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. Author manuscript; available in PMC: 2013 Dec 15.
Published in final edited form as: Biochem Pharmacol. 2012 Oct 5;84(12):1696–1704. doi: 10.1016/j.bcp.2012.09.032

Rat CYP24A1 acts on 20-hydroxyvitamin D3 producing hydroxylated products with increased biological activity

Elaine W Tieu a, Edith K Y Tang a, Jianjun Chen b, Wei Li b, Minh N Nguyen a, Zorica Janjetovic c, Andrzej Slominski c, Robert C Tuckey a
PMCID: PMC3501553  NIHMSID: NIHMS412699  PMID: 23041230

Abstract

20-Hydroxyvitamin D3 (20(OH)D3), the major product of CYP11A1 action on vitamin D3, 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 D3 (1,25(OH)2D3), to metabolize 20(OH)D3. 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)2D3 with a catalytic efficiency 1.4-fold higher than that seen for 25-hydroxyvitamin D3 (25(OH)D3). CYP24A1 hydroxylated 20(OH)D3 to several dihydroxy-derivatives with the major two identified by NMR as 20,24-dihydroxyvitamin D3 (20,24(OH)2D3) and 20,25-dihydroxyvitamin D3 (20,25(OH)2D3). The catalytic efficiency of CYP24A1 for 20(OH)D3 metabolism was more than 10-fold lower than for either 25(OH)D3 or 1,25(OH)2D3 and no secondary metabolites were produced. The two major products, 20,24(OH)2D3 and 20,25(OH)2D3, caused significantly greater inhibition of colony formation by SKMEL-188 melanoma cells than either 1,25(OH)2D3 or the parent 20(OH)D3, showing that CYP24A1 plays an activating, rather than an inactivating role on 20(OH)D3.

Keywords: CYP24A1, vitamin D metabolism, 20-hydroxyvitamin D, phospholipid vesicles, melanoma

1. Introduction

CYP24A1 catalyzes the inactivation of 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) with initial production of 1,24,25-trihydroxyvitamin D3 (1,24,25(OH)3D3), as illustrated in Fig. 1 [1–4]. This product can undergo further CYP24A1-catalyzed oxidations resulting in cleavage of the side chain between C23 and C24 to produce calcitroic acid, the final product which is excreted (Fig. 1) [3, 5, 6]. Some mammalian species including the human also display a CYP24A1-catalyzed C23 oxidation pathway where initial hydroxylation is at C23 and the final product is 1,25-dihydroxyvitamin D3-26,23-lactone [6–9]. The rat enzyme almost exclusively catalyzes the C24 oxidation pathway, while this pathway accounts for 79% of the metabolism of 1,25(OH)2D3 by the human enzyme [7]. CYP24A1 also catalyzes the hydroxylation of 25-hydroxyvitamin D3 (25(OH)D3) [1, 2, 9, 10] by the C24 pathway with some of the primary product, 24,25-dihydroxyvitamin D3 (24,25(OH)2D3), escaping from the enzyme and being one of the major dihydroxyvitamin D metabolites seen in human serum [11]. While this product lacks the typical activity seen for 1,25(OH)2D3, it has been reported to be active in stimulating cartilage development and bone fracture repair [12].

Figure 1.

Figure 1

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Pathways of vitamin D3 metabolism by CYP11A1 and CYP24A1. CYP11A1 catalyzes the conversion of vitamin D3 to 20(OH)D3, 20,23(OH)2D3 and 17,20,23(OH)3D3. Vitamin D3 is activated by other P450s to produce the biologically active hormone, 1,25(OH)2D3. This hormone is inactivated by CYP24A1 through sequential oxidation reactions which convert 1,25(OH)2D3 to the excretory product, calcitroic acid.

20S-Hydroxyvitamin D3 (20(OH)D3) is the major product of CYP11A1 action on vitamin D3 and can be further metabolized by purified CYP11A1 to a number of products with the main ones being 20,23-dihydroxyvitamin D3 (20,23(OH)2D3) and 17,20,23-trihydroxyvitamin D3, as illustrated in Fig. 1 [13–17]. Our most recent studies have shown that 20(OH)D3 can be produced by human placentae and rat adrenal glands fragments ex-vivo through a reaction catalyzed by CYP11A1 [18, 19]. Furthermore, we have shown that this secosteroid can be produced by cultured keratinocytes both with and without the addition of vitamin D3 to the culture media, and have provided data indicating that 20(OH)D3 is present in human serum [18, 19]. Thus, there is now compelling evidence that CYP11A1 provides an alternative pathway for vitamin D3 activation via production of 20(OH)D3, separate from the classical pathway producing 1,25(OH)2D3. 20(OH)D3 shares many, but not all of the actions of 1,25(OH)2D3. It inhibits the proliferation and stimulates the differentiation of a range of normal and cancer cells including keratinocytes, melanoma and leukemia cells [20–25]. Similar properties are seen for 20-hydroxyvitamin D2, the main product of CYP11A1-mediated metabolism of vitamin D2 [26–28]. 20(OH)D3 inhibits NF-κB activity by stimulating the expression of inhibitory IκBα protein [29, 30] and therefore serves as an excellent candidate for treatment of inflammatory diseases. Knockdown of the vitamin D receptor inhibits the effects of 20(OH)D3 indicating that like 1,25(OH)2D3, it works via the vitamin D receptor [24, 28, 30]. 20(OH)D3 also shows comparable activity to 1,25(OH)2D3 in stimulating translocation of the VDR coupled to green fluorescent protein from the cytoplasm to the nucleus [15, 28, 31]. However, unlike 1,25(OH)2D3, high concentrations of 20(OH)D3 (up to 30 μg/kg) do not raise serum calcium levels in rodents [22, 23]. Therefore, 20(OH)D3 has the potential to serve as a relatively non-toxic drug for treatment of hyperproliferative and inflammatory disorders. While 20(OH)D3 is a relatively poor substrate for 1α-hydroxylation by CYP27B1 [32], the product 1,20-dihydroxyvitamin D3, has calcemic activity although lower than that seen for 1,25(OH)2D3 [22]. Another important difference to 1,25(OH)2D3 is that 20(OH)D3 is a very poor inducer of CYP24A1 expression in a number of cell systems [15, 21, 22, 24, 28]. Therefore, if used therapeutically it may not be subject to the same rapid inactivation that occurs with 1,25(OH)2D3 and many vitamin D analogs when its levels are elevated [4].

Metabolism of 20(OH)D3 to 20,23(OH)2D3 and other products by CYP11A1 occurs with low catalytic efficiency and the metabolites retain strong biological activity [15, 22, 25, 29, 31]. Since CYP24A1 is the enzyme that inactivates 1,25(OH)2D3, it is important to know whether CYP24A1 can metabolize 20(OH)D3 and whether the resulting products are also biologically inactive. We have addressed these questions in the current study using purified rat CYP24A1 expressed in E. coli. The rat enzyme was chosen for these studies because it is easier to express and purify than the less stable human enzyme [33], plus we have used rodents to test the toxicity and other effects of 20(OH)D3 [22, 23] where potential metabolism by CYP24A1 is of importance. The rat and human enzymes share 90% amino acid sequence identity [34] and both metabolize 25(OH)D3 and 1,25(OH)2D3 predominantly by the C24 oxidation pathway, therefore data for 20(OH)D3 metabolism for the rat enzyme should provide a useful model for 20(OH)D3 metabolism by human CYP24A1.

The anti-melanoma activity of 1,25(OH)2D3 is well established (reviewed in [35–37], but its clinical use is limited by its calcemic activity. Since 20(OH)D3 is non-calcemic and non-toxic at high concentrations [22, 23], it is a good candidate for treatment of melanoma with comparable anti-melanoma activity to 1,25(OH)2D3, mediated via binding to the vitamin D receptor [20, 25, 28]. These effects of 20(OH)D3 include differential inhibition of melanoma growth compared to melanocytes [25]. Since inhibition of melanoma colony formation in soft agar by 20(OH)D3 is well characterized and represents a good in vitro measure of anti-tumorigenic activity, we have chosen this system for the preliminary biological testing of the products of CYP24A1 action on 20(OH)D3 and have shown that both of the major products, 20,24(OH)2D3 and 20,25(OH)2D3, display enhanced, rather than reduced, anti-melanoma activity.

2. Materials and Methods

2.1. Materials

20(OH)D3 was synthesized enzymatically from the action of CYP11A1 on vitamin D3 and was purified by TLC and reverse phase HPLC as before [15, 16]. 20,26-Dihydroxyvitamin D3 (20,26(OH)2D3) was produced by the action of human CYP27A1 on 20(OH)D3 [38]. 25(OH)D3 and NADPH were purchased from Merck (Darmstadt, Germany). Vitamin D3, dioleoyl phosphatidylcholine, bovine heart cardiolipin, cyclodextrin, glucose-6-phosphate were purchased from Sigma (St Louis, MO, U.S.A.). Glucose-6-phosphate dehydrogenase was from Roche (Mannheim, Germany). All solvents were of HPLC grade and were purchased from Merck (Darmstadt, Germany). The MTT reagent was from Promega (Madison, WI, U.S.A.). The charcoal-stripped serum was from HyClone (Logan, UT, U.S.A.).

2.2. Preparation of enzymes

Mouse adrenodoxin and human adrenodoxin reductase were expressed in Escherichia coli and purified as before [15, 39, 40]. The cDNA for rat CYP24A1 was synthesized by GenScript Corporation (Piscataway, NJ) according to the published sequence with the N-terminal mitochondrial target sequence removed [41] and the new N-terminus enriched with purines as described by Annalora et al. [3]. The cDNA was designed to encode a 6-histidine tag at the C-terminus of the CYP24A1. The construct was ligated to pTrc99A via NcoI (5′ end) and HindIII (3′ end) restriction sites. Competent E. coli (JM109) cells were transformed with the pTrc99A-CYP24A1 construct and the CYP24A1 expressed and purified using Ni-affinity chromatography as described for CYP27B1 [42]. The expression level measured after the Ni-affinity chromatography was 230 nmol/L culture.

2.3. Measurement of 20(OH)D3 metabolism by CYP24A1 in phospholipid vesicles

Dioleoyl phosphatidylcholine (1.08 μmol), bovine heart cardiolipin (0.19 μmol) and secosteroid substrate (as required) were placed in glass tubes and the ethanol solvent removed under nitrogen. Buffer (0.5 mL) comprising 20 mM HEPES pH 7.4, 100 mM NaCl, 0.1 mM DTT and 0.1 mM EDTA was added and the tubes sonicated for 10 min in a bath-type sonicator [32, 43]. The incubation mixture contained vesicles (510 μM phospholipid), CYP24A1 (0.15–0.5 μM), mouse adrenodoxin (15 μM), human adrenodoxin reductase (0.4 μM), glucose-6-phosphate (2 mM), glucose-6-phosphate dehydrogenase (2 U/mL) and NADPH (50 μM), in the same buffer used for vesicle preparation. Adrenodoxin and adrenodoxin reductase served to transport electrons from NADPH to the CYP24A1 [6]. Samples (typically 0.25–1.0 mL) were incubated at 37°C, with shaking (see Section 3.1. for incubation times), then reactions terminated by the addition of 2.5 mL ice-cold dichloromethane and vortexing. After phase separation aided by centrifugation, the lower organic phase was retained and the upper aqueous phase was extracted twice more with 2.5 mL aliquots of dichloromethane. The dichloromethane was removed under nitrogen and the residual sample dissolved in solvent for HPLC analysis. Reverse phase HPLC on a C18 column was carried out as described before [32] to measure product formation by CYP24A1. Kinetic parameters were determined by fitting the Michaelis-Menten equation to the experimental data using Kaleidagraph 3.6 (Synergy Software) [17].

2.4. Large-scale preparation of 20,24(OH)2D3 and 20,25(OH)2D3

Rat CYP24A1 (1.0 μM) was incubated with 50 μM 20(OH)D3 (added from a 0.75 mM stock in 4.5% cyclodextrin) in 40 mL buffer comprising 20 mM HEPES pH 7.4, 100 mM NaCl, 0.1 mM DTT, 0.1 mM EDTA, 15 μM mouse adrenodoxin, 0.4 μM human adrenodoxin reductase, 2 mM glucose-6-phosphate, 2 U/mL glucose-6-phosphate dehydrogenase and 50 μM NADPH, for 2 h at 37°C. The reaction was stopped with 80 mL ice-cold dichloromethane and products were extracted in a scaled up version of that described above (Section 2.3.) incubations with phospholipid vesicles.

The 20,24(OH)2D3 and 20,25(OH)2D3 products were purified by reverse phase HPLC using a Grace Smart column (15 cm × 4.6 mm, particle size 7 μm) with a gradient of 50 to 65% acetonitrile in water for 45 min at a flow rate of 0.5 mL/min. This gave baseline separation of 20,24(OH)2D3 and 20,25(OH)2D3. These products were further purified on the same column using isocratic conditions, 71% methanol in water for 20,25(OH)2D3 and 73% methanol in water for 20,24(OH)2D3, at a flow rate of 0.5 mL/min for 1 h. The yield of product, determined spectrophotometrically at 263 nm using an extinction coefficient of 18,000 M−1cm−1[44], was 80 nmol 20,25(OH) 2D3and 280 nmol 20,24(OH) 2D3, enough for structure determination by mass spectrometry and NMR, and biological testing.

2.5. Mass Spectrometry

Mass spectra of the dihydroxy metabolites were measured using a Bruker Esquire IonTrap-LC/MS system (Bruker Daltonics, Billerica, MA, U.S.A.). The measurements were performed on an electrospray ionization (ESI) source with nitrogen as the nebulizing gas. Data were transferred to an offline PC and processed with ACD software (Advanced Chemistry Development, Toronto, Canada).

2.6. NMR

NMR measurements were performed using an inverse triple-resonance 3 mm probe on a Varian Unity Inova 500 MHz spectrometer (Agilent Technologies, Inc., Santa Clara, CA, U.S.A.). Sample was dissolved in CD3OD and transferred to a 3-mm Shigemi NMR tube (Shigemi Inc., Allison Park, PA). Temperature was regulated at 22°C and was controlled with an accuracy of ± 0.1°C. Chemical shifts were referenced to residual solvent peaks for CD3OD (3.31 ppm for proton and 49.15 ppm for carbon). Standard two-dimensional NMR experiments [1H-1H correlation spectroscopy (COSY), 1H-1H total correlation spectroscopy (TOCSY, mixing time=80 ms), 1H-13C heteronuclear single quantum correlation spectroscopy (HSQC), and 1H-13C heteronuclear multiple bond correlation spectroscopy (HMBC)] were acquired in order to fully elucidate the structures of the metabolites. All data were processed using ACD software (Advanced Chemistry Development, Toronto, ON, Canada), with zero-filling in the direct dimension and linear prediction in the indirect dimension.

2.7. Colony formation assay in soft agar

The SKMEL-188 melanoma cells were cultured as described previously [45]. To measure their tumorigenicity the cells were trypsinized and re-suspended (~1000 cells/well) in 250 μL medium containing 0.4% agarose and 5% charcoal-stripped serum. Cell suspensions were added to a 0.8% agar layer in 4 × 24 well plates. Compounds were added from ethanol stocks (100 μM) to final concentrations of 0.1 nM or 10 nM, in 100 μL media. Each condition was tested in quadruplicate. An ethanol solvent control (amount of ethanol equivalent to test) as well as a media-only control was included in the assay. Cells were allowed to grow at 37°C with 5% CO2 over two weeks with secosteroids in fresh media (100 μL) being added after every 72 h. Soft agar colonies were scored and stained with 0.5 mg/mL MTT reagent, 500 μL/well after two weeks. Colonies were then counted under the microscope. Data are presented as means ± SEM and have been analyzed with Student’s t-test (for unpaired two groups) or one-way analysis of variance, using Prism 4.00 (GraphPad Software, San Diego, CA).

3. Results

3.1. Metabolism of 20(OH)D3 in phospholipid vesicles by rat CYP24A1

CYP24A1 activity was measured with 20(OH)D3 incorporated into phospholipid vesicles as this system mimics the native environment of the cytochrome in the inner mitochondrial membrane [1, 46] and has been observed to give high catalytic activity for other mitochondrial cytochromes P450 [17, 32, 38, 42, 43, 47, 48]. Incubation of 20(OH)D3 in phospholipid vesicles, with rat CYP24A1, resulted in two major products and several minor ones (Fig. 2). The same products were seen with 20(OH)D3 solubilized in cyclodextrin (Fig. 2), a system often used because of its ability to solubilize a high concentration of secosteroid [15–17, 26, 49]. The amount of total substrate consumed was higher in the cyclodextrin system. A separate HPLC step employing a gradient of methanol in water was required to separate the combined peak D/E obtained with the acetonitrile gradient, into two products of equal proportion plus another very minor product (Fig. 2C, insert). Mass spectra with electrospray ionization of all products (A – E) all showed the same major ion with m/z = 439 (corresponding to M + Na), revealing a molecular weight of 416 and hence that all of these products are dihydroxyvitamin D compounds (Fig. 3). The two major products were identified by NMR as 20,25(OH)2D3 for A and 20,24(OH)2D3 for B, with the full analysis described in Section 3.3. Using authentic standards produced by the actions of CYP11A1 and CYP27A1 on vitamin D3 [15, 38], we were able to exclude that products C, D or E were 20,22(OH)2D3, 20,23(OH)2D3 or 20,26(OH)2D3, or at least the isomeric forms produced by these enzymes.

Figure 2.

Figure 2

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Metabolism of 20(OH)D3 by rat CYP24A1. (A) 20(OH)D3 was incorporated into phospholipid vesicles at a ratio of 0.05 mol/mol phospholipid and was incubated with 1.0 μM CYP24A1 for 30 min at 37°C in a reconstituted system containing adrenodoxin (15 μM) and adrenodoxin reductase (0.4 μM). Samples were extracted using dichloromethane and analyzed by reverse phase HPLC. (B) Control incubation for vesicles without adrenodoxin showing the substrate but the absence of product. (C) CYP24A1 (1.0 μM) was incubated for 30 min with 50 μM 20(OH)D3 dissolved in cyclodextrin (0.45%) in a reconstituted system as for vesicles.

Figure 3.

Figure 3

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Mass spectra with electrospray ionization of products A-E of CYP24A1 action on 20(OH)D3.

A time course for CYP24A1 activity on 20(OH)D3 in vesicles was carried out at 37°C (Fig. 4). The reaction was largely over by 4 min of incubation despite only 10% of the substrate being used. The proportion of the different products remained reasonably constant throughout the incubation with no lags evident, consistent with the identification of all the products as dihydroxyvitamin D compounds and not secondary metabolites resulting from multiple oxidations.

Figure 4.

Figure 4

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Time course for metabolism of 20(OH)D3 by CYP24A1 in phospholipid vesicles. CYP24A1 was incubated with vesicles containing 20(OH)D3 for various times as in Fig. 1.

Kinetic experiments were performed with substrates incorporated into phospholipid vesicles to compare the ability of rat CYP24A1 to metabolize 20(OH)D3, 25(OH)D3 and 1,25(OH)2D3. This was also of interest because, despite the substrate access channel for this enzyme being in the hydrophobic domain of the membrane [46], the activity of the purified enzyme has not ever been characterized in a defined membrane reconstituted system. In phospholipid vesicles 20(OH)D3 was metabolized by CYP24A1 with a kcat of 10.8 ± 0.8 mol/min/mol CYP24A1 and a Km of 0.028 ± 0.007 mol substrate/mol phospholipid (Fig. 5). This compares to kcat and Km values for the initial rate of metabolism of 25(OH)D3 of 52.6 ± 2.9 mol/min/mol CYP24A1 and 0.0080 ± 0.0016 mol substrate/mol phospholipid, respectively (Fig. 5) and to 13.60 ± 0.54 mol/min/mol CYP24A1 and 0.0015 ± 0.0003 mol substrate/mol phospholipid for 1,25(OH)2D3, respectively. In the case of 25(OH)D3 the major product of the 1 min incubation was 24,25(OH)2D3 and for 1,25(OH)2D3 was 1,24,25(OH)3D3, as expected. Thus 20(OH)D3 is a relatively poor substrate for CYP24A1 being metabolized with a kcat/Km value of 386 min−1(mol substrate/mol phospholipid)−1 which is 17-fold lower than that for 25(OH)D3 and 24-fold lower than for 1,25(OH)2D3.

Figure 5.

Figure 5

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Michaelis-Menten plots for the metabolism of CYP24A1 substrates in phospholipid vesicles. CYP24A1 was incubated with phospholipid vesicles containing 20(OH)D3, 25(OH)D3 or 1,25(OH)2D3 and incubated for 1 min with a reconstituted system containing adrenodoxin and adrenodoxin reductase. Products were extracted and analyzed by reverse phase HPLC. Hyperbolic curves were fitted by non-linear least squares analysis using Kaleidagraph 3.6. The R values for the curve fits were 0.999, 0.992 and 0.949 for 20(OH)D3, 25(OH)D3 and 1,25(OH)2D3, respectively.

3.2. Synthesis of products A and B for NMR analysis

Due to the ability of cyclodextrin to solubilize a high concentration of secosteroid and the greater conversion of substrate to product in this system (Fig. 2), we scaled up an incubation in this system to 40 mL. Following extraction and HPLC purification using two solvent systems (see Section 2.4.), 80 nmol product A and 280 nmol product B were obtained, enough for structure determination by NMR (see Section 3.3.). Insufficient products D-E were produced for NMR so these products were only analyzed by mass spectrometry (see Fig. 3).

3.3. Identification of the two major metabolites produced by CYP24A1 action on 20(OH)D3 as 20,24(OH)2D3 and 20,25(OH)2D3

Analysis of product A by NMR reveals that this metabolite is 20,25(OH)2D3. We have recently shown that 20,25(OH)2D3 is also generated from CYP27A1 action on 20(OH)D3 [38] and product A and CYP27A1-derived 20,25(OH)2D3 displayed identical HPLC retention times. Since the structure elucidation of 20,25(OH)2D3 by NMR has already been presented we will not describe it here for the CYP24A1-derived product.

Analysis of product B by mass spectrometry (Fig. 3B) reveals that it is a dihydroxyvitamin D3 derivative. The observed molecular ion had a mass of 439 [M + Na]+ giving a molecular weight of 416. The site of hydroxylation on 20(OH)D3 was unambiguously assigned to be at the 24-position based on the NMR spectra for this metabolite. First, none of the four methyl carbons (C18, C21, C26, C27) are hydroxylated based on 1H NMR (Fig. 6A). 1H-13C HSQC revealed the presence of a new methine group at 3.32 ppm (13C at 78.2 ppm, Fig. 6B). 1H-1H TOCSY (Fig. 6C) clearly showed that this methine is in the same spin system as 26/27-CH3 (1H at 0.92 ppm), indicating the hydroxylation occurred in the side chain. From the 1H-1H COSY (Fig. 6D) spectrum, this methine (1H at 3.32 ppm) showed a strong correlation to 25-CH (1H at 1.62 ppm) and 23-CH2 (1H at 1.74 and 1.39 ppm). From 1H-13C HMBC (Fig. 6E), 26/27-CH3 (1H at 0.92 ppm) showed a strong correlation to the new methine (13C at 78.2 ppm), in addition to the expected correlation to 27/26-CH3 (13C at 18.5/19.3 ppm) and 25-CH (13C at 34.0 ppm). Taken together the above analysis shows that the hydroxylation site can be unambiguously assigned to the 24-position. The full assignments for this metabolite are summarized in Table 1 (for comparison, we also included the assignments for 20(OH)D3).

Figure 6.

Figure 6

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NMR spectra of 20,24(OH)2D3. (A) 1D Proton; (B) 1H-13C HSQC; (C) 1H-1H TOCSY; (D) 1H-1H COSY; (E) 1H-13C HMBC.

Table 1.

NMR chemical shift assignments for 20,24(OH)2D3. (Solvent: CD3OD)

Atom 20,24(OH)2D3 20(OH)D3
1H 13C 1H 13C
1 2.11α, 2.41β 33.5 2.11α, 2.40β 32.2
2 1.96α, 1.53β 36.5 1.97α, 1.53β 35.2
3 3.76 70.4 3.76 69.2
4 2.52α, 2.18β 46.8 2.53α, 2.19β 45.6
5 NA 136.4 NA 136.3
6 6.22 122.5 6.21 121.2
7 6.03 119.3 6.02 118.0
8 NA 141.1 NA 141.0
9 1.68α, 2.84β 28.4 1.68α, 2.85β 28.4
10 NA 146.2 NA 145.6
11 1.55α, 1.69β 24.2 1.56α, 1.66β 23.1
12 1.36α, 2.10β 42.3 1.37α, 2.07β 40.9
13 NA 45.4 NA 45.6
14 2.01 57.6 2.00 56.4
15 1.77 23.1 1.51 21.7
16 1.68α, 1.77β 23.0 1.69α, 1.78β 21.7
17 1.69 60.4 1.67 58.5
18 0.70 14.1 0.69 12.8
19 4.75, 5.04 112.4 4.74, 5.04 111.3
20 NA 74.7 NA 74.5
21 1.24 25.9 1.23 24.8
22 1.36 39.6 1.32, 1.46 43.9
23 1.74, 1.38 41.0 1.33 21.7
24 3.22 78.2 1.16 39.6
25 1.62 33.5 1.55 27.8
26 0.93 19.3 0.89 21.7
27 0.92 18.5 0.89 21.7
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NA – Not applicable (ternary carbons).

3.4. Biological activity of CYP24A1-derived secosteroids on melanoma cells

The abilities of 20,24(OH)2D3 and 20,25(OH)2D3 to inhibit melanoma cell proliferation were determined using a soft agar assay, which is regarded as a good measure of tumorigenicity (Fig. 7). We also included 20,26(OH)2D3 in these tests, which like 20,25(OH)2D3, is a product of CYP27A1 action on 20(OH)D3. In agreement with previous studies [25], 20(OH)D3 and 1,25(OH)2D3 significantly inhibited colony formation by melanoma cells. The three new compounds, 20,24(OH)2D3, 20,25(OH)2D3 and 20,26(OH)2D3, at a concentration of 0.1 nM also significantly inhibited colony formation compared to the vehicle control (p<0.0005). Furthermore, they inhibited colony formation significantly more than both 1,25(OH)2D3 and the parent compound, 20(OH)D3. These significant differences were also seen when the secosteroid concentration was increased to 10 nM (Fig. 7).

Figure 7.

Figure 7

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The new secosteroids dramatically inhibit colony formation by SKMEL 188 melanoma cells. Colony formation was determined in soft agar. Data represent means ± S.D. (n= 4), and they were analyzed using GraphPad Prism and Student’s t test. Statistical significance: *, p < 0.05; **,p < 0.01; ***,p < 0.001.

4. Discussion

Analysis of the crystal structure of CYP24A1 [46] indicates that substrate enters the active site of CYP24A1 from an access channel open to the hydrophobic domain of the membrane bilayer. Entry of substrate into the active site from the membrane phase is also supported from studies on other mitochondrial P450s [17, 38, 42, 43, 47, 50]. Strong partitioning of the substrates for CYP24A1 such as 25(OH)D3 into the phospholipid bilayer has also been demonstrated [42]. To replicate this in vivo mechanism of substrate access to the active site of CYP24A1, we have analyzed the ability of the expressed rat enzyme to catalyze the hydroxylation of substrates incorporated into phospholipid vesicles. Using this reconstituted system we report the highest kcat values yet described for the metabolism of both 25(OH)D3 and 1,25(OH)2D3. Previously, CYP24A1 purified from isolated rat kidney mitochondria was reported to have a turnover number for 25(OH)D3 of 20 min−1 when assayed in aqueous buffer [1], approximately 40% of the value that we observed. Stimulation by 50% was observed by these authors when phosphatidylcholine was included in the incubation mixture.

It is interesting to compare the kinetic parameters for metabolism of 25(OH)D3 and 1,25(OH)2D3 by expressed rat CYP24A1, as there has been some debate in the past over which is the preferred substrate for the enzyme and over the methodology employed to determine this [51, 52]. For the purified rat enzyme using the phospholipid vesicle system, our data shows that CYP24A1 displays a slightly higher kcat/Km value with 1,25(OH)2D3 (9265 min−1(mol/mol phospholipid)−1) than with 25(OH)D3 (6722 min−1(mol/mol phospholipid)−1), at least for the initial 24-hydroxylation. Product analysis indicated that in the short incubation time used, the 24-hydroxyproduct dominated and thus we are essentially measuring only the first hydroxylation reaction. We also checked that product formation closely matched substrate depletion measured relative to an internal standard. The data shows that 1,25(OH)2D3 should be the preferred substrate for metabolism by CYP24A1 when substrate concentrations in the membrane are low (well below Km) where the velocity is determined by the term kcat/Km ([E][S]), where [E] is enzyme concentration and [S] is substrate concentration. This is consistent with the view of Jones and Tenenhouse [51] that 1,25(OH)2D3 is the preferred substrate for CYP24A1. The 3.8-fold higher kcat for 25(OH)D3 over that for 1,25(OH)2D3 which is in agreement with data reported by Taniguchi et al. (3.4-fold) [52], Annalora et al. (4.2-fold) [3] and Akiyoshi-Shibata et al. (2.4-fold) [2], indicates that rat CYP24A1 has a greater capacity to inactivate 25(OH)D3 when substrates are saturating. Other parameters in the living cell such as the presence of vitamin D binding protein, the concentrations of adrenodoxin and/or adrenodoxin reductase and the megalin transport system may also influence the relative rates of 1,25(OH)2D3 and 25(OH)D3 metabolism in vivo.

We compared the kinetics of 20(OH)D3 metabolism by rat CYP24A1 to those for 25(OH)D3 and 1,25(OH)2D3 metabolism in the phospholipid vesicle system and found that 20(OH)D3 showed a kcat only slightly lower than that for 1,25(OH)2D3 but a Km 19-fold higher. Its kcat was 5-fold lower than that for 25(OH)D3 and its Km was 3.5-fold higher. It is thus a considerably poorer substrate for CYP24A1 than 25(OH)D3 or 1,25(OH)2D3. However, its kcat/Km value of 386 min−1 (mol substrate/mol phospholipid)−1 is considerably higher than that for its metabolism to 20,23(OH)2D3 by CYP11A1 (24 min−1 (mol substrate/mol phospholipid)−1), an alternative pathway for 20(OH)D3 metabolism [17]. Like 25(OH)D3 and 1,25(OH)2D3, the favored site for initial hydroxylation of 20(OH)D3 by CYP24A1 is at C24. However, subsequent metabolism is clearly different as no subsequent oxidations were observed with 20(OH)D3, all products were dihydroxyvitamins D3 as shown by mass spectrometry. The presence of the 20-hydroxyl group prevents 23-hydroxylation as no 20,23(OH)2D3 or 20,23,24(OH)3D3 were seen among the products. Rather, the presence of the 20-hydroxyl group shifts the position of the side chain in the active site such that 20,25(OH)2D3 is the second major product of CYP24A1 action on 20(OH)D3. Other sites of hydroxylation in the minor products remain to be established.

Since the role of CYP24A1 is to inactivate 1,25(OH)2D3 [4], we tested the biological activity of the products of CYP24A1 action on 20(OH)D3. Inhibition of colony formation by SKMEL-188 melanoma cells in soft agar was used as the assay since the effect of the parent compound, 20(OH)D3, has been well documented in this system (see Section 1.) and the anchorage independent growth represents the best in vitro estimate of tumorigenicity. Both 20,24(OH)2D3 and 20,25(OH)2D3 caused significantly higher inhibition of colony formation in soft agar than the parent 20(OH)D3 and 1,25(OH)2D3 at concentrations of both 10−8 and 10−10 M. Since metabolism of 20(OH)D3 by CYP27A1 also produces 20,25(OH)2D3 as well as 20,26(OH)2D3 [38], we included the latter in our tests. It was also more potent than 20(OH)D3 and 1,25(OH)2D3in the inhibition of colony formation by melanoma cells.

In conclusion, the present study shows that CYP24A1 acts relatively slowly on 20(OH)D3 producing only dihydroxyvitamin D products, with the major ones being identified by NMR as 20,24(OH)2D3 and 20,25(OH)2D3. Both of these products inhibit anchorage-independent melanoma growth significantly more than the parent compound indicating that CYP24A1 plays an activating role on 20(OH)D3 rather than an inactivating role, which is in contrast to its well established inactivating activity on 1,25(OH)2D3. The anti-melanoma effects of 20,24(OH)2D3 and 20,25(OH)2D3, and the CYP27A1 product 20,26(OH)2D3, identify them as good candidates for further testing of their therapeutic utility in melanoma treatment.

Acknowledgments

This work was supported by NIH [Grant R01AR052190] to AS, by the University of Western Australia and by the College of Pharmacy at the University of Tennessee Health Science Center. The contents are solely the responsibility of the authors and do not necessarily represent the official views of the NIH.

Abbreviations

(20(OH)D3)

20S-hydroxyvitamin D3

(25(OH)D3)

25-hydroxyvitamin D3

(20,24(OH)2D3)

20S,24-dihydroxyvitamin D3

(20,25(OH)2D3)

20S,25-dihydroxyvitamin D3

(20,26(OH)2D3)

20S,26-dihydroxyvitamin D3

(1,25(OH)2D3)

1α,25-dihydroxyvitamin D3

ESI

electrospray ionization

HSQC

heteronuclear single quantum correlation

cyclodextrin

2-hydroxypropyl-β-cyclodextrin

Footnotes

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