Expression, purification, and bioassay of human stanniocalcin from baculovirus-infected insect cells and recombinant CHO cells

Stanniocalcin-1 in the female reproductive system and pregnancy

BACKGROUND

Stanniocalcin-1 (STC-1) is a widely expressed glycoprotein hormone involved in a diverse spectrum of physiological and pathophysiological processes including angiogenesis, mineral homeostasis, cell proliferation, inflammation and apoptosis. Over the last 20 years, numerous studies have reported STC-1 expression within female reproductive tissues including the uterus, ovaries and placenta and implicated STC-1 in processes such as ovarian follicular development, blastocyst implantation, vascular remodelling in early pregnancy and placental development. Notably, dysregulation of STC-1 within reproductive tissues has been linked to the onset of severe reproductive disorders including endometriosis, polycystic ovary syndrome, poor trophoblast invasion and placental perfusion in early pregnancy. Furthermore, significant changes in tissue expression and in maternal systemic concentration take place throughout pregnancy and further substantiate the vital role of this protein in reproductive health and disease.

OBJECTIVE AND RATIONALE

Our aim is to provide a comprehensive overview of the existing literature, to summarise the expression profile and roles of STC-1 within the female reproductive system and its associated pathologies. We highlight the gaps in the current knowledge and suggest potential avenues for future research.

SEARCH METHODS

Relevant studies were identified through searching the PubMed database using the following search terms: ‘stanniocalcin-1’, ‘placenta’, ‘ovary’, ‘endometrium’, ‘pregnancy’, ‘reproduction’, ‘early gestation’. Only English language papers published between 1995 and 2020 were included.

OUTCOMES

This review provides compelling evidence of the vital function that STC-1 plays within the female reproductive system. The literature presented summarise the wide expression profile of STC-1 within female reproductive organs, as well as highlighting the putative roles of STC-1 in various functions in the reproductive system. Moreover, the observed link between altered STC-1 expression and the onset of various reproductive pathologies is presented, including those in pregnancy whose aetiology occurs in the first trimester. This summary emphasises the requirement for further studies on the mechanisms underlying the regulation of STC-1 expression and function.

WIDER IMPLICATIONS

STC-1 is a pleiotropic hormone involved in the regulation of a number of important biological functions needed to maintain female reproductive health. There is also growing evidence that dysregulation of STC-1 is implicated in common reproductive and obstetric disorders. Greater understanding of the physiology and biochemistry of STC-1 within the field may therefore identify possible targets for therapeutic intervention and/or diagnosis.

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.

Overexpression of stanniocalcin-1 inhibits reactive oxygen species and renal ischemia/reperfusion injury in mice

Reactive oxygen species, endothelial dysfunction, inflammation, and mitogen-activated protein kinases have important roles in the pathogenesis of ischemia/reperfusion kidney injury. Stanniocalcin-1 (STC1) suppresses superoxide generation in many systems through induction of mitochondrial uncoupling proteins and blocks the cytokine-induced rise in endothelial permeability. Here we tested whether transgenic overexpression of STC1 protects from bilateral ischemia/reperfusion kidney injury. This injury in wild type mice caused a halving of the creatinine clearance; severe tubular vacuolization and cast formation; increased infiltration of macrophages and T cells; higher vascular permeability; greater production of superoxide and hydrogen peroxide; and higher ratio of activated ERK/activated JNK and p38, all compared to sham-treated controls. Mice transgenic for human STC1 expression, however, had resistance to equivalent ischemia/reperfusion injury indicated as no significant change from controls in any of these parameters. Tubular epithelial cells in transgenic mice expressed higher mitochondrial uncoupling protein 2 and lower superoxide generation. Pre-treatment of transgenic mice with paraquat, a generator of reactive oxygen species, before injury restored the susceptibility to ischemia/reperfusion kidney injury, suggesting that STC1 protects by an anti-oxidant mechanism. Thus, STC1 may be a therapeutic target for ischemia/reperfusion kidney injury.

Human stanniocalcin-1 suppresses angiotensin II-induced superoxide generation in cardiomyocytes through UCP3-mediated anti-oxidant pathway

Rationale

We have previously shown increased cardiac stanniocalcin-1 (STC1) in patients with idiopathic dilated cardiomyopathy. STC1 localizes to the inner mitochondrial membrane and transgenic over-expression of STC1 is associated with increased energy utilization.

Objective

We examined the hypothesis that STC1 uncouples mitochondrial oxidative phosphorylation - to suppress superoxide generation and modulate neurohormonal effects on cardiomyocytes.

Methods and Results

Compared to WT mouse heart, STC1 Tg heart displays: 2-fold higher uncoupling protein 3 (UCP3) levels, but no effect on UCP2 protein; 40% lower ATP levels; but similar activities of respiratory chain complexes I-IV. In cultured adult rat and freshly-isolated mouse cardiomyocytes, rSTC1 induces UCP3, but not UCP2. Treatment of cardiomyocytes with STC1 decreases mitochondrial membrane potential and suppresses baseline and angiotensin II (Ang II)-induced superoxide generation. Furthermore, baseline superoxide generation is higher in freshly-isolated adult UCP3^−/− mouse cardiomyocytes compared to WT, suggesting an important role for UCP3 in regulating cardiomyocyte ROS under physiologic conditions. Treatment of UCP3^−/− cardiomyocytes with rSTC1 failed to suppress superoxide generation, suggesting that the effects of STC1 on superoxide generation in cardiomyocytes are UCP3-dependent.

Conclusion

STC1 activates a novel anti-oxidant pathway in cardiac myocytes through induction of UCP3, and may play an important role in suppressing ROS in the heart under normal physiologic conditions and ameliorate the deleterious effects of Ang II-mediated cardiac injury. Importantly, our data point to a critical role for the mitochondria in regulating ROS generation in response to Ang II.

Characterization of stanniocalcin-1 receptors in the rainbow trout

Mammalian stanniocalcin-1 (STC-1) is one of several ligands targeted to mitochondria. High affinity STC-1 receptors are present on the mitochondrial membranes of nephron cells, myocytes, and hepatocytes, to enable ligand sequestration within the matrix. However, STC-1 receptors have not been characterized in fish. Nor is it known if mitochondrial targeting occurs in fish. The aim of the study was to address these questions. Saturation binding assays were carried out to obtain estimates of K_D and B_max. They revealed the presence of saturable, high-affinity receptors on both membranes and mitochondria of liver, muscle, and gill filament. In situ ligand binding (ISLB) was used to localize receptors at the histological level and revealed some unexpected findings. In cranium, for instance, receptors were found mainly in the cartilage matrix, as opposed to the chondrocytes. In brain, the majority of receptors were located on neuropil areas as opposed to neuronal cell bodies. In skeletal muscle, receptors were confined to periodic striations, tentatively identified as the Z lines. Receptors were even found on STC-1 producing corpuscles of Stannius cells, raising the possibility of there being an autocrine feedback loop or, perhaps, a soluble binding protein that is released with the ligand to regulate its bioavailability.

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.

Human stanniocalcin-1 blocks TNF-alpha-induced monolayer permeability in human coronary artery endothelial cells

OBJECTIVE

Our previous studies revealed upregulation of stanniocalcin-1 (STC1) in cardiac vessels in dilated cardiomyopathy. However, the functional significance of STC1 is unknown. The objective of this study was to determine the effects of STC1 on TNF-alpha-induced monolayer permeability of human coronary artery endothelial cells (HCAECs).

METHODS AND RESULTS

Cells were pretreated with STC1 for 30 minutes followed by treatment with TNF-alpha (2 ng/mL) for 24 hours. Monolayer permeability was studied using a transwell system. STC1 pretreatment significantly blocked TNF-alpha-induced monolayer permeability in a concentration- and time-dependent manner. STC1 effectively blocked TNF-alpha-induced downregulation of endothelial tight junction proteins zonula occluden-1 and claudin-1 at both mRNA and protein levels. STC1 also significantly decreased TNF-alpha-induced superoxide anion production. The inhibitory effect of STC1 was specific to TNF-alpha, as it failed to inhibit VEGF-induced endothelial permeability. Furthermore, STC1 partially blocked NF-kappaB and JNK activation in TNF-alpha-treated endothelial cells. JNK inhibitor and antioxidant also effectively blocked TNF-alpha-induced NF-kappaB activation and monolayer permeability in HCAECs.

CONCLUSIONS

STC1 maintains endothelial permeability in TNF-alpha-treated HCAECs through preservation of tight junction protein expression, suppression of superoxide anion production, and inhibition of the activation of NFkappaB and JNK, suggesting an important role for STC1 in regulating endothelial functions during cardiovascular inflammation.

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.

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.

Stanniocalcin-1 regulates endothelial gene expression and modulates transendothelial migration of leukocytes

The mammalian counterpart of the fish calcium-regulating hormone stanniocalcin-1 (STC1) inhibits monocyte chemotactic protein-1- and stromal-derived factor-1alpha (SDF-1alpha)-mediated chemotaxis and diminishes chemokinesis in macrophage-like RAW264.7 and U937 cells in a manner that may involve attenuation of the intracellular calcium signal. STC1 is strongly induced in the kidney following obstructive injury. We hypothesized that STC1 may serve to attenuate the influx of inflammatory cells to the site of tissue injury. In this study, we examined the effect of STC1 on the migration of freshly isolated human macrophages, neutrophils, and T and B lymphocytes through quiescent or IL-1beta-treated human umbilical vein endothelial cell (HUVEC) monolayers. STC1 inhibited transmigration of macrophages and T lymphocytes through quiescent or IL-1beta-activated HUVECs but did not attenuate the transmigration of neutrophils and B lymphocytes. STC1 regulates gene expression in cultured endothelial cells and is detected on the apical surface of endothelial cells in vivo. The data suggest that STC1 plays a critical role in transendothelial migration of inflammatory cells and is involved in the regulation of numerous aspects of endothelial function.

Evidence for cross-talk between stanniocalcins

There are 2 forms of stanniocalcin (STC) produced by the STC-1 gene; a 50 kDa polypeptide known as STC50 and a recently discovered group of higher molecular weight variants that are collectively referred to as big STC. Both have different tissue patterns of expression and different intracellular targeting pathways. STC50 functions locally in tissues such as muscle, liver, and kidney and is targeted to mitochondria. Big STC, on the other hand, is made by the ovaries. It signals both locally on nearby corpus luteal cells and systemically. Interestingly, however, receptor binding assays employing STC50 as the tracer have shown that the smaller ligand can bind equally to tissue receptors targeted by either form of the hormone. This suggests there may be cross-talk between ligands. The present study provides credence to this notion by demonstrating how the 2 hormones can compete for tissue receptors normally targeted by 1 form of the hormone (big STC). The results also reveal how STC50 can completely block the inhibitory effects of big STC on luteal cell progesterone release when added simultaneously. The findings therefore add credence to the possibility that there may be circumstances during which the 2 ligands functionally antagonize each other's actions.

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.

Characterization of big stanniocalcin variants in mammalian adipocytes and adrenocortical cells

The hormone stanniocalcin (STC) is widely distributed, and in rodents the highest levels of expression are in the ovaries. In both cows and rodents, ovarian STC consists of three high-molecular-weight variants collectively known as big STC. In the ovary, big STC is made by theca cells and interstitial cells and is targeted to lipid storage droplets of nearby luteal cells to inhibit progesterone release. An endocrine pathway is operative during pregnancy and lactation. Whether or not big STC is made by tissues other than ovary has never been addressed. Therefore, the purpose of this study was to determine via a detailed characterization of adrenal glands and adipocytes whether big STC is present in other cells that store and release lipids. The results showed that STC was made in bovine and mouse adrenals, mainly in steroidogenic, adrenocortical cells. The majority of ligand and receptor were likewise confined to cortical zone cells. As in luteal cells, high levels of ligand and receptor were found in the adrenocortical cell lipid droplet fraction. However, adrenals made only the largest (135 kDa) of the three big STC variants. Nonetheless, adrenal STC had much greater receptor affinity than a mixture of the three big STC variants. Adipocytes contained all three big STC variants, and both ligand and receptor were heavily concentrated on the lipid droplets. Moreover, adipocyte lipid storage droplets had 50-fold more receptors than those in steroidogenic cells, indicating that big STC is heavily targeted to adipose cells. The findings collectively support the hypothesis that big STC is not unique to ovarian steroidogenic cells but is in fact common to cells with a role in lipid storage and release.

Human stanniocalcin-2 exhibits potent growth-suppressive properties in transgenic mice independently of growth hormone and IGFs

Stanniocalcin (STC)-2 was discovered by its primary amino acid sequence identity to the hormone STC-1. The function of STC-2 has not been examined; thus we generated two lines of transgenic mice overexpressing human (h)STC-2 to gain insight into its potential functions through identification of overt phenotypes. Analysis of mouse Stc2 gene expression indicates that, unlike Stc1, it is not highly expressed during development but exhibits overlapping expression with Stc1 in adult mice, with heart and skeletal muscle exhibiting highest steady-state levels of Stc2 mRNA. Constitutive overexpression of hSTC-2 resulted in pre- and postnatal growth restriction as early as embryonic day 12.5, progressing such that mature hSTC-2-transgenic mice are approximately 45% smaller than wild-type littermates. hSTC-2 overexpression is sometimes lethal; we observed 26-34% neonatal morbidity without obvious dysmorphology. hSTC-2-induced growth retardation is associated with developmental delay, most notably cranial suture formation. Organ allometry studies show that hSTC-2-induced dwarfism is associated with testicular organomegaly and a significant reduction in skeletal muscle mass likely contributing to the dwarf phenotype. hSTC-2-transgenic mice are also hyperphagic, but this does not result in obesity. Serum Ca2+ and PO4 were unchanged in hSTC-2-transgenic mice, although STC-1 can regulate intra- and extracellular Ca2+ in mammals. Interestingly, severe growth retardation induced by hSTC-2 is not associated with a decrease in GH or IGF expression. Consequently, similar to STC-1, STC-2 can act as a potent growth inhibitor and reduce intramembranous and endochondral bone development and skeletal muscle growth, implying that these tissues are specific physiological targets of stanniocalcins.

Stanniocalcin: no longer just a fish tale

Stanniocalcin was originally described as a hormone with calcitonin-like actions in fish. During the last decade, mammalian forms of stanniocalcin have been identified, and this discovery has led to important advances in our understanding of this enigmatic polypeptide hormone. This review briefly covers some early studies on stanniocalcin in fish and then provides a more in-depth look at some of the more intriguing, new aspects of its functions in mammals. The roles of stanniocalcin in renal function, metabolism, angiogenesis, pregnancy and lactation, bone formation, and neural protection are discussed, along with new information relating to its receptor-mediated sequestration and accumulation in target cell organelles.

Evidence for stanniocalcin and a related receptor in annelids

Stanniocalcin (STC) is a prime example of a hormone whose discovery in fish led to its subsequent discovery in mammals. STC is considered to be first and foremost a vertebrate polypeptide hormone with regulatory effects on ion transport, mitochondrial function and steroid hormone synthesis. The gene is widely expressed in both fishes and mammals, and the hormone can operate via both local and endocrine signaling pathways. In spite of the growing catalogue of vertebrate hormones and receptors with homologues in invertebrates, the notion that there might be an invertebrate STC homolog has received scant attention to date. In the present study, we have provided evidence for STC in annelid worms (freshwater leeches). Western blot analysis revealed the presence of two STC immunoreactive (STCir) proteins in leech tissue extracts of 100 and 193 kDa. These same extracts significantly lowered the rate of gill calcium transport upon injection into fish. Similarly, fish STC increased the rate of whole body calcium uptake when administered to leeches, and STC receptors of high affinity were identified on isolated leech plasma membranes. Two discrete populations of STC-positive cells were also identified in leeches using antibodies to fish STC and fish STC cRNA probes. One of the cell types was confined to the skin. The second cell type was confined to the coelomic cavity and identified as an adipose cell, which in leeches is a major repository of fat. Collectively, the data constitutes compelling evidence for the existence of STC-related proteins and receptors in annelids that share structural and functional similarities with those in vertebrates.

Stanniocalcin-1, an inhibitor of macrophage chemotaxis and chemokinesis

In macrophages, changes in intracellular calcium have been associated with activation of cellular processes that regulate cell adhesion and motility and are important for the response of macrophages to antigenic stimuli. The mammalian counterpart of the fish calcium-regulating hormone stanniocalcin-1 (STC1) is expressed in multiple organs including the thymus and spleen, and hence, we hypothesized that it may have a role in modulating the immune/inflammatory response. Using murine macrophage-like (RAW264.7) and human monoblast-like (U937) cells to study chemotaxis in vitro, we found that STC1 attenuated chemokinesis and diminished the chemotactic response to monocyte chemotactic protein-1 (MCP-1) and stromal cell-derived factor-1alpha. Consistent with these findings, STC1 blunted the rise in intracellular calcium following MCP-1 stimulation in RAW264.7 cells. In vivo studies suggested differential expression of STC1 in obstructed kidney and localization to macrophages. MCP-1 and STC1 transcripts were both upregulated following ureteric obstruction, suggesting a functional association between the two genes. Our data suggest a role for mammalian STC1 in modulating the immune/inflammatory response.

Stanniocalcin-1 is a naturally occurring L-channel inhibitor in cardiomyocytes: relevance to human heart failure

Cardiomyocytes of the failing heart undergo profound phenotypic and structural changes that are accompanied by variations in the genetic program and profile of calcium homeostatic proteins. The underlying mechanisms for these changes remain unclear. Because the mammalian counterpart of the fish calcium-regulating hormone stanniocalcin-1 (STC1) is expressed in the heart, we reasoned that STC1 might play a role in the adaptive-maladaptive processes that lead to the heart failure phenotype. We examined the expression and localization of STC1 in cardiac tissue of patients with advanced heart failure before and after mechanical unloading using a left ventricular assist device (LVAD), and we compared the results with those of normal heart tissue. STC1 protein is markedly upregulated in cardiomyocytes and arterial walls of failing hearts pre-LVAD and is strikingly reduced after LVAD treatment. STC1 is diffusely expressed in cardiomyocytes, although nuclear predominance is apparent. Addition of recombinant STC1 to the medium of cultured rat cardiomyocytes slows their endogenous beating rate and diminishes the rise in intracellular calcium with each contraction. Furthermore, using whole cell patch-clamp studies in cultured rat cardiomyocytes, we find that addition of STC1 to the bath causes reversible inhibition of transmembrane calcium currents through L-channels. Our data suggest differential regulation of myocardial STC1 protein expression in heart failure. In addition, STC1 may regulate calcium currents in cardiomyocytes and may contribute to the alterations in calcium homeostasis of the failing heart.

Characterization of mammalian stanniocalcin receptors. Mitochondrial targeting of ligand and receptor for regulation of cellular metabolism

The polypeptide hormone stanniocalcin (STC) is widely expressed in mammalian tissues. STC acts locally in kidney and gut to modulate calcium and phosphate excretion, and its overexpression in mice results in high serum phosphate, dwarfism, and increased metabolic rate. STC has also been linked to cancer, pregnancy, lactation, angiogenesis, organogenesis, cerebral ischemia, and hypertonic stress. In this report we have characterized the STC receptor and the functional targeting of ligand and receptor to mitochondria. For receptor binding analysis, a stanniocalcin-alkaline phosphatase fusion protein was engineered. Subsequent binding assays using the fusion protein indicated that kidney and liver contained the highest number of binding sites with affinities of 0.8 and 0.25 nm, respectively. Intriguingly, purified mitochondria from both tissues yielded similar high affinity binding sites. Fractionation analysis revealed that the majority of binding sites were localized to the inner mitochondrial membrane. In further studies, we characterized the time course of STC-alkaline phosphatase fusion protein sequestration by intact mitochondria. In situ ligand binding also revealed discrete, displaceable binding to plasma membranes and mitochondria of nephron cells and liver hepatocytes. The existence of mitochondrial receptors prompted a similar search for the ligand. Immunogold electron microscopy revealed that STC was preferentially concentrated in the mitochondria of all nephron segments targeted by STC. Subcellular fractionation revealed that >90% of cellular STC immunoreactivity was mitochondrial, confined to the inner matrix, and similar in size to recombinant STC (50 kDa). In functional studies, recombinant STC had concentration-dependent stimulatory effects on electron transfer by sub-mitochondrial particles. Collectively the evidence implies a role for STC in cell metabolism.

Ovarian stanniocalcin is structurally unique in mammals and its production and release are regulated through the luteinizing hormone receptor

Stanniocalcin (STC) is a recently discovered mammalian hormone that is widely distributed in many tissues. In rodents the STC gene is most highly expressed in ovary, specifically in androgen-producing thecal and interstitial cells. In addition, ovarian levels of expression rise 15-fold over pregnancy. The objective of this study was to develop a primary culture system for ovarian thecal-interstitial cells (TICs) to identify factors governing STC production and release. We used highly purified primary cultures of rat and bovine TICs, the purity of which was routinely assessed with antigenic and enzymatic markers. The functionality of cells was assured by their responsiveness to LH in the form of progesterone release. We found that forskolin significantly increased STC gene expression and secretion by both rat and bovine TICs, an effect that was only replicated by human (h) chorionic gonadotropin (CG). Coincubation of TICs with hCG and phosphodiesterase inhibitors further increased STC secretion, whereas coincubation of TICs with hCG and protein kinase A inhibitors attenuated hCG-stimulated release. Intriguingly, ovarian STC proved to be substantially larger than the 50-kDa homodimer produced in most other tissues. These results indicate that ovarian STC is physically distinct, a feature that could explain its presence in serum during pregnancy and lactation.

Stanniocalcin 1 (STC1) protein and mRNA are developmentally regulated during embryonic mouse osteogenesis: the potential of stc1 as an autocrine/paracrine factor for osteoblast development and bone formation

STC1, a mammalian homologue of stanniocalcin (STC) which plays a major role in calcium/phosphate homeostasis in fish, has been recently isolated. We have characterized the spatiotemporal distribution of STC1 mRNA and protein during mouse embryonic development generally and osteogenesis specifically. Northern blotting analysis of whole embryos showed that STC1 mRNA is highly and differentially expressed during embryogenesis. By in situ hybridization, STC1 mRNA was detected early in mesenchymal condensations and was then found to be highly expressed in perichondrial cells, periosteal cells, and then osteoblasts during endochondral bone formation. In bones forming by intramembranous ossification, STC1 mRNA was not detected until osteogenic cells appeared. The cellular distribution of STC1 protein closely corresponded to that of its mRNA, but the protein was also detected in hypertrophic chondrocytes. In the MC3T3-E1 osteogenic cell model, STC1 protein and mRNA were detectable throughout proliferation and differentiation stages but levels were relatively higher late during nodule formation/mineralization phases. For comparison, STC1 mRNA was also found in epithelial cells of both embryonic and adult intestine that had not previously been described among tissues responsive to calcium/phosphate transport. These results suggest that STC1 is expressed in a time- and cell-specific manner and may play an autocrine/paracrine role during osteoblast development and bone formation.

Analysis of recombinant Phex: an endopeptidase in search of a substrate

X-linked hypophosphatemia (XLH) is caused by inactivating mutations of Phex, a phosphate-regulating endopeptidase. Further advances in our knowledge of the pathogenesis of XLH require identification of the biological function of Phex and its physiologically relevant substrates. We evaluated several potential substrates using mouse recombinant wild-type Phex proteins (rPhex-WT) and inactive mutant Phex proteins (rPhex-3'M) lacking the COOH-terminal catalytic domain as controls. By Western blot analysis, we demonstrated that Phex is a membrane-bound 100-kDa glycosylated monomer. Neither casein, a substrate for the related endopeptidase thermolysin, human stanniocalcin 1 (hSTC-1), an osteoblast-derived phosphate-regulating factor, nor FGF-23 peptide (amino acid 172-186), comprising the region mutated in autosomal dominant hypophosphatemia, was cleaved by rPhex-WT. In addition, membranes expressing rPhex-WT, rPhex-3'M, and the empty vector hydrolyzed parathyroid hormone-(1-34), indicating the lack of Phex-specific cleavage of parathyroid hormone. In contrast, rPhex-WT did display an EDTA-dependent cleavage of the neutral endopeptidase substrate [Leu]enkephalin. Further studies with wild-type and mutant rPhex proteins should permit the identification of physiologically relevant substrates involved in the pathogenesis of XLH.

Industrial choices for protein production by large-scale cell culture

Traditional barriers to large-scale mammalian culture have now been addressed, with the standard stirred-tank reactor emerging as industry's technology of choice. The issues of adapting cells to suspension culture, shear sensitivity and oxygen supply have been largely resolved. But for many low-volume and specialty applications, such as the production of viral vaccines and gene therapies, reactor technology remains diversified, with reactor types ranging from roller bottles to stacked plates and hollow fibers.

Dynamic changes in stanniocalcin gene expression in the mouse uterus during early implantation

Blastocyst implantation is accompanied by dramatic changes in gene expression to facilitate decidualization and remodelling of uterine architecture. Stanniocalcin (STC) is a new mammalian polypeptide hormone with roles in ion transport, reproduction and development. Here we report dynamic changes in STC mRNA and protein distributions in the early post-implantation mouse uterus. In the non-pregnant state, STC gene expression was confined to the uterine lumenal epithelium. Following implantation STC gene expression shifted to mesometrial stromal cells bordering the uterine lumen. Between E6.5-E8.5 expression shifted once more to cells of the mesometrial lateral sinusoids, and then declined thereafter. Intriguingly immunoreactive STC did not entirely co-localize with areas of high STC gene activity and instead appeared to accumulate in presumptive targets of the hormone (uterine epithelium, stromal and decidual cells, trophoblastic giant cells). STC is only the fourth gene identified as being expressed mesometrially in the uterus following implantation.

Stanniocalcin gene expression during mouse urogenital development: a possible role in mesenchymal-epithelial signalling

Stanniocalcin (STC) is a polypeptide hormone first discovered in fish and more recently in mammals. In mammals, the STC gene is widely expressed and the hormone is involved in a variety of functions, but STC does not normally circulate in the blood. In both kidney and gut, STC regulates phosphate fluxes across the transporting epithelia, whereas in brain it protects neurons against cerebral ischemia and promotes neuronal cell differentiation. However, the gene is most highly expressed in ovary and expression is dramatically up-regulated by both pregnancy and nursing. STC mRNA levels are also high in the developing mouse embryo, but literally nothing is known of the tissue pattern of gene expression. Therefore, the aim of this study was to map the temporal and spatial patterns of gene expression during mouse embryologic development, starting with the urogenital system where the gene is so highly expressed in adults. STC mRNA was evident as early as E10.5 in both the mesonephros and genital ridge. Between E10.5 and 14.5 in developing kidney, STC was produced in undifferentiated mesenchyme cells and sequestered by ureteric bud epithelial cells that did not express the gene but nonetheless contained high levels of STC protein. Thereafter, the distribution pattern resembled that in adults such that gene expression predominated in collecting duct cells, whereas protein was present in most nephron segments. The pattern of gene expression during gonadal development was sexually dimorphic. In males, expression was first evident on E12.5 in interstitial mesenchyme cells surrounding the developing sex cords, whereas the protein accumulated in developing gonocytes within the sex cords that did not express the gene. This pattern became more pronounced over the course of gestation. In contrast, ovarian gene expression was only weakly evident during development. Collectively, the evidence suggests that in addition to its regulatory effects in adults, STC has novel and distinctive roles in the mesenchymal-epithelial interactions that are vital to normal organogenesis.

Development of a human stanniocalcin radioimmunoassay: serum and tissue hormone levels and pharmacokinetics in the rat

Stanniocalcin (STC) is a polypeptide hormone that was first discovered in fish and recently identified in humans and other mammals. In fish STC is produced by one gland, circulates freely in the blood and plays an integral role in mineral homeostasis. In mammals, STC is produced in a number of different tissues and serves a variety of different functions. In kidney, STC regulates phosphate reabsorption by proximal tubule cells, whereas in ovary it appears to be involved in steroid hormone synthesis. However there is no information on circulating levels of STC in mammals or the regulation of its secretion. In this report we have developed a radioimmunoassay (RIA) for human STC. The RIA was validated for measuring tissue hormone levels. However human and other mammalian sera were completely devoid of immunoreactive STC (irSTC). To explore the possibility that mammalian STC might have a short half-life pharmacokinetic analysis was carried out in rats. STC pharmacokinetics were best described by a two compartment model where the distribution phase (t1/2(alpha)) equaled 1 min and the elimination phase (t1/2(beta)) was 60 min. However the STC in the elimination phase no longer crossreacted in the RIA indicating it had undergone substantial chemical modification, which could explain our inability to detect irSTC in mammalian sera. When we compared the pharmacokinetics of human and fish STC in mammalian and fish models the human hormone was always eliminated faster, indicating that human STC has unique structural properties. There also appears to be a unique clearance mechanism for STC in mammals. Hence there are major differences in the delivery and biology of mammalian STC. Unlike fishes, mammalian STC does not normally circulate in the blood and functions instead as a local mediator of cell function. Future studies will no doubt show that this has had important ramifications on function as well.

Evidence for stanniocalcin gene expression in mammalian bone

Stanniocalcin (STC) acts as a regulator of calcium and phosphate homeostasis in an endocrine manner in bony fish. Recently, complementary DNAs encoding human and mouse STC have been characterized, and the messenger RNA (mRNA) expression was identified in various tissues, such as kidney, small intestine, prostate, thyroid, and ovary. Because previous studies concerning the effects of fish STC on mammalian bone have been discussed, there is a good possibility that mammalian STC is a local factor in bone. Here, we demonstrated STC mRNA expression in neonatal mouse calvaria, the primary cultured mouse osteoblast-rich fractions, and human and mouse osteoblastic cell lines. We also mapped the cellular distribution of the STC mRNA in femur and calvaria in developing mice. Several transcripts with a major 4-kb band were detected in all samples. The cellular distribution of the mRNA expression corresponded closely to osteoblasts in both femur and calvaria. Significant labeling of the STC mRNA was also identified in chondrocytes but not in osteoclasts and other bone marrow elements. These results are the first evidence that hormone may be actually expressed in osteoblasts and chondrocytes, and they strongly implicate the involvement of local STC in both endochondral and membrane bone as an autocrine/paracrine factor.

Molecular cloning and characterization of stanniocalcin-related protein

Stanniocalcin (STC) is a glycoprotein hormone first discovered in fish and recently identified in humans and mice. In this report we have described the cloning of an STC-like cDNA, designated here as STC related protein (STCrP). Human STCrP (hSTCrP) cDNAs were isolated as expressed sequence tags (ESTs) and predicted a polypeptide of 302 amino acids, with 58%, homology to human STC (hSTC). Ten of the eleven 1/2 cysteine residues that in STC allow for dimerization of monomeric subunits were conserved in hSTCrP. By Northern analysis, three hSTCrP mRNAs were detected and were most abundant in pancreas, spleen and kidney as well as a variety of different transformed cell lines. The high degree of sequence homology suggests that STC and STCrP may have been derived from a common ancestral gene.

Comparative analysis of mammalian stanniocalcin genes

The recent discovery of mammalian stanniocalcin (STC) prompted an investigation of its gene structure and expression pattern to study its function and regulation. We show that both the human and mouse genes are composed of four exons spanning about 13 kb, with 85% nucleotide sequence identity in coding regions. Remarkably high sequence conservation between species also exists in the approximately 3-kb 3'-untranslated region. Comparative analysis of the 5'-untranslated region and flanking DNA from the rat and human STC genes showed long stretches of CAG trinucleotide repeats and an additional (CA)25 dinucleotide repeat unique to the rat promoter. An analysis of STC expression in the mouse showed that ovary contained the highest level of messenger RNA, with lower, but detectable, levels in most tissues. In situ hybridization revealed strong, specific hybridization over the thecal-interstitial cells of the ovarian stroma, whereas immunohistochemical analysis indicated that STC was present not only in the stroma, but also in the corpora lutea and oocyte of the developing follicle. Consequently, STC may act as a signaling molecule between the thecal-interstitial cell compartment and the corpus luteum and oocyte, thereby regulating the activity of these structures in some way. These findings suggest that in addition to its role in mineral metabolism, STC has acquired an important function in reproduction during its evolution to mammals.