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Calcium and phosphate are essential to many vital physiological processes, making the maintenance of their homeostasis crucial for survival. A tightly controlled balance of calcium and phosphorus is maintained by hormonal control of transport in the intestine, bone, and kidney. The kidneys participate by modulating calcium and phosphate reabsorption from the glomerular filtrate according to the body needs. This process is mediated by ion transporters. Besides the classical endocrine factors (i.e., PTH and vitamin D metabolites), new factors have been identified that are involved in maintaining calcium and primarily phosphate balance. Fibroblast growth factor-23 (FGF23) regulates urinary phosphate excretion by interacting with FGF receptors. Klotho, a transmembrane protein, facilitates this interaction, with the result of reducing phosphate reabsorption by the kidney leading to hypophosphatemia. More recently, dental matrix protein-1, an osteocyte product, has been shown to participate in FGF23-mediated regulation of phosphorus homeostasis. Transgenic mouse models have been of great value in the elucidation of Klotho and FGF23 function. This review highlights current knowledge into calcium and phosphate homeostasis in health and disease.

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Glucocorticoid-induced osteoporosis is the leading cause of medication-induced osteoporosis. The incidence of new fractures after one year of glucocorticoid therapy can be as high as 17%, and observational studies suggest that fractures, which are often asymptomatic, occur in 30-50% of chronic glucocorticoid-treated patients. Fractures can occur within 3 months of initiation of steroid therapy and with daily doses as low as 2.5 mg of prednisone, indicating that there is no "safe dose" of glucocorticoid therapy in terms of skeletal safety. Even inhaled steroids can lead to bone loss, if used for prolonged periods of time. Importantly in glucocorticoid- treated patients, fractures tend to occur at bone mineral density levels that usually carry lower risk in women with post-menopausal osteoporosis, thus implying effects independent of bone mass. Glucocorticoids seem to affect skeletal sites that are mostly composed of trabecular bone, although fractures can occur at cortical sites as well. The combination of high dose, long duration of treatment, and a continuous pattern of administration significantly increase the relative risk of fractures. The rapid and profound bone loss that occurs after initiation of glucocorticoid therapy has some very practical implications with regards to the site and the timing of bone mineral density measurements and fracture prevention strategies.

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Because lifelong exposure to estrogen is a strong determinant of bone mass, we asked whether metabolic conversion of estrogen to either inactive or active metabolites would reflect postmenopausal bone mineral density (BMD) and rate of bone loss. Biochemical markers of inactive estrogen metabolites, urinary 2-hydroxyestrogen (2OHE1) and 2-methoxyestrogen (2MeOE1), and active metabolites, urinary 16alpha-hydroxyestrone (16alphaOHE1), estradiol (E2), and estriol (E3), were determined in 71 untreated, healthy postmenopausal women (age, 47-59 years) followed prospectively for 1 year. Urinary 2MeOE1 was correlated negatively with baseline vertebral (anteroposterior [AP] projection, r = -0.23 andp < 0.05; lateral view, r = -0.27 and p < 0.05) and proximal femur bone density measured by dual-energy X-ray absorptiometry (DXA; total, r = -0.38 and p < 0.01; neck, r = -0.28 and p = 0.02; trochanter, r = -0.44 and p < 0.01). BMDs of women in the lowest quartile of urinary 2MeOE1 (< 15 ng/g) were significantly higher than those in the highest quartile at all skeletal sites (p < 0.05). Likewise, women in the lowest quartile of urinary 2OHE1/16alphaOHE1 ratio (< 1.6) did not experience bone loss after 1 year, in contrast to women in the higher quartiles. We propose that the rate of inactivation of estrogens through 2-hydroxylation may contribute to postmenopausal osteoporosis.

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OBJECTIVE: To describe a patient with galactorrhea and severe hyperprolactinemia in whom workup revealed a nontumoral mechanism. METHODS: We present the medical history of a woman with long-standing diabetes in whom bilateral galactorrhea and hyperprolactinemia developed. In addition, the details of her clinical course and management are reviewed. RESULTS: A 33-year-old woman with diabetes, end-stage renal disease, and gastroparesis was admitted to the hospital because of intractable nausea and vomiting. Several months before admission, she had been noted to have galactorrhea and irregular menses. Routine medications included captopril, verapamil, furosemide, prochlorperazine, metoclopramide, cisapride, and Ortho-Novum. Laboratory evaluation showed normal thyroid function, increased serum prolactin levels (up to 1,197 ng/mL), and normal findings on magnetic resonance imaging of the pituitary. Electrophoresis of the patient's serum on a protein A Sepharose column showed no evidence of macro-prolactinemia. Orally administered medications were discontinued, and the patient was given total parenteral nutrition. These measures resulted in a decrease of 300 ng/mL in serum prolactin levels in 4 days. The prolactin levels eventually normalized after withdrawal of verapamil, prochlorperazine, and metoclopramide. CONCLUSION: A modest increase in serum prolactin level often can be produced by a variety of medications, but gross hyperprolactinemia of 200 ng/mL or higher usually raises suspicion of an underlying prolactin-secreting tumor. This case report demonstrates that conventional limits for nontumoral hyperprolactinemia can be exceeded by concurrent exposure to multiple lactotropic medications in the setting of renal failure.

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Bone-forming cells are organized in a multicellular network interconnected by gap junctions. In these cells, gap junctions are formed by connexin43 (Cx43) and connexin45 (Cx45). Cx43 gap junctions form pores that are more permeable to negatively charged dyes such as Lucifer yellow and calcein than are Cx45 pores. We studied whether altering gap junctional communication by manipulating the relative expression of Cx43 and Cx45 affects the osteoblast phenotype. Transfection of Cx45 in cells that express primarily Cx43 (ROS 17/2.8 and MC3T3-E1) decreased both dye transfer and expression of osteocalcin (OC) and bone sialoprotein (BSP), genes pivotal to bone matrix formation and calcification. Conversely, transfection of Cx43 into cells that express predominantly Cx45 (UMR 106-01) increased both cell coupling and expression of OC and BSP. Transient cotransfection of promoter-luciferase constructs and connexin expression vectors demonstrated that OC and BSP gene transcription was down-regulated by Cx45 cotransfection in ROS 17/2. 8 and MC3T3-E1 cells, in association with a decrease in dye coupling. Conversely, cotransfection of Cx43 in UMR 106-01 cells up-regulated OC and BSP gene transcription. Activity of other less specific osteoblast promoters, such as osteopontin and osteonectin, was less sensitive to changes in gap junctional communication. Thus, altering gap junctional permeability by manipulating the expression of Cx43 and Cx45 in osteoblastic cells alters transcriptional activity of osteoblast-specific promoters, presumably via modulation of signals that can diffuse from cell to cell. A communicating intercellular network is required for the full elaboration of a differentiated osteoblastic phenotype.

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Mechanical loading is essential to maintain skeletal integrity. Because gap junctions in bone are affected by mechanical factors, we studied whether stretch, an anabolic stimulus for osteoblasts, modulates direct intercellular communication in these cells. Gap junctional communication during stretch was assessed using a newly developed method, the "parachute assay," which allows monitoring of dye diffusion without disruption of the plasma membrane. Application of cyclic stretch for 2 or 24 h to well-coupled ROS 17/2.8 cells resulted in a 56.5% and 30.4% increase in dye coupling, respectively, compared with resting conditions. Stretch increased dye diffusion less dramatically (12.4% compared with unstimulated cells) in the poorly coupled UMR 106-01 cells. The stretch-induced increase of cell coupling was abolished in the presence of the gap junctional inhibitor, heptanol. Steady-state mRNA levels of connexin43 (Cx43), the gap junction protein that mediates cell-to-cell diffusion of negatively charged dyes between osteoblasts, were not different between control and stretched ROS 17/2.8 or UMR 106-01 cultures after various periods of cyclic stretch. However, phosphorylated forms of Cx43 protein were more abundant in stretched ROS 17/2.8 than in controls. This was associated with increased punctate Cx43-specific immunostain at appositional membranes of stretched cells. Thus, cyclic stretch increases gap junctional communication between osteoblastic cells by modulating intracellular localization of Cx43.

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Connexin43 (Cx43) forms gap junctions that mediate intercellular communication between osteoblasts. We have examined the effects of prostaglandin E2 (PGE2) and parathyroid hormone (PTH) on gap junctional communication in the rat osteogenic sarcoma cells UMR 106-01. Incubation with either PGE2 or PTH rapidly (within 30 min) increased transfer of negatively charged dyes between UMR 106-01 cells. This stimulatory effect lasted for at least 4 h. Both PGE2 and PTH increased steady-state levels of Cx43 mRNA, but only after 2-4 h of incubation. Transfection with a Cx43 gene construct linked to luciferase showed that this effect of PTH was the result of transcriptional upregulation of Cx43 promoter. Stimulation of dye coupling and Cx43 gene transcription were reproduced by forskolin and 8Br-cAMP. Exposure to PGE2 for 30 min increased Cx43 abundance at appositional membranes in UMR 106-01, whereas total Cx43 protein levels increased only after 4-6 h of incubation with either PGE2 or PTH. Inhibition of protein synthesis by cycloheximide did not affect this early stimulation of dye coupling, but it significantly inhibited the sustained effect of PTH and forskolin on cell coupling. In summary, both PTH and PGE2, presumably through cAMP production, enhance gap junctional communication in osteoblastic cell cultures via two mechanisms: initial rapid redistribution of Cx43 to the cell membrane, and later stimulation of Cx43 gene expression. Modulation of intercellular communication represents a novel mechanism by which osteotropic factors regulate the activity of bone forming cells.

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Giant cell tumors (GCTs) of bone, also called osteoclastomas, complicate Paget bone disease (PBD), though infrequently. Giant cell reparative granulomas (GCRGs), which are histologically similar to GCTs, also occur rarely in pagetic patients. A 45-yr-old black woman with neurofibromatosis, type I, and polyostotic PBD developed slowly-growing masses in the right posterior iliac and left upper parasacral regions. She had multiple cutaneous neurofibromas and café-au-lait spots. Serum alkaline phosphatase activity and urine hydroxyproline levels were elevated. Skeletal radiographs and bone scintigraphy showed changes of widespread PBD. Computerized tomography and magnetic resonance imaging (MRI) delineated masses in the right gluteal and the left lower lumbar paraspinal regions. Five additional smaller masses were found in the abdomen and in the pelvis. Biopsy of the right gluteal mass revealed a GCT. However, we found that this lesion had several histologic features distinct from those of giant cell reparative granulomas or GCT. In our patient's tumor, the huge polykaryons, like osteoclasts, expressed abundant tartrate-resistant acid phosphatase activity, whereas those of GCRG lack this enzyme. Although the polykaryons in conventional GCTs and GCTs in PBD express tartrate-resistant acid phosphatase activity, the location of these tumors in bone differs from the extraskeletal masses encountered in our patient. Furthermore, the larger size of the polykaryons and the greater number of nuclei in our patient's GCT differ from conventional GCTs, but not GCTs in PBD. Her extraskeletal osteoclastoma rapidly shrunk to one third its original size during 2 weeks of oral dexamethasone treatment. Significant clinical improvement lasted about 5 months before additional courses of dexamethasone therapy were necessary. Injections of synthetic salmon calcitonin alone did not affect the tumor's size. Thus, PBD can be complicated by extraskeletal tumors that seem to contain osteoclasts and are responsive to dexamethasone treatment.

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Intermittent administration of parathyroid hormone (PTH) peptides increases bone density in animal and human models of osteoporosis. In vitro studies have demonstrated that PTH analogs lacking the first two amino acids can stimulate cell proliferation in certain cell systems, whereas fragments with an intact N terminus can be antimitogenic. We have tested whether the truncated PTH(3-38) fragment may be a better "anabolic analog" than PTH(1-38) by monitoring bone density and biomechanical properties of the femur in 6-month-old ovariectomized (OVX) rats. Either PTH fragment was administered subcutaneously (8 micrograms/100 g of body weight) 5 days/week, for 4 weeks, starting 1 week after surgery. During the entire study, untreated OVX rats lost 12.1 +/- 4.4% of their initial bone density. PTH(1-38) reversed the initial bone loss, leading to complete restoration of presurgery values after 4 weeks of treatment. Conversely, administration of PTH(3-38) resulted in 13.2 +/- 5.8% bone loss, while continuous estrogen infusion (10 micrograms/kg/day) prevented bone loss but did not reverse it. Sham-operated animals also experienced significant bone loss in the vehicle and PTH(3-38)-treated groups (-4.5 +/- 6.7%, and -7.6 +/- 2.8%, respectively), whereas a significant gain in bone density (+4.4 +/- 5.6%) was observed in the rats treated with PTH(1-38). A bone quality factor (index of strain energy loss) and the impact strength (resistance to fracture) were 25% and 44% lower in femurs explanted from OVX animals treated with either vehicle or PTH(3-38), compared with sham-operated animals. On the contrary, no difference was observed between OVX and control animals after treatment with PTH(1-38), indicating a preservation of the capacity to withstand mechanical stress. Thus, PTH(1-38) counteracts estrogen-dependent loss of mineral density and bone biomechanical properties and increases bone density in estrogen-replete animals. An intact N terminus sequence is necessary for this anabolic action of PTH.

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