Targeting of Big Stanniocalcin and Its Receptor to Lipid Storage Droplets of Ovarian Steroidogenic Cells

Molecular cloning and characterization of STC1 gene and its functional analyses in yak (Bos grunniens) cumulus granulosa cells

Cumulus granulosa cells (CGCs), an important type of ovarian somatic cells, carries out various functions related to oogenesis, follicular development, and steroidogenesis. Studying the biological mechanisms involved in the development and function of CGCs makes a great contribution to understanding the reproductive regulation in female animals. Stanniocalcin-1 (STC1) is an important Ca2+-regulated glycoprotein hormone that exhibits high expression levels in ovaries. In this study, we cloned the coding sequence of the yak STC1, predicted the structure of STC1 protein, detected the expression and localization of STC1 in yak ovaries, and analyzed the functions of STC1 in yak CGCs. The CDS (coding sequence) region of yak STC1 gene was found to be 744 bp and encoded 247 amino acids. Homology comparison revealed that STC1 protein was highly conserved among mammals. The STC1 mRNA displayed dynamic expression profiles in different stages of yak ovaries, and the highest expression was found in the follicular phase. Regarding localization, STC1 protein was widely distributed in various kinds of yak ovarian cells, including oocytes, mural granulosa cells, CGCs, and thecal cells. Repressing the expression of STC1 resulted in defective proliferation and survival of yak CGCs. In addition, knockdown the expression of STC1 repressed the secretion of progesterone and promoted the secretion of estrogen. Overexpression of STC1 partially rescued the proliferation of CGCs and resulted in opposite effects on the secretion of progesterone and estrogen. Several apoptosis and steroidogenesis-related genes, including BAX, BCL2, HSD3B1, HSD17B1, CYP11A1 and CYP17A1 showed altered expressions after repressing or increasing the expression of STC1 in yak CGCs. To the best of our knowledge, this study is the first to focus on the role of STC1 in yak CGCs, and the outcomes offer fresh insights into the mechanism governing yak reproduction.

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-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.

Characteristics of lipid droplets and the expression of proteins involved in lipolysis in the murine cervix during mid-pregnancy

Lipid droplets (LDs) are reservoirs of arachidonoyl lipids for prostaglandin (PG) E2 synthesis, and progesterone can stimulate PGE2 synthesis; however, the relationship between progesterone and LD metabolism in the murine cervix remains unclear. In the present study we examined LD distribution and changes in the expression of proteins involved in lipolysis and autophagy in the murine cervix during pregnancy, and compared the findings with those in dioestrous mice. During mid-pregnancy, LDs were predominantly distributed in the cervical epithelium. Electron microscopy revealed the transfer of numerous LDs from the basal to apical region in the luminal epithelium, marked catabolism of LDs, an elevated number of LDs and autophagosomes and a higher LD:mitochondrion size ratio in murine cervical epithelial cells (P<0.05). In addition, immunohistochemical and western blotting analyses showed significantly higher cAMP-dependent protein kinase, adipose triglyceride lipase and hormone-sensitive lipase expression, and a higher light chain 3 (LC3) II:LC3I ratio in the stroma and smooth muscles and, particularly, in murine cervical epithelial cells, during mid-pregnancy than late dioestrus. In conclusion, these results suggest that the enhanced lipolysis of LDs and autophagy in murine cervical tissues were closely related to pregnancy and were possibly controlled by progesterone because LD catabolism may be necessary for energy provision and PGE2 synthesis to maintain a closed pregnant cervix.

Effects of miR-101-3p on goat granulosa cells in vitro and ovarian development in vivo via STC1

Background

MiRNAs act as pivotal post-transcriptional gene mediators in the regulation of diverse biological processes, including proliferation, development and apoptosis. Our previous study has showed that miR-101-3p is differentially expressed in dairy goat ovaries compared single with multiple litters. The objective of this research was to explore the potential function and molecular mechanism of miR-101-3p via its target STC1 in goat ovarian growth and development.

Results

cDNA libraries were constructed using goat granulosa cells transfected with miR-101-3p mimics and negative control by RNA-sequencing. In total, 142 differentially expressed unigenes (DEGs) were detected between two libraries, including 78 down-regulated and 64 up-regulated genes. GO and KEGG enrichment analysis showed the potential impacts of DEGs on ovarian development. STC1 was singled out from DEGs for further research owing to it regulates reproductive-related processes. In vitro, bioinformatics analysis and 3′-UTR assays confirmed that STC1 was a target of miR-101-3p. ELISA was performed to detect the estrogen (E2) and progesterone (P4) levels. CCK8, EdU and flow cytometry assays were performed to detect the proliferation and apoptosis of granulosa cells. Results showed that miR-101-3p regulated STAR, CYP19A1, CYP11A1 and 3β-HSD steroid hormone synthesis-associated genes by STC1 depletion, thus promoted E2 and P4 secretions. MiR-101-3p also affected the key protein PI3K, PTEN, AKT and mTOR in PI3K-AKT pathway by STC1, thereby suppressing proliferation and promoting apoptosis of granulosa cells. In vivo, the distribution and expression levels of miR-101-3p in mouse ovaries were determined through fluorescence in situ hybridisation (FISH). Immunohistochemistry results showed that STC1 expression was suppressed in mouse ovaries in miR-101-3p-agonist and siRNA-STC1 groups. Small and stunted ovarian fragments, decreased numbers of follicles at diverse stages were observed using Hematoxylin-eosin (HE) staining, thereby showing unusual ovarian development after miR-101-3p overexpression or STC1 depletion. Inhibition of miR-101-3p manifested opposite results.

Conclusions

Taken together, our results demonstrated a regulatory mechanism of miR-101-3p via STC1 in goat granulosa cells, and offered the first in vivo example of miR-101-3p and STC1 functions required for ovarian development.

Autophagy Attenuation Hampers Progesterone Synthesis during the Development of Pregnant Corpus Luteum

Simple Summary

The present study demonstrates that induction of autophagy-related proteins in corpus luteum is regulated by Akt/mTOR signaling and autophagy may exert influences on progesterone production by controlling the pool of lipid droplets in luteal cells during the luteal development of pregnant rats. Furthermore, mitophagy-related proteins were also induced during the initiation of luteal regression in pregnant rats, which may play an essential role in the maintenance of mitochondrial homeostasis. These findings will shed light on the role of autophagy during the luteal development of pregnant ovaries in vivo in mammals.

Abstract

The contribution of autophagy to catabolic balance has been well-established in various types of cells, whereas the involvement of autophagy in progesterone synthesis during rat pregnancy still remains unknown. Therefore, the present study was designed to evaluate the role of autophagy in progesterone production during the luteal development of pregnant rats. The results showed autophagy-related proteins was maintained at a low level on day 10 after pregnancy, significantly induced on day 16 and subsided to a relative low level on day 21, which was consistent with the changes of serum progesterone levels. The findings further indicated the contribution of autophagy to progesterone production was regulated by inactivation of Akt/mTOR signaling during the luteal development of pregnant rats in in vivo and in vitro experiments. Further investigations revealed autophagy may be involved in the surge of progesterone production in pregnant rats, as inhibition of autophagy by 3-MA compromised serum progesterone levels. Furthermore, 3-MA treatment also leveled down the number of lipid droplets in luteal cells, implying that autophagy may affect the production of progesterone by manipulating the formation of lipid droplets in luteal cells. In addition, the results suggested that mitophagy was mobilized during the primary stage of luteolysis and inhibition of autophagy promoted the increase of redundant mitochondrial and cytoplasmic cytochrome C in luteal cells of pregnant rats. Taken together, the present study indicated that autophagy-related proteins were induced by the inactivation of Akt/mTOR signaling and then contributed to the progesterone production possibly by affecting the formation of intracellular lipid droplets during the luteal development of pregnant rats. To our knowledge, this will provide a new insight into the important mechanism of autophagy regulating progesterone production in ovaries of pregnant mammals.

Stanniocalcin-1 and -2 effects on glucose and lipid metabolism in white adipose tissue from fed and fasted rats

Stanniocalcin-1 and -2 belong to a family of molecules that exhibit both paracrine and autocrine effects in mammalian cells. Human stanniocalcin-1 (hSTC-1) is expressed in a wide range of tissues, including white adipose tissue. In fed rats, hSTC-1 increases carbon flux from glucose to lipids in retroperitoneal white adipose tissue. Human stanniocalcin-2 (hSTC-2) is expressed in almost all tissues and regulates various biological processes. The aim of this work was to study the action of hSTC-1 and hSTC-2 in the lipid and glucose metabolism of epididymal white adipose tissue (eWAT) in rats in different nutritional states. This study shows for the first time an opposite effect of hSTC-1 and hSTC-2 on glyceride-glycerol generation from glucose in eWAT of fed rats. hSTC-1 stimulated the storage of triacylglycerol in eWAT in the postprandial period, increasing glucose uptake and glyceride-glycerol generation from 14C-glucose. hSTC-2 decreased triacylglycerol synthesis, reducing glyceride-glycerol generation from 14C-glucose, direct phosphorylation of glycerol, and fatty acid synthesis from 14C-glucose in eWAT of fed rats. However, both hormones increased glucose uptake in fed and fasting states. These findings provide evidence for a direct role of hSTC-1 and hSTC-2 in the regulation of lipid and glucose metabolism in eWAT of rats.

Effects of melatonin on the synthesis of estradiol and gene expression in pig granulosa cells

The interaction of granulosa cells (GCs) with oocytes is important to regulate follicle development. The exogenous melatonin promoting the maturation of oocytes by GCs has been approved in pig, however, the transcriptome profile and the functions of the genes regulated by melatonin in GCs have not yet to be fully characterized. In this study, we found melatonin could stimulate the synthesis of estradiol in pig GCs. The RNA-seq was used to explore the effects of melatonin on gene expression, a total of 89 differentially expressed genes (DEGs) were identified. Gene ontology analysis showed DEGs which associated with regulation of cell proliferation, cell cycle, and anti-apoptosis were significantly enriched. The functions of two DEGs, NOTCH2 and FILIP1L, were studied in pig GCs. The results showed that NOTCH2 inhibited the synthesis of estradiol, but FILIP1L promoted the synthesis of estradiol. Furthermore, inhibiting NOTCH2 in granulosa cells cocultured with cumulus-oocyte-complexes had no obvious effect on the maturation of pig oocyte, but could upregulate the cleavage rate of oocyte. We proved that FILIP1L had no effect on the maturation and cleavage of pig oocytes. Our work deepens the understanding of melatonin's effects on GCs and oocyte. The DEGs we found will be beneficial to reveal mechanisms of melatonin acting on GCs and oocytes and design the pharmacological interventions.

Stanniocalcin-1 expression in normal human endometrium and dysregulation in endometriosis

OBJECTIVE

To determine expression of stanniocalcin-1 (STC1) in human endometrium with and without endometriosis and its regulation by steroid hormones.

DESIGN

Laboratory study.

SETTING

University.

PATIENT(S)

Nineteen women with endometriosis and 33 control women.

INTERVENTION(S)

Endometrial biopsy and fluid sampling.

MAIN OUTCOME MEASURE(S)

Analysis of early secretory (ESE) and midsecretory (MSE) endometrial secretomes from fertile women with the use of nano-liquid chromatography-dual mass spectrometry; real-time quantitative polymerase chain reaction, and immunohistochemistry for STC1 and its receptor calcium-sensing receptor (CASR) mRNA and proteins in endometrium with and without endometriosis; evaluation of STC1 and CASR mRNA expression in endometrial stromal fibroblasts (eSF) from women with and without endometriosis decidualized with the use of E2P or 8-bromo-cyclic adenosine monophosphate (cAMP).

RESULT(S)

STC1 protein was strongly up-regulated in MSE versus ESE in endometrial fluid of fertile women. STC1 mRNA significantly increased in MSE from women with, but not from those without, endometriosis, compared with proliferative endometrium or ESE, with no significant difference throughout the menstrual cycle between groups. STC1 mRNA in eSF from control women increased >230-fold on decidualization with the use of cAMP versus 45-fold from women with endometriosis, which was not seen on decidualization with E2/P. CASR mRNA did not exhibit significant differences in any condition and was not expressed in isolated eSF. STC1 protein immunoexpression in eSF was significantly lower in women with endometriosis compared with control women.

CONCLUSION(S)

STC1 protein is significantly up-regulated in MSE endometrial fluid and is dysregulated in eutopic endometrial tissue from women with endometriosis. It is likely regulated by cAMP and may be involved in the pathogenesis of decidualization defects.

Comparative gene expression profiling in human cumulus cells according to ovarian gonadotropin treatments

In in vitro fertilization cycles, both HP-hMG and rFSH gonadotropin treatments are widely used to control human follicle development. The objectives of this study are (i) to characterize and compare gene expression profiles in cumulus cells (CCs) of periovulatory follicles obtained from patients stimulated with HP-hMG or rFSH in a GnRH antagonist cycle and (ii) to examine their relationship with in vitro embryo development, using Human Genome U133 Plus 2.0 microarrays. Genes that were upregulated in HP-hMG-treated CCs are involved in lipid metabolism (GM2A) and cell-to-cell interactions (GJA5). Conversely, genes upregulated in rFSH-treated CCs are implicated in cell assembly and organization (COL1A1 and COL3A1). Interestingly, some genes specific to each gonadotropin treatment (NPY1R and GM2A for HP-hMG; GREM1 and OSBPL6 for rFSH) were associated with day 3 embryo quality and blastocyst grade at day 5, while others (STC2 and PTX3) were related to in vitro embryo quality in both gonadotropin treatments. These genes may prove valuable as biomarkers of in vitro embryo quality.

Cholesterol ester droplets and steroidogenesis

Intracellular lipid droplets (LDs) are dynamic organelles that contain a number of associated proteins including perilipin (Plin) and vimentin. Cholesteryl ester (CE)-rich LDs normally accumulate in steroidogenic cells and their mobilization is the preferred initial source of cholesterol for steroidogenesis. Plin1a, 1b and 5 were found to preferentially associate with triacylglycerol-rich LDs and Plin1c and Plin4 to associate with CE-rich LDs, but the biological significance of this remains unanswered. Vimentin null mice were found to have decreased ACTH-stimulated corticosterone levels, and decreased progesterone levels in females, but normal hCG-stimulated testosterone levels in males. Smaller LDs were seen in null cells. Lipoprotein cholesterol delivery to adrenals and ovary was normal, as was the expression of steroidogenic genes; however, the movement of cholesterol to mitochondria was reduced in vimentin null mice. These results suggest that vimentin is important in the maintenance of CE-rich LDs and in the movement of cholesterol for steroidogenesis.

Stanniocalcin-1 in the subfornical organ inhibits the dipsogenic response to angiotensin II

Recently, receptors for the calcium-regulating glycoprotein hormone stanniocalcin-1 (STC-1) have been found within subfornical organ (SFO), a central structure involved in the regulation of electrolyte and body fluid homeostasis. However, whether SFO neurons produce STC-1 and how STC-1 may function in fluid homeostasis are not known. Two series of experiments were done in Sprague-Dawley rats to investigate whether STC-1 is expressed within SFO and whether it exerts an effect on water intake. In the first series, experiments were done to determine whether STC-1 was expressed within cells in SFO using immunohistochemistry, and whether protein and gene expression for STC-1 existed in SFO using Western blot and quantitative RT-PCR, respectively. Cells containing STC-1 immunoreactivity were found throughout the rostrocaudal extent of SFO. STC-1 protein expression within SFO was confirmed with Western blot, and SFO was also found to express STC-1 mRNA. In the second series, microinjections (200 nl) of STC-1, ANG II, a combination of the two or the vehicle were made into SFO in conscious, unrestrained rats. Water intake was measured at 0700 for a 1-h period after each injection in animals. Microinjections of STC-1 (17.6 or 176 nM) alone had no effect on water intake compared with controls. However, STC-1 not only attenuated the drinking responses to ANG II for about 30 min, but also decreased the total water intake over the 1-h period. These data suggest that STC-1 within the SFO may act in a paracrine/autocrine manner to modulate the neuronal responses to blood-borne ANG II. These findings also provide the first direct evidence of a physiological role for STC-1 in central regulation of body fluid homeostasis.

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.

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.

Proteomics in the characterization of adipose dysfunction in obesity

Adipose tissue plays a central role in body weight homeostasis, inflammation, and insulin resistance via serving as a fat-buffering system, regulating lipid storage and mobilization and releasing a large range of adipokines and cytokines. Adipose tissue is also the major inflammation-initiated site in obesity. Adipose-derived adipokines and cytokines are known to be involved in the modulation of a wide range of important physiological processes, particularly immune response, glucose and lipid homeostasis and insulin resistance. Adipose tissue dysfunction, characterized by an imbalanced secretion of pro- and anti-inflammatory adipokines and cytokines, decreased insulin-stimulated glucose uptake, dysregulation of lipid storage and release and mitochondrial dysfunction, has been linked to obesity and its associated metabolic disorders. Proteomic technology has been a powerful tool for identifying key components of the adipose proteome, which may contribute to the pathogenesis of adipose tissue dysfunction in obesity. In this review, we summarized the recent advances in the proteomic characterization of adipose tissue and discussed the identified proteins that potentially play important roles in insulin resistance and lipid homeostasis.

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.

Stanniocalcin-1 regulates re-epithelialization in human keratinocytes

Stanniocalcin-1 (STC1), a glycoprotein hormone, is believed to be involved in various biological processes such as inflammation, oxidative responses and cell migration. Riding on these emerging evidences, we hypothesized that STC1 may participate in the re-epithelialization during wound healing. Re-epithelialization is a critical step that involves keratinocyte lamellipodia (e-lam) formation, followed by cell migration. In this study, staurosporine (STS) treatment induced human keratinocyte (HaCaT) e-lam formation on fibronectin matrix and migration via the activation of focal adhesion kinase (FAK), the surge of intracellular calcium level [Ca²⁺]i and the inactivation of Akt. In accompanied with these migratory features, a time- and dose-dependent increase in STC1 expression was detected. STC1 gene expression was found not the downstream target of FAK-signaling as illustrated by FAK inhibition using PF573228. The reduction of [Ca²⁺]i by BAPTA/AM blocked the STS-mediated keratinocyte migration and STC1 gene expression. Alternatively the increase of [Ca²⁺]i by ionomycin exerted promotional effect on STS-induced STC1 gene expression. The inhibition of Akt by SH6 and GSK3β by lithium chloride (LiCl) could respectively induce and inhibit the STS-mediated e-lam formation, cell migration and STC1 gene expression. The STS-mediated e-lam formation and cell migration were notably hindered or induced respectively by STC1 knockdown or overexpression. This notion was further supported by the scratched wound assay. Collectively the findings provide the first evidence that STC1 promotes re-epithelialization in wound healing.

The role of lipid droplets in metabolic disease in rodents and humans

Lipid droplets (LDs) are intracellular organelles that store neutral lipids within cells. Over the last two decades there has been a dramatic growth in our understanding of LD biology and, in parallel, our understanding of the role of LDs in health and disease. In its simplest form, the LD regulates the storage and hydrolysis of neutral lipids, including triacylglycerol and/or cholesterol esters. It is becoming increasingly evident that alterations in the regulation of LD physiology and metabolism influence the risk of developing metabolic diseases such as diabetes. In this review we provide an update on the role of LD-associated proteins and LDs in metabolic disease.

Stanniocalcin 1 and ovarian tumorigenesis

BACKGROUND

Stanniocalcin 1 (STC1) is a secreted glycoprotein hormone. High expression of STC1 has been associated with several cancers including ovarian cancer, but its role in the development of ovarian cancer is not clear.

METHODS

We used five human ovarian epithelial cancer cell lines (OVCA420, OVCA432, OVCA433, SKOV3, and HEY), immortalized human ovarian surface epithelial cells (T29 and T80), ovarian cancer tissues from 342 patients, serum from 73 ovarian cancer patients and from58 control subjects, and 116 mice, with six or eight per group. Protein expression was assessed. Cells overexpressing STC1 protein were generated by ectopic expression of human STC1 cDNA. STC1 expression was silenced by using small interfering RNA against STC1. Cell proliferation, migration, colony formation, and apoptosis were assessed. Xenograft tumor growth in mice was studied. Neutralizing anti-STC1 antibody was used to inhibit STC1 function. All statistical tests were two-sided.

RESULTS

STC1 protein expression was higher in all human ovarian cancer cell lines examined than in immortalized human ovarian epithelial cell lines, higher in ovarian cancer tissue than in normal ovarian tissue (P < .001), and higher in serum from ovarian cancer patients than from control subjects (P = .021). Ovarian cancer cells with STC1 overexpression, compared with corresponding control cells, had increased cell proliferation, migration, and colony formation in cell culture and increased growth of xenograft tumors in mice. These activities in normal or malignant ovarian cells with STC1 overexpression, compared with control cells, were also accompanied by increased expression of cell cycle regulatory proteins and antiapoptotic proteins but decreased cleavage of several caspases. Within 24 hours of treatment, apoptosis in cultures of HEY ovarian cancer cells treated with neutralizing anti-STC1 monoclonal antibody was higher (17.3% apoptotic cells) than that in cultures treated with mouse IgG control cells (4.4%) (12.9% difference, 95% confidence interval = 11.6% to 14.2%).

CONCLUSIONS

STC1 protein may be involved in ovarian tumorigenesis.

Expression and localization of stanniocalcin 1 in swine ovary

Stanniocalcin 1 (STC 1) is a glycoprotein involved in mineral homeostasis and was first identified in fish. Its mammalian homologue has been implicated in the regulation of various biological processes, including angiogenesis and steroidogenesis both of which are fundamental events in ovarian function. Interestingly, the highest level of STC 1 expression in mammals occurs in ovarian tissue but no information is available on swine species. Therefore, the present study was undertaken to investigate the expression and the immunolocalization of STC 1 in swine ovary. In addition, we evaluated whether swine granulosa cells synthesize STC 1 and its possible modulation by hypoxia, a physiological condition in ovarian follicle growth. Our data show STC 1 for the first time in swine ovary; moreover, we demonstrate STC 1 production by granulosa cells, both in basal condition and in response to oxygen deprivation. The latter is suggestive of a potential modulatory role for STC 1 in hypoxia-driven angiogenesis.

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.

Mammalian stanniocalcin-1 activates mitochondrial antioxidant pathways: new paradigms for regulation of macrophages and endothelium

The mammalian homolog of the fish calcium regulatory hormone stanniocalcin-1 (STC1) is ubiquitously expressed and likely functions in an autocrine/paracrine fashion. Mammalian STC1 does not appear to exert significant effects on serum calcium, and its physiological role remains to be determined. In macrophages, STC1 decreases intracellular calcium and cell mobility; attenuates the response to chemoattractants; and diminishes superoxide generation through induction of uncoupling protein-2 (UCP2). In cytokine-treated endothelial cells, STC1 attenuates superoxide generation and the activation of inflammatory pathways [c-Jun NH(2)-terminal kinase (JNK) and NF-kappaB]; maintains the expression of tight junction proteins, preserving the endothelial monolayer seal; and decreases transendothelial migration of leukocytes. Combined, the effects of STC1 on endothelial cells and macrophages predict potent anti-inflammatory action. Indeed, application of the anti-glomerular basement membrane (GBM) glomerulonephritis model to STC1 transgenic mice that display increased expression of STC1 transgene in endothelial cells and macrophages yields renal protection. Our data suggest that STC1 activates antioxidant pathways in endothelial cells and macrophages and displays cytoprotective and anti-inflammatory actions.

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.

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.

Osteoblast autonomous Pi regulation via Pit1 plays a role in bone mineralization

The complex pathogenesis of mineralization defects seen in inherited and/or acquired hypophosphatemic disorders suggests that local inorganic phosphate (P(i)) regulation by osteoblasts may be a rate-limiting step in physiological bone mineralization. To test whether an osteoblast autonomous phosphate regulatory system regulates mineralization, we manipulated well-established in vivo and in vitro models to study mineralization stages separately from cellular proliferation/differentiation stages of osteogenesis. Foscarnet, an inhibitor of NaP(i) transport, blocked mineralization of osteoid formation in osteoblast cultures and local mineralization after injection over the calvariae of newborn rats. Mineralization was also down- and upregulated, respectively, with under- and overexpression of the type III NaP(i) transporter Pit1 in osteoblast cultures. Among molecules expressed in osteoblasts and known to be related to P(i) handling, stanniocalcin 1 was identified as an early response gene after foscarnet treatment; it was also regulated by extracellular P(i), and itself increased Pit1 accumulation in both osteoblast cultures and in vivo. These results provide new insights into the functional role of osteoblast autonomous P(i) handling in normal bone mineralization and the abnormalities seen in skeletal tissue in hypophosphatemic disorders.

Roles and origins of leukocyte lipid bodies: proteomic and ultrastructural studies

Lipid bodies (LBs), multifunctional organelles present in most eukaryotic cells, are sites of eicosanoid formation in leukocytes; but little is known about the composition of leukocyte LBs or the biogenesis and internal structures of LBs from mammalian cells. Proteomic analyses of LBs purified from human monocytic U937 cells detected, common to LBs in other cells, proteins involved in cholesterol and triglyceride metabolism, Rab GTPases, and many membrane and endoplasmic reticulum (ER)-associated proteins. Newly lipid body (LB)-associated proteins included MRP-14, potentially involved in arachidonate transport, and ribosomal subunit proteins and translation regulatory proteins. Ultrastructurally, in U937 cells as well as human neutrophils and eosinophils, ribosomes are attached to and distributed within LBs, and LBs contain extensive ER-like membranes. The presence of ribosomes, ER-like membranes and many membrane-associated and ER luminal proteins within LBs, supports a new model by which enveloped ER-membranes and domains form LBs and indicates that LBs may be sites of protein synthesis.

Passive immunization of lactating mice with stanniocalcin-1 antiserum reduces mammary gland development, milk fat content, and postnatal pup growth

During pregnancy and lactation in rodents, stanniocalcin-1 (STC-1) production by the ovaries is upregulated markedly and released into the circulation. The mammary glands are one target of this systemically delivered hormone. The purpose of this study was to lower serum levels of STC-1 in lactating mice through passive immunization so as to monitor the effects on mammary gland function and postnatal pup growth. Passive immunization significantly reduced circulating hormone levels, and pup growth was significantly compromised (30%), even though control and experimental litters had ingested equal amounts of milk. When mammary glands were analyzed, the alveolar area was significantly reduced in antibody-treated mothers. An analysis of milk composition revealed no changes in lactose, protein, or electrolyte levels but an approximately 40% reduction in triglyceride levels. The latter was due to a significant reduction in mammary gland lipoprotein lipase activity and led to a significant buildup of triglycerides in the serum. Body fat content was also significantly reduced in pups from antibody-treated mothers, whereas pup fecal fat content was increased. In mothers, passive immunization also caused significant behavioral effects, in particular, increased locomotor and hindleg rearing activities. Collectively, the results suggest that systemically derived STC-1 has important effects on mammary gland development and the transfer of serum-based triglycerides into milk. Locomotor effects suggest that STC-1 also has a role in maternal behavior.

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.

Characterization of the Drosophila lipid droplet subproteome

Lipid storage droplets are universal organelles essential for the cellular and organismal lipometabolism including energy homeostasis. Despite their apparently simple design they are proposed to participate in a growing number of cellular processes, raising the question to what extent the functional multifariousness is reflected by a complex organellar proteome composition. Here we present 248 proteins identified in a subproteome analysis using lipid storage droplets of Drosophila melanogaster fat body tissue. In addition to previously known lipid droplet-associated PAT (Perilipin, ADRP, and TIP47) domain proteins and homologues of several mammalian lipid droplet proteins, this study identified a number of proteins of diverse biological function, including intracellular trafficking supportive of the dynamic and multifaceted character of these organelles. We performed intracellular localization studies on selected newly identified subproteome members both in tissue culture cells and in fat body cells directly. The results suggest that the lipid droplets of fat body cells are of combinatorial protein composition. We propose that subsets of lipid droplets within single cells are characterized by a protein "zip code," which reflects functional differences or specific metabolic states.

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.

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.

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.

Evidence for stanniocalcin binding activity in mammalian blood and glomerular filtrate

BACKGROUND

The 50 kD form of the hormone stanniocalcin-1 (STC50) is widely distributed in organs such as kidney, lung, and liver. Kidney collecting duct cells produce STC50 for local targeting to proximal tubule cells to increase phosphate reabsorption. As such the current dogma is that in most organs STC50 is a purely local mediator that is not released into the circulation. However, liver hepatocytes contain high levels of both STC50 and its receptor but little evidence of STC production, suggesting that the hormone may in fact be delivered to hepatocytes systemically. Moreover, previous data suggest that red blood cells may in fact bind STC. In this report, we have sought to identify STC binding activity in mammalian blood.

METHODS

Human, pig, and dog red blood cells were analyzed in STC receptor binding assays. Mouse red blood cells and adult mouse kidney were also analyzed histologically for the presence of STC ligand and receptor.

RESULTS

Saturable, high affinity STC receptors were identified on red blood cells from all species. More intriguingly, STC binding activity was also identified in glomerular filtrate, indicative of a soluble, filterable STC binding protein. This binding protein was subsequently observed being reabsorbed in proximal straight tubules.

CONCLUSION

These findings suggest that our inability to detect STC in mammalian serum is due to its being attached to soluble and tethered forms of a high-affinity binding protein. This could be a means of delivering STC to distant targets as well as a mechanism for removing unwanted hormone from the circulation.

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.

Identification of a stanniocalcin paralog, stanniocalcin-2, in fish and the paracrine actions of stanniocalcin-2 in the mammalian ovary

Stanniocalcin is a glycoprotein hormone important in the maintenance of calcium and phosphate homeostasis in fish. Two related mammalian stanniocalcin genes, STC1 and STC2, were found to be expressed in various tissues as paracrine regulators. We have demonstrated the existence of a second stanniocalcin gene in fish, designated fish STC2, with only 30% identity to fish STC1. However, phylogenetic analysis and comparison of the genomic structure of STC genes in vertebrates indicated that STC1 and STC2 genes were probably derived from a common ancestor gene. Based on the prominent expression of mammalian STC1 in the ovary, we tested STC2 expression in rat ovary and the regulation of STC2 expression by gonadotropins. Treatment of immature rats with pregnant mare serum gonadotropin increased STC2 transcripts, whereas subsequent treatment with human chorionic gonadotropin suppressed STC2 expression. Real-time PCR analyses also demonstrated that STC2 is expressed mainly in thecal layers. In situ hybridization studies also revealed that STC2 is expressed in thecal cell layers of antral and preovulatory follicles after gonadotropin stimulation. To elucidate the physiological functions of STC2, recombinant human and fish STC2 proteins were generated and found to be N-glycosylated homodimers. In cultured granulosa cells, treatment with human or fish STC2 suppressed FSH-induced progesterone, but not estradiol or cAMP, production. The STC2 suppression of progesterone production was associated with the inhibition of FSH-induced CYP11A and 3beta-hydroxysteroid dehydrogenase expression. Thus, STC2 is a functional homodimeric glycoprotein, and thecal cell-derived STC2 could play a paracrine role during follicular development.

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.

Stanniocalcin in terminally differentiated mammalian cells

Stanniocalcin (STC) is a glycoprotein hormone originally found in teleost fish, where it regulates the calcium/phosphate homeostasis, and protects the fish against toxic hypercalcemia. STC was considered an exclusive fish protein, until the cloning of cDNA for human (in 1995) and murine (in 1996) STC. We originally reported a high constitutive content of STC in mammalian brain neurons, and found that the expression of STC occurred concomitantly with terminal differentiation of neural cells. Since then, we have investigated the expression of STC in relation to terminal cell differentiation also in mammalian hematopoietic tissue, and fat tissues. In this review we summarize our findings on STC expression during postmitotic differentiation in three different cell systems; in neural cells, in megakaryocytes and in adipocytes. We also present findings, suggesting that STC plays a role for maintaining the integrity of terminally differentiated mammalian cells.

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.

Regulation of luteal cell big stanniocalcin production and secretion

In mammals, the ovaries have the highest levels of stanniocalcin (STC) gene expression, most or all of which is confined to androgen-producing thecal-interstitial cells (TICs). Ovarian TICs also synthesize a different STC that consists of three high molecular weight species collectively known as big STC. Upon release in response to LH stimulation, TIC-derived big STC is sequestered locally by target cells, particularly steroidogenic cells of the corpus luteum, via a receptor-mediated process. Although there is little or no STC gene expression in luteal cells in the in vivo setting, this report describes how the gene is turned on, STC mRNA becomes readily detectable, and big STC is secreted when bovine luteal cells are cultured in vitro. STC gene expression and secretion were both positively regulated by activation of the adenylate cyclase/protein kinase A signaling pathway (forskolin and 8-bromo-cAMP). However, prostaglandin E2 was the only natural luteal cell ligand capable of replicating the effects of forskolin and 8-bromo-cAMP (LH had no consistent effect). Sex steroids such as 17beta-estradiol, androstenedione, and progesterone significantly decreased luteal cell STC expression and secretion. However, only androstenedione was capable of reducing STC production and secretion to undetectable levels. This report is the first to show that once removed from their normal context within the ovary, luteal cells are capable of synthesizing and secreting big STC. It is also the first to delineate the regulatory mechanisms involved in STC production and secretion by luteal cells. These results therefore suggest that under certain physiological conditions, the corpus luteum could very well serve as a source of STC production.