Substitution at carbon 2 of 19-nor-1α,25-dihydroxyvitamin D3 with 3-hydroxypropyl group generates an analogue with enhanced chemotherapeutic potency in PC-3 prostate cancer cells

The active form of vitamin D(3), 1α,25-dihydroxyvitamin D(3)(1α,25(OH)(2)D(3)), has anti-proliferative and anti-invasive activities in prostate cancer cells. Because of 1α,25(OH)(2)D(3) therapeutic potential in treating cancers, numerous analogues have been synthesized with an attempt to increase anti-proliferative and/or decrease calcemic properties. Among these analogues, 19-nor-1α,25(OH)(2)D(2) while being less calcemic has equivalent potency as 1α,25(OH)(2)D(3) in several in vitro and in vivo systems. We recently showed that 19-nor-2α-(3-hydroxypropyl)-1α,25(OH)(2)D(3) (MART-10) was at least 500-fold and 10-fold more active than 1α,25(OH)(2)D(3) in inhibiting the proliferation of an immortalized normal prostate PZ-HPV-7 cells and the invasion of androgen insensitive PC-3 prostate cancer cells, respectively. In this study, we further investigated the effects of MART-10 and 1α,25(OH)(2)D(3) on the dose- and time-dependent induction of CYP24A1 gene expression in PC-3 prostate cancer cells. We found that MART-10 induced CYP24A1 gene expression at a lower concentration with a longer duration compared to 1α,25(OH)(2)D(3), suggesting that MART-10 is less susceptible to CYP24A1 degradation. Molecular docking model of human CYP24A1 and MART-10 indicates that its side chain is far away from the heme ion and is less likely to be hydroxylated by the enzyme. Furthermore, MART-10 was a more potent inhibitor of PC-3 cell proliferation and invasion compared to 1α,25(OH)(2)D(3). In addition, MART-10 down-regulated matrix metalloproteinase-9 (MMP-9) expression which could be one mechanism whereby MART-10 influences cancer cell invasion. Finally, we observed that subcutaneous administration of MART-10 up-regulated the CYP24A1 mRNA expression in rat kidneys without affecting their plasma calcium levels. Thus, our findings demonstrate that MART-10 is biologically active in vivo and may be an effective vitamin D analogue for clinical trials to treat prostate cancer.

Mechanism of the anti-proliferative action of 25-hydroxy-19-nor-vitamin D(3) in human prostate cells

According to the prevailing paradigm, 1α-hydroxylation of 25-hydroxyvitamin D(3) (25(OH)D(3)) and its analogs is a pre-requisite step for their biological effects. We previously reported that 25-hydroxy-19-nor-vitamin D(3) (25(OH)-19-nor-D(3)) had anti-proliferative activity in a cell line, PZ-HPV-7, which was derived from human non-cancerous prostate tissue, and suggested that 25(OH)-19-nor-D(3) acted after 1α-hydroxylation by vitamin D 1α-hydroxylase (CYP27B1). However, metabolic studies of 25(OH)-19-nor-D(3) using recombinant CYP27B1 revealed that 25(OH)-19-nor-D(3) was rarely subjected to 1α-hydroxylation. Therefore, in this report, we attempted to clarify the mechanism of 25(OH)-19-nor-D(3) action in intact cells using PZ-HPV-7 prostate cells. After incubating the cells with 25(OH)-19-nor-D(3), eight metabolites of 24-hydroxylase (CYP24A1) were detected, whereas no products of CYP27B1 including 1α,25-dihydroxy-19-nor-vitamin D(3) (1α,25(OH)(2)-19-nor-D(3)) were found. Furthermore, the time-dependent nuclear translocation of vitamin D receptor (VDR) and the subsequent transactivation of cyp24A1 gene in the presence of 25(OH)-19-nor-D(3) were almost identical as those induced by 1α,25(OH)(2)-19-nor-D(3). These results strongly suggest that 25(OH)-19-nor-D(3) directly binds to VDR as a ligand and transports VDR into the nucleus to induce transcription of cyp24A1 gene. In addition, knock down of cyp27B1 gene did not affect the anti-proliferative activity of 25(OH)-19-nor-D(3), whereas knock down of VDR attenuated the inhibitory effect. Thus, our results clearly demonstrate that the anti-proliferative activity of 25(OH)-19-nor-D(3) is VDR dependent but 1α-hydroxylation independent, suggesting that 25(OH)D(3) analogs such as 25(OH)-19-nor-D(3) could be attractive candidates for anticancer therapy.

Superior sagittal sinus thrombosis: a clinical and experimental study

Sinus-vein thrombosis is increasingly recognized as a much more frequent neurological disorder than was anticipated before. We examined the pathophysiology of superior sagittal sinus thrombosis (SSST) from 19 patients and a rat SSST model. We treated 19 cases with SSST who were diagnosed by angiography. The symptoms of nine patients, who suffered multiple intracerebral hemorrhage, were abrupt. In another ten patients who recovered satisfactorily, the condition progressed slowly and they were treated with heparin and urokinase. Multivariate analysis demonstrated that female, sudden onset (<24 hours) and posterior 1/3 occlusion are related to bad outcome. Experimentally, SSST was induced by ligation and slow injection of kaolin-cephalin suspension into SSS in rats. Regional cerebral blood flow (rCBF) and tissue hemoglobin oxygen saturation (Hb Sao(2)) using a "scanning" technique were measured at 48 locations, and fluorescence angiography was performed before and until 90 min after SSST induction. After 48 hours the animals were sacrificed for histological studies. Decrease of rCBF and tissue Hb SO(2) and brain damage were seen in group B (n = 10) with an extension of thrombosis from SSS into cortical veins. Brain injury was not observed in group A (n = 8) with SSS thrombus alone and sham-operated animals (n = 5). In conclusion, a brain with acute extension of thrombus from SSS into cortical veins becomes critical for cerebral blood supply and metabolism. CBF, tissue HbSO(2) and repeated angiography can be helpful monitors for the early detection of critical conditions after SSST. As to the therapy, restraint on the ongoing thrombus is essential to protect the brain with SSST, and we encourage the use of combination therapy of heparin and urokinase as early as possible in cases without intracerebral hemorrhage.

An experimental model of intraoperative venous injury in the rat

Intraoperative obliteration of cerebral veins occasionally causes unexpected severe complications, especially in elderly patients. However, very title information is available on the pathophysiology of cerebral venous circulation disturbance. Occlusion of cortical veins in rats by a photochemical thrombotic technique is a less invasive, clinically relevant and reproducible model that is suitable for the study of venous circulation disturbance. In the present study, 54 male Wistar rats were used. We examined changes of the cerebral venous flow pattern by fluorescence anglography and brain damage histologically in a one- or two-(cortical) vein occlusion model using a photochemical thrombotic technique. Approximately 30% (9 of 27) of animals in the single-vein occlusion group and 90% (15 of 17) of those in the two-vein occlusion group had microcirculation perturbation, which results very soon in the formation of venous thrombus accompanied by severe venous infarction. In addition, infarction size in the two-vein occlusion group (9.7 +/- 3.2%) was significantly larger than in the single-vein group (2.9 +/- 1.3%) (unpaired T-test, P < 0.01).In conclusion, the photochemical dye technique of attaining cerebral venous occlusion is a worth while addition to the study of circulation perturbations of the brain.

Sequential metabolism of sesamin by cytochrome P450 and UDP-glucuronosyltransferase in human liver

Our previous study revealed that CYP2C9 played a central role in sesamin monocatecholization. In this study, we focused on the metabolism of sesamin monocatechol that was further converted into the dicatechol form by cytochrome P450 (P450) or the glucuronide by UDP-glucuronosyltransferase (UGT). Catecholization of sesamin monocatechol enhances its antioxidant activity, whereas glucuronidation strongly reduces its antioxidant activity. In human liver microsomes, the glucuronidation activity was much higher than the catecholization activity toward sesamin monocatechol. In contrast, in rat liver microsomes, catecholization is predominant over glucuronidation. In addition, rat liver produced two isomers of the glucuronide, whereas human liver produced only one glucuronide. These results suggest a significant species-based difference in the metabolism of sesamin between humans and rats. Kinetic studies using recombinant human UGT isoforms identified UGT2B7 as the most important UGT isoform for glucuronidation of sesamin monocatechol. In addition, a good correlation was observed between the glucuronidation activity and UGT2B7-specific activity in in vitro studies using 10 individual human liver microsomes. These results strongly suggest that UGT2B7 plays an important role in glucuronidation of sesamin monocatechol. Interindividual difference among the 10 human liver microsomes is approximately 2-fold. These results, together with our previous results on the metabolism of sesamin by human P450, suggest a small interindividual difference in sesamin metabolism. We observed the methylation activity toward sesamin monocatechol by catechol O-methyl transferase (COMT) in human liver cytosol. On the basis of these results, we concluded that CYP2C9, UGT2B7, and COMT played essential roles in the metabolism of sesamin in the human liver.

Three-step hydroxylation of vitamin D3 by a genetically engineered CYP105A1: enzymes and catalysis

Our previous studies revealed that the double variant of cytochrome P450 (CYP)105A1, R73V/R84A, has a high ability to convert vitamin D(3) to its biologically active form, 1α,25-dihydroxyvitamin D(3) [1α,25(OH)(2)D(3)], suggesting the possibility for R73V/R84A to produce 1α,25(OH)(2)D(3). Because Actinomycetes, including Streptomyces, exhibit properties that have potential advantages in the synthesis of secondary metabolites of industrial and medical importance, we examined the expression of R73V/R84A in Streptomyces lividans TK23 cells under the control of the tipA promoter. As expected, the metabolites 25-hydroxyvitamin D(3) [25(OH)D(3)] and 1α,25(OH)(2)D(3) were detected in the cell culture of the recombinant S. lividans. A large amount of 1α,25(OH)(2)D(3), the second-step metabolite of vitamin D(3), was observed, although a considerable amount of vitamin D(3) still remained in the culture. In addition, novel polar metabolites 1α,25(R),26(OH)(3)D(3) and 1α,25(S),26(OH)(3)D(3), both of which are known to have high antiproliferative activity and low calcemic activity, were observed at a ratio of 5:1. The crystal structure of the double variant with 1α,25(OH)(2)D(3) and a docking model of 1α,25(OH)(2)D(3) in its active site strongly suggest a hydrogen-bond network including the 1α-hydroxyl group, and several water molecules play an important role in the substrate-binding for 26-hydroxylation. In conclusion, we have demonstrated that R73V/R84A can catalyze hydroxylations at C25, C1 and C26 (C27) positions of vitamin D(3) to produce biologically useful compounds.

Bioconversion of vitamin D to its active form by bacterial or mammalian cytochrome P450

Bioconversion processes, including specific hydroxylations, promise to be useful for practical applications because chemical syntheses often involve complex procedures. One of the successful applications of P450 reactions is the bioconversion of vitamin D₃ to 1α,25-dihydroxyvitamin D₃. Recently, a cytochrome P450 gene encoding a vitamin D hydroxylase from the CYP107 family was cloned from Pseudonocardia autotrophica and is now applied in the bioconversion process that produces 1α,25-dihydroxyvitamin D₃. In addition, the directed evolution study of CYP107 has significantly enhanced its activity. On the other hand, we found that Streptomyces griseolus CYP105A1 can convert vitamin D₃ to 1α,25-dihydroxyvitamin D₃. Site-directed mutagenesis of CYP105A1 based on its crystal structure dramatically enhanced its activity. To date, multiple vitamin D hydroxylases have been found in bacteria, fungi, and mammals, suggesting that vitamin D is a popular substrate of the enzymes belonging to the P450 superfamily. A combination of these cytochrome P450s would produce a large number of compounds from vitamin D and its analogs. Therefore, we believe that the bioconversion of vitamin D and its analogs is one of the most promising P450 reactions in terms of practical application.

Nutritional status affects fluvastatin-induced hepatotoxicity and myopathy in rats

Rats that consumed a high-fat and high-sucrose diet (HF diet) developed hepatic steatosis. Treatment of HF diet-fed rats with fluvastatin (8 mg/kg) was lethal, followed by an elevation in levels of plasma aspartate aminotransferase and creatine kinase activities and skeletal muscle toxicity. This study was conducted to determine whether nutritional status affects statin-induced adverse effects in rats. Fluvastatin treatment of rats fed the HF diet led to an increase in systemic exposure, suggesting altered metabolism and elimination. In fact, although hepatic multidrug resistance-associated protein (Mrp) 2 and multidrug resistance (Mdr) 1b protein levels were not significantly changed by fluvastatin treatment for 8 days of rats fed a HF diet, the organic anion-transporting protein (Oatp) 1, Mrp3, CYP1A, CYP2C, UDP-glucuronosyltransferase (UGT) 1A1, and UGT1A5 protein levels were moderately decreased and the Oatp2, CYP3A, and UGT2B1 protein levels were markedly suppressed. No significant difference in the baseline level of Oatp1, Oatp2, Mrp2, Mrp3, Mdr1b, CYP1A, CYP2C, CYP3A, UGT1A1, UGT1A5, or UGT2B1 protein was found between the standard diet- and HF diet-fed groups. In addition, the mRNA levels of Oatp2, CYP2C11, and CYP3A1/2 were markedly decreased in HF diet-fed and fluvastatin-treated rats. There was no significant difference in the glucuronidation activities against fluvastatin among the four groups. In liver cell nuclei, levels of constitutive androstane receptor, pregnane X receptor, and hepatocyte nuclear factor 4α proteins were decreased in fluvastatin-treated HF diet-fed rats, which correlated with the decrease in Oatp2, CYP2C, and CYP3A. Taken together, these results indicate that nutritional status may influence adverse effects of fluvastatin by increasing systemic exposure through modulation of hepatic uptake and elimination.

Atypical kinetics of cytochromes P450 catalysing 3'-hydroxylation of flavone from the white-rot fungus Phanerochaete chrysosporium

We cloned full-length cDNAs of 130 cytochrome P450s (P450s) derived from Phanerochaete chrysosporium and successfully expressed 70 isoforms in Saccharomyces cerevisiae. To elucidate substrate specificity of P. chrysosporium P450s, we examined various substrates including steroid hormones, several drugs, flavonoids and polycyclic aromatic hydrocarbons using the recombinant S. cerevisiae cells. Of these P450s, two CYPs designated as PcCYP50c and PcCYP142c with 14% identity in their amino acid sequences catalyse 3'-hydroxylation of flavone and O-deethylation of 7-ethoxycoumarin. Kinetic data of both enzymes on both reactions fitted not to the Michaelis-Menten equation but to Hill's equation with a coefficient of 2, suggesting that two substrates bind to the active site. Molecular modelling of PcCYP50c and a docking study of flavone to its active site supported this hypothesis. The enzymatic properties of PcCYP50c and PcCYP142c resemble mammalian drug-metabolizing P450s, suggesting that their physiological roles are metabolism of xenobiotics. It is noted that these unique P. chrysosporium P450s have a potential for the production of useful flavonoids.

Synthesis and biological activities of 14-epi-MART-10 and 14-epi-MART-11: implications for cancer and osteoporosis treatment

The 14-epimer of MART-10, namely 14-epi-MART-10 (14-epi-2alpha-(3-hydroxypropyl)-1alpha,25-dihydroxy-19-norvitamin D3) and its 2-epimeric analog (14-epi-MART-11) were efficiently synthesized using the Julia coupling reaction to connect between the C5 and C6 positions (steroid numbering). An A-ring precursor was prepared from (-)-quinic acid as shown in the previous MART-10 synthesis. The novel 14-epi-CD-ring coupling partner with an elongated two carbon unit as a sulfone was synthesized from 14-epi-25-hydroxy Grundmann's ketone in good yield. The subsequent coupling reaction followed by a deprotection step afforded a mixture of 14-epi-MART-10 and 14-epi-MART-11 in 40% yield. To separate 14-epi-MART-10 and 14-epi-MART-11, each primary hydroxyl group was esterified with a pivaloyl group and the resulting pivalates 2alpha and 2beta were separated by high performance liquid chromatography. After the separation, the C2-stereochemistry of each (2alpha or 2beta) was determined by 1H NMR (nuclear magnetic resonance) studies including NOE (nuclear Overhauser effect) experiments. The pivaloyl group was removed under basic conditions to obtain the target molecules of 14-epi-MART-10 and 14-epi-MART-11, respectively. The VDR (vitamin D receptor)-binding affinity, HL-60 (human promyelocytic leukemia) cell differentiation activity, antiproliferative activity in PZ-HPV-7 (immortalized normal prostate) cells and transactivation activity of the osteocalcin promoter in HOS (human osteoblast cell line) cells (serum-free conditions) were investigated. In addition, the effects on bone mineral density (BMD) and the blood and urine calcium concentrations of ovariectomized (OVX) rats were examined. 14-epi-MART-10 has much greater antiproliferative and cell differentiation activities compared to 1alpha,25-dihydroxyvitamin D3 (1alpha,25(OH)2D3).

Evaluation of 19-nor-2alpha-(3-hydroxypropyl)-1alpha,25-dihydroxyvitamin D3 as a therapeutic agent for androgen-dependent prostate cancer

The high incidence of prostate cancer and lack of an effective, long-term treatment for metastatic disease highlights the need for more potent non-calcemic vitamin D analogs as potential alternative or combinational prostate cancer therapies. Among the analogs, 19-nor-1alpha,25-dihydroxyvitamin D2 (19-nor-1alpha,25(OH)2D2) known as paricalcitol or Zempler, has less calcemic effects and an equipotential activity as 1alpha,25-dihydroxyvitamin D3 (1alpha,25(OH)2D3) in several in vivo and in vitro systems. It was recently demonstrated that a modified analog of paricalcitol, 19-nor-2alpha-(3-hydroxypropyl)-1alpha,25-dihydroxyvitamin D3 (MART-10) compared to 1alpha,25(OH)2D3 was more effective in inhibiting proliferation of an immortalized normal prostate cell line (PZ-HPV-7) (1,000-fold) and invasion of PC-3 prostate cancer cells (10-fold). In this study, the effects of MART-10 and 1alpha,25(OH)2D3 on proliferation, vitamin D receptor transactivation, vitamin D-binding protein (DBP) binding, CYP24A1 (24-OHase) substrate hydroxylation kinetics, and induction of CYP24A1 gene expression were compared in an androgen-dependent prostate cancer cell model, LNCaP. The results demonstrated that MART-10 was 1,000-fold more active than 1alpha,25(OH)2D3 in inhibiting LNCaP cell proliferation. MART-10 was more active than 1alpha,25(OH)2D3 in up-regulating a vitamin D receptor-responsive Luciferase construct and inducing CYP24A1 gene expression in LNCaP prostate cancer cells. In addition, MART-10 has a lower affinity for DBP and less substrate degradation by CYP24A1 compared to 1alpha,25(OH)2D3, indicating that MART-10 has more bioavailability and a longer half-life. Thus, these data suggest that MART-10 may be a potential candidate as a therapeutic agent for prostate cancer, especially for patients who fail in conventional therapies.

Functional analyses of cytochrome P450 genes responsible for the early steps of brassicicene C biosynthesis

We previously revealed that Orf8 and Orf6, which were identified in the brassicicene C biosynthetic gene cluster in Alternaria brassicicola strain ATCC96836, were fusicoccadiene (FD) synthase and 16-O-methyltransferase, respectively. In the present Letter, the early biosynthetic steps after the formation of FD were investigated. Plasmids carrying the FD synthase gene, one (or two) of five cytochrome P450 genes (orf1, orf2, orf5, orf7, and orf11) identified in the cluster and a cytochrome P450 reductase gene cloned from strain ATCC96836 were constructed and introduced into Saccharomyces cerevisiae. Based on the structures of the compounds produced by the transformants, Orf1 is suggested to be an 8beta-hydroxylation enzyme that yields FD 8beta-ol (4), followed by 16-hydroxylation by Orf7 to produce FD 8beta16-diol (5).

Enzymatic properties of cytochrome P450 catalyzing 3'-hydroxylation of naringenin from the white-rot fungus Phanerochaete chrysosporium

We cloned full-length cDNAs of more than 130 cytochrome P450s (P450s) derived from Phanerochaete chrysosporium, and successfully expressed 70 isoforms using a co-expression system of P. chrysosporium P450 and yeast NADPH-P450 reductase in Saccharomyces cerevisiae. Of these P450s, a microsomal P450 designated as PcCYP65a2 consists of 626 amino acid residues with a molecular mass of 68.3kDa. Sequence alignment of PcCYP65a2 and human CYP1A2 revealed a unique structure of PcCYP65a2. Functional analysis of PcCYP65a2 using the recombinant S. cerevisiae cells demonstrated that this P450 catalyzes 3'-hydroxylation of naringenin to yield eriodictyol, which has various biological and pharmacological properties. In addition, the recombinant S. cerevisiae cells expressing PcCYP65a2 metabolized such polyaromatic compounds as dibenzo-p-dioxin (DD), 2-monochloroDD, biphenyl, and naphthalene. These results suggest that PcCYP65a2 is practically useful for both bioconversion and bioremediation.

Quantitative analysis of UDP-glucuronosyltransferase (UGT) 1A and UGT2B expression levels in human livers

UDP-glucuronosyltransferases (UGTs) catalyze glucuronidation of a variety of xenobiotics and endobiotics. UGTs are divided into two families, UGT1 and UGT2. The purpose of this study was to estimate the absolute expression levels of each UGT isoform in human liver and to evaluate the interindividual variability. Real-time reverse transcriptase-polymerase chain reaction analysis was performed to determine the copy numbers of nine functional UGT1A isoforms and seven UGT2B isoforms. We noticed that not only primers but also templates as a standard for quantification should prudently be selected. Once we established appropriate conditions, the mRNA levels of each UGT isoform in 25 individual human livers were determined. UGT1A1 (0.9-138.5), UGT1A3 (0.1-66.6), UGT1A4 (0.1-143.3), UGT1A6 (1.0-70.4), UGT1A9 (0.3-132.4), UGT2B4 (0.3-615.0), UGT2B7 (0.2-97.4), UGT2B10 (0.7-253.2), UGT2B15 (0.3-107.8), and UGT2B17 (0.5-157.1) were substantially expressed (x10(4) copy/mug RNA) with large interindividual variability. Abundant isoforms were UGT2B4 and UGT2B10, followed by UGT1A1, UGT2B15, and UGT1A6. The sum of the UGT2B mRNA levels was higher than that of UGT1A mRNA levels. It is interesting to note that the mRNA levels normalized with glyceraldehyde-3-phosphate dehydrogenase mRNA for almost UGT isoforms that are substantially expressed in liver showed significant correlations to each other. Western blot analysis was performed using antibodies specific for UGT1A1, UGT1A4, UGT1A6, or UGT2B7. Correlation between the protein and mRNA levels was observed in only UGT1A1 (r = 0.488; p < 0.01). In conclusion, this study comprehensively determined the absolute values of mRNA expression of each UGT isoform in human livers and found considerable interindividual variability.

Metabolism of 1alpha,25-dihydroxyvitamin D2 by human CYP24A1

The metabolism of 1alpha,25-dihydroxyvitamin D2 (1alpha,25(OH)2D2) by human CYP24A1 was examined using the recombinant enzyme expressed in Escherichia coli cells. HPLC analysis revealed that human CYP24A1 produces at least 10 metabolites, while rat CYP24A1 produces only three metabolites, indicating a remarkable species-based difference in the CYP24A1-dependent metabolism of 1alpha,25(OH)2D2 between humans and rats. LC-MS analysis and periodate treatment of the metabolites strongly suggest that human CYP24A1 converts 1alpha,25(OH)2D2 to 1alpha,24,25,26(OH)4D2, 1alpha,24,25,28(OH)4D2, and 24-oxo-25,26,27-trinor-1alpha(OH)D2 via 1alpha,24,25(OH)3D2. These results indicate that human CYP24A1 catalyzes the C24-C25 bond cleavage of 1alpha,24,25(OH)2D2, which is quite effective in the inactivation of the active form of vitamin D2. The combination of hydroxylation at multiple sites and C-C bond cleavage could form a large number of metabolites. Our findings appear to be useful to predict the metabolism of vitamin D2 and its analogs in the human body.

The aryl hydrocarbon receptor activator benzo[a]pyrene enhances vitamin D3 catabolism in macrophages

Benzo[a]pyrene (BaP), a polycyclic aromatic hydrocarbon produced by cigarette combustion, is implicated as a causative agent in smoking-related cancer and atherosclerosis. 1,25-Dihydroxyvitamin D3 [1,25(OH)2D3], a potent ligand for the nuclear receptor vitamin D receptor (VDR), has been shown to decrease the risk of osteoporosis, some types of cancer and cardiovascular disease, suggesting an opposing effect of vitamin D3 to cigarette smoking. In this study, we investigated the effects of BaP on the vitamin D3 signaling pathway. BaP effectively enhanced the 1,25(OH)2D3-dependent induction of cytochrome P450 24A1 (CYP24A1) in human monocyte/macrophage-derived THP-1 cells and breast cancer MCF-7 cells. BaP combination was less or not effective on mRNA expression of CD14, arachidonate 5-lipoxygenase, and cathelicidin antimicrobial peptide in THP-1 cells. BaP also increased the expression of CYP24A1 induced by a non-vitamin D VDR ligand, lithocholic acid acetate. Another aryl hydrocarbon receptor (AhR) ligand, 2,3,7,8-tetrachlorodibenzo-p-dioxin, enhanced CYP24A1 expression by 1,25(OH)2D3 in THP-1 cells. Treatment of cells with an AhR antagonist and a protein synthesis inhibitor inhibited the enhancing effect of BaP on CYP24A1 induction, indicating that the effects of BaP are mediated by AhR activation and de novo protein synthesis. BaP pretreatment increased 1,25(OH)2D3-dependent recruitment of VDR and retinoid X receptor to the CYP24A1 promoter. Analysis of 1,25(OH)2D3 metabolism showed that BaP enhanced the hydroxylation of 1,25(OH)2D3 by CYP24A1 in THP-1 cells. Thus, AhR activation by BaP stimulates vitamin D3 catabolism. Modulation of vitamin D signaling by AhR may represent a mechanism underlying cigarette smoking-related diseases.