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Expression of calbindin-D28k and its regulation by estrogen in the human endometrium during the menstrual… Yang, Hyun; Kim, Tae-Hee; Lee, Hae-Hyeog; Choi, Kyung-Chul; Hong, Yeon-pyo; Leung, Peter C; Jeung, Eui-Bae Mar 2, 2011

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RESEARCH Open AccessExpression of calbindin-D28k and its regulationby estrogen in the human endometrium duringthe menstrual cycleHyun Yang1†, Tae-Hee Kim2†, Hae-Hyeog Lee2, Kyung-Chul Choi1, Yeon-pyo Hong3, Peter CK Leung4,Eui-Bae Jeung1*AbstractHuman endometrium resists embryo implantation except during the ‘window of receptivity’. A change inendometrial gene expression is required for the development of receptivity. Uterine calbindin-D28k (CaBP-28k) isinvolved in the regulation of endometrial receptivity by intracellular Ca2+. Currently, this protein is known to bemainly expressed in brain, kidneys, and pancreas, but potential role(s) of CaBP-28k in the human uterus during themenstrual cycle remain to be clarified. Thus, in this study we demonstrated the expression of CaBP-28k in thehuman endometrium in distinct menstrual phases. During the human menstrual cycle, uterine expression levels ofCaBP-28k mRNA and protein increased in the proliferative phase and fluctuated in these tissues, compared withthat observed in other phases. We assessed the effects of two sex-steroid hormones, 17beta-estradiol (E2) andprogesterone (P4), on the expression of CaBP-28k in Ishikawa cells. A significant increase in the expression of CaBP-28k mRNA was observed at the concentrations of E2 (10(-9 to -7) M). In addition, spatial expression of CaBP-28kprotein was detected by immunohistochemistry. CaBP-28k was abundantly localized in the cytoplasm of theluminal and glandular epithelial cells during the proliferative phases (early-, mid-, late-) and early-secretory phase ofmenstrual cycle. Taken together, these results indicate that CaBP-28k, a uterine calcium binding protein, isabundantly expressed in the human endometrium, suggesting that uterine expression of CaBP-28k may beinvolved in reproductive function during the human menstrual cycle.BackgroundIntracellular calcium binding proteins (calbindins) arecritical for regulating the availability of calcium ions(Ca2+) within cells. There are two types of cytosolic cal-bindins, calbindin-D9k (CaBP-9k) and calbindin-D28k(CaBP-28k), which are cytosolic proteins differentiallyregulated by steroid hormones in the uterus [1,2]. Inaddition to its traditional role in extracellular calciumhomeostasis [3], vitamin D influences a broad range ofcellular events, ranging from oncogene expression [4]and immunoregulation [5] to cellular differentiation[6,7] and intracellular calcium metabolism [8]. Previousstudies have documented that vitamin D deficiencydecreases fertility in female rats [9-11], and vitaminD-dependent calcium-binding proteins were discoveredin reproductive tissues: CaBP-9k in the uterus of rats[12-14], and the larger CaBP-28k in the uterus ofdomestic fowl [15,16].The expression of CaBP-9k and CaBP-28k in repro-ductive tissues is affected by steroid hormones[12,14,15,17]. For example, the expression of CaBP-9kmRNA in the mouse uterus was significantly increasedby treatment with P4 and E2 plus P4, but not by E2alone [18,19]. Conversely, rat uterine CaBP-D9k mRNAwas induced only by E2 [20]. In hens, uterine expressionof the larger calbindin-D28k increased by the addition oftestosterone [15].Embryo implantation is a complex process involvinginteractions between the blastocyst and the uterus.A successful implantation requires the development ofthe blastocyst stage, its escape from the zonapellucida,* Correspondence: ebjeung@chungbuk.ac.kr† Contributed equally1Laboratory of Veterinary Biochemistry and Molecular Biology, College ofVeterinary Medicine, Chungbuk National University, Cheongju, Chungbuk361-763, Republic of KoreaFull list of author information is available at the end of the articleYang et al. Reproductive Biology and Endocrinology 2011, 9:28http://www.rbej.com/content/9/1/28© 2011 Yang et al; licensee BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative CommonsAttribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction inany medium, provided the original work is properly cited.and the establishment of a receptive uterus [21]. Theprimate endometrium undergoes certain hormone-dependent changes during a particular time windowwithin the preimplantation phase that prepares it toreceive the growing blastocyst [22-24]. A complex inter-action between effectors, including steroid hormones,growth factors, and cytokines, regulates development ofthe “receptivity” state of the uterine epithelium [25-28].The action of calcium ions in female reproductiveorgans has also been widely studied for several decades.It has been suggested that calcium ions are involved inuterine smooth muscle contraction and fetal implanta-tion [1,29,30]. Additionally, the balance of calcium ionsduring uterine contraction and relaxation is extremelyimportant throughout pregnancy and during labor.However, the mechanism of regulation of calcium levelsin uterine tissue remains largely unknown. In the pre-sent model of calcium flux in uterine tissue, calciumions flow into the cytoplasm through ion transport pro-teins, exchangers, or calcium-binding proteins, i.e.,CaBP-9k and CaBP-28k. Furthermore, the mechanismand regulation of calcium-related genes in the uterusare not fully characterized, and it is likely that the regu-lation of calcium ion flux is complex and involves adiverse set of proteins. For instance, implantation inCaBP-9k knockout mice appears to occur normally,while CaBP-9k/CaBP-28k double knockout mice experi-ence failed embryo implantation [28]. These resultsdemonstrate that the expression patterns of CaBP-28kmay be involved in reproductive function and fetalimplantation during the menstrual cycle in female mice.In our previous studies, the expression of uterineCaBP-28k mRNA and protein in mice during the estruscycle is regulated by sex-steroid hormones [18,31]. How-ever, the role of CaBP-28k in the human uterus has yetto be fully characterized. It is likely that CaBP-28k isfunctionally important in female reproductive organs.We examined the effect of steroid hormones E2 and P4on the expression of CaBP-28k in the human endome-trial tissues during the different stages of the menstrualcycle and in Ishikawa endometrial cancer cells. In addi-tion, we also examined the intracellular localization ofuterine CaBP-28k in human female endometrial tissue.MethodsMaterials17b-Estradiol (E2), progesterone (P4), and mifepristone(RU486) were obtained from Sigma-Aldrich Corporation(St. Louis, MO). ICI 182 780 was purchased fromTOCRIS (Avonmouth, UK).Endometrial tissueHuman endometrial tissues were collected by curettagefrom women (the ages of 28-45 years) undergoinghysteroscopy for investigation of tubal p"36atency ortubal ligation. Endometrial tissues were classifiedaccording to the most recent menstrual period, and his-tology was performed according to the criteria of Noyeset al. [32]. Approval was given by the Human EthicsCommittee at SCH Medical Center (Bucheon, Korea),and signed consent was obtained in every case. Humanuterine finally classified samples (total n = 42) weredivided into seven groups (n = 6 in each period): men-strual, proliferative (early, mid, late), and secretory phase(early, mid, late). Endometrial tissues were transportedto the laboratory in buffered neutral formalin (10%) onice. Tissues were then quickly frozen in liquid nitrogenand stored at -70°C until further use.Cell culture and treatmentIshikawa cells were obtained from Sigma-Aldrich (St.Louis, MO). The cells were grown as monolayer culturesin Dulbecco’s Modified Eagle Medium (DMEM; GibcoBRL, Grand Island, NY), supplemented with 10% fetalbovine serum (FBS; Gibco BRL), 100 IU/mL penicillin,and 100 μg/mL streptomycin (Gibco BRL) at 37°C in ahumidified atmosphere of 95% O2 and 5% CO2. Tochallenge Ishikawa cells to E2 and P4, the cells were pla-ted and grown to 70-80% confluence in six-well plastictissue culture dishes (NUNC™, Roskilde, Denmark). Inorder to ensure depletion of steroid hormones andgrowth factors in the cells, the growth medium wasreplaced with starvation media containing phenol red-free DMEM with 5% dextran-coated charcoal-strippedFBS, 100 IU/mL penicillin, and 100 μg/mL streptomy-cin, as described previously [33]. The Ishikawa cellswere maintained on starvation media for 3 days beforeexposure to three concentrations of E2 (10-9 M, 10-8 M,and 10-7 M) and P4 (10-7 M, 10-6 M, and 10-5 M). Tofurther investigate whether ER or PR is involved in theexpression of CaBP-28k protein, the cells were pre-treated with ICI 182 780 (10-6 M) or mifepristone (10-6M) for 60 min prior to the treatment with E2 (10-8 M)or P4 (10-6 M). Each chemical was dissolved in DMSOand added to the starvation media with the final DMSOconcentration being 0.1%. DMSO alone was used as anegative control. Ishikawa cells were harvested 48 hafter treatment with E2 or P4 to measure mRNA levels,and whole cells were harvested for mRNA and Westernblot analyses. All experiments were performed in tripli-cate (group n = 3).Total RNA extraction and quantitative real-time PCREndometrial tissues were transported, rapidly excised,and washed in cold, sterile NaCl (0.9%). Total RNA wasprepared with Trizol reagent (Invitrogen, Carlsbad, CA),and the concentration of RNA was determined byabsorbance at 260 nm. Total RNA (1 μg) was reverseYang et al. Reproductive Biology and Endocrinology 2011, 9:28http://www.rbej.com/content/9/1/28Page 2 of 8transcribed into first-strand cDNAs using Moloney mur-ine leukemia virus (MMLV) reverse-transcriptase (Invi-trogen, Carlsbad, CA) and random primers (9-mers;Takara Bio Inc., Otsu, Shiga, Japan). Two microliters ofcDNA template was added to 10 μL of 2× SYBR PremixEx Taq (TaKaRa Bio) and 10 pmol of each specific pri-mer. The reactions were carried out for 40 cyclesaccording to the following parameters: denaturation at95°C for 30 s, annealing at 58°C for 30 s, and extensionat 72°C for 45 s. The oligonucleotide primers for CaBP-28k were 5’- AGT GGT TAC CTG GAA GGA AAG G-3’ (sense) and 5’- AGC AGG AAA TTC TCT TCTGTG G -3’ (antisense). The primers for GAPDH were5’- GGT GTG AAC CAT GAG AAG TAT GAC -3’(sense) and 5’- AGT AGA GGC AGG GAT GAT GTTCT -3’ (antisense). Fluorescence intensity was measuredat the end of the extension phase of each cycle. Thethreshold value for the fluorescence intensity of all sam-ples was set manually. The reaction cycle at which PCRproducts exceeded this fluorescence intensity thresholdwas identified as the threshold cycle [34] in the expo-nential phase of the PCR amplification. The expressionof calbindin-D28k was quantified against that ofGAPDH. Relative quantification was based on the com-parison of CT [cycle threshold] at a constant fluorescentintensity. The amount of transcript is inversely relatedto the magnitude of observed CT, and for every two-fold dilution in the transcript, CT is expected toincrease by one. Relative expression was calculatedusing the equation R = 2- [ΔCT sample - ΔCT control] [26].To determine a normalized arbitrary value for eachgene, every obtained value was normalized to that ofGAPDH.Reverse transcription polymerase chain reaction (RT-PCR)Total RNA was extracted and cDNA synthesized asdescribed in the preceding paragraph. RT-PCR was per-formed as described by [35]. Denaturation was per-formed at 95°C for 30 s, annealing at 58°C for 30 s (forCaBP-28k) or at 55°C for 30 s (for GAPDH), and exten-sion at 72°C for 30 s. Cycling kinetics were performedusing 20, 25, and 30 cycles, to ensure linearity of PCRproduct detection, and the final PCR condition was 30cycles for CaBP-28k or 25 cycles for GAPDH. Eightmicroliters of PCR products was loaded on 2% agarosegel and stained with ethidium bromide following elec-trophoresis. The intensity of the PCR bands was deter-mined directly by scanning the agarose gel and wasanalyzed using the molecular analysis program version4.5.1 (Gel Doc 1000, Bio-Rad, Hercules, CA).Western blot analysisEndometrial tissues were transported, rapidly excised,and washed in cold sterile 0.9% NaCl solution. Proteinwas extracted with Pro-prep (iNtRON Bio Inc, Sungnam,Kyungki-Do, Korea) according to the manufacturer’sinstructions. The cells were harvested, washed two timeswith ice-cold PBS, and then resuspended in 20 mM Tris-HCl buffer (pH 6.4) containing protease inhibitors (0.1mM phenylmethylsulfonyl fluoride, 5 μg/mL pepstatin A,and 1 μg/mL chymostatin). Whole cell lysate was pre-pared using 20 strokes of a Dounce homogenizer,followed by centrifugation at 13,000g for 20 min at 4°C.Proteins (70 μg per lane) were separated on a 10% SDS-polyacrylamide gel electrophoresis (PAGE) gel and trans-ferred to a polyvinylidene fluoride transfer membrane(Perkin Elmer Co., Wellesley, MA) in a TransBlot Cell(TE-22, Hoefer Inc, San Francisco, CA) according to themanufacturer’s protocol. The resulting blot was blockedin TBS-T (Tris-Buffered Saline Tween-20) containing 5%skim milk for 60 min and then incubated with primaryantibody: CaBP-28k (goat-polyclonal, 1:500, Santa Cruz,CA, USA) or GAPDH (mouse-monoclonal, 1:2000, AssayDesign Inc, Ann Abor, MI). After being washed in buffer,the membranes were incubated with the appropriatehorseradish peroxidase-conjugated secondary antibodies(anti-goat, 1:2000 or anti-mouse, 1:5000, Santa Cruz, CA)for 1 h at room temperature (RT). After being washed,the blots were developed by incubation in ECL chemilu-minescence reagent (Santa Cruz, CA) and subsequentlyexposed to Biomax™ Light film (Kodak) for 1-5 min.Signal specificity was confirmed by blotting in theabsence of primary antibody, and bands were normalizedto GAPDH-immunoreactive bands visualized in the samemembrane after stripping. Density measurements foreach band were performed with NIH ImageJ software.Background samples from an area near each lanewere subtracted from each band to obtain mean banddensity.Immunohistochemical stainingLocalization of CaBP-28k protein was examined byimmunohistochemistry. Endometrial tissues wereembedded in paraffin, before sections (5 μm) weredeparaffinized in xylene and hydrated in descendinggrades of ethanol. Endogenous peroxidase activity wasblocked with 3% hydrogen peroxide in TBS-T for30 min. Nonspecific reactions were blocked by incubat-ing the sections in 10% normal goat serum for 2 h atRT. The sections were subsequently incubated at RT for4 h with a polyclonal goat antibody directed againstCaBP-28k (diluted 1:300; Santa Cruz, CA, USA) dis-solved in 10% normal goat serum. After being washedwith TBS-T, the sections were incubated with a biotiny-lated secondary antibody (goat IgG, Vector Laboratories,Burlingame, CA) for 30 min at 37°C and then incubatedwith ABC-Elite for 30 min at 37°C. Diaminobenzidine(DAB; Sigma) was used as a chromogen, and theYang et al. Reproductive Biology and Endocrinology 2011, 9:28http://www.rbej.com/content/9/1/28Page 3 of 8sections were counterstained with hematoxylin, followedby mounting in Canada balsam.Data analysisData were presented as the means ± SEM and analyzedby one-way analysis of variance (ANOVA), followed byTukey’s multiple comparison test. Statistical analysis wasperformed using Prism Graph Pad (v4.0; GraphPad Soft-ware Inc., San Diego, CA, USA). P < 0.05 was consid-ered to be statistically significant.ResultsPattern of endometrial calbindin-D28k expressionTo investigate the expression of endometrial CaBP-28k dur-ing the menstrual cycle, we divided the endometrial tissuesinto seven groups [menstrual phase, proliferative phase(early, mid, late), and secretory phase (early, mid, late)according to the most recent menstrual period, and histol-ogy according to the criteria of Noyes et al. [32]. In normali-zation to GAPDH, CaBP-28k mRNA levels in theproliferative phase (early, mid, late) and secretory phase(early) were induced up to 1.5-, 2.5-, 2.5-, and 2.0-fold higherthan in the menstrual phase, respectively (Figure 1). Thisinduced expression of CaBP-28k was significantly decreasedin secretory phases (early, mid, and late) compared to thatin proliferative phases (mid and late phases). In parallel withmRNA levels, the protein levels of CaBP-28k were higher inthe proliferative phase (early-, mid-, late-) than in the otherphases (Figure 2). This induced protein level of CaBP-9kwas also reduced in secretory phases compared to that ofproliferative phases as shown in Figure 2.Effect of sex-steroid hormone E2 and P4 on regulation ofcalbindin-D28k expression in Ishikawa cellsWe assessed the effects of sex-steroid hormones E2 andP4 on the regulation of CaBP-28k in Ishikawa cells.A panel of chemicals comprising E2 and P4 wereapplied to Ishikawa cells at increasing concentrations(E2: 10-9 M to 10-7 M, and P4: 10-7 M to 10-5M). Asshown in Figure 3, a significant increase in the expres-sion of CaBP-28k mRNA was observed in the cells trea-ted with E2 (10-9 M to 10-7M). Compared to vehicle(negative) controls, CaBP-28k mRNA levels wereincreased up to 2.0- and 2.5-fold, respectively (Figure 3).Also, we elucidated the involvement of the estrogenreceptor (ER) in the E2-induced CaBP-28k increase inIshikawa cells by treating the cells with an ER antagonistor PR antagonist (ICI and mifepristone, respectively) for60 min prior to the treatment with E2 (10-8 M) or P4(10-6 M). Pretreatment with an ER antagonist comple-tely reversed the E2-induced increase in CaBP-28kexpression (Figure 4), suggesting that ERs are involvedin the E2-mediated regulation of CaBP-28k expressionin Ishikawa cells. Furthermore, after pretreatment with aPR antagonist, the P4-induced CaBP-28k expression wasnot changed, indicating that PRs are not involved in theexpression of CaBP-28k protein in Ishikawa cells.Figure 1 Pattern of endometrial CaBP-28k mRNA expression.Endometrial tissues were divided into 7 groups (M: menstrualphase, EP: early-proliferative phase, MP: mid-proliferative phase, LP:late-proliferative phase, ES: early-secretory phase, MS: mid-secretoryphase, LS: late-secretory phase) based on the endometrial tissuesclassified according to the last menstrual period and histologyaccording to the criteria of Noyes et al [32]. Uterine calbindin-D28kmRNA expression during the menstrual cycle was examined by (A:RT-PCR; B: real-time PCR). A significant increase in calbindin-D28kexpression level was observed during menstrual cycle (aP < 0.05 vs.M, bP < 0.05 vs. MP or LP).Figure 2 Pattern of endometrial CaBP-28k protein expression.Endometrial tissues were divided into 7 groups (M: menstrualphase, EP: early-proliferative phase, MP: mid-proliferative phase, LP:late-proliferative phase, ES: early-secretory phase, MS: mid-secretoryphase, LS: late-secretory phase) based on the endometrial tissuesclassified according to the last menstrual period and histologyaccording to the criteria of Noyes et al [32]. Uterine CaBP-28kprotein expression during the menstrual cycle was examined byWESTERN BLOT. A significant increase in calbindin-D28k expressionlevel was observed during menstrual cycle (aP < 0.05 vs. M, bP <0.05 vs. MP or LP).Yang et al. Reproductive Biology and Endocrinology 2011, 9:28http://www.rbej.com/content/9/1/28Page 4 of 8Localization of calbindin-D28k expression in humanendometrial tissueTo assess the spatial pattern of expression of CaBP-28kin the endometrial tissue, we analyzed tissue sections atdifferent stages of the menstrual phases (early-, mid-,late-proliferative phase or secretory phase) by immuno-histochemistry using an anti-CaBP-28k antibody.Endometrial CaBP-28k was highly expressed in cyto-plasm of endometrial epithelial cells in early-, mid-, late-proliferative phase compared to secretory phases. Addi-tionally, in the early-secretory phase, CaBP-28k wasweakly expressed in the endometrial epithelial cells ascompared to the glandular epithelial cells (Figure 5).DiscussionWe have previously determined that calcium-relatedproteins are regulated by sex-steroid hormones in theuterus of rodents [26,31,36,37]. It is well known that thestatus of the uterine cavity is important for successfulimplantation and is influenced by the secretory activityof the glandular epithelium [15,38,39]. In this study,CaBP-28k mRNA and protein were expressed in humanendometrial tissues. The levels of calcium-related pro-teins fluctuated in E2-predominant stages [20,26,37].CaBP-28k mRNA and protein were expressed at thesestages [proliferative (early, mid, late) and secretory (onlyearly) phase], followed by a decline in the secretoryphase (mid-, late-) of the human endometrium, asdemonstrated in this study. This result is in agreementwith the pattern of CaBP-28k expression in the uterusof both mice and humans [28]. Therefore, these resultsindicate that this parallel pattern of CaBP-28k expres-sion may be also involved in fetal implantation inhumans.Moreover, the level of CaBP-28k mRNA was increasedby E2 (E2, 10-9 M to 10-7 M) in human endometrialcancer cells (Ishikawa cells) as shown in this study. Sex-steroid hormones can induce changes in the structureand function of the uterus and regulate menstrual cycleprogression. To examine the effect of these hormoneson uterine CaBP-28k expression, we treated Ishikawacells with E2 (10-8 M) or P4 (10-6 M). Treatment withE2 resulted in increased uterine expression of CaBP-28kmRNA and protein, while P4 did not alter CaBP-28kexpression. The expression of several calcium-relatedproteins is altered in an E2-dependent [31,37] or P4-dependent [26,36] manner in the estrus cycle. It is parti-cularly interesting that calcium-related proteins areexpressed during the estrus cycle, due to the wide varia-tion in regulation of calcium-related protein expressionin the uterus across species. Calcium-related proteins,which are crucial for early implantation, are alsoexpressed in the endometrial layer [27,28,39]. However,it is still unclear why the uterine luminal epitheliumincreases Ca2+ uptake during implantation. In whataspect would this uptake facilitate blastocyst attachmentat implantation? It is known that, for proper blastocystimplantation in human and mice, there must be interac-tion between adhesion-competent trophoblast cells andendometrial extracellular matrix components. The accu-mulation of integrin receptors for fibronectin on theFigure 3 Effect of sex-steroid hormone E2 and P4 onregulation of CaBP-28k mRNA expression. The cells were treatedwith only DMSO (Ve, negative control), E2 (10-9 to -7 mol), P4(10-7 to-5 mol) for 2 days. Graphs represent the analysis of real-time PCRdata. aP < 0.05 indicate a significant difference in CaBP-28kexpression level compared to a negative control.Figure 4 Effect of sex-steroid hormones and ER and PRantagonists on regulation of CaBP-28k protein expression. Thecells were treated with only DMSO (Ve, negative control), E2 (10-8M),P4 (10-6M) and/or ICI (ICI 182,780: 10-6M), RU (RU486: 10-6M) for2 days and harvested 24 h following the final injection. Graphsrepresent the analysis of Western blot data. aP < 0.05 indicate asignificant difference in CaBP-28k expression level compared to anegative control.Yang et al. Reproductive Biology and Endocrinology 2011, 9:28http://www.rbej.com/content/9/1/28Page 5 of 8blastocyst surface enables it to bind to and penetrate theextracellular matrix (ECM) [40], but only when the blas-tocyst has reached an adhesion-competent state.Recently, it has been shown that the development of anadhesion-competent embryo is facilitated by heparin-binding epidermal growth factor (EGF)-like growth fac-tor (HB-EGF) and that this process is dependent on cal-cium influx from extracellular sources [41]. The uterineepithelium is a barrier to interstitial blastocyst implanta-tion, and it is also an important participant in thematernal-embryonic dialogue. During the periimplanta-tion period, uterine epithelial cells produce HB-EGF[42] and the uterine glands secrete calcitonin [43], twoparacrine or juxtacrine agents that are each capable ofaccelerating blastocyst differentiation. It is intriguingthat the biological activity of both factors is dependenton Ca2+ signaling [44], which appears to regulate preim-plantation embryogenesis beginning as early as thematuring oocyte [45,46]. These data suggest that a pos-sible source of this external calcium needed for HB-EGF-mediated differentiation of the blastocyst to anadhesion-competent state is from stores in the luminalepithelium of the endometrium.As previously shown, the initial up-regulation ofCaBP-28k increases the storage capacity for Ca2+ inluminal epithelial cells, and the subsequent specificdown-regulation leads to an increase in free Ca2+ con-centration [28]. In mammalian enterocytes, free Ca2+ions are bound to cytosolic CaBP-9k and transferredacross the cells by facilitated diffusion [47]. This trans-port of Ca2+ by CaBP-9k helps to maintain homeostasisby keeping intracellular Ca2+ ion concentrations below10-7 M, preventing premature cell death by apoptosis.Thus, it can be speculated that the role of CaBP-28kprotein in the uterine luminal epithelium is also toenhance Ca2+ uptake by increasing the cell-bufferingcapacity and stimulating the calcium entry mechanismin these cells. This release may trigger apoptosis inthese specific epithelial cells because high concentra-tions of free Ca2+ are reported to cause apoptosis inmany different cell types [48] and CaBP-28k is able toinhibit apoptosis in osteoblastic cells [49]. This apopto-sis could, in turn, destabilize the epithelial barrier at theimplantation site and facilitate trophoblast invasion andimplantation.An immunohistochemistry study revealed that theCaBP-28k protein was highly expressed in the cytoplasmof endometrial epithelial cells in the early-, mid-, late-proliferative compared with other phases, however, itwas weakly expressed in endometrial epithelial cells asFigure 5 Localization of CaBP-28k protein in the human endometrium. Immunohistochemistry was performed to detect CaBP-28k proteinin different phases in the uterine sections during the menstrual cycle (a, early-proliferative phase; b, mid-proliferative phase; c, late-proliferativephase; d, early-secretory phase; e, mid-secretory phase; f, late-secretory phase) as described in Materials and Methods. Arrowheads ®endometrial epithelial cells, arrows ® endometrial glandular cells. Arrows and arrowheads indicate CaBP-28k-positive regions.Yang et al. Reproductive Biology and Endocrinology 2011, 9:28http://www.rbej.com/content/9/1/28Page 6 of 8compared to that in glandular epithelial cells in theearly-secretory phase. In the previous study, it was parti-cularly interesting to observe that the regions where theuterine epithelium surrounded the implanting embryowere negative for CaBP-9k mRNA, whereas uniformexpression was detected in the entire luminal epitheliumin the regions where no embryo was attached [19].Although CaBP-9k expression is not detected in humanendometrium, either of CaBP-9k or CaBP-28k expres-sion appears to be sufficient to facilitate implantation inmice [27]. We did not examine expression pattern(s) ofCaBP-28k in the human uterus during pregnancy, partlybecause it is difficult to collect uterine samples frompregnant subjects due to ethical issues. Although weestablished that CaBP-28k expression occurred only dur-ing the menstrual cycle, we hypothesized that uterineCaBP-28k may be effectively mediated through mater-nal-fetal dialogue despite species differences betweenhuman and rodents. Thus, we assumed that CaBP-28kexpression may be involved in endometrial receptivityduring the menstrual cycle in humans.AcknowledgementsThis work was supported by the National Research Foundation ofKorea (NRF) grant funded by the Korea government (MEST)(No. 2010-0011433).Author details1Laboratory of Veterinary Biochemistry and Molecular Biology, College ofVeterinary Medicine, Chungbuk National University, Cheongju, Chungbuk361-763, Republic of Korea. 2Department of Obstetrics and Gynecology,College of Medicine, Soonchunhyang University, Bucheon 420-767, Republicof Korea. 3Department of Preventive Medicine, College of Medicine, Chung-Ang University, Seoul 156-756, Republic of Korea. 4Department of Obstetricsand Gynecology, Faculty of Medicine, University of British Columbia,Vancouver, British Columbia, Canada.Authors’ contributionsHY and TK carried out the overall experiments including molecularexperiments. TK, HL and YH provided human endometrial tissues andplaned clinical experiments. KC participated in the real-time PCR andimmunohistochemical analysis. PL participated in the design of the study,performed the statistical analysis and helped to finalize the manuscript. EJ*designed and coordinated the overall study as a corresponding author andhelped to draft the manuscript. All authors read and approved the finalmanuscript.Competing interestsThe authors declare that they have no competing interests.Received: 3 December 2010 Accepted: 2 March 2011Published: 2 March 2011References1. Opperman LA, Saunders TJ, Bruns DE, Boyd JC, Mills SE, Bruns ME: Estrogeninhibits calbindin-D28k expression in mouse uterus. Endocrinology 1992,130(3):1728-1735.2. L’Horset F, Blin C, Brehier A, Thomasset M, Perret C: Estrogen-inducedcalbindin-D 9k gene expression in the rat uterus during the estrouscycle: late antagonistic effect of progesterone. Endocrinology 1993,132(2):489-495.3. 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Reproductive Biology and Endocrinology 2011 9:28.Submit your next manuscript to BioMed Centraland take full advantage of: • Convenient online submission• Thorough peer review• No space constraints or color figure charges• Immediate publication on acceptance• Inclusion in PubMed, CAS, Scopus and Google Scholar• Research which is freely available for redistributionSubmit your manuscript at www.biomedcentral.com/submitYang et al. Reproductive Biology and Endocrinology 2011, 9:28http://www.rbej.com/content/9/1/28Page 8 of 8


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