Effects of a synthetic N-terminal fragment of stanniocalcin on the metabolism of mammalian bone in vitro

New Insights Into Physiological and Pathophysiological Functions of Stanniocalcin 2

Stanniocalcin, a glycosylated peptide hormone, first discovered in a bony fish has originally been shown to play critical role in calcium and phosphate homeostasis. Two paralogs of stanniocalcin (STC1 and STC2) identified in mammals are widely expressed in variety of tissues. This review provides historical perspective on the discovery of fish and mammalian stanniocalcin, describes molecular regulation of STC2 gene, catalogs distribution as well as expression of STC2 in tissues, and provides key structural information known till date regarding mammalian STC2. Additionally, this mini review summarizes pivotal functions of STC2 in calcium and phosphate regulation, cytoprotection, cell development, and angiogenesis. Finally, STC2's role as a novel marker for human cancers has also been outlined. Reviewing these studies will provide an opportunity to understand STC2's structure, biological functions as well as key molecular pathways involving STC2, which will help us design innovative therapeutic interventions using this novel hormone.

STC1 interference on calcitonin family of receptors signaling during osteoblastogenesis via adenylate cyclase inhibition

Stanniocalcin 1 (STC1) and calcitonin gene-related peptide (CGRP) are involved in bone formation/remodeling. Here we investigate the effects of STC1 on functional heterodimer complex CALCRL/RAMP1, expression and activity during osteoblastogenesis. STC1 did not modify CALCRL and ramp1 gene expression during osteoblastogenesis when compared to controls. However, plasma membrane spatial distribution of CALCRL/RAMP1 was modified in 7-day pre-osteoblasts exposed to either CGRP or STC1, and both peptides induced CALCRL and RAMP1 assembly. CGRP, but not STC1 stimulated cAMP accumulation in 7-day osteoblasts and in CALCRL/RAMP1 transfected HEK293 cells. Furthermore, STC1 inhibited forskolin stimulated cAMP accumulation of HEK293 cells, but not in CALCRL/RAMP1 transfected HEK293 cells. However, STC1 inhibited cAMP accumulation in calcitonin receptor (CTR) HEK293 transfected cells stimulated by calcitonin. In conclusion, STC1 signals through inhibitory G-protein modulates CGRP receptor spatial localization during osteoblastogenesis and may function as a regulatory factor interacting with calcitonin peptide members during bone formation.

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.

Stanniocalcin 1 alters muscle and bone structure and function in transgenic mice

Fish stanniocalcin (STC) inhibits uptake of calcium and stimulates phosphate reabsorption. To determine the role of the highly homologous mammalian protein, STC-1, we created and characterized transgenic mice that express STC-1 under control of a muscle-specific promoter. STC-1 transgenic mice were smaller than wild-type littermates and had normal growth plate cartilage morphology but increased cartilage matrix synthesis. In STC-1 mice, the rate of bone formation, but not bone mineralization, was decreased. Increased cortical bone thickness and changes in trabeculae number, density, and thickness in STC-1 mice indicated a concomitant suppression of osteoclast activity, which was supported by microcomputed tomography analyses and histochemistry. Skeletal muscles were disproportionately small and showed altered function and response to injury in STC-1 mice. Electron microscopy indicated that muscle mitochondria were dramatically enlarged in STC-1 mice. These changes in STC-1 mice could not be explained by deficits in blood vessel formation, as vascularity in organs and skeletal tissues was increased as was induction of vascularity in response to femoral artery ligation. Our results indicate that STC-1 can affect calcium homeostasis, bone and muscle mass and structure, and angiogenesis through effects on osteoblasts, osteoclasts, myoblasts/myocytes, and endothelial cells.

Characterization of gene expression induced by RET with MEN2A or MEN2B mutation

Germ-line point mutations of the RET gene are responsible for multiple endocrine neoplasia (MEN) type 2A and 2B that develop medullary thyroid carcinoma and pheochromocytoma. We performed a differential display analysis of gene expression using NIH 3T3 cells expressing the RET-MEN2A or RET-MEN2B mutant proteins. As a consequence, we identified 10 genes induced by both mutant proteins and eight genes repressed by them. The inducible genes include cyclin D1, cathepsins B and L, and cofilin genes that are known to be involved in cell growth, tumor progression, and invasion. In contrast, the repressed genes include type I collagen, lysyl oxidase, annexin I, and tissue inhibitor of matrix metalloproteinase 3 (TIMP3) genes that have been implicated in tumor suppression. In addition, six RET-MEN2A- and five RET-MEN2B-inducible genes were identified. Among 21 genes induced by RET-MEN2A and/or RET-MEN2B, six genes including cyclin D1, cathepsin B, cofilin, ring finger protein 11 (RNF11), integrin-alpha6, and stanniocalcin 1 (STC1) genes were also induced in TGW human neuroblastoma cells in response to glial cell line-derived neurotrophic factor stimulation. Because the STC1 gene was found to be highly induced by both RET-MEN2B and glial cell line-derived neurotrophic factor stimulation, and the expression of its product was detected in medullary thyroid carcinoma with the MEN2B mutation by immunohistochemistry, this may suggest a possible role for STC1 in the development of MEN 2B phenotype.

Prospect of a stanniocalcin endocrine/paracrine system in mammals

Stanniocalcin (STC) is a calcium- and phosphate-regulating hormone produced in bony fish by the corpuscles of Stannius, which are located close to the kidney. It is a major antihypercalcemic hormone in fish. As the corpuscles of Stannius are absent, and antihypercalcemic hormones are basically not necessary, in mammals, the discovery of a mammalian homolog, STC1, was surprising and intriguing. STC1 displays a relatively high amino acid sequence identity (approximately 50%) with fish STC. In contrast to fish STC, STC1 is expressed in many tissues, including kidney. More recently, a human gene encoding the second stanniocalcin-like protein, STC2, was identified. STC2 has a lower identity (approximately 35%) with STC1 and fish STC. Similar to STC1, STC2 is also expressed in a variety of tissues. Research into the functions of STCs in mammals is still at an early stage, and the ultimate physiological and pathological roles of STCs have not yet been established. A few studies indicate that STC1, similar to fish STC, stimulates phosphate absorption in the kidney and intestine, but the function of STC2 is still unknown. However, several interesting findings have been reported on their cellular localization, gene structure, and expression in different physiological and pathological conditions, which will be clues in elucidating the functions of STCs in mammals. STC1 expression is enhanced by hypertonicity in a kidney cell line or by ischemic injuries and neural differentiation in the brain. STC1 expression in the ovary is also enhanced during pregnancy and lactation. Calcitriol upregulates STC1 and downregulates STC2 expression in the kidney. Interestingly, STC1 and STC2 are expressed in many tumor cell lines, and the expression of STC2 is enhanced by estradiol in breast cancer cells. STC2 is also expressed in pancreatic islets. These results suggest that the biological repertoires of STCs in mammals will be considerably larger than in fish and may not be limited to mineral metabolism. This brief review describes recent progress in mammalian STC research.

Overexpression of human stanniocalcin affects growth and reproduction in transgenic mice

In mammals stanniocalcin (STC) is widely expressed, and in the kidney and gut it regulates serum calcium levels by promoting phosphate reabsorption. To shed further light on its functional significance in mammals we have created several lines of mice that express a human STC (hSTC) transgene. Three lines expressed the hSTC transgene, but only two lines exhibited high expression and contained circulating hSTC, and in these animals there was a reduction in postnatal growth (30-50%) that persisted after weaning. Moreover, even wild-type pups exhibited a growth retardation phenotype when nursed by a transgenic foster mother, and this implies that hSTC overexpression deleteriously affects maternal behavior and/or lactation. The reproductive potential of female transgenic mice was also compromised, as evidenced by significantly smaller litter sizes, but transgenic male fertility was unchanged even though the transgene was most highly expressed in testes. Interestingly, transgene-derived serum hSTC increased significantly after puberty and was severalfold higher in females than in males, suggesting a gender-specific mechanism for maintaining elevated circulating levels of STC. Blood analysis revealed that both transgenic lines had elevated phosphate and decreased alkaline phosphatase levels, indicative of altered kidney and bone metabolism. These studies provide the first evidence that STC is involved in growth and reproduction and reaffirm its role in mineral homeostasis.

Assignment of disulfide linkages in chum salmon stanniocalcin

Stanniocalcin (STC) is a hypocalcemic glycoprotein hormone secreted by the corpuscles of Stannius, endocrine glands unique to the bony fishes. Recently, two cDNAs encoding proteins, STC-1 and STC-2, homologous to fish STC have been identified in humans and rodents. The STC-1, but not STC-2, is identical to fish STC with respect to location of all Cys residues. The present study reports the localization of inter- and intra-molecular disulfide linkages in chum salmon STC. The chum salmon STC was deglycosylated and digested with several proteases in series. Six fragments, each of which consisted of two peptides connected by a single disulfide bond, were isolated by HPLC. Disulfide bonds were determined by sequence analysis after reduction and S-alkylation of each peptide fragment. Chum salmon STC is a homodimer connected by a single intermonomeric disulfide bond at Cys169. The monomer consists of 179 amino-acids, containing five intramonomeric disulfide bonds formed between Cys12-Cys26, Cys21-Cys41, Cys32-Cys81, Cys65-Cys95, and Cys102-Cys137.

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.

The role of the PHEX gene (PEX) in families with X-linked hypophosphataemic rickets

For over a hundred years, the bane of rickets (a disease of bone), has been prominent in those countries that have participated in, and seeded, the industrial revolution. Industrialisation had major effects of the demography of populations, and many people moved to dark, heavily industrialised cities to find work. It soon became apparent that rickets could be cured by supplementing the diet with cod liver oil and exposure to sunlight. This in turn led to the discovery that photoactivation of 7-dehydrocholesterol was required to produce vitamin D, an indispensable regulator of bone mineral metabolism. Although inadequate exposure to light and poor dietary intake are the main causes of rickets and osteomalacia, recent research has confirmed the role of familial, and tumour forms of the disease. This review will describe the recent advances in our knowledge of the molecular defects in X-linked hypophosphataemic rickets (HYP), and oncogenic hypophosphataemic osteomalacia (OHO). Although HYP and OHO have different primary defects, both diseases have similarities that suggest a linked or overlapping pathophysiology. Also, without doubt, the recent cloning of the gene defective in HYP (the PHEX gene), has given researchers a new reagent to explore the molecular regulation of bone and its links to kidney endocrine function. The fact that the PHEX gene codes for a Zn metallopeptidase raises new and intriguing questions, and adds new momentum to the research on diseases of bone mineral metabolism.