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

A human vitamin D receptor mutant activated by cholecalciferol

The human vitamin D receptor (hVDR) is a member of the nuclear receptor superfamily, involved in calcium and phosphate homeostasis; hence implicated in a number of diseases, such as Rickets and Osteoporosis. This receptor binds 1α,25-dihydroxyvitamin D(3) (also referred to as 1,25(OH)(2)D(3)) and other known ligands, such as lithocholic acid. Specific interactions between the receptor and ligand are crucial for the function and activation of this receptor, as implied by the single point mutation, H305Q, causing symptoms of Type II Rickets. In this work, further understanding of the significant and essential interactions between the ligand and the receptor was deciphered, through a combination of rational and random mutagenesis. A hVDR mutant, H305F, was engineered with increased sensitivity towards lithocholic acid, with an EC(50) value of 10 μM and 40±14 fold activation in mammalian cell assays, while maintaining wild-type activity with 1,25(OH)(2)D(3). Furthermore, via random mutagenesis, a hVDR mutant, H305F/H397Y, was discovered to bind a novel small molecule, cholecalciferol, a precursor in the 1α,25-dihydroxyvitamin D(3) biosynthetic pathway, which does not activate wild-type hVDR. This variant, H305F/H397Y, binds and activates in response to cholecalciferol concentrations as low as 100 nM, with an EC(50) value of 300 nM and 70±11 fold activation in mammalian cell assays. In silico docking analysis of the variant displays a dramatic conformational shift of cholecalciferol in the ligand binding pocket in comparison to the docked analysis of cholecalciferol with wild-type hVDR. This shift is hypothesized to be due to the introduction of two bulkier residues, suggesting that the addition of these bulkier residues introduces molecular interactions between the ligand and receptor, leading to activation with cholecalciferol.

Probing a water channel near the A-ring of receptor-bound 1 alpha,25-dihydroxyvitamin D3 with selected 2 alpha-substituted analogues

The crystal structure of the vitamin D receptor (VDR) in complex with 1 alpha,25(OH)2D3 revealed the presence of several water molecules near the A-ring linking the ligand C-2 position to the protein surface. Here, we report the crystal structures of the human VDR ligand binding domain bound to selected C-2 alpha substituted analogues, namely, methyl, propyl, propoxy, hydroxypropyl, and hydroxypropoxy. These specific replacements do not modify the structure of the protein or the ligand, but with the exception of the methyl substituent, all analogues affect the presence and/or the location of the above water molecules. The integrity of the channel interactions and specific C-2 alpha analogue directed additional interactions correlate with the binding affinity of the ligands. In contrast, the resulting loss or gain of H-bonds does not reflect the magnitude of HL60 cell differentiation. Our overall findings highlight a rational approach to the design of more potent ligands by building in features revealed in the crystal structures.

2-Methylene-19-nor-(20S)-1,25-dihydroxyvitamin D3 potently stimulates gene-specific DNA binding of the vitamin D receptor in osteoblasts

2-Methylene-19-nor-(20S)-1,25-dihydroxyvitamin D3 (2MD) is a highly potent analog of 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) whose actions are mediated through the vitamin D receptor (VDR). In this report, we have replicated this increased potency of 2MD in vitro using osteoblastic cells and explored its underlying molecular mechanism. 2MD stimulates the expression of several vitamin D-sensitive genes including 25-hydroxyvitamin D3-24 hydroxylase (Cyp24), osteopontin and receptor activator of NF kappa B ligand and suppresses osteoprotegerin at concentrations two logs lower than that for 1,25(OH)2D3. 2MD is also more potent in stimulating transfected chimeric reporter genes under either Cyp24 or the osteocalcin promoter control. Enhanced potency is retained regardless of medium serum content. Interestingly, the uptake of both 1,25(OH)2D3 and 2MD into cells is similar, as is their rapid association with the VDR. This indicates that comparable levels of occupied VDR do not elicit equivalent levels of transactivation. Using chromatin immunoprecipitation (ChIP), however, we observed a strong correlation between DNA-bound receptor and the level of induced transcription suggesting a 2MD-induced increase in affinity of the VDR for DNA. Additional studies using a mammalian two-hybrid system and ChIP indicate that 2MD is also more potent in promoting interaction with RXR and the coactivators SRC-1 and DRIP205. Finally, protease digestion studies revealed a unique VDR conformation in the presence of 2MD. These studies suggest that the molecular mechanism of 2MD potency is due to its ability to promote enhanced levels of specific DNA binding by the VDR and could suggest possible explanations for the tissue- and gene-selective actions of 2MD.

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.

Effect of 1,25-dihydroxyvitamin D3 on preventing allograft from acute rejection following rat orthotoic liver transplantation

AIM: To study the mechanism and the preventive role of 1, 25-dihydroxyvitamin D3 in acute rejection following orthotopic liver transplantation.

METHODS: Rats were randomly divided as donors or recipients for orthotopic liver allotransplantation model. Four groups were designed in the study, Group I: syngenic control (Wistar to Wistar); Group II: acute rejection (SD to Wistar); Group III: acute rejection treated with cyclosporine A, and Group IV: acute rejection treated with 1,25-(OH)₂ D₃. Liver function, rejection activity index and mRNA of IFN-γ, IL-10 intragraft in recipients were measured on day 1, 5, 7, 15, 30 posttransplant for assessing graft function, severity of acute rejection and immune state of recipients.

RESULTS: Survival time of recipients in Group IV was significantly prolonged (4/6 recipients survived for over 100 days. vs Group II, P < 0.001; vs Group III, P > 0.05). After treatment with 1,25-(OH)₂ D₃, mean value of all the assay tested on each experimental time was compared, liver function in group IV was significantly improved (AST 127 ± 41 U/L-360 ± 104 U/L, BIL 13 ± 5 mmol/l-38 ± 11 mmol/l; vs Group II, P < 0.05; vs Group III, P > 0.05. Rejection activity index was significantly decreased (0-3.3 ± 1.6; vs Group II, P < 0.05; vs Group III, P > 0.05). Level of hepatic IFN-γ mRNA in group IV was decreased, while level of hepatic IL-10 mRNA was increased (vs Group II, P < 0.05; vs Group III, P > 0.05).

CONCLUSION: Our results indicated that 1,25-(OH)₂ D₃ induced the secretion of cytokine toward to Th2 type, which would alleviate acute rejection, protect liver function and prolong survival of recipient after orthotopic liver transplantation.

Crystal structures of the vitamin D receptor complexed to superagonist 20-epi ligands

The crystal structures of the ligand-binding domain (LBD) of the vitamin D receptor complexed to 1alpha,25(OH)(2)D(3) and the 20-epi analogs, MC1288 and KH1060, show that the protein conformation is identical, conferring a general character to the observation first made for retinoic acid receptor (RAR) that, for a given LBD, the agonist conformation is unique, the ligands adapting to the binding pocket. In all complexes, the A- to D-ring moieties of the ligands adopt the same conformation and form identical contacts with the protein. Differences are observed only for the 17beta-aliphatic chains that adapt their conformation to anchor the 25-hydroxyl group to His-305 and His-397. The inverted geometry of the C20 methyl group induces different paths of the aliphatic chains. The ligands exhibit a low-energy conformation for MC1288 and a more strained conformation for the two others. KH1060 compensates this energy cost by additional contacts. Based on the present data, the explanation of the superagonist effect is to be found in higher stability and longer half-life of the active complex, thereby excluding different conformations of the ligand binding domain.

Interaction of two novel 14-epivitamin D3 analogs with vitamin D3 receptor-retinoid X receptor heterodimers on vitamin D3 responsive elements

This study provides a detailed and exact evaluation of the interactions between vitamin D3 receptor (VDR), retinoid X receptor (RXR), and vitamin D3 responsive elements (VDREs) mediated by two novel 14-epianalogs of 1,25-dihydroxyvitamin D [1,25(OH)2D3], 19-nor-14-epi-23-yne-1,25(OH)2D3 (TX 522) and 19-nor-14,20-bisepi-23-yne-1,25(OH)2D3 (TX 527). Both analogs were more potent (14- and 75-fold, respectively) than 1,25(OH)2D3 in inhibiting cell proliferation and inducing cell differentiation. However, DNA-independent experiments indicated that both analogs had a lower affinity to VDR and that the stability of the induced VDR conformation, as measured by limited protease digestion assays, was similar (TX 527) or even weaker (TX 522) than that induced by the parent compound. However, DNA-dependent assays such as gel shift experiments revealed that those analogs were slightly more potent (3-7 times) than 1,25(OH)2D3 in enhancing binding of VDR-RXR heterodimers to a direct repeat spaced by three nucleotides (DR3) type VDRE. The functional consequences of the ligand-VDR-RXR-VDRE interactions observed in vitro were subsequently evaluated in transfection experiments. Both 14-epianalogs enhanced transcription of VDRE containing reporter constructs more efficiently than 1,25(OH)2D3 in COS-1 and MCF-7 cells regardless of the presence of ketoconazole. Transactivation activity is suggested to be a cell-specific process because maximal transcriptional induction and the half-maximal transactivation concentration for each reporter construct were different in both cell lines. The superagonistic transactivation activity closely resembled the biological potency of these analogs on the inhibition of MCF-7 cell proliferation. These data clearly indicate that superagonistic activity starts beyond the binding of the ligand-heterodimer (VDR-RXR) complex to VDRE and thus probably involves coactivator/corepressor molecules.

Vitamin D(3) analogs (MC 1288, KH 1060, EB 1089, GS 1558, and CB 1093): studies on their mechanism of action

Selected 20-epi and 20-normal vitamin D(3) analogs were studied. First, point mutations were introduced into human vitamin D receptor (VDR) to identify residues important for ligand binding. In helices three, four and five, His229, Asp232, Ser237 and Arg274 seem to have an important role in the binding of calcitriol. Surprisingly, the 20-epi analog MC 1288 did not bind to Ser237. Second, the effects of analogs on VDR degradation were studied. The transcriptionally active 20-epi analogs protected VDR against degradation more efficiently than the 20-normal analogs and calcitriol. With proteasome inhibitor MG-132 formation of Sug-1-RXRbeta-VDR-VDRE complex was detected. The 20-epi analogs effectively prevented its formation. Thus, the 20-epi analogs induce a VDR conformation, which prevents binding of factors mediating VDR degradation. Third, the analogs were found to be powerful regulators of cell cycle progression in MG-63 cells. They arrested cell cycle in the G0/G1 phase at lower concentrations and earlier time points than calcitriol. This was accompanied by hypophosphorylation of Rb followed by strong inhibition of Cdk2 activity. This correlated with increased levels of p27. Cdk2 and cyclin E levels were downregulated but those of p21 and cyclin D1 were not affected. Thus, a similar sequence of events with calcitriol and the analogs in inhibiting MG-63 cell growth was detected but the analogs had much longer lasting and stronger effects than calcitriol. A unifying scheme for the varying effects of vitamin D(3) analogs is presented.

Conceptually new 20-epi-22-oxa sulfone analogues of the hormone 1alpha,25-dihydroxyvitamin D(3): synthesis and biological evaluation

New C,D-ring side-chain-modified sulfone 4a, with natural 1alpha, 3beta-hydroxyl groups but lacking the 25-hydroxyl group characteristic of the natural hormone 1alpha,25-dihydroxyvitamin D(3) (1), has been prepared and characterized. Novel synthetic features include: (1) chemoselective oxidation of only a primary silyl ether in a primary-secondary bis-silyl ether intermediate and (2) smooth reductive etherification without interference by a neighboring sulfonyl group. Sulfone 4a, but not its 1beta, 3alpha-diastereomer 4b, is powerfully antiproliferative and transcriptionally active in vitro but desirably noncalcemic in vivo. Although sulfone 4a, designed to resemble Leo Pharmaceutical Co.'s KH-1060 (3), is recognized by catabolic enzymes, the selective biological profile of sulfone 4a is likely not due to its metabolites that are formed in only minor amounts.