Regulation of parathyroid hormone messenger RNA levels by protein kinase A and C in bovine parathyroid cells

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.

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.

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

The human PTH mRNA 3'-UTR has a cis element homologous to the rat cis-acting instability element and a more proximal element identical to the single binding element identified in bovine PTH mRNA 3'-UTR. The function of the elements was shown in vitro.

INTRODUCTION

In the rat, Ca(2+) and phosphate regulate PTH mRNA stability by the interaction of trans-acting proteins with a defined cis-acting instability element in the distal region of the PTH mRNA 3'-untranslated region (UTR). This element has been characterized in the rat and is conserved in human, canine, feline, and murine 3'-UTRs but not in bovine and porcine 3'-UTRs.

MATERIALS AND METHODS

Parathyroid protein-binding assays to the PTH mRNA transcripts were performed. Functionality was studied in reporter genes that were transiently transfected into HEK293 cells.

RESULTS

Protein-RNA binding experiments identified an element in bovine PTH mRNA at the proximal end of the 3'-UTR that is different from the rat protein-binding element. The human 3'-UTR contains both elements, but only the distal element binds proteins. Functional studies with HEK293 cells transiently transfected with reporter genes containing the different elements and flanking nucleotides (nt) showed that the human distal element destabilized a reporter mRNA similar to the effect of this element in the rat. A reporter mRNA containing the single bovine PTH mRNA protein-binding element was also destabilized, and this was prevented by coexpression of AU-rich element binding factor 1 (AUF1).

CONCLUSION

Our results identify a new protein-binding element in the PTH mRNA 3'-UTR. In bovine PTH mRNA, it is the only element, and it is functional in destabilizing a reporter gene. It is also present in other species, including human PTH mRNA, where it is not functional, possibly because of differences in flanking sequences. The human PTH mRNA 3'-UTR distal element is highly homologous to the rat cis-acting instability element and destabilized a reporter gene, indicating its functionality. Therefore, different species have alternative cis-acting protein-binding elements that may determine the regulation of PTH mRNA stability in response to changes in serum calcium and phosphate.

Cell lines and primary cell cultures in the study of bone cell biology

Bone is a metabolically active and highly organized tissue consisting of a mineral phase of hydroxyapatite and amorphous calcium phosphate crystals deposited in an organic matrix. Bone has two main functions. It forms a rigid skeleton and has a central role in calcium and phosphate homeostasis. The major cell types of bone are osteoblasts, osteoclasts and chondrocytes. In the laboratory, primary cultures or cell lines established from each of these different cell types provide valuable information about the processes of skeletal development, bone formation and bone resorption, leading ultimately, to the formulation of new forms of treatment for common bone diseases such as osteoporosis.

Bone ageing: genetics versus environment

Bone ageing results from a complex interaction between genetic and environmental factors (such as diet, climate and physical exercise) throughout human life. According to current literature, the most popular measures of bone ageing are osseometric measurements (OSM), bone mineral density (BMD) and osseographic scores (OSS), based on descriptive criteria of bone age. Plain roentgenography allows simultaneous assessment of all three measures. Ethnic differences with regard to these bone ageing characteristics have prompted us to study to the process anew, with the aim of elucidation the nature of the genetic and environmental components involved, and the possible interaction(s) between them. Despite abundant data on ethnic differences regarding these measures, modern knowledge on the genetics of these processes has derived primarily from the family studies of BMD, which pointed to strong involvement of the familial factors on bone mass. Segregation analysis performed by us in two ethnically different samples of pedigrees revealed a significant effect of the putative major gene on BMD of both compact and cancellous bone. The major finding of our bivariate segregation analysis was that it lead to the acceptance of the hypothesis predicating a single major locus with pleiotropy to both cancellous and compact BMD, but clearly rejecting the polygenic hypotheses. Our study of cortical index (CI) provided evidence that a single potential major gene controls not only the baseline trait level, but also the age at onset of the involutive bone changes, and the rate of the CI change with age. When we examined the environmental vs genetic influences on OSS variation in 32 human populations, we found very little environmental effect on the rate of bone change (r2 = 0.107), but a substantial effect on this rate of the genetic differences between populations (r = 0.480). Clarification of the genetic basis of bone ageing could have wide-ranging applications in the prevention and treatment of bone degenerative diseases such as osteoporosis and osteoarthritis, before irreversible damage takes place. There is thus a need to target the genetic analysis of BMD and the biochemical regulating factors of bone turnover through the use of molecular genetic techniques.

Translational regulation of parathyroid hormone gene expression and RNA: protein interactions

The aim of this study was to investigate the mechanism by which translation of parathyroid hormone (PTH) mRNA is regulated with regard to the subcellular distribution of PTH mRNA and RNA:protein interactions. Sucrose density ultracentrifugation of RNA from bovine parathyroid cells indicated that there was no evidence for a pool of nonribosomal PTH mRNA, and the extracellular calcium concentration had no effect on polysome size. UV cross-linking studies revealed two proteins in parathyroid cell cytosol which bound specifically to the 5'-untranslated region (UTR) of PTH mRNA with molecular masses of 66 and 68 kD while proteins with apparent molecular masses of 48 and 70 kD bound to the 3'-UTR. In vitro translation assays indicated that parathyroid cell cytosol contains factors that inhibit translation of PTH mRNA. Fractionation of cytosol revealed that this effect was associated with proteins within the molecular mass range 30-90 kD. To determine which sequences in PTH mRNA mediate translational regulation, RNA was synthesized from luciferase gene constructs containing the 5'- and/or 3'-UTR of PTH mRNA, and translated in vitro. Addition of parathyroid cell cytosol reduced the translation of RNA containing the 5'- and 3'-UTR of PTH mRNA by 44 +/- 7% but had no effect on the translation of RNA containing only the luciferase coding region. Translation of RNA containing only the 5'-UTR of PTH mRNA was unchanged; however, cytosol reduced the translation of RNA containing the 3'-UTR by 31 +/- 9%. These data demonstrate a role for RNA:protein interactions in the regulation of PTH synthesis and that translational control is mediated primarily through interactions with the 3'-UTR of PTH mRNA.