Parathyroid growth and suppression in renal failure

In advanced uremia, parathyroid hormone (PTH) levels should be controlled at a moderately elevated level in order to promote normal bone turnover. As such, a certain degree of parathyroid hyperplasia has to be accepted. Uremia is associated with parathyroid growth. In experimental studies, proliferation of the parathyroid cells is induced by uremia and further promoted by hypocalcemia, phosphorus retention, and vitamin D deficiency. On the other hand, parathyroid cell proliferation might be arrested by treatment with a low-phosphate diet, vitamin D analogs, or calcimimetics. When established, parathyroid hyperplasia is poorly reversible. There exists no convincing evidence of programmed parathyroid cell death or apoptosis in hyperplastic parathyroid tissue or of involution of parathyroid hyperplasia. However, even considerable parathyroid hyperplasia can be controlled when the functional demand for increased PTH levels is removed by normalization of kidney function. Today, secondary hyperparathyroidism can be controlled in patients with long-term uremia in whom considerable parathyroid hyperplasia is to be expected. PTH levels can be suppressed in most uremic patients and this suppression can be maintained by continuous treatment with phosphate binders, vitamin D analogs, or calcimimetics. Thus modern therapy permits controlled development of parathyroid growth. When nonsuppressible secondary hyperparathyroidism is present, nodular hyperplasia with suppressed expression of the calcium-sensing receptor (CaR) and vitamin D receptor (VDR) has been found in most cases. An altered expression of some autocrine/paracrine factors has been demonstrated in the nodules. The altered quality of the parathyroid mass, and not only the increased parathyroid mass per se, might be responsible for uncontrollable hyperparathyroidism in uremia and after kidney transplantation.     

Efficacy of 19-Nor-1,25-(OH)2D2 in the prevention and treatment of hyperparathyroid bone disease in experimental uremia

BACKGROUND: The control of parathyroid hyperplasia and high circulating parathyroid hormone (PTH) levels is crucial in preventing secondary hyperparathyroidism (SH) in renal failure. Parathyroid gland enlargement and elevated levels of PTH are major contributors to increase bone resorption, a feature of renal osteodystrophy. METHODS: These studies assessed the efficacy of the 1,25(OH)2D3 analog, 19-Nor-1,25(OH)2D2 (19-Nor), in the prevention (protocol I) and treatment (protocol II) of SH and renal osteodystrophy in uremic rats. In protocol I, normal and uremic rats were fed a high phosphorus diet for 2 months; uremic rats were administered intraperitoneal injections of either vehicle or 19-Nor (200 ng three times a week). In protocol II, normal and uremic rats were fed a high phosphorus diet for 4 months; 2 months after the onset of uremia, rats were administered either intraperitoneal vehicle or 19-Nor (200 ng three times a week). Serum PTH and bone histology were used to assess the degree of SH. RESULTS: 19-Nor was effective in preventing (protocol I) and suppressing (protocol II) the significant SH induced by uremia and further enhanced by a high phosphorus diet. In protocol I, bone histology in uremic controls showed a threefold increase in the cancellous bone mass compared to normal rats. This expansion in unmineralized bone was accompanied by 5-, 1.5-, and 7-fold increases in eroded surface, mineralization lag time (MLT), and bone formation rate (BFR/BS), respectively. Moreover, cortical bone porosity in untreated uremic rats increased 267-fold compared to normal animals. 19-Nor ameliorated these changes in cancellous and cortical bone. In protocol II, the reported indices worsened even further. In contrast, 2 months of 19-Nor treatment improved bone histology by reducing cortical bone porosity, woven bone formation, MLT, and BFR/BS. CONCLUSION: In an experimental model of chronic renal failure (CRF), 19-Nor prevents SH and ameliorates the histomorphometric changes induced by uremia and high phosphorus diet. In addition, 19-Nor suppresses serum PTH and improves bone histology in uremic rats with established severe SH. Further studies in patients with CRF are necessary to define the clinical applicability of 19-Nor on bone histology in humans.     

Pathogenesis of refractory secondary hyperparathyroidism

Calcitriol is currently used to reduce parathyroid hormone (PTH) levels in uremic patients. However, a significant number of patients fail to respond to calcitriol therapy. The data suggest that a poor response to calcitriol can be anticipated in patients with severe hyperparathyroidism (with a high basal PTH levels) and uncontrolled serum phosphate. The abnormal parathyroid response to calcitriol in uremic patients with severe parathyroid hyperplasia may be attributed, to a large extent, to the development of nodular hyperplasia as a result of clonal transformation from a diffuse polyclonal hyperplasia. The factors involved in the development of polyclonal parathyroid hyperplasia, at earlier stages of secondary hyperparathyroidism, appear to be the same factors that stimulate PTH secretion and synthesis: hypocalcemia, hyperphosphatemia and low serum calcitriol levels. Studies performed in vitro using parathyroid tissue from uremic patients who required parathyroidectomy demonstrate that in nodular hyperplasia there is an abnormal response to calcium and calcitriol, which suggests that there are factors intrinsic to the hyperplastic cell (such as decrease in calcium sensor receptors and vitamin D receptors) responsible for an abnormal regulation of parathyroid function. Accumulation of phosphate is a key factor in the pathogenesis of secondary hyperparathyroidism and a poor response to calcitriol treatment is associated with the failure to control the serum phosphorus. High phosphate stimulates PTH secretion as demonstrated by in vivo and in vitro studies. In addition, animal studies strongly suggest that phosphate increases parathyroid cell proliferation. There are growth-related genes potentially involved in uremic hyperparathyroidism; however, changes in the expression of these genes may be the consequence rather than the cause of parathyroid hyperplasia.     

p21WAF1 and TGF-alpha mediate parathyroid growth arrest by vitamin D and high calcium.

BACKGROUND: High dietary phosphorus (P) worsens uremia-induced parathyroid (PT) hyperplasia through increases in the growth promoter transforming growth factor-alpha (TGF-alpha). In contrast, P restriction prevents PT hyperplasia by inducing the cell cycle inhibitor p21. Since 1,25(OH)2D3-antiproliferative action in various cell types involve increases in p21, we studied whether induction of p21 by 1,25(OH)2D3 or the vitamin D analog, 19-Nor-1,25(OH)2D2, could counteract the PT hyperplasia induced by high dietary P in early uremia. METHODS: Normal (N) and uremic (U; 5/6 nephrectomized) female Sprague-Dawley rats were fed high P (HP), low P (LP) or high Ca (HCa) diets and administered intraperitoneally (IP) either vehicle or vitamin D metabolites for seven days, as follows: N-HP; U-HP + vehicle; U-HP + 1,25(OH)2D3 (4 ng/day); U-HP + 19-Nor-1,25(OH)2D2 (30 ng/day); U-LP; U-HCa. Serum PTH and PT gland weight assessed secondary hyperparathyroidism. Immunohistochemical quantitation of two markers of mitotic activity, Ki67 and PCNA measured PT hyperplasia. Immunohistochemical expression of PT p21 and TGF-alpha addressed potential mechanisms regulating PT cell growth. RESULTS: 1,25(OH)2D3 and 19-Nor-1,25(OH)2D2 were effective in suppressing both PTH secretion and PT hyperplasia induced by uremia and high dietary P independent of increases in ionized Ca. Both vitamin D compounds enhanced PT p21 expression and prevented high P-induced increases in PT TGF-alpha content. Induction of PT p21 and reduction of TGF-alpha content also occurred when uremia-induced PT hyperplasia was suppressed by high dietary Ca. CONCLUSIONS: In early uremia, vitamin D suppression of high P-induced PT hyperplasia and high dietary Ca arrest of PT growth involve induction of PT p21 and prevention of increases in TGF-alpha.     

Influence of parathyroid mass on the regulation of PTH secretion

In advanced uremia, parathyroid hormone (PTH) levels should be controlled at a moderately elevated level in order to promote normal bone turnover. As such, a certain degree of parathyroid gland (PG) hyperplasia has to be accepted. No convincing evidence of apoptosis or of involution of PG hyperplasia exists. However, even considerable parathyroid hyperplasia can be controlled when the functional demand for increased PTH levels is abolished. When 20 isogenic PG were implanted into one parathyroidectomized (PTX) rat normalization of Ca(2+) and PTH levels and normal suppressibility of PTH secretion by high Ca(2+) was obtained. Similarly, normal levels of Ca(2+) and PTH and suppressibility of PTH secretion were obtained when Eight isogenic PG from uremic rats were implanted into normal rats or when long-term uremia and severe secondary hyperparathyroidism (sec. HPT) was reversed by an isogenic kidney transplantation. Normalization of PTH levels after experimental kidney transplantation took place despite a persistent decrease of vitamin D receptor (VDR) mRNA and calcium sensing receptor (CaR) mRNA in PG. Thus, in experimental models PTH levels are determined by the functional demand and not by parathyroid mass, per se. When non-suppressible sec. HPT is present in patients referred to PTX, nodular hyperplasia with differences in gene expression between different nodules has been observed in most cases. An altered expression of some autocrine/paracrine factors has been demonstrated in the nodules. Enhanced expression of PTH-related peptide (PTHrP) has been demonstrated in PG from patients with severe secondary HPT. PTHrP has been shown to stimulate PTH secretion in vivo and in vitro. PTH/PTHrP receptor was demonstrated in the parathyroids. The low Ca(2+) stimulated PTH secretion was enhanced by 300% by PTHrP 1-40. The altered quality of the parathyroid mass and not only the increased parathyroid mass, per se, might be responsible for non-controllable hyperparathyroidism in uremia and after kidney transplantation.     

FGF23 Fails to Inhibit Uremic Parathyroid Glands

Fibroblast growth factor 23 (FGF23) modulates mineral metabolism by promoting phosphaturia and decreasing the production of 1,25-dihydroxyvitamin D₃. FGF23 decreases parathyroid hormone (PTH) mRNA and secretion, but despite a marked elevation in FGF23 in uremia, PTH production increases. Here, we investigated the effect of FGF23 on parathyroid function in normal and uremic hyperplastic parathyroid glands in rats. In normal parathyroid glands, FGF23 decreased PTH production, increased expression of both the parathyroid calcium-sensing receptor and the vitamin D receptor, and reduced cell proliferation. Furthermore, FGF23 induced phosphorylation of extracellular signal–regulated kinase 1/2, which mediates the action of FGF23. In contrast, in hyperplastic parathyroid glands, FGF23 did not reduce PTH production, did not affect expression of the calcium-sensing receptor or vitamin D receptor, and did not affect cell proliferation. In addition, FGF23 failed to activate the extracellular signal–regulated kinase 1/2–mitogen-activated protein kinase pathway in hyperplastic parathyroid glands. We observed very low expression of the FGF23 receptor 1 and the co-receptor Klotho in uremic hyperplastic parathyroid glands, which may explain the lack of response to FGF23 in this tissue. In conclusion, in hyperparathyroidism secondary to renal failure, the parathyroid cells resist the inhibitory effects of FGF23, perhaps as a result of the low expression of FGF23 receptor 1 and Klotho in this condition.Fibroblast growth factor 23 (FGF23) is produced by bone cells and plays a fundamental role in the regulation of mineral metabolism. FGF23 inhibits tubular resorption of phosphate and decreases 1α hydroxylase activity, which limits 1,25-dihydroxyvitamin D₃ [1,25(OH)₂D₃] production. Both phosphate excess and high 1,25(OH)₂D₃ stimulate the production of FGF23.¹ FGF23 signals through a widely expressed receptor (FGFR) that becomes functional only in cells expressing the Klotho protein.^2,3 Klotho, which is expressed in the parathyroid cell, converts FGFR1(IIIc), a canonical receptor for various FGFs, into a specific receptor for FGF23. The tissue-specific unique biological activity of FGF23 is likely to be regulated by the limited local distribution of Klotho. In renal failure, the decrease in glomerular filtration causes phosphate retention, which stimulates the production of FGF23. This elevation in FGF23 levels should help to control phosphate in patients with renal failure.⁴Klotho and FGFR are abundantly expressed in parathyroid cells. Some studies^5,6 showed that FGF23 decreases parathyroid hormone (PTH) secretion and PTH mRNA. In dialysis patients, FGF23 levels can reach extremely high values^4,7 but PTH is not reduced; in fact, the highest PTH values correspond to patients with a marked increase in FGF23 levels.⁸ Thus, it is not clear whether FGF23 is capable of reducing PTH production in uremia. Our hypothesis is that there may be a resistance to the action of FGF23 in patients with uremia.This study was designed to evaluate the effect of FGF23 on parathyroid function in normal and hyperplastic parathyroid glands. The study was performed in vivo and in vitro using intact rat parathyroid glands from normal and uremic animals with parathyroid hyperplasia.     

Pathogenesis of parathyroid hyperplasia in renal failure

In chronic kidney disease, secondary hyperparathyroidism (HPTH) is characterized by parathyroid hyperplasia and enhanced synthesis and secretion of parathyroid hormone (PTH). Elevated PTH levels cause renal osteodistrophy and cardiovascular complications, with significantly increased morbidity and mortality in renal failure. The three main direct causes of renal HPTH are hypocalcemia, hyperphosphatemia and vitamin D deficiency. A link between the mechanisms controlling proliferation and hormonal production also exists in normal parathyroid cells which respond to the stimulus of chronic hypocalcemia, not only by an increase in PTH release but also with a consequent parathyroid cell proliferation. The mechanisms responsible for this link, however, remain poorly understood. In this review, we analyze the current understanding concerning the new insights into the molecular mechanisms of parathyroid hyperplasia and PTH secretion in renal failure regulated by calcium, phosphate and vitamin D.     

Reversal of secondary hyperparathyroidism by phosphate restriction restores parathyroid calcium-sensing receptor expression and function.

Secondary hyperparathyroidism (secondary HPT), a common disorder in chronic renal failure (CRF) patients, is characterized by hypersecretion of parathyroid hormone (PTH), parathyroid hyperplasia, and decreased expression of the calcium-sensing receptor (CaR). Dietary phosphate loading promotes secondary HPT, and phosphate restriction prevents and arrests secondary HPT in CRF. This study examined the ability of phosphate restriction to restore parathyroid CaR expression and function. Uremic rats fed a 1.2% P diet for 2 weeks developed secondary HPT with down-regulated CaR expression. Continuation on the 1.2% P diet for 2 more weeks worsened the secondary HPT and further decreased CaR, but switching the rats to a 0.2% P diet for 2 weeks normalized PTH, arrested parathyroid hyperplasia, and restored CaR expression to normal. The calcium-PTH relationship was abnormal in uremic rats fed a high phosphate (HP) diet with a right-shifted calcium set point but was corrected by 2 weeks of phosphate restriction. A time course revealed that following the switch to a low phosphate diet, PTH levels were normalized by day 1, and growth was arrested by day 2, but CaR expression was restored between days 7 and 14. We conclude that although phosphate restriction restores CaR expression and function in parathyroid glands of uremic rats, it is a late event and not involved in the arrest of secondary HPT.     

Evaluation of parathyroid hyperplasia by ultrasonographic examination in patients with end-stage renal failure before and at initiation of dialysis

SUMMARY:   Secondary hyperparathyroidism (2HPT), which is related to renal osteodystrophy (ROD), may occur in patients in the comparatively early stage of chronic renal failure (CRF). Secondary hyperparathyroidism patients with parathyroid hyperplasia showed resistance to vitamin D₃ treatment during long-term dialysis. At present, evaluation by ultrasonography is considered to be useful for confirming parathyroid hyperplasia. There are no clinical data associated with imaging evaluation of 2HPT in CRF patients. In the present study, the relationship among clinical and biochemical data, and parathyroid hyperplasia by ultrasonography, was evaluated in 12 patients (six males and six females) with end-stage renal failure (ESRF) before and at initiation of dialysis. Five patients showed an enlargement of parathyroid glands in ultrasonography. Levels of serum-intact parathyroid hormone (PTH) in patients with parathyroid hyperplasia (positive group) were significantly higher than in those without hyperplasia (negative group; 97.6 ± 36.65 vs 17.4 ± 4.45 pmol/L; P  < 0.05). The levels of intact PTH were above 35.0 pmol/L in all five patients with hyperplasia. All patients in the positive group had never taken vitamin D₃ supplements. Calcium-containing phosphate binders were not prescribed before the present study, except in one patient. Parathyroid hyperplasia caused by 2HPT was recognized in patients before and at initiation of dialysis in this study. It appears that untreated 2HPT in CRF patients may progress to advanced 2HPT in ESRF before and/or after the early stage of dialysis. The levels of serum intact PTH are useful in predicting parathyroid hyperplasia.     

Regulation of parathyroid function in chronic kidney disease (CKD)

In chronic kidney disease (CKD), several abnormalities in bone and mineral metabolism develop in the majority of patients. The parathyroid plays a very important role in regulating bone and mineral metabolism; thus, control of parathyroid function is one of the main targets of the management of CKD-mineral and bone disorder (CKD-MBD). In the development of secondary hyperparathyroidism, it has recently been suggested that fibroblast growth factor 23 (FGF23) plays a crucial role, both as a phosphaturic factor and as a suppressor of active vitamin D (1,25D) production in the kidney. FGF23 is originally secreted to prevent hyperphosphatemia in CKD, but this occurs at the expense of low 1,25D and hyperparathyroidism (“trade-off” hypothesis revisited). Furthermore, recent data suggest that FGF23 could be another useful marker for the prognosis of hyperparathyroidism, because a high serum level may reflect the cumulative dose of vitamin D analogues previously administered. We have also demonstrated that severe hyperparathyroidism was associated with the production and secretion of a new form of parathyroid hormone (PTH) molecule, which can be detected by third-generation assays for PTH, but not by the second-generation assays. For the regression of already established nodular hyperplasia, the more advanced type of parathyroid hyperplasia, it is certainly necessary, in the near future, to develop new agents that specifically induce apoptosis in parathyroid cells. Until such agents are developed, prevention and early recognition of nodular hyperplasia is mandatory for the effective and safe management of hyperparathyroidism in CKD.     

Dissociation of renal cyclic AMP and phosphate responses to parathyroid hormone in uremia

Summary Various investigators have shown that chronic uremia is associated with a normal or exaggerated phosphaturic response to parathyroid hormone (PTH). To explore the relationship between progressive uremia, renal tubular cyclic AMP (cAMP), and inorganic phosphate (Pi) response to PTH and acidosis, in vivo and in vitro experiments were designed in rats with experimental uremia of 4–6 weeks’ duration. Both uremic and pair-fed control rats were treated with 1,25-dihydroxycholecalciferol (1,25(OH)₂D₃) and/or chronic NH₄Cl feeding. Urinary Pi and cAMP and plasma immunoreactive PTH (iPTH) were measured as well as PTH- and NaF-stimulated cAMP from isolated renal tubules. Excretion of cAMP decreased by 30% in uremic as compared to control rats despite a 3-fold rise in Pi excretion. Acidosis superimposed on uremia did not further decrease cAMP excretion, nor did it significantly alter the elevated Pi excretion. 1,25(OH)₂D₃ treatment of uremic rats further lowered cAMP excretion although Pi excretion rose, hypercalcemia occurred, and plasma iPTH fell. In nonuremic control rats, 1,25(OH)₂D₃ treatment led to hypercalcemia, a progressive decrease in cAMP, and increase in Pi excretion. Isolated renal tubules from uremic or acidotic uremic rats revealed a 50% reduction in both PTH- and NaF-stimulated cAMP generation compared to control rat renal tubules. This observation was unchanged by 1,25(OH)₂D₃ treatment. Renal tubules of 1,25(OH)₂D₃-treated control rats demonstrated a decreased cAMP production in response to both PTH and NaF which was inversely related to the calcium content of the renal tubules. Renal tubular calcium levels of uremic rats, initially 3-fold elevated, also increased during 1,25(OH)₂D₃ treatment. These results are consistent with the hypothesis that progressive uremia results in a dissociation between PTH, urinary cAMP, and Pi excretion which cannot be explained by either metabolic acidosis or 1,25(OH)₂D₃ deficiency.     

A new experimental model for secondary hyperparathyroidism

We have developed an animal model to study the pathogenesis of secondary hyperparathyroidism by inducing stable uremia in Sprague-Dawley rats by selective microligation of terminal branches of the left renal artery, followed by right nephrectomy. After 4 weeks the animals were killed, the parathyroid glands were removed and weighed, and blood samples were obtained. Of 30 rats, uremia developed in 22 (73%; uremic group) and eight (27%) died or did not become uremic. A sham-operated group of 15 rats served as control (control group). Creatinine levels were 1.8 +/- 0.5 mg/dl in the uremic group versus 0.5 +/- 0.1 mg/dl in the control group (p less than 0.0001). Parathyroid glands were hyperplastic in all rats with uremia and were heavier than parathyroid glands of control animals (70.3 +/- 26 vs 19.1 +/- 8 micrograms; p less than 0.0001). In the group with uremia, parathyroid hormone levels were increased over those of the control group (112.6 +/- 13 vs 28.9 +/- 6.2 pg/ml; p less than 0.0001), whereas osteocalcin levels were similar (36.6 +/- 11 vs 37.5 +/- 1 ng/ml). Serum calcium, phosphate, and alkaline phosphatase levels were similar in both groups. Our model can be used to test hypotheses concerning the treatment of secondary hyperparathyroidism and the relative pathogenetic relevance of vitamin D deficiency and phosphate retention.