Human stanniocalcin inhibits renal phosphate excretion in the rat

The down-regulation of STC2 mRNA may serve as a biomarker for death from mechanical asphyxia

Death from mechanical asphyxia (DMA) is a common cause of death in forensic pathology. However, due to the lack of biomarkers, the authentication of DMA now relies on a series of non-specific signs, which may cause troubles in the judicial trials, especially when the criminal scene is not fully elucidated. To search for the potential biomarkers for DMA, brain samples of DMA and craniocerebral injury groups were screened by microarray. The obtained mRNAs were validated by animal and human samples. Primary cell culture was conducted to explore the biochemical changes under hypoxia. 415 differentially expressed mRNAs between two groups were discovered. Ten mRNAs were examined in both human and animal samples died of different causes of death. Stanniocalcin-2 (STC2) showed significant down-regulation in DMA samples compared to other groups, regardless of PMI, age, or temperature. Cellular experiments indicated that ROS level peaked after 15-min-hypoxic culture, when the expression level of STC2 was significant down-regulated simultaneously. The ER-stress-related proteins also showed potential connection with STC2. In general, it is indicated that the down-regulation of STC2 may serve as a biomarker for DMA.

Identification and characterization of a membrane receptor that binds to human STC1

A study using TriCEPS-based ligand–receptor methodology and surface plasmon resonance assays identified that human stanniocalcin-1 binds to insulin-like growth factor-2 receptors in human leukemia monocytic cells with high affinity.

Stanniocalcin-1 (STC1) is a hypocalcemic hormone originally identified in bony fishes. The mammalian homolog is found to be involved in inflammation and carcinogenesis, among other physiological functions. In this study, we used the TriCEPS-based ligand–receptor methodology to identify the putative binding proteins of human STC1 (hSTC1) in the human leukemia monocytic cell line, ThP-1. LC–MS/MS analysis of peptides from shortlisted hSTC1-binding proteins detected 32 peptides that belong to IGF2/MPRI. Surface plasmon resonance assay demonstrated that hSTC1 binds to immobilized IGF2R/MPRI with high affinity (10–20 nM) and capacity (Rmax 70–100%). The receptor binding data are comparable with those of (CREG) cellular repressor of E1A-stimulated gene a known ligand of IGF2R/MPRI, with Rmax of 75–80% and affinity values of 1–2 nM. The surface plasmon resonance competitive assays showed CREG competed with hSTC1 in binding to IGF2R/MPRI. The biological effects of hSTC1 on ThP-1 cells were demonstrated via IGF2R/MPRI to significantly reduce secreted levels of IL-1β. This is the first study to reveal the high-affinity binding of hSTC1 to the membrane receptor IGF2R/MPRI.

Stanniocalcin 2 drives malignant transformation of human glioblastoma cells by targeting SNAI2 and Matrix Metalloproteinases

Glioblastoma multiforme (GBM) is the most malignant brain tumor and is refractory to conventional therapies. Although previous studies have proposed that the interaction between gene mutations and the external environment leads to the occurrence of GBM, the pathogenesis of GBM is still unclear and much remains to be studied. Herein, we show an association between human glycoprotein stanniocalcin-2 (STC2) and aggressive GBM progression, and demonstrate the underlying mechanism. Elevated STC2 expression and secretion greatly increase GBM cell growth and invasive phenotypes. Mechanistically, both, conditioned media (CM) containing STC2 and recombinant STC2, can induce the transformation of GBM cells into more malignant phenotypes by upregulating the expression of the epithelial-mesenchymal transition transcription factor, snail family transcription repressor 2 (SNAI2) as well as matrix metalloproteinases (MMPs). Moreover, we further demonstrate that the oncogenic function of STC2 in GBM is mediated through the MAPK signaling pathway. Collectively, these results identify the mechanism of STC2 targeting SNAI2 and MMPs through the MAPK pathway in GBM, and provide insights into a potential therapeutic strategy for GBM.

Reduced phosphorus intake throughout gestation and lactation of sows is mitigated by transcriptional adaptations in kidney and intestine

BACKGROUND

The environmental impact of pig farming need to be reduced, with phosphorus (P) being of particular interest. Specified dietary regimens and management systems contribute to meet environmental concerns and reduce economic constrains. However, pregnant and lactating sows represent vulnerable individuals, whose reproductive potential and metabolic health status relies on adequate supply of macro- and micronutrients. The aim of this study was to investigate, whether sows fed with a dietary P content that is below or above current recommendations are capable to maintain mineral homeostasis during the reproduction cycle and which endogenous mechanisms are retrieved therefore in kidney and jejunum. Nulliparous gilts were fed iso-energetic diets with recommended (M), reduced (L), or high (H) amounts of mineral P supplements throughout gestation and lactation periods. Blood metabolites and hormones referring to the P homeostasis were retrieved prior to term (110 days of gestation) and at weaning (28 days of lactation). Transcriptional responses in kidney cortex and jejunal mucosa were analyzed using RNA sequencing.

RESULTS

The variable dietary P content neither led to an aberration on fertility traits such as total weaned piglets nor to an effect on the weight pattern throughout gestation and lactation. Serum parameters revealed a maintained P homeostasis as reflected by unaltered inorganic P and calcium levels in L and H fed groups. The serum calcitriol levels were increased in lactating L sows. The endocrine responses to the dietary challenge were reflected at the transcriptional level. L diets led to an increase in CYP27B1 expression in the kidney compared to the H group and to an altered gene expression associated with lipid metabolism in the kidney and immune response in the jejunum.

CONCLUSIONS

Our results suggest that current P requirements for gestating and lactating sows are sufficient and over supplementation of mineral P is not required. Shifts in renal and jejunal expression patterns between L and H groups indicate an affected intermediate metabolism, which long-term relevance needs to be further clarified.

Novel discoveries in acid-base regulation and osmoregulation: A review of selected hormonal actions in zebrafish and medaka

Maintenance of internal ionic and acid-base homeostasis is critical for survival in all biological systems. Similar to mammals, aquatic fishes have developed sophisticated homeostatic mechanisms to mitigate metabolic or environmental disruptions in ionic and acid-base status of systemic body fluids via hormone-controlled transport of ions or acid equivalents. The present review summarizes newly discovered actions of several hormones in zebrafish (Danio rerio) and medaka (Oryzias latipes) that have greatly contributed to our overall understanding of ionic/acid-base regulation. For example, isotocin and cortisol were reported to enhance transport of various ions by stimulating the proliferation and/or differentiation of ionocyte progenitors. Meanwhile, stanniocalcin-1, a well-documented hypocalcemic hormone, was found to suppress ionocyte differentiation and thus downregulate secretion of H⁺ and uptake of Na⁺ and Cl^-. Estrogen-related receptor and calcitonin gene-related peptide also regulate the differentiation of certain types of ionocytes to either stimulate or suppress H⁺ secretion and Cl^- uptake. On the other hand, endothelin and insulin-like growth factor 1 activate the respective secretion of H⁺ and Na⁺/Cl through fast actions. These new findings enhance our understanding of how hormones regulate fish ionic and acid-base regulation while further providing new insights into vertebrate evolution, mammalian endocrinology and human disease-related therapeutics.

Molecular Physiology of the Hypocalcemic Action of Fibroblast Growth Factor 23 in Zebrafish (Danio rerio)

Fibroblast growth factor 23 (FGF23), a hormone required for phosphorus metabolism, was recently proposed to act on Ca2+ uptake; however, the available evidence of how FGF23 controls the body fluid Ca2+ homeostasis needs to be further clarified. The use of zebrafish as a model system revealed that FGF23 is specifically expressed in the corpuscles of Stannius (CS), an organ involved in Ca2+ homeostasis in fish, and that its expression is stimulated by ambient water with a high Ca2+ level. The overexpression of FGF23 inhibited Ca2+ uptake by downregulating the messenger RNA (mRNA) expression of epithelium calcium channel. Calcium-sensing receptor (CaSR), which senses changes in extracellular Ca2+ levels and modulates calciotropic hormones in organs controlling Ca2+ homeostasis in vertebrates, was found to be coexpressed with FGF23 in the CS. In addition, upregulated expression of FGF23 mRNA was detected in morphants of stanniocalcin 1 (stc1, another hypocalcemic factor synthesized in the CS), and knockdown of CaSR suppressed such upregulation and enhanced Ca2+ uptake. Taken together, our data indicate that FGF23 functions as a hypocalcemic hormone in zebrafish and that the CaSR/STC1-FGF23 axis is involved in body fluid Ca2+ homeostasis in vertebrates.

The Control of Calcium Metabolism in Zebrafish (Danio rerio)

Zebrafish is an emerging model for the research of body fluid ionic homeostasis. In this review, we focus on current progress on the regulation of Ca²⁺ uptake in the context of Ca²⁺ sensing and hormonal regulation in zebrafish. Na⁺-K⁺-ATPase-rich cells (NaRCs), the specialized ionocytes in the embryonic skin and adult gills, play a dominant role in Ca²⁺ uptake in zebrafish. Transepithelial Ca²⁺ transport in NaRC, through apical epithelial Ca²⁺ channels (ECaC), basolateral plasma membrane Ca²⁺-ATPase (PMCA), and Na⁺/Ca²⁺ exchanger (NCX), is analogous to mammalian renal and intestinal Ca²⁺-absorption cells. Several hormones were demonstrated to differentially regulate Ca²⁺ uptake through modulating the expression of Ca²⁺ transporters and/or the proliferation/differentiation of NaRC in zebrafish. In addition, the counterbalance among these hormones is associated with the maintenance of body fluid Ca²⁺ homeostasis. Calcium-sensing receptor (CaSR) is expressed in several hormone-secreting tissues in zebrafish, and activated CaSR differentially controls calciotropic hormones. The major principles of Ca²⁺ transport and the hormonal control appear to be conserved from zebrafish to other vertebrates including mammals. The new knowledge gained from zebrafish studies provides new insights into the related issues in vertebrates.

Regulation of Intestinal Epithelial Calcium Transport Proteins by Stanniocalcin-1 in Caco2 Cells

Stanniocalcin-1 (STC1) is a calcium and phosphate regulatory hormone. However, the exact molecular mechanisms underlying how STC1 affects Ca²⁺ uptake remain unclear. Here, the expression levels of the calcium transport proteins involved in transcellular transport in Caco2 cells were examined following over-expression or inhibition of STC1. These proteins include the transient receptor potential vanilloid members (TRPV) 5 and 6, the plasma membrane calcium ATPase 1b (PMCA1b), the sodium/calcium exchanger (NCX1), and the vitamin D receptor (VDR). Both gene and protein expressions of TRPV5 and TRPV6 were attenuated in response to over-expression of STC1, and the opposite trend was observed in cells treated with siRNA_STC1. To further investigate the ability of STC1 to influence TRPV6 expression, cells were treated with 100 ng/mL of recombinant human STC1 (rhSTC1) for 4 h following pre-transfection with siRNA_STC1 for 48 h. Intriguingly, the increase in the expression of TRPV6 resulting from siRNA_STC1 was reversed by rhSTC1. No significant effect of STC1 on the expression of PMCA1b, NCX1 or VDR was observed in this study. In conclusion, the effect of STC1 on calcium transport in intestinal epithelia is due to, at least in part, its negative regulation of the epithelial channels TRPV5/6 that mediate calcium influx.

Stanniocalcin-1 Potently Inhibits the Proteolytic Activity of the Metalloproteinase Pregnancy-associated Plasma Protein-A

Stanniocalcin-1 (STC1) is a disulfide-bound homodimeric glycoprotein, first identified as a hypocalcemic hormone important for maintaining calcium homeostasis in teleost fish. STC1 was later found to be widely expressed in mammals, although it is not believed to function in systemic calcium regulation in these species. Several physiological functions of STC1 have been reported, although many molecular details are still lacking. We here demonstrate that STC1 is an inhibitor of the metzincin metalloproteinase, pregnancy-associated plasma protein-A (PAPP-A), which modulates insulin-like growth factor (IGF) signaling through proteolytic cleavage of IGF-binding proteins (IGFBPs). STC1 potently (Ki = 68 pm) inhibits PAPP-A cleavage of IGFBP-4, and we show in a cell-based assay that STC1 effectively antagonizes PAPP-A-mediated type 1 IGF receptor (IGF1R) phosphorylation. It has recently been found that the homologous STC2 inhibits PAPP-A proteolytic activity, and that this depends on the formation of a covalent complex between the inhibitor and the proteinase, mediated by Cys-120 of STC2. We find that STC1 is unable to bind covalently to PAPP-A, in agreement with the absence of a corresponding cysteine residue. It rather binds to PAPP-A with high affinity (KD = 75 pm). We further demonstrate that both STC1 and STC2 show inhibitory activity toward PAPP-A2, but not selected serine proteinases and metalloproteinases. We therefore conclude that the STCs are proteinase inhibitors, probably restricted in specificity to the pappalysin family of metzincin metalloproteinases. Our data are the first to identify STC1 as a proteinase inhibitor, suggesting a previously unrecognized function of STC1 in the IGF system.

Osmoregulation in zebrafish: ion transport mechanisms and functional regulation

Fish, like mammals, have to maintain their body fluid ionic and osmotic homeostasis through sophisticated iono-/osmoregulation mechanisms, which are conducted mainly by ionocytes of the gill (the skin in embryonic stages), instead of the renal tubular cells in mammals. Given the advantages in terms of genetic database availability and manipulation, zebrafish is an emerging model for research into regulatory and integrative physiology. At least five types of ionocytes, HR, NaR, NCC, SLC26, and KS cells, have been identified to carry out Na(+) uptake/H(+) secretion/NH4 (+) excretion, Ca(2+) uptake, Na(+)/Cl(-) uptake, K(+) secretion, and Cl(-) uptake/HCO3 (-) secretion, respectively, through distinct sets of transporters. Several hormones, namely isotocin, prolactin, cortisol, stanniocalcin-1, calcitonin, endothelin-1, vitamin D, parathyorid hormone 1, catecholamines, and the renin-angiotensin-system, have been demonstrated to positively or negatively regulate ion transport through specific receptors at different ionocytes stages, at either the transcriptional/translational or posttranslational level. The knowledge obtained using zebrafish answered many long-term contentious or unknown issues in the field of fish iono-/osmoregulation. The homology of ion transport pathways and hormone systems also means that the zebrafish model informs studies on mammals or other animal species, thereby providing insights into related fields.

Stanniocalicin 2 suppresses breast cancer cell migration and invasion via the PKC/claudin-1-mediated signaling

Stanniocalcin (STC), a glycoprotein hormone, is expressed in a wide variety of tissues to regulate Ca²⁺ and PO4^- homeostasis. STC2, a member of STC family, has been reported to be associated with tumor development. In this study, we investigated whether the expression of STC2 is associated with migration and invasion of breast cancer cells. We found that breast cancer cell line 231 HM transfected with STC2 shRNA displayed high motility, fibroblast morphology, and enhanced cell migration and invasion. Introduction of STC2 in 231 cells reduced cell migration and invasion. In response to irradiation, silencing of STC2 in 231 HM cells reduced apoptosis, whereas overexpression of STC2 in 231 cells promoted apoptosis, compared with in control cells. Mechanistic study showed that STC2 negatively regulated PKC to control the expression of Claudin-1, which subsequently induced the expressions of EMT-related factors including ZEB1, ZO-1, Slug, Twist, and MMP9. Suppression of PKC activity by using a PKC inhibitor (Go 6983) restored the normal motility of STC2-silenced cells. Furthermore, in vivo animal assay showed that STC2 inhibited tumorigenesis and metastasis of breast cancer cells. Collectively, these results indicate that STC2 may inhibit EMT at least partially through the PKC/Claudin-1-mediated signaling in human breast cancer cells. Thus, STC2 may be exploited as a biomarker for metastasis and targeted therapy in human breast cancer.

Stanniocalcin-1 controls ion regulation functions of ion-transporting epithelium other than calcium balance

Stanniocalcin-1 (STC-1) was first identified to involve in Ca(2+) homeostasis in teleosts, and was thought to act as a hypocalcemic hormone in vertebrate. Recent studies suggested that STC-1 exhibits broad effects on ion balance, not confines to Ca(2+), but the mechanism of this regulation process remains largely unknown. Here, we used zebrafish embryos as an alternative in vivo model to investigate how STC-1 regulates transepithelial ion transport function in ion-transporting epithelium. Expression of stc-1 mRNA in zebrafish embryos was increased in high-Ca(2+) environments but decreased by acidic and ion-deficient treatments while overexpression of stc-1 impaired the hypotonic acclimation by decreasing whole body Ca(2+), Na(+), and Cl(-) contents and H(+) secretion ability. Injection of STC-1 mRNA also down-regulated mRNA expressions of epithelial Ca(2+) channel, H(+)-ATPase, and Na(+)-Cl(-) cotransporter, suggesting the roles of STC-1 in regulation of ions other than Ca(2+). Knockdown of STC-1 caused an increase in ionocyte progenitors (foxi3a as the marker) and mature ionocytes (ion transporters as the markers), but did not affect epithelium stem cells (p63 as the marker) in the embryonic skin. Overexpression of STC-1 had the corresponding opposite effect on ionocyte progenitors, mature ionocytes in the embryonic skin. Taken together, STC-1 negatively regulates the number of ionocytes to reduce ionocyte functions. This process is important for body fluid ionic homeostasis, which is achieved by the regulation of ion transport functions in ionocytes. The present findings provide new insights into the broader functions of STC-1, a hypocalcemic hormone.

Cyclooxygenase-2 mediates induction of the renal stanniocalcin-1 gene by arginine vasopressin

In rats and mice, the renal stanniocalcin-1 (STC-1) gene is expressed in most nephron segments, but is differentially induced in response to dehydration. In cortical segments STC-1 mRNA levels are upregulated by the hypertonicity of dehydration, while hypovolemia causes gene induction in the inner medulla (papilla). In both cases induction is mediated by arginine vasopressin (AVP) acting via the V2 receptor (V2R). The intent of STC-1 gene upregulation during dehydration has yet to be established. Therefore, to narrow down the range of possible actions, we mapped out the pathway by which V2R occupancy upregulates the gene. V2R occupancy activates two different renal pathways in response to dehydration. The first is antidiuretic in nature and is mediated by direct V2R occupancy. The second pathway is indirect and counter-regulates AVP-mediated antidiuresis. It involves COX-2 (cyclooxygenase-2) and the prostanoids, and is activated by the V2R-mediated rise in medullary interstitial osmolality. The resulting prostanoids counter-regulate AVP-mediated antidiuresis. They also upregulate renal cytoprotective mechanisms. The present studies employed models of COX inhibition and COX gene deletion to address the possible involvement of the COX pathway. The results showed that both general and specific inhibitors of COX-2 blocked STC-1 gene induction in response to dehydration. Gene induction in response to dehydration was also abolished in COX-2 null mice (cortex and papilla), but not in COX-1 null mice. STC-1 gene induction in response to V2R occupancy was also uniquely abolished in COX-2 nulls (both regions). These findings therefore collectively suggest that AVP-mediated elevations in STC-1 gene expression are wholly dependent on functional COX-2 activity. As such, a permissive role for STC-1 in AVP-mediated antidiuresis can be ruled out, and its range of possible actions has been narrowed down to AVP counter-regulation and renal cytoprotection.

Zebrafish as an animal model to study ion homeostasis

Zebrafish (Danio rerio) possesses several advantages as an experimental organism, including the applicability of molecular tools, ease of in vivo cellular observation and functional analysis, and rapid embryonic development, making it an emerging model for the study of integrative and regulatory physiology and, in particular, the epithelial transport associated with body fluid ionic homeostasis. Zebrafish inhabits a hypotonic freshwater environment, and as such, the gills (or the skin, during embryonic stages) assume the role of the kidney in body fluid ionic homeostasis. Four types of ionocyte expressing distinct sets of transporters have been identified in these organs: H⁺-ATPase-rich, Na⁺-K⁺-ATPase-rich, Na⁺-Cl⁻ cotransporter-expressing and K⁺-secreting cells; these ionocytes perform transepithelial H⁺ secretion/Na⁺ uptake/NH₄⁺ excretion, Ca²⁺ uptake, Na⁺/Cl⁻ uptake, and K⁺ secretion, respectively. Zebrafish ionocytes are analogous to various renal tubular cells, in terms of ion transporter expression and function. During embryonic development, ionocyte progenitors develop from epidermal stem cells and then differentiate into different types of ionocyte through a positive regulatory loop of Foxi3a/-3b and other transcription factors. Several hormones, including cortisol, vitamin D, stanniocalcin-1, calcitonin, and isotocin, were found to participate in the control pathways of ionic homeostasis by precisely studying the target ion transport pathways, ion transporters, or ionocytes of the hormonal actions. In conclusion, the zebrafish model not only enhances our understanding of body fluid ion homeostasis and hormonal control in fish but also informs studies on mammals and other animal species, thereby providing new insights into related fields.

Stanniocalcin supports the functional adaptation of adult-sized kidneys transplanted into the pediatric recipients

BACKGROUND

The functional adaptation of adult-sized kidney (ASK) grafts to the recipient's size is not completely reversible and is associated with irreversible histologic damage in the smallest recipients. The aim of the study was to unveil the molecular mechanisms underlying the functional adaptation of ASK transplants to small pediatric recipients.

METHODS

Because physiologic and functional adaption of ASK in infants is seen maximally at 3 months after transplantation, we selected 21 pediatric recipients of an ASK with protocol biopsies at engraftment and 3 and 6 months, without delayed graft function or interval rejection, and we conducted whole-genome expression profiles of the renal allograft biopsies at 3 months and correlated results with subsequent absolute glomerular filtration rate (aGFR).

RESULTS

Seven hundred twenty-four unique genes correlated significantly with aGFR (q value <5%). Canonical pathway analysis identified overrepresentation of relevant pathways involved in regulation of tubular salt reabsorption and enzymatic pathways for organ development and hypertrophy. The single gene that correlated best with aGFR was stanniocalcin 1 (STC1). STC1 expression also correlated with the recipient's size at the time of transplantation and the chronic allograft damage index at 6 months.

CONCLUSIONS

Functional adaptation of adult-sized grafts to the pediatric recipient is associated with molecular adaptation for normal-volume homeostasis of the recipient. Our finding also suggests that stanniocalcin (STC1) plays an important role on functional adaption in pediatric kidney transplantation.

Synergistic effect of p53 on TSA-induced stanniocalcin 1 expression in human nasopharyngeal carcinoma cells, CNE2

Human stanniocalcin 1 (STC1) has recently been identified as a putative protein factor involved in cellular apoptosis. The use of histone deacetylase inhibitor (i.e. trichostatin A (TSA)) and doxorubicin (Dox) is one of the common treatment methods to induce apoptosis in human cancer cells. A study on TSA and Dox-mediated apoptosis may shed light on the regulation and function of STC1 in cancer treatment. In this study, TSA and Dox cotreatment in human nasopharyngeal carcinoma cells (CNE2) elicited synergistic effects on STC1 gene expression and cellular apoptosis. An activation of p53 (TP53) transcriptional activity in Dox- or Dox+TSA-treated cells was revealed by the increased expression levels of p53 mRNA/protein as well as p53-driven luciferase activities. To elucidate the possible involvement of p53 in STC1 gene transcription, a vector expressing wild-type or dominant negative (DN) p53 was transiently transfected into the cells. Both STC1 promoter luciferase constructs and chromatin immunoprecipitation assays did not support the direct role of p53 in STC1 gene transactivation. However, the synergistic effects of p53 on the induction of NF-κB phosphorylation and the recruitment of acetylated histone H3 in STC1 promoter were observed in TSA-cotreated cells. The overexpression of exogenous STC1 sensitized apoptosis in Dox-treated cells. Taken together, this study provides data to show the cross talk of NF-κB, p53, and histone protein in the regulation of STC1 expression and function.

Stanniocalcin 2 is associated with ectopic calcification in α-klotho mutant mice and inhibits hyperphosphatemia-induced calcification in aortic vascular smooth muscle cells

Ectopic calcification of soft tissues can have severe clinical consequences especially when localized to vital organs such as heart, arteries and kidneys. Mammalian stanniocalcin (STC) 1 and 2 are glycoprotein hormones identified as calcium/phosphate-regulating hormones. The mRNA of STCs is upregulated in the kidney of α-klotho mutant (kl/kl) mice, which have hypercalcemia, hyperphosphatemia and hypervitaminosis D and exhibit a short life span, osteopenia and ectopic calcification. In the present study, we investigated the distribution and localization of STCs in kl/kl mice. Quantitative RT-PCR revealed that renal mRNA expression of STC2 was increased in both kl/kl mice and fibroblast growth factor 23 (Fgf23)-null mice compared with wild type mice. Interestingly, STC2 protein was focally localized with the calcified lesions of renal arterioles, renal tubular cells, heart and aorta in kl/kl mice. In vitro analysis of rat aortic vascular smooth muscle (A-10) cells showed that inorganic phosphate (Pi) stimulation significantly increased STC2 mRNA levels as well as that of osteocalcin, osteopontin and the type III sodium-dependent phosphate co-transporter (PiT-1), and induced STC2 secretion. Interestingly, the knockdown with a small interfering RNA or the over-expression of STC2 showed acceleration and inhibition of Pi-induced calcification in A-10 cells, respectively. These results suggest that the up-regulation of STC2 gene expression resulting from abnormal α-klotho-Fgf23 signaling may contribute to limitation of ectopic calcification and thus STC2 represents a novel target gene for cardio-renal syndrome.

Evolution and roles of stanniocalcin

In fish, stanniocalcin-1 (STC1) is a key endocrine factor that acts on gill, intestine and kidney to regulate serum calcium and phosphate homeostasis. The recent identification and study of mammalian STCs (STC1 and STC2) revealed that the hormones are made in virtually all tissues and they act primarily as paracrine/autocrine factors to regulate various biological functions. Based on their ubiquitous expression patterns and generally undetectable levels in blood serum, it is unlikely that the mammalian STCs play important roles in serum Ca(2+)/P(i) homeostasis. However current evidences still support the local action of STCs in Ca(2+) and P(i) transport, probably via their action on Ca(2+)-channels and Na(+)/P(i) co-transporter. At present, information about the sequence, expression and distribution of the STC receptor(s) is lacking. However, recent emerging evidence hints the involvement of STC1 and STC2 in the sub-cellular functions of mitochondria and endoplasmic reticulum respectively, particularly responding to oxidative stress and unfolded protein response. With increasing evidence that demonstrates the local actions of STCs, the focus of the research has been moved to cellular inflammation and carcinogenesis. This review integrates the information available on STCs in fish and mammals, focusing mainly on their embryonic origin, tissue distribution, their potential regulatory mechanisms and the modes of action, and their physiological and pathophysiological functions, particularly in cancer biology.

Up-regulation of stanniocalcin 1 expression by 1,25-dihydroxy vitamin D(3) and parathyroid hormone in renal proximal tubular cells

Stanniocalcin 1 and stanniocalcin 2 are two glycoprotein hormones, which act as calcium phosphate-regulating factor on intestine and kidney. We have previously reported that stanniocalcin 2 expression is positively and negatively controlled by 1,25(OH)₂D₃ and parathyroid hormone in renal proximal tubular cells. However, it has been unclear whether they regulate the stanniocalcin 1 gene expression. In this study, we identified the opossum stanniocalcin 1 cDNA sequence. The opossum stanniocalcin 1 amino acid sequence had 83% homology with human stanniocalcin 1, and has a conserved putative N-linked glycosylation site. Real-time PCR analysis using opossum kidney proximal tubular (OK-P) cells revealed that the mRNA levels of stanniocalcin 1 gene is up-regulated by both 1,25(OH)₂D₃ and parathyroid hormone in dose-dependent and time-dependent manners. We also demonstrated that the stanniocalcin 1 expression was increased in parathyroid hormone injected rat kidney. Furthermore, the mRNA expression of stanniocalcin 1 and stanniocalcin 2 were oppositely regulated by phorbol 12,13-myristic acetate, a specific PKC activator. Interestingly, the up-regulation of stanniocalcin 1 gene by 1,25(OH)₂D₃ and phorbol 12,13-myristic acetate were not prevented in the presence of actinomycin D, an RNA synthesis inhibitor. These results suggest that the stanniocalcin 1 gene expression is up-regulated by 1,25(OH)₂D₃ and parathyroid hormone through mRNA stabilization in renal proximal tubular cells.

Production of calcium maintenance factor Stanniocalcin-1 (STC1) by the equine endometrium during the early pregnant period

A factor responsible for progression to pregnancy establishment in the mare has not been definitively characterized. To identify factors possibly involved in the establishment of equine pregnancy, the endometrium was collected from day 13 (day 0=day of ovulation) cyclic and day 13, 19 and 25 pregnant animals. From initial subtractive hybridization studies, a calcium regulating factor, Stanniocalcin-1 (STC1) mRNA, was found as a candidate molecule expressed uniquely in the pregnant endometrium. Endometrial expression of STC1 mRNA was noted on day 19 and was markedly increased in the day 25 gravid endometrium. STC1 protein was found in the extracts of day 25 gravid endometrium and immunochemically localized in the uterine glands. In addition, STC1 protein was detected in uterine flushing media collected from day 25 pregnant mares. High concentrations of estradiol-17 β (E(2)) were detected in day 25 conceptuses. E(2) levels were much higher in the gravid endometrium than in other regions, whereas progesterone levels did not differ among the samples from different endometrial regions. Expression of STC1 mRNA, however, was not significantly upregulated in cultured endometrial explants treated with various concentrations of E(2) (0.01-100 ng/ml) with or without 10 ng/ml progesterone. These results indicate that an increase in STC1 expression appears to coincide with capsule disappearance in the conceptus, and suggest that STC1 from the uterine glands likely plays a role in conceptus development during the pregnancy establishment period in the mare.

Human stanniocalcin-1 interacts with nuclear and cytoplasmic proteins and acts as a SUMO E3 ligase

Human stanniocalcin-1 (STC1) is a glycoprotein that has been implicated in different physiological process, including angiogenesis, apoptosis and carcinogenesis. Here we identified STC1 as a putative molecular marker for the leukemic bone marrow microenvironment and identified new interacting protein partners for STC1. Seven selected interactions retrieved from yeast two-hybrid screens were confirmed by GST-pull down assays in vitro. The N-terminal region was mapped to be the region that mediates the interaction with cytoplasmic, mitochondrial and nuclear proteins. STC1 interacts with SUMO-1 and several proteins that have been shown to be SUMOylated and localized to SUMOylation related nuclear bodies. Although STC1 interacts with SUMO-1 and has a high theoretical prediction score for a SUMOylation site, endogenous co-immunoprecipitation and in vitro SUMOylation assays with the purified recombinant protein could not detect STC1 SUMOylation. However, when we tested STC1 for SUMO E3 ligase activity, we found in an in vitro assay, that it significantly increases the SUMOylation of two other proteins. Confocal microscopic subcellular localization studies using both transfected cells and specific antibodies for endogenous STC1 revealed a cytoplasmic and nuclear deposition, the latter in the form of some specific dot-like substructure resembling SUMOylation related nuclear bodies. Together, these findings suggest a new role for STC1 in SUMOylation pathways, in nuclear bodies.

Stanniocalcin-1 regulates extracellular ATP-induced calcium waves in human epithelial cancer cells by stimulating ATP release from bystander cells

Background

The epithelial cell response to stress involves the transmission of signals between contiguous cells that can be visualized as a calcium wave. In some cell types, this wave is dependent on the release of extracellular trinucleotides from injured cells. In particular, extracellular ATP has been reported to be critical for the epithelial cell response to stress and has recently been shown to be upregulated in tumors in vivo.

Methodology/Principal Findings

Here, we identify stanniocalcin-1 (STC1), a secreted pleiotrophic protein, as a critical mediator of calcium wave propagation in monolayers of pulmonary (A549) and prostate (PC3) epithelial cells. Addition of STC1 enhanced and blocking STC1 decreased the distance traveled by an extracellular ATP-dependent calcium wave. The same effects were observed when calcium was stimulated by the addition of exogenous ATP. We uncover a positive feedback loop in which STC1 promotes the release of ATP from cells in vitro and in vivo.

Conclusions/Significance

The results indicated that STC1 plays an important role in the early response to mechanical injury by epithelial cells by modulating signaling of extracellular ATP. This is the first report to describe STC1 as a modulator or purinergic receptor signaling.

Human stanniocalcin-1 or -2 expressed in mice reduces bone size and severely inhibits cranial intramembranous bone growth

Stanniocalcin-1 (STC1) and -2 (STC2) are highly related, secreted, homodimeric glycoproteins that are significantly upregulated by different forms of stress including high phosphate levels. Transgenic mice that constitutively express either human STC1 or STC2 exhibit intra-uterine growth restriction and permanent post-natal growth retardation. STC1 is expressed in chondrocytic and osteoblastic cells during murine development and can enhance differentiation of calvarial cells in culture. Therefore, there is mounting evidence that stanniocalcins (STCs) modulate bone development in vivo. To further define the effects of stanniocalcins on skeletal development, we performed a series of measurements on components of the axial, appendicular, and cranial skeleton in transgenic and wildtype mice. We show that skeletal growth is retarded and that the intramembranous bones of the cranium exhibit a particularly severe delay in suture closure. The posterior frontal suture remains patent throughout the lifetime of human STC1 and STC2 transgenic mice. We did not observe significant effects on chondrogenesis: however, calvarial cells exhibited reduced viability, proliferation and delayed differentiation, indicating that developing osteoblasts are particularly sensitive to the levels of STCs. Given the evidence linking STC1 to cellular phosphate homeostasis, we assessed the expression of a variety of phosphate regulators in transgenic and wildtype calvarial cells and found significantly lower levels of Mepe, Dmp1, Sfrp4 in transgenic cells without a change in Pit1 or Pit2. Collectively these data support a direct regulatory role for STCs in osteoblasts and suggest that overexposure to these factors inhibits normal skeletal development without significant changes in patterning.

Expression of stanniocalcin-1 in gastrointestinal tracts of neonatal and mature rats

Stanniocalcin-1 (STC-1) produced by ovaries endocrinologically targets to mammary glands and is secreted into milk during lactation. The decline of mother's serum level by STC-1 antiserum administration reduced the milk fat content and the pups' body fat content. Nevertheless, the pups' fecal fat content was increased, suggesting that milk-derived STC-1 could influence intestinal fat absorption. We investigated the STC-1 expression in rat gastrointestinal tissues using immunocytochemistry and in situ hybridization. STC-1 was widely expressed in the chief cells of gastric pits and the cells of intestinal glands. Goblet cells in the small intestine contained STC-1 protein in their mucus. The distribution shows that this peptide is secreted exocrinologically into the gastrointestinal lumen. Quantitative RT-PCR analysis revealed that the expression ratio was higher in the periods of heavy nutritional demand, such as growing and lactation. The endogenous STC-1, similar to milk-derived STC-1, may be involved in digestion and/or absorption in gastrointestinal organs.

Low-resolution structural studies of human Stanniocalcin-1

BACKGROUND

Stanniocalcins (STCs) represent small glycoprotein hormones, found in all vertebrates, which have been functionally implicated in Calcium homeostasis. However, recent data from mammalian systems indicated that they may be also involved in embryogenesis, tumorigenesis and in the context of the latter especially in angiogenesis. Human STC1 is a 247 amino acids protein with a predicted molecular mass of 27 kDa, but preliminary data suggested its di- or multimerization. The latter in conjunction with alternative splicing and/or post-translational modification gives rise to forms described as STC50 and "big STC", which molecular weights range from 56 to 135 kDa.

RESULTS

In this study we performed a biochemical and structural analysis of STC1 with the aim of obtaining low resolution structural information about the human STC1, since structural information in this protein family is scarce. We expressed STC1 in both E. coli and insect cells using the baculo virus system with a C-terminal 6 x His fusion tag. From the latter we obtained reasonable amounts of soluble protein. Circular dichroism analysis showed STC1 as a well structured protein with 52% of alpha-helical content. Mass spectroscopy analysis of the recombinant protein allowed to assign the five intramolecular disulfide bridges as well as the dimerization Cys202, thereby confirming the conservation of the disulfide pattern previously described for fish STC1. SAXS data also clearly demonstrated that STC1 adopts a dimeric, slightly elongated structure in solution.

CONCLUSION

Our data reveal the first low resolution, structural information for human STC1. Theoretical predictions and circular dichroism spectroscopy both suggested that STC1 has a high content of alpha-helices and SAXS experiments revealed that STC1 is a dimer of slightly elongated shape in solution. The dimerization was confirmed by mass spectrometry as was the highly conserved disulfide pattern, which is identical to that found in fish STC1.

Stanniocalcin-1 suppresses superoxide generation in macrophages through induction of mitochondrial UCP2

Mammalian STC1 decreases the mobility of macrophages and diminishes their response to chemokines. In the current experiments, we sought to determine the impact of STC1 on energy metabolism and superoxide generation in mouse macrophages. STC1 decreases ATP level in macrophages but does not affect the activity of respiratory chain complexes I-IV. STC1 induces the expression of mitochondrial UCP2, diminishing mitochondrial membrane potential and superoxide generation; studies in UCP2 null and gp91phox null macrophages suggest that suppression of superoxide by STC1 is UCP2-dependent yet is gp91phox-independent. Furthermore, STC1 blunts the effects of LPS on superoxide generation in macrophages. Exogenous STC1 is internalized by macrophages within 10 min and localizes to the mitochondria, suggesting a role for circulating and/or tissue-derived STC1 in regulating macrophage function. STC1 induces arrest of the cell cycle at the G1 phase and reduces cell necrosis and apoptosis in serum-starved macrophages. Our data identify STC1 as a key regulator of superoxide generation in macrophages and suggest that STC1 may profoundly affect the immune/inflammatory response.

Stanniocalcin-1 acts in a negative feedback loop in the prosurvival ERK1/2 signaling pathway during oxidative stress

Mammalian Stanniocalcin-1 (STC1) is a glycoprotein that has been implicated in various biological processes including angiogenesis. Aberrant STC1 expression has been reported in breast, ovarian and prostate cancers, but the significance of this is not well understood. Here, we report that oxidative stress caused a 40-fold increase in STC1 levels in mouse embryo fibroblasts (MEFs). STC1-/- MEFs were resistant to growth inhibition and cell death induced by H(2)O(2) or by 20% O(2) (which is hyperoxic for most mammalian cells); this is the first phenotype reported for STC1-null cells. STC1-/- cells had higher levels of activated MEK and ERK1/2 than their wild-type (WT) counterparts, and these levels were all reduced by stable expression of exogenous STC1 in STC1-/- cells. Furthermore, pharmacological inhibition by PD98059 or UO126 of MEK and therefore of ERK1/2 activation restored sensitivity of STC1-/- cells to oxidative stress. We also found that H(2)O(2)-induced STC1 expression in WT cells was abolished by inhibition of ERK1/2 activation. Thus, the ERK1/2 signaling pathway upregulates STC1 expression, which in turn downregulates the level of activated MEK and consequently ERK1/2 in a novel negative feedback loop. Therefore, STC1 expression downregulates prosurvival ERK1/2 signaling and reduces survival under conditions of oxidative stress.

Anti-inflammatory and renal protective actions of stanniocalcin-1 in a model of anti-glomerular basement membrane glomerulonephritis

We have previously shown that stanniocalcin-1 (STC1) inhibits the transendothelial migration of macrophages and T cells, suppresses superoxide generation in macrophages, and attenuates macrophage responses to chemoattractants. To study the effects of STC1 on inflammation, in this study we induced a macrophage- and T-cell-mediated model of anti-glomerular basement membrane disease in STC1 transgenic mice, which display elevated serum STC1 levels and preferentially express STC1 in both endothelial cells and macrophages. We examined the following parameters both at baseline and after anti-glomerular basement membrane antibody treatment: blood pressure; C(3a) levels; urine output; proteinuria; blood urea nitrogen; and kidney C(3) deposition, fibrosis, histological changes, cytokine expression, and number of T cells and macrophages. Compared with wild-type mice, after anti-glomerular basement membrane treatment STC1 transgenic mice exhibited: i) diminished infiltration of inflammatory macrophages in the glomeruli; ii) marked reduction in crescent formation and sclerotic glomeruli; iii) decreased interstitial fibrosis; iv) preservation of kidney function and lower blood pressure; v) diminished C(3) deposition in the glomeruli; and vi) reduced expression of macrophage inhibitory protein-2 and transforming growth factor-beta2 in the kidney. Compared with baseline, wild-type mice, but not STC1 transgenic mice, had higher proteinuria and a marked reduction in urine output. STC1 had minimal effects, however, on both T-cell number in the glomeruli and interstitium and on cytokine expression characteristic of either TH1 or TH2 activation. These data suggest that STC1 is a potent anti-inflammatory and renal protective protein.

Stanniocalcin 1 is a luminal epithelial marker for implantation in pigs regulated by progesterone and estradiol

Stanniocalcin 1 (STC1) is a glycoprotein that decreases calcium and increases phosphate in cells/tissues. This investigation examined endocrine regulation of STC1 in endometria of pigs during the estrous cycle and pregnancy. STC1 mRNA was present exclusively in luminal epithelium (LE) between d 12 and 15 of the estrous cycle, increased between d 12 and d 20, and was not detectable by d 30 of pregnancy. STC1 protein was also detected in uterine flushings. To determine effects of estrogen and progesterone, pigs were ovariectomized and treated with these hormones alone or together. Progesterone, but not estrogen, induced STC1 in LE. Cotreatment with progesterone and estrogen further stimulated STC1 over progesterone alone. To determine effects of pseudopregnancy, nonpregnant gilts were given daily injections of estradiol benzoate from d 11 to d 14. STC1 was not expressed in LE on d 90 of pseudopregnancy, suggesting that the estradiol given to induce pseudopregnancy and/or long-term exposure to progesterone are required for down-regulation of STC1. To determine effects of long-term progesterone, without effects of estradiol, pigs were ovariectomized on d 12, given daily injections of progesterone through d 39, and hysterectomized on d 40 after estrus. STC1 was expressed in LE of progesterone-treated pigs, suggesting that estrogen is involved in down-regulation of STC1. We conclude that STC1 is induced in LE by progesterone and further stimulated by estrogen, and its down-regulation in LE by d 25 likely requires exposure of the progestinized uterus to estrogen. The temporal and cell type-specific expression of STC1 makes this gene a unique marker for implantation in pigs.

Effects of stanniocalcin 1 on calcium uptake in zebrafish (Danio rerio) embryo

Stanniocalcin (STC) formerly called hypocalcin or teleocalcin, is a 50-kDa disulfide-linked homodimeric glycoprotein that was originally identified in fish and secreted from the corpuscles of Stannius (CS). One of the main functions of STC-1 is Ca(2+) uptake inhibition; however, the mechanisms remain unknown. In the present study, we provide molecular evidence to elucidate how zebrafish STC-1 regulates Ca(2+) uptake in zebrafish embryos. In a wide variety of tissues including the kidney, brain, gill, muscle, and skin, zstc-1 was expressed. Incubating zebrafish embryos in low-Ca(2+) (0.02 mM) freshwater stimulated whole body Ca(2+) influx and zebrafish epithelial Ca(2+) channel (zECaC) mRNA expression, while downregulated zstc-1 expression. A morpholino microinjection approach was used to knockdown the zSTC-1 protein, and the results showed that the Ca(2+) content, Ca(2+) influx, and zECaC mRNA expression all increased in morphants. These data suggest that zSTC-1 negatively regulates ECaC gene expression to reduce Ca(2+) uptake in zebrafish embryos.

The murine stanniocalcin 2 gene is a negative regulator of postnatal growth

Stanniocalcin (STC), a secreted glycoprotein, was first studied in fish as a classical hormone with a role in regulating serum calcium levels. There are two closely related proteins in mammals, STC1 and STC2, with functions that are currently unclear. Both proteins are expressed in numerous mammalian tissues rather than being secreted from a specific endocrine gland. No phenotype has been detected yet in Stc1-null mice, and to investigate whether Stc2 could have compensated for the loss of Stc1, we have now generated Stc2(-/-) and Stc1(-/-) Stc2(-/-) mice. Although Stc1 is expressed in the ovary and lactating mouse mammary glands, like the Stc1(-/-) mice, the Stc1(-/-) Stc2(-/-) mice had no detected decrease in fertility, fecundity, or weight gain up until weaning. Serum calcium and phosphate levels were normal in Stc1(-/-) Stc2(-/-) mice, indicating it is unlikely that the mammalian stanniocalcins have a major physiological role in mineral homeostasis. Mice with Stc2 deleted were 10-15% larger and grew at a faster rate than wild-type mice from 4 wk onward, and the Stc1(-/-) Stc2(-/-) mice had a similar growth phenotype. This effect was not mediated through the GH/IGF-I axis. The results are consistent with STC2 being a negative regulator of postnatal growth.

Stanniocalcin-1 secretion and receptor regulation in kidney cells

Kidney collecting duct principal cells are the main source of stanniocalcin-1 (STC-1) production and secretion. From there, the hormone targets thick ascending limb and distal convoluted tubule cells, as well as collecting duct cells. More specifically, STC-1 targets their mitochondria to exert putative antiapoptotic effects. Two distal tubule cell lines serve as models of STC-1 production and/or mechanism of action. Madin-Darby canine kidney-1 (MDCK-1) cells mimic collecting duct cells in their synthesis of STC-1 ligand and receptor, whereas inner medullary collecting duct-3 (IMCD-3) cells respond to additions of STC-1 by increasing their respiration rate. In the present study, MDCK cell STC-1 secretion was examined under normal and hypertonic conditions, vectorally, and in response to hormones and signal transduction pathway activators/inhibitors. STC-1 receptor regulation was monitored in both cell lines in response to changing ligand concentration. The results showed that NaCl-induced hypertonicity had concentration-dependent stimulatory effects on STC-1 secretion, as did the PKC activator TPA. Calcium and ionomycin were inhibitory, whereas calcium receptor agonists had no effect. Angiotensin II, aldosterone, atrial natriuretic factor, antidiuretic hormone, and forskolin also had no effects. Moreover, STC-1 secretion exhibited no vectoral preference. STC-1 receptors were insensitive to homologous downregulation in both cell lines. In contrast, they were upregulated when STC-1 secretion was inhibited by calcium. The findings suggest that hypertonicity-induced STC-1 secretion is regulated through PKC activation and that high intracellular calcium levels are a potent inhibitor of release. More intriguingly, the results suggest that the receptor may not accompany STC-1 in its passage to the mitochondria.

Stanniocalcin 1 prevents cytosolic Ca2+ overload and cell hypercontracture in cardiomyocytes

BACKGROUND

The aim of the present study was to examine whether stanniocalcin 1 (STC1) affects cardiomyocytes under physiological or pathophysiological conditions.

METHODS AND RESULTS

Using fresh isolated rat cardiomyocytes, the effects of STC1 on cell hypercontracture, cell shortening and Ca(2+) transients were measured after exposing the cells to ouabain. STC1 alone did not affect cell shortening or the Ca(2+) transient. Exposure to ouabain significantly increased the fraction of hypercontractured cells (40.5+/-1.4% vs 3.5+/-1.7% in the control, p<0.01). However, treatment with STC1 decreased the percentage of cell hypercontracture that was induced by ouabain, in a concentration-dependent manner (17.4+/-2.6% at 2.5 nmol/L STC1, p<0.01). Moreover, STC1 prevented the increase in diastolic intracellular Ca(2+) level that was induced by ouabain (-5.3+/-2.7% vs 7.9+/-3.7% induced by ouabain, p<0.05; -15.3+/-5.1% in the control) in the cardiomyocytes.

CONCLUSIONS

STC1 prevented the increase in diastolic Ca(2+) overload and ouabain-induced cell hypercontracture, which suggests that STC1 could effectively prevent cytosolic Ca(2+) overload and protect cardiomyocytes from pathophysiological conditions such as in the failing heart.

The respiratory effects of stanniocalcin-1 (STC-1) on intact mitochondria and cells: STC-1 uncouples oxidative phosphorylation and its actions are modulated by nucleotide triphosphates

Stanniocalcin-1 (STC-1) is one of only a handful of hormones that are targeted to mitochondria. High affinity receptors for STC-1 are present on cytoplasmic membranes and both the outer and inner mitochondrial membranes of nephron cells and hepatocytes. In both cell types, STC-1 is also present within the mitochondrial matrix and receptors presumably enable its sequestration. Furthermore, studies in bovine heart sub-mitochondrial particles have shown that STC-1 has concentration-dependent stimulatory effects on electron transport chain activity. The aim of the present study was to determine if the same effects could be demonstrated in intact, respiring mitochondria. At the same time, we also sought to demonstrate the functionality, if any, of an ATP binding cassette that has only recently been identified within the N-terminus of STC-1 by Prosite analysis. Intact, respiring mitochondria were isolated from rat muscle and liver and exposed to increasing concentrations of recombinant human STC-1 (STC-1). Following a 1h exposure to 500 nM STC-1, mitochondria from both organs displayed significant increases in respiration rate as compared to controls. Moreover, STC-1 uncoupled oxidative phosphorylation as ADP:O ratios were significantly reduced in mitochondria from both tissues. The resulting uncoupling was correlated with enhanced mitochondrial (45)Ca uptake in the presence of hormone. Respiratory studies were also conducted on a mouse inner medullary collecting cell line, where STC-1 had time and concentration-dependent stimulatory effects within the physiological range. In the presence of nucleotide triphosphates such as ATP and GTP (5mM) the respiratory effects of STC-1 were attenuated or abolished. Receptor binding studies revealed that this was due to a four-fold decrease in binding affinity (KD) between ligand and receptor. The results suggest that STC-1 stimulates mitochondrial electron transport chain activity and calcium transport, and that these effects are negatively modulated by nucleotide triphosphates.

Expression and regulation of stanniocalcin 1 and 2 in rat uterus during embryo implantation and decidualization

Stanniocalcin-1 (STC-1) is a recently discovered polypeptide hormone, while stanniocalcin-2 (STC-2) is a subsequently identified homologue of stanniocalcin-1. Although previous studies have shown that both STC-1 and -2 are involved in various physiological processes, such as ion transport, reproduction and development, their expression in the uterus and roles in implantation and early pregnancy are unclear. Here we have investigated the expression and regulation of both STC-1 and STC-2 in rat uterus during early pregnancy under various physiological conditions. We show that only basal levels of STC-1 and STC-2 mRNA were detected in the uterus from day one (D1) to day five (D5) of pregnancy. STC-2 immunostaining was gradually increased in the glandular epithelium from day two (D2), with a peak occurring on D5. High levels of both STC-1 and STC-2 mRNA were observed in the stoma cells at the implantation site on day six (D6) of pregnancy, whereas their immunostaining signals were also significant in the luminal epithelium. Basal levels of both STC-1 and STC-2 mRNA and STC-1 immunostaining were detected in the uterus with delayed implantation. After the delayed implantation was terminated by estrogen treatment, both STC-1 and STC-2 mRNA signals were significantly induced in the stroma underlying the luminal epithelium at the implantation site, and STC-2 immunostaining was also observed in the luminal epithelium surrounding the implanting blastocyst. Embryo transfer experiments further confirmed that STC-1 and STC-2 expression at the implantation sites was induced by the implanting blastocyst. Both STC-1 mRNA and immunostaining were seen in the decidualized cells from day seven (D7) to day nine (D9) of pregnancy. STC-2 mRNA was also found in the whole decidua from D7 to D9 of pregnancy; STC-2 protein, however, was strictly localized to the primary deciduas on D7 and D8, with a weak expression in the whole deciduas on D9. Consistent with the normal pregnancy process, strong STC-1 and STC-2 mRNA signals were detected in the decidualized cells under artificial decidualization, whereas only basal levels of STC-1 mRNA and immunostaining were observed in the control horn. These data suggest, for the first time, that STC-1 together with STC-2 may play important roles in the processes of implantation and decidualization in the rat.

Molecular characterization and expression analysis of stanniocalcin-1 in turbot (Scophthalmus maximus)

Stanniocalcin-1 (STC-1) is an anti-hypercalcemic hormone that is produced by the corpuscles of Stannius (CS) in the teleost. The hormone is a homodimeric glycoprotein involved in the calcium and phosphate regulation in both teleost fish and mammals. In the present study, we have cloned and characterized a full-length cDNA of STC-1 from the turbot (Scophthalmus maximus) CS and examined its expression pattern in various tissues. The turbot STC cDNA (1,246 nucleotides) encodes a putative preprohormone of 248 amino acids (aa) with a signal peptide of 18 aa and a pro-sequence peptide of 14 aa followed by a mature protein of 216 aa. The deduced aa sequence of turbot STC-1 shows the highest sequence identity (86.3%) with that of European flounder STC-1 among the fish and mammalian STC-1. The aa sequence of turbot STC-1 showed moderate sequence identity with those of salmoniforms (chum salmon and rainbow trout), cypriniforms (zebrafish), ancient fish (gar and bowfin), and humans (43.7-66.9%), respectively, whereas it showed less identity with those of zebrafish and human STC-2 (23.8-25.4%). RT-PCR analysis revealed that the turbot STC-1 gene is expressed in the CS, pituitary, brain, kidney, liver, heart, muscle, and gonad. In a transient transfection of turbot STC-1 cDNA construct into human embryonic kidney 293T cells followed by Western blot analysis, a recombinant STC-1 secreted into a medium was detected as a single band of 28 kDa under reducing conditions and a 50 kDa protein under non-reducing conditions. Deglycosylation of the recombinant STC-1 led to a decrease in the molecular mass. Our data indicate that the turbot STC-1 gene is widely expressed and the secreted protein exists as a homodimeric glycoprotein.

Stanniocalcin (STC) in the Endometrial Glands of the Ovine Uterus: Regulation by Progesterone and Placental Hormones1

Stanniocalcin (STC) is a hormone in fish that regulates calcium levels. Mammals have two orthologs of STC with roles in calcium and phosphate metabolism and perhaps cell differentiation. In the kidney and gut, STC regulates calcium and phosphate homeostasis. In the mouse uterus, Stc1 increases in the mesometrial decidua during implantation. These studies determined the effects of pregnancy and related hormones on STC expression in the ovine uterus. In Days 10-16 cyclic and pregnant ewes, STC1 mRNA was not detected in the uterus. Intriguingly, STC1 mRNA appeared on Day 18 of pregnancy, specifically in the endometrial glands, increased from Day 18 to Day 80, and remained abundant to Day 120 of gestation. STC1 mRNA was not detected in the placenta, whereas STC2 mRNA was detected at low abundance in conceptus trophectoderm and endometrial glands during later pregnancy. Immunoreactive STC1 protein was detected predominantly in the endometrial glands after Day 16 of pregnancy and in areolae that transport uterine gland secretions across the placenta. In ovariectomized ewes, long-term progesterone therapy induced STC1 mRNA. Although interferon tau had no effect on endometrial STC1, intrauterine infusions of ovine placental lactogen (PL) increased endometrial gland STC1 mRNA abundance in progestinized ewes. These studies demonstrate that STC1 is induced by progesterone and increased by a placental hormone (PL) in endometrial glands of the ovine uterus during conceptus (embryo/fetus and extraembryonic membranes) implantation and placentation. Western blot analyses revealed the presence of a 25-kDa STC1 protein in the endometrium, uterine luminal fluid, and allantoic fluid. The data suggest that STC1 secreted by the endometrial glands is transported into the fetal circulation and allantoic fluid, where it is hypothesized to regulate growth and differentiation of the fetus and placenta, by placental areolae.

The stanniocalcin family of proteins

Stannniocalcin (STC) is a polypeptide hormone that was originally identified in bony fishes as a systemic regulator of mineral metabolism, and is best known for its regulatory effects on calcium/phosphate transport by the gills, gut and kidneys. The mammalian homolog to fish STC was discovered in 1995 and has resulted in progressively growing interest ever since as to its possible role in humans. Moreover, new discoveries in the mammalian STC field are resulting in significant reappraisals as to its role in fishes. Perhaps the most significant of these has been the discovery of a second gene encoding stanniocalcin-related protein, or STC-2, first in mammals and subsequently in fish. This review covers the comparative endocrinology of the STCs in fishes and mammals from the perspectives of structure, function and regulation. It then delves into some of the newer aspects of STC-1/STC-2 biology that have been uncovered using both classical and transgenic approaches. Of these, one of the most intriguing discoveries relates to the receptor-mediated sequestration of STC by target cell organelles. The functions of other newly discovered mammalian and fish STC variants are also discussed, as is the recent discovery of STC-related homologs in invertebrates. Based on our current state of knowledge, it is apparent that STC has an ancient lineage and that the STC family of proteins is proving to have significant roles in metabolism, reproduction and development.

The murine stanniocalcin 1 gene is not essential for growth and development

The stanniocalcin 1 (STC1) gene is expressed in a wide variety of tissues, including the kidney, prostate, thyroid, bone, and ovary. STC1 protein is considered to have roles in many physiological processes, including bone development, reproduction, wound healing, angiogenesis, and modulation of inflammatory response. In fish, STC1 is a hormone that is secreted by the corpuscles of Stannius and is involved in calcium and phosphate homeostasis. To determine the role of STC1 in mammals, we generated Stc1-null mice by gene targeting. The number of Stc1-/- mice obtained was in accordance with Mendelian ratios, and both males and females produced offspring normally. No anatomical or histological abnormalities were detected in any tissues. Our results demonstrated that Stc1 function is not essential for growth or reproduction in the mouse.

Stanniocalcin 1 acts as a paracrine regulator of growth plate chondrogenesis

During embryogenesis, the expression of mammalian stanniocalcin (STC1) in the appendicular skeleton suggests its involvement in the regulation of longitudinal bone growth. Such a role is further supported by the presence of dwarfism in mice overexpressing STC1. Yet, the STC 1 inhibitory effect on growth may be related to both postnatal metabolic abnormalities and prenatal defective bone formation. In our study, we used an organ culture system to evaluate the effects of STC on growth plate chondrogenesis, which is the primary determinant of longitudinal bone growth. Fetal rat metatarsal bones were cultured in the presence of recombinant human STC (rhSTC). After 3 days, rhSTC suppressed metatarsal growth, growth plate chondrocyte proliferation and hypertrophy/differentiation, and extracellular matrix synthesis. In addition, rhSTC increased the number of apoptotic chondrocytes in the growth plate. In cultured chondrocytes, rhSTC increased phosphate uptake, reduced chondrocyte proliferation and matrix synthesis, and induced apoptosis. All these effects were reversed by culturing chondrocytes with rhSTC and phosphonoformic acid, an inhibitor of phosphate transport. The rhSTC-mediated inhibition of metatarsal growth and growth plate chondrocyte proliferation and hypertrophy/differentiation was abolished by culturing metatarsals with rhSTC and phosphonoformic acid. Taken together, our findings indicate that STC1 inhibits longitudinal bone growth directly at the growth plate. Such growth inhibition, likely mediated by an increased chondrocyte phosphate uptake, results from suppressed chondrocyte proliferation, hypertrophy/differentiation, and matrix synthesis and by increased apoptosis. Last, the expression of both STC1 and its binding site in the growth plate would support an autocrine/paracrine role for this growth factor in the regulation of growth plate chondrogenesis.

Stanniocalcin in the euryhaline flounder (Platichthys flesus): primary structure, tissue distribution, and response to altered salinity

Stanniocalcin (STC) is a homodimeric glycoprotein hormone that was first discovered in fish, where it is largely produced by a unique endocrine gland, the corpuscles of Stannius (CS). In bony fish, it is thought to be an important regulator of calcium and phosphate uptake from the aquatic environment. This report describes the molecular cloning of STC from euryhaline flounder (Platichthys flesus) CS cDNA and genomic DNA. The flounder STC encodes a prehormone of 251 amino acids (aa) with a signal peptide of 17 aa, followed by another 15 aa sequence before the mature protein of 219 aa. The deduced aa sequence of flounder STC shows 62.9-89.0% similarity and 50.4-83.1% identity with other known fish STC sequences, but only 42.3% identity with mouse STC1, 24.4% identity with fugu and zebrafish STC2, and 22.3% identity with mouse STC2. Primary structural analysis demonstrated that flounder STC gene contains five exons in contrast to the four exons present in mammalian STC gene. RT-PCR revealed the expression of flounder STC mRNA to be widely spread in many tissues and organs, similar to the situation in mammals and other fish. Quantitative PCR (Q-PCR) was conducted to measure relative STC expression levels in the CS, which showed STC mRNA expression levels in seawater-adapted fish CS were about 3-fold higher than in freshwater-adapted fish CS.

Nuclear targeting of stanniocalcin to mammary gland alveolar cells during pregnancy and lactation

In most mammalian tissues, the stanniocalcin-1 gene (STC-1) produces a 50-kDa polypeptide hormone known as STC50. Within the ovaries, however, the STC-1 gene generates three higher-molecular-mass variants known as big STC. Big STC is targeted locally to corpus luteal cells to block progesterone release. During pregnancy and lactation, however, ovarian big STC production increases markedly, and the hormone is released into the serum. During lactation, this increase in hormone production is dependent on a suckling stimulus, suggesting that ovarian big STC may have regulatory effects on the lactating mammary gland. In this report, we have addressed this possibility. Our results revealed that virgin mammary tissue contained large numbers of membrane- and mitochondrial-associated STC receptors. However, as pregnancy progressed into lactation, there was a decline in receptor densities on both organelles and a corresponding rise in nuclear receptor density, most of which were on milk-producing, alveolar cells. This was accompanied by nuclear sequestration of the ligand. Sequestered STC resolved as one approximately 135-kDa band in the native state and therefore had the appearance of a big STC variant. However, chemical reduction collapsed this one band into six closely spaced, lower-molecular-mass species (28-41 kDa). Mammary gland STC production also underwent a dramatic shift during pregnancy and lactation. High levels of STC gene expression were observed in mammary tissue from virgin and pregnant rats. However, gene expression then fell to nearly undetectable levels during lactation, coinciding with the rise in nuclear targeting. These findings have thus shown that the mammary glands are indeed targeted by STC, even in the virgin state. They have further shown that there are marked changes in this targeting pathway during pregnancy and lactation, accompanied by a switch in ligand source (endogenous to exogenous). They also represent the first example of nuclear targeting by STC.