Reduced vitamin D receptor expression in parathyroid adenomas: implications for pathogenesis

mRNA expression of vitamin D receptor, calcium-sensing receptor, cyclin D1, and PTH in symptomatic and asymptomatic primary hyperparathyroidism

BACKGROUND AND OBJECTIVE

The exact underlying mechanism for the differential clinical profiles of symptomatic and asymptomatic primary hyperparathyroidism (PHPT) patients has not been fully elucidated, and efforts to define the molecular mechanisms underlying the phenotypic heterogeneity of PHPT have been limited. The aim of this study was to explore the underlying molecular mechanisms involved in the pathogenesis of symptomatic and asymptomatic sporadic PHPT in Asian Indians.

METHODS

A prospective cohort study was conducted at a tertiary care hospital in North India. PHPT patients who underwent parathyroidectomy were included. The main outcome was the comparison of vitamin D receptor (VDR), calcium-sensing receptor (CaSR), cyclin D 1 (CD1), and parathyroid hormone (PTH) mRNA levels between symptomatic and asymptomatic PHPT patients and controls determined via quantitative real-time polymerase chain reaction (qRT-PCR).

RESULTS

Forty-two PHPT patients were studied. The mean (SD) age was 49.7 (12.8) years. Twenty patients were asymptomatic. The median PTH levels were significantly greater in the symptomatic group than in the asymptomatic group (878 vs 653 pg/mL). CaSR and VDR mRNAs were significantly lower in both symptomatic and asymptomatic patients than in controls. CD1 and PTH mRNAs were significantly increased in symptomatic patients, but not in asymptomatic PHPT patients compared with controls. Symptomatic PHPT patients had significantly greater CD1 mRNA expression and reduced CaSR expression than asymptomatic patients.

CONCLUSION

Symptomatic PHPT patients had significantly greater CD1 mRNA expression and lower CaSR expression than asymptomatic patients, underscoring the importance of the molecular mechanisms underlying the phenotypic heterogeneity of PHPT.

Sporadic Parathyroid Adenoma: A Pilot Study of Novel Biomarkers in Females

Background and Objectives: Parathyroid adenoma is a distinct cause of primary hyperparathyroidism, with the vast majority being sporadic ones. Proteomic analysis of parathyroid adenomas has proposed a large number of related proteins. The aim of this study is to evaluate the immunohistochemical staining of ANXA2, MED12, MAPK1 and VDR in parathyroid adenoma tissue. Materials and Methods: Fifty-one parathyroid adenomas were analyzed for ANXA2, MED12, MAPK1 and VDR expressions. Tissue was extracted from formalin-fixed paraffin-embedded parathyroid adenoma specimens; an immunohistochemical study was applied, and the percentage of allocation and intensity were evaluated. Results: ANXA2 stained positively in 60.8% of all cell types, while MED12 had positive staining in 66%. MAPK1 expression was found to be negative in total, although a specific pattern for oxyphil cells was observed, as they stained positive in 17.7%. Finally, VDR staining was positive at 22.8%, based on nuclear staining. Conclusions: These immunohistochemical results could be utilized as biomarkers for the diagnosis of sporadic parathyroid adenoma. It is of great importance that a distinct immunophenotype of nodule-forming cells in a positive adenoma could suggest a specific pattern of adenoma development, as in hereditary patterns.

Analysis of Preoperative Predictors of Single and Multigland Primary Hyperparathyroidism

INTRODUCTION

Preoperative differentiation of single-gland (SG) versus multigland (MG) primary hyperparathyroidism (PHPT) can assist with surgical planning, treatment prognostication, and patient counseling. The aim of this study was to identify preoperative predictors of SG-PHPT.

METHODS

Retrospective analysis of 408 patients with PHPT who underwent parathyroidectomy at a tertiary referral center. Comprehensive preoperative parameters, including demographic, laboratory, clinical, and imaging results were analyzed. Univariate analysis and binary logistic regression identified preoperative predictors of SG-PHPT. Receiver operator curves were used to analyze the predictive values of existing and novel preoperative predictive models.

RESULTS

Elevated parathyroid hormone (PTH) (99.1 pg/mL in SG versus 93.0 pg/mL in MG), elevated calcium (10.8 mg/dL in SG versus 10.6 mg/dL in MG), lower phosphate levels (2.80 mg/dL in SG versus 2.95 mg/dL in MG), and positive imaging (ultrasound 75.6% in SG versus 56.5% in MG; sestamibi 70.8% in SG versus 45.5% in MG) were significantly associated with SG-PHPT. The Washington University Score (a predictive scoring system made from calcium, PTH, phosphate, ultrasound, and sestamibi) and the Washington University Index ([calcium × PTH]/phosphate) were comparable to previous scoring systems used to predict SG versus MG-PHPT.

CONCLUSIONS

The association of lower phosphate with SG-PHPT is a novel finding. Previously identified predictors of SG-PHPT, including elevated PTH and positive imaging were confirmed. The Washington University Score and Index are comparable to previously described models and can be used to help surgeons predict if a patient may have SG versus MG-PHPT.

[The role of calcium sensitive and vitamin D receptors in the pathogenesis of sporadic multiple parathyroid gland disease]

BACKGROUND

Sporadic multiple parathyroid gland disease is ¼ cases of primary hyperparathyroidism (PHPT). However, a single tactic for diagnosing and operating volume in patients with this variant of PHPT has not yet been developed. One of the possible directions in the search for pathogenetically substantiated methods of diagnosis and treatment is the study of the molecular genetic features of the disease and associated clinical and laboratory factors.

AIM

To study the features of the expression of calcium sensitive (CaSR) and vitamin D (VDR) receptors on the surface of parathyroid cells in primary hyperparathyroidism with solitary and multiple lesions of the parathyroid glands, as well as its changes under the influence of a decrease in the filtration function of the kidneys.

MATERIALS AND METHODS

In a single center observational prospective study with retrospective data collection, there were patients who during 2019-2021. operated on for PHPT, secondary hyperparathyroidism (SHPT) and all cases of tertiary hyperparathyroidism (THPT) operated during 2014-2021. The expression of CaSR, VDR and its relationship with the main laboratory parameters, the clinical variant of hyperparathyroidism, and the morphological substrate were studied.

RESULTS

The study included 69 patients: 19 with multiple and 25 with solitary PTG near PHPT, 15 with SHPT, 10 with THPT. A statistically significant decrease in the frequency of detection of normal expression of CaSR and VDR receptors occurs in any morphological variant of hyperparathyroidism and is observed in 93-60% of drugs. A decrease in the normal expression of CaSR in hyperplasia is detected statistically significantly less frequently than in adenoma (p≤0.01). The median expression intensity in adenoma was 2.5 (2:3), in hyperplasia 3.5 (3-4) (p≤0.01). The difference in the molecular mechanisms of the development of hyperparathyroidism with a predominance of a morphological substrate in the form of adenoma (PHPT with solitary adenoma) or hyperplasia (SHPT and PHPT with multiple PTG lesions) is realized in the frequency of maintaining normal CaSR expression in the PTG tissue. These mechanisms are implemented at the local level, their variability does not change under the influence of RRT. A common molecular genetic mechanism for the development of hyperparathyroidism with a predominance of a morphological substrate in the form of adenoma or hyperplasia has been found to reduce the frequency of maintaining normal VDR expression in PTG (up to 7-13%), p<0.01. This mechanism is implemented at the local level, its variability changes under the influence of RRT, reaching statistically significant differences in patients with THPT.

CONCLUSION

The study demonstrates the features of changes in the expression of CaSR and VDR in PHPT with multiple lesions of the parathyroid glands. The relationship between the expression of these receptors and the clinical variant of hyperparathyroidism, the morphological substrate, the main laboratory parameters, and renal function was shown.

Large parathyroid adenomas: Potential mechanisms to reconcile adenoma size and disease phenotype

Parathyroid adenomas weighing more than 3.5 g are reported variously as "atypical", "large" or "giant" parathyroid adenomas. All such adenomas are rare variants accounting for no more than 1.5% of all parathyroid adenomas. Large parathyroid adenomas are often associated with more severe form of the disease, including osteitis fibrosa cystica (OFC) and share many biochemical, histological, and molecular features of both benign and malignant parathyroid neoplasms, and are considered a distinct clinical entity. However, the pathogenesis of oversized parathyroid adenomas and the often-associated skeletal phenotype remains unclear. We present 5 cases of primary hyperparathyroidism (PHPT) with OFC, an uncommon manifestation of contemporary PHPT, associated with larger parathyroid adenomas, seen in the Bone and Mineral Disorders Clinic of the Henry Ford Health in the last 30 years to illustrate the critical role of vitamin D nutrition in the pathogenesis of both the OFC and adenoma size. The estimated prevalence of OFC was very low 0.2%, 5 of the >3000 surgically confirmed cases of PHPT seen during this time. The mean ± SD values were: age: 36.8 ± 22.1 years (4 of the 5 <36years), serum calcium 11.6 ± 1.1 mg/dl, alkaline phosphatase 799 ± 487 IU/L, PTH 1440 ± 477 pg/ml, 25-hydroxyvitamin D 13.0 ± 8.9 ng/ml, 1,25-dihyroxyvitamin D 26.5 ± 13.7 pg/ml, urine calcium 562 ± 274 mg/day, and parathyroid adenoma weight 4.53 ± 2.2 g. Parathyroidectomy led to the resolution of both the biochemical indices and OFC in each patient without recurrence over >10 years of follow-up. Because OFC is a very rare in the West, but very common areas of endemic vitamin D deficiency, we also examined the relationship between vitamin D nutrition, as assessed by serum 25-hydroxyvitamin D level, and parathyroid adenoma weight as well as prevalence of OFC in two large secularly diverse cohorts of patients with PHPT (Detroit, USA and Chandigarh, India). Based on this relationship and the relative prevalence of OFC in these two large cohorts, we propose that vitamin D nutrition (and perhaps calcium nutrition) best explains both the adenoma size and prevalence of OFC.

Role of LncRNA score and PVT1 in primary hyperparathyroidism-- a preliminary study

BACKGROUND

Dysregulated lncRNA score and PVT1 expression may be involved in cancer. However, relationships of lncRNAs with hyperparathyroidism and parathyroid cancer (PC) diagnosis remain mysterious.

METHODS

Using quantitative real-time polymerase chain reaction (RT-qPCR), expression profile of PVT1 was evaluated in 57 patients with primary hyperparathyroidism, including 11 with parathyroid cancer (PC) and 46 with parathyroid adenoma (PA).

RESULTS

Higher levels of lncRNA score and PVT1 expression were associated with increased serum calcium level after water ingestion and PC risk (P < 0.05). Accordingly, lncRNA score and PVT1 expression were increased with varying degrees of hypercalcemia in PC. A higher level of lncRNA score (but not PVT1) was an independent risk factor of PC, with an AUC up to 0.872 (95% CI: 0.756-0.945, P < 0.001). Moreover, lncRNA score was more valuable (with AUC 0.974, sensitivity of 85.71% and specificity of 100%, respectively) than intact parathyroid hormone (iPTH) in predicting risk of PC among patients with hypercalcemia (especially based on greater AUC, P = 0.010).

CONCLUSION

Increased lncRNA score is correlated with an elevated level of serum calcium, which may serve as a potential biomarker for PC diagnosis, especially with hypercalcemia.

Reduced Calcium Sensing Receptor (CaSR) Expression Is Epigenetically Deregulated in Parathyroid Adenomas

AIM

Reduced calcium sensing receptor (CaSR) expression has been implicated in parathyroid tumorigenesis, but the underlying mechanism remains elusive. Accordingly, we aimed to explore the epigenetic changes (DNA methylation and histone modifications) involved in CaSR regulation in sporadic parathyroid adenomas and correlate epigenetic state with disease indices.

EXPERIMENTAL DESIGN

Forty sporadic parathyroid adenomas and 10 control parathyroid tissues were studied. Real-time quantitative PCR (qPCR) for mRNA and immunohistochemistry for protein expression of CaSR were performed. The methylation status of the CaSR promoter 2 was determined by bisulphite sequencing analysis of sodium bisulphite-converted DNA. To determine the role of histone modifications in the CaSR regulation, chromatin immunoprecipitation-qPCR assay was performed.

RESULTS

Real-time qPCR revealed reduced CaSR mRNA expression with a fold reduction of 0.12 (P < 0.0001) in parathyroid adenomas. Immunohistochemistry revealed reduced protein expression of CaSR in 90% (36/40) of adenomas. The promoter 2 region of CaSR displayed significant hypermethylation in 45% (18/40) of the adenomas compared with the controls (6.7%; 1 of 10) (P < 0.002). Bisulphite sequencing analysis revealed maximum methylated CpG at glial cell missing 2 binding site on the CaSR promoter 2 compared to other CpG sites. The methylation status of CaSR correlated directly with plasma intact parathyroid hormone levels in patients with parathyroid adenoma. With chromatin immunoprecipitation-qPCR analysis, H3K9me3 levels showed increased enrichment by 10-fold in adenomas and correlated with CaSR-mRNA expression (r = 0.61; P < 0.003). Treatment with 5-aza-2'deoxycytidine restored the expression of CaSR in a parathyroid cell line.

CONCLUSION

Our data suggest that hypermethylation and increased H3K9me3 of the CaSR promoter 2 are involved in silencing CaSR expression in sporadic parathyroid adenoma.

Etelcalcetide Is Effective at All Levels of Severity of Secondary Hyperparathyroidism in Hemodialysis Patients

INTRODUCTION

Calcimimetics improve parameters of secondary hyperparathyroidism (sHPT) but are mostly initiated when patients have severe disease, potentially limiting effectiveness. We evaluated the effects of etelcalcetide on lowering intact parathyroid hormone, calcium, and phosphate at different disease severity levels.

METHODS

This analysis examined data from 2 parallel, phase 3, randomized, placebo-controlled, 26-week trials conducted in 1023 adult (≥18 years old) patients with sHPT on maintenance hemodialysis. Etelcalcetide effects by baseline intact parathyroid hormone stratum (<600, 600-1000, and >1000 ng/l) on mean percentage change in intact parathyroid hormone; changes in calcium and phosphate; and achieving serum intact parathyroid hormone ≤300 ng/l, phosphate <1.78 mmol/l, and both combined, were assessed.

RESULTS

Etelcalcetide reduced serum intact parathyroid hormone by a similar percentage across baseline strata. A similar proportion achieved >30% intact parathyroid hormone reduction across strata for the etelcalcetide arms. Parathyroid hormone increased modestly in each placebo-group stratum, most prominently in the lowest stratum. Serum calcium and phosphate concentrations decreased across strata in etelcalcetide-treated patients, with the most pronounced reductions in patients with highest baseline parathyroid hormone. However, the proportion of patients achieving parathyroid hormone, phosphate, and both targets was highest in the lowest baseline parathyroid hormone stratum, where etelcalcetide dose requirements were lowest. Etelcalcetide dose requirement was lowest among patients in the lowest intact parathyroid hormone stratum.

CONCLUSION

Etelcalcetide effectively lowered serum intact parathyroid hormone, calcium, and phosphate, irrespective of the severity of secondary hyperparathyroidism. The ability to achieve target goals was greatest, and dose requirement smallest, when etelcalcetide was initiated among patients with the lowest level of disease severity.

Alterations of DNA methylation in parathyroid tumors

Parathyroid tumors are common endocrine neoplasias associated with primary hyperparathyroidism, a metabolic disorder characterized by parathormone hypersecretion. Parathyroid neoplasia are frequently benign adenomas or multiple glands hyperplasia, while malignancies are rare. The epigenetic scenario in parathyroid tumors has just begun to be decoded: DNA methylation, histones and chromatin modifiers expression have been investigated so far. The main findings suggest that DNA methylation and chromatin remodeling are active and deregulated in parathyroid tumors, cooperating with genetic alterations to drive the tumor phenotype: the tumor suppressors menin and parafibromin, involved in parathyroid tumorigenesis, interact with chromatin modifiers, defining distinct epigenetic derangements. Many epigenetic alterations identified in parathyroid tumors are common to those in human cancers; moreover, some aspects of the epigenetic profile resemble epigenetic features of embryonic stem cells. Epigenetic profile may contribute to define the heterogeneity of parathyroid tumors and to provide targets for new therapeutic approaches.

An Excess of CYP24A1, Lack of CaSR, and a Novel lncRNA Near the PTH Gene Characterize an Ectopic PTH-Producing Tumor

Thus far, only 23 cases of the ectopic production of parathyroid hormone (PTH) have been reported. We have characterized the genome-wide transcription profile of an ectopic PTH-producing tumor originating from a retroperitoneal histiocytoma. We found that the calcium-sensing receptor (CaSR) was barely expressed in the tumor. Lack of CaSR, a crucial braking apparatus in the presence of both intraparathyroid and, probably, serendipitous PTH expression, might contribute strongly to the establishment and maintenance of the ectopic transcriptional activation of the PTH gene in nonparathyroid cells. Along with candidate drivers with a crucial frameshift mutation or copy number variation at specific chromosomal areas obtained from whole exome sequencing, we identified robust tumor-specific cytochrome P450 family 24 subfamily A member 1 (CYP24A1) overproduction, which was not observed in other non–PTH-expressing retroperitoneal histiocytoma and parathyroid adenoma samples. We then found a 2.5-kb noncoding RNA in the PTH 3′-downstream region that was exclusively present in the parathyroid adenoma and our tumor. Such a co-occurrence might act as another driver of ectopic PTH-producing tumorigenesis; both might release the control of PTH gene expression by shutting down the other branches of the safety system (e.g., CaSR and the vitamin D3–vitamin D receptor axis).

A Possible Link Between Parathyroid Hormone Secretion and Local Regulation of GABA in Human Parathyroid Adenomas

CONTEXT

γ-Aminobutyric acid-B receptor 1 (GABABR1) forms a heterodimeric complex with calcium-sensing receptor (CaSR) in human brain tissue. However, the expression and implication of GABABR1 in human parathyroid adenoma has not yet been examined.

OBJECTIVE

The objective of the study was to examine a possible link between GABABR1 and PTH secretion in human parathyroid adenoma Design and Methods: Sixty-five patients who underwent parathyroidectomy for primary hyperparathyroidism (PHPT) and 29 control patients with normal parathyroid glands were retrospectively included. All patients diagnosed with PHPT had parathyroid adenomas. We evaluated the protein expression of GABABR1, glutamic acid decarboxylase 65/67 (GAD65/67), and various factors proposed as regulators of PTH secretion including CaSR, vitamin D receptor (VDR), CYP24A1, CYP27B1, fibroblast growth factor, and α-klotho in parathyroid tissues from patients with parathyroid adenomas using immunohistochemistry.

RESULTS

Expressions of CaSR, GABABR1, and VDR were significantly lower in PHPT patients than in control subjects (P < .001 for CaSR and GABABR1; P = .006 for VDR). Protein expression of GAD65/67, which indicates local production and regulation of GABAergic pathway, was significantly increased in PHPT (P < .001). There were no significant differences in CYP24A1, CYP27B1, fibroblast growth factor, and α-klotho expression between the two groups. Expression of GAD65/67 was significantly correlated with VDR, CYP24A1, CYP27B1, and α-klotho in PHPT (all P < .01) but not in the control groups. CaSR expression was positively associated with serum phosphorus level (r = 0.274, P = .029) and GAD65/67 was negatively correlated with serum PTH level (r = -0.342, P = .005).

CONCLUSIONS

Local production and action of GABA may be regulated in human parathyroid adenomas. This suggests a possible link between PTH secretion and local regulation of GABA in parathyroid adenomas.

Diffuse parathyroid hormone expression in parathyroid tumors argues against important functional tumor subclones

OBJECTIVE

Primary hyperparathyroidism is usually characterized by a monoclonal parathyroid tumor secreting excess parathyroid hormone (PTH). The main regulator of PTH secretion is calcium and the calcium-PTH set point is shifted in parathyroid tumor cells. We sought to investigate the relationship between tumor PTH and PTH mRNA expression and clinical presentation as well as the regulatory factors including phosphate, vitamin D, and fibroblast growth factor 23.

DESIGN

A total of 154 parathyroid tumors were analyzed by PTH immunohistochemistry and chromogenic in situ hybridization of PTH mRNA. A subset of samples (n = 34) was analyzed using quantitative real-time PCR.

RESULTS

Low tumor PTH mRNA level was significantly associated with low tumor PTH immunoreactivity (P = 0.026), but the two did not correlate with regard to histological distribution within individual tumors. Tumors displaying reduced PTH mRNA levels as compared with normal rim were significantly larger (P = 0.013) and showed higher expression of the calcium-sensing receptor (CASR) (P = 0.046). Weaker tumor PTH mRNA level was significantly associated with higher concentration of circulating 25-hydroxyvitamin D (P = 0.005). No significant correlation was seen between PTH immunoreactivity and patient biochemistry. Tumor weight was strongly associated with circulatory concentrations of calcium and PTH.

CONCLUSIONS

No areas with apparently higher PTH expression were identified, perhaps suggesting that hyper functioning parathyroid tumor subclones should be rare. Circulating 25-hydroxyvitamin D levels may influence tumor PTH expression in vivo. If PTH immunoreactivity reflects the tumor calcium-PTH set point, our data imply that the main determinant of disease severity should be tumor weight.

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.

Clinicopathological correlates of hyperparathyroidism

Hyperparathyroidism is a common endocrine disorder with potential complications on the skeletal, renal, neurocognitive and cardiovascular systems. While most cases (95%) occur sporadically, about 5% are associated with a hereditary syndrome: multiple endocrine neoplasia syndromes (MEN-1, MEN-2A, MEN-4), hyperparathyroidism-jaw tumour syndrome (HPT-JT), familial hypocalciuric hypercalcaemia (FHH-1, FHH-2, FHH-3), familial hypercalciuric hypercalcaemia, neonatal severe hyperparathyroidism and isolated familial hyperparathyroidism. Recently, molecular mechanisms underlying possible tumour suppressor genes (MEN1, CDC73/HRPT2, CDKIs, APC, SFRPs, GSK3β, RASSF1A, HIC1, RIZ1, WT1, CaSR, GNA11, AP2S1) and proto-oncogenes (CCND1/PRAD1, RET, ZFX, CTNNB1, EZH2) have been uncovered in the pathogenesis of hyperparathyroidism. While bi-allelic inactivation of CDC73/HRPT2 seems unique to parathyroid malignancy, aberrant activation of cyclin D1 and Wnt/β-catenin signalling has been reported in benign and malignant parathyroid tumours. Clinicopathological correlates of primary hyperparathyroidism include parathyroid adenoma (80-85%), hyperplasia (10-15%) and carcinoma (<1-5%). Secondary hyperparathyroidism generally presents with diffuse parathyroid hyperplasia, whereas tertiary hyperparathyroidism reflects the emergence of autonomous parathyroid hormone (PTH)-producing neoplasm(s) from secondary parathyroid hyperplasia. Surgical resection of abnormal parathyroid tissue remains the only curative treatment in primary hyperparathyroidism, and parathyroidectomy specimens are frequently encountered in this setting. Clinical and biochemical features, including intraoperative PTH levels, number, weight and size of the affected parathyroid gland(s), are crucial parameters to consider when rendering an accurate diagnosis of parathyroid proliferations. This review provides an update on the expanding knowledge of hyperparathyroidism and highlights the clinicopathological correlations of this prevalent disease.

Changes in parathyroid proteome in patients with primary hyperparathyroidism due to sporadic parathyroid adenomas

PURPOSE

The pathogenesis of parathyroid tumours is only partially understood. A direct approach using proteomics could be a promising tool to increase our understanding of parathyroid tumorigenesis. The aim of the study was to investigate differentially expressed proteins to explore the underlying molecular basis of the disease and identify potential target proteins responsible for the genesis of adenoma.

METHODS

Proteins were extracted from adenomatous and normal parathyroid tissues. Differentially expressed proteins were separated by two-dimensional gel electrophoresis (2-D) and identified by matrix-assisted laser desorption/ionization-time of flight (MALDI-TOF) mass spectrometry. Statistical analysis was performed using spss 10.01 software.

RESULTS

Comparative analysis of the 2-D profiles of proteins isolated from adenomatous and normal parathyroid tissues showed 15 differentially expressed proteins, of which 11 were overexpressed. The characterized proteins were associated with diverse cellular functions including regulation of cell organization, programmed cell death, transcription and signal transduction.

CONCLUSION

The differentially expressed proteins in parathyroid adenomas may potentially serve as new targets to investigate the mechanisms of parathyroid adenoma transformation.

Involvement of α-klotho, fibroblast growth factor-, vitamin-D- and calcium-sensing receptor in 53 patients with primary hyperparathyroidism

The presentation of patients with primary hyperparathyroidism is often atypical and ranges from normocalcemic, primary hyperparathyroidism to severe, symptomatic hypercalcemia. G-protein-coupled, calcium-sensing receptor (CaSR), vitamin D receptor (VDR), and fibroblast growth factor receptor (FGFR)/klotho complexes seem to be involved in the development of pHPT. Parathyroid glands from 53 patients with pHPT and normal parathyroid tissue from 7 patients were obtained during parathyroidectomy. Conventional detailed morphological and immunohistochemical analyses of parathyroid glands were performed after dividing each slide in a 3 × 3 array. From morphology, the number of lipocytes was significantly lower in parathyroid tissue glands in the pHPT group (p < 0.001). Protein expressions of klotho, CaSR, and VDR were significantly reduced in the pHPT compared with the control group (p = 0.004, p = 0.007, p < 0.001). No differences were seen between the two groups (p = 0.35) regarding expression of FGFR. Correlations between expression showed significant positively correlations between klotho and CaSR and FGFR and VDR. No correlations between klotho expression and serum calcium levels could be detected (R = -0.13, p = 0.66), but there were positive correlations between expressions of CaSR/serum phosphate and klotho/serum phosphate. Impaired protein expression of CaSR and VDR seem to be involved in the development of pHPT. The role of the FGFR/klotho-axis remains still unclear. Correlations between protein expression of CaSR and serum phosphate and klotho and serum phosphate levels could be detected. Whether these findings give new insights into the pathogenesis of the disease is yet unknown and has to be elucidated.

Simultaneous expression analysis of vitamin D receptor, calcium-sensing receptor, cyclin D1, and PTH in symptomatic primary hyperparathyroidism in Asian Indians

BACKGROUND

To explore underlying molecular mechanisms in the pathogenesis of symptomatic sporadic primary hyperparathyroidism (PHPT).

MATERIALS AND METHODS

Forty-one parathyroid adenomas from patients with symptomatic PHPT and ten normal parathyroid glands either from patients with PHPT (n=3) or from euthyroid patients without PHPT during thyroid surgery (n=7) were analyzed for vitamin D receptor (VDR), calcium-sensing receptor (CASR), cyclin D1 (CD1), and parathyroid hormone (PTH) expressions. The protein expressions were assessed semiquantitatively by immunohistochemistry, based on percentage of positive cells and staining intensity, and confirmed by quantitative real-time PCR.

RESULTS

Immunohistochemistry revealed significant reductions in VDR (both nuclear and cytoplasmic) and CASR expressions and significant increases in CD1 and PTH expressions in adenomatous compared with normal parathyroid tissue. Consistent with immunohistochemistry findings, both VDR and CASR mRNAs were reduced by 0.36- and 0.45-fold change (P<0.001) and CD1 and PTH mRNAs were increased by 9.4- and 17.4-fold change respectively (P<0.001) in adenomatous parathyroid tissue. PTH mRNA correlated with plasma PTH (r=0.864; P<0.001), but not with adenoma weight, while CD1 mRNA correlated with adenoma weight (r=0.715; P<0.001). There were no correlations between VDR and CASR mRNA levels and serum Ca, plasma intact PTH, or 25-hydroxyvitamin D levels. In addition, there was no relationship between the decreases in VDR and CASR mRNA expressions and the increases in PTH and CD1 mRNA expressions.

CONCLUSIONS

The expression of both VDR and CASR are reduced in symptomatic PHPT in Asian Indians. In addition, CD1 expression was greatly increased and correlated with adenoma weight, implying a potential role for CD1 in adenoma growth and differential clinical expression of PHPT.

Type I membrane klotho expression is decreased and inversely correlated to serum calcium in primary hyperparathyroidism

CONTEXT

The type I membrane protein Klotho was recently shown to mediate PTH secretion in parathyroid cells in response to low extracellular calcium. In contrast, Klotho inhibits PTH secretion indirectly through the action of fibroblast growth factor-23. Abnormal Klotho expression in parathyroid disorders remains to be elucidated.

OBJECTIVE

The aim of the study was to determine: 1) Klotho expression in parathyroid adenomas from patients with primary hyperparathyroidism (pHPT) compared to normal tissue; and 2) its relation to the serum calcium and PTH levels.

DESIGN

Surgically removed parathyroid glands (n = 40) and four normal parathyroid tissue specimens were analyzed for Klotho mRNA and protein levels by quantitative real-time PCR and immunohistochemistry. In vitro effects of calcium on Klotho mRNA expression were studied in bovine parathyroid cells.

RESULTS

Klotho mRNA levels were significantly decreased (n = 23) or undetectable (n = 17) in parathyroid adenomas compared to normal tissues (P < 0.001). Reduced Klotho protein expression was confirmed by immunohistochemistry. Klotho mRNA levels were inversely correlated to serum calcium (r = -0.97; P < 0.0001), and calcium dose-dependently decreased Klotho mRNA expression in normal parathyroid cells in vitro (P < 0.01). Serum calcium was the only significant marker of Klotho expression in multivariate analysis with calcium, phosphate, PTH, and adenoma weight as independent variables.

CONCLUSIONS

Parathyroid Klotho expression is decreased or undetectable in pHPT. We provide evidence that 1) serum calcium is strongly associated with parathyroid Klotho expression in pHPT; and 2) abnormal PTH secretion in hypercalcemic pHPT subjects is mediated by Klotho-independent mechanisms.

Potentiating effects of nonactive/active vitamin D analogues and ketoconazole in parathyroid cells

BACKGROUND AND OBJECTIVE

1,25-dihydroxyvitamin D(3)[1alpha,25(OH)(2)D(3), calcitriol], and its less calcaemic synthetic analogues have therapeutic potential in several diseases, including hyperparathyroidism (HPT). We have suggested that non-1alpha-hydroxylated (nonactive) vitamin D analogues may present an alternative in tumour cells expressing 25-hydroxyvitamin D(3) 1alpha-hydroxylase (1alpha-hydroxylase). The aim of this study was to investigate biological effects of a non-1alpha-hydroxylated vitamin D analogue in normal and tumour parathyroid cells.

PATIENTS AND METHODS

Effects of vitamin D analogues and ketoconazole on parathyroid hormone (PTH) secretion (radioimmunoassay) and PTH mRNA expression (reverse transcription-polymerase chain reaction) were studied in primary bovine parathyroid cells. Proliferation of tumour cells isolated from HPT patients was determined by thymidine incorporation.

RESULTS

EB1285, non-1alpha-hydroxylated precursor of the vitamin D analogue EB1089, suppressed PTH secretion and PTH mRNA level as well as increased expression of 25-hydroxyvitamin D(3)-24-hydroxylase (24-hydroxylase) in bovine parathyroid cells. EB1285 also inhibited cell proliferation of parathyroid tumour cells from primary (pHPT) and secondary HPT (sHPT) patients. Combined treatment with the cytochrome P450-dependent enzyme inhibitor ketoconazole and EB1285 or with active vitamin D compounds potentiated the suppressive effect on PTH secretion from bovine parathyroid cells. Ketaconazole alone displayed PTH suppression and increased 24-hydroxylase expression.

CONCLUSION

The results support the idea that a non-1alpha-hydroxylated vitamin D analogue may elicit vitamin D receptor (VDR) effects in 1alpha-hydroxylase expressing parathyroid tumour cells. Further studies are warranted to elucidate whether precursor vitamin D analogues as well as inhibitors of 24-hydroxylase present therapeutic alternatives in patients suffering from HPT.

Mutational analysis of the vitamin D receptor does not support its candidacy as a tumor suppressor gene in parathyroid adenomas

CONTEXT

The vitamin D receptor gene (VDR) is a compelling candidate tumor suppressor gene for parathyroid adenomas based on existing evidence of the vitamin D system's antiproliferative actions in parathyroid and other tissues, its reported inhibition of PTH gene transcription, and the decreased expression of VDR mRNA and VDR protein observed in parathyroid adenomas.

OBJECTIVE

Because demonstration of intragenic mutations is required to establish the authenticity and primary role in pathogenesis for any candidate tumor suppressor gene, we examined the VDR gene in parathyroid adenomas for the presence of such mutations and other loss-of-function abnormalities.

METHODS AND RESULTS

Genomic DNA samples from 37 sporadic parathyroid adenomas and matched normal control DNA from the same individuals were subjected to direct sequencing of the entire VDR coding region and all intron-exon boundaries. No VDR coding region or junctional mutations were identified. The tumors were also analyzed for loss of heterozygosity, a frequent mechanism of tumor suppressor gene inactivation, by molecular allelotyping at three microsatellite markers located near the VDR gene, D12S85, D12S96, and D12S368, and a polymorphism within VDR itself. In all 37 cases, at least one marker was informative and no tumor-specific loss of heterozygosity was observed.

CONCLUSION

We found no evidence of allelic loss within or near the VDR locus and no mutations within the splice junctions and coding regions of the VDR gene in 37 typical sporadic parathyroid adenomas. Thus, VDR is most unlikely to commonly serve as a classical tumor suppressor gene in sporadic parathyroid adenomas.

Role of calcium-sensing receptor in mineral ion metabolism and inherited disorders of calcium-sensing

The extracellular calcium-sensing receptor (CaR), a G protein-coupled receptor that resides on the parathyroid cell surface negatively regulates secretion of parathyroid hormone (PTH). The CaR is functionally expressed in bone, kidney, and gut--the three major calcium-translocating organs involved in calcium homeostasis. Further studies are needed to define fully the homeostatic roles of the CaR in tissues that are involved in systemic extracellular calcium [Ca(2+)](o) homeostasis. The role of the CaR in regulating calcium metabolism has been greatly clarified by the identification and studies of genetically determined disorders that either activate or inactivate the receptor. Antibodies to the CaR that either activate or inactivate it produce syndromes resembling the corresponding genetic diseases. Expression of the CaR is significantly reduced in primary and secondary hyperparathyroidism, which could contribute to the defective [Ca(2+)](o)-sensing in these conditions. Calcimimetics act as CaR agonists or allosteric activators and thereby potentiate the effects of [Ca(2+)](o) on parathyroid cell function. This kind of pharmacological manipulation of the CaR is now used for the treatment of hyperparathyroid states, whereby the calcimimetics increase the activation of the CaR at any given level of extracellular calcium. Calcimimetics are also an effective element in the treatment of secondary hyperparathyroidism, particularly in dialysis patients, by virtue of reducing plasma levels of PTH, calcium and phosphate.

New Therapies for Uremic Secondary Hyperparathyroidism

Secondary hyperparathyroidism (SHPT) is a common and serious complication of chronic kidney disease (CKD). It affects more than 300,000 end-stage renal disease patients treated by dialysis and probably more than 3 million patients with CKD worldwide. For a long time, traditional therapies for SHPT had consisted of correcting the hypocalcemia using calcium salts and vitamin D derivatives, preventing the hyperphosphatemia by calcium- or aluminum-containing intestinal phosphate binders, and recently by using no metal-containing intestinal phosphate binders; however, these therapies are limited by the occurrence of hypercalcemia, hyperphosphatemia, and the lack of specificity and long-term efficacy. Moreover, surgical parathyroidectomy (PTX), which remains the gold standard therapy, is not exempt from risk. PTX exposes patients to anesthesia risks, presurgical and postsurgical complications, and in many cases a permanent state of hypoparathyroidism. Thus, the medical treatment of SHPT became an ideal target for the development of new therapies and strategies. The purpose of this article is to provide an overview of these new therapies, including vitamin D analogs, intestinal phosphate binders, calcimimetics, parathyroidectomies, tyrosine kinase inhibitors, azydothymidine, anticalcineurins, N-terminal truncated parathyroid hormone fragments, bisphosphonates, calcitonin, osteoprotegerin, and others. The use of these new therapies alone or in combination may help to optimize the future treatment of SHPT in CKD patients.

Vitamin D repletion in patients with primary hyperparathyroidism and coexistent vitamin D insufficiency

Vitamin D insufficiency is common in patients with primary hyperparathyroidism (PHPT) and may be associated with more severe and progressive disease. Uncertainty exists, however, as to whether repletion of vitamin D should be undertaken in patients with PHPT. Here we report the effects of vitamin D repletion on biochemical outcomes over 1 yr in a group of 21 patients with mild PHPT [serum calcium <12 mg/dl (3 mmol/liter)] and coexistent vitamin D insufficiency [serum 25 hydroxyvitamin D [25(OH)D] <20 microg/liter (50 nmol/liter)]. In response to vitamin D repletion to a serum 25(OH)D level greater than 20 microg/liter (50 nmol/liter), mean levels of serum calcium and phosphate did not change, and serum calcium did not exceed 12 mg/dl (3 mmol/liter) in any patient. Levels of intact PTH fell by 24% at 6 months (P < 0.01) and 26% at 12 months (P < 0.01). There was an inverse relationship between the change in serum 25(OH)D and that in intact PTH (r = -0.43, P = 0.056). At 12 months, total serum alkaline phosphatase was significantly lower, and urine N-telopeptides tended to be lower than baseline values (P = 0.02 and 0.13, respectively). In two patients, 24-h urinary calcium excretion rose to exceed 400 mg/d, but the group mean 24-h urinary calcium excretion did not change. These preliminary data suggest that vitamin D repletion in patients with PHPT does not exacerbate hypercalcemia and may decrease levels of PTH and bone turnover. Some patients with PHPT may experience an increase in urinary calcium excretion after vitamin D repletion.

The effect of vitamin D status on the severity of bone disease and on the other features of primary hyperparathyroidism (pHPT) in a vitamin D deficient region

A clear relationship between vitamin D status and the clinical indices of primary hyperparathyroidism (pHPT) severity has not been convincingly established. We proposed that such a relationship might exist, in so far as vitamin D deficiency could contribute to the severity of metabolic bone disease and promote the growth of the parathyroid tumor. Accordingly, we undertook a retrospective study and analyzed the clinical, biochemical, radiological and histopathological findings in a group of 49 patients who underwent parathyroidectomy at our center. Patients who had skeletal X-rays were grouped, according to their X-ray findings, in group A (19 patients; 45%) if they had severe bone changes, or group B (23 patients; 55%) if they had mild or no bone changes. Patients were also stratified according to their 25-hydroxyvitamin D (25-OHD) levels in tertiles. The 2 groups were compared using Fisher's exact test or analysis of variance as appropriate. Group A patients were younger (p=0.001), had more musculoskeletal symptoms (p=0.0003), and complained more frequently of fatigue (p=0.02). They had higher alkaline phosphatase (AP; p=0.0002), PTH index (p=0.0007), and serum Ca level (p=0.006). There were more patients from the lower and middle vitamin D tertiles and fewer patients from the upper vitamin D tertile in group A (p=0.02). Post-operative severe hypo-calcemia was more prevalent in group A patients (p<0.0001). Resected parathyroid tumors were larger in size in group A patients (p=0.01), and weighed more (p=0.01). There was a positive correlation between the weight of the parathyroid tumor and the PTH index (p=0.002), and AP level (p=0.0007). We concluded that vitamin D deficiency is a contributing factor to both the severity of bone disease and the high activity of parathyroid tumors seen in many patients with pHPT in vitamin D deficient regions.

Central and peripheral immunoreactivity of melanin-concentrating hormone in hypothalamic obese and lactating rats

Melanin-concentrating hormone (MCH) is believed to be an important orexigenic peptide mainly localized in the lateral hypothalamic area. Its involvement in the hyperphagia induced by hypothalamic lesions and lactation remains unclear. In this study, we investigated MCH immunoreactivity in the hypothalamus using immunohistochemistry and MCH concentration in the peripheral circulation using an enzyme immunoassay in rats with a lesion in the ventromedial hypothalamus or the paraventricular nucleus, and in lactating rats. Bilateral lesions of the ventromedial or paraventricular nuclei were performed using an electrolytic method. Quantification of immunoreactivity by image analysis revealed that the number and mean staining intensity of MCH-immunoreactive neurones in the lateral hypothalamic area and the zona incerta were significantly decreased by both types of lesions compared to sham controls, whereas circulating MCH concentration was not significantly different on day 7 postlesion. By contrast, in lactating rats on days 11-12 postpartum, the expression of MCH in the lateral hypothalamic area and the zona incerta was significantly increased compared to nonlactating controls. Circulating MCH concentration was not changed in lactating rats. These results suggest that hyperphagia induced by lactation, but not hypothalamic lesion, might be induced by excessive expression of MCH in the lateral hypothalamic area and the zona incerta.

Calcium sensing by endocrine cells

The elucidation of the structure and function of the Ca2+(o)-sensing receptor (CaR) has provided important insights into the normal control of Ca2+(o) homeostasis, particularly the key role of the receptor in kidney and parathyroid. Further studies are needed to define more clearly the homeostatic role of the CaR in additional tissues, both those that are involved and those that are uninvolved in systemic Ca2+(o) homeostasis. The availability of the cloned CaR has also permitted documentation of the molecular basis of inherited disorders of Ca2+(o) sensing, including those in which the receptor is less and or more sensitive than normal to Ca2+(o). Antibodies to the CaR that either activate it or inactivate it produce syndromes resembling the corresponding genetic diseases. Expression of the receptor is abnormally low in 1 degree and 2 degrees hyperparathyroidism, which could contribute to the defective Ca2+(o) sensing in these conditions. The recent discovery of calcimimetics, which sensitize the CaR to Ca2+(o), has provided what will likely be an effective medical therapy for the secondary/tertiary hyperparathyroidism of end stage renal failure as well as for 1 degree hyperparathyroidism.

Orexin-A immunoreactivity and prepro-orexin mRNA expression in hyperphagic rats induced by hypothalamic lesions and lactation

Orexins are endogenous neuropeptides that potently facilitate appetite and food consumption. In the present study, we examined orexin immunoreactivity and prepro-orexin mRNA expression in the lateral hypothalamus by immunohistochemistry and competitive reverse transcription-polymerase chain reaction (RT-PCR) methods in different models of hyperphagia in rats. Hyperphagia was induced by lesions of either the ventromedial hypothalamus (VMHL) or the paraventricular nucleus (PVNL), and we also compared lactating rats to nonlactating controls. Both VMHL and PVNL increased food intake and body weight compared to shams. On day 7 post lesion, serum leptin and insulin concentrations exhibited 3.2- and 2.8-fold increases in VMHL rats, and nonsignificant 1.8- and 1.8-fold increases in PVNL rats; there were significant decreases (48% and 33%) in lactating rats on day 12 postpartum compared to controls, respectively. Serum glucose concentrations were not significantly changed compared to controls in these rats. Quantification by image analysis suggests that VMHL significantly decreased the number and mean staining intensity of orexin-A immunoreactive neurones compared to those in the sham-lesioned group; while PVNL did not change orexin-A immunoreactivity. Competitive RT-PCR analysis showed that VMHL significantly decreased the prepro-orexin mRNA expression compared to those in the sham-lesioned group, and PVNL did not change it. Lactating rats on days 11-12 of lactation had significantly greater number and mean staining intensity of orexin-A immunoreactive neurones, prepro-orexin mRNA expression food intake and body weight than nonlactating postpartum rats. Thus, changes in orexin-A immunoreactivity and prepro-orexin mRNA expression were not consistent between the hyperphagia models. These results suggest that the hyperphagia from VMHL or PVNL and lactating rats differ in their involvement of orexin-A, and the change in circulating leptin and insulin concentrations may be involved in the change of orexin-A immunoreactivity in these rats.

Age-related changes in the 25-hydroxyvitamin D versus parathyroid hormone relationship suggest a different reason why older adults require more vitamin D

Vitamin D requirements are thought to vary with age, but there is little comparative evidence for this. One goal in establishing a vitamin D requirement is to avoid secondary hyperparathyroidism. We studied 1741 euthyroid, thyroid clinic outpatients without evidence of calcium abnormalities, ranging in age from 19 to 97 yr, whose serum and urine had been analyzed for calcium, vitamin D, and parathyroid status. We found no effect of age on the 25-hydroxyvitamin D [25(OH)D] concentration associated with specific vitamin D intakes, and there was no relationship between 25(OH)D and 1,25hydroxyvitamin D [1,25(OH)2D]. In every age group, serum 1,25(OH)2D declined with increasing creatinine (P < 0.001). What changed with age included creatinine, which correlated with 25(OH)D (r = 0.146, P < 0.001) only in the youngest age group (19-50 yr) but not in the older age groups (P > 0.1). Creatinine did not correlate with PTH in the youngest age group, but the relationship became significant as age increased (e.g. for the elderly, r = 0.365, P < 0.001). Linear regression of log PTH vs. log 25(OH)D agreed with the natural shape of the relationship observed with scatterplot smoothing, and this showed no plateau in PTH as 25(OH)D increased. We compared PTH concentrations among age groups, based on 20 nmol/liter increments in 25(OH)D. Mean PTH in adults older than 70 yr was consistently higher than in adults younger than 50 yr (P < 0.05 by ANOVA and Dunnett's t test). PTH levels of the elderly who had 25(OH)D concentrations greater than 100 nmol/liter matched PTH of younger adults having 25(OH)D concentrations near 70 nmol/liter. This study shows that all age groups exhibit a high prevalence of 25(OH)D insufficiency and secondary hyperparathyroidism. Older adults are just as efficient in maintaining 25(OH)D, but they need more vitamin D to produce the higher 25(OH)D concentrations required to overcome the hyperparathyroidism associated with their diminishing renal function.

Familial hypocalciuric hypercalcemia caused by an R648stop mutation in the calcium-sensing receptor gene

In this study, we report an 84-year-old female proband in a Japanese family with familial hypocalciuric hypercalcemia (FHH) caused by an R648stop mutation in the extracellular calcium-sensing receptor (CaR) gene. At the age of 71 years, she presented with hypercalcemia (11.4 mg/dl), hypocalciuria (Cca/Ccr = 0.003), hypermagnesemia (2.9 mg/dl), and a high-serum parathyroid hormone (PTH) level (midregion PTH, 3225 [160-520] pg/ml). At the age of 74 years, a family screening was carried out and revealed a total of 9 hypercalcemic individuals (all intact PTH values <62 pg/dl) among 17 family members tested, thus, being diagnosed as FHH. Two and one-half of three clearly enlarged parathyroid glands were resected, because persistently high PTH levels (intact PTH, 292 pg/ml; midregion PTH, 5225 pg/ml) and the presence of a markedly enlarged parathyroid gland by several imaging modalities (ultrasonography, computed tomography [CT], magnetic resonance imaging [MRI], and subtraction scintigraphy) suggested coexistent primary hyperparathyroidism (pHPT); however, hypercalcemia persisted postoperatively. Histological and immunohistochemical examination revealed that the resected parathyroid glands showed lipohyperplasia as well as normally expressed Ki67, vitamin D receptor (VDR), and the CaR. Sequence analysis disclosed that the proband and all affected family members had a heterozygous nonsense (R648stop) mutation in the CaR gene. This mutation is located in the first intracellular loop; thus, it would be predicted to produce a truncated CaR having only one transmembrane domain (TMD) and lacking its remaining TMDs, intracellular loops, and C-terminal tail. Western analysis of biotinylated HEK293 cells transiently transfected with this mutant receptor showed cell surface expression of the truncated protein at a level comparable with that of the wild-type CaR. The mutant receptor, however, exhibited no increase in intracellular free calcium concentration (Ca2+i) when exposed to high extracellular calcium concentrations (Ca2+o). The proband's clinical course was complicated because of associated renal tubular acidosis (RTA) and nephrotic syndrome. However, it was unclear whether their association affected the development of elevated serum PTH and parathyroid gland enlargement. This report is the first to show that an R648stop CaR mutation yields a truncated receptor that is expressed on the cell surface but is devoid of biological activity, resulting in FHH.

Human calcium-sensing receptor gene. Vitamin D response elements in promoters P1 and P2 confer transcriptional responsiveness to 1,25-dihydroxyvitamin D

The calcium-sensing receptor (CASR), expressed in parathyroid chief cells, thyroid C-cells, and cells of the kidney tubule, is essential for maintenance of calcium homeostasis. Here we show parathyroid, thyroid, and kidney CASR mRNA levels increased 2-fold at 15 h after intraperitoneal injection of 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) in rats. Human thyroid C-cell (TT) and kidney proximal tubule cell (HKC) CASR gene transcription increased approximately 2-fold at 8 and 12 h after 1,25(OH)2D3 treatment. The human CASR gene has two promoters yielding alternative transcripts containing either exon 1A or exon 1B 5'-untranslated region sequences that splice to exon 2 some 242 bp before the ATG translation start site. Transcriptional start sites were identified in parathyroid gland and TT cells; that for promoter P1 lies 27 bp downstream of a TATA box, whereas that for promoter P2, which lacks a TATA box, lies in a GC-rich region. In HKC cells, transcriptional activity of a P1 reporter gene construct was 11-fold and of P2 was 33-fold above basal levels. 10(-8) m 1,25(OH)2D3 stimulated P1 activity 2-fold and P2 activity 2.5-fold. Vitamin D response elements (VDREs), in which half-sites (6 bp) are separated by three nucleotides, were identified in both promoters and shown to confer 1,25(OH)2D3 responsiveness to a heterologous promoter. This responsiveness was lost when the VDREs were mutated. In electrophoretic mobility shift assays with either in vitro transcribed/translated vitamin D receptor and retinoid X receptor-alpha, or HKC nuclear extract, specific protein-DNA complexes were formed in the presence of 1,25(OH)2D3 on oligonucleotides representing the P1 and P2 VDREs. In summary, functional VDREs have been identified in the CASR gene and provide the mechanism whereby 1,25(OH)2D up-regulates parathyroid, thyroid C-cell, and kidney CASR expression.