data indicate that perturbation of vitamin D signaling may be a target for the prevention and treatment of prostate cancer. Large studies of vitamin D supplementation will be required to determine whether these observations can be translated into prevention strategies. This paper reviews the available data in the use of vitamin D compounds in the treatment of prostate cancer. Clinical data are limited which support the use of vitamin D compounds in the management of men with prostate cancer. However, clinical trials guided by existing preclinical data are limited.

Calcitriol and cancer therapy: A missed opportunity

The vitamin D receptor is expressed in most tissues of the body - and the cancers that arise from those tissues. The vitamin D signaling pathway is active in those tissues and cancers. This is at least consistent with the hypothesis that perturbing this signaling may have a favorable effect on the genesis and growth of cancers. Epidemiologic data indicate that vitamin D signaling may be important in the initiation and outcome of a number of types of cancer. Many studies have shown that calcitriol (1,25 dihydroxycholecalciferol) and other vitamin D compounds have antiproliferative, pro-apoptotic, anti-cell migration and antiangiogenic activity in a number of preclinical studies in many different cancer types. Unfortunately, the assessment of the activity of calcitriol or other vitamin D analogues in the treatment of cancer, as single agents or in combination with other anticancer agents has been stymied by the failure to adhere to commonly accepted principles of drug development and clinical trials conduct.

Vitamin D and cancer: a review of molecular mechanisms

The population-based association between low vitamin D status and increased cancer risk can be inconsistent, but it is now generally accepted. These relationships link low serum 25OHD (25-hydroxyvitamin D) levels to cancer, whereas cell-based studies show that the metabolite 1,25(OH)2D (1,25-dihydroxyvitamin D) is a biologically active metabolite that works through vitamin D receptor to regulate gene transcription. In the present review we discuss the literature relevant to the molecular events that may account for the beneficial impact of vitamin D on cancer prevention or treatment. These data show that although vitamin D-induced growth arrest and apoptosis of tumour cells or their non-neoplastic progenitors are plausible mechanisms, other chemoprotective mechanisms are also worthy of consideration. These alternative mechanisms include enhancing DNA repair, antioxidant protection and immunomodulation. In addition, other cell targets, such as the stromal cells, endothelial cells and cells of the immune system, may be regulated by 1,25(OH)2D and contribute to vitamin D-mediated cancer prevention.

Cardiorenal syndrome and vitamin D receptor activation in chronic kidney disease

Cardiorenal syndrome (CRS) refers to a constellation of conditions whereby heart and kidney diseases are pathophysiologically connected. For clinical purposes, it would be more appropriate to emphasize the pathophysiological pathways to classify CRS into: (1) hemodynamic, (2) atherosclerotic, (3) uremic, (4) neurohumoral, (5) anemic–hematologic, (6) inflammatory–oxidative, (7) vitamin D receptor (VDR) and/or FGF23-, and (8) multifactorial CRS. In recent years, there have been a preponderance data indicating that vitamin D and VDR play an important role in the combination of renal and cardiac diseases. This review focuses on some important findings about VDR activation and its role in CRS, which exists frequently in chronic kidney disease patients and is a main cause of morbidity and mortality. Pathophysiological pathways related to suboptimal or defective VDR activation may play a role in causing or aggravating CRS. VDR activation using newer agents including vitamin D mimetics (such as paricalcitol and maxacalcitol) are promising agents, which may be related to their selectivity in activating VDR by means of attracting different post-D-complex cofactors. Some, but not all, studies have confirmed the survival advantages of D-mimetics as compared to non-selective VDR activators. Higher doses of D-mimetic per unit of parathyroid hormone (paricalcitol to parathyroid hormone ratio) is associated with greater survival, and the survival advantages of African American dialysis patients could be explained by higher doses of paricalcitol (>10 μg/week). More studies are needed to verify these data and to explore additional avenues for CRS management via modulating VDR pathway.

Phosphorylation at serine 482 affects stability of NF90 and its functional role in mitosis

OBJECTIVES

NF90 is a multifunctional double-strand RNA binding protein with documented roles in transcription, mRNA stability, translation, RNA processing and transport, and mitosis. It is a phosphoprotein that interacts with, and is a substrate for, several protein kinases. The study described here was initiated to gain better understanding of specific NF90 phosphorylation sites and their relationship to mechanisms by which NF90 performs its various functions.

MATERIALS AND METHODS

Phosphoproteomic studies have identified NF90 serine 482 (S482) as a major phosphorylation site in vivo. Site-specific mutations were introduced at this site and the mutated proteins were expressed in MCF7 cells by transfection. Western blotting was used to examine NF90 expression, stability, and responsiveness to protein kinase activators and inhibitors. Flow cytometry was used to examine effects of NF90 mutation on cell cycle progression.

RESULTS

Non-phosphorylatable mutant S482A was unstable compared to phosphomimetic S482E mutant. NF90-S482A expression was greatly enhanced by inhibiting proteasomal degradation or by activating PKC. Identical treatments had little effect on NF90-S482E. In contrast to WT NF90 or NF90-S482E, cells stably expressing NF90-S482A accumulated in M phase when treated with TPA.

CONCLUSIONS

Phosphorylation at S482 is important for NF90 stability and in regulating its functional role during mitosis. Based on the sequence surrounding S482, mitotic kinase PLK1 is a strong candidate for the enzyme that phosphorylates NF90 at this site.

Human breast tumor slices: a model for identification of vitamin D regulated genes in the tumor microenvironment

While many studies have addressed the direct effects of 1alpha,25(OH)2D3 on breast cancer (BC) cells, stromal-epithelial interactions, which are important for the tumor development, have been largely ignored. In addition, high concentrations of the hormone, which cannot be attained in vivo, have been used. Our aim was to establish a more physiological breast cancer model, represented by BC tissue slices, which maintain epithelial-mesenchymal interactions, cultured with a relatively low 1alpha,25(OH)2D3 concentration, in order to evaluate the vitamin D pathway. Freshly excised human BC samples were sliced and cultured in complete culture media containing vehicle, 0.5 nM or 100 nM 1alpha,25(OH)2D3 for 24 h. BC slices remained viable for at least 24 h, as evaluated by preserved tissue morphology in hematoxylin and eosin (HE) stained sections and bromodeoxyuridine (BrdU) incorporation by 10% of tumor cells. VDR mRNA expression was detected in all samples and CYP24A1 mRNA expression was induced by 1alpha,25(OH)2D3 in both concentrations (but mainly with 100 nM). Our results indicate that the vitamin D signaling pathway is functional in BC slices, a model which preserves stromal-epithelial interactions and mimics in vivo conditions.

Nuclear targeting of cyclin-dependent kinase 2 reveals essential roles of cyclin-dependent kinase 2 localization and cyclin E in vitamin D-mediated growth inhibition

1,25-Dihydroxyvitamin D(3) (1,25-(OH)(2)D(3)), inhibits proliferation of a variety of cell types including adenocarcinoma of the prostate. We have previously shown that 1,25-(OH)(2)D(3) increases the stability of the cyclin-dependent kinase inhibitor p27(KIP1), decreases cyclin-dependent kinase 2 (CDK2) activity, and promotes G(1) phase accumulation in human prostate cancer cells. These effects correlate with cytoplasmic relocalization of CDK2. In this study, we investigated the role of CDK2 cytoplasmic relocalization in the antiproliferative effects of 1,25-(OH)(2)D(3). CDK2 was found to be necessary for prostate cancer cell proliferation. Although induced by 1,25-(OH)(2)D(3), the cyclin-dependent kinase inhibitor p27(KIP1) was dispensable for 1,25-(OH)(2)D(3)-mediated growth inhibition. Reduction in CDK2 activity by 1,25-(OH)(2)D(3) was associated with decreased T160 phosphorylation, a residue whose phosphorylation in the nucleus is essential for CDK2 activity. Ectopic expression of cyclin E was sufficient to overcome 1,25-(OH)(2)D(3)-mediated cytoplasmic mislocalization of CDK2 and all antiproliferative effects of 1,25-(OH)(2)D(3), yet endogenous levels of cyclin E or binding to CDK2 were not affected by 1,25-(OH)(2)D(3). Similarly, knockdown of the CDK2 substrate retinoblastoma, which causes cyclin E up-regulation, resulted in resistance to 1,25-(OH)(2)D(3)-mediated growth inhibition. Human prostate cancer cells resistant to growth inhibition by 1,25-(OH)(2)D(3) but retaining fully functional vitamin D receptors were developed. These cells did not exhibit 1,25-(OH)(2)D(3)-mediated cytoplasmic relocalization of CDK2. Targeting CDK2 to the nucleus of 1,25-(OH)(2)D(3)-sensitive cancer cells blocked G(1) accumulation and growth inhibition by 1,25-(OH)(2)D(3). These data establish central roles for CDK2 nuclear-cytoplasmic trafficking and cyclin E in the mechanism of 1,25-(OH)(2)D(3)-mediated growth inhibition in prostate cancer cells.

Molecular actions of vitamin D contributing to cancer prevention

The population-based relationship between low vitamin D status and increased cancer risk is now generally accepted. While these relationships are between serum 25 hydroxyvitamin D and cancer, cell-based studies show that the metabolite 1,25 dihydroxyvitamin D is biologically active and influences cell biology relevant to cancer through vitamin D receptor-mediated gene transcription. This review examines this paradox and also discusses the cell and gene targets influenced by 1,25 dihydroxyvitamin D that may account for the anti-cancer actions of vitamin D. A review of the literature shows that while vitamin D-induced growth arrest and apoptosis of tumor cells or their non-neoplastic progenitors are plausible mechanisms, other gene targets related to DNA repair and immunomodulation, and other cell targets such as the stromal cells and cells of the immune system, may be regulated by 1,25 dihydroxyvitamin D and contribute to vitamin D mediated cancer prevention.

BAF180 is a critical regulator of p21 induction and a tumor suppressor mutated in breast cancer

Screening for tumor suppressor genes in breast cancer revealed multiple truncating mutations of PB1, which encodes the BAF180 subunit of the PBAF chromatin remodeling complex. Mutation was associated with loss of heterozygosity of the wild-type allele. BAF180 complementation of BAF180-mutant tumor cells caused G(1) arrest that was dependent on increased expression of the cyclin/cyclin-dependent kinase inhibitor p21/WAF1/CIP1. Endogenous wild-type BAF180 bound to the p21 promoter and was required for proper p21 expression and G(1) arrest after transforming growth factor-beta and gamma-radiation treatment. BAF180 thus functions on two tumor suppressor signaling pathways as a physiologic mediator of p21 expression. We conclude that BAF180 suppresses tumorigenesis, at least in part, through its ability to regulate p21.

Synergism between vitamin D and secreted protein acidic and rich in cysteine-induced apoptosis and growth inhibition results in increased susceptibility of therapy-resistant colorectal cancer cells to chemotherapy

Patients with advanced colorectal cancer continue to have poor outcomes because of therapy-refractory disease. We previously showed that secreted protein acidic and rich in cysteine (SPARC) gene and protein could function as a chemotherapy sensitizer by enhancing tumor regression in response to radiation and chemotherapy in tumor xenograft models of chemotherapy-resistant tumors. This function of SPARC was gleamed from a microarray analysis that also revealed down-regulation of the vitamin D receptor (VDR) in therapy-refractory colorectal cancer cells. This study examines the potential synergistic effect of SPARC and vitamin D, which up-regulates VDR, in enhancing chemotherapy response in colorectal cancer. Using MIP101 colorectal cancer cell lines and SPARC-overexpressing MIP101 cells, we were able to show that, in the presence of SPARC, exposure to low doses of 1alpha,25-dihydroxyvitamin D(3) significantly reduces cell viability, enhances chemotherapy-induced apoptosis, and inhibits the growth of colorectal cancer cells. Moreover, in tumor xenograft mouse models, up-regulation of VDR was seen in tumors that had the greatest regression following treatment that combined SPARC with chemotherapy. Therefore, our findings reveal a synergistic effect between SPARC and low doses of 1alpha,25-dihydroxyvitamin D(3) that further augments the sensitivity of tumors to chemotherapy. This combination may prove to be a useful adjunct in the treatment of colorectal cancer, especially in those patients with therapy-refractory disease.

Identification of novel mediators of Vitamin D signaling and 1,25(OH)2D3 resistance in mammary cells

Since the discovery of the Vitamin D receptor (VDR) in mammary cells, the role of the Vitamin D signaling pathway in normal glandular function and in breast cancer has been extensively explored. In vitro studies have demonstrated that the VDR ligand, 1,25(OH)(2)D(3), modulates key proteins involved in signaling proliferation, differentiation and survival of normal mammary epithelial cells. Anti-proliferative and pro-differentiating effects of 1,25(OH)(2)D(3) have also been observed in VDR positive breast cancer cells, indicating that transformation per se does not abolish Vitamin D signaling. However, many breast cancer cell lines are less sensitive to 1,25(OH)(2)D(3) than normal mammary epithelial cells. Reduced sensitivity to 1,25(OH)(2)D(3) has been linked to alterations in Vitamin D metabolizing enzymes as well as down regulation of VDR expression or function. In this report, we describe results from a proteomics screening approach used to search for proteins involved in dictating sensitivity or resistance to Vitamin D mediated apoptosis in breast cancer cells. Several proteins not previously linked to 1,25(OH)(2)D(3) signaling were identified with this approach, and a distinct subset of proteins was linked to 1,25(OH)(2)D(3) resistance. Follow-up studies to determine the relevance of these proteins to Vitamin D signaling in general are in progress.

Characterization of Vitamin D insensitive prostate cancer cells

The antitumor effects of 1,25-dihydroxyvitamin D(3) (calcitriol) are being exploited for prevention and treatment of prostate cancer (CaP). These studies examined the antiproliferative effects of calcitriol in primary cell cultures derived from transgenic adenocarcinoma of mouse prostate (TRAMP) mice chronically treated with calcitriol (20 microg/kg) or vehicle 3x/week from 4 weeks-of-age until palpable tumors developed. This is a report on the response of two representative control (Vitamin D naïve, naïve) and calcitriol-treated (Vitamin D insensitive, VDI) cells to calcitriol. VDI cells were less sensitive to calcitriol based on less cell growth inhibition and less inhibition of DNA synthesis as measured by MTT and BrdU incorporation assays. Similarly, VDI cells were less sensitive to growth inhibition by the vitamin analog, 19-nor-1alpha,25-dihydroxyvitamin D(2) (paricalcitol). There was no change in apoptosis following treatment of naïve and VDI cells with calcitriol. Vitamin D receptor (VDR) expression was up-regulated by calcitriol in both naïve and VDI cells. In addition, calcitriol induced the Vitamin D metabolizing enzyme, 24-hydroxylase (cyp24) mRNA and enzyme activity similarly in naïve and VDI cells as measured by RT-PCR and HPLC, respectively. In summary, VDI cells are less responsive to the antiproliferative effects of calcitriol. Understanding Vitamin D insensitivity will further clinical development of Vitamin D compounds for prevention and treatment of CaP.

Pharmacokinetics and safety of ILX23-7553, a non-calcemic-vitamin D3 analogue, in a phase I study of patients with advanced malignancies

PURPOSE

Differentiation therapy is an alternative to chemotherapy with potentially less toxicity, improved quality of life, and survival. We conducted a phase I trial of ILX23-7553, a formulation of 1,25-dihydroxy-16-ene-23-yne-vitamin D(3), a 1,25-dihydroxyvitamin D(3) analog with preclinically demonstrated antitumor and differentiating effects and diminished hypercalcemic effects.

PATIENTS AND METHODS

The protocol consisted of five daily oral treatments during 14-day cycles at 15 dose levels from 1.3 to 45.0 mug/m(2)/day. We treated 42 heavily pretreated patients who had a variety of malignancies with 162 treatment cycles, and obtained pharmacokinetics from three patients at the two highest dose levels.

RESULTS

There were no grade 3 or 4 toxicities. Grade 1-2 toxicities included diarrhea, nausea, fatigue, constipation, and one grade 1 hypercalcemia. Average day 6 calcium was 9.26 +/- 0.55 mg/dl in cycle 1 and 9.30 +/- 0.67 mg/dl in cycle 2. Pharmacokinetics at dose levels 14 (40 mug/m(2)/day) (1 patient) and 15 (45 mug/m(2)/day) (2 patients) demonstrated an average C(max) of 30.4 +/- 7.8 pg/ml (0.07 nM) and 104 +/- 38.2 pg/ml (0.25 nM), and AUCs of 222.5 +/- 225.2 pg.h/ml and 855 +/- 536 pg h/ml, respectively. Eight patients (19%) had stable disease. While in vitro effects have been reported at these concentrations, they were at least 10-fold lower than ED(50)s, and the study was terminated before an MTD was reached.

CONCLUSION

The drug is safe and has potential benefits at serum concentrations where effects begin to be noted in vitro. Further study is needed with a reformulated higher unit dose compound to determine the safety and efficacy of higher serum concentrations.

Inhibition of MG-63 cell cycle progression by synthetic vitamin D3 analogs mediated by p27, Cdk2, cyclin E, and the retinoblastoma protein

Progression through eukaryotic cell division cycle is regulated by synergistic activities of both positive and negative regulatory factors. The active form of vitamin D(3) (1alpha,25(OH)(2)D(3), 1,25D) and a number of its synthetic analogs have been shown to arrest cells in the G(1) phase of the cell cycle. In the present study, 1alpha,25(OH)(2)D(3) and the analogs KH1060, EB1089, and CB1093 were used to study the mechanism of the cell cycle arrest and to compare the effectiveness of these compounds in human MG-63 osteosarcoma cells. The 20-epi analogs KH1060 and CB1093, as well as the 20-normal analog EB1089, were found to be more potent than 1alpha,25(OH)(2)D(3) in inhibiting cell proliferation and arresting the MG-63 cells in the G(1) phase. These analogs were more active than 1alpha,25(OH)(2)D(3) in increasing the cyclin dependent kinase inhibitor p27 protein levels (approximately 2.3-2.5-fold compared to 1alpha,25(OH)(2)D(3)) by both increasing its formation and decreasing its degradation rate. The increased p27 formation was accompanied by stabilization of binding of nuclear proteins to the Sp1+NF-Y responsive promoter region of the p27 gene. The increase in p27 protein levels and the simultaneous decrease in cyclin E protein levels was accompanied by decreased Cdk2 kinase activity, retinoblastoma (Rb) protein hypophosphorylation and, finally, cell cycle arrest in the G(1) phase. In summary, the analogs KH1060, EB1089, and CB1093 keep Rb protein in its growth-suppressing, hypophosphorylated form and prevent cell cycle progression through the restriction point. Therefore, these synthetic vitamin D(3) analogs may be potential candidates for treating diseases, where cell cycle regulation is needed.

Characterization of a novel hexameric repeat DNA sequence in the promoter of the immediate early gene, IEX-1, that mediates 1alpha,25-dihydroxyvitamin D(3)-associated IEX-1 gene repression

1alpha,25-Dihydroxyvitamin D(3)(1alpha,25(OH)(2)D(3)), the active metabolite of vitamin D(3), mediates anti-proliferative effects in cells by regulating the expression of 1alpha,25(OH)(2)D(3)-responsive genes. The expression of the proliferation-promoting Immediate Early gene X-1 (IEX-1) is reduced by 1alpha,25(OH)(2)D(3) through unknown mechanisms. Here we report the presence of a novel inhibitory hexameric repeat DNA response element in the promoter of the human IEX-1 gene that mediates 1alpha,25(OH)(2)D(3)-associated IEX-1 gene repression. To localize a vitamin D sensitive DNA response element we transfected the keratinocyte-like cell line, HaCaT, (referred as HaCaT) with a series of plasmids containing full-length and truncated IEX-1 promoter elements fused to the luciferase reporter gene in the absence or presence of 1alpha,25(OH)(2)D(3), and we performed electrophoretic gel mobility assays in the presence of receptors for 1alpha,25(OH)(2)D(3) (vitamin D receptor, VDR) and 9-cis-retinoic acid (RXRalpha). We mapped a negative response element between nt -405 and -391(15 bp) of theIEX-1 promoter (5'-TGAACC AGG GAGTCA-3') that mediates transcriptional inhibition in response to 1alpha,25(OH)(2)D(3) and which requires expression of both nuclear receptors for 1alpha,25(OH)(2)D(3) and 9-cis-retinoic acid. Our data indicate that the physiological repression of IEX-1 gene expression by 1alpha,25(OH)(2)D(3) is directly mediated by nuclear VDR/RXRalpha heterodimers through a specific transcriptional element.

Does melatonin induce apoptosis in MCF-7 human breast cancer cells in vitro?

Melatonin inhibits proliferation of the estrogen-responsive MCF-7 human breast cancer cells. The objective of this work was to assess whether melatonin not only regulates MCF-7 cell proliferation but also induces apoptosis. In this experiment we used 1,25-dihydroxycholecalciferol (D3) as a positive control because it inhibits MCF-7 cell proliferation and induces apoptosis. MCF-7 cells were cultured with either I nM melatonin, 100 nM D3 or its diluent to determine their effects on cell proliferation, cell viability, cell-cycle phase distribution, population of apoptotic cells, and expression of p53, p21WAF1, bcl-2, bcl-X(L) and bax proteins. After 24 or 48 hr of incubation, both melatonin and D3-treatment significantly decreased the number of viable cells in relation to the controls, although no differences in cell viability were observed between the treatments. The incidence of apoptosis, measured as the population of cells falling in the sub-G1 region of the DNA histogram, or by the TUNEL reaction, was similar in melatonin-treated and control cells whereas, as expected, apoptosis was higher among cells treated with D3 than in controls. The expression of p53 and p21WAF1 proteins significantly increased after 24 or 48 hr of incubation with either melatonin or D3. No significant changes in bcl-2, bcl-XL and bax mRNAs were detected after treatment with melatonin whereas in D3-treated cells, a significant drop in bcl-XL was observed. These data support the hypothesis that melatonin reduces MCF-7 cell proliferation by modulating cell-cycle length through the control of the p53-p21 pathway, but without clearly inducing apoptosis.

MCF-7/VD(R): a new vitamin D resistant cell line

Several in vitro and in vivo experiments have demonstrated potent cell regulatory effects of vitamin D compounds in cancer cells. Moreover, a promising phase I study with the vitamin D analogue Seocalcitol (EB 1089) in patients with advanced breast and colon cancer has already been carried out and more clinical trials evaluating the clinical effectiveness of EB 1089 in other cancer types are in progress (Mørk Hansen et al. [2000a]). However, only little is known about the mechanisms underlying the actions of vitamin D or about the possible development of drug resistance in the patients. Therefore, in an attempt to gain more insight into these aspects, we have developed the MCF-7/VD(R) cell line, a stable subclone of the human MCF-7 breast cancer cell line, which is resistant to the growth inhibitory and apoptosis inducing effects of 1alpha,25(OH)(2)D(3). Despite this characteristic, receptor studies on the VDR have clearly demonstrated that the MCF-7/VD(R) cells contain fully functional VDRs, although in a lower number than seen with the parental MCF-7 cells. The regulation of the 24-hydroxylase enzyme appeared to be intact in the MCF-7/VD(R) cells and no differences with regard to growth rate and morphological appearance between the MCF-7/VD(R) cells and the parental MCF-7 cells were observed. Interestingly, however, the sensitivity of the MCF-7/VD(R) cells to the pure anti-estrogen ICI 182,780 was found to be increased. The MCF-7/VD(R) cell line shows characteristics different from those of previously described vitamin D resistant breast cancer cell lines but also some similarities. Together such vitamin D resistant cell lines therefore serve as a useful tool for studying the exact mechanism of action of vitamin D and the development of vitamin D resistance.

Vitamin D(3)-induced apoptosis of murine squamous cell carcinoma cells. Selective induction of caspase-dependent MEK cleavage and up-regulation of MEKK-1

Vitamin D(3) inhibits cell growth and induces apoptosis in several human cancer lines in vitro and in vivo. However, little is known about the molecular events involved in vitamin D(3)-induced apoptosis. Here, we demonstrate that the growth-promoting/pro-survival signaling molecule mitogen-activated protein kinase kinase (MEK) is cleaved in a caspase-dependent manner in murine squamous cell carcinoma (SCC) cells induced to undergo apoptosis by treatment with vitamin D(3). Cleavage resulted in nearly complete loss of full-length MEK and ERK1/2 phosphorylation. ERK1/2 expression was affected only slightly. The phosphorylation and expression of Akt, a kinase regulating a second cell survival pathway, was also inhibited after treatment with vitamin D(3). However, the pro-apoptotic signaling molecule MEKK-1 was up-regulated in both apoptotic and non-apoptotic cells with greater induction and partial N-terminal proteolysis of MEKK-1 observed in apoptotic cells. In contrast to vitamin D(3), cisplatin and etoposide down-regulated Akt levels only modestly, did not promote significant loss of MEK expression, and did not up-regulate MEKK-1. We propose that vitamin D(3) induces apoptosis in SCC cells by a unique mechanism involving selective caspase-dependent MEK cleavage and up-regulation of MEKK-1. Additional evidence is provided that vitamin D(3)-induced apoptosis may be mediated via p38 MAPK.

BAG-1 p50 isoform interacts with the vitamin D receptor and its cellular overexpression inhibits the vitamin D pathway

Human BAG-1 is an anti-apoptotic protein with four protein isoforms (BAG-1 p50, p46, p33, and p29). BAG-1 p46 was originally isolated in a screen for proteins binding to the glucocorticoid receptor; it binds and modulates the action of several members of the nuclear steroid hormone receptor superfamily. The vitamin D receptor (VDR) is another member of this superfamily, and the vitamin D pathway is important for prevention and therapy of osteoporosis, renal failure, cancer, and psoriasis. Therefore, we investigated the effect of the recently isolated BAG-1 p50 on the vitamin D pathway. By use of Far Western blot analysis and glutathione S-transferase BAG-1 p50 binding assays, BAG-1 p50 was demonstrated to interact with the VDR, and the BAG-1 p50 N-terminus was required. In U87 cells that were stably transfected with BAG-1 p50, binding of the VDR to its response element in electrophoretic mobility shift assays was blocked, enhancement of transcriptional activation was inhibited, cell growth rate was enhanced, cell growth inhibition induced by 1,25-dihydroxyvitamin D3 [1,25(OH)2D3] was blocked, and 1,25(OH)2D3-mediated VDR induction was inhibited. These results suggest that BAG-1 p50 is a novel regulator of the vitamin D signaling pathway, and its overexpression may lead to cellular resistance to 1,25(OH)2D3 therapy.

Role of mitochondria and caspases in vitamin D-mediated apoptosis of MCF-7 breast cancer cells

Vitamin D(3) compounds are currently in clinical trials for human breast cancer and offer an alternative approach to anti-hormonal therapies for this disease. 1alpha,25-Dihydroxyvitamin D(3) (1alpha,25(OH)(2)D(3)), the active form of vitamin D(3), induces apoptosis in breast cancer cells and tumors, but the underlying mechanisms are poorly characterized. In these studies, we focused on the role of caspase activation and mitochondrial disruption in 1alpha,25(OH)(2)D(3)-mediated apoptosis in breast cancer cells (MCF-7) in vitro. The effect of 1alpha,25(OH)(2)D(3) on MCF-7 cells was compared with that of tumor necrosis factor alpha, which induces apoptosis via a caspase-dependent pathway. Our major findings are that 1alpha,25(OH)(2)D(3) induces apoptosis in MCF-7 cells by disruption of mitochondrial function, which is associated with Bax translocation to mitochondria, cytochrome c release, and production of reactive oxygen species. Moreover, we show that Bax translocation and mitochondrial disruption do not occur after 1alpha,25(OH)(2)D(3) treatment of a MCF-7 cell clone selected for resistance to 1alpha,25(OH)(2)D(3)-mediated apoptosis. These mitochondrial effects of 1alpha,25(OH)(2)D(3) do not require caspase activation, since they are not blocked by the cell-permeable caspase inhibitor z-Val-Ala-Asp-fluoromethylketone. Although caspase inhibition blocks 1alpha,25(OH)(2)D(3)-mediated events downstream of mitochondria such as poly(ADP-ribose) polymerase cleavage, external display of phosphatidylserine, and DNA fragmentation, MCF-7 cells still execute apoptosis in the presence of z-Val-Ala-Asp-fluoromethylketone, indicating that the commitment to 1alpha,25(OH)(2)D(3)-mediated cell death is caspase-independent.

Functions of 1alpha,25-dihydroxyvitamin D(3) in mammary gland: from normal development to breast cancer

This review examines the role of 1alpha,25(OH)(2)D(3) (1,25D) and the vitamin D(3) receptor in growth regulation of normal and transformed mammary epithelial cells. 1,25D exerts both anti-proliferative and pro-apoptotic functions in transformed mammary cells such as MCF-7. The anti-proliferative effects of 1,25D 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, p27, cyclins and Rb. The pro-apoptotic effects of 1,25D involve alterations in the relative ratios of the bcl-2 family members which regulate mitochondrial integrity. In MCF-7 human breast cancer cells, 1,25D mediated apoptosis is associated with translocation of the pro-apoptotic protein Bax to the mitochondria, generation of reactive oxygen species, dissipation of the mitochondrial membrane potential and release of cytochrome c. These mitochondrial events trigger apoptosis in a caspase-independent manner, since caspase inhibitors do not rescue 1,25D treated cells from death. The potential role of 1,25D in growth and differentiation of normal mammary epithelial cells has been examined in VDR null mice. Initial data indicates a significant decrease in ductal differentiation in VDR null mice compared to age matched wild type mice, reflected as an increased number of undifferentiated terminal end buds in the VDR null mouse. These data suggest that 1,25D promotes differentiation during early mammary gland development. In summary, our studies suggest an expanding role for the vitamin D(3) endocrine system in control of proliferation, differentiation and apoptosis of mammary epithelial cells.

Anti-oestrogen resistant human breast cancer cell lines are more sensitive towards treatment with the vitamin D analogue EB1089 than parent MCF-7 cells

Most breast cancer patients treated with anti-oestrogens will eventually develop resistance towards treatment. Therefore it is important to find new therapeutic agents effective for treatment of patients relapsing on anti-oestrogen. The vitamin D analogue EB1089 (Seocalcitol(TM)) is a promising new agent for treatment of breast cancer patients with advanced disease, and in this study we show that two different anti-oestrogen-resistant human breast cancer cell lines are more sensitive towards treatment with EB1089, than the parent MCF-7 cell line. The two resistant cell lines both express a lower content of the anti-apoptotic protein Bcl-2, and we suggest that this may explain the higher sensitivity towards EB1089. The importance of Bcl-2 for response to EB1089 is supported by our observation that oestradiol abrogates the effect of EB1089 in cell lines which increase Bcl-2 in response to oestradiol treatment. Overall these results indicate that treatment with Seocalcitol(TM)may prove effective when patients become refractory to anti-oestrogen therapy, and that Bcl-2 may be used as a predictive marker.

Identification of a hormone-responsive promoter immediately upstream of exon 1c in the human vitamin D receptor gene

To gain insight into the molecular regulation of the human vitamin D3 receptor (hVDR), we have cloned and sequenced the 5' flanking region of exon 1c and examined promoter activity of this region in breast cancer cells. Sequence analysis of the first 1300 bp upstream of exon 1c reveals several characteristics of a class II promoter, including GC-rich regions and the presence of a TATA box at -29 bp. Putative transcription factor binding sites identified in this potential hVDR promoter include AP-2, Sp-1, and glucocorticoid response elements. No consensus vitamin D3 (VDRE) or estrogen (ERE) responsive elements were identified in the promoter sequence. Primer extension analysis performed with a primer specific for exon 1c confirms that transcription initiated in the 5' flanking region of exon 1c occurs in MCF-7 cells. Transient transfection of MCF-7 cells with this putative promoter region cloned into the pRLnull luciferase reporter vector generates significant reporter gene activity that is enhanced by treatment with forskolin, retinoic acid, and 17beta-estradiol. The enhancement of exon 1c promoter activity by 17beta-estradiol is blocked by the selective estrogen response modifier (SERM) tamoxifen and is not observed in estrogen receptor-negative breast cancer cells. In summary, we have cloned and characterized a TATA containing promoter upstream of exon 1c of the hVDR and provide evidence that this region represents a hormonally regulated hVDR promoter.

Catalytic asymmetric syntheses and biological activities of singly dehydroxylated 19-nor-1alpha,25-dihydroxyvitamin D(3) A-ring analogs in cancer cell differentiation and apoptosis

BACKGROUND

1alpha,25-Dihydroxyvitamin D(3) (1alpha,25(OH)(2)D(3)) has been shown to modulate not only proliferation and differentiation, but also apoptosis in malignant cells, indicating that it could be useful for treating cancer. Little information is available concerning the structural motifs of the 1alpha, 25(OH)(2)D(3) molecule responsible for modulation of differentiation and apoptosis, however. We set out to synthesize singly dehydroxylated A-ring analogs of 19-nor-1alpha,25(OH)(2)D(3) in a catalytic asymmetric fashion, and to investigate their biological activities in leukemia HL-60 cells.

RESULTS

A series of singly dehydroxylated 19-nor-1alpha,25-dihydroxyvitamin D(3) A-ring analogs were synthesized using a combinatiorial sequence of regioselective propiolate-ene reaction and catalytic asymmetric carbonyl-ene cyclization. Surprisingly, the analogs could be clearly divided into two categories; one group, bearing 1alpha-hydroxy or 3beta-hydroxy groups in the A-ring, were potent differentiators and the second group, bearing 1beta-hydroxy or 3alpha-hydroxy groups, were potent stimulators of apoptosis.

CONCLUSIONS

We have clearly identified the structural motifs of 19-nor-1alpha,25(OH)(2)D(3) analogs responsible for differentiation and apoptosis in HL-60 cells. These findings will provide useful information not only for development of therapeutic agents for treatment of leukemia and other cancers, but also for structure-function studies of 1alpha,25(OH)(2)D(3).

Regulation of a novel immediate early response gene, IEX-1, in keratinocytes by 1alpha,25-dihydroxyvitamin D3

1alpha,25-Dihydroxyvitamin D3 [1alpha,25(OH)2D3] regulates cellular growth and differentiation. We show that in keratinocytes, 1alpha, 25(OH)2D3 reduces concentrations of the messenger RNA of IEX-1, the product of which blocks Fas- or tumor necrosis factor type alpha-induced apoptosis in various cells. In sub-confluent keratinocyte cultures, the addition of 1alpha,25(OH)2D3, in amounts that induce growth arrest, reduces IEX-1 mRNA concentrations. In confluent cells, 1alpha,25(OH)2D3 initially reduces and then increases IEX-1 mRNA concentrations. IEX-1 protein is localized in the nucleus and perinuclear region of keratinocytes. In sub-confluent cells, 1alpha,25(OH)2D3 translocates IEX-1 protein from the nucleus to the perinuclear region and cytoplasm. Since IEX-1 has recently been shown to regulate cell survival and number, we suggest that IEX-1 may play a role in keratinocyte growth and differentiation and that 1alpha,25(OH)2D3 may reduce keratinocyte growth via a reduction in IEX-1 mRNA and a change in the intracellular distribution of IEX-1 protein.