Endogenously produced nonclassical vitamin D hydroxy-metabolites act as "biased" agonists on VDR and inverse agonists on RORα and RORγ

The classical pathway of vitamin D activation follows the sequence D3→25(OH)D3→1,25(OH)₂D3 with the final product acting on the receptor for vitamin D (VDR). An alternative pathway can be started by the action of CYP11A1 on the side chain of D3, primarily producing 20(OH)D3, 22(OH)D3, 20,23(OH)₂D3, 20,22(OH)₂D3 and 17,20,23(OH)₃D3. Some of these metabolites are hydroxylated by CYP27B1 at C1α, by CYP24A1 at C24 and C25, and by CYP27A1 at C25 and C26. The products of these pathways are biologically active. In the epidermis and/or serum or adrenals we detected 20(OH)D3, 22(OH)D3, 20,22(OH)₂D3, 20,23(OH)₂D3, 17,20,23(OH)₃D3, 1,20(OH)₂D3, 1,20,23(OH)₃D3, 1,20,22(OH)₃D3, 20,24(OH)₂D3, 1,20,24(OH)₃D3, 20,25(OH)₂D3, 1,20,25(OH)₃D3, 20,26(OH)₂D3 and 1,20,26(OH)₃D3. 20(OH)D3 and 20,23(OH)₂D3 are non-calcemic, while the addition of an OH at C1α confers some calcemic activity. Molecular modeling and functional assays show that the major products of the pathway can act as "biased" agonists for the VDR with high docking scores to the ligand binding domain (LBD), but lower than that of 1,25(OH)₂D3. Importantly, cell based functional receptor studies and molecular modeling have identified the novel secosteroids as inverse agonists of both RORα and RORγ receptors. Specifically, they have high docking scores using crystal structures of RORα and RORγ LBDs. Furthermore, 20(OH)D3 and 20,23(OH)₂D3 have been tested in a cell model that expresses a Tet-on RORα or RORγ vector and a RORE-LUC reporter (ROR-responsive element), and in a mammalian 2-hybrid model that test interactions between an LBD-interacting LXXLL-peptide and the LBD of RORα/γ. These assays demonstrated that the novel secosteroids have ROR-antagonist activities that were further confirmed by the inhibition of IL17 promoter activity in cells overexpressing RORα/γ. In conclusion, endogenously produced novel D3 hydroxy-derivatives can act both as "biased" agonists of the VDR and/or inverse agonists of RORα/γ. We suggest that the identification of large number of endogenously produced alternative hydroxy-metabolites of D3 that are biologically active, and of possible alternative receptors, may offer an explanation for the pleiotropic and diverse activities of vitamin D, previously assigned solely to 1,25(OH)₂D3 and VDR.

Metabolism of 20-hydroxyvitamin D3 and 20,23-dihydroxyvitamin D3 by rat and human CYP24A1

CYP11A1 hydroxylates vitamin D3 producing 20S-hydroxyvitamin D3 [20(OH)D3] and 20S,23-dihydroxyvitamin D3 [20,23(OH)2D3] as the major and most characterized metabolites. Both display immuno-regulatory and anti-cancer properties while being non-calcemic. A previous study indicated 20(OH)D3 can be metabolized by rat CYP24A1 to products including 20S,24-dihydroxyvitamin D3 [20,24(OH)2D3] and 20S,25-dihydroxyvitamin D3, with both producing greater inhibition of melanoma colony formation than 20(OH)D3. The aim of this study was to characterize the ability of rat and human CYP24A1 to metabolize 20(OH)D3 and 20,23(OH)2D3. Both isoforms metabolized 20(OH)D3 to the same dihydroxyvitamin D species with no secondary metabolites being observed. Hydroxylation at C24 produced both enantiomers of 20,24(OH)2D3. For rat CYP24A1 the preferred initial site of hydroxylation was at C24 whereas the human enzyme preferred C25. 20,23(OH)2D3 was initially metabolized to 20S,23,24-trihydroxyvitamin D3 and 20S,23,25-trihydroxyvitamin D3 by rat and human CYP24A1 as determined by NMR, with both isoforms showing a preference for initial hydroxylation at C25. CYP24A1 was able to further oxidize these metabolites in a series of reactions which included the cleavage of C23-C24 bond, as indicated by high resolution mass spectrometry of the products, analogous to the catabolism of 1,25(OH)2D3 via the C24-oxidation pathway. Similar catalytic efficiencies were observed for the metabolism of 20(OH)D3 and 20,23(OH)2D3 by human CYP24A1 and were lower than for the metabolism of 1,25(OH)2D3. We conclude that rat and human CYP24A1 metabolizes 20(OH)D3 producing only dihydroxyvitamin D3 species as products which retain biological activity, whereas 20,23(OH)2D3 undergoes multiple oxidations which include cleavage of the side chain.

Kinetic analysis of human CYP24A1 metabolism of vitamin D via the C24-oxidation pathway

CYP24A1 is the multicatalytic cytochrome P450 responsible for the catabolism of vitamin D via the C23- and C24-oxidation pathways. We successfully expressed the labile human enzyme in Escherichia coli and partially purified it in an active state that permitted detailed characterization of its metabolism of 1,25-dihydroxyvitamin D3 [1,25(OH)2 D3] and the intermediates of the C24-oxidation pathway in a phospholipid-vesicle reconstituted system. The C24-oxidation pathway intermediates, 1,24,25-trihydroxyvitamin D3, 24-oxo-1,25-dihydroxyvitamin D3, 24-oxo-1,23,25-trihydroxyvitamin D3 and tetranor-1,23-dihydroxyvitamin D3, were enzymatically produced from 1,25(OH)2 D3 using rat CYP24A1. Both 1,25(OH)2 D3 and 1,23-dihydroxy-24,25,26,27-tetranorvitamin D3 were found to partition strongly into the phospholipid bilayer when in aqueous medium. Changes to the phospholipid concentration did not affect the kinetic parameters for the metabolism of 1,25(OH)2 D3 by CYP24A1, indicating that it is the concentration of substrates in the membrane phase (mol substrate·mol phospholipid(-1) ) that determines their rate of metabolism. CYP24A1 exhibited Km values for the different C24-intermediates ranging from 0.34 to 15 mmol·mol phospholipid(-1) , with 24-oxo-1,23,25-trihydroxyvitamin D3 [24-oxo-1,23,25(OH)3 D3] displaying the lowest and 1,24,25-trihydroxyvitamin D3 [1,24,25(OH)3 D3] displaying the highest. The kcat values varied by up to 3.8-fold, with 1,24,25(OH)3 D3 displaying the highest kcat (34 min(-1) ) and 24-oxo-1,23,25(OH)3 D3 the lowest. The data show that the cleavage of the side chain of 24-oxo-1,23,25(OH)3 D3 occurs with the highest catalytic efficiency (kcat /Km ) and produces 1-hydroxy-23-oxo-24,25,26,27-tetranorvitamin D3 and not 1,23-dihydroxy-24,25,26,27-tetranorvitamin D3, as the primary product. These kinetic analyses also show that intermediates of the C24-oxidation pathway effectively compete with precursor substrates for binding to the active site of the enzyme, which manifests as an accumulation of intermediates, indicating that they dissociate after each catalytic step.

Novel vitamin D analogs as potential therapeutics: metabolism, toxicity profiling, and antiproliferative activity

AIM

To discover novel [20(OH)D3] analogs as antiproliferative therapeutics.

MATERIALS AND METHODS

We studied in vitro liver microsome stability, in vivo toxicity using mice, vitamin D receptor (VDR) translocation, in vitro antiproliferative effect, CYP enzyme metabolism.

RESULTS

20S- and 20R(OH)D3 had reasonable half-lives of 50 min and 30 min (average) respectively in liver microsomes. They were non-hypercalcemic at a high dose of 60 μg/kg. Three new 20(OH)D3 analogs were designed, synthesized and tested. They showed higher or comparable potency for inhibition of proliferation of normal keratinocytes and in the induction of VDR translocation from cytoplasm to nucleus, compared to 1,25(OH)2D3. These new analogs demonstrated different degrees of metabolism through a range of vitamin D-metabolizing CYP enzymes.

CONCLUSION

Their lack of calcemic toxicity at high doses and their high biological activity suggest that this novel 20(OH)D3 scaffold may represent a promising platform for further development of therapeutically-useful agents.

Hydroxylation of CYP11A1-derived products of vitamin D3 metabolism by human and mouse CYP27B1

CYP11A1 can hydroxylate vitamin D3 at carbons 17, 20, 22, and 23, producing a range of secosteroids which are biologically active with respect to their ability to inhibit proliferation and stimulate differentiation of various cell types, including cancer cells. As 1α-hydroxylation of the primary metabolite of CYP11A1 action, 20S-hydroxyvitamin D3 [20(OH)D3], greatly influences its properties, we examined the ability of both human and mouse CYP27B1 to 1α-hydroxylate six secosteroids generated by CYP11A1. Based on their kcat/Km values, all CYP11A1-derived metabolites are poor substrates for CYP27B1 from both species compared with 25-hydroxyvitamin D3. No hydroxylation of metabolites with a 17α-hydroxyl group was observed. 17α,20-Dihydroxyvitamin D3 acted as an inhibitor on human CYP27B1 but not the mouse enzyme. We also tested CYP27B1 activity on 20,24-, 20,25-, and 20,26-dihydroxyvitamin D3, which are products of CYP24A1 or CYP27A1 activity on 20(OH)D3. All three compounds were metabolized with higher catalytic efficiency (kcat/Km) by both mouse and human CYP27B1 than 25-hydroxyvitamin D3. CYP27B1 action on these new dihydroxy derivatives was confirmed to be 1α-hydroxylation by mass spectrometry and nuclear magnetic resonance analyses. Both 1,20,25- and 1,20,26- trihydroxyvitamin D3 were tested for their ability to inhibit melanoma (SKMEL-188) colony formation, and were significantly more active than 20(OH)D3. This study shows that CYP11A1-derived secosteroids are 1α-hydroxylated by both human and mouse CYP27B1 with low catalytic efficiency, and that the presence of a 17α-hydroxyl group completely blocks 1α-hydroxylation. In contrast, the secondary metabolites produced by subsequent hydroxylation of 20(OH)D3 at C24, C25, or C26 are very good substrates for CYP27B1.

Rat CYP24A1 acts on 20-hydroxyvitamin D(3) producing hydroxylated products with increased biological activity

20-Hydroxyvitamin D(3) (20(OH)D(3)), the major product of CYP11A1 action on vitamin D(3), is biologically active and is produced in vivo. As well as potentially having important physiological actions, it is of interest as a therapeutic agent due to its lack of calcemic activity. In the current study we have examined the ability of CYP24A1, the enzyme that inactivates 1,25-dihydroxyvitamin D(3) (1,25(OH)(2)D(3)), to metabolize 20(OH)D(3). Rat CYP24A1 was expressed in Escherichia coli, purified by Ni-affinity chromatography and assayed with substrates incorporated into phospholipid vesicles which served as a model of the inner mitochondrial membrane. In this system CYP24A1 metabolized 1,25(OH)(2)D(3) with a catalytic efficiency 1.4-fold higher than that seen for 25-hydroxyvitamin D(3) (25(OH)D(3)). CYP24A1 hydroxylated 20(OH)D(3) to several dihydroxy-derivatives with the major two identified by NMR as 20,24-dihydroxyvitamin D(3) (20,24(OH)(2)D(3)) and 20,25-dihydroxyvitamin D(3) (20,25(OH)(2)D(3)). The catalytic efficiency of CYP24A1 for 20(OH)D(3) metabolism was more than 10-fold lower than for either 25(OH)D(3) or 1,25(OH)(2)D(3) and no secondary metabolites were produced. The two major products, 20,24(OH)(2)D(3) and 20,25(OH)(2)D(3), caused significantly greater inhibition of colony formation by SKMEL-188 melanoma cells than either 1,25(OH)(2)D(3) or the parent 20(OH)D(3), showing that CYP24A1 plays an activating, rather than an inactivating role on 20(OH)D(3).

Metabolism of cholesterol, vitamin D3 and 20-hydroxyvitamin D3 incorporated into phospholipid vesicles by human CYP27A1

CYP27A1 is a mitochondrial cytochrome P450 which can hydroxylate vitamin D3 and cholesterol at carbons 25 and 26, respectively. The product of vitamin D3 metabolism, 25-hydroxyvitamin D3, is the precursor to the biologically active hormone, 1α,25-dihydroxyvitamin D3. CYP27A1 is attached to the inner mitochondrial membrane and substrates appear to reach the active site through the membrane phase. We have therefore examined the ability of bacterially expressed and purified CYP27A1 to metabolize substrates incorporated into phospholipid vesicles which resemble the inner mitochondrial membrane. We also examined the ability of CYP27A1 to metabolize 20-hydroxyvitamin D3 (20(OH)D3), a novel non-calcemic form of vitamin D derived from CYP11A1 action on vitamin D3 which has anti-proliferative activity on keratinocytes, leukemic and myeloid cells. CYP27A1 displayed high catalytic activity towards cholesterol with a turnover number (k(cat)) of 9.8 min(-1) and K(m) of 0.49 mol/mol phospholipid (510 μM phospholipid). The K(m) value of vitamin D3 was similar for that of cholesterol, but the k(cat) was 4.5-fold lower. 20(OH)D3 was metabolized by CYP27A1 to two major products with a k(cat)/K(m) that was 2.5-fold higher than that for vitamin D3, suggesting that 20(OH)D3 could effectively compete with vitamin D3 for catalysis. NMR and mass spectrometric analyses revealed that the two major products were 20,25-dihydroxyvitamin D3 and 20,26-dihydroxyvitamin D3, in almost equal proportions. Thus, the presence of the 20-hydroxyl group on the vitamin D3 side chain enables it to be metabolized more efficiently than vitamin D3, with carbon 26 in addition to carbon 25 becoming a major site of hydroxylation. Our study reports the highest k(cat) for the 25-hydroxylation of vitamin D3 by any human cytochrome P450 suggesting that CYP27A1 might be an important contributor to the synthesis of 25-hydroxyvitamin D3, particularly in tissues where it is highly expressed.

Human cytochrome P450scc (CYP11A1) catalyzes epoxide formation with ergosterol

Cytochrome P450scc (P450scc) catalyzes the cleavage of the side chain of both cholesterol and the vitamin D(3) precursor, 7-dehydrocholesterol. The aim of this study was to test the ability of human P450scc to metabolize ergosterol, the vitamin D(2) precursor, and define the structure of the major products. P450scc incorporated into the bilayer of phospholipid vesicles converted ergosterol to two major and four minor products with a k(cat) of 53 mol · min(-1) · mol P450scc(-1) and a K(m) of 0.18 mol ergosterol/mol phospholipid, similar to the values observed for cholesterol metabolism. The reaction of ergosterol with P450scc was scaled up to make enough of the two major products for structural analysis. From mass spectrometry, NMR, and comparison of the NMR data to that for similar molecules, we determined the structures of the two major products as 20-hydroxy-22,23-epoxy-22,23-dihydroergosterol and 22-keto-23-hydroxy-22,23-dihydroergosterol. Molecular modeling and nuclear Overhauser effect (or enhancement) spectroscopy spectra analysis helped to establish the configurations at C20, C22, and C23 and determine the final structures of major products as 22R,23S-epoxyergosta-5,7-diene-3β,20α-diol and 3β,23S-dihydroxyergosta-5,7-dien-22-one. It is likely that the formation of the second product is through a 22,23-epoxy (oxirane) intermediate followed by C22 hydroxylation with the formation of strained 22-hydroxy-22,23-epoxide (oxiranol), which is immediately transformed to the more stable α-hydroxyketone. Molecular modeling of ergosterol into the P450scc crystal structure positioned the ergosterol side chain consistent with formation of the above products. Thus, we have shown that P450scc efficiently catalyzes epoxide formation with ergosterol giving rise to novel epoxy, hydroxy, and keto derivatives, without causing cleavage of the side chain.