Takehisa Kawata

Parathyroid hormone regulates fibroblast growth factor-23 in a mouse model of primary hyperparathyroidism.

The importance of fibroblast growth factor 23 (FGF-23) in the pathogenesis of phosphate wasting disorders has been established, but controversy remains about how parathyroid hormone (PTH), which also stimulates urinary phosphate excretion, regulates the circulating level of FGF-23. We found that the serum FGF-23 concentration was higher in PTH-cyclin D1 transgenic mice, a model of primary hyperparathyroidism, than in wild-type mice. The serum FGF-23 concentration was significantly and directly correlated with serum PTH and calcium, and inversely correlated with phosphate levels in 90- to 118-week-old mice (all P < 0.005). Quantitative real-time reverse-transcriptase PCR revealed abundant expression of fgf23 in bone, especially in calvaria. The fgf23 expression in calvaria was significantly higher in the transgenic mice compared to the wild-type mice, and correlated well with serum FGF-23 levels. There was a direct correlation between the expression of fgf23 and the expression of osteocalcin and ALP, suggesting that activation of osteoblasts is important in the regulation of FGF-23. Serum FGF-23 levels decreased in the transgenic mice after parathyroidectomy. In conclusion, PTH plays a major role in the regulation of serum FGF-23 level in primary hyperparathyroidism, likely via activation of osteoblasts in bone.

Therapeutic Effects of Anti‐FGF23 Antibodies in Hypophosphatemic Rickets/Osteomalacia

X‐linked hypophosphatemia (XLH), characterized by renal phosphate wasting, is the most common cause of vitamin D‐resistant rickets. It has been postulated that some phosphaturic factor plays a causative role in XLH and its murine homolog, the Hyp mouse. Fibroblast growth factor 23 (FGF23) is a physiological phosphaturic factor; its circulatory level is known to be high in most patients with XLH and Hyp mice, suggesting its pathophysiological role in this disease. To test this hypothesis, we treated Hyp mice with anti‐FGF23 antibodies to inhibit endogenous FGF23 action. A single injection of the antibodies corrected the hypophosphatemia and inappropriately normal serum 1,25‐dihydroxyvitamin D. These effects were accompanied by increased expressions of type IIa sodium‐phosphate cotransporter and 25‐hydroxyvitamin‐D‐1α‐hydroxylase and a suppressed expression of 24‐hydroxylase in the kidney. Repeated injections during the growth period ameliorated the rachitic bone phenotypes typically observed in Hyp mice, such as impaired longitudinal elongation, defective mineralization, and abnormal cartilage development. Thus, these results indicate that excess actions of FGF23 underlie hypophosphatemic rickets in Hyp mice and suggest a novel therapeutic potential of the FGF23 antibodies for XLH.

Cinacalcet suppresses calcification of the aorta and heart in uremic rats

High serum parathyroid hormone levels are associated with vascular calcification. Cinacalcet is a calcimimetic agent that inhibits parathyroid hormone secretion and is used to treat patients with secondary hyperparathyroidism. Here we measured the effects of oral cinacalcet on calcification of the aorta and heart in rats with a remnant kidney (5/6 nephrectomy) model of uremia that were fed a high-phosphate diet containing lactose to accelerate the process of aortic calcification. Alizarin red staining showed that the smooth muscle in the aortic arch of rats with a remnant kidney was calcified. The tissue levels of calcium and phosphorus in the aorta and hearts of these rats were significantly increased compared to sham-operated rats. Expression of the osteoblastic lineage genes osteocalcin, osteopontin and runt-related gene 2 were also increased in the aorta of these rats. Cinacalcet suppressed these calcification-related changes by reducing serum parathyroid hormone, calcium, phosphorus, and the calcium-phosphorus product. Parathyroidectomy also suppressed calcification in this model. We suggest that cinacalcet inhibits calcification of the aorta and heart in uremic patients with secondary hyperparathyroidism by decreasing serum parathyroid hormone levels.

Direct in vitro evidence of the suppressive effect of cinacalcet HCl on parathyroid hormone secretion in human parathyroid cells with pathologically reduced calcium-sensing receptor levels

Clinical studies have been performed to determine the effect of cinacalcet HCl (cinacalcet), an allosteric modulator of the calcium-sensing receptor (CaR), on primary hyperparathyroidism (PHPT) and secondary hyperparathyroidism of uremia (SHPT). However, no in vitro studies on human parathyroid cells have been reported to date. In this study, the inhibitory effect of cinacalcet on PTH secretion was analyzed in primary cultured parathyroid cells obtained from patients. The investigation involved three PHPT and three SHPT patients subjected to therapeutic parathyroidectomy. Notably, all SHPT patients were resistant to intravenous vitamin D analogue therapy. Removed parathyroid tumors were used for immunohistochemistry and parathyroid cell primary culture. Immunohistochemical analyses revealed diminished expression of CaR and vitamin D receptor (VDR) in all parathyroid tumors. PTH secretion from cultured parathyroid cells of PHPT and SHPT patients was suppressed by extracellular Ca2+ and cinacalcet in a dose-dependent manner. Rates of suppression of PTH secretion in PHPT and SHPT by cinacalcet (1000 nmol/l) were 61% ± 21% and 61% ± 19%, respectively. Cinacalcet demonstrates significant potency in the suppression of PTH secretion in primary cultured human parathyroid cells in vitro, despite reduced levels of the target protein, CaR. Data from this in vitro analysis support the clinical application of cinacalcet in PHPT and SHPT therapy.

Animal models of hyperfunctioning parathyroid diseases for drug development

Background: Disorders of mineral and bone metabolism have been implicated as a risk factor in the high mortality in patients with chronic kidney disease (CKD). Hyperphosphatemia, disorders of vitamin D metabolism and secondary hyperparathyroidism of uremia (SHPT) are therapeutic targets in these patients to improve the mortality. Animal models for CKD are indispensable and uremic rats produced by 5/6-nephrectomies are one of the most useful animal models for the development of new therapeutic agents. As there are limitations of uremic rats such as short lifespan and less severity of secondary hyperparathyroidism distinct from CKD patients on maintenance hemodialysis, the development of new model animals is expected. Objective: This review discusses the molecular pathogenesis of hyperfunctioning parathyroid diseases and the applications of animal models exhibiting hyperparathyroidisms in the aspect of the development of new therapeutics. Conclusion: PTH–cyclin D1 transgenic mice, with parathyroid-targeted overexpression of cyclin D1 oncogene, not only developed abnormal parathyroid cell proliferation but, notably, also developed biochemical hyperparathyroidism with characteristic abnormalities in bone. The mice exhibit age-dependent development of biochemical hyperparathyroidism, which enables testing of the drug precisely. In addition, the mice develop parathyroid cell hyperplasia, followed by monoclonal expansion, which is observed in refractory SHPT patients.

Regulatory Mechanisms of Circulating Fibroblast Growth Factor 23 in Parathyroid Diseases

Abstract:  Fibroblast growth factor‐23 (FGF‐23) is a circulating factor regulating phosphate and vitamin D homeostasis. Phosphate intake and an administration of 1,25‐dihydroxyvitamin D3 (1,25(OH)2D3) increase circulating FGF‐23 levels, and elevated FGF‐23 prevents hyperphosphatemia and vitamin D toxicity by hyperphosphaturia and suppression of circulating 1,25(OH)2D level, comprising a feedback loop to maintain phosphate and vitamin D homeostasis. Excess secretion of parathyroid hormone (PTH), another phosphaturic factor, elevates the serum calcium level in primary hyperparathyroidism. PTH also elevates circulating FGF‐23 level, which cooperatively enhances the phosphaturia, resulting in hypophosphatemia.

Cinacalcet in hyperfunctioning parathyroid diseases.

The calcium‐sensing receptor (CaR) on the parathyroid cell surface senses ionized calcium concentration in the extracellular fluid and regulates minute‐to‐minute parathyroid hormone (PTH) secretion. Synthetic allosteric modulators of CaR have been developed, and one of the positive modulators (calcimimetics) is cinacalcet HCl or cinacalcet. Cinacalcet increases the sensitivity of CaR to be activated by extracellular calcium, and thus suppresses PTH release. Cinacalcet is an effective treatment for secondary hyperparathyroidism (SHPT) in patients with uremia on hemodialysis. In this review, based on basic experiments using cinacalcet, we postulate the beneficial effects of cinacalcet on hyperfunctioning parathyroid diseases.

Magnesium and Osteoporosis

Magnesium is the second most abundant cation in bone and has been recognized as one of essential elements of bone. It is estimated that 1300g of calcium, 14g of magnesium, and 600g of phosphorus exist in bones of a 70-kg human adult.1 Basic structure of bone is composed of hydroxyapatite [Ca5(OH)(PO4)3], but Newman proposed that magnesium is incorporated in bone crystal in the following formula: [Ca9 ++(H2O+)2(PO4 ≡)6(OH−)2][Ca++ · Mg0.3 ++ · Na0.3 + · CO3 − · Cit0.3].2