No abstract

Most patients with chronic kidney disease develop secondary hyperparathyroidism with disabling systemic complications. Calcimimetic agents are effective tools in the management of secondary hyperparathyroidism, acting through allosteric modification of the calcium-sensing receptor (CaR) on the parathyroid gland (PT) to decrease parathyroid hormone (PTH) secretion and PT cell proliferation. This study showed that rats that were fed an adenine high-phosphorus diet had increased serum PTH and PTH mRNA levels at 7 and 21 d. For studying the effect of activation of the CaR by the calcimimetics R-568 on PTH gene expression, R-568 was given by gavage to uremic rats for the last 4 d of a 7-d adenine high-phosphorus diet. R-568 decreased both PTH mRNA and serum PTH levels. The effect of the calcimimetic on PTH gene expression was posttranscriptional and correlated with differences in protein-RNA binding and posttranslational modifications of the trans acting factor AUF1 in the PT. The AUF1 modifications as a result of uremia were reversed by treatment with R-568 to those of normal rats. Therefore, uremia and activation of the CaR mediated by calcimimetics modify AUF1 posttranslationally. These modifications in AUF1 correlate with changes in protein-PTH mRNA binding and PTH mRNA levels. T he elevated parathyroid hormone (PTH) and disordered mineral metabolism associated with secondary hyperparathyroidism (HPT) complicate the clinical course of most patients with late-stage chronic kidney disease (CKD) and, when advanced, are associated with markedly increased morbidity and mortality (1). The hallmark of secondary HPT is the high levels of circulating PTH, which result from increased PTH secretion, increased PTH gene expression and synthesis, and increased parathyroid gland (PT) cell proliferation (2). The elucidation of PTH gene regulation in CKD is central to the understanding and control of the pathogenesis of secondary HPT (2-4). A limited number of preformed secretory granules contain mature PTH in the PT, and the increased PTH secretion demands the synthesis of new hormone (5-7). Accordingly, treatments that are designed to regulate PTH gene expression and translation may be of substantial clinical benefit.Data from clinical trials have demonstrated that calcimimetic therapy can reduce PTH, serum calcium and phosphorus, and the calcium-phosphorus product (Ca ϫ P) (8,9) and lead to the achievement of Kidney Disease Outcomes Quality Initiative target levels for PTH and Ca ϫ P in many more patients (10). In addition, calcimimetics, which act through the allosteric modulation of the calcium-sensing receptor (CaR), have been shown effectively to decrease PT cell proliferation in a rat model of secondary HPT (11). Therefore, there is considerable interest in determining the mechanisms by which this novel therapeutic class regulates PTH. To date, there have been no reports on the effects of calcimimetics on PTH gene expression. In this study, we examined the effect of the calcimimetic R-568 on PTH mRNA levels, protein-RNA b...

1,25(OH)2D3 decreases parathyroid hormone (PTH) gene transcription through the vitamin D receptor (VDR). Total body VDR(-/-) mice have high PTH levels, hypocalcemia, hypophosphatemia, and bone malformations. To investigate PTH regulation by the VDR specifically in the parathyroid, we generated parathyroid-specific VDR knockout mice (PT-VDR(-/-)). In both strains, there was a decrease in parathyroid calcium receptor (CaR) levels. The number of proliferating parathyroid cells was increased in the VDR(-/-) mice but not in the PT-VDR(-/-) mice. Serum PTH levels were moderately but significantly increased in the PT-VDR(-/-) mice with normal serum calcium levels. The sensitivity of the parathyroid glands of the PT-VDR(-/-) mice to calcium was intact as measured by serum PTH levels after changes in serum calcium. This indicates that the reduced CaR in the PT-VDR(-/-) mice enables a physiologic response to serum calcium. Serum C-terminal collagen crosslinks, a marker of bone resorption, were increased in the PT-VDR(-/-) mice with no change in the bone formation marker, serum osteocalcin, consistent with a resorptive effect due to the increased serum PTH levels in the PT-VDR(-/-) mice. Therefore, deletion of the VDR specifically in the parathyroid decreases parathyroid CaR expression and only moderately increases basal PTH levels, suggesting that the VDR has a limited role in parathyroid physiology.

Serum FGF23 is markedly elevated in chronic kidney disease and has been associated with poor long-term outcomes. FGF23 expression is increased by activation of the FGF receptor 1 (FGFR1) in rats with normal renal function and in vitro in bone-derived osteoblast-like cells. We studied the regulation of FGF23 by FGFR1 in vivo in acute and chronic uremia in mice and rats. Folic acid-induced acute kidney injury increased calvaria FGF23 mRNA and serum FGF23 and parathyroid hormone (PTH) levels at 6 h. The FGFR1 receptor inhibitor PD173074 prevented the folic acid-induced increase in both FGF23 mRNA and serum levels but had no effect on serum PTH levels. A more prolonged uremia due to an adenine high-phosphorus diet for 14 days resulted in high levels of FGF23 mRNA and serum FGF23 and PTH. PD173074 decreased serum FGF23 and mRNA levels with no effect on PTH in the adenine high phosphorus-induced uremic rats. Therefore, a derangement in FGF23 regulation starts early in the course of acute kidney injury, is in part independent of the increase in serum PTH, and involves activation of FGFR1. It is possible that FGFR1 in the osteocyte is activated by locally produced canonical FGFs, which are increased in uremia. This is the first demonstration that activation of FGFR1 is essential for the high levels of FGF23 in acute and chronic experimental uremia.FGF23; FGFR; acute kidney injury; uremia FGF23 IS PRODUCED BY OSTEOCYTES and osteoblasts, binds to its receptor, the fibroblast growth factor receptor 1 (FGFR1)-klotho heterodimer, in the kidney to cause a phosphaturia and decrease the synthesis of 1,25(OH) 2 vitamin D (25, 10). In addition, FGF23 acts on the FGFR1-klotho in the parathyroid to decrease parathyroid hormone (PTH) gene expression and parathyroid cell proliferation (2). In chronic kidney disease (CKD), there are extremely high levels of serum FGF23, which is one of the markers of the increased mortality in these patients (13). FGF23 also acts independently of klotho through FGFR1 and the calcineurin pathway in the heart and parathyroid (6,8,21). The synthesis and secretion of FGF23 by osteocytes are increased by a number of systemic factors (26). PTH acts through the nuclear orphan receptor nurr1, which binds to the FGF23 gene promoter to increase FGF23 transcription by PTH/PKA signaling (17). Conserved Nurr1 elements in the FGF23 proximal promoter and immediately preceding the start site for transcription mediate the increase in FGF23 transcription by PTH (20). FGF23 expression is also enhanced by 1,25(OH) 2 vitamin D, which binds to a vitamin D-response element (VDRE) in the FGF23 promoter (15). One of the Nurr1 elements at the Ϫ200-to Ϫ399-bp region of the mouse FGF23 promoter is part of a VDRE in the FGF23 promoter (14). This vitamin D receptor (VDR)/Nurr1-element mediates the effect of 1,25(OH) 2 vitamin D and is a cisregulatory module anchored by adjacent ETS1, a transcription factor that cooperates with VDR (4, 24), and VDRE/Nurr1 sites (14). Calcium, phosphorus retention in the milieu of CKD, metabolic aci...

Regulation of PTH synthesis and secretion relevant to the management of secondary hyperparathyroidism in chronic kidney disease. Small decreases in serum Ca(++) and more prolonged increases in serum phosphate (P) stimulate the parathyroid (PT) to secrete parathyroid hormone (PTH), while 1,25(OH)(2)-vitamin D(3) decreases PTH synthesis and secretion. A prolonged decrease in serum Ca(++) and 1,25(OH)(2)D(3), or increase in serum P, such as in patients with chronic renal failure, leads to the appropriate secondary increase in serum PTH. This secondary hyperparathyroidism involves increases in PTH gene expression, synthesis, and secretion, and, if chronic, to proliferation of the parathyroid cells. A low serum Ca(++) leads to an increase in PTH secretion, PTH mRNA stability, and parathyroid cell proliferation. Pi also regulates the parathyroid in a similar manner. The effect of Ca(++) on the parathyroid is mediated by a membrane Ca(2+) receptor (CaR). 1,25(OH)(2)D(3) decreases PTH gene transcription. Ca(2+) and P regulate the PTH gene post-transcriptionally by regulating the binding of parathyroid cytosolic proteins, trans factors, to a defined cis sequence in the PTH mRNA 3'-untranslated region (UTR), thereby determining the stability of the transcript. The parathyroid trans factors and cis elements have been defined.