IL-6 negatively regulates IL-11 production in vitro and in vivo.

IL-6 and IL-11 are two cytokines that increase osteoclast formation and augment bone resorption. PTH stimulates the production of both cytokines by human osteoblast-like cells. Circulating levels of IL-6 are elevated in patients with states of PTH excess and correlate strongly to markers of bone resorption. In contrast, serum levels of IL-11 were significantly reduced in patients with primary hyperparathyroidism compared with values in euparathyroid controls. Further, after successful parathyroid adenomectomy, circulating levels of IL-6 fell, whereas IL-11 levels increased. Five-day infusions of human PTH-(1--84) in rodents resulted in a significant decline in mean circulating levels of IL-11, whereas IL-6 levels significantly increased. Pretreatment of cells and mice with neutralizing serum to IL-6 enhanced PTH-induced IL-11 production compared with the effect of pretreatment with nonimmune sera. These data indicate that IL-6 negatively regulates IL-11 production in vivo and in vitro. Analysis of steady state mRNA levels in SaOS-2 cells indicated that this effect is posttranscriptional. As both IL-6 and IL-11 stimulate osteoclast formation, down-regulation of IL-11 by IL-6 may help modulate the resorptive response to PTH.

Parameters of high bone-turnover predict bone loss in renal transplant patients: a longitudinal study.

BACKGROUND: Osteoporosis is a serious complication of kidney transplantation. Various factors have been postulated to contribute to posttransplant bone loss, among them treatment with corticosteroids, the use of cyclosporine and cyclosporine-like agents, and persistent hyperparathyroidism. In a previous cross-sectional study of long-term renal transplant recipients, we observed that osteoporosis or osteopenia was present in 88% of patients. Because biochemical markers of bone formation (serum osteocalcin) and bone resorption (urine pyridinoline, PYD, and deoxypyridinoline, DPD) were elevated in the majority of study subjects, we hypothesized that elevated rates of bone-turnover contribute to posttransplant bone loss in long-term renal transplant patients. This study was performed to examine this hypothesis. METHODS: The study population was composed of 62 patients who were more than 1-year postrenal transplantation and who had preserved renal function. They were followed prospectively for 1 year. Biochemical markers of bone-turnover were measured at study entry, and patients were classified as having high bone-turnover based on elevated urinary levels of at least one marker of bone resorption (i.e., PYD or DPD) and/or serum osteocalcin (group 1). If none of these were present, they were classified as having normal bone-turnover (group 2). Bone mineral density (BMD) was measured by dual energy x-ray absorptiometry (DEXA) at time of entry into the study and again after 1 year of follow-up. The changes in BMD at the lumbar spine, hip, and wrist over the period of the study were compared between the high and normal bone-turnover groups. RESULTS: Forty-three patients (69%) were classified as having high bone-turnover (Group 1), and 19 patients (31%) were classified as having normal bone-turnover (Group 2). There was a statistically significant difference in change in BMD between the two groups at the lumbar spine (-1.11+/-0.42%, high bone-turnover, vs. 0.64+/-0.54%, normal bone-turnover; P=0.02) and the hip (-0.69+/-0.38%, high bone-turnover, vs. 1.36+/-0.66%, normal bone-turnover; P=0.006). Whereas group 2 had stable bone mass, group 1 exhibited bone loss at these skeletal sites. CONCLUSIONS: Our results indicate that bone loss is greater in renal transplant recipients with elevated biochemical markers of bone-turnover, suggesting that these markers may be useful in identifying patients at risk for continued bone loss. These data support the hypothesis that continued bone loss in long-term renal transplant recipients is associated with high bone-turnover. If accelerated bone resorption does play a role in posttransplant bone loss, this would provide a strong rationale for use of antiresorptive therapy for the prevention and treatment of this complication.

Parathyroid hormone induces hepatic production of bioactive interleukin-6 and its soluble receptor.

Interleukin-6 (IL-6) is an important mediator of parathyroid hormone (PTH)-induced bone resorption. Serum levels of IL-6 and its soluble receptor (IL-6sR) are regulated in part by PTH. The PTH/PTH-related protein type 1 receptor is highly expressed in the liver, and in the current study we investigated whether the liver produces IL-6 or IL-6sR in response to PTH. Perfusion of the isolated rat liver with PTH-(1-84) stimulated rapid, dose-dependent production of bioactive IL-6 and the IL-6sR. These effects were observed at near physiological concentrations of the hormone such that 1 pM PTH induced hepatic IL-6 production at a rate of approximately 0.6 ng/min. In vitro, hepatocytes, hepatic endothelial cells, and Kupffer cells, but not hepatic stellate cells, were each found to produce both IL-6 and IL-6sR in response to higher (10 nM) concentrations of PTH. Our data suggest that hepatic-derived IL-6 and IL-6sR contribute to the increase in circulating levels of these cytokines induced by PTH in vivo and raise the possibility that PTH-induced, liver-derived IL-6 may exert endocrine effects on tissues such as bone.

Estrogen modulates parathyroid hormone-induced interleukin-6 production in vivo and in vitro.

Interleukin (IL)-6 promotes osteoclastogenesis and is thought to play a role in the bone loss that follows estrogen withdrawal. In vitro studies have demonstrated that IL-6 is produced in response to PTH by cells in the osteoblast lineage and that PTH-induced bone resorption is inhibited by a neutralizing antibody to the IL-6 receptor. In addition, we have recently reported that IL-6 plays a role in PTH-induced bone resorption in humans with chronic PTH excess and in experimental animals during the short-term infusion of PTH. In the current study, we examined whether estrogen withdrawal augments PTH-induced IL-6 production. When cultured in the absence of estrogen, human osteosarcoma cells (Saos-2) treated with PTH demonstrated significantly greater release of IL-6 than cells grown under estrogen-replete conditions, 30-fold vs. 15-fold (P = 0.005). A similar effect but of lesser magnitude was seen with primary human osteoblasts. In vivo, PTH induced IL-6 production was also increased in the estrogen-deficient state (ovx) such that at the end of a 5-day PTH infusion, the mean circulating level of IL-6 was significantly higher in ovx vs. sham/ovx mice (60.1 vs. 16.9 pg/ml; P < 0.0001). The greater increase in circulating levels of IL-6 in PTH-treated ovx mice was paralleled by a greater rise in bone resorption markers with the mean level of urine collagen cross-links in the PTH-treated ovx group being more than 2.5-fold higher than in the PTH-treated sham/ovx animals (236 vs. 88.5 microg/mmol creatinine, P < 0.0001). Mean serum collagen cross-link values were 17.4 microg/liter in PTH-treated ovx vs. 7.4 microg/liter in PTH-treated sham/ovx animals (P < 0.0001). Treatment of animals with estrogen prevented the exaggerated response to PTH infusion such that the increase in both circulating levels of IL-6 and bone turnover markers in estrogen-treated animals were similar to those observed in sham/ovx animals and significantly lower than those in PTH-treated ovx animals. These findings may help to explain the increased skeletal sensitivity to the resorbing effects of PTH seen in the estrogen-deficient state.

A threshold for low-protein-diet-induced elevations in parathyroid hormone.

BACKGROUND: We reported previously that lowering dietary protein intake in young healthy women to 0.7 g/kg depressed intestinal calcium absorption and was accompanied by elevations in parathyroid hormone (PTH). Moderate amounts of dietary protein (1.0 g/kg) did not appear to perturb calcium homeostasis. OBJECTIVE: The purpose of this study was to evaluate the effect of graded intakes of dietary protein (0.7, 0.8, 0.9, and 1.0 g/kg) on calcium homeostasis. DESIGN: The experiment consisted of 2 wk of a well-balanced diet containing moderate amounts of calcium, sodium, and protein followed by 4 d of an experimental diet containing 1 of 4 amounts of protein. Eight young healthy women received the 4 amounts of protein in random order. The average age of the subjects was 23.1 +/- 2.3 y, their weight was 64 +/- 3 kg, and their body mass index (in kg/m(2)) was 24.3 +/- 0.9. RESULTS: Elevations in PTH developed by day 4 of the diets containing 0.7 and 0.8 g protein/kg but not during the diets containing 0.9 or 1.0 g protein/kg. By day 4 of the 0.7- and 0. 8-g/kg diets, midmolecule PTH, calcitriol, and nephrogenous cyclic adenosine monophosphate were 1.5-3.5-fold higher than on day 0. Calcitropic hormones on day 4 of the diets containing 0.8 and 0.9 g protein/kg were within the normal range and 23-57% lower than values observed with the 0.7- and 0.8-g/kg diets (P < 0.005). Mean 24-h urinary calcium was 3.29 +/- 0.35 mmol with the diet containing 0.7 g protein/kg and 3.54 +/- 0.46 mmol with the diet containing 1.0 g protein/kg. CONCLUSIONS: Our data suggest that in young healthy women consuming a well-balanced diet, the current recommended dietary allowance for protein (0.8 g/kg) results in short-term perturbations in calcium homeostasis.

Posttransplant bone disease: evidence for a high bone resorption state.

Loss of bone is a significant problem after renal transplant. Although bone loss in the first post transplant year has been well documented, conflicting data exist concerning bone loss after this time. It is equally unclear whether bone loss in long-term renal transplant recipients correlates with bone turnover as it does in postmenapausal osteoporosis. To examine these issues, we conducted a cross-sectional study to define the prevalence of osteoporosis in long-term (> 1 year) renal transplant recipients with preserved renal function (mean creatinine clearance 73 +/- 23 ml/min). Bone mineral density (BMD) was measured at the hip, spine and wrist by DEXA in 69 patients. Markers for bone formation (serum osteocalcin) and bone resorption [urinary levels of pyridinoline (PYD) and deoxypyridinoline (DPD)] were also measured as well as parameters of calcium metabolism. Correlations were made between these parameters and BMD at the various sites. The mean age of the patients was 45 +/- 11 years. Eighty eight percent of patients were on cyclosporine (12% on tacrolimus) and all but 2 were on prednisone [mean dose 9 +/- 2 mg/day)]. Osteoporosis (BMD more than 2.5 SD below peak adult BMD) at the spine or hip was diagnosed in 44% of patients and osteopenia was present in an additional 44%. Elevated levels of intact parathyroid hormone (i PTH) were observed in 81% of patients. Elevated urinary levels of PYD or DPD were present in 73% of patients and 38% had elevated serum levels of osteocalcin. Levels of calcium, and of 25(OH) and 1,25(OH)2 vitamin D were normal. In a stepwise multiple regression model that included osteocalcin, PYD, DPD, intact PTH, age, years posttransplant, duration of dialysis, cumulative prednisone dose, smoking, and diabetes: urinary PYD was the strongest predictor of bone mass. These results demonstrate that osteoporosis is common in long-term renal transplant recipients. The data also suggest that elevated rates of bone resorption contribute importantly to this process.

A role for interleukin-6 in parathyroid hormone-induced bone resorption in vivo.

Parathyroid hormone (PTH) exerts its regulatory effects on calcium homeostasis in part by stimulating the release of calcium from the skeleton. PTH stimulates bone resorption indirectly, by inducing the production by stromal/osteoblastic cells of paracrine agents which recruit and activate the bone-resorbing cell, the osteoclast. The identity of the stromal cell/osteoblast-derived paracrine factor(s) responsible for mediating the effects of PTH on osteoclasts is uncertain. Recently, it has been demonstrated that the cytokine interleukin-6 (IL-6), which potently induces osteoclastogenesis, is produced by osteoblastic cells in response to PTH. Further, we have reported that circulating levels of IL-6 are elevated in patients with primary hyperparathyroidism, and correlate with biochemical markers of bone resorption. Thus, IL-6 may play a permissive role in PTH-induced bone resorption. In the current studies, we demonstrate that low-dose PTH infusion in rodents increased serum levels of IL-6, coincident with a rise in biochemical markers of bone resorption. In mice, both acute neutralization and chronic deficiency of IL-6 were associated with markedly lower levels of biochemical markers of bone resorption in response to PTH infusion than were observed in animals with normal IL-6 production. Acute neutralization of IL-6 did not affect PTH-induced changes in markers of bone formation. These findings demonstrate that PTH regulates systemic levels of IL-6 in experimental animals, that IL-6 is an important mediator of the bone-resorbing actions of PTH in vivo and suggest that IL-6 plays a role in coupling PTH-induced bone resorption and formation.

Changes in bone turnover in young women consuming different levels of dietary protein.

Although high protein diets are known to increase urinary calcium excretion and induce negative calcium balance, the impact of dietary protein on bone turnover and fractures is controversial. We therefore evaluated the effect of dietary protein on markers of bone turnover in 16 healthy young women. The experiment consisted of 2 weeks of a well balanced diet containing moderate amounts of calcium, sodium, and protein followed by 4 days of an experimental diet containing one of three levels of protein (low, medium, or high). On day 4, serum and urinary calcium, serum PTH, 1,25-dihydroxyvitamin D, serum osteocalcin, bone-specific alkaline phosphatase, and urinary N-telopeptide excretion were measured. Urinary calcium excretion was significantly higher on the high than on the low protein diet. Secondary hyperparathyroidism occurred on the low protein diet. Urinary N-telopeptide excretion was significantly greater during the high protein than during the low protein intake (48.2 +/- 7.2 vs. 32.7 +/- 5.3 nM bone collagen equivalents/mM creatinine; P < 0.05). There was no increase in osteocalcin or bone-specific alkaline phosphatase when comparing the low to the high diet, suggesting that bone resorption was increased without a compensatory increase in bone formation. Our data suggest that at high levels of dietary protein, at least a portion of the increase in urinary calcium reflects increased bone resorption.

Dissociation of bone formation from resorption during 2-week treatment with human parathyroid hormone-related peptide-(1-36) in humans: potential as an anabolic therapy for osteoporosis.

PTH administration increases bone mass in rodents and in humans. PTH-related protein (PTHrP) binds to and signals via the skeletal PTH receptor. Administration of PTHrP on a once daily basis increases bone mineral content in rats. In humans, PTHrP-(1-36) is equipotent to PTH-(1-34) and is active when administered s.c. These findings suggest that PTHrP might have therapeutic benefit in the treatment of osteoporosis. In this study, 13 postmenopausal estrogen-deficient women received a single daily s.c. dose of PTHrP-(1-36) for a 14-day period to determine whether PTHrP-(1-36) 1) could be given in doses that do not alter systemic mineral homeostasis, but increase markers of bone turnover; and 2) is tolerated without adverse effects. Daily s.c. PTHrP-(1-36) administration caused no significant changes in serum calcium or phosphorus concentrations, fractional calcium excretion, the tubular maximum for phosphorus, fractional calcium excretion, or plasma 1,25-dihydroxyvitamin D concentrations. Nephrogenous cAMP and endogenous PTH-(1-84) declined. Importantly, markers of bone formation trended upward, as reported in subjects treated with PTH. In marked contrast to findings in PTH-treated subjects, in PTHrP-treated subjects, markers of bone resorption declined in a highly significant fashion. These observations indicate that PTHrP-(1-36) treatment uncouples bone formation from resorption, in favor of formation. This uncoupling, if it were to continue over the longer term, would predict that PTHrP-(1-36) might be a potent anabolic therapeutic agent for osteoporosis.

Colony-stimulating factor-1 induces cytoskeletal reorganization and c-src-dependent tyrosine phosphorylation of selected cellular proteins in rodent osteoclasts.

Colony-stimulating factor-1 (CSF-1) stimulates motility and cytoplasmic spreading in mature osteoclasts. Therefore, we examined the cellular events and intracellular signaling pathways that accompany CSF-1-induced spreading in normal osteoclasts. To explore the role c-src plays in these processes, we also studied osteoclasts prepared from animals with targeted disruption of the src gene. In normal osteoclasts, CSF-1 treatment induces rapid cytoplasmic spreading, with redistribution of F-actin from a well-delineated central attachment ring to the periphery of the cell. CSF-1 increases membrane phosphotyrosine staining in osteoclasts and induces the phosphorylation of several cellular proteins in cultured, osteoclast-like cells, including c-fms, c-src, and an 85-kD Grb2-binding protein. Src kinase activity is increased threefold after CSF-1 treatment. In src- cells, no attachment ring is present, and CSF-1 fails to induce spreading or a change in the pattern of F-actin distribution. Although c-fms becomes phosphorylated after CSF-1 treatment, the 85-kD protein is significantly less phosphorylated in src- osteoclast-like cells. These results indicate that c-src is critical for the normal cytoskeletal architecture of the osteoclast, and, in its absence, the spreading response induced by CSF-1 is abrogated, and downstream signaling from c-fms is altered.

Increased circulating concentrations of parathyroid hormone in healthy, young women consuming a protein-restricted diet.

Increasing dietary protein induces hypercalciuria and a negative calcium balance. Despite this, the influence of dietary protein on the parathyroid hormone (PTH) I-a-hydroxylase axis is not well understood. We therefore examined the effect of three amounts of dietary protein: low (0.7 g/kg), medium (1.0 g/kg), and high (2.1 g/kg) on mineral metabolism and the PTH-1-alpha-hydroxylase axis in 16 healthy women aged 26.7 +/- 1.3 y. By day 4, urinary calcium decreased significantly with the low-protein diet and increased significantly with the high-protein diet compared with the medium-protein diet (control). Also by day 4, there were striking elevations in serum PTH and calcitriol [1,25-dihydroxyvitamin D] in subjects consuming the low-protein diet. Serum PTH, by two different assays, was 1.5-2.4 times higher and by day 14 1.6-2.7 times higher during the low-protein diet compared with the medium-protein diet. This was accompanied by a significant increase in both nephrogenous cyclic adenosine monophosphate (cAMP), a sensitive and specific indicator of PTH bioactivity, and serum calcitriol by day 14. In comparison, there were relatively minor changes in the calcitropic hormones with the medium- and high-protein diets. The stimulus for the elevation in PTH induced by protein restriction is unclear, but probably does not involve a simple renal mechanism and could reflect either a decline in intestinal calcium absorption, a reduction of bone turn-over, or both. Our data indicate that dietary protein is a powerful regulator of calcium metabolism. Further study is needed to both clarify the mechanisms by which these changes are induced and to better define the amount of dietary protein that will optimize skeletal health in young women.

Parathyroid hormone increases circulating levels of fibronectin in vivo: modulating effect of ovariectomy.

To explore the effect of PTH on circulating levels of fibronectin (FN), adult female Sprague-Dawley rats were implanted with Alzet minipumps prepared to deliver 7 pmol/h x kg BW of either human PTH (1-34) or human PTH (1-84). Both forms of the hormone led to significant and progressive increases in circulating levels of FN over the 72-h study period (P < 0.001). However, at every time point, circulating levels of FN with human PTH (hPTH) (1-84) infusion were significantly higher than with hPTH (1-34), such that at the end of the infusion, mean levels in the hPTH (1-34) group were 32.2 +/- 1.4 ng/ml, in the hPTH (1-84) group 93.8 +/- 5.4 ng/ml, and in the vehicle infused group 14.6 +/- 0.7 ng/ml. The greater agonist efficacy of hPTH (1-84) was not explained by differences in circulating levels of the hormones, and both forms of the hormone were equipotent at stimulating cAMP production by ROS 17/2.8 cells. However, hPTH (1-84) remained a more effective agonist than hPTH (1-34) at stimulating FN production in these cells (P < 0.001). Nephrectomy did not blunt the ability of PTH to increase circulating FN in vivo, indicating that the kidney was not the source of the FN produced in response to PTH. Pretreament with the potent bisphosphonate APD to block bone resorption also did not blunt the in vivo response to PTH. Parathyroidectomy did not blunt the response. Cultured fetal rat bones showed a significant 2.4-fold increase in FN production when treated with PTH. Consistent with our earlier in vitro studies (Endocrinology, 135: 1639-1644, 1994), estrogen deficiency, induced by ovariectomy, significantly diminished the ability of PTH to increase circulating FN levels in vivo (P < 0.001). We conclude that PTH increases circulating levels of FN in vivo and may be a physiologic regulator for the plasma form of this glycoprotein. The effects of PTH on circulating FN may reflect the anabolic properties of the hormone in bone and the blunted response following estrogen withdrawal could be a manifestation of the diminished bone formation vis-à-vis resorption seen in the estrogen deficient state.

Parathyroid hormone-related protein-(1-36) is biologically active when administered subcutaneously to humans.

PTH-related protein (PTHrP) is responsible for most cases of humoral hypercalcemia of malignancy (HHM). It mimics the actions of PTH as a result of its structural homology with PTH and its ability to bind to and signal via the PTH/PTHrP receptor in bone and kidney. PTHrP-(1-36) appears to be one of several secretory forms of PTHrP. This peptide has been administered iv to normal volunteers previously and has been shown to produce effects that are qualitatively and quantitatively the same as those produced by PTH-(1-34). To determine whether PTHrP-(1-36) could be used sc in humans as a diagnostic reagent for elucidating the differences between HHM and hyperparathyroidism, we performed a 12-h dose-finding study examining whether sc PTHrP-(1-36) could elicit effects on mineral homeostasis. PTHrP-(1-36) administered sc in three doses (0.82, 1.64, and 3.28 micrograms/kg) to 21 normal women produced increases in circulating PTHrP-(1-36), reductions in serum phosphorus and the renal phosphorus threshold, increments in fractional calcium excretion and nephrogenous cAMP excretion, and increases in plasma 1,25-dihydroxyvitamin D. These changes were highly significant in statistical terms and were observed at doses that had no effect on serum calcium or endogenous PTH. These studies demonstrate the feasibility of using PTHrP-(1-36) as a diagnostic probe for future studies aimed at elucidating the differing pathophysiologies of HHM and hyperparathyroidism.

A midregion parathyroid hormone-related peptide mobilizes cytosolic calcium and stimulates formation of inositol trisphosphate in a squamous carcinoma cell line.

A midregion fragment of PTH-related protein (PTHrP), which is intensively conserved across species, has been identified as a secretory product of several different cell types, including keratinocytes and squamous carcinomas. As recent data suggest that a midregion PTHrP fragment may be biologically active, we hypothesized that midregion PTHrPs interact with unique cell surface receptors that mediate autocrine or paracrine action. Dose-dependent transient elevations in intracellular calcium ([Ca2-]i) were observed in fura-2-loaded SqCC/Y1 squamous carcinoma cells exposed to human (h) PTHrP-(67-86)NH2, [Tyr36]hPTHrP-(1-36)NH2, and hPTHrP-(1-141) at concentrations ranging from 1 pM to 1 microM. The effects of maximal stimulatory concentrations of [Tyr36]PTHrP-(1-36)NH2 and PTHrP-(67-86)NH2 on [Ca2+]i were additive. The inhibitory PTH analog, [D-Trp12,Tyr34]bovine PTH-(7-34)NH2, attenuated the [Ca2+]i response to [Tyr36]hPTHrP-(1-36)NH2, but not that to PTHrP-(67-86)NH2. These data suggest that PTHrP-(67-86)NH2 activates a different receptor pathway in SqCC/Y1 cells from the one activated by [Tyr36]hPTHrP-(1-36)NH2. Radiolabeled PTHrP-(67-86)NH2 did not bind to SqCC/Y1 cells, and PTHrP-(67-86)NH2 did not compete for binding of 125I-labeled [Tyr36]PTHrP-(1-36)NH2 to PTH/PTHrP receptors on SaOS-2 osteosarcoma cells. Activation of the phospholipase C pathway by PTHrP-(67-86)NH2 was confirmed by exposing SqCC/Y1 cells to peptide for 1 min and measuring the accumulation of inositol trisphosphates. PTHrP-(67-86)NH2 treatment (100 nM) resulted in maximal stimulation of inositol trisphosphates of 3.1 +/- 0.1-fold over the control value, with an EC50 of 1.5 +/- 1.2 nm. In contrast, PTHrP-(67-86)NH2 (0.1 nM to 1 microM) did not stimulate adenylyl cyclase in SqCC/Y1 cells despite vigorous stimulation of cAMP formation by isoproterenol (1 microM) to 66-fold over the basal value. To determine whether messenger RNA (mRNA) prepared from SqCC/Y1 cells would direct the translation of a receptor protein that mediated a [Ca2+]i response to PTHrP-(67-86)NH2, we performed expression studies in Xenopus oocytes. Fluo-3 fluorescence in Xenopus oocytes expressing SqCC/Y1 mRNA was visualized by confocal video microscopy after exposure to 1 microM PTHrP-(67-86)NH2. Clear increases in [Ca2+]i were detected in mRNA-injected, but not in sham-injected, oocytes. Finally, we examined the effect of PTHrP-(67-86)NH2 treatment on fibronectin secretion from SqCC/YN1 cells. A significant 3.5-fold increase in fibronectin secretion into conditioned medium was observed when SqCC/Y1 cells were exposed to 100 nM PTHrP-(67-86)NH2, and this effect was dose dependent, with an EC50 of 0.1 nM. We conclude that PTHrP-(67-86)NH2 activates phospholipase C-dependent pathways in SqCC/Y1 cells through a receptor distinct from that activated by PTHrP-(1-36) in the same cells. As a midregion secretory fragment of PTHrP has been partially purified from several different cell types, this receptor may have broad biological significance.

Circulating levels of interleukin-6 and tumor necrosis factor-alpha are elevated in primary hyperparathyroidism and correlate with markers of bone resorption--a clinical research center study.

The pathogenesis of PTH-induced bone loss is uncertain. Experimental evidence suggests that PTH induces the production by osteoblasts of the bone-resorbing cytokine, interleukin-6. We measured the circulating levels of interleukin-6, tumor necrosis factor-alpha, and interleukin-1 beta and examined their relationship to biochemical markers of bone turnover in 38 patients with primary hyperparathyroidism (7 of whom also were studied after successful parathyroid adenomectomy), 6 patients with hypoparathyroidism, and 12 subjects with normal parathyroid function. The patients with untreated primary hyperparathyroidism had mean serum levels of interleukin-6 that were 16-fold higher than control values (mean +/- SEM; primary hyperparathyroidism 18.6 +/- 2.1 pg/mL, controls 1.1 +/- 0.1; P < 0.001). Circulating levels of interleukin-6 soluble receptor (primary hyperparathyroidism 41.7 +/- 1.2 ng/ mL, controls 25.1 +/- 1.0; P < 0.001), and tumor necrosis factor-alpha (primary hyperparathyroidism 11.6 +/- 0.8 pg/mL, controls 2.5 +/- 0.2; P < 0.001) were also elevated. After successful parathyroid adenomectomy, levels of each of these cytokines fell into the normal range. The mean levels of interleukin-6, its soluble receptor, and tumor necrosis factor-alpha in the subjects with hypoparathyroidism were lower than control values (P < 0.001 for each variable). There was no difference between subjects with primary hyperparathyroidism and controls in the circulating level of interleukin-1 beta. In the subjects with untreated primary hyperparathyroidism, serum levels of interleukin-6 correlated strongly with those of intact PTH (r = 0.47, P = 0.003) and biochemical markers of bone resorption: serum deoxypyridinoline (r = 0.93, P < 0.001), serum type I collagen carboxyterminal telopeptide (r = 0.87, P < 0.001), urinary pyridinoline (r = 0.81, P < 0.001), and urinary deoxypyridinoline (r = 0.63, P = 0.005). Levels of tumor necrosis factor-alpha correlated less strongly with the same variables: PTH (r = 0.41, P = 0.01), serum deoxypyridinoline (r = 0.48, P = 0.002), serum type I collagen carboxyterminal telopeptide (r = 0.46, P = 0.004), urinary pyridinoline (r = 0.61, P = 0.008), and urinary deoxypyridinoline (r = 0.61, P = 0.007). Levels of interleukin-6 also correlated with those of tumor necrosis factor-alpha (r = 0.44, P = 0.005). Multiple regression analysis indicated that interleukin-6, but not tumor necrosis factor-alpha, was independently predictive of bone resorption. We conclude that serum levels of interleukin-6 and tumor necrosis factor-alpha are increased in patients with primary hyperparathyroidism and are normalized by successful surgical treatment. The finding that these cytokines correlate with biochemical markers of bone resorption suggests that they play a role in the pathogenesis of bone loss in primary hyperparathyroidism.

24,25 Dihydroxyvitamin D supplementation corrects hyperparathyroidism and improves skeletal abnormalities in X-linked hypophosphatemic rickets--a clinical research center study.

Therapy for X-linked hypophosphatemia (XLH) only partially corrects skeletal lesions and is often complicated by hyperparathyroidism. 24,25(OH)2 D3 improves skeletal lesions in a murine model of XLH and suppresses PTH secretion in animals. Therefore, we undertook a placebo-controlled trial of 24,25(OH)2 D3 supplementation to standard treatment in patients with XLH to improve bone disease and reduce hyperparathyroid complications. Fifteen subjects with XLH receiving standard treatment [1,25(OH)2 D3 or dihydrotachysterol plus phosphate] were evaluated, supplemented with placebo, and reevaluated one yr later. 24,25(OH)2 D3 supplementation was then begun and studies repeated after another year. Each patient underwent a detailed evaluation of calcium homeostasis over a 24-h period. Rachitic abnormalities were assessed radiographically in children. Adults underwent bone biopsies. 24,25(OH)2 D3 normalized PTH values in nine subjects (peak PTH was 46.5 +/- 6.6 pmol/L at entry, 42.3 +/- 5.9 pmol/L after placebo, and 23.3 +/- 5.4 pmol/L after 24,25(OH)2 D3). Nephrogenous cAMP decreased at night, coincident with the decrease in PTH, and serum phosphorus was slightly greater with 24,25(OH)2 D3. Radiographic features of rickets improved during 24,25(OH)2 D3 supplementation in children, and osteoid surface decreased in adults. 24,25(OH)2 D3 is a useful adjunct to standard therapy in XLH by effecting correction of hyperparathyroidism and improvement of rickets and osteomalacia.

Parathyroid hormone (PTH)-related protein(1-36) is equipotent to PTH(1-34) in humans.

Humoral hypercalcemia of malignancy (HHM) results from the production of PTH-related protein (PTHrP) by human tumors. One previous study has reported the results of human (h) PTHrP(1-34) infusion into humans. In that report, hPTHrP(1-34) was found to be qualitatively similar to but 3- to 10-fold less potent than hPTH(1-34). Because hPTHrP(1-36) and not hPTH(1-34) is likely to be the actual amino-terminal secretory form of PTHrP, and because this previously reported lack of potency was unexpected, we repeated these studies using hPTHrP(1-36) and compared the results with those obtained with hPTH(1-34). Healthy subjects (n = 30) were infused over 6 h with either vehicle alone, hPTH(1-34) at a dose of 8 pmol/kg.h, or hPTHrP(1-36) at doses of 8 or 80 pmol/kg.h. Both hPTH(1-34) and hPTHrP(1-36) caused an increase in serum ionized calcium, a decrease in serum phosphorus, an increase in the fractional excretion of phosphorus, a decrease in the tubular maximum for phosphorus, an increase in nephrogenous cAMP excretion, and suppression of endogenous PTH(1-84). Unlike events observed in HHM, hPTHrP(1-36) induced an increase in plasma 1,25-dihydroxyvitamin D2. In addition, fractional excretion of calcium was reduced by both hPTH(1-34) and hPTHrP(1-36). In their actions on serum calcium, renal calcium and phosphorus handling, and nephrogenous cAMP excretion, hPTHrP(1-36) and hPTH(1-34) appeared equivalent in potency. These studies indicate that short-term infusion of hPTHrP(1-36) into humans reproduces most but not all of the features of HHM. In contrast to the reported findings with hPTHrP(1-34), we found the potency of hPTHrP(1-36) to be comparable with that of hPTH(1-34) in vivo in humans. In addition, unlike the situation in HHM, hPTHrP(1-36) produces an increment in plasma 1,25-dihydroxyvitamin D2. Finally, hPTHrP(1-36) has been shown for the first time to have anticalciuric effects in humans. This would suggest that, in addition to osteoclastic bone resorption, tubular reabsorbtion of calcium by hPTHrP may contribute to the hypercalcemia in patients with HHM.

Estrogen modulates parathyroid hormone-induced fibronectin production in human and rat osteoblast-like cells.

Fibronectin (FN) is an important constituent of the extracellular matrix in bone. Its hormonal regulation in this tissue has not been extensively studied. We, therefore, examined the effects of PTH, estrogen, and transforming growth factor beta on the production of FN by human and rat osteoblast-like cells. Confluent cells were stabilized for 48 h under estrogen-replete (10(-9) M 17 beta-estradiol) conditions and then continued under these same conditions or withdrawn from estrogen for varying periods of time. PTH over the range 10(-11)-(10(-8) M caused a dose-dependent increase in FN production [P < 0.001 by analysis of variance (ANOVA)] such that at the highest dose, FN production was increased 11-fold. Estrogen withdrawal for 96 h caused a significant diminution in PTH-induced FN production (P < 0.005 by two-way ANOVA). Estrogen withdrawal over the of period 48-144 h caused a progressive diminution in PTH-induced FN production such that differences in mean values for estrogen-replete vs. deficient conditions were greater at 144 than 48 h (P < 0.05). The estrogen effect was titratable over the range 10(-11)-10(-9) M, and the inactive congener 17 alpha-estradiol failed to prevent the inhibitory effect of estrogen withdrawal on PTH-induced FN production. Interestingly, estrogen withdrawal had absolutely no effect on transforming growth factor-beta-induced FN production. Northern analysis demonstrated no effect of PTH on steady-state FN messenger RNA levels in Saos-2 cells under either estrogen-replete or estrogen-deficient conditions, suggesting that PTH effects an increase in FN production via a posttranscriptional mechanism in these cells. We conclude that PTH stimulates FN production in human and rat osteoblast-like cells, and under estrogen-deficient conditions this effect is significantly diminished. The modulatory effect of estrogen is not a universal phenomenon because transforming growth factor-beta-induced FN production is unaffected by estrogen withdrawal.

Nocturnal hyperparathyroidism: a frequent feature of X-linked hypophosphatemia.

Parathyroid function is described as normal in patients with phosphopenic rickets before initiation of therapy with phosphate salts; however, hyperparathyroidism is an occasional complication of treatment. We observed a higher than expected frequency of hyperparathyroidism in patients with phosphate-wasting rickets, present before treatment in some patients and, more frequently, after the onset of treatment. To better define parathyroid status in hypophosphatemic rickets, we sampled 12 affected children and 7 affected adults every 4 h for 1 day and measured PTH in assays detecting midmolecule fragments (cPTH) and intact hormone (iPTH). All children and 4 adults were receiving a vitamin D preparation and phosphate salts; 3 adults were untreated. Mean cPTH, iPTH, and nephrogenous cAMP excretions in each group of patients were greater than in controls. Exaggerated nocturnal rises in both cPTH and iPTH characterized the profile in patients. Seventeen patients demonstrated frankly elevated cPTH at night, with peak values at midnight, whereas no control individual did. Although mean iPTH values in patients increased at night, they did not exceed the upper limit of normal. Hyperparathyroidism in hypophosphatemic rickets occurs in both children and adults, may be present in untreated patients, is predominantly nocturnal, and is characterized by exaggerated secretion of midmolecule fragments. This manifestation of hypophosphatemic rickets is more widespread than currently recognized; we speculate that it may contribute to the pathogenesis of nephrocalcinosis and precede the development of tertiary hyperparathyroidism.

Parathyroid hormone-related protein stimulates prostaglandin E2 release from human osteoblast-like cells: modulating effect of peptide length.

Parathyroid hormone-related protein (PTHrP) is a potent bone-resorbing protein that frequently mediates the humoral hypercalcemia of malignancy syndrome. Since prostaglandins may mediate the bone-resorptive action of certain hormones, we examined the effect of PTHrP on prostaglandin E2 (PGE2) secretion by human osteoblast-like cells. There was low-level basal secretion of PGE2 by Saos-2 cells (8.1 +/- 0.6 pg/ml). Using four different preparations of PTHrP, it was observed that with increasing peptide length, from 36 to 141 amino acids, a significant increase in efficacy for PGE2 release was seen in these cells. All forms of PTHrP were agonists for PGE2 release, with effects seen at concentrations as low as 10(-12) M in 48 h conditioned media. The amino terminus of the molecule appeared critical for this effect since the truncated derivative PTHrP-(7-34) did not induce significant PGE2 secretion. However, the influence of peptide length could not be explained by differential activation of adenylate cyclase since [Tyr36]PTHrP-(1-36)amide was equipotent to the longest peptide preparation, PTHrP-(1-141), in stimulating cyclic AMP accumulation in the Saos-2 cells. In contrast, PTHrP-(1-141) was significantly more effective than [Tyr35]PTHrP-(1-36)-amide in inducing a rise in cytosolic calcium. Further, this effect was noted at concentrations lower than those that caused significant cyclic AMP accumulation in the Saos-2 cells. PTHrP-(1-141) induced the release of PGE2 from primary human bone cell cultures to levels entirely comparable to those seen in the Saos-2 cells. PTHrP-(1-141) also induced PGE2 release by cultured fetal rat long bones at 72 h. We conclude that the carboxy-terminal region of PTHrP has important effects on cellular signal transduction pathways and on the release of a potent bone-active cytokine, PGE2.