Identification and characterization of cis-acting elements in the human and bovine PTH mRNA 3'-untranslated region

Molecular insights into mineralotropic hormone inter-regulation

The regulation of mineral homeostasis involves the three mineralotropic hormones PTH, FGF23 and 1,25-dihydroxyvitamin D3 (1,25(OH)2D3). Early research efforts focused on PTH and 1,25(OH)2D3 and more recently on FGF23 have revealed that each of these hormones regulates the expression of the other two. Despite early suggestions of transcriptional processes, it has been only recently that research effort have begun to delineate the genomic mechanisms underpinning this regulation for 1,25(OH)2D3 and FGF23; the regulation of PTH by 1,25(OH)2D3, however, remains obscure. We review here our molecular understanding of how PTH induces Cyp27b1 expression, the gene encoding the enzyme responsible for the synthesis of 1,25(OH)2D3. FGF23 and 1,25(OH)2D3, on the other hand, function by suppressing production of 1,25(OH)2D3. PTH stimulates the PKA-induced recruitment of CREB and its coactivator CBP at CREB occupied sites within the kidney-specific regulatory regions of Cyp27b1. PKA activation also promotes the nuclear translocation of SIK bound coactivators such as CRTC2, where it similarly interacts with CREB occupied Cyp27b1 sites. The negative actions of both FGF23 and 1,25(OH)2D3 appear to suppress Cyp27b1 expression by opposing the recruitment of CREB coactivators at this gene. Reciprocal gene actions are seen at Cyp24a1, the gene encoding the enzyme that degrades 1,25(OH)2D3, thereby contributing to the overall regulation of blood levels of 1,25(OH)2D3. Relative to PTH regulation, we summarize what is known of how 1,25(OH)2D3 regulates PTH suppression. These studies suggest that it is not 1,25(OH)2D3 that controls PTH levels in healthy subjects, but rather calcium itself. Finally, we describe current progress using an in vivo approach that furthers our understanding of the regulation of Fgf23 expression by PTH and 1,25(OH)2D3 and provide the first evidence that P may act to induce Fgf23 expression via a complex transcriptional mechanism in bone. It is clear, however, that additional advances will need to be made to further our understanding of the inter-regulation of each of these hormonal genes.

Kidney Failure Alters Parathyroid Pin1 Phosphorylation and Parathyroid Hormone mRNA-Binding Proteins, Leading to Secondary Hyperparathyroidism

BACKGROUND

Secondary hyperparathyroidism (SHP) is a common complication of CKD that increases morbidity and mortality. In experimental SHP, increased parathyroid hormone (PTH) expression is due to enhanced PTH mRNA stability, mediated by changes in its interaction with stabilizing AUF1 and destabilizing KSRP. The isomerase Pin1 leads to KSRP dephosphorylation, but in SHP parathyroid Pin1 activity is decreased and hence phosphorylated KSRP fails to bind PTH mRNA, resulting in high PTH mRNA stability and levels. The up- and downstream mechanisms by which CKD stimulates the parathyroid glands remain elusive.

METHODS

Adenine-rich high-phosphate diets induced CKD in rats and mice. Parathyroid organ cultures and transfected cells were incubated with Pin1 inhibitors for their effect on PTH expression. Mass spectrometry was performed on both parathyroid and PTH mRNA pulled-down proteins.

RESULTS

CKD led to changes in rat parathyroid proteome and phosphoproteome profiles, including KSRP phosphorylation at Pin1 target sites. Furthermore, both acute and chronic kidney failure led to parathyroid-specific Pin1 Ser16 and Ser71 phosphorylation, which disrupts Pin1 activity. Pharmacologic Pin1 inhibition, which mimics the decreased Pin1 activity in SHP, increased PTH expression ex vivo in parathyroid glands in culture and in transfected cells through the PTH mRNA-protein interaction element and KSRP phosphorylation.

CONCLUSIONS

Kidney failure leads to loss of parathyroid Pin1 activity by inducing Pin1 phosphorylation. This predisposes parathyroids to increase PTH production through impaired PTH mRNA decay that is dependent on KSRP phosphorylation at Pin1-target motifs. Pin1 and KSRP phosphorylation and the Pin1-KSRP-PTH mRNA axis thus drive SHP.

Phosphorylation of poly(rC) binding protein 1 (PCBP1) contributes to stabilization of mu opioid receptor (MOR) mRNA via interaction with AU-rich element RNA-binding protein 1 (AUF1) and poly A binding protein (PABP)

Gene regulation at the post-transcriptional level is frequently based on cis- and trans-acting factors on target mRNAs. We found a C-rich element (CRE) in mu-opioid receptor (MOR) 3'-untranslated region (UTR) to which poly (rC) binding protein 1 (PCBP1) binds, resulting in MOR mRNA stabilization. RNA immunoprecipitation and RNA EMSA revealed the formation of PCBP1-RNA complexes at the element. Knockdown of PCBP1 decreased MOR mRNA half-life and protein expression. Stimulation by forskolin increased cytoplasmic localization of PCBP1 and PCBP1/MOR 3'-UTR interactions via increased serine phosphorylation that was blocked by protein kinase A (PKA) or (phosphatidyl inositol-3) PI3-kinase inhibitors. The forskolin treatment also enhanced serine- and tyrosine-phosphorylation of AU-rich element binding protein (AUF1), concurrent with its increased binding to the CRE, and led to an increased interaction of poly A binding protein (PABP) with the CRE and poly(A) sites. AUF1 phosphorylation also led to an increased interaction with PCBP1. These findings suggest that a single co-regulator, PCBP1, plays a crucial role in stabilizing MOR mRNA, and is induced by PKA signaling by conforming to AUF1 and PABP.

PTH and Vitamin D

PTH and Vitamin D are two major regulators of mineral metabolism. They play critical roles in the maintenance of calcium and phosphate homeostasis as well as the development and maintenance of bone health. PTH and Vitamin D form a tightly controlled feedback cycle, PTH being a major stimulator of vitamin D synthesis in the kidney while vitamin D exerts negative feedback on PTH secretion. The major function of PTH and major physiologic regulator is circulating ionized calcium. The effects of PTH on gut, kidney, and bone serve to maintain serum calcium within a tight range. PTH has a reciprocal effect on phosphate metabolism. In contrast, vitamin D has a stimulatory effect on both calcium and phosphate homeostasis, playing a key role in providing adequate mineral for normal bone formation. Both hormones act in concert with the more recently discovered FGF23 and klotho, hormones involved predominantly in phosphate metabolism, which also participate in this closely knit feedback circuit. Of great interest are recent studies demonstrating effects of both PTH and vitamin D on the cardiovascular system. Hyperparathyroidism and vitamin D deficiency have been implicated in a variety of cardiovascular disorders including hypertension, atherosclerosis, vascular calcification, and kidney failure. Both hormones have direct effects on the endothelium, heart, and other vascular structures. How these effects of PTH and vitamin D interface with the regulation of bone formation are the subject of intense investigation.

Analyses of porcine public SNPs in coding-gene regions by re-sequencing and phenotypic association studies

The Porcine SNP database has a huge number of SNPs, but these SNPs are mostly found by computer data-mining procedures and have not been well characterized. We re-sequenced 1,439 porcine public SNPs from four commercial pig breeds and one Korean domestic breed (Korean Native pig, KNP) by using two DNA pools from eight unrelated animals in each breed. These SNPs were from 419 protein-coding genes covering the 18 autosomes, and the re-sequencing in breeds confirmed 690 public SNPs (47.9%) and 226 novel mutations (173 SNPs and 53 insertions/deletions). Thus, totally, 916 variations were found from our study. Of the 916 variations, 148 SNPs (16.2%) were found across all the five breeds, and 199 SNPs (21.7%) were breed specific polymorphisms. According to the SNP locations in the gene sequences, these 916 variations were categorized into 802 non-coding SNPs (785 in intron, 17 in 3'-UTR) and 114 coding SNPs (86 synonymous SNPs, 28 non-synonymous SNPs). The nucleotide substitution analyses for these SNPs revealed that 70.2% were from transitions, 20.0% from transversions, and the remaining 5.79% were deletions or insertions. Subsequently, we genotyped 261 SNPs from 180 genes in an experimental KNP × Landrace F2 cross by the Sequenom MassARRAY system. A total of 33 traits including growth, carcass composition and meat quality were analyzed for the phenotypic association tests using the 132 SNPs in 108 genes with minor allele frequency (MAF)>0.2. The association results showed that five marker-trait combinations were significant at the 5% experiment-wise level (ADCK4 for rear leg, MYH3 for rear leg, Hunter B, Loin weight and Shearforce) and four at the 10% experiment-wise level (DHX38 for average daily gain at live weight, LGALS9 for crude lipid, NGEF for front leg and LIFR for pH at 24 h). In addition, 49 SNPs in 44 genes showing significant association with the traits were detected at the 1% comparison-wise level. A large number of genes that function as enzymes, transcription factors or signalling molecules were considered as genetic markers for pig growth (RNF103, TSPAN31, DHX38, ABCF1, ABCC10, SCD5, KIAA0999 and FKBP10), muscling (HSPA5, PTPRM, NUP88, ADCK4, PLOD1, DLX1 and GRM8), fatness (PTGIS, IDH3B, RYR2 and NOL4) and meat quality traits (DUSP4, LIFR, NGEF, EWSR1, ACTN2, PLXND1, DLX3, LGALS9, ENO3, EPRS, TRIM29, EHMT2, RBM42, SESN2 and RAB4B). The SNPs or genes reported here may be beneficial to future marker assisted selection breeding in pigs.

Minireview: the play of proteins on the parathyroid hormone messenger ribonucleic Acid regulates its expression

PTH regulates serum calcium and phosphate levels and bone strength. The parathyroid is unique in that the trigger for PTH secretion is a low extracellular calcium rather than high calcium as for other hormones. The parathyroid senses small changes in serum calcium through the seven-trans-membrane G protein-coupled calcium receptor to alter PTH secretion. PTH then acts on bone and kidney to correct serum calcium. Parathyroid cells have few secretory granules as compared with other endocrine cells, and therefore PTH production is regulated largely at the levels of PTH gene expression and parathyroid cell proliferation. The regulation of PTH gene expression by changes in calcium and phosphate and in chronic kidney failure is posttranscriptional involving the binding of trans-acting proteins to a defined cis element in the PTH mRNA 3'-untranslated region. These protein-PTH mRNA interactions are orchestrated by the peptidyl-prolyl isomerase Pin1. This review discusses the mechanisms of regulation of PTH mRNA stability determining serum PTH levels and mineral metabolism.

KSRP-PMR1-exosome association determines parathyroid hormone mRNA levels and stability in transfected cells

Background

Parathyroid hormone (PTH) gene expression is regulated post-transcriptionally through the binding of the trans-acting proteins AU rich binding factor 1 (AUF1), Upstream of N-ras (Unr) and KH-type splicing regulatory protein (KSRP) to an AU rich element (ARE) in PTH mRNA 3'-UTR. AUF1 and Unr stabilize PTH mRNA while KSRP, recruiting the exoribonucleolytic complex exosome, promotes PTH mRNA decay.

Results

PTH mRNA is cleaved by the endoribonuclease polysomal ribonuclease 1 (PMR1) in an ARE-dependent manner. Moreover, PMR1 co-immunoprecipitates with PTH mRNA, the exosome and KSRP. Knock-down of either exosome components or KSRP by siRNAs prevents PMR1-mediated cleavage of PTH mRNA.

Conclusion

PTH mRNA is a target for the endonuclease PMR1. The PMR1 mediated decrease in PTH mRNA levels involves the PTH mRNA 3'-UTR ARE, KSRP and the exosome. This represents an unanticipated mechanism by which the decay of an ARE-containing mRNA is facilitated by KSRP and is dependent on both the exosome and an endoribonuclease.

Human PTH gene regulation in vivo using transgenic mice

To study the regulation of the human PTH (hPTH) gene in vivo, we generated transgenic mice with the hPTH gene expressed in the mouse parathyroid using a bacterial artificial chromosome (BAC) containing the hPTH gene within its 144-kb chromosomal region. The BAC construct maintains the native hPTH gene surrounding sequences and isolates it from positional effects. The transgenic mice had normal levels of serum mouse PTH (mPTH) in addition to both intact and bioactive hPTH. Despite the presence of both mPTH and hPTH, serum calcium and 1,25(OH)(2) vitamin D levels were normal. The lack of response to hPTH may be due to tachyphylaxis of the mPTH receptor (PTH1R) and/or impaired recognition of the mPTH1R. In contrast, the regulation of hPTH levels in the mouse was intact. A calcium-depleted diet increased serum mPTH and both intact and bioactive hPTH. mPTH and hPTH mRNA levels were also markedly increased. The calcimimetic R-568 dramatically decreased mPTH and hPTH serum levels. Administered recombinant fibroblast growth factor (FGF)23 decreased hPTH. Therefore, the regulation of hPTH gene expression and serum hPTH levels is intact in the transgenic mice, indicating preservation of the signal transduction of the parathyroid calcium receptor and the Klotho-FGF receptor between mouse and man.

The calcium-sensing receptor regulates parathyroid hormone gene expression in transfected HEK293 cells

Background

The parathyroid calcium receptor determines parathyroid hormone secretion and the response of parathyroid hormone gene expression to serum Ca²⁺ in the parathyroid gland. Serum Ca²⁺ regulates parathyroid hormone gene expression in vivo post-transcriptionally affecting parathyroid hormone mRNA stability through the interaction of trans-acting proteins to a defined cis element in the parathyroid hormone mRNA 3'-untranslated region. These parathyroid hormone mRNA binding proteins include AUF1 which stabilizes and KSRP which destabilizes the parathyroid hormone mRNA. There is no parathyroid cell line; therefore, we developed a parathyroid engineered cell using expression vectors for the full-length human parathyroid hormone gene and the human calcium receptor.

Results

Co-transfection of the human calcium receptor and the human parathyroid hormone plasmid into HEK293 cells decreased parathyroid hormone mRNA levels and secreted parathyroid hormone compared with cells that do not express the calcium receptor. The decreased parathyroid hormone mRNA correlated with decreased parathyroid hormone mRNA stability in vitro, which was dependent upon the 3'-UTR cis element. Moreover, parathyroid hormone gene expression was regulated by Ca²⁺ and the calcimimetic R568, in cells co-transfected with the calcium receptor but not in cells without the calcium receptor. RNA immunoprecipitation analysis in calcium receptor-transfected cells showed increased KSRP-parathyroid hormone mRNA binding and decreased binding to AUF1. The calcium receptor led to post-translational modifications in AUF1 as occurs in the parathyroid in vivo after activation of the calcium receptor.

Conclusion

The expression of the calcium receptor is sufficient to confer the regulation of parathyroid hormone gene expression to these heterologous cells. The calcium receptor decreases parathyroid hormone gene expression in these engineered cells through the parathyroid hormone mRNA 3'-UTR cis element and the balanced interactions of the trans-acting factors KSRP and AUF1 with parathyroid hormone mRNA, as in vivo in the parathyroid. This is the first demonstration that the calcium receptor can regulate parathyroid hormone gene expression in heterologous cells.

Phosphate and the parathyroid

The phosphate (Pi) retention in patients with chronic kidney disease leads to secondary hyperparathyroidism (2HPT). 2HPT is the physiological response of the parathyroid not only to Pi retention but also to decreased synthesis of 1,25(OH)(2) vitamin D, and the attendant hypocalcemia. 2HPT is characterized by increased PTH synthesis, secretion, and parathyroid cell proliferation. Extracellular fluid (ECF) Ca(2+) is recognized by the parathyroid calcium receptor and a small decrease in the ECF Ca(2+) results in relaxation of the calcium receptor and allows the unrestrained secretion and synthesis of PTH and in the longer term, parathyroid cell proliferation. Both 1,25(OH)(2) vitamin D and fibroblast growth factor 23 inhibit PTH gene expression and secretion. Secondary hyperparathyroidism can initially be controlled by a single therapeutic intervention, such as a Pi-restricted diet, a calcimimetic, or an active vitamin D analog. In this review we discuss the mechanisms whereby Pi regulates the parathyroid. Pi has a direct effect on the parathyroid which requires intact parathyroid tissue architecture. The effect of Pi, as of Ca(2+), on PTH gene expression is post-transcriptional and involves the regulated interaction of parathyroid cytosolic proteins to a defined cis acting sequence in the PTH mRNA. Changes in serum Ca(2+) or Pi regulate the activity of trans acting interacting proteins in the parathyroid, which alters their binding to a defined 26 nucleotide cis acting instability sequence in the PTH mRNA 3'-untranslated region. The trans factors are either stabilizing or destabilizing factors and their regulated binding to the PTH cis acting element determines the PTH mRNA half-life. The responses of the parathyroid to changes in serum Pi are now being revealed but the sensing mechanisms remain a mystery.

A translocation causing increased alpha-klotho level results in hypophosphatemic rickets and hyperparathyroidism

Phosphate homeostasis is central to diverse physiologic processes including energy homeostasis, formation of lipid bilayers, and bone formation. Reduced phosphate levels due to excessive renal loss cause hypophosphatemic rickets, a disease characterized by prominent bone defects; conversely, hyperphosphatemia, a major complication of renal failure, is accompanied by parathyroid hyperplasia, hyperparathyroidism, and osteodystrophy. Here, we define a syndrome featuring both hypophosphatemic rickets and hyperparathyroidism due to parathyroid hyperplasia as well as other skeletal abnormalities. We show that this disease is due to a de novo translocation with a breakpoint adjacent to alpha-Klotho, which encodes a beta-glucuronidase, and is implicated in aging and regulation of FGF signaling. Plasma alpha-Klotho levels and beta-glucuronidase activity are markedly increased in the affected patient; unexpectedly, the circulating FGF23 level is also markedly elevated. These findings suggest that the elevated alpha-Klotho level mimics aspects of the normal response to hyperphosphatemia and implicate alpha-Klotho in the selective regulation of phosphate levels and in the regulation of parathyroid mass and function; they also have implications for the pathogenesis and treatment of renal osteodystrophy in patients with kidney failure.

Regulation of parathyroid hormone mRNA stability by calcium, phosphate and uremia

PURPOSE OF REVIEW

This review focuses on the regulation of parathyroid hormone gene expression by dietary-induced hypocalcemia, hypophosphatemia and uremia. Understanding the mechanism by which calcium and phosphate regulate parathyroid hormone gene expression is important for both normal physiology and in pathological states, especially chronic kidney disease.

RECENT FINDINGS

Calcium and phosphate regulate parathyroid hormone secretion, gene expression and, if prolonged, parathyroid cell proliferation. Chronic kidney disease is characterized by a high serum phosphate level that often leads to secondary hyperparathyroidism. In the rat, regulation of parathyroid hormone gene expression by calcium, phosphate and uremia is posttranscriptional, affecting mRNA stability. Differences in binding of protective trans-acting proteins to a conserved protein-binding cis-acting instability element in the parathyroid hormone mRNA 3'-untranslated region alter parathyroid hormone mRNA stability. Two trans-acting proteins - adenosine-uridine rich binding factor 1 and Up-stream of N-ras- stabilize parathyroid hormone mRNA in vivo and in vitro. Parathyroid hormone mRNA also interacts with mRNA decay-promoting proteins and ribonucleases that lead to parathyroid hormone mRNA degradation.

SUMMARY

Calcium, phosphate and uremia determine parathyroid hormone mRNA stability through the binding of the protective factors adenosine-uridine rich binding factor 1 and Up-stream of N-ras and the recruitment of a degradation complex that cleaves parathyroid hormone mRNA.

Mutational analysis of the PTH 3'-untranslated region in parathyroid disorders

OBJECTIVE

Sequence alterations in untranslated regions (UTRs) of genes are important contributors to human diseases, including hereditary thrombophilia, hereditary hyperferritinaemia-cataract and fragile X mental retardation syndromes. Recently, functional studies of the 3'-UTR of the PTH gene, encoding parathyroid hormone, have highlighted it as a potential target for pathogenic mutations in patients with parathyroid dysfunction. Regulation of PTH gene expression occurs in part through protein binding to a specific 26 nucleotide instability element in the 3'-UTR of PTH mRNA, in a sequence-dependent manner. Thus, the PTH 3'-UTR has emerged as an important potential contributor to parathyroid dysfunction. Therefore, we sought to rigorously examine the PTH 3'-UTR in patients with primary and secondary parathyroid disorders, including primary parathyroid hyperplasia, secondary parathyroid hyperplasia, sporadic parathyroid adenoma and familial hypoparathyroidism of unknown genetic basis.

PATIENTS AND DESIGN

Twenty-one parathyroid glands from 14 patients with primary parathyroid hyperplasia, 40 sporadic parathyroid adenomas from 40 patients, 42 parathyroid glands from 29 patients with secondary parathyroid hyperplasia and peripheral blood leucocytes from 24 affected members of eight kindreds with familial hypoparathyroidism of unknown genetic basis were examined for mutations in the 3'-UTR of the PTH gene.

RESULTS

No alterations from the normal sequence were detected in any of the 127 samples examined.

CONCLUSIONS

Based on the absence of identifiable DNA sequence alterations in these forms of parathyroid dysfunction, it is unlikely that mutation of the PTH 3'-UTR contributes frequently to their pathogenesis.

Control of protein expression through mRNA stability in calcium signalling

Specific sequences (cis-acting elements) in the 3'-untranslated region (UTR) of RNA, together with stabilizing and destabilizing proteins (trans-acting factors), determine the mRNA stability, and consequently, the level of expression of several proteins. Such interactions were discovered initially for short-lived mRNAs encoding cytokines and early genes like c-jun and c-myc. However, they may also determine the fate of more stable mRNAs in a tissue and disease-dependent manner. The interactions between the cis-acting elements and the trans-acting factors may also be modulated by Ca(2+) either directly or via a control of the phosphorylation status of the trans-acting factors. We focus initially on the basic concepts in mRNA stability with the trans-acting factors AUF1 (destabilizing) and HuR (stabilizing). Sarco/endoplasmic reticulum Ca(2+) pumps, SERCA2a (cardiac and slow twitch muscles) and SERCA2b (most cells including smooth muscle cells), are pivotal in Ca(2+) mobilization during signal transduction. SERCA2a and SERCA2b proteins are encoded by relatively stable mRNAs that contain cis-acting stability determinants in their 3'-regions. We present several pathways where 3'-UTR mediated mRNA decay is key to Ca(2+) signalling: SERCA2a and beta-adrenergic receptors in heart failure, renin-angiotensin system, and parathyroid hormones. Other examples discussed include cytokines vascular endothelial growth factor, endothelin and endothelial nitric oxide synthase. Roles of Ca(2+) and Ca(2+)-binding proteins in mRNA stability are also discussed. We anticipate that these novel modes of control of protein expression will form an emerging area of research that may explore the central role of Ca(2+) in cell function during development and in disease.

In vitro evidence that upstream of N-ras participates in the regulation of parathyroid hormone messenger ribonucleic acid stability

Calcium and phosphate regulate PTH gene expression posttranscriptionally through the binding of trans-acting factors to a defined cis-acting instability element in the PTH mRNA 3'-untranslated region (UTR). We have previously defined AU-rich binding factor 1 as a PTH mRNA binding and stabilizing protein. We have now identified, by affinity chromatography, Upstream of N-ras (Unr) as another PTH mRNA 3'-UTR binding protein. Recombinant Unr bound the PTH 3'-UTR transcript, and supershift experiments with antibodies to Unr showed that Unr is part of the parathyroid RNA binding complex. Finally, because there is no parathyroid cell line, the functionality of Unr in regulating PTH mRNA levels was demonstrated in cotransfection experiments in heterologous human embryonic kidney 293 cells. Depletion of Unr by small interfering RNA decreased simian virus 40-driven PTH gene expression in human embryonic kidney 293 cells transiently cotransfected with the human PTH gene. Overexpression of Unr increased the rat full-length PTH mRNA levels but not a PTH mRNA lacking the terminal 60-nucleotide cis-acting protein binding region. Unr also stabilized a chimeric GH reporter mRNA that contained the rat PTH 63-nucleotide cis-acting element but not a truncated PTH element. Therefore, Unr binds to the PTH cis element and increases PTH mRNA levels, as does AU-rich binding factor 1. Our results suggest that Unr, together with the other proteins in the RNA binding complex, determines PTH mRNA stability.