Synthetic 20-epi analogs of calcitriol are potent inducers of target-gene activation in osteoblastic cells

Synthesis and preliminary biological evaluation of 20-epi-eldecalcitol [20-epi-1alpha,25-dihydroxy-2beta-(3-hydroxypropoxy)vitamin D3: 20-epi-ED-71]

Eldecalcitol [1alpha,25-dihydroxy-2beta-(3-hydroxypropoxy)vitamin D3, developing code: ED-71] is an analog of active vitamin D3, 1alpha,25-dihydroxyvitamin D3 [1,25(OH)2D3] that possesses a hydroxypropoxy substituent at the 2beta-position of 1,25(OH)2D3. Eldecalcitol has potent biological effects on bone and is now in preparation for approval as a promising medicine for the treatment of osteoporosis in Japan. To explore chemical structure-biological activity relationships between eldecalcitol and related analogs, we have already synthesized 1-epi-eldecalcitol, 3-epi-eldecalcitol, and 1,3-diepi-eldecalcitol with inherent biological interests of each targeted analog and evaluated their biological responses. It has been reported that 20-epi-1,25(OH)2D3, a diastereomer of 1,25(OH)2D3 that possesses an inverted methyl substituent at the 20-position of the side chain, shows remarkably enhanced biological activities compared to parental compound, 1,25(OH)2D3. As a continuation of our modification studies on eldecalcitol, we took great interest in 20-epi-eldecalcitol and its biological responses. In this paper, the synthesis of 20-epi-eldecalcitol by the Trost coupling reaction between the A-ring fragment and the C/D-ring fragment as well as in vitro preliminary biological evaluation of 20-epi-eldecalcitol are described. In the induction of human myeloid leukemia cell (HL-60) differentiation, inhibition of the human histiocytic lymphoma cell (U937) proliferation, and increase in osteocalcin concentration in the human osteosarcoma cell (MG-63), 20-epi-eldecalcitol showed significantly enhanced activity compared to eldecalcitol.

Elevated plasma calcifediol is associated with aggressive periodontitis

BACKGROUND

Vitamin D is associated with a number of inflammatory diseases and plays a significant role in regulating bone metabolism. Serum calcifediol was demonstrated to be potentially associated with periodontal disease. The purpose of this study was to evaluate whether an association exists between plasma calcifediol concentrations and aggressive periodontitis (AgP) and whether plasma levels of bone-related biomarkers (osteocalcin, alkaline phosphatase, calcium, and phosphorus) regulated by vitamin D are related to AgP.

METHODS

Sixty-six patients with generalized AgP, 52 patients with chronic periodontitis, and 60 healthy controls were included in this study. Periodontal examination consisted of probing depth, attachment loss, and bleeding index measurements. Hematic calcifediol and bone-related biomarker levels were detected using radioimmunity assay kits or a biochemical analyzer.

RESULTS

Plasma calcifediol levels in patients with AgP were higher than those of healthy controls (29.28 versus 21.60 nmol/l; P <0.05) and were statistically significantly correlated with bleeding index (r = 0.321; P <0.05). Plasma osteocalcin concentrations in patients with AgP were higher than those of healthy controls (0.90 versus 0.70 ng/ml; P <0.05). Serum inorganic phosphorus values of both periodontitis groups were lower than those of healthy controls (1.06 +/- 0.18 mmol/l and 1.10 +/- 0.15 mmol/l versus 1.26 +/- 0.17 mmol/l; P <0.05).

CONCLUSION

Plasma calcifediol levels might be associated with periodontal inflammation.

The enhanced hypercalcemic response to 20-epi-1,25-dihydroxyvitamin D3 results from a selective and prolonged induction of intestinal calcium-regulating genes

20-Epi-1,25-dihydroxyvitamin D(3) (20-epi-1,25(OH)(2)D(3)) is a vitamin D analog that exhibits unique biologic properties. The mechanism(s) responsible for these activities remains unclear. Here we explore the ability of 20-epi-1,25(OH)(2)D(3) to induce calcemic responses in mice in vivo and identify a potential mechanism. Surprisingly, the levels of calcemia induced at 24 h after single injections of equivalent doses of 1,25(OH)(2)D(3) or 20-epi-1,25(OH)(2)D(3) were similar, suggesting that both compounds were equal in both potency and efficacy. This similarity was also observed at genes involved in calcium homeostasis including, S100g (calbindin D9K), Trpv6, Cldn2 (claudin 2), Trpv5, and Tnfsf11 (Rankl) as well as Cyp24a1. Despite this, the activities of the two compounds at 48 h were strikingly different. Thus, whereas the activity of 1,25-dihydroxyvitamin D(3) declined at this time point, the response to 20-epi-1,25(OH)(2)D(3) was increased. This unique profile was not due to an exaggerated induction of calcium regulating genes in the intestine, kidney, or bone but to a sustained action on these genes in the intestine. This conclusion was supported by studies using in vivo chromatin immunoprecipitation analysis, which revealed a prolonged presence of vitamin D receptor and RNA polymerase II at the Trpv6 and Cyp24a1 promoters and a sustained increase in histone 4 acetylation in these gene regions as well. We conclude that 20-epi-1,25(OH)(2)D(3) displays superagonist properties largely as a result of its duration of action in the intestine. This action is likely due to a decrease in the rate of intestinal-specific degradation of the ligand rather than to an increase in the functional stability of the vitamin D receptor.

Biological activities of 2alpha-substituted analogues of 1alpha,25-dihydroxyvitamin D3 in transcriptional regulation and human promyelocytic leukemia (HL-60) cell proliferation and differentiation

Biological activities of 2alpha-substituted 1alpha,25-dihydroxyvitamin D3 analogues were evaluated in vitro. Their binding affinity was examined with calf thymus cytosolic vitamin D receptor (VDR) and rat plasma vitamin D-binding protein (DBP). In addition, the transcriptional activity of the analogues was measured using a rat 25-hydroxyvitamin D3-24-hydroxylase gene promoter, a human osteocalcin gene promoter, and VDR-GAL4 system. This study investigated the biological activities of 2alpha-substituted analogues in comparison with 2beta-substitued analogues at the molecular level, with regard to the structural differences of alkyl, hydroxyalkyl, hydroxyalkoxy substituents at the 2-position of 1alpha,25-dihydroxyvitamin D3.

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.

A central dinucleotide within vitamin D response elements modulates DNA binding and transactivation by the vitamin D receptor in cellular response to natural and synthetic ligands

There is considerable divergence in the sequences of steroid receptor response elements, including the vitamin D response elements (VDREs). Two major VDRE-containing and thus 1,25-dihydroxyvitamin D(3) (1,25-(OH)(2)D(3))-regulated genes are the two non-collagenous, osteoblast-derived bone matrix proteins osteocalcin and osteopontin. We observed a stronger induction of osteopontin than osteocalcin mRNA expression by 1,25-(OH)(2)D(3). Subsequently, we have shown that vitamin D receptor/retinoid X receptor alpha (VDR/RXRalpha) heterodimers bind more tightly to the osteopontin VDRE than to the osteocalcin VDRE. Studies using point mutants revealed that the internal dinucleotide at positions 3 and 4 of the proximal steroid half-element are most important for modulating the strength of receptor binding. In addition, studies with VDRE-driven luciferase reporter gene constructs revealed that the central dinucleotide influences the transactivation potential of VDR/RXRalpha with the same order of magnitude as that observed in the DNA binding studies. The synthetic vitamin D analog KH1060 is a more potent stimulator of transcription and inducer of VDRE binding of VDR/RXR in the presence of nuclear factors isolated from ROS 17/2.8 osteoblast-like cells than the natural ligand 1,25-(OH)(2)D(3). Interestingly, however, KH1060 is comparable or even less potent than 1,25-(OH)(2)D(3) in stimulating VDRE binding of in vitro synthesized VDR/RXRalpha. Thus, the extent of 1,25-(OH)(2)D(3)- and KH1060-dependent binding of VDR/RXRalpha is specified by a central dinucleotide in the VDRE, and the ligand-induced effects on DNA binding are in part controlled by the cellular context of nuclear proteins.

Vitamin D compounds exert anti-apoptotic effects in human osteosarcoma cells in vitro

Several studies have demonstrated that vitamin D regulates growth and differentiation in bone cells in vitro. In addition, in vivo studies have shown that vitamin D stimulates bone formation, increases the number of osteoblast precursor cells and prevents bone mineral loss. These observations indicate that vitamin D may have anabolic effects on bone, and thus therapeutic potential in the treatment of osteoporosis. However, little is known about the effects of vitamin D on apoptosis in bone cells and about the contribution of this process to the effect of vitamin D on bone mineral loss. To investigate this aspect in more detail, we studied the effect of 1alpha,25(OH)(2)D(3) and a series of analogues on apoptosis in human osteosarcoma cells. No significant induction of apoptosis was observed with any of the compounds after a 5 day treatment period. In contrast, some of the analogues showed a tendency to protect the cells from undergoing apoptosis. This anti-apoptotic effect of vitamin D was further confirmed by the ability of 1alpha,25(OH)(2)D(3) to suppress camptothecin- and staurosporin-induced DNA fragmentation in the cells. In cultures treated simultaneously with 1alpha,25(OH)(2)D(3) in combination with camptothecin or staurosporin, the level of DNA fragmentation was markedly reduced compared with cultures treated with camptothecin or staurosporin alone. On the basis of the present results, it is therefore concluded that vitamin D displays anti-apoptotic effects in human osteoblast-like osteosarcoma cells in vitro. This observation suggests that besides regulating growth and differentiation, vitamin D exerts its anabolic effects on bone by protecting osteoblastic cells from undergoing apoptosis.

Characterization of a novel analogue of 1alpha,25(OH)(2)-vitamin D(3) with two side chains: interaction with its nuclear receptor and cellular actions

The hormone 1alpha,25(OH)(2)-vitamin D(3) (125D) binds to its nuclear receptor (VDR) to stimulate gene transcription activity. Inversion of configuration at C-20 of the side chain to generate 20-epi-1alpha,25(OH)(2)D(3) (20E-125D) increases transcription 200-5000-fold over 125D with its 20-normal (20N) side chain. This enhancement has been attributed to the VDR ligand-binding domain (LBD) having different contact sites for 20N and 20E side chains that generate different VDR conformations. We synthesized 1alpha, 25-dihydroxy-21-(3-hydroxy-3-methylbutyl)vitamin D(3) (Gemini) with two six-carbon side chains (both 20N and 20E orientations). Energy minimization calculations indicate the Gemini side chain possesses significantly more energy minima than either 125D or 20E-125D (2346, 207, and 127 minima, respectively). We compared activities of 125D, 20E-125D, and Gemini, respectively, in several assays: binding to wild-type (100%, 147%, and 38%) and C-terminal-truncated mutant VDR; transcriptional activity (of the transfected osteopontin promoter in ROS 17/2.8 cells: ED(50) 10, 0.005, and 1.0 nM); mediation of conformational changes in VDR assessed by protease clipping (major trypsin-resistant fragment of 34, 34, and 28 kDa). For inhibition of cellular clonal growth of human leukemia (HL-60) and breast cancer (MCF7) cell lines, the ED(50)(125D)/ED(50)(Gem) was respectively 380 and 316. We conclude that while Gemini readily binds to the VDR and generates unique conformational changes, none of them is able to permit a superior gene transcription activity despite the presence of a 20E side chain.

Structure-function studies of new C-20 epimer pairs of vitamin D3 analogs

A growing number of calcitriol (1alpha,25-dihydroxyvitamin D3) analogs have become available in recent years. Many of these analogs exhibit lower calcemic effects than calcitriol and inhibit cell proliferation and enhance cell differentation more efficiently than calcitriol. We have compared structure-function relationships of a series of new C-20 epimer (20-epi) vitamin D3 analogs with their natural C-20 counterparts. In human MG-63 osteosarcoma cells, quantification of cellular osteocalcin mRNA levels by Northern blot analysis and osteocalcin biosynthesis by radioimmunoassay indicated that most studied analogs at a concentration of 10 nm induced osteocalcin gene expression more efficiently than the parent compound, calcitriol. Interestingly, when the biological responses were compared with the binding affinities of the analogs to in-vitro translated human vitamin D receptor and with their ability to protect the receptor against partial proteolytic digestion, significant correlations were not observed. Further, molecular modelling of the compounds by energy minimization did not reveal marked differences in the three-dimensional structures of the analogs. These results suggest that higher than normal ligand binding affinity or 'natural' conformation of the ligand-receptor complex are not necessarily required for the 'superagonist' transactivation activity. The mechanism of action of the efficient analogs may involve stabilization and/or differential binding of transcriptional coactivators or transcription intermediary factors to the hVDR during transactivation.

Differential effects of 20-epi vitamin D analogs on the vitamin D receptor homodimer

Vitamin D analogs have received increased attention because of their possible therapeutic benefits in treating osteoporosis and various proliferative disorders. Several analogs were examined for their effects on DNA binding of the vitamin D receptor (VDR) homodimer complex with the murine osteopontin vitamin D response element. All of the tested analogs increased complex binding by recombinant human VDR in the electrophoretic mobility shift assay and notable differences in mobility of these complexes were observed. A panel of C-terminal anti-VDR antisera were screened for their ability to interact with analog-bound VDR homodimer complexes or as a heterodimer complex with recombinant human retinoid X receptor alpha (rhRXR alpha). Like calcitriol, analog-bound heterodimer complexes were largely resistant to interaction with these antisera; however, striking differences were observed with the various antisera in an analogous homodimer binding experiment. KH1060 and CB1093, analogs with 20-epi conformations, produced homodimer complexes that were 3- to 6-fold more resistant to supershifting with Ab180 compared with the hormone or EB1089. Chymotrypsin digestion in combination with Western blotting using a C-terminal anti-VDR antiserum revealed similar digestion patterns for all ligands. However, KH1060- and CB1093-bound VDR complexes were more resistant to digestion than either calcitriol or EB1089. Finally, the ability of these compounds to yield stable homodimer complexes was assessed by challenging preformed homodimer with the exogenous addition of rhRXR alpha extracts. Although new heterodimer complexes appeared in a time-dependent fashion, the preformed homodimer complexes exhibited stable binding throughout the time course of the experiment. The results indicate that VDR homodimers are targets of vitamin D analogs with differential effects on C-terminal protein conformation that may partially explain the varied biological responses of these compounds.

Inhibition of proliferation and induction of differentiation of osteoblastic cells by a novel 1,25-dihydroxyvitamin D3 analog with an extensively modified side chain (CB1093)

1,25-Dihydroxyvitamin D3 (1,25D) is involved in the regulation of proliferation and differentiation of a variety of cell types including cancer cells. In recent years, numerous new vitamin D3 analogs have been developed in order to obtain favorable therapeutic properties. The effects of a new 20-epi analog, CB1093 (20-epi-22-ethoxy-23-yne-24a,26a,27a-trihomo+ ++-1alpha,25(OH)2D3), on the proliferation and differentiation of human MG-63 osteosarcoma cell line were compared here with those of the parent compound 1,25D. Proliferation of the MG-63 cells was inhibited similarly by 22%, 50% and 59% after treatment with 0.1 microM 1,25D or CB1093 for 48 h, 96 h, and 144 h, respectively. In transfection experiments, the compounds were equipotent in stimulating reporter gene activity under the control of human osteocalcin gene promoter. In cell culture experiments, however, CB1093 was more potent than 1,25D at low concentrations and more effective for a longer period of time in activating the osteocalcin gene expression at mRNA and protein levels. Also, a 6-h pretreatment and subsequent culture for up to 120 h without 1,25D or CB1093 yielded higher osteocalcin mRNA and protein levels with analog-treated cells than with 1,25D-treated cells. The electrophoretic mobility shift assay (EMSA) revealed stronger VDR-VDRE binding with analog-treated MG-63 cells than with 1,25D-treated cells. The differences in the DNA binding of 1,25D-bound vs. analog-bound VDR, however, largely disappeared when the binding reactions were performed with recombinant hVDR and hRXRbeta proteins. These results demonstrate that the new analog CB1093 was equally or even more effective than 1,25D in regulating all human osteosarcoma cell functions ranging from growth inhibition to marker gene expression and that the differences in effectivity most probably resulted from interactions of the hVDR:hRXRbeta-complex with additional nuclear proteins.

The vitamin D analog, KH1060, is rapidly degraded both in vivo and in vitro via several pathways: principal metabolites generated retain significant biological activity

Vitamin D analogs are valuable drugs with established and potential uses in hyperproliferative disorders. Lexacalcitol (KH1060) is over 100 times more active than 1alpha,25-dihydroxyvitamin D3 [1alpha,25-(OH)2D3], as judged by in vitro antiproliferative and cell differentiating assays. The underlying biochemical reasons for the increased biological activity of KH1060 are unknown, but are thought to include 1) metabolic considerations in addition to explanations based upon 2) enhanced stability of KH1060-liganded transcriptional complexes. In this study we explored the in vivo and in vitro metabolism of KH1060. We established by physicochemical techniques the existence of multiple side-chain hydroxylated metabolites of KH1060, including 24-, 24a-, 26-, and 26a-hydroxylated derivatives as well as side-chain truncated forms. KH1060 metabolism could be blocked by the cytochrome P450 inhibitor, ketoconazole. KH1060 was not an effective competitor of C24 oxidation of 1alpha,25-(OH)2D3. Certain hydroxylated metabolites of KH1060 retained significant biological activity in vitamin D-dependent reporter gene systems (chloramphenicol acetyltransferase). Likewise, those metabolites accumulating in the target cell culture models in metabolism studies, particularly 24a-hydroxy-KH1060 and 26-hydroxy-KH1060, retained biological activities superior to those of 1alpha,25-(OH)2D3 in native gene expression systems in vitamin D target cells (osteopontin and P450cc24). We conclude that KH1060 is rapidly metabolized by a variety of cytochrome P450-mediated enzyme systems to products, many of which retain significant biological activity in vitamin D-dependent assay systems. These results provide an explanation for the considerable biological activity advantage displayed by KH1060 compared with 1alpha,25-(OH)2D3 in various in vitro assay systems.

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.