Complex regulation of Calbindin-D(9k) in the mouse placenta and extra-embryonic membrane during mid- and late pregnancy

Adaptations in Maternofetal Calcium Transport in Relation to Placental Size and Fetal Sex in Mice

Appropriate placental transport of calcium is essential for normal fetal skeletal mineralization. In fetal growth restriction (FGR), the failure of a fetus to achieve its growth potential, a number of placental nutrient transport systems show reduced activity but, in the case of calcium, placental transport is increased. In a genetic mouse model of FGR this increase, or adaptation, maintains appropriate fetal calcium content, relative to the size of the fetus, despite a small, dysfunctional placenta. It is unknown whether such an adaptation is also apparent in small, but normally functioning placentas. We tested the hypothesis that calcium transfer would be up-regulated in the lightest vs. heaviest placentas in the same C57Bl/6J wild-type (WT) mouse litter. Since lightest placentas are often from females, we also assessed whether fetal sex influenced placental calcium transfer. Placentas and fetuses were collected at embryonic day (E)16.5 and 18.5; the lightest and heaviest placentas, and female and male fetuses, were identified. Unidirectional maternofetal calcium clearance (^CaK_mf) was assessed following ⁴⁵Ca administration to the dam and subsequent radiolabel counts within the fetuses. Placental expression of calcium pathway components was measured by Western blot. Data (median) are lightest placenta expressed as percentage of the heaviest within a litter and analyzed by Wilcoxon signed-rank test. In WT mice having normally grown fetuses, ^CaK_mf, per gram placenta near term, in the lightest placentas was increased (126%; P < 0.05) in association with reduced fetal calcium accretion earlier in gestation (92%; P < 0.05), that was subsequently normalized near term. Increased placental expression of calbindin-D_9K, an important calcium binding protein, was observed in the lightest placentas near term (122%; P < 0.01). There was no difference in fetal calcium accretion between male and female littermates but a trend toward higher ^CaK_mf in females (P = 0.055). These data suggest a small, normal placenta adapts calcium transfer according to its size, as previously demonstrated in a mouse model of FGR. Fetal sex had limited influence on this adaptive increase. These adaptations are potentially driven by fetal nutrient demand, as evidenced by the normalization of fetal calcium content. Understanding the regulatory mechanisms involved may provide novel avenues for treating placental dysfunction.

Change of genes in calcium transport channels caused by hypoxic stress in the placenta, duodenum, and kidney of pregnant rats

Preeclampsia is a pregnancy-specific disease characterized by concurrent development of hypertension, proteinuria, and oxidative stress in the placenta. In this study, we induced hypoxic stress in rats during pregnancy to reproduce physiological conditions associated with preeclampsia. The maternal weight of hypoxic pregnant rats was lower than that of normoxic animals. The level of calcium ions were also increased in urine collected from the hypoxic animals. In contrast, urinary concentrations of sodium, chloride, and potassium ions declined in hypoxic rats, and developed to proteinuria. The expression of genes known as two biomarkers, sFLT1 (for preeclampsia) and HIF-1alpha (for hypoxia), were highly induced in the placenta, duodenum, and kidney by hypoxic stress. The overexpression of sFLT1 and HIF-1alpha demonstrated that our experimental conditions closely mimicked ones that are associated with preeclampsia. In the present study, we measured the expression of calcium transporters (TRPV5, TRPV6, PMCA1, NCKX3, NCX1, and CaBP-9k) in the placenta, duodenum, and kidney under hypoxic conditions on Gestational Day 19.5 in rats. Placental TRPV5, TRPV6, and PMCA1 expression was up-regulated in the hypoxic rats, whereas the levels of NCX1 and CaBP-9k were unchanged. In addition, NCKX3 expression was increased in the placenta of hypoxic rats. Duodenal expression of CaBP-9k, TRPV5, TRPV 6, and PMCA1 was decreased in the hypoxic rats, whereas levels of NCXs were not altered. Renal expression of NCKX3 and TRPV6 was increased, whereas NCX1 was decreased in the hypoxic rats compared to the normoxic controls. Taken together, these results indicate that physiological changes observed in the hypoxic rats were similar to ones associated with preeclampsia. Expression of calcium transport genes in the placenta, duodenum, and kidney perturbed by hypoxic stress during pregnancy may cause calcium loss in the urine, and thereby induce calcium-deficient characteristics of preeclampsia.

Apoptosis- and endoplasmic reticulum stress-related genes were regulated by estrogen and progesterone in the uteri of calbindin-D(9k) and -D(28k) knockout mice

Calcium (Ca(2+)) is an important regulator of apoptotic signaling. Calbindin-D(9k) (CaBP-9k) and -D(28k) (CaBP-28k) have a high affinity for Ca(2+) ions. Uterine calbindins appear to be involved in the regulation of myometrial activity by intracellular Ca(2+). In addition, uterine calbindins are expressed in the mouse endometrium and are regulated by steroid hormones during implantation and development. The aim of the present study was to evaluate the regulation of apoptosis in the uteri of CaBP-9k, CaBP-28k, and CaBP-9k/28k knockout (KO) mice. Our findings indicated that Bax protein was enhanced in the uteri of CaBP-28k and CaBP-9k/28k KO mice compared to wild-type (WT) and CaBP-9k KO mice, but no difference was observed in Bcl-2 protein expression. The expressions of caspase 3, 6, and 7 proteins were higher in both CaBP-28k and CaBP-9k/28k KO mice than in WT and CaBP-9k KO mice. These results suggest that the absence of CaBP-28k increases apoptotic signaling. We also investigated the expression of endoplasmic reticulum (ER) stress genes by Western blot analysis in calbindin KO mice. C/EBP homologous protein and immunoglobulin heavy chain-binding protein protein levels were elevated in CaBP-28k KO mice compared to WT mice. When immature mice were treated with 17β-estradiol (E2) or progesterone (P4) for 3 days, we found that the expressions of Bax and caspase 3 protein were increased by E2 treatment in WT and CaBP-9k KO mice, and by P4 treatment in CaBP-28k KO mice. These results indicate that CaBP-28k blocks the up-regulation of apoptosis-related genes and ER stress genes, implying that CaBP-28k may decrease the expression of genes involved in apoptosis and ER stress in murine uterine tissue.

Calcium transport genes are differently regulated in maternal and fetal placenta in the knockout mice of calbindin-D(9k) and -D(28k)

Calbindin-D(9k) (CaBP-9k) and -D(28k) (CaBP-28k) are cytosolic proteins with EF-hand motifs that have a high affinity for calcium ions. Many types of calcium channels and intracellular calcium binding proteins, such as sodium/calcium exchangers (NCXs) and transient receptor potential cation channels (TRPVs), have been detected in the placenta. In this study, the expression of calcium channels involved in maternal-fetal calcium transport were investigated in wild-type mice versus CaBP-9k, CaBP-28k, and CaBP-9k/28k double knockout (KO) mouse models. The expression of calcium transport genes in three dissected sections of the placenta (maternal, central, and fetal) was examined on gestational day 19 (GD 19). The expression of CaBP-9k, TRPV6, TRPV5, and NCX1 mRNA was high in fetal compared to maternal placenta, while CaBP-28k was abundant in the maternal placenta. CaBP-9k was enhanced in all sections of placenta in CaBP-28k KO mice, whereas CaBP-28k was reduced in CaBP-9k KO mice. The expression of TRPV6, TRPV5, and NCX1 were induced in both maternal and fetal placentas in CaBP-9k KO mice, but were upregulated in maternal and central placentas of CaBP-28k KO mice. The levels of these proteins showed similar patterns with those of their mRNA. Placental CaBP-9k, TRPV6, TRPV5, and NCX1 proteins were abundantly expressed in the intraplacental yolk sac located in the fetal placenta. CaBP-28k did not colocalize with other calcium transport genes, although it was enriched in the placental trophoblasts of the decidual zone in the maternal placenta. These results indicate that placental TRPV6, TRPV5, and NCX1 compensate for CaBPs in CaBP-9k and/or CaBP-28k KO mice, and may take over the roles of CaBP-9k and CaBP-28k to transfer calcium ions in the placenta. Taken together, these results indicate that TRPV6, NCX1, and CaBP-9k in the fetal placenta and CaBP-28k in the maternal placenta may play key roles in controlling calcium transport across the placenta during pregnancy.

Developmental dioxin exposure of either parent is associated with an increased risk of preterm birth in adult mice

We have previously described diminished uterine progesterone response and increased uterine sensitivity to inflammation in adult female mice with a history of developmental exposure to TCDD (2,3,7,8-tetrachlorodibenzo-p-dioxin). Since parturition in mammals is an inflammatory process mediated in part by a decline in progesterone action, toxicant-mediated disruption of progesterone receptor (PR) expression at the maternal-fetal interface would likely impact the timing of birth. Therefore, in the current study, we examined pregnancy outcomes in adult female mice with a similar in utero exposure to TCDD. We also examined the impact of in utero TCDD exposure of male mice on pregnancy outcomes in unexposed females since the placenta, a largely paternally derived organ, plays a major role in the timing of normal parturition via inflammatory signaling. Our studies indicate that developmental exposure of either parent to TCDD is associated with preterm birth in a subsequent adult pregnancy due to altered PR expression and placental inflammation.

Placental-specific Igf2 knockout mice exhibit hypocalcemia and adaptive changes in placental calcium transport

Evidence is emerging that the ability of the placenta to supply nutrients to the developing fetus adapts according to fetal demand. To examine this adaptation further, we tested the hypothesis that placental maternofetal transport of calcium adapts according to fetal calcium requirements. We used a mouse model of fetal growth restriction, the placental-specific Igf2 knockout (P0) mouse, shown previously to transiently adapt placental System-A amino acid transporter activity relative to fetal growth. Fetal and placental weights in P0 mice were reduced when compared with WT at both embryonic day 17 (E17) and E19. Ionized calcium concentration [Ca(2+)] was significantly lower in P0 fetal blood compared with both WT and maternal blood at E17 and E19, reflecting a reversal of the fetomaternal [Ca(2+)] gradient. Fetal calcium content was reduced in P0 mice at E17 but not at E19. Unidirectional maternofetal calcium clearance ((Ca) K (mf)) was not different between WT and P0 at E17 but increased in P0 at E19. Expression of the intracellular calcium-binding protein calbindin-D(9K), previously shown to be rate-limiting for calcium transport, was increased in P0 relative to WT placentas between E17 and E19. These data show an increased placental transport of calcium from E17 to E19 in P0 compared to WT. We suggest that this is an adaptation in response to the reduced fetal calcium accumulation earlier in gestation and speculate that the ability of the placenta to adapt its supply capacity according to fetal demand may stretch across other essential nutrients.

An evaluation of estrogenic activity of parabens using uterine calbindin-d9k gene in an immature rat model

In the present study, calbindin-D9k (CaBP-9k), a potent biomarker for screening estrogen-like environmental chemicals in vivo and in vitro, was adopted to examine the potential estrogen-like property of the following parabens: propyl-, isopropyl-, butyl-, and isobutylparaben. Immature female rats were administered for 3 days from postnatal day 14 to 16 with 17alpha-ethinylestradiol (EE, 1 mg/kg body weight [BW]/day) or parabens (62.5, 250, and 1000 mg/kg BW/day). In uterotrophic assays, significantly increased uterus weights were detected in the EE-treated group and in the groups treated with the highest dose of isopropyl-, butyl-, and isobutylparaben. In addition, these parabens induced uterine CaBP-9k messenger RNA (mRNA) and protein levels, whereas cotreatment of parabens and fulvestrant, a pure estrogen receptor (ER) antagonist, completely reversed the paraben-induced gene expression and increased uterine weights. To investigate the ER-mediated mechanism(s) by which parabens exert their effects, the expression level of ER-alpha and progesterone receptor (PR) was analyzed. Exposure to EE or parabens caused a dramatic decrease in expression of both ER-alpha mRNA and protein levels, whereas cotreatment with fulvestrant reversed these effects. These data showed the difference of CaBP-9k and ER-alpha expression, suggesting that CaBP-9k may not express via ER-alpha pathway. In the effect of parabens on CaBP-9k expression through PR mediation, a significantly increased expression of uterine PR gene, a well-known ER-regulating gene, at both transcriptional and translational levels was indicated in the highest dose of isopropyl- and butylparaben. These parabens-induced PR gene expression was completely blocked by fulvestrant. This result indicates that CaBP-9k expression may involve with PR mediates in the estrogenic effect of paraben in immature rat uteri. Taken together, parabens exhibited an estrogen-like property in vivo, which may be mediated by a PR and/or ER-alpha signaling pathway. In addition, our results expanded the current understanding of the potential adverse effects of parabens associated with their estrogen-like activities. Further investigation is needed to elucidate in greater detail the adverse effects of parabens in humans and wildlife.

Uterine and placental expression of TRPV6 gene is regulated via progesterone receptor- or estrogen receptor-mediated pathways during pregnancy in rodents

Transient receptor potential cation channel, subfamily V, member 6 (TRPV6) is an epithelial Ca2+ channel protein expressed in calcium absorbing organs. In the present study, we investigated the expression and regulation of uterine and placental TRPV6 during gestation in rodents. Uterine TRPV6 peaked at pregnancy day (P) 0.5, P5.5 and, P13.5 and was detected in uterine epithelium and glands of rats, while placental TRPV6 mRNA levels increased in mid-gestation. Uterine and placental TRPV6 mRNA levels in rats appear to cyclically change during pregnancy, suggesting that TRPV6 may participate in the implantation process. In addition, uterine TRPV6 mRNA is only expressed in placenta-unattached areas of the uterus, and uterine TRPV6 immunoreactivity was observed in luminal and glandular epithelial cells. In the placenta, TRPV6 was detected in the labyrinth and spongy zone. These results may indicate that TRPV6 has at least two functions: implantation of the embryo and maintenance of pregnancy. To investigate the pathway(s) mediating TRPV6 expression in rodents, anti-steroid hormone antagonists were injected prior to maximal TRPV6 expression. In rats, TRPV6 expression was reduced by RU486 (an anti-progesterone) through progesterone receptors, and ICI 182,780 (an anti-estrogen) blocked TRPV6 expression via estrogen receptors in mice. The juxtaposition of uterine and placental TRPV6 expressed in these tissues supports the notion that TRPV6 participates in transferring calcium ions between the maternal and fetal compartments. Taken together, TRPV6 gene may function as a key element in controlling calcium transport in the uterus between the embryo and the placenta during pregnancy.

Increased maternofetal calcium flux in parathyroid hormone-related protein-null mice

The role of parathyroid hormone-related protein (PTHrP) in fetal calcium homeostasis and placental calcium transport was examined in mice homozygous for the deletion of the PTHrP gene (PTHrP^−/− null; NL) compared to PTHrP^+/+ (wild-type; WT) and PTHrP^+/− (heterozygous; HZ) littermates. Fetal blood ionized calcium was significantly reduced in NL fetuses compared to WT and HZ groups at 18 days of pregnancy (dp) with abolition of the fetomaternal calcium gradient. In situ placental perfusion of the umbilical circulation at 18 dp was used to measure unidirectional clearance of ⁴⁵Ca across the placenta in maternofetal (^CaK_mf) and fetoplacental (^CaK_fp) directions; ^CaK_fp was < 5% of ^CaK_mf for all genotypes. At 18 dp, ^CaK_mf across perfused placenta and intact placenta (^CaK_mf(intact)) were similar and concordant with net calcium accretion rates in vivo. ^CaK_mf was significantly raised in NL fetuses compared to WT and HZ littermates. Calcium accretion was significantly elevated in NL fetuses by 19 dp. Placental calbindin-D_9K expression in NL fetuses was marginally enhanced (P < 0.07) but expression of TRPV6/ECaC2 and plasma membrane Ca²⁺-ATPase (PMCA) isoforms 1 and 4 were unaltered. We conclude that PTHrP is an important regulator of fetal calcium homeostasis with its predominant effect being on unidirectional maternofetal transfer, probably mediated by modifying placental calbindin-D_9K expression. In situ perfusion of mouse placenta is a robust methodology for allowing detailed dissection of placental transfer mechanisms in genetically modified mice.

Molecular mechanism of regulation of the calcium-binding protein calbindin-D9k, and its physiological role(s) in mammals: a review of current research

Calbindin-D_9k (CaBP-9k) is a cytosolic calcium-binding protein that is expressed in a variety of tissues, such as uterus, placenta, intestine, kidney, pituitary gland and bone. At present, the precise role(s) of CaBP-9k remains to be clarified. CaBP-9k-null mice are normal, which indicates that other calcium-transporter genes can compensate for the lack CaBP-9k. Uterine CaBP-9k has been shown to be involved in the regulation of myometrial activity by intracellular calcium. In the uterus and placenta, CaBP-9k expression is regulated by the sex steroid hormones oestrogen (E2) and progesterone (P4). Intestinal CaBP-9k is involved in intestinal calcium absorption, and is regulated at the transcriptional and post-transcriptional levels by 1,25-dihydroxyvitamin D3, the hormonal form of vitamin D. Thus, evidence to date suggests that CaBP-9k may be regulated in a tissue-specific manner. In this review, we will summarize current data on the molecular mechanism of regulation of CaBP-9k in mammals, including recent research data generated in our laboratories.

Phenotype of a calbindin-D9k gene knockout is compensated for by the induction of other calcium transporter genes in a mouse model

CaBP-9k may be involved in the active calcium absorption and embryo implantation. Although we generated CaBP-9k KO mice to explore its function, no distinct phenotypes were observed in these KO mice. It can be hypothesized that TRPV5 and 6 and plasma membrane calcium ATPase 1b may play a role in the regulation of calcium transport to compensate CaBP-9k deficiency in its KO model.

INTRODUCTION

Active calcium transport in the duodenum and kidney is carried in three steps: calcium entry through epithelial Ca2+ channels (TRPV5 and TRPV6), buffering and/or transport by calbindin-D9k (CaBP-9k) and -D28k (CaBP-28k), and extrusion through the plasma membrane calcium ATPase 1b (PMCA1b) and sodium/calcium exchanger 1. Although the molecular mechanism of calcium absorption has been studied using knockouts (KOs) of the vitamin D receptor and CaBP-28k in animals, the process is not fully understood.

MATERIALS AND METHODS

We generated CaBP-9k KO mice and assessed the phenotypic characterization and the molecular regulation of active calcium transporting genes when the mice were fed different calcium diets during growth.

RESULTS

General phenotypes showed no distinct abnormalities. Thus, the active calcium transport of CaBP-9k-null mice proceeded normally in this study. Therefore, the compensatory molecular regulation of this mechanism was elucidated. Duodenal TRPV6 and CaBP-9k mRNA of wildtype (WT) mice increased gradually during preweaning. CaBP-9k is supposed to be an important factor in active calcium transport, but its role is probably compensated for by other calcium transporter genes (i.e., intestinal TRPV6 and PMCA1b) during preweaning and renal calcium transporters in adult mice.

CONCLUSIONS

Depletion of the CaBP-9k gene in a KO mouse model had little phenotypic effect, suggesting that its depletion may be compensated for by calcium transporter genes in the intestine of young mice and in the kidney of adult mice.

The classical and a non-classical pathways associated with NF-kappaB are involved in estrogen-mediated regulation of calbindin-D9k gene in rat pituitary cells

Calbindin-D9k (CaBP-9k) is a high affinity calcium binding protein that is highly expressed in the duodenum, kidney, uterus, and pituitary glands. Previous studies have shown that CaBP-9k expression is regulated by several steroid hormones, such as 1,25-dihydroxyvitamin D3, glucocorticoids, 17beta-estradiol (E2), and progesterone (P4), in a tissue-specific manner. However, the promoter elements that mediate transcriptional regulation by these steroid hormones are not clearly understood, mainly due to the lack of an appropriate cell line expressing CaBP-9k. Recently it was shown that CaBP-9k was constitutively expressed in the rat pituitary gland, and is expressed in an E2-dependent manner in a pituitary gland tumor-derived cell line, GH3. In the current study, we examined the activity of the estrogen responsive element (ERE) in rat CaBP-9k gene in GH3 cells, using a luciferase gene reporter assay, electrophoretic mobility shift assay, and mutagenesis. A nuclear factor-kappaB (NF-kappaB) binding site in the CaBP-9k promoter region was identified (nucleotides -848 to -834 from the transcriptional start site), and we demonstrated that addition of an NF-kappaB blocker to GH3 cells reduced E2-induced CaBP-9k transcription. In the present study, we further showed a previously reported imperfect ERE (nucleotides +51 to +65) between exon I and intron A of CaBP-9k, indicating that the interaction of estrogen receptor (ER) alpha with this region is involved in the regulation of CaBP-9k promoter activity and its expression. Taken together, these results suggest that in GH3 cells, both the classical ERalpha-ERE pathway and a non-classical pathway involving NF-kappaB are involved in E2-mediatd regulation of CaBP-9k expression in the pituitary gland.

Calbindin D9k is not required for 1,25-dihydroxyvitamin D3-mediated Ca2+ absorption in small intestine

The exact role of calbindin D9k in vitamin D-mediated calcium absorption has been debated but remains unsettled. In 129/OlaHsd mice, calbindin D9k was found highest in duodenum (36-50%) and kidney (24-34%) followed by stomach, lung and uterus. Age does not affect the relative distribution of calbindin D9k but it does decline with age in duodenum of both male and female 129/Ola mice. Recently, we produced a null calbindin D9k mutant 129/OlaHsd mouse; this mouse proved to be indistinguishable from the wild-type in phenotype and in a serum calcium level regardless of age or gender. We have now examined directly whether the mutant mouse can absorb calcium from the intestine in response to the active form of vitamin D. The calbindin D9k null mutant mouse is fully able to absorb calcium from the intestine in response to 1,25-dihydroxyvitamin D3. It is, therefore, clear that calbindin D9k is not required for vitamin D-induced intestinal calcium absorption.

Glucocorticoids differentially regulate expression of duodenal and renal calbindin-D9k through glucocorticoid receptor-mediated pathway in mouse model

Dexamethasone (Dex) is a member of the glucocorticoids (GCs), and is broadly used as an anti-inflammatory medication. Continuous administration with GCs induces adverse effects and suffering in humans (i.e., osteoporosis) due to negative calcium balance derived from low re- and absorption in the duodenum and kidney. A cytosolic calcium-binding protein, calbindin-D9k (CaBP-9k), is dominantly expressed in the renal and intestinal tissues involved in calcium re- and absorption and plays an active role in calcium transport. In the present study, we employed adrenalectomized (ADX) and sham-treated (Sham) male mice to examine the effect of Dex on CaBP-9k gene expression in the duodenum and kidney. Dex significantly reduced the levels of duodenal CaBP-9k mRNA and protein, and it restored ADX-induced decrease in renal CaBP-9k protein compared with the level of Sham control. Dex treatment increased calcium and phosphate levels in the sera of both Sham and ADX mice. In a time course experiment, Dex significantly decreased duodenal CaBP-9k at the transcriptional and translational levels at 3 days, whereas it temporarily increased CaBP-9k mRNA and protein levels at 12 and 24 h. Altered CaBP-9k expression by Dex was completely reversed by mifepristone, an antagonist for the GC receptor (GR). In addition, duodenal CaBP-9k and GR were colocalized on the enterocyte (duodenocyte), supporting a role for GR in regulating CaBP-9k. In ovariectomized (OVX) and ADX female mice daily treated with Dex for 3 days, duodenal CaBP-9k was expressed at the same level as in male mice. Also, no cross-activity of progesterone and Dex on their receptors was observed. Taken together, these results indicate that mouse CaBP-9k gene may be regulated by Dex in a tissue-specific manner, and reduced duodenal CaBP-9k via the GR pathway may take part in negative calcium absorption of GC-induced osteoporosis, whereas renal CaBP-9k may not be involved in the regulation of calcium homeostasis.

Calcium Homeostasis in Human Placenta: Role of Calcium‐Handling Proteins

The human placenta is a transitory organ, representing during pregnancy the unique connection between the mother and her fetus. The syncytiotrophoblast represents the specialized unit in the placenta that is directly involved in fetal nutrition, mainly involving essential nutrients, such as lipids, amino acids, and calcium. This ion is of particular interest since it is actively transported by the placenta throughout pregnancy and is associated with many roles during intrauterine life. At term, the human fetus has accumulated about 25-30 g of calcium. This transfer allows adequate fetal growth and development, since calcium is vital for fetal skeleton mineralization and many cellular functions, such as signal transduction, neurotransmitter release, and cellular growth. Thus, there are many proteins involved in calcium homeostasis in the human placenta.

Biology and physiology of Calbindin-D9k in female reproductive tissues: involvement of steroids and endocrine disruptors

Although Calbindin-D9k (CaBP-9k), a cytosolic calcium binding protein which has calcium binding sites, is expressed in various tissues, i.e., intestine, uterus, and placenta, potential roles of this gene and its protein are not clearly understood. Uterine CaBP-9k may be involved in controlling myometrial activity related with intracellular calcium level and is not under the control of vitamin D despite the presence of vitamin D receptors. But, it is under the control of the sex steroid hormones, estrogen (E2) and progesterone (P4), in female reproductive systems including the uterus and placenta. Thus, in this review, we summarize recent research literature in regards to the expression and regulation of CaBP-9k in mammals and introduce the research data of recent studies by us and others.

A calcium binding protein, Calbindin-D9k, is mainly regulated by estrogen in the pituitary gland of rats during estrous cycle

As a member of family of cytosolic calcium binding proteins, Calbindin-D9k (CaBP-9k) is expressed in female reproductive system and regulated by steroid hormones, estrogen (E2) and progesterone (P4), but its expression and role in pituitary gland have not been elucidated yet. Thus, in this study, we elucidated the expression of CaBP-9k mRNA and protein in pituitary gland of rats. During estrous cycle of rats, pituitary CaBP-9k level fluctuated, and its mRNA was highly elevated during an E2-dominant stage (proestrus and estrus), whereas its level disappeared at a P4-dominant stage (metestrus and diestrus). In parallel with CaBP-9k mRNA, an increased level of CaBP-9k protein was observed during proestrus and estrus, suggesting that pituitary CaBP-9k may be up-regulated by E2. In addition, spatial CaBP-9k expression was attested by immunohistochemistry. Pituitary CaBP-9k protein was localized in the cytoplasm of a specific cell type in the anterior lobe, and the positive cells were abundant at proestrus and estrus. The CaBP-9k-positive cells were mainly localized in the acidophils producing growth hormones and prolactin. To verify hormonal regulation of pituitary CaBP-9k in this tissue, immature rats were treated with a physiological dose of E2 in the absence or presence P4 for 3 days. In a time-dependent experiment, pituitary CaBP-9k protein was induced at 48 h after the final E2 injection. A significant increase in CaBP-9k protein was caused by E2, whereas P4 antagonized E2-stimulated CaBP-9k expression as similarly observed in the uterus. Taken together, these results indicated for the first time that pituitary CaBP-9k expression is regulated during estrous cycle, and its expression is mainly controlled by E2 and antagonized by P4, suggesting that pituitary CaBP-9k in female rats may be involved in the central function of the reproduction system.

Transfer of maternally injected endocrine disruptors through breast milk during lactation induces neonatal Calbindin-D9k in the rat model

The uterus is a highly estrogen-responsive tissue, which can be measured through changes in CaBP-9k expression. In this study, we investigated the potential for estrogenic compounds 4-tert-octylphenol (OP), nonylphenol (NP), bisphenol A (BPA), diethylstilbestrol (DES) and 17beta-estradiol (E2) to be transferred through breast milk from dam to neonate during lactation using the induction of CaBP-9k in uterine tissue as a biomarker. Dams were treated with OP, NP and BPA, dissolved in corn oil, at doses of 200, 400 and 600 mg/kg body weight per day l for 5 days after delivery. Dams and neonates were euthanized after 24h. Treatment with these estrogenic compounds increased the expression of CaBP-9k mRNA in the maternal uterus, in a dose-dependent manner. All doses of estrogenic compounds resulted in an increase in CaBP-9k protein levels. These compounds have an estrogenic effect on the maternal uterus during the lactation period as shown by the induction of both CaBP-9k mRNA and protein. In the neonatal uterus, the expression of CaBP-9k mRNA and protein significantly increased with DES exposure. There was a significant increase in CaBP-9k mRNA in neonatal uterus when the dams were treated with high doses of estrogenic compounds, but protein levels of CaBP-9k were undetectable. Taken together, these findings suggest that maternally injected estrogenic compounds may be transferred to neonates through breast milk and thus affecting uterine function, as shown by the induction of CaBP-9k gene expression in the neonatal uterus.