Synthesis of 1,25-dihydroxyvitamin D3 by human placenta in vitro

Association of vitamin D and gene variants in the vitamin D metabolic pathway with preterm birth

OBJECTIVES

The aim of this study was to explore the association of vitamin D (VitD) levels during pregnancy and its metabolic pathway genes with the risk for preterm birth (PTB) among pregnant women in southeast China.

METHODS

This study was conducted in Zhoushan Maternal and Child Health Hospital, Zhejiang, from August 2011 to May 2018. Plasma 25-hydroxyvitamin vitamin D [25(OH)D] levels in three trimesters and single-nucleotide morphisms in the VitD metabolic pathway were measured. Relevant information was collected using questionnaires and an electronic medical recorder system. Multiple statistical methods including linear regression, logistic regression, and crossover analysis were applied.

RESULTS

The prospective cohort study included 3465 pregnant women, of which 202 were PTB (week of gestation at delivery: 33.38 ± 4.05), accounting for 5.8%. After adjusting for potential confounders, VitD sufficiency (≥30 ng/mL) in the second and third trimesters was associated with longer gestational age at delivery compared with VitD deficiency (<20 ng/mL). However, no significant association was found between VitD with the risk for PTB. rs7041, rs10210408, and rs2228171 were associated with gestational week and the risk for PTB. Significant associations were found of rs10210408, rs2209314, rs1155563, rs2544381 and the status of VitD in the second and third trimester with the gestational week. We also found that rs7041 and VitD in the second trimester might exert interaction on gestational week and the risk for PTB (P_inter = 0.038; P_inter = 0.019); rs16846876 and VitD in the second trimester might exert interaction on gestational week (P_inter = 0.024); rs4334089 and VitD in the third trimester might exert interaction on gestational week (P_inter = 0.024). Similar results were found when we tested pregnant women's plasma 25(OH)D in the first and second trimesters.

CONCLUSIONS

Women with VitD deficiency were associated with shorter gestational weeks. Single-nucleotide morphisms in VitD metabolic pathway genes were significantly associated with gestation week and the risk for PTB, mainly in vitamin D-binding protein (GC) and low-density lipoprotein-related protein 2 (LRP2)genes. Additionally, maternal VitD with GC gene and maternal VitD with vitamin D receptor (VDR) gene might exert interactions on the risk for PTB.

Negative correlation between testosterone and TNF-α in umbilical cord serum favors a weakened immune milieu in the human male fetoplacental unit

Clinical and epidemiological evidence supports that pregnancies carrying a male fetus are more vulnerable to infections and preterm birth, probably due to testosterone immunosuppressive properties. In human placentas, testosterone lowers the expression of CYP27B1, the vitamin D (VD)-activating enzyme, diminishing cathelicidin synthesis, a potent VD-dependent antimicrobial peptide (AMP). VD also stimulates other AMPs, including defensins. To get insights into the increased male vulnerability mechanisms, we investigated the relationship between fetal sex and the immunoendocrine milieu at the fetoplacental unit. For this, umbilical vein serum and placental samples were collected from healthy newborns. In males' serum, testosterone levels were significantly higher and negatively associated with TNF-α, a cytokine that strengthens the immune response. Males showed lower serum TNF-α and increased levels and gene expression of the immunosuppressive cytokine IL-10. Only in female samples there was a positive association (P < 0.05) between AMPs and both TNF-α and CYP27B1 and between 25-hydroxyvitamin D₃ and IL-1β serum levels. Accordingly, VD-metabolites (25-hydroxyvitamin D₃, calcitriol) significantly stimulated IL-1β gene expression in cultured trophoblasts. Interestingly, IL-1β mRNA correlated positively with defensins (P < 0.05) in males, but not with cathelicidin expression, which was significantly diminished in comparison to females. Our data suggest that high umbilical serum testosterone and IL-10 in males could explain reduced TNF-α levels and lack of association between VD-dependent innate immunity markers and proinflammatory cytokines expression in the fetoplacental unit. Altogether, our observations imply a restricted basal immune milieu in males compared to females, which may help understand the higher male susceptibility to adverse perinatal outcomes.

Lipopolysaccharide and cAMP modify placental calcitriol biosynthesis reducing antimicrobial peptides gene expression

PROBLEM

Calcitriol, the hormonal form of vitamin D₃ (VD), stimulates placental antimicrobial peptides expression; nonetheless, the regulation of calcitriol biosynthesis in the presence of bacterial products and its consequence on placental innate immunity have scarcely been addressed.

METHOD OF STUDY

We investigated how some bacterial products modify placental VD metabolism and its ability to induce antimicrobial peptides gene expression.

RESULTS

Cultured human trophoblasts biosynthesized calcitriol only in the presence of its precursor calcidiol, a process that was inhibited by cyclic-AMP but stimulated by lipopolysaccharide (LPS). Intracrine calcitriol upregulated cathelicidin, S100A9, and β-defensins (HBDs) gene expression, while LPS further stimulated HBD2 and S100A9. Unexpectedly, LPS significantly repressed cathelicidin basal mRNA levels and drastically diminished calcidiol ability to induce it. Meanwhile, cyclic-AMP, which is used by many microbes to avoid host defenses, suppressed calcitriol biosynthesis, resulting in significant inhibition of most VD-dependent microbicidal peptides gene expression.

CONCLUSION

While LPS stimulated calcitriol biosynthesis, cyclic-AMP inhibited it. LPS downregulated cathelicidin mRNA expression, whereas cyclic-AMP antagonized VD-dependent-upregulation of most antimicrobial peptides. These findings reveal LPS and cyclic-AMP involvement in dampening placental innate immunity, highlighting the importance of cyclic-AMP in the context of placental infection and suggesting its participation to facilitate bacterial survival.

Decreased serum vitamin D levels in early spontaneous pregnancy loss

Background/Objectives:

Effects of vitamin D deficiency in pregnancy have been associated with some adverse pregnancy outcomes. The objective of this study was to analyze the relationship between vitamin D deficiency in childbearing aged women and pregnancy loss (PL) in the first trimester.

Subjects/Methods:

This is a cross-sectional study. Plasma was collected from 60 nulliparous women with singleton at 7–9 weeks of gestation (30 with viable gestation and 30 with PL) and 60 non-gravid childbearing aged women (30 with a successful pregnancy history, and 30 with one or more spontaneous first-trimester PL history). Quantitation of serum 25-hydroxyvitamin D (25(OH)D) and 25-hydroxyvitamin D-1 alpha hydroxylase (CYP27B1) was assayed.

Results:

By pregnancy/non-gravid, normal pregnant women had higher 25(OH)D (49.32 μg/l) and CYP27B1 (82.00 pg/ml) than PL women (34.49 μg/l and 37.87 pg/ml, both P<0.01); the non-gravid women with a successful pregnancy history also had higher 25(OH)D (39.56 μg/l) and CYP27B1 (39.04 pg/ml) than women with PL history (12.30 μg/l and 12.35 pg/ml, both P<0.01). The 96.7% of non-gravid women with PL history and 43.3% of PL women had serum 25(OH)D concentrations below 30 μg/l. There was a strong association between low vitamin D levels and PL (odds ratio 1.71; 95% confidence interval: 1.2–2.4, P<0.001). The regression analyses showed that PL was significantly inversely correlated with 25(OH)D (P<0.01) and CYP27B1 levels (P<0.01).

Conclusions:

Vitamin D deficiency associated with PL in the first trimester of pregnancy. Decreased serum vitamin D levels among childbearing aged women with the failed clinical pregnancies history may predispose to increased risk for PL.

Serum calcitriol concentrations measured with a new direct automated assay in a large population of adult healthy subjects and in various clinical situations

The measurement of calcitriol [1,25(OH2)D], is important for the differential diagnosis of several disorders of calcium/phosphorus metabolism but is time-consuming and tricky. We measured serum calcitriol with a new automated direct assay on the Liaison XL platform in 888 healthy French Caucasian subjects aged 18-89 years, 32 patients with a surgically-proven PHPT, 32 pregnant women at the end of the first and at the end of the third trimester, and 24 dialysis patients before and after one year of supplementation with vitamin D3 or placebo. The mean calcitriol concentration (±SD) in the healthy population was 52.9±14.5 ng/L with a 95% CI interval of 29-83.6 ng/L. In PHPT patients, calcitriol concentration was 81.6±29.0 ng/L, 15 of them (46.9%) having a concentration >83.6 ng/L. In pregnant women, calcitriol was 80.4±26.4 ng/L at the end of the first trimester, and 113.1±33.0 ng/L at the end of the third trimester, 12 (37.5%) and 26 (81.3%) of them having a calcitriol concentration >83.6 ng/L at the first and third trimesters respectively. In 14 dialysis patients, calcitriol was 9.5±7.7 ng/L and rose to 19.3 ng/L after one year of supplementation with 50,000 IU vitamin D3/month. In 10 other dialysis patients, calcitriol was 9.9±2.9 ng/L and remained stable (12.4±3.7 ng/L) after one year of placebo. In conclusion, this new automated calcitriol assay, in addition to presenting excellent analytical performances, gives the expected variations in patients compared to "normal" values obtained in an extensive reference population.

Regulation of calcitriol biosynthesis and activity: focus on gestational vitamin D deficiency and adverse pregnancy outcomes

Vitamin D has garnered a great deal of attention in recent years due to a global prevalence of vitamin D deficiency associated with an increased risk of a variety of human diseases. Specifically, hypovitaminosis D in pregnant women is highly common and has important implications for the mother and lifelong health of the child, since it has been linked to maternal and child infections, small-for-gestational age, preterm delivery, preeclampsia, gestational diabetes, as well as imprinting on the infant for life chronic diseases. Therefore, factors that regulate vitamin D metabolism are of main importance, especially during pregnancy. The hormonal form and most active metabolite of vitamin D is calcitriol. This hormone mediates its biological effects through a specific nuclear receptor, which is found in many tissues including the placenta. Calcitriol synthesis and degradation depend on the expression and activity of CYP27B1 and CYP24A1 cytochromes, respectively, for which regulation is tissue specific. Among the factors that modify these cytochromes expression and/or activity are calcitriol itself, parathyroid hormone, fibroblast growth factor 23, cytokines, calcium and phosphate. This review provides a current overview on the regulation of vitamin D metabolism, focusing on vitamin D deficiency during gestation and its impact on pregnancy outcomes.

Influence of vitamin D levels on in vitro fertilization outcomes in donor-recipient cycles

OBJECTIVE

To elucidate the role of vitamin D in reproduction by examining the relationship between recipient vitamin D levels and pregnancy rates in donor-recipient IVF cycles.

DESIGN

Retrospective cohort study.

SETTING

Academic tertiary care center.

PATIENT(S)

Ninety-nine recipients of egg donation at University of Southern California Fertility.

INTERVENTION(S)

Serum was collected from egg donor recipients before ET and was tested for vitamin D levels [25(OH)D].

MAIN OUTCOME MEASURE(S)

Clinical pregnancy as defined by sonographic presence of a heartbeat at 7-8 weeks of gestation.

RESULT(S)

In a diverse population of 99 recipients (53% Caucasian, 20% Asian, 16% Hispanic, 7% African American), adjusted clinical pregnancy rates were lower among vitamin D-deficient recipients than among vitamin D-replete recipients (37% vs. 78%). Live-birth rates were 31% among vitamin D-deficient recipients, compared with 59% among vitamin D-replete recipients. There were no differences in adjusted clinical pregnancy and live-birth rates among recipients who were vitamin D deficient [25(OH)D<20 ng/mL] vs. among those who were vitamin D insufficient [20 ng/mL ≤ 25(OH)D<30 ng/mL].

CONCLUSION(S)

Nonreplete vitamin D status [25(OH)D<30 ng/mL] was associated with lower pregnancy rates in recipients of egg donation. Since the oocyte donor-recipient model is able to separate the impact of vitamin D on oocyte vs. endometrium, these data suggest that the effects of vitamin D may be mediated through the endometrium.

Endocrine regulation of HOX genes

Hox genes have a well-characterized role in embryonic development, where they determine identity along the anteroposterior body axis. Hox genes are expressed not only during embryogenesis but also in the adult, where they are necessary for functional differentiation. Despite the known function of these genes as transcription factors, few regulatory mechanisms that drive Hox expression are known. Recently, several hormones and their cognate receptors have been shown to regulate Hox gene expression and thereby mediate development in the embryo as well as functional differentiation in the adult organism. Estradiol, progesterone, testosterone, retinoic acid, and vitamin D have been shown to regulate Hox gene expression. In the embryo, the endocrine system directs axial Hox gene expression; aberrant Hox gene expression due to exposure to endocrine disruptors contributes to the teratogenicity of these compounds. In the adult, endocrine regulation of Hox genes is necessary to enable such diverse functions as hematopoiesis and reproduction; endocrinopathies can result in dysregulated HOX gene expression affecting physiology. By regulating HOX genes, hormonal signals utilize a conserved mechanism that allows generation of structural and functional diversity in both developing and adult tissues. This review discusses endocrine Hox regulation and its impact on physiology and human pathology.

Identification of a 25-hydroxyvitamin D3 1alpha-hydroxylase gene transcription product in cultures of human syncytiotrophoblast cells

Although accumulating data show that placenta is able to synthesize 1,25-dihydroxyvitamin D3, the presence of cytochrome P(450) enzyme capable of converting 25-hydroxyvitamin D3 (250HD(3)) to the biologically active form of vitamin D in this tissue, has not been yet clearly established. In this study, we have investigated the presence of 25-hydroxyvitamin D3 1alpha-hydroxylase (1alpha-(OH)ase) gene expression products in cultured human syncytiotrophoblast. Total RNA was isolated from cultured placental cells and subjected to Northern blots or RT-PCR by using 1alpha-(OH)ase-specific primers. The amplified complementary DNA fragments were analyzed by gel electrophoresis and nucleotide sequencing. Total RNA from kidney HEK 293 cells was subjected to reverse transcriptase reaction, and a 298-bp complementary DNA 1alpha-(OH)ase probe was generated by PCR. Primary cultures of human syncytiotrophoblasts exhibited 1alpha-(OH)ase activity, and a transcript for this gene could be demonstrated in these cells. Northern blot analysis revealed the presence of a 2.5-kb product, similar in size to that previously reported in kidney. RT-PCR analysis demonstrated the presence of a single transcript with nucleotide sequence identical to that previously reported for human 1alpha-(OH)ase complementary DNA clones. In addition, data are presented which suggest that differentiation of cytotrophoblast to the syncytial state was not necessary for this gene to be expressed, which may indicate a role of this enzyme all through pregnancy. The overall results of this study provide evidence for the presence of 1alpha-(OH)ase in the human placenta, suggesting that conversion of 25OHD(3) to 1,25-dihydroxyvitamin D3 in the trophoblast is most probably attributed to an enzymatic 1alpha-hydroxylation reaction.

Intraplatelet free calcium and calcium-regulating hormones in plasma are not related to the antihypertensive effect of nifedipine in hypertensive pregnancy

Intracellular free calcium regulates contraction-relaxation processes in vascular smooth muscle. We compared intraplatelet free calcium ([Ca2+]i) and pH ([pH]i) in hypertensive pregnant women to those in normotensive pregnant and non-pregnant women. Plasma parathormone and vitamin D metabolite were simultaneously assessed. In hypertensive pregnancy, [Ca2+]i tended to be lower than in normotensive pregnant (P = 0.08) and non-pregnant subjects (P = 0.06). In hypertensive pregnancy, 1,25, (OH)2 vitamin D in plasma was in the same range as in non-pregnant women and significantly lower than in normotensive pregnancy (p < 0.01). The other two vitamin D metabolites, parathormone and [pH]i were equal in the three groups. A five-day nifedipine treatment (10 mg t.i.d.) increased [Ca2+]i in hypertensive pregnant (P < 0.05) and normotensive non-pregnant subjects (P = 0.06), whereas [pH]i (P < 0.05) and 25 (OH) vitamin D (P < 0.05) decreased in the former and 24,25 (OH)2 vitamin D increased in the latter group (P < 0.05). Initial [Ca2+]i did not correlate with blood pressure in any group. The antihypertensive effect of nifedipine did not correlate with any variable measured. In conclusion, [Ca2+]i and calcium-regulating hormones seem not to be related to the antihypertensive effect of nifedipine in hypertensive pregnancy. In this type of hypertension, intraplatelet calcium may not reflect calcium balance in smooth muscle cells regulating vascular tone and blood pressure.

Metabolism of 3H-1 alpha,25-dihydroxyvitamin D3 in cultured human keratinocytes

In the present investigation we studied the metabolism of 1 alpha,25-dihydroxy-[1 beta-3H]vitamin D3 (3H-1,25(OH)2D3) in culture-grown human keratinocytes (CHK). Our results showed that the cellular uptake of 3H-1,25(OH)2D3, upon incubation with CHK, occurred very rapidly; and it paralleled a decrease in the concentration of 3H-1,25(OH)2D3 in the medium. The amount of 3H-calcitroic acid, on the other hand, increased slowly in the medium, while the concentration of 3H-calcitroic acid in the cell remained undetectable during the whole period of incubation. When the cells were preincubated with 1,25(OH)2D3 (10(-8)M), conversion of 3H-1,25(OH)2D3 to 3H-calcitroic acid increased almost twofold, indicating that 1,25(OH)2D3 catalyzed its own catabolism.

Uterine cells other than stromal decidual cells are required for 1,25-dihydroxyvitamin D3 production during early human pregnancy

Human decidual cells are known to produce 1,25-(OH)2D3 at the end of pregnancy, the present study evaluates this capacity, and the part played by stromal decidual cells, in early pregnancy. Cells were obtained from nine human decidua by aspiration or curettage during early pregnancy (7-10 weeks), separated on Ficoll-Paque and plastic adherence, and incubated for 1 h with 25-(OH)D3. Incubation medium and cells were extracted and chromatographed on two successive HPLC systems. The cells examined were of both physiological and pathological (ectopic pregnancy) origin. Endometrial cells obtained in four non-pregnant situations (myomas) were also studied to determine whether the 1,25-(OH)2D3 synthesis by the uterus is associated with the appearance of decidual cells. Results show that human decidual cells from early pregnancy convert 25(OH)D3 (2.5 nM or 2.5 microM) into a metabolite with the physicochemical characteristics of synthetic 1,25-(OH)2D3. This ability is shared by cells isolated during early pregnancy, whether physiological or ectopic (tubal pregnancy). Non-adherent cells, which include mainly stromal decidual cells, are less able to produce 1,25-(OH)2D3 than are the adherent cells, suggesting that macrophages, granulocytes or as yet unidentified cell types are required for the 1,25-(OH)2D3 production by decidual tissue during early human pregnancy. In addition, one out of four experiments with non-pregnant endometrial cells could produce 1,25-(OH)2D3 suggesting that, although not the rule in the non-pregnant state, in vitro production of 1,25-(OH)2D3 by uterine cells can be found in the absence of decidual cells.

Calciotropic hormones during reproduction

This review summarizes the reported effects of the menstrual cycle, pregnancy and lactation on serum concentration of the calciotropic hormones PTH and 1,25(OH)2D. A midcycle rise in PTH and 1,25(OH)2D has been observed, but in the majority of studies there was no change in PTH and 1,25(OH)2D concentrations throughout the menstrual cycle. Both total and free 1,25(OH)2D levels are increased during pregnancy. The renal 1,25(OH)2D production is stimulated, and there is some evidence of 1,25(OH)2D production by decidua/placenta and fetal kidney in vitro; the decidual/placental production should not be overestimated in vivo. The increased renal 1 alpha-hydroxylase activity is possibly mediated by estrogens and PTH, although the effect of pregnancy on PTH remains uncertain. Increased serum 1,25(OH)2D concentrations probably result in a rise of intestinal calcium absorption during pregnancy. There is a postdelivery drop in PTH and 1,25(OH)2D levels, but they are increased when lactation is prolonged, or in mothers nursing twins. The l alpha-hydroxylase activity during lactation may be stimulated by PTH, but also by prolactin.

Parathyroid-like regulation of parathyroid-hormone-related protein release and cytoplasmic calcium in cytotrophoblast cells of human placenta

Immunohistochemical staining of human placenta revealed intense reactivity for amino terminal and midregional parathyroid-hormone-related protein (PTHrp) in the cytotrophoblast cells and weaker staining in the syncytiotrophoblasts. The cytotrophoblasts also displayed conspicuous surface staining with the monoclonal antibodies E11 and G11, which recognize a Ca2+ receptor mechanism regulating hormone release of parathyroid cells. Cytotrophoblasts enriched on Percoll gradients or by linking surface-bound E11 to magnetic beads revealed biphasic elevation of cytoplasmic Ca2+ ([Ca2+]i) upon a stepwise rise of external Ca2+ from 0.5 to 3.0 mM, with a half-maximal effect at 1.75 mM. Individual cytotrophoblasts identified by their E11 reactivity disclosed a temporary increase of [Ca2+]i upon elevation of external Mg2+, while Mn2+ triggered both a [Ca2+]i transient and an influx of itself. These effects were efficiently blocked by the G11 antibody. Depolarization with K+ or addition of the voltage-dependent Ca2+ channel blocker verapamil had only marginal effects on [Ca2+]i. Raised extracellular calcium inhibited release of PTHrp from the cells, and this inhibition was blocked by the G11 antibody. The virtually parathyroid-identical Ca2+ regulation of [Ca2+]i may mediate feedback control of PTHrp release from the cytotrophoblasts and thereby participate in the regulation of placental Ca2+ transport.

Involvement of endogenously produced 1,25-dihydroxyvitamin D-3 in the growth and differentiation of human keratinocytes

In this study, we investigated the possibility that cultured keratinocytes from normal human adult skin produce 1,25-dihydroxyvitamin D-3 (1,25(OH)2D3, a biologically active form of vitamin D-3) from 25-hydroxyvitamin D-3 [25(OH)D3], and that 1,25(OH)2D3 endogenously produced by keratinocytes is involved in the self regulation of their growth and differentiation. To determine whether 1,25(OH)2D3 is produced from 25(OH)D3 by skin keratinocytes, 25(OH)[3H]D3 was added to keratinocyte cultures and incubated for 1 h and 5 h. The intracellular and extracellular metabolites were analyzed by three chromatographic systems. The three chromatograms revealed that the major metabolite produced from 25(OH)D3 was 1,25(OH)2D3. Most of the 1,25(OH)2D3 endogenously produced from 25(OH)D3 remained within the cells. To examine the time course of 1,25(OH)2D3 production, the amount of 1,25(OH)[3H]D3 was measured at 15 min, 1 h, 5 h and 10 h, being at a maximum 1 h after the addition of 25(OH)D3. These data indicate that keratinocytes rapidly convert 25(OH)D3 to 1,25(OH)2D3 and that 1,25(OH)2D3 is not released into the medium. To determine whether endogenously produced 1,25(OH)2D3 is involved in the regulation of growth and differentiation of normal human keratinocytes, we examined the effects of 1,25(OH)2D3 and 25(OH)D3 on their growth and differentiation. Keratinocyte growth was inhibited to 52.6% and 23.4% by 10(-8) M and 10(-7) M 1,25(OH)2D3 and to 80.5% and 23.9% by 10(-8) M and 10(-7) M 25(OH)D3, respectively. Differentiation of these cells was evaluated by quantifying the number which express involucrin, a precursor protein of cornified envelope. The population of involucrin expressing cells (differentiated cells) increased from 6.2% to 14.5% by 2.5.10(-7) M 1,25(OH)2D3, and to 11.8% by 2.5.10(-7) M 25(OH)D3. These results clearly indicate that 25(OH)D3 is as effective on human keratinocytes as 1,25(OH)2D3 in inhibiting growth and inducing differentiation, although to a slightly lesser extent than 1,25(OH)2D3. The possibility that the effect of 25(OH)D3 is mediated through binding to the 1,25(OH)2D3 receptor can be excluded, since a competitive binding assay revealed that the affinity of 25(OH)D3 for the 1,25(OH)2D3 receptor in a cytosolic extract of keratinocytes was 100-times lower than that of 1,25(OH)2D3. Thus, these results suggest that 1,25(OH)2D3 endogenously produced in keratinocytes from 25(OH)D3 is involved in the regulation of their growth and differentiation in vitro.

Calcium and phosphorus metabolism in the premature infant

During the last trimester of pregnancy, there is a sixfold increase in fetal calcium and phosphorus accumulation. Unsupplemented human breast milk may not provide sufficient calcium and phosphorus for the rapidly growing preterm infant to match the accumulation that should have taken place in utero and to permit normal bone mineralization. Rickets of prematurity may present clinically between the 6th and 12th postnatal week. The clinical diagnosis may be confirmed using simple biochemical tests. Inadequate mineral substrate intake, particularly of phosphorus, is the most common cause, although a delay in the maturation of the renal enzyme, 1-alpha hydroxylase, with low plasma concentrations of 1,25-dihydroxyvitamin D, may also occur. The biochemical response to treatment can be determined by documenting a fall in plasma alkaline phosphatase activity and a rise in plasma phosphate concentration and urinary phosphate excretion.

25-Hydroxyvitamin D3-1 alpha-hydroxylase in porcine hepatic tissue: subcellular localization to both mitochondria and microsomes

In vitro studies were performed to assess the ability of hepatic homogenates, mitochondria, and microsomes to 1 alpha-hydroxylate 25-hydroxyvitamin D3 [25(OH)D3]. Addition of 25(OH)D3 to either hepatic mitochondria or microsomes caused a concentration-dependent increase in the production of 1,25-dihydroxyvitamin D3 [1,25(OH)2D3]. Hepatic homogenates also produced purported 1,25(OH)2D3, although at a much reduced efficiency as compared with hepatic mitochondria or microsomes. Purported 1,25(OH)2D3 synthesized by hepatic mitochondria or microsomes was identified by its mobility on several high-performance liquid chromatographic systems and, ultimately, by its ability to interact with the bovine thymus 1,25(OH)2D3 receptor protein. Production of 1,25(OH)2D3 by hepatic mitochondria and microsomes was dependent on time of incubation, protein content, and pH of incubation medium, and it required an adequate source of reducing equivalents. Generation of 1,25(OH)2D3 by these organelles could be totally blocked by the cytochrome P-450 inhibitor ketoconazole. The microsomal 1 alpha-hydroxylase could not be saturated even at the highest concentration (240 microM) of 25(OH)D3 used. The mitochondrial 1 alpha-hydroxylase, however, displayed saturation at approximately 40 microM 25(OH)D3. Eadie-Hofstee reciprocal plot analysis of the hepatic mitochondrial 1 alpha-hydroxylase gave a Km of 17 microM 25(OH)D3 and a Vmax of 481 pg of 1,25(OH)2D3 per min per mg of protein. Because of its inability to achieve substrate saturation, meaningful kinetic parameters could not be calculated for the hepatic microsomal 1 alpha-hydroxylase. These data demonstrate the liver to be an even more dynamic organ than was previously believed with respect to vitamin D metabolism in that the liver has the potential to produce 1,25(OH)2D3 in situ by at least two separate mechanisms.

Antigenic and catalytic disparity in the distribution of cytochrome P-450-dependent 25-hydroxyvitamin D3-1 alpha- and 24-hydroxylases

Chick 25-hydroxyvitamin D3-1 alpha-hydroxylase, a cytochrome P-450 monooxygenase with a molecular weight of 57 kDa, can be isolated as described by Mandel et al. (1990 b). Under normal physiological circumstances, it occurs exclusively in kidney mitochondria. An isozyme of the 1 alpha-hydroxylase, known as the 24-hydroxylase, which uses the same substrate to yield an isomeric product, is also a cytochrome P-450 monooxygenase, has a molecular weight of 55 kDa, and like-wise occurs in kidney mitochondria. The amino-terminal sequences of the first 10 residues of the two isozymes are 100% homologous. Monoclonal antibodies of the IgM class raised against the 1 alpha-hydroxylase, which quantitatively discriminate against other P-450 cytochromes of mitochondrial or microsomal origin, recognize and interact with the 24-hydroxylase as an antigen. In the present study we show that the intestine, which is the only non-renal tissue with demonstrable 24-hydroxylase activity, gives a positive peroxidase-antiperoxidase immunohistochemical reaction using the monoclonal antibodies against the 1 alpha-hydroxylase. The reactions revealed that the antigen in the kidney is restricted to the cortical proximal tubular cells while in the intestine, the antigen is localized in the enterocytes of the villi. In kidney medullary or intestinal crypt cells, or in liver, heart and lung tissues where 1 alpha-hydroxylase or 24-hydroxylase activity could not be detected using cell or tissue homogenates, the immunohistochemical reactions were also negative.(ABSTRACT TRUNCATED AT 250 WORDS)

Development of endocrine pathways in the regulation of calcium homeostasis

The mammalian fetus is maintained hypercalcaemic relative to its mother by the action of a calcium pump believed to be located at the basement membranes of the epithelial cells of the fetal chorion. It has recently been demonstrated that the activity of this putative pump is stimulated by a new fetal hormone, parathyroid hormone-related protein, described originally as the product of a human BEN cell line which was derived from a lung tumour associated with hypercalcaemia of malignancy. Whereas the circulating level of immunoreactive parathyroid hormone in the fetus is very low, in keeping with the hypercalcaemia, the plasma concentrations of bioactive parathyroid hormone and parathyroid hormone-related protein can be measured using a sensitive cytochemical bioassay and the separate concentrations assessed by pre-incubation with appropriate antisera. The total plasma concentration of both hormones is inversely related to the prevailing calcium ion concentration but the set point of parathyroid hormone-related protein is probably higher than that for parathyroid hormone. Probably as a result of the hypercalcaemia, the circulating concentration of calcitonin is also higher than in maternal plasma and may serve to limit bone resorption to favour net bone accretion as part of the overall growth of the fetus. Vitamin D and its most active metabolite, 1,25(OH)2D, can pass across the placenta in either direction, in contrast to most peptide hormones. In addition to the supply of some 1,25(OH)2D by the mother to her fetus, the fetal placenta and fetal kidneys can all synthesize 1,25(OH)2D. The relative concentrations circulating in maternal and fetal plasma pools vary with the species, presumably as a result of differing importance of the three sources of supply to the fetus and the relative concentrations of vitamin D-binding protein circulating in mother and fetus. The importance of parathyroid hormone-related protein derived from fetal parathyroid glands has been clearly demonstrated in the fetal sheep. Such animals develop rickets following the removal of their parathyroid glands, despite the demonstration of this substance in fetal placental membranes. However, the relative importance of the parathyroid glands versus the placenta and its membranes as the principal source of parathyroid hormone-related protein remains to be elucidated and may vary with species.

Identification and regulation of 1,25-dihydroxyvitamin D3 receptor activity and biosynthesis of 1,25-dihydroxyvitamin D3. Studies in cultured bovine aortic endothelial cells and human dermal capillaries

Because 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) has been shown to play roles in both proliferation and differentiation of novel target cells, the potential expression of 1,25(OH)2D3 receptor (VDR) activity was investigated in cultured bovine aortic endothelial cells (BAEC). Receptor binding assays performed on nuclear extracts of BAEC revealed a single class of specific, high-affinity VDR that displayed a 4.5-fold increase in maximal ligand binding (Nmax) in rapidly proliferating BAEC compared with confluent, density-arrested cells. When confluent BAEC were incubated with activators of protein kinase C (PKC), Nmax increased 2.5-fold within 6-24 h and this upregulation was prevented by sphingosine, an inhibitor of PKC, as well as by actinomycin D or cycloheximide. Immunohistochemical visualization using a specific MAb disclosed nuclear localized VDR in venular and capillary endothelial cells of human skin biopsies, documenting the expression of VDR, in vivo, and validating the BAEC model. Finally, additional experiments indicated that BAEC formed the 1,25(OH)2D3 hormonal metabolite from 25(OH)D3 substrate, in vitro, and growth curves of BAEC maintained in the presence of 10(-8) M 1,25(OH)2D3 showed a 36% decrease in saturation density. These data provide evidence for the presence of a vitamin D microendocrine system in endothelial cells, consisting of the VDR and a 1 alpha-hydroxylase enzyme capable of producing 1,25(OH)2D3. That both components of this system are coordinately regulated, and that BAEC respond to the 1,25(OH)2D3 hormone by modulating growth kinetics, suggests the existence of a vitamin D autocrine loop in endothelium that may play a role in the development and/or functions of this pathophysiologically significant cell population.

Extra-renal production of calcitriol in chronic renal failure

Renal 1-alpha-hydroxylase activity is tightly regulated in normal humans and intact animals. No significant changes in serum 1,25(OH)2D levels occur in response to vitamin D challenge. However, conflicting reports have appeared in the literature with regard to stimulation of 1,25(OH)2D production after 25(OH)D administration in uremia. To provide further insight into this issue, 25(OH)D at a dose of 100 micrograms every other day for two weeks followed by 50 micrograms every other day for the next two weeks was given orally to seven uremic mongrel dogs. After two weeks of 25(OH)D therapy, 1,25(OH)2D levels increased from 16.4 +/- 0.9 to 28.0 +/- 1.9 pg/ml (P less than 0.001) in parallel with a fourfold increase in 25(OH)D concentrations from a basal of 50.1 +/- 6.5 to 203.2 +/- 18.1 ng/ml. No significant changes in serum i-PTH, ICa or P were observed. Linear regression analysis of the relationship between serum concentrations of 1,25(OH)2D versus 25(OH)D, for each dog during this period, showed highly significant correlation coefficients. To evaluate the possibility that extra-renal sites contribute to the described enhanced 1,25(OH)2D net synthesis after 25(OH)D treatment, similar studies were performed in four anephric patients undergoing hemodialysis. Basal serum 1,25(OH)2D levels were 5.5 +/- 2.4 pg/ml and increased to 19.6 +/- 5.0 pg/ml after 25(OH)D administration. A significant correlation was also found for the relationship between serum levels of 1,25(OH)2D and 25(OH)D in anephrics (r = 0.72, P less than 0.001). The same therapy in four normal volunteers showed no significant changes in serum 1,25(OH)2D concentrations.(ABSTRACT TRUNCATED AT 250 WORDS)

Vitamin D metabolism in pregnant and pseudopregnant rats: identification of 25-hydroxycholecalciferol-1-hydroxylase in decidual tissue

Elevated levels of 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) are found in late pregnancy but the factors responsible for this are not known. To determine if the maternal-fetal calcium flux or the presence of a previously described extrarenal 25-hydroxycholecalciferol-1-hydroxylase (25(OH)-D3-1-hydroxylase) play a role, serum calcium and 1,25(OH)2D3 were measured in pregnant, nonpregnant, and decidua-bearing pseudopregnant rats. Serum calcium was 8.74 +/- 0.26 mg/dl (mean +/- SEM) in nonpregnant rats. In pregnant rats, serum calcium was not significantly different from nonpregnant controls on day 12 and only slightly higher on day 15. Pseudopregnant rats were significantly hypercalcemic on days 12 (11.93 +/- 0.19 mg/dl) and 15 (11.45 +/- 0.23 mg/dl) compared with nonpregnant rats (P less than 0.001). In nonpregnant controls the serum level of 1,25(OH)2D3 was 44.6 +/- 6.3 pg/ml. Levels in pregnant rats were not significantly different on days 12 or 15 but tended to be higher by day 15 (75.2 +/- 19.7 pg/ml). Pseudopregnant rats had levels of 72.6 +/- 13.5 pg/ml on day 12 and 102.8 +/- 10.9 pg/ml on day 15, the latter of which was significantly higher than nonpregnant values (P less than 0.05). 25(OH)D3-1-hydroxylase activity was determined in whole tissue homogenates of placenta and decidua. Placenta from pregnant rats and decidua from pregnant and pseudopregnant rats both formed putative 1,25(OH)2D3 in short-term incubation with 25(OH)D3 as identified by comigration with authentic 1,25(OH)2D3 on high pressure liquid chromatography (HPLC).(ABSTRACT TRUNCATED AT 250 WORDS)

Synthesis of 1,25-dihydroxycholecalciferol and 24,25-dihydroxycholecalciferol by calvarial cells. Characterization of the enzyme systems

The synthesis of 1,25-dihydroxycholecalciferol [1,25(OH)2D3] and 24,25-dihydroxycholecalciferol [24,25(OH)2D3] from 25-hydroxycholecalciferol [25(OH)D3] has previously been shown to occur in cells isolated from bone. The main findings of the present study are that the enzyme systems which catalyse these syntheses are: (1) active at 'in vitro' substrate concentrations over the range of 2-50 nM; (2) regulatable in a complex way by 1,25(OH)2D3, 24,25(OH)2D3, 25,26-dihydroxycholecalciferol and 25(OH)D3, but not by cholecalciferol ('vitamin D3'); and (3) have relatively short half-lives (approx. 5 h).

Assay and properties of rat yolk sac 25-hydroxyvitamin D3 1 alpha-hydroxylase

An in vitro assay has been developed for the rat yolk sac 25-hydroxyvitamin D3 1 alpha-hydroxylase (1 alpha-hydroxylase). The subcellular location and some properties of the enzyme are described. 1,25-Dihydroxyvitamin D3 produced from incubations of yolk sac homogenates was extracted, purified by Sephadex LH-20 chromatography and straight- and reverse-phase high-performance liquid chromatography (HPLC), and measured by a competitive binding assay using chick intestinal receptor. The reaction is linear with time for up to 45 min at a substrate concentration of 80 microM and 4-6 mg/mL microsomal protein. The enzyme, located in the microsomes, requires molecular oxygen and NADPH. Metyrapone (1 X 10(-3) M) was found to inhibit 1-hydroxylation, but a 90% carbon monoxide-10% oxygen atmosphere did not, leaving open the question of involvement of cytochrome P-450. Diphenyl-p-phenylenediamine, a lipid peroxidase inhibitor, inhibited 1 alpha-hydroxylation.

25-Hydroxyvitamin D3 metabolism in a human leukemia cell line

1,25-dihydroxyvitamin D3 (1,25(OH)2D3) is a potent inducer of monocytic differentiation of the human promyelocytic leukemia cell line, HL-60. We have noted that 25-hydroxyvitamin D3 (25(OH)D3) in high doses is also capable of promoting monocytic differentiation of this cell line. To test the possibility that the latter activity is due to conversion of 25OHD3 to 1,25(OH)2D3 by HL-60, we exposed HL-60 cells to 25OHD3 and analyzed the products by HPLC and radioreceptor assay. When chromatographed in the traditional solvent system (isopropanol-hexane), a new peak appears which migrates with authentic 1,25(OH)2D3. However, in a solvent system containing dichloromethane, 90% of the peak migrates with another metabolite, 19-Nor-10-Keto-25OHD3 (19-Nor-25OHD3). Production of this metabolite is enhanced by living cells and is synthesized by both virgin HL-60 and those which have undergone differentiation. We next determined if authentic 19-Nor-25OHD3 also promotes differentiation of this cell. As assessed by appearance of the monocyte-specific surface antigen (63D3) and macrophage-specific esterase activity, we find that this metabolite does, in fact, induce monocytic differentiation of HL-60 with a potency of approximately 1/200 that of 1,25(OH)2D3 and similar to that of 25OHD3. In agreement with the effect upon cell maturation, 19-Nor-25OHD3 displaces 3H-1,25(OH)2D3 from its HL-60 receptor with an efficiency comparable to 25OHD3. Hence, HL-60 cells convert 25OHD3 to 19-Nor-25OHD3, and 19-Nor-25OHD3 induces monocytic differentiation of HL-60 with comparable efficiency to its precursor, 25OHD3.

1,25-Dihydroxyvitamin D3 production by human keratinocytes. Kinetics and regulation

Human foreskin keratinocytes in vitro metabolize 25-hydroxyvitamin D3 to a number of metabolites, including 1,25-dihydroxyvitamin D3 (1,25(OH)2D3). This metabolite remains mostly within the cell and does not accumulate in the medium under the conditions of these experiments. With time, 1,25(OH)2D3 is catabolized, and more polar metabolites appear in both the cells and the medium. The production of 1,25(OH)2D3 has an apparent Michaelis constant (Km) for 25-hydroxyvitamin D3 of 5.4 X 10(-8) M. The levels of 1,25(OH)2D3 within the cell are increased both by increased production and decreased catabolism when parathyroid hormone(1-34) and isobutylmethylxanthine are added. Exogenously added 1,25(OH)2D3 at concentrations as low as 10(-12) M reduces endogenous 1,25(OH)2D3 production, increases 1,25(OH)2D3 catabolism, and increases 24,25-dihydroxyvitamin D3 production by an actinomycin D-sensitive process. These data indicate that the regulation of 1,25(OH)2D3 production by keratinocytes is similar to, but not identical to the regulation of 1,25(OH)2D3 by the kidney.

Neonatal human foreskin keratinocytes produce 1,25-dihydroxyvitamin D3

Primary cultures of neonatal human foreskin keratinocytes converted 25-hydroxyvitamin D in high yield to a metabolite with the chromatographic behavior of 1,25-dihydroxyvitamin D3. The identity of this metabolite as 1,25-dihydroxyvitamin D3 was confirmed both by its potency in displacing 1,25-dihydroxyvitamin D3 in the chick cytosol receptor assay and by mass spectral analysis. These results suggest that 1,25-dihydroxyvitamin D3 may be formed in the epidermis to regulate vitamin D production by the epidermis and to provide an alternative to 1,25-dihydroxyvitamin D3 production by the kidneys.

In vitro secretion of peptide hormones by the human placenta: I. ACTH

In order to address the question of autonomy of placental hormone secretion, fresh human term placentae were utilized for the preparation of small tissue fragments. The fragment pool was divided over four parallel chambers in a superfusion apparatus and could thus serve as both control and experiment under identical in vitro conditions. Oxygen consumption was substantial and could be maintained for at least 5 h. Adrenocorticotrophic hormone (ACTH) concentrations in the effluent buffer were estimated by radioimmunoassay and bioassay. Both non-specific (membrane depolarization with 45 mM KCl) and specific (isoproterenol at 10(-7) M) stimulation increased the ACTH secretion from 10 to 20 pg/min/g to 60 to 80 pg/min/g. Propranolol blocked the adrenergic stimulation almost completely, indicating the specificity of the effect. Thus, in terms of in vitro ACTH secretion, the human placenta can be stimulated and therefore does not seem to behave in an autonomous manner.

Vitamin D metabolism during pregnancy

Vitamin D metabolism is altered in the pregnant animal, presumably in response to fetal demands for calcium. Circulating levels of 1,25-dihydroxyvitamin D are elevated in the pregnant animal. The stimulus of this increase and the hydroxylase(s) (placental or renal) responsible are unknown. Maternal plasma 25-hydroxyvitamin D levels have been reported to be both unchanged and decreased during pregnancy but very much dependent upon exposure to ultraviolet light and vitamin D supplementation. The major vitamin D metabolites (25-hydroxyvitamin D, 24,25-dihydroxyvitamin D, and 1,25-dihydroxyvitamin D) circulate in fetal plasma but generally at lower concentrations than in the mother (exception is the sheep). All of these metabolites are able to cross the placenta. The fetal kidney and placenta both have 25-hydroxyvitamin D1 alpha- and 24-hydroxylase activity. However, the relative contribution of mother, fetus, and placenta to fetal vitamin D metabolism has yet to be fully determined.

gamma-Interferon stimulates production of 1,25-dihydroxyvitamin D3 by normal human macrophages

We show for the first time that normal human pulmonary alveolar macrophages (PAM) markedly enhance their basal rate of the production of [3H]-1,25(OH)2D3 when cultured in the presence of recombinant gamma-interferon (gamma-IFN). The rate of conversion of [3H]-25(OH)D3 to [3H]-1,25(OH)2D3 was dose-dependent in a linear fashion. A maximal production of 1,25(OH)2D3 by PAM occurred after exposure of PAM to gamma-IFN for one day. This maximum plateau-level was sustained for at least five days. The authenticity of the putative 1,25(OH)2D3 obtained from PAM was tested by demonstrating the exact comigration of [3H]-1,25(OH)2D3 with chemically synthesized 1,25(OH)2D3 in four different HPLC-systems.

Vitamin D and skeletal tissues

It is now accepted that vitamin D is an integral part of a complex endocrine system, one with far-reaching implications in mineral metabolism. Reviews of the sources, functions and metabolism of vitamin D, as currently understood, are presented as a prelude to discussions of the role of vitamin D in calcium and phosphorous homeostatis and possible specific roles for vitamin D in mineralized tissues. Data describing a possible regulatory function for vitamin D in bone and bone protein metabolism are presented. Some of the controversy which presently exists regarding the biochemical mechanism of the action of this vitamin is discussed. Finally, the possible relationship of vitamin D and disorders of skeletal tissues is described.

Can the fetus regulate its calcium uptake?

To investigate the role of the fetus in vitamin D metabolism concentrations of vitamin D metabolites, 25(OH)D, 24,25(OH)2D and 1,25(OH)2D, were measured in human umbilical artery and vein. There were no differences between artery and vein in 25(OH)D and 24,25(OH)2D levels. 1,25(OH)2D concentrations were statistically significantly higher in the artery than in the vein. It has been shown in animal experiments that 1,25(OH)2D is an important factor in the maintenance of the placental calcium gradient. We suggest that the fetus actively produces 1,25(OH)2D and hence has the capacity to control its calcium influx.

Transfer of 25-hydroxyvitamin D3 and 1,25-dihydroxyvitamin D3 across the perfused human placenta

The rates of placental transfer of 25-hydroxyvitamin D3 [25-(OH)D3] and 1,25-dihydroxyvitamin D3 [1,25-(OH)2D3] were determined in an in vitro perfusion system. Antipyrine was included in each perfusion, and the data are expressed as clearance index, the ratio of hydroxyvitamin D3 to antipyrine clearance. In most experiments, serum, 0.2%, was added to the perfusates as a source of vitamin D3-binding protein. Binding as measured by dextran-coated charcoal assay for 25-(OH)D3 was over 90%, for 1,25-(OH)2D3, only 25% to 50%. The clearance index from the maternal to fetal circulation averaged 0.02 and 0.26 for 25-(OH)D3 and 1,25-(OH)2D3, respectively. When vitamin D3-binding protein was omitted from the perfusate, the clearance indices of 25-(OH)D3 were 0.12 and 0.46 in two experiments. Binding to vitamin D3-binding protein is a major determining factor for the transfer rates of 25-(OH)D3 and 1,25-(OH)2D3.