23-carboxy-24,25,26,27-tetranorvitamin D3 (calcioic acid) and 24-carboxy-25,26,27-trinorvitamin D3 (cholacalcioic acid): end products of 25-hydroxyvitamin D3 metabolism in rat kidney through C-24 oxidation pathway

During the past two and half decades the elucidation of the metabolic pathways of 25OHD(3) and its active metabolite 1alpha,25(OH)(2)D(3) progressed in parallel. In spite of many advances in this area of vitamin D research, the unequivocal identification of the end products of 25OHD(3) metabolism through C-24 oxidation pathway has not been achieved. It is now well established that both 25OHD(3) and 1alpha,25(OH)(2)D(3) are metabolized through the same C-24 oxidation pathway initiated by the enzyme 24-hydroxylase (CYP24A1). Based on the information that the end product of 1alpha,25(OH)(2)D(3) metabolism through C-24 oxidation pathway is 1alpha-OH-23- COOH-24,25,26,27-tetranor D(3) or calcitroic acid; the metabolism of 25OHD(3) into 23-COOH-24,25,26,27-tetranor D(3) has been assumed. Furthermore, a previous study indicated 24-COOH-25,26,27-trinor D(3) as a water soluble metabolite of 24R,25(OH)(2)D(3) produced in rat kidney homogenates. Therefore, 24-COOH-25,26,27-trinor D(3) was also assumed as another end product of 25OHD(3) metabolism through C-24 oxidation pathway. We embarked on our present study to provide unequivocal proof for these assumptions. We first studied the metabolism of 25OHD(3) at low substrate concentration (3x10(-10)M) using [1,2-(3)H]25OHD(3) as the substrate in the perfused rat kidneys isolated from both normal and vitamin D(3) intoxicated rats. A highly polar water soluble metabolite, labeled as metabolite X was isolated from the kidney perfusate. The amount of metabolite X produced in the kidney of a vitamin D intoxicated rat was about seven times higher than that produced in the kidney of a normal rat. We then produced metabolite X in a quantity sufficient for its structure identification by perfusing kidneys isolated from vitamin D intoxicated rats with high substrate concentration of 25OHD(3) (5x10(-6)M). Using the techniques of electron impact and thermospray mass spectrometry, we established that the metabolite X contained both 23-COOH-24,25,26,27-tetranor D(3) and 24-COOH-25,26,27-trinor D(3) in a ratio of 4:1. The same metabolite X containing both acids in the same ratio of 4:1 was also produced when 24R,25(OH)(2)D(3) was used as the starting substrate. Previously, the trivial name of cholacalcioic acid was assigned to 24-COOH-25,26,27-trinorvitamin D(3). Using the same guidelines, we now assign the trivial name of calcioic acid to 23-COOH-24,25,26,27-tetranor D(3). In summary, for the first time our study provides unequivocal evidence to indicate that both calcioic and cholacalcioic acids as the end products of 25OHD(3) metabolism in rat kidney through C-24 oxidation pathway.     

Antagonistic action of novel 1alpha,25-dihydroxyvitamin D3-26, 23-lactone analogs on differentiation of human leukemia cells (HL-60) induced by 1alpha,25-dihydroxyvitamin D3.

We examined the effects of two novel 1alpha,25-dihydroxyvitamin D3-26,23-lactone (1alpha,25-lactone) analogues on human promyelocytic leukemia cell (HL-60) differentiation using the evaluation system of the vitamin D nuclear receptor (VDR)/vitamin D-responsive element (DRE)-mediated genomic action stimulated by 1alpha,25-dihydroxyvitamin D3 (1alpha,25(OH)2D3) and its analogues. We found that the 1alpha,25-lactone analogues (23S)-25-dehydro-1alpha-hydroxyvitamin-D3-26,23-lactone (TEI-9647), and (23R)-25-dehydro-1alpha-hydroxyvitamin-D3-26,23-lactone (TEI-9648) bound much more strongly to the VDR than the natural (23S, 25R)-1alpha,25(OH)2D3-26,23-lactone, but did not induce cell differentiation even at high concentrations (10(-6) M). Intriguingly, the differentiation of HL-60 cells induced by 1alpha,25(OH)2D3 was inhibited by either TEI-9647 or TEI-9648 but not by the natural lactone. In contrast, retinoic acid or 12-O-tetradecanoylphorbol-13-acetate-induced HL-60 cell differentiation was not blocked by TEI-9647 or TEI-9648. In separate studies, TEI-9647 (10(-7) M) was found to be an effective antagonist of both 1alpha,25(OH)2D3 (10(-8) M) mediated induction of p21(WAF1, CIP1) in HL-60 cells and activation of the luciferase reporter assay in COS-7 cells transfected with cDNA containing the DRE of the rat 25(OH)D3-24-hydroxylase gene and cDNA of the human VDR. Collectively the results strongly suggest that our novel 1alpha,25-lactone analogues, TEI-9647 and TEI-9648, are specific antagonists of 1alpha, 25(OH)2D3 action, specifically VDR/DRE-mediated genomic action. As such, they represent the first examples of antagonists, which act on the nuclear VDR.     

Skin is an autonomous organ in synthesis, two-step activation and degradation of vitamin D(3): CYP27 in epidermis completes the set of essential vitamin D(3)-hydroxylases

The current understanding of the vitamin D(3) system shows skin as the unique site of vitamin D(3) production and liver is thought to be the main site of conversion to 25(OH)D(3). Skin is capable of activating 25(OH)D(3) via 1alpha-hydroxylation and the resulting 1alpha,25(OH)(2)D(3) plays a role in epidermal homeostasis in normal and diseased skin. It also rapidly up-regulates the major vitamin D(3) metabolizing enzyme 24-hydroxylase at the mRNA level, which is an established indicator for 1alpha,25(OH)(2)D(3)-presence. We investigated the capability of primary human keratinocytes to produce 25(OH)D(3) and subsequent metabolites from vitamin D(3). Thus, by orchestrating the entire system of production, activation and inactivation, skin could be independent of other organs in supply of hormonally active vitamin D(3). First, we demonstrated substantial conversion of (3)H-D(3) to (3)H-25(OH)D(3) in primary human keratinocytes. 25-Hydroxylation was slow, followed first order rate kinetics and was not saturable under our experimental conditions. Then we showed expression of 25-hydroxylase mRNA and compared it to levels of 1alpha-hydroxylase and 24-hydroxylase. Pre-incubation with vitamin D(3) resulted in dose and time dependent up-regulation of 24-hydroxylase mRNA, whereas neither 1alpha-hydroxylase nor 25-hydroxylase expression was affected. Since both, D(3) and 25(OH)D(3) are lacking intrinsic 24-hydroxylase-inducing capacity, up-regulation had to be the consequence of a two-step activation process via 25-hydroxylation and subsequent 1alpha-hydroxylation. 24-Hydroxylase-activities closely followed the corresponding mRNA levels. When 1alpha,25(OH)(2)D(3) itself or its precursor 25(OH)D(3) were used as inducing agents, 24-hydroxylase mRNA and enzyme activity followed a transient time course. In contrast, induction observed with physiological doses of D(3) remained high, even after a 20 h-time period. These differing characteristics may be explained by the slow but constant formation of 1alpha,25(OH)(2)D(3) from a large reservoir of D(3) in the target cell, providing constant supplies for induction.     

Natural metabolites of 1alpha,25-dihydroxyvitamin D(3) retain biologic activity mediated through the vitamin D receptor

1alpha,25-dihydroxyvitamin D(3) (1alpha,25(OH)(2)D(3)), the active metabolite of vitamin D, mediates many of its effects through the intranuclear vitamin D receptor (VDR, NR1I1), that belongs to the large superfamily of nuclear receptors. Vitamin D receptor can directly regulate gene expression by binding to vitamin D response elements (VDREs) located in promoter or enhancer regions of various genes. Although numerous synthetic analogs of 1alpha,25(OH)(2)D(3) have been analysed for VDR binding and transactivation of VDRE-driven gene expression, the biologic activity of many naturally occurring metabolites has not yet been analyzed in detail. We therefore studied the transactivation properties of 1alpha,24R, 25-trihydroxyvitamin D(3) (1alpha,24R,25(OH)(3)D(3)), 1alpha, 25-dihydroxy-3-epi-vitamin D(3) (1alpha,25(OH)(2)-3-epi-D(3)), 1alpha,23S,25-trihydroxyvitamin D(3) (1alpha,23S,25(OH)(3)D(3)), and 1alpha-hydroxy-23-carboxy-24,25,26,27-tetranorvitamin D(3) (1alpha(OH)-24,25,26,27-tetranor-23-COOH-D(3); calcitroic acid) using the human G-361 melanoma cell line. Cells were cotransfected with a VDR expression plasmid and luciferase reporter gene constructs driven by two copies of the VDRE of either the mouse osteopontin promoter or the 1alpha,25(OH)(2)D(3) 24-hydroxylase (CYP24) promoter. Treatment with 1alpha,25(OH)(2)D(3) or the metabolites 1alpha,24R,25(OH)(3)D(3), 1alpha,25(OH)(2)-3-epi-D(3), and 1alpha,23S,25(OH)(3)D(3) resulted in transactivation of both constructs in a time- and dose-dependent manner, and a postitive regulatory effect was observed even for calcitroic acid in the presence of overexpressed VDR. The metabolites that were active in the reporter gene assay also induced expression of CYP24 mRNA in the human keratinocyte cell line HaCaT, although with less potency than the parent hormone. A ligand-binding assay based on nuclear extracts from COS-1 cells overexpressing human VDR demonstrated that the metabolites, although active in the reporter gene assay, were much less effective in displacing [(3)H]-labeled 1alpha,25(OH)(2)D(3) from VDR than the parent hormone. Thus, we report that several natural metabolites of 1alpha,25(OH)(2)D(3) retain significant biologic activity mediated through VDR despite their apparent low affinity for VDR.     

Anticancer effect of retinoic acid via AP-1 activity repression is mediated by retinoic acid receptor alpha and beta in gastric cancer cells

To uncover the mechanisms relating to the anticancer effect of retinoic acids in gastric cancer cells, the mediation of activator protein-1 (AP-1) activity repression by retinoic acid receptors (RARs) was investigated. All-trans retinoic acid (ATRA) inhibited AP-1 activity in BGC-823 cells (RARalpha(+), RARbeta(+)), but not in MKN-45 cells (RARalpha(lo), RARbeta(-)). Transient transfection of RARbeta expression vector into MKN-45 cells significantly resulted in direct repression of AP-1 activity in a receptor concentration-dependent manner, and this could be strengthened by ATRA. Stable transfection of RARbeta into MKN-45 cells directly inhibited cell growth and colony formation, and ATRA also enhanced these effects. Transient transfection of RARalpha into MKN-45 cells however, displayed receptor concentration-dependent AP-1 activity inhibition only in the presence of ATRA. Stable transfection of RARalpha into MKN-45 cells resulted in ATRA-dependent inhibition of cell growth and colony formation. For AP-1 binding activity induced by TPA, the repressive effect of ATRA was only observed in BGC-823 and RARalpha and RARbeta stably transfected MKN-45 cells, but not in intact MKN-45 cells. This indicates the necessity for sufficient cellular RARalpha and/or RARbeta in order for AP-1 activity repression to occur. Deletion of DNA binding domain (DBD) of RARbeta, but not ligand binding domain (LBD), eliminated the anti-AP-1 function of RARbeta. It is therefore concluded that both RARalpha and RARbeta are mediators in the anticancer function of ATRA via AP-1 activity inhibition, and that RARbeta, not RARalpha, can inhibit AP-1 activity to a certain extent directly by itself. Thus DBD, not LBD, is critical for anti-AP-1 activity.     

Regulation of aromatase and 5alpha-reductase by 25-hydroxyvitamin D(3), 1alpha,25-dihydroxyvitamin D(3), dexamethasone and progesterone in prostate cancer cells

Estrogens and androgens are proposed to play a role in the pathogenesis of prostate cancer. The effective metabolites, estradiol and 5alpha-dihydrotestosterone are produced from testosterone by aromatase and 5alpha-reductase, respectively. Metabolites of vitamin D have shown to inhibit the growth of prostate cancer cells. The aim of the present study was to verify whether 25-hydroxyvitamin D(3) (25OHD(3)), 1alpha,25-dihydroxyvitamin D(3) [1alpha,25-(OH)(2)D(3)], dexamethasone, and progesterone regulate the expression of aromatase and 5alpha-reductase in human prostate cancer cells. LNCaP and PC3 cells were treated with 25OHD(3), 1alpha,25-(OH)(2)D(3), dexamethasone, or progesterone. Aromatase and 5alpha-reductase mRNA was quantified by real-time RT-PCR and aromatase enzyme activity was measured by the [(3)H] water assay. Aromatase enzyme activity in LNCaP and PC3 cells was increased by both 10nM dexamethasone, 1-100 nM 1alpha,25-(OH)(2)D(3) and 100 nM-10 microM progesterone. The induction was enhanced when hormones were used synergistically. Real-time RT-PCR analysis showed no regulation of the expression of aromatase mRNA by any steroids tested in either LNCaP or PC3 cells. The expression of 5alpha-reductase type I mRNA was not regulated by 1alpha,25-(OH)(2)D(3) and no expression of 5alpha-reductase type II was detected in LNCaP.     

(23S)- and (23R)-25-dehydro-1alpha-hydroxyvitamin D(3)-26,23-lactone function as antagonists of vitamin D receptor-mediated genomic actions of 1alpha,25-dihydroxyvitamin D(3)

We synthesized various analogues of 1alpha,25-(OH)(2)D(3)-26,23-lactone and examined the effects of them on HL-60 cell differentiation using the evaluation system of the genomic action of 1alpha,25-(OH)(2)D(3). We found that (23S)- and (23R)-25-dehydro-1alpha-OH-D(3)-26,23-lactone (TEI-9647 and TEI-9648) strongly bound to the VDR, but did not induce HL-60 cell differentiation. Intriguingly, TEI-9647 and TEI-9648 did inhibit that induced by 1alpha,25-(OH)(2)D(3), whereas they did not suppress that caused by retinoic acid or TPA. On the contrary, the similar 25-dehydrated 24-dehydro analogues, TEI-D1807 and TEI-D1808, weakly but significantly induced HL-60 cell differentiation, never showing inhibitory effect on HL-60 cell differentiation induced by 1alpha,25-(OH)(2)D(3). In other experiments, TEI-9647 and TEI-9648 markedly suppressed 25-OH-D(3)-24-hydroxylase gene expression induced by 1alpha,25-(OH)(2)D(3) in HL-60 cells. TEI-9647 also inhibited the heterodimer formation between VDR and RXRalpha, and the VDR interaction with co-activator SRC-1 according to the results obtained from the mammalian two-hybrid system in Saos-2 cells. Taking all these results into consideration, we reached a manifest conclusion that TEI-9647 and TEI-9648 are the specific and first antagonists of 1alpha,25-(OH)(2)D(3) action, specifically VDR-VDRE mediated genomic action.     

25-hydroxy-vitamin d metabolism in human colon cancer cells during tumor progression

RT-PCR analysis showed elevated expression of 25-hydroxyvitamin D-1alpha-hydroxylase (1alpha-OHase) and of 25-hydroxyvitamin D-24-hydroxylase (24-OHase) in well differentiated human colon carcinomas in comparison to normal mucosa. Further tumor progression is associated with a rise in 1alpha-OHase but with no significant change in 24-OHase mRNA expression. Accordingly, HPLC analysis of 25-hydroxy-vitamin D3 metabolism in freshly isolated tumor cells indicated that well to moderately differentiated colon cancers in situ are able to produce 1alpha,25-dihydroxyvitamin D3 (1alpha,25-(OH)2D3) and convert it through 24-OHase activity into side-chain modified metabolites, 1,24,25-(OH)3-D3 and 1,25-(OH)2- 24-oxo-D3. Likewise, 25-(OH)-D3 is metabolized into 24,25-(OH)2D3, 23,25-(OH)2D3, and 23,25-(OH)2-24-oxo-D3. Poorly-differentiated cancers expressed low levels of 1alpha-OHase mRNA, whereas 24-OHase was even over-expressed. RT-PCR and HPLC analysis of vitamin D metabolism in primary culture cell clones strongly suggested that the extent of endogenously produced 1alpha,25-(OH)2-D3 was inversely related to 24-OHase activity, which could thus limit the antimitotic efficacy of 1alpha,25-(OH)2-D3 particularly at late stages of colon cancer progression.     

Vitamin D antagonist, TEI-9647, inhibits osteoclast formation induced by 1alpha,25-dihydroxyvitamin D3 from pagetic bone marrow cells

(23S)-25-Dehydro-1alpha-hydroxyvitamin D(3)-26,23-lactone (TEI-9647) functions an antagonist of the 1alpha,25-dihydroxyvitamin D(3) (1alpha,25-(OH)(2)D(3)) nuclear receptor (VDR)-mediated differentiation of human leukemia (HL-60) cells [J. Biol. Chem. 274 (1999) 16392]. We examined the effect of vitamin D antagonist, TEI-9647, on osteoclast formation induced by 1alpha,25-(OH)(2)D(3) from bone marrow cells of patients with Paget's disease. TEI-9647 itself never induced osteoclast formation even at 10(-6)M, but dose-dependently (10(-10) to 10(-6)M) inhibited osteoclast formation induced by physiologic concentrations of 1alpha,25-(OH)(2)D(3) (41 pg/ml, 10(-10)M) from bone marrow cells of patients with Paget's disease. At the same time, 10(-8)M of TEI-9647 alone did not cause 1alpha,25-(OH)(2)D(3) dependent gene expression, but almost completely suppressed TAF(II)-17, a potential coactivator of VDR and 25-hydroxyvitamin D(3)-24-hydroxylase (25-OH-D(3)-24-hydroxylase) gene expression induced by 10(-10)M 1alpha,25-(OH)(2)D(3) in bone marrow cells of patients with Paget's disease. Moreover, TEI-9647 dose-dependently inhibited bone resorption induced by 10(-9)M 1alpha,25-(OH)(2)D(3) by osteoclasts produced by RANKL and M-CSF treatment of measles virus nucleocapsid gene transduced bone marrow cells. These results suggest that TEI-9647 acts directly on osteoclast precursors and osteoclasts, and that TEI-9647 may be a novel agent to suppress the excessive bone resorption and osteoclast formation in patients with Paget's disease.     

A variant form of the 1,25-dihydroxyvitamin D3 receptor with low apparent hormone affinity in cultured monkey kidney cells (LLC-MK2). A model for tissue resistance to vitamin D

Chandler et al. (Chandler, J.S., Chandler, S.K., Pike, J.W., and Haussler, M.R. (1984) J. Biol. Chem. 259, 2214-2222) previously demonstrated that 1,25-dihydroxyvitamin D3 (1,25-(OH)2D3) caused the induction of 25-hydroxyvitamin D3-24-hydroxylase (24-hydroxylase) in a rhesus monkey kidney cell line (LLC-MK2) apparently deficient in the high affinity 1,25-(OH)2D3 receptor. We have re-examined this phenomenon and report here that 24-hydroxylase induction is mediated by a receptor variant in LLC-MK2 cells with low hormone affinity. Dose response analysis showed that in contrast to LLC-PK1 (a typical receptor-positive cell line), the LLC-MK2 line was less sensitive to 1,25-(OH)2D3 by 2 orders of magnitude. Employing optimal concentrations of 1,25-(OH)2D3 for 24-hydroxylase induction in each cell type, the early time courses of this bioresponse were identical in LLC-MK2 and LLC-PK1 and were consistent with a nuclear action of hormone-receptor complexes. Moreover, the rank order of potency of vitamin D3 congeners as inducers of 24-hydroxylase activity in LLC-MK2 cells agreed well with their relative affinity for the 1,25-(OH)2D3 receptor. An examination of 1,25-(OH)2D3 receptor content via DNA-cellulose chromatography in LLC-MK2 cells incubated at ligand concentrations 10-25-fold higher than the normal 2 nM revealed a minimum of 1600 receptor-like molecules/LLC-MK2 cell. These results show that LLC-MK2 cells possess a variant receptor form with apparent low affinity for 1,25-(OH)2D3. This system should serve as a model for clinical syndromes characterized by the requirement for massive doses of vitamin D to prevent rickets.     

A rapid and sensitive in vitro assay of 25-hydroxyvitamin D3-1 alpha-hydroxylase and 24-hydroxylase using rat kidney homogenates

A sensitive and rapid in vitro assay of 25-hydroxyvitamin D3 [25-(OH)D3]-1 alpha- and 24-hydroxylase activities was developed using rat kidney homogenates. A potent inhibitor of the enzymes in rat plasma was removed by thoroughly perfusing rats with saline. Kidney homogenates prepared from vitamin D-deficient rats preferentially produced tritiated 1 alpha,25-dihydroxyvitamin D3 [1 alpha,25(OH)2D3] from 25(OH) [3H]D3. Addition of 10 microliter or more of rat plasma to 3 ml of 10% kidney homogenates suppressed 1 alpha-hydroxylase activity dose-dependently. Thyroparathyroidectomy (TPTX) of vitamin D-deficient rats greatly abolished 1 alpha-hydroxylase activity. Administration of parathyroid hormone to the TPTX rats increased 1 alpha-hydroxylase activity and that of 1 alpha,25(OH)2D3 enhanced 24-hydroxylase markedly. Since this assay is technically simple, rapid and sensitive, it will be useful in studying the regulatory mechanism in the renal metabolism of 25(OH)D3 in mammals.     

1 alpha,25-dihydroxyvitamin D3 modulation in lipid metabolism in established bone marrow-derived stromal cells, MC3T3-G2/PA6.

MC3T3-G2/PA6 (PA6) cells established from newborn mouse calvaria are preadipocytic stromal cells, which differentiate into adipocytes in response to glucocorticoids. We examined the effects of 1 alpha,25-dihydroxyvitamin D3[1 alpha,25(OH)2D3] on adipogenesis in PA6 cells. When PA6 cells were cultured with 10(-8) M dexamethasone, adipocytes containing oil red O-positive droplets first appeared on day 7 (3 days after confluence was attained) and the maximal synthesis of neutral lipids occurred on day 12. Simultaneous addition of 1 alpha,25(OH)2D3 at 10(-9)M completely blocked this dexamethasone-induced neutral lipid synthesis throughout the 14-day culture period. Dose-response studies of vitamin D3 derivatives showed that 1 alpha,25(OH)2D3 was the most potent in inhibiting neutral lipid synthesis in PA6 cells, followed by 1 alpha-hydroxyvitamin D3, 25-hydroxyvitamin D3, and 24R,25-dihydroxyvitamin D3, in that order. Dexamethasone greatly enhanced incorporation of [14C]-acetic acid into triacylglycerol in PA6 cells. The incorporation was markedly inhibited by the addition of 10(-9) M 1 alpha,25(OH)2D3. Instead, 1 alpha,25(OH)2D3 greatly increased incorporation of [14C]-acetic acid into phospholipids, such as phosphatidylcholine and phosphatidylethanolamine, irrespective of the presence or absence of dexamethasone. These results suggest that 1 alpha,25(OH)2D3 modulation of lipid metabolism in bone marrow stromal cells is receptor mediated.     

C-25 hydroxylation of 1alpha,24(R)-dihydroxyvitamin D3 is catalyzed by 25-hydroxyvitamin D3-24-hydroxylase (CYP24A1): metabolism studies with human keratinocytes and rat recombinant CYP24A1

Recently, 25-hydroxyvitamin D3-24-hydroxylase (CYP24A1) has been shown to catalyze not only hydroxylation at C-24 but also hydroxylations at C-23 and C-26 of the secosteroid hormone 1alpha, 25-dihydroxyvitamin D3 (1alpha,25(OH)2D3). It remains to be determined whether CYP24A1 has the ability to hydroxylate vitamin D3 compounds at C-25. 1alpha,24(R)-dihydroxyvitamin D3 (1alpha,24(R)(OH)2D3) is a non-25-hydroxylated synthetic vitamin D3 analog that is presently being used as an antipsoriatic drug. In the present study, we investigated the metabolism of 1alpha,24(R)(OH)2D3 in human keratinocytes in order to examine the ability of CYP24A1 to hydroxylate 1alpha,24(R)(OH)2D3 at C-25. The results indicated that keratinocytes metabolize 1alpha,24(R)(OH)2D3 into several previously known both 25-hydroxylated and non-25-hydroxylated metabolites along with two new metabolites, namely 1alpha,23,24(OH)3D3 and 1alpha,24(OH)2-23-oxo-D3. Production of the metabolites including the 25-hydroxylated ones was detectable only when CYP24A1 activity was induced in keratinocytes 1alpha,25(OH)2D3. This finding provided indirect evidence to indicate that CYP24A1 catalyzes C-25 hydroxylation of 1alpha,24(R)(OH)2D3. The final proof for this finding was obtained through our metabolism studies using highly purified recombinant rat CYP24A1 in a reconstituted system. Incubation of this system with 1alpha,24(R)(OH)2D3 resulted in the production of both 25-hydroxylated and non-25-hydroxylated metabolites. Thus, in our present study, we identified CYP24A1 as the main enzyme responsible for the metabolism of 1alpha,24(R)(OH)2D3 in human keratinocytes, and provided unequivocal evidence to indicate that the multicatalytic enzyme CYP24A1 has the ability to hydroxylate 1alpha,24(R)(OH)2D3 at C-25.     

Antagonistic action of novel 1alpha,25-dihydroxyvitamin D(3)-26, 23-lactone analogs on 25-hydroxyvitamin-D(3)-24-hydroxylase gene expression induced by 1alpha,25-dihydroxy-vitamin D(3) in human promyelocytic leukemia (HL-60) cells

We have demonstrated that 1alpha,25-dihydroxyvitamin D(3)-26, 23-lactone analogs, (23S)- and (23R)-25-dehydro-1alpha-hydroxyvitamin D(3)-26,23-lactone (TEI-9647, TEI-9648, respectively), inhibit HL-60 cell differentiation induced by 1alpha,25-dihydroxyvitamin D(3) [1alpha,25(OH)(2)D(3)], but not differentiation caused by all-trans retinoic acid (D. Miura et al., 1999, J. Biol. Chem. 274, 16392). To assess whether the antagonistic actions of TEI-9647 and TEI-9648 in HL-60 cells are related to 1alpha,25(OH)(2)D(3) breakdown, we investigated their effects on catabolism of 1alpha,25(OH)(2)D(3). In HL-60 cells, the C-24 but not the C-23 side-chain oxidation pathway of 1alpha,25(OH)(2)D(3) has been reported. Here we demonstrate that 1alpha,25(OH)(2)D(3) was metabolized both to 24,25,26,27-tetranor-1alpha,23-(OH)(2)D(3) and 1alpha,25(OH)(2)D(3)-26,23-lactone; thus HL-60 cells constitutively possess both the 24- and the 23-hydroxylases. Metabolism of 1alpha, 25(OH)(2)D(3) was strongly suppressed by 10(-7) M TEI-9647 or 10(-6) M TEI-9648. 1alpha,25(OH)(2)D(3) alone slightly induced 24-hydroxylase gene expression by 8 h with full enhancement by 24-48 h; this induction was inhibited by 10(-6) M TEI-9647 and 10(-6) M TEI-9648 (86.2 and 31.9%, respectively) 24 h after treatment. However, analogs of TEI-9647 and TEI-9648 without the 25-dehydro functionality induced 24-hydroxylase gene expression. These results indicate that TEI-9647 and TEI-9648 clearly mediate their stereoselective antagonistic actions independent of their actions to block the catabolism of 1alpha,25(OH)(2)D(3). Therefore, TEI-9647 and TEI-9648 appear to be the first antagonists specific for the nuclear 1alpha,25(OH)(2)D(3) receptor-mediated genomic actions of 1alpha,25(OH)(2)D(3) in HL-60 cells.     

Differential regulation of protein expression, growth and apoptosis by natural and synthetic retinoids

All-trans retinoic acid (ATRA) can down regulate the anti-apoptotic protein Bcl-2 and the cell cycle proteins cyclin D1 and cdk2 in estrogen receptor-positive breast cancer cells. We show here that retinoids can also reduce expression of the inhibitor of apoptosis protein, survivin. Here we have compared the regulation of these proteins in MCF-7 and ZR-75 breast cancer cells by natural and synthetic retinoids selective for the RA receptors (RARs) alpha, beta, and gamma then correlated these with growth inhibition, induction of apoptosis and chemosensitization to Taxol. In both cell lines ATRA and 9-cis RA induced the most profound decreases in cyclin D1 and cdk2 expression and also mediated the largest growth inhibition. The RARalpha agonist, Ro 40-6055 also strongly downregulated these proteins although did not produce an equivalent decrease in S-phase cells. Only ATRA induced RARbeta expression. ATRA, 9-cis RA and 4-HPR initiated the highest level of apoptosis as determined by mitochondrial Bax translocation, while only ATRA and 9-cis RA strongly reduced Bcl-2 and survivin protein expression. Enumeration of dead cells over 96 h correlated well with downregulation of both survivin and Bcl-2. Simultaneous retinoid-mediated reduction of both these proteins also predicted optimal Taxol sensitization. 4-HPR was much weaker than the natural retinoids with respect to Taxol sensitization, consistent with the proposed requirement for reduced Bcl-2 in this synergy. Neither the extent of cell cycle protein regulation nor AP-1 inhibition fully predicted the antiproliferative effect of the synthetic retinoids suggesting that growth inhibition requires regulation of a spectrum of RAR-regulated gene products in addition even to pivotal cell cycle proteins.