Bone and the regulation of global energy balance

The skeleton, populated by large numbers of osteoblasts and long-lived osteocytes, requires a constant supply of energy-rich molecules to fuel the synthesis, deposition and mineralization of bone matrix during bone modelling and remodelling. When these energetic demands are not met, bone acquisition is suppressed. Recent findings suggest that key developmental signals emanating from Wnt low-density lipoprotein-related receptor 5 and hypoxia-inducible factor pathways impact osteoblast bioenergetics to accommodate the energy requirements for bone cells to fulfil their function. In vivo studies in several mutant mouse strains have confirmed a link between bone cells and global metabolism, ultimately leading to the identification of hormonal interactions between the skeleton and other tissues. The hormones insulin and leptin affect postnatal bone acquisition, whilst osteocalcin produced by the osteoblast in turn stimulates insulin secretion by the pancreas. These observations have prompted additional questions regarding the nature of the mechanisms of fuel sensing and processing in the osteoblast and their contribution to overall energy utilization and homeostasis. Answers to such questions should advance our understanding of metabolic diseases and may ultimately improve management of affected patients. In this review, we highlight recent studies in this field and offer a perspective on the evolutionary implications of bone as a metabolic endocrine organ.

The role of angiogenesis in a murine tibial model of distraction osteogenesis

Distraction osteogenesis (DO) is one of the most dramatic in vivo applications of mechanical stimulation as a means of inducing bone regeneration. A simple and reproducible murine model of tibia distraction osteogenesis was developed using a monolateral fixator. Bone formation was assessed histologically over a 35-day time course. The steady state expression of a broad family of angiogenesis-associated genes was assessed by microarray hybridization analyses over the same time course, while the immediate gene response that was induced during each cycle of distraction was assessed at 30 min and 8 h after the first and last rounds of activation of the fixator. Distraction osteogenesis promoted new bone formation primarily through an intramembranous process with maximal osteogenesis during the active distraction period. Histological analysis also showed that dense cortical bone continued to be formed, during the consolidation phase, for 2 weeks after distraction ended. The analysis of steady state mRNA expression levels over the time course of DO showed that VEGF-A and neuropilin, an alternate receptor for VEGF-A, both angiopoietin (Ang) 1 and 2 factors, and the Ang receptor Tie2 were the critical angiogenic factors during DO. A key transcriptional regulator of many of the angiogenic factors, hypoxia-induced factor1alpha (Hif-1a), the FGF binding protein pleiotropin/OSF1, and multiple MMP(s), were also induced during the active distraction period. Examination of the expression of angiogenic factors that were induced after each cycle of activation, demonstrated that Hif-1a, Nrp1, and VEGF-A were all cyclically induced after each increment of distraction. These results suggest that these factors are early mediators that are produced by distraction and contribute toward the processes that promote bone formation. These experiments represent the first step in defining the molecular mechanisms that regulate skeletal regeneration and the functional relationship between angiogenesis and osteogenesis during distraction osteogenesis.

Selected Contribution: Osteocytes upregulate HIF-1alpha in response to acute disuse and oxygen deprivation

Loss of mechanical loading, or disuse, rapidly precipitates locally mediated bone resorption. However, the pathway by which this process is initiated and mediated is poorly understood. In this study, we used a complementary in vivo and in vitro approach to determine whether disuse-induced osteocyte hypoxia resulted in upregulation of the hypoxia-dependent transcription factor HIF-1alpha. We found that acute disuse (1-5 days) resulted in a significant increase in the percentage of osteocytes staining positive for HIF-1alpha vs. normal bone (30.9 +/- 6.1 vs. 14.1 +/- 3.8%) and that this response was uniform around the cortex. In addition, we found that acute oxygen deprivation (4-12 h of 2% O2) resulted in a 2.1- to 3.7-fold upregulation of HIF-1alpha protein expression in MLO-Y4 osteocyte-like cells compared with cells cultured in parallel under normal oxygen conditions. Given known HIF-1alpha targets genes, we suggest that osteocyte hypoxia and subsequent upregulation of hypoxia-dependent pathways may serve to initiate and mediate disuse-induced bone resorption.

Hypoxia induces vascular endothelial growth factor gene transcription in human osteoblast-like cells through the hypoxia-inducible factor-2alpha

VEGF is produced by osteoblasts and has been postulated to function as an angiogenic stimulus during normal skeletal development and in fracture repair. In this study, we characterized the molecular mechanisms by which experimental hypoxia increases VEGF mRNA in human MG63 osteoblast-like cells. Exposure of MG63 cells to 1% O(2) for 24 h resulted in a four-fold increase in VEGF mRNA. Immunoblotting of nuclear extracts demonstrated a time-dependent increase in the level of the Hif-2alpha protein, which preceded the rise in VEGF mRNA. To determine the effect of hypoxia on VEGF gene transcription, MG63 cells were transiently transfected with a segment of the VEGF promoter construct fused to luciferase and then exposed to 1% O(2). Hypoxia induced VEGF promoter activity five-fold by 24 h. Forced expression of Hif-2alpha, but not Hif-1alpha, increased both basal and hypoxia induced VEGF promoter activity. By contrast, the ability of the VEGF reporter to respond to hypoxia or recombinant Hif-2alpha was abolished in cells transfected with a VEGF promoter construct containing a mutation in the hypoxia response element. In summary, exposure of osteoblast-like cells to hypoxia induces VEGF expression via induction of Hif-2alpha and transcriptional activation of the VEGF promoter.

Targeted overexpression of insulin-like growth factor I to osteoblasts of transgenic mice: increased trabecular bone volume without increased osteoblast proliferation

Insulin-like growth factor I (IGF-I) is an important growth factor for bone, yet the mechanisms that mediate its anabolic activity in the skeleton are poorly understood. To examine the effects of locally produced IGF-I in bone in vivo, we targeted expression IGF-I to osteoblasts of transgenic mice using a human osteocalcin promoter. The IGF-I transgene was expressed in bone osteoblasts in OC-IGF-I transgenic mice at high levels in the absence of any change in serum IGF-I levels, or of total body growth. Bone formation rate at the distal femur in 3-week-old OC-IGF-I transgenic mice was approximately twice that of controls. By 6 weeks, bone mineral density as measured by dual energy x-ray, and quantitative computed tomography was significantly greater in OC-IGF-I transgenic mice compared with controls. Histomorphometric measurements revealed a marked (30%) increase femoral cancellous bone volume in the OC-IGF-I transgenic mice, but no change in the total number of osteoblasts or osteoclasts. Transgenic mice also demonstrated an increase in the osteocyte lacunea occupancy, suggesting that IGF-I may extend the osteocyte life span. We conclude that IGF-I produced locally in bone osteoblasts exerts its anabolic effect primarily by increasing the activity of resident osteoblasts.

Parathyroid hormone-related protein induces G1 phase growth arrest of vascular smooth muscle cells

In this study, we investigated the mechanisms responsible for the growth-inhibitory action of parathyroid hormone-related protein (PTHRP) in A10 vascular smooth muscle cells (VSMC). Fluorescence-activated cell sorting analysis of serum-stimulated VSMC treated with PTHRP or dibutyryl-cAMP (DBcAMP) demonstrated an enrichment of cells in G1 and a reduction in the S phase. Measurement of DNA synthesis in platelet-derived growth factor-stimulated VSMC treated with DBcAMP revealed that cells became refractory to growth inhibition by 12-16 h, consistent with blockade in mid-G1. cAMP treatment blunted the serum-induced rise in cyclin D1 during cell cycle progression without altering levels of the cyclin-dependent kinase cdk4 or cyclin E and its associated kinase, cdk2. Exposure of cells to PTHRP or cAMP resulted in a reduction in retinoblastoma gene product (Rb) phosphorylation. Immunoblotting of extracts from cAMP-treated cells with antibodies to cdk inhibitors revealed a striking increase in p27(kip1) abundance coincident with the G1 block. Immunoprecipitation with an anti-cyclin D1 antibody of cell lysates prepared from cAMP-treated cells followed by immunoblotting with antisera to p27(kip1) disclosed a threefold increase in p27(kip1) associated with cyclin D1 compared with lysates treated with serum alone. We conclude that PTHRP, by increasing intracellular cAMP, induces VSMC cycle arrest in mid-G1. This occurs secondary to a suppression in cyclin D1 and induction of p27(kip1) expression, which in turn inhibits Rb phosphorylation.

Vasorelaxant properties of parathyroid hormone-related protein in the mouse: evidence for endothelium involvement independent of nitric oxide formation

PTH-related peptide is produced in vascular smooth muscle and is believed to participate in the local control of vascular tone. The recent identification of mid-region PTHrP peptides, as well as the discovery of multiple receptors in blood vessels, raises new questions concerning the mechanisms by which PTHrP relaxes the vasculature. In this study, we examined these mechanisms in two vascular beds of the mouse. PTHrP-(1-34) and PTH-(1-34), but not PTHrP-(38-64) or PTHrP-(38-94), caused concentration-dependent relaxation of pre-contracted aortas and reduced the spontaneous phasic activity of the portal vein. PTHrP and PTH-induced aortic relaxations were largely endothelium dependent, whereas an intact endothelium was not necessary for maximal portal vein relaxation. The endothelium-dependent component of PTHrP and PTH-induced aortic relaxations were unaffected by pretreatment with either L-NNA or indomethacin but were abolished by pretreatment with tetrabutyl ammonium. These results demonstrate that the N-terminal portions of PTHrP and PTH are required for their vasorelaxant activity in the mouse. In addition, maximal relaxant activity of PTHrP and PTH in murine aorta is dependent on the endothelium, which appears to involve the generation of an endothelium-derived hyperpolarizing factor.

Targeted overexpression of parathyroid hormone-related protein (PTHrP) to vascular smooth muscle in transgenic mice lowers blood pressure and alters vascular contractility

PTH-related protein (PTHrP) and its receptor are expressed in vascular smooth muscle cells and are believed to participate in the local regulation of vascular tone. To explore the function of locally produced PTHrP in vascular smooth muscle in vivo, we developed transgenic mice that overexpress PTHrP in smooth muscle using a smooth muscle alpha-actin promoter to direct expression of the transgene. In the PTHrP-overexpressing mice, messenger RNA expression was mainly restricted to smooth muscle-containing tissues. Several founders also expressed the transgene in bone and heart and exhibited striking abnormalities in the development of these tissues. In PTHrP-overexpressing mice, blood pressure was significantly lower than that in wild-type controls (121 +/- 3 vs. 135 +/- 2 mm Hg; P < 0.01). Moreover, the magnitude of the vasorelaxant response to iv infusions of PTHrP-(1-34)NH2 was significantly attenuated in the transgenic animals. A similar desensitization to PTHrP was observed in aortic ring and portal vein preparations. Surprisingly, PTHrP-overexpressing mice were also significantly less responsive to the hypotensive action of infused acetylcholine in vivo and to the relaxant actions of acetylcholine on aortic vessel preparations in vitro. In summary, we have successfully targeted overexpression of PTHrP to the smooth muscle of transgenic mice. When expressed in its normal autocrine/paracrine setting, PTHrP lowers systemic blood pressure and decreases vascular responsiveness to further relaxation by PTHrP and other endothelium-dependent vasorelaxants such as acetylcholine. We postulate that the heterologous desensitization to acetylcholine-induced relaxation in PTHrP-overexpressing blood vessels involves desensitization of second messenger/effector signaling pathways common to PTHrP and acetylcholine.

Reduced blood pressure and increased sensitivity of the vasculature to parathyroid hormone-related protein (PTHrP) in transgenic mice overexpressing the PTH/PTHrP receptor in vascular smooth muscle

PTH-related protein (PTHrP) is produced in vascular smooth muscle, where it is postulated to exert vasorelaxant properties by activation of the PTH/PTHrP type 1 receptor. As a model for studying the actions of locally produced PTHrP in vascular smooth muscle in vivo, we developed transgenic mice that overexpress the PTH/PTHrP receptor (PTHrP-R) in smooth muscle. Oocyte injection with a SMP8-PTHrP-R fusion construct yielded six founder mice. F1 offspring were viable and demonstrated selective overexpression of the SMP8-PTHP-R messenger RNA in smooth muscle-rich tissues. Baseline blood pressure measured in conscious mice by tail sphygmomanometry was significantly lower in the receptor-overexpressing mice than that in controls (117 +/- 4 vs. 133 +/- 3 mm Hg; P < 0.05). In anesthetized animals, iv infusion of PTHrP-(1-34)NH2 caused a significantly greater reduction in blood pressure and total peripheral resistance in transgenic mice than in control animals. Vascular contractility was studied in paired, isometrically mounted aortas from 9-week-old transgenic and wild-type mice. The force of contraction in response to phenlyephrine was not significantly different between transgenic and wild-type mice. However, PTHrP-(1-34) NH2 relaxed aortic vessel preparations from transgenic mice to a greater extent than in controls (77.1 +/- 3% vs. 38.4 +/- 4%; P < 0.001). To determine the impact of overexpression of PTH/PTHrP type 1 receptor and its ligand on the development of the cardiovascular system, double transgenic mice were created by crossing SMP8-PTHrP-R transgenic mice with mice overexpressing PTHrP (SMP8-PTHrP). Double transgenic mice died around day E9 with abnormalities in the developing heart. In conclusion, overexpression of PTH/PTHrP type 1 receptor in vascular smooth muscle of transgenic mice reduces blood pressure, probably through sustained activation of the receptor by endogenous ligand. The cardiovascular defects observed in mice overexpressing both PTHrP and its receptor suggest that PTHrP may play a role in the normal development of the cardiovascular system.

Parathyroid hormone-related protein expression in vascular smooth muscle of spontaneously hypertensive rats: evidence for lack of response to angiotensin II

OBJECTIVE

We studied the expression of parathyroid hormone (PTH)-related protein in vascular smooth muscle cells of spontaneously hypertensive rats (SHR) using Wistar-Kyoto (WKY) and Sprague-Dawley rats as normotensive controls.

METHODS

Aortae from 4- and 18-week-old SHR versus age-matched WKY and Sprague-Dawley rats were excised to obtain total RNA or smooth muscle cells. The cells were subcultured in Dulbecco's Modified Eagle's Medium containing 10% fetal calf serum, then serum-deprived for 72 h and stimulated with 0.1 micromol/I angiotensin II. PTH-related protein, c-myc and angiotensin II type qa receptor (AT1aR) messenger (m)RNA levels were measured by Northern blot, using total RNA extracted by phenol/chloroform. The effects of PTH-related protein(1-34)NH2 intravenous injections on arterial blood pressure and the heart rate were studied in anesthetized SHR and WKY rats.

RESULTS

The Northern blots showed a significantly higher abundance of PTH-related protein mRNA in aortae of SHR versus WKY rats in the prehypertensive state but no significant difference in adult animals. In cultured aortic smooth muscle cells, angiotensin II induced a four- to sixfold increase in PTH-related protein mRNA levels in smooth muscle cells from normotensive animals, but failed to elicit a significant response in smooth muscle cells derived from SHR in either the prehypertensive or the hypertensive state. This lack of response to angiotensin II in SHR smooth muscle cells was not due to decreased expression or responsiveness of the AT1aR, since SHR smooth muscle cells had more AT1aR mRNA than Sprague-Dawley smooth muscle cells, and angiotensin II-induced activation of c-myc was faster and greater in smooth muscle cells derived from 4- or 18-week-old SHR than in Sprague-Dawley smooth muscle cells. In contrast, PTH-related protein(1-34)NH2 induced a long-lasting dose-dependent hypotensive and tachycardic response in both SHR and WKY rats, indicating that SHR retained responsiveness to the vasodilator.

CONCLUSIONS

PTH-related protein gene expression in response to angiotensin II is impaired in SHR arteries. A deficiency in this potent local vasodilator may contribute to the development and/or maintenance of arterial hypertension in this model.

The effect of cell-matrix interaction on parathyroid hormone (PTH) receptor binding and PTH responsiveness in proximal renal tubular cells and osteoblast-like cells

The interaction of cells with the surrounding extracellular matrix (ECM) or basement membrane (BM) brings about profound changes in cellular biological responses, such as cell differentiation, proliferation, and gene expression. We studied the effect of ECM on PTH receptor binding and on biological responses mediated by PTH, in two cell preparations: 1) the proximal tubular OK opossum kidney cell line; and 2) MC3T3-E1 cells, a clonal line of nontransformed murine osteoblasts. Cells were plated on either plastic surfaces or on tissue culture dishes coated with specific ECM components. In both cell types plated on collagen-type IV (Col-IV), PTH receptor binding, on day 4 of culture, was markedly diminished, when compared with cells on plastic (approximately 45% inhibition, P < 0.01). In addition, Col-IV dose dependently inhibited cAMP generation stimulated by PTH (P < 0.001 vs. plastic), whereas cAMP generation by PGE2, cholera toxin, and forskolin was not altered. In Northern blot analysis, a PTH/PTH-related-protein receptor messenger RNA transcript was detected in both the kidney and bone cells. However, only OK cells manifested a decreased abundance of receptor messenger RNA when plated on Col-IV, compared with plastic. The physiological significance of inhibited cAMP production by Col-IV was evaluated by measuring the influence of different matrices on the activity of Na+/H+ exchanger (NHE) in OK cells and cell mitogenic activity in MC3T3-E1 cells (both responses are negatively modulated by cAMP). OK cells plated on Col-IV showed 70% inhibition of NHE, compared with cells plated on plastic (P < 0.01). PTH inhibits NHE activity in cells on plastic but stimulates exchanger activity by 40% in cells plated on Col-IV. In MC3T3-E1 cells grown on plastic, PTH exerts a dose-dependent antiproliferative effect, which is mediated by cAMP. This effect is mitigated when cells are grown on Col-IV (40-50% less antiproliferative effect). In summary, Col-IV, a maj or BM constituent, has a profound inhibitory effect on PTH binding and PTH-mediated biological responses in both kidney tubular cells and osteoblasts. Altered cellular function by Col-IV may be of physiological relevance in states associated with altered composition of BM or expansion of ECM (e.g. diabetes mellitus and interstitial fibrosis).

Risedronate, a highly effective oral agent in the treatment of patients with severe Paget's disease

Thirteen patients with severe Paget's disease of bone [mean serum alkaline phosphatase (SAP) level 17 times the upper limit of normal] were treated with 30 mg oral risedronate daily for 8 weeks. Patients were followed for 16 weeks without treatment. The change from baseline SAP was the primary end point. Those patients whose SAP levels did not reach the normal range were retreated with 30 mg for another 8 weeks. There was a mean percent decrease in SAP of 77% after the first course of risedronate treatment and 87% after the second course of treatment. All patients who completed the study had a decrease in SAP of at least 77% from the baseline. The urinary hydroxyproline/creatinine level was decreased by 64% and 79%, respectively, during the first and second treatment courses. There were transient asymptomatic decreases in serum calcium and phosphorus levels. The urinary calcium/creatinine ratio also decreased in these patients. Serum intact PTH and 1,25-dihydroxyvitamin D levels increased transiently during risedronate treatment. Oral risedronate was well tolerated by the patients. Only one patient discontinued treatment because of an adverse event (diarrhea) thought to be related to risedronate therapy.

Overexpression of insulin-like growth factor-binding protein-4 (IGFBP-4) in smooth muscle cells of transgenic mice through a smooth muscle alpha-actin-IGFBP-4 fusion gene induces smooth muscle hypoplasia

Insulin-like growth factor I (IGF-I) has been postulated to function as a smooth muscle cell (SMC) mitogen and to play a role in the pathogenesis of bladder hypertrophy, estrogen-induced uterine growth, and restenosis after arterial angioplasty. IGF-binding protein-4 (IGFBP-4) inhibits IGF-I action in vitro and is the most abundant IGFBP in the rodent arterial wall. To explore the function of this binding protein in vivo, transgenic mouse lines were developed harboring fusion genes consisting of a rat IGFBP-4 complementary DNA cloned downstream of either a -724 bp fragment of the mouse smooth muscle alpha-actin 5'-flanking region (SMP2-BP-4) or -1074 bp, 63 bp of 5'-untranslated region, and 2.5 kb of intron 1 of smooth muscle alpha-actin (SMP8-BP-4). SMP2-BP-4 mice expressed low levels of the exogenous IGFBP-4 messenger RNA (mRNA), which was not specifically targeted to SMC-rich tissue environments, and were therefore not analyzed further. Six SMP8-BP-4 transgenic lines derived from separate founders were characterized. Mating of hemizygous SMP8-BP-4 mice with controls produced about 50% transgenic offspring, with equal sex distribution. Expression of IGFBP-4 mRNA in nontransgenic littermates was maximal in liver and kidney. By contrast, transgenic IGFBP-4 mRNA expression, distinguished because of a smaller transcript size, was confined to SMC-containing tissues, with the following hierarchy: bladder > aorta > stomach = uterus. There was no transgene expression in skeletal muscle, brain, or cardiac myocytes. The abundance of IGFBP-4 measured by Western ligand blotting or by immunoblotting, was 8- to 10-fold higher in aorta and bladder of SMP8-BP-4 mice than in their nontransgenic littermates, with no change in plasma IGFBP-4 levels. Transgenic mice exhibited a significant reduction in wet weight of SMC-rich tissues, including bladder, intestine, aorta, uterus, and stomach, with no change in total body or carcass weight. In situ hybridization showed that transgene expression was targeted exclusively to the muscular layers of the arteries, veins, bladder, ureter, stomach, intestine, and uterus. Overexpression of IGFBP-4 was associated with SMC hypoplasia, a reciprocal phenotype to that of transgenic mice overexpressing IGF-I under control of the same promoter (SMP8-IGF-I). Double transgenic mice derived from mating SMP8-BP-4 with SMP8-IGF-I animals showed a modest decrease in wet weight at selected SMC tissues. Although we cannot exclude that the effects of IGFBP-4 may be IGF independent, these data suggest that IGFBP-4 is a functional antagonist of IGF-I action on SMC in vivo.

The N-terminal portion of parathyroid hormone-related protein mediates the inhibition of apical Na+/H+ exchange in opossum kidney cells

Parathyroid hormone (PTH) and PTH-related protein (PTHrP) can activate a common receptor in several different cell types. Both PTH and N-terminal PTHrP peptides have been shown to acutely inhibit the apical Na+/H+ exchanger in the renal proximal tubule. In this study, the ability of various PTHrP fragments to inhibit apical Na+/H+ exchange was investigated. In addition, the signal transduction events associated with PTHrP inhibition of apical Na+/H+ exchange in polarized OK-P cells were characterized. Both PTHrP-(1-34)NH2 and recombinant full-length PTHrP-(1-141) inhibited apical Na+/H+ exchange activity by approximately 50%. These changes occurred in close temporal association with significant (threefold) increases in cellular cAMP accumulation. PTHrP-(1-34)NH2 had no effect on intracellular Ca2+, inositol phosphate production, or protein kinase C activity. PTHrP peptides, including PTHrP-(38-64)NH2, PTHrP-(67-86)NH2, PTHrP-(102-107)NH2, and PTHrP-(107-139)NH2, which lack the PTH-like N terminus, had no effect on the antiporter activity or cAMP accumulation. The results demonstrate that the N-terminal portion of the PTHrP molecule is responsible for inhibition of the apical Na+/H+ antiporter in OK-P cells.

Analysis of osteocalcin expression in transgenic mice reveals a species difference in vitamin D regulation of mouse and human osteocalcin genes

A line of transgenic mice expressing a human osteocalcin genomic fragment (hOClocus) and a murine MC3T3-E1 cell line containing a stably integrated human osteocalcin promoter construct have been developed to characterize the osteogenic and hormonal regulation of human osteocalcin in vivo and in vitro. In this study, we used these models to demonstrate a species difference in the regulation of the mouse and human osteocalcin genes by vitamin D. Repeated administration of 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) to mice carrying the hOClocus transgene resulted in striking increases in serum human osteocalcin, whereas serum mouse osteocalcin levels were unchanged after 24 h and only modestly increased 48 h after the second dose of hormone. 1,25(OH)2D3 increased human calvarial mRNA expression by 1.8-fold and slightly decreased mouse osteocalcin mRNA levels by approximately 1.2-fold. Furthermore, treatment of primary calvarial osteoblasts from these mice with 1,25(OH)2D3 increased human osteocalcin production but inhibited mouse osteocalcin protein accumulation. To investigate further the mechanism for the apparent species difference in vitamin D3 induction of mouse and human osteocalcin, we examined the effect of 1,25(OH)2D3 in an MC3T3-E1 cell line (MC4) containing a stably integrated 3900 bp osteocalcin promoter-luciferase construct. Treatment of MC4 cells with ascorbic acid resulted in parallel increases of the endogenous mouse osteocalcin protein and luciferase reporter activity over a 12-day period. Continuous exposure of MC4 cells to 1,25(OH)2D3 resulted in time-and dose-dependent increases in the activity of the phOC3900 luciferase construct. By contrast, the hormone had no effect on mouse osteocalcin protein concentrations and inhibited its induction by ascorbic acid. However, when cells were treated acutely with 1,25(OH)2D3 at later times during growth in ascorbic acid, the induction of mouse osteocalcin protein was only partially inhibited. In conclusion, our results indicate that common osteogenic signals regulate both mouse and human osteocalcin gene expression, but the mouse gene is resistant to induction by vitamin D. This species difference in vitamin D regulation of osteocalcin appears to result from the failure of 1,25(OH)2D3 to transcriptionally activate the mouse osteocalcin gene.

Tumor-induced osteomalacia: clinical and basic studies

A patient with classic clinical and biochemical features of tumor-induced osteomalacia (hypophosphatemia, phosphaturia, and undetectable serum concentrations of 1,25-dihydroxyvitamin D [1,25(OH)2D]) was studied before and after resection of a benign extraskeletal chondroma from the plantar surface of the foot. Presurgical laboratory evaluation was notable for normal serum concentrations of calcium, intact parathyroid hormone (PTH), parathyroid hormone-related protein (PTHrP), and osteocalcin, increased serum alkaline phosphate activity, and frankly elevated urinary cyclic adenosine monophosphate (cAMP) and pyridinium cross-link excretion. Quantitative histomorphometry showed severe osteomalacia and deep erosions of the cancellous surface by active osteoclasts. After resection, serum 1,25(OH)2D normalized within 24 h, while renal tubular phosphorus reabsorption and serum phosphorus did not normalized until days 2 and 3, respectively; serum Ca declined slightly, and serum intact PTH, osteocalcin, and urinary pyridinium cross-link excretion increased dramatically. Urinary cAMP excretion declined immediately after resection and then began to increase concomitant with the increase in serum intact PTH. A second bone biopsy taken 3 months after resection demonstrated complete resolution of the osteomalacia, increased mineral apposition rate (1.09 mu/day), resorption surface (9.2%), mineralizing surface (71%), and bone formation rate (0.83 mm3/mm2/day), and marked decrease in cancellous bone volume (13.1%) and trabecular connectivity compared with first biopsy. Tumor extracts did not affect phosphate transport in renal epithelial cell lines or 1 alpha-hydroxylase activity in a myelomonocytic cell line. The patient's course suggests that the normal 1,25(OH)2D and phosphorus metabolism is due to a tumor product that may be acting via stimulation of adenylate activity. Increased bone resorption prior to surgical resection suggests that the tumor may also produce an osteoclast activator. The rise in resorption surface and pyridinium cross-link excretion, increase in serum osteocalcin and bone mineralization, normalization of osteoid width, and fall in cancellous bone volume after resection are consistent with healing of osteomalacia by rapid remodeling.

Effect of UVB phototherapy and oral calcitriol (1,25-dihydroxyvitamin D3) on vitamin D photosynthesis in patients with psoriasis

BACKGROUND

Phototherapy and activated froms of vitamin D help clear psoriasis.

OBJECTIVE

The influence of UVB phototherapy and oral calcitriol (1,25-dihydroxyvitamin D3) on vitamin D photosynthesis was assessed in 16 patients.

METHODS

Patients were randomly selected to receive orally either placebo or calcitriol (0.5 to 2 micrograms daily) for the duration of the 8-week study; all patients received approximately 21 UVB treatments. Before and after treatment, serum levels of 25-hydroxyvitamin D and calcitriol were measured by high-pressure liquid chromatography.

RESULTS

Although calcitriol had no additive effect on phototherapy as a treatment modality, a significant increase in serum 25-hydroxyvitamin D levels occurred in both groups; in three patients extraordinarily high levels developed (> 120 ng/ml). Oral calcitriol significantly increased calcitriol serum levels. Increased serum calcitriol did not inhibit cutaneous synthesis of vitamin D or its hepatic conversion to serum 25-hydroxyvitamin D.

CONCLUSION

UVB induces high levels of vitamin D photosynthesis. Because oral or topical calcitriol alone helps clear psoriasis, studies to explore the possible influence of UVB phototherapy on its production should be considered. If UVB phototherapy induces cutaneous calcitriol synthesis this could explain the lack of added benefit to treatment when oral calcitriol is administered with phototherapy.

Cell-specific signal transduction of parathyroid hormone (PTH)-related protein through stably expressed recombinant PTH/PTHrP receptors in vascular smooth muscle cells

PTH-related protein activates a G protein-coupled PTH/PTHrP receptor in many cell types and produces diverse biological actions. To study the signal transduction events associated with biological activity of the PTH/PTHrP receptor in vascular smooth muscle, a principal PTHrP-responsive tissue, rat aortic smooth muscle cells (A10) were stably transfected with a plasmid encoding a PTH/PTHrP receptor and tested for ligand binding, PTHrP-(1-34)-induced cAMP levels, inositol phosphate production, and cytosolic calcium transients. Of nineteen G418-resistant lines recovered, all exhibited high affinity binding [approximately dissociation constant (Kd) > 10(-10)) of iodinated [Tyr36]hPTHrP(1-36)NH2 and ligand-induced cAMP accumulation (2- to 100-fold), which was directly proportional to PTH/PTHrP receptor number (range 4 x 10(3) to 7 x 10(7) sites/cell]. PTHrP had no effect on intracellular calcium or inositol phosphate formation in any cell line regardless of receptor number despite the presence of detectable G alpha q). Transient overexpression of individual G alpha q proteins (G alpha q, G alpha 11 or G alpha 14) into PTH/PTHrP receptor-expressing A10 cells conferred the ability of PTHrP to increase intracellular calcium and inositol phosphate formation. Ligand activation of the recombinant PTH/PTHrP receptor elicited appropriate downstream biological effects in A10 cells including inhibition of DNA synthesis and osteopontin messenger RNA (mRNA) expression. Thus, a single PTH/PTHrP receptor, though capable of coupling to different G proteins, signals exclusively through a cAMP-dependent pathway in vascular smooth muscle.

1,25-Dihydroxyvitamin D induces lipoprotein lipase expression in 3T3-L1 cells in association with adipocyte differentiation

1,25-dihydroxyvitamin D3 [1,25-(OH)2D3] is known to modulate the development of bone and other mesenchymal cell types. Since osteoblasts and adipocytes are thought to arise in bone marrow from a common progenitor, this work examined the effects of 1,25-(OH)2D3 on adipocyte development, and in particular on the expression of lipoprotein lipase (LPL), which is an early marker for the differentiated adipocyte. 3T3-L1 preadipocytes were cultured in the presence of 1,25-(OH)2D3 (10(-9) to 10(-7) M) for up to 7 days. LPL activity was measured in the medium and cell extracts, and LPL messenger RNA levels were measured by Northern blotting. When compared to control cells, 10(-7) M 1,25-(OH)2D3 increased medium LPL activity by 2- to 3-fold and cellular LPL by 1.5-fold. Significant increases in medium and cellular LPL were observed at 10(-9) M and were maximal at 10(-7) M. Along with the increase in LPL activity, there was an increase in LPL messenger RNA by 2-fold at 5 days, and by 5-fold at 7 days. In addition to an increase in LPL, 1,25-(OH)2D3 increased expression of aP2, an adipocyte-specific marker associated with differentiation. After the addition of 1,25-(OH)2D3, there was a decrease in 3T3-L1 cell number, which is consistent with differentiation, and a decrease in vitamin D receptors. Finally, these cells developed a different morphology. 1,25-(OH)2D3-treated cells assumed a rounded appearance, although without detachment from the dish and without the degree of lipid accumulation usually associated with the addition of insulin, isbutylmethylxanthine, and dexamethasone. It is concluded that 1,25-(OH)2D3 induced LPL expression in 3T3-L1 cells through an induction of differentiation-dependent mechanism(s). These findings suggest an important role for 1,25-(OH)2D3 in normal adipocyte differentiation.

Cell-matrix interaction in bone: type I collagen modulates signal transduction in osteoblast-like cells

Cell interaction with extracellular matrix (ECM) modulates cell growth and differentiation. By using in vitro culture systems, we tested the effect of type I collagen (Coll-I) on signal transduction mechanisms in the osteosarcoma cell line UMR-106 and in primary cultures from neonatal rat calvariae. Cells were cultured for 72 h on Coll-I gel matrix and compared with control cells plated on plastic surfaces. Agonist-dependent and voltage-dependent rises in cytosolic Ca2+ concentration ([Ca2+]i; measured by fura 2 fluorometry) were significantly blunted in cells cultured on Coll-I compared with cells grown on plastic. In UMR-106 cells, the collagen matrix effect was mimicked by 24-h incubation with soluble Coll-I or short peptides containing the arginine-glycine-aspartate motif. Accumulation of cellular adenosine 3',5'-cyclic monophosphate (cAMP) stimulated by parathyroid hormone, cholera toxin, and forskolin was augmented (50-150%) in cells plated on Coll-I vs. control. The collagen effect on both [Ca2+]i- and adenylate cyclase-signaling pathways in UMR-106 cells was abrogated in the presence of protein kinase C (PKC) depletion or inhibition. Also, Coll-I induced a twofold increase in membrane-bound PKC without changing cytosolic PKC activity. Thus, by altering PKC activity, Coll-I modulates the [Ca2+]i- and cAMP-signaling pathways in osteoblasts. This, in turn, may influence bone remodeling processes.

Involvement of cell cycle and mitogen-activated pathways in induction of parathyroid hormone-related protein gene expression in rat aortic smooth muscle cells

PTH-related protein (PTHrP) is induced in aortic vascular smooth muscle cells (VSMC) in association with mitogen-stimulated proliferation. In this study we examined the role of the cell cycle in the control of PTHrP gene expression. In asynchronously cycling cells grown in serum-containing medium, PTHrP-immunoreactive cells were enriched in G2+M, as revealed by fluorescence-activated cell sorting using a specific monoclonal antibody. PTHrP messenger RNA (mRNA) increased transiently in cells after release from chemically induced cell cycle blockade; levels increased by 10-fold at 2 h, coincident with expression of histone-4 mRNA and enrichment of VSMC in the early S phase. However, PTHrP mRNA levels then declined abruptly while the proportion of cells in the S phase and histone-4 mRNA levels remained constant for 8 h. When cell cycle-arrested cells were exposed to fresh serum-containing medium, angiotensin-II, or phorbol ester without removing the cell cycle blocking agents, PTHrP mRNA levels were induced over a time course identical to that observed in cells released from the blockade. This suggests that progression through the cycle per se is not necessary for mitogen-induced PTHrP mRNA expression, and that conventional chemical synchronization is not adequate to examine the cell cycle dependency of PTHrP mRNA abundance in VSMC. By contrast, in two different PTHrP-producing carcinoma cell lines, PTHrP and its mRNA were not altered as a function of cell cycle, demonstrating that different mechanisms control PTHrP expression in these cancer cells. In conclusion, constitutive immunoreactive levels of PTHrP are low in normally cycling VSMC (but not cancer cells) and accumulate during the latter stages of the cell cycle, suggesting a role for this protein in the process of smooth muscle cell division. However, separate mechanisms, which are independent of cell cycle, operate through a protein kinase-C-dependent pathway(s) to mediate the stimulation of PTHrP gene expression by vasoconstrictors such as angiotensin-II.

Parathyroid hormone (PTH)/PTH-related protein (PTHrP) receptor and its messenger ribonucleic acid in rat aortic vascular smooth muscle cells and UMR osteoblast-like cells: cell-specific regulation by angiotensin-II and PTHrP

PTH-related protein (PTHrP) is produced in vascular smooth muscle, where it is believed to act as a local vasorelaxant by activating either the classical PTH or a unique PTHrP receptor. We used a newly cloned complementary DNA encoding the rat PTH/PTHrP receptor to study the expression of its messenger RNA (mRNA) in primary aortic vascular smooth muscle cells (VSMC) and in UMR-106 osteoblast-like cells under basal conditions and in response to treatment with agonists. Both cell types expressed a 2.4-kilobase PTH/PTHrP receptor mRNA transcript and exhibited hormone-induced desensitization of PTHrP-(1-34)NH2-stimulated cAMP. In VSMC, angiotensin-II, which induces PTHrP expression, also rapidly (30 min) desensitized the cAMP response and down-regulated (75-90%) receptor mRNA within 1 h. Treatment of cells with phorbol 12-myristate 13-acetate (0.1 microM) mimicked these effects, whereas neither PTHrP-(1-34)NH2, forskolin, nor (Bu)2cAMP altered receptor mRNA expression. By contrast, in UMR-106 cells, PTHrP-(1-34)NH2 induced time- and dose-dependent decreases in receptor mRNA that were preceded by pronounced desensitization (cAMP and ligand binding) of cell surface receptors. These effects were mimicked by (Bu)2cAMP and forskolin, but not by phorbol 12-myristate 13-acetate, suggesting that both receptor mRNA down-regulation and receptor desensitization in UMR cells were mediated through a protein kinase-A pathway. We suggest that VSMC and UMR cells express a common receptor, which is subject to cell-specific regulation. Such diversity in the PTH/PTHrP receptor regulatory mechanisms provides a means for restricting the length and duration of the cellular response to hormone in a cell/tissue-specific manner.

Regulated production and intracrine action of 1,25-dihydroxyvitamin D3 in the chick myelomonocytic cell line HD-11

To better understand the extrarenal production of active vitamin D metabolites by cells of the monocyte/macrophage lineage, we investigated the 25-hydroxyvitamin D (25OHD)-1-hydroxylation reaction in the v-myc-transformed chick myelomonocytic cell line HD-11; the 1-hydroxylation reaction in this cell line has a high affinity for 25-hydroxylated vitamin D substrates, is localized to mitochondria, and is associated with cytochrome P450 activity. In this study we demonstrated that the HD-11 cell 1-hydroxylation reaction in vitro is not affected by the majority of extracellular regulatory factors that modulate expression of the renal 25OHD-1-hydroxylase in vivo. A 50% increase in extracellular calcium and phosphate concentrations, physiological inhibitory events for renal 1,25-dihydroxyvitamin D [1,25-(OH)2D] synthesis, did not decrease basal expression of the HD-11 cell 1-hydroxylation reaction, nor did a 50% decrease in extracellular calcium and phosphate concentrations, stimulatory signals for the 1-hydroxylase in vivo, increase 1,25-(OH)2D3 synthesis in vitro. Receptor-saturating concentrations of PTH and PTH-related peptide were similarly without effect. In contrast, the HD-11 1-hydroxylation reaction was significantly stimulated in a dose-dependent fashion by the macrophage stimulatory agents lipopolysaccharide [P < 0.001 at a maximum effective concentration (EC100) of 25 micrograms/ml] and interferon-gamma (P < 0.001 at EC100 of 1000 IU/ml) and by insulin-like growth factor-I (P < 0.01 at EC100 of 15 nM) with the rank order of stimulation being interferon-gamma > lipopolysaccharide > insulin-like growth factor-I. Dexamethasone (> or = 10 nM) and the cytochrome P450 inhibitors (EC100, 20 microM), ketoconazole, clotrimazole, and menadione, all significantly inhibited the HD-11 cell 1-hydroxylation reaction. The naphthoquinone menadione, which blocks electron transfer to the P450-associated enzyme, was the most effective inhibitor of the reaction in both intact cells (3 +/- 1% of basal expression; P < or = 0.002) and after reconstitution of HD-11 cell mitochondrial extracts with a ferredoxin, reductase, O2, and NADPH (5 +/- 1% of basal; P < or = 0.02). We have also shown that 1,25-(OH)2D3 produced from substrate 25OHD3 appears to exert an endogenous (intracrine) inhibitory effect on HD-11 cell growth; incubation of HD-11 cells with a concentration of ketoconazole (10 microM) known to reduce 1,25-(OH)2D3 production by roughly 50% restored 50% of the growth deficit induced by 1,25-(OH)2D3 (EC100, 100 nM).(ABSTRACT TRUNCATED AT 400 WORDS)

Mechanical stimuli induce vascular parathyroid hormone-related protein gene expression in vivo and in vitro

PTH-related peptide (PTHrP), the factor mediating the syndrome of humoral hypercalcemia of malignancy, is also expressed in smooth muscle cells (SMC) of the urinary bladder and uterus in response to mechanical distention and fetal occupancy, respectively. Vascular SMC also produce PTHrP, and its expression is induced by serum and vasoconstrictors, such as angiotensin-II. To determine whether mechanical distension affected vascular PTHrP gene expression, the abdominal aorta of adult male rats was balloon-distended, and aortae were collected at various times after the intervention. PTHrP mRNA was determined by competitive reverse transcriptase-polymerase chain reaction, using sequential dilutions of a cloned internally truncated PTHrP RNA fragment as standard. The molar concentration of PTHrP mRNA was obtained by extrapolating at a standard/wild-type band intensity ratio of 1:1. Aortic PTHrP mRNA was induced from a basal level of 19, to 22, 46, 36, 13, 12, 22, and 20 attamoles/mg total RNA 1, 2, 12, 24, and 48 h and 7 and 48 days after balloon distension, respectively. To determine whether mechanical events directly regulate vascular PTHrP gene expression, primary rat aortic SMC were plated and placed on a rocking device at 20 oscillations/min to create a gentle flowing motion of the culture medium. Rocking induced PTHrP mRNA of SMC exposed to either serum-free medium or 10% serum by 2.5-and 4.0-fold at 4 h, and 2.9- and 3.7-fold at 24 h, respectively. These effects were oscillation rate dependent, potentiated by angiotensin-II, and specific, as similar changes were not observed in alpha-actin mRNA content. Flow motion-induced PTHrP mRNA at 24 h was partially decreased by 10(-6) M colchicine (which inhibits microtubule assembly), but not by cytochalasin-E (which disrupts actin polymerization). As PTHrP is a known vasorelaxant, we propose that mechanical events induce the release of PTHrP by SMC, possibly to serve as a compliance factor or an agent for vascular remodeling.

A role for nitric oxide in the regulated expression of the 25-hydroxy-vitamin D-1-hydroxylation reaction in the chick myelomonocytic cell line HD-11

We have recently described the existence of a cytochrome P450-associated, mitochondrial-based 25-hydroxyvitamin D (25-OHD)-1-hydroxylation reaction in the chick macrophage-like cell line HD-11. Considering that this reaction is regulated by the same set of factors (ie. interferon-gamma, lipopolysaccharide, and glucocorticoids) that modulate expression of the macrophage nitric oxide synthase (mac NOS), we investigated the possibility that endogenous nitric oxide (NO) production may be linked to 1,25-dihydroxyvitamin D3 (1,25-(OH)2D) synthesis by HD-11 cells in vitro. To test this hypothesis we investigated the effects excluding from the extracellular medium the essential amino acid L-arginine, substrate for endogenous NO production, on the basal and stimulated expression of the HD-11 cell 25-OHD-1-hydroxylation reaction. Depletion of L-arginine from the extracellular medium for as little as 6 h resulted in a significant decrease (p < 0.02) in basal 1,25-(OH)2D synthesis; after 15 h in an L-arginine-free environment hormone production was reduced to < 10% of basal levels without any adverse affect on cell viability. Reintroduction of L-arginine, but not D-arginine, into the extracellular medium restored 1,25-(OH)2D3 synthetic capacity fully if done after < or = 6 h of incubation in the absence of L-arginine. Competitive inhibition of NOS with Nw-nitro-L-arginine methyl ester (p < 0.002) and Nw-nitro-L-arginine (p < 0.02) significantly inhibited 1,25-(OH)2D synthesis, indicating that macrophage NO generating capacity is functionally linked to endogenous synthesis of the active vitamin D metabolite.

Effects of N-terminal, midregion, and C-terminal parathyroid hormone-related peptides on adenosine 3',5'-monophosphate and cytoplasmic free calcium in rat aortic smooth muscle cells and UMR-106 osteoblast-like cells

N-Terminal analogs of PTH-related protein (PTHrP) and PTH bind to a common receptor and exhibit similar biological properties. However, recent studies suggest that certain midregion and C-terminal PTHrP peptides have activities distinct from those of PTH in the placenta and in osteoclasts, respectively. In this study we determined the biological activities of full-length recombinant PTHrP-(1-141) and several synthetic N-terminal, midregion, and C-terminal PTHrP fragments in two PTHrP-producing cell types. Peptides were tested for their ability to stimulate cAMP production and raise intracellular free calcium ([Ca2+]i) in primary rat aortic smooth muscle cells (VSMC) and UMR-106 rat osteoblast-like (UMR) cells. In UMR cells PTHrP-(1-34)NH2, PTHrP-(1-141), and bovine PTH-(1-34) all increased cAMP (approximately 50 fold) and [Ca2+]i (180 nM). By contrast, in VSMC, these N-terminal peptides increased cAMP (3-fold) but had no detectable effect on [Ca2+]i. PTHrP-(1-34) and PTHrP-(1-141) significantly blunted the angiotensin II-induced rise in cAMP (but not the calcium signal) consistent with the concept that PTHrP opposes angiotensin II activity in VSMC. PTHrP-(67-86)NH2, PTHrP-(107-138)NH2, and PTHrP-(107-111)NH2 had no effect on either cAMP or [Ca2+]i in either cell type. VSMC and UMR-106 cells both expressed a 2.5-kilobase PTH/PTHrP receptor messenger RNA (mRNA) transcript. However, high affinity specific binding of 125I-labeled [Tyr36] PTHrP-(1-36)NH2 was detected in UMR cells but not in VSMC. We conclude that the PTH-like, N terminus of the PTHrP molecule is critical in induction of cAMP and [Ca2+]i pathways in UMR cells, and for cAMP stimulation in VSMC. In addition, PTHrP, like other established vasodilators, signals in VSMC mainly (if not exclusively) by increasing the production of cAMP.

Exogenous calciferol (vitamin D) and vitamin D endocrine status among elderly nursing home residents in the New York City area

OBJECTIVE

To determine the role and relative importance of sources of exogenous calciferol (vitamin D) in maintaining vitamin D endocrine status in the mid-winter and early spring in a representative sample of institutionalized elderly persons in the New York City area.

DESIGN

Cross-sectional survey.

SETTING

A privately-run urban nursing home and the long-term care unit of a suburban public hospital.

PARTICIPANTS

Residents aged 60 years and older scheduled for a routine annual physical examination and an additional group of individuals ascertained by random sampling. Those with a history of anti-convulsant or glucocorticoid use, liver disease, chronic renal disease (or serum creatinine > 1.5 mg/dL), parathyroid disease, Paget's disease, gastric surgery, or pharmacological vitamin D use were excluded. Of 301 sampled individuals, 221 were found eligible to participate, and 109 were successfully enrolled.

RESULTS

The average vitamin D intake was 379 IU/day (range 55-1006 IU/day) and total vitamin D intake was below the Recommended Dietary Allowance in 16% of subjects. Fifty percent of total vitamin D intake came from fortified milk, and 26% came from vitamin supplements. Vitamin supplement use was not associated with low dietary intake. Among subjects taking a supplement containing 400 IU/day, none had serum calcidiol levels below 15 ng/mL, while among subjects with vitamin D intake between 200 and 400 IU/day, 46% had serum calcidiol levels below 15 ng/mL and 14% had calcidiol levels below 10 ng/mL. Vitamin D intake from non-supplement sources (but not from supplements) appeared to have a negative association with serum calcitriol levels.

CONCLUSIONS

Many nursing home residents may require vitamin supplements in order to achieve optimal levels of calciferol replacement. The choice of a vehicle for calciferol replacement may affect calcitriol levels.

Subcellular localization and partial purification of the 25-hydroxyvitamin D3 1-hydroxylation reaction in the chick myelomonocytic cell line HD-11

Hypercalcemia in human granuloma-forming diseases like sarcoidosis results from the endogenous overproduction of 1,25-dihydroxyvitamin D [1,25-(OH)2D] by disease-activated tissue macrophages. The recent identification of an immortalized chick myelomonocytic cell line, HD-11, that constitutively expresses a 25-hydroxyvitamin D (25-OHD) 1-hydroxylation reaction has alleviated dependence on studying primary macrophage cultures with no replicative potential in vitro. In these experiments we established conditions for the maximal expression of the HD-11 cell 25-OHD3-1-hydroxylation reaction and localized this activity to the mitochondrial fraction. On a per cell basis, the activity of HD-11 cell 25-OHD3 1-hydroxylation reaction was comparable to that in primary cultures of chick renal tubular epithelial cells, which express the authentic renal 25-OHD3 1-hydroxylase. Maximal product yield was achieved after incubation of HD-11 cells with 200 nM 25-OHD3 for 3 h. Although adherent monolayers possessed 3- to 4-fold more capacity for hormone production than cells in suspension, suspended cells exhibited easily detectable 25-OHD3 catalytic activity (0.58 +/- 0.08 pmol per 10(6) cells per h; +/- SEM), 50% of which remained solubilized in a sonicate of suspended cells cleared of nuclei and plasma membrane. Subcellular localization disclosed 91% of the residual activity to be concentrated in the mitochondrial subfraction. A detergent-solubilized extract of this mitochondrial subfraction contained 1.9 +/- 0.3 pmol 1,25-(OH)2D3 synthetic capacity per mg protein. The catalytic activity (1-hydroxylase activity) was concentrated 20.2-fold after chromatography on octyl-amino agarose and was associated with 0.054 nmol cytochrome P450 per mg protein.(ABSTRACT TRUNCATED AT 250 WORDS)

Angiotensin II regulates parathyroid hormone-related protein expression in cultured rat aortic smooth muscle cells through transcriptional and post-transcriptional mechanisms

Parathyroid hormone-related protein (PTHrP), a tumor product responsible for malignancy-associated hypercalcemia, is also produced in many normal tissues, including vascular smooth muscle cells (SMC). As PTHrP exhibits vasodilatory properties, we postulated that other vasoactive agents may control PTHrP gene expression in SMC. Addition of angiotensin II to serum-deprived SMC resulted in a marked induction of PTHrP mRNA by 2 h, with a peak (6-10-fold) at 4-6 h. Angiotensin II effects on PTHrP gene expression were inhibited by saralasin, an angiotensin II receptor antagonist, and blocked by actinomycin D and cycloheximide, suggesting a requirement for gene transcription and protein synthesis. Nuclear run-off assays revealed a 3-fold increase in PTHrP gene transcription 1 h after angiotensin II treatment. Angiotensin II also prolonged PTHrP mRNA half-life by 2-3-fold. Angiotensin-induced PTHrP mRNA is partially dependent on cyclooxygenase products and protein kinase C activation. Other vasoconstrictor substances, including serotonin and bradykinin, also stimulated PTHrP expression, whereas the vasodilator atrial natriuretic peptide did not. Addition of recombinant PTHrP-(1-141) significantly inhibited angiotensin II-induced SMC DNA synthesis. PTHrP expression is increased by angiotensin II through transcriptional and post-transcriptional mechanisms. In addition, PTHrP modulates the effect of angiotensin II on SMC proliferation. This suggests that PTHrP acts locally in SMC, possibly to oppose the vasoactive and/or growth-promoting effects of vasoconstrictor agents such as angiotensin II.

Regulation of avian calbindin-D28K gene expression in primary chick kidney cells: importance of posttranscriptional mechanisms and calcium ion concentration

Vitamin D-dependent calcium-binding protein (calbindin-D28K), is regulated by 1,25-dihydroxyvitamin D3 [1,25-(OH)2D3], and several other factors in a tissue-specific manner, but the controlling mechanisms are still poorly understood. In this study we examined the relative contributions of transcriptional and posttranscriptional mechanisms in the 1,25-(OH)2D3 control of calbindin-D28K mRNA expression in primary chick kidney cells and investigated the effect of extracellular Ca2+ on calbindin-D28K gene expression in the presence and absence of hormone. 1,25-(OH)2D3 treatment (10(-8) M) of cells grown in serum-free medium resulted in a marked 20- to 30-fold increase in calbindin-D28K mRNA peaking at 12-18 h, which then rapidly declined to basal levels by 24 h. The abrupt decline in mRNA appeared to be associated with a reduction in size of the calbindin-D28K transcripts. Nuclear run-off assays showed only a slight (1.5-fold) increase in calbindin-D28K gene transcription 2 h after 1,25-(OH)2D3, whereas parallel assays clearly demonstrated a marked (7-fold) induction in the rate of metallothionein gene transcription 2 h after treatment of chick kidney cells with 10 microM zinc. The induction of calbindin-D mRNA by 1,25-(OH)2D3 required ongoing protein synthesis, since it was blocked by cycloheximide. Calbindin-D28K mRNA was stable for 12 h in the presence of actinomycin-D in both vitamin D-deficient and 1,25-(OH)2D3-treated cells. Both basal and 1,25-(OH)2D3-induced calbindin-D28K mRNA were modulated by the extracellular Ca2+, with maximum expression occurring at 1-2 mM. We conclude that 1,25-(OH)2D3 induces kidney calbindin-D28K mRNA by producing a small increase in its transcriptional rate, which is accompanied by pronounced posttranscriptional effects(s). The striking modulation of calbindin-D28K expression by extracellular Ca2+ is consistent with a putative role for this protein in the regulation of this ion in the kidney cell.

A 7-34 analog of the parathyroid hormone-related protein has potent antagonist and partial agonist activity in vivo

The biological properties of a new synthetic analog of parathyroid hormone-related protein [PTHrP-(7-34)NH2] were examined in vivo using a well characterized thyroparathyroidectomized (TPTX) rat model. The phosphaturic and urine cyclic AMP response induced by infusion of PTHrP-(1-34)NH2 (0.16 nmol/h) was inhibited by 70% (P less than 0.01, n = 6) by co-infusion of PTHrP-(7-34)NH2 at a 10-fold molar excess (1.6 nmol/h). The 7-34 PTHrP analog also antagonized the PTHrP-(1-34)NH2-induced hypercalcemia and rises in blood 1,25-dihydroxyvitamin D concentrations. However, when infused alone at a higher dose rate (8 nmol/h), PTHrP-(7-34)NH2 displayed significant PTH agonist activity. This profile contrasts to that of [Tyr-34]bPTH-(7-34)NH2 which is comparatively less potent (10-20-fold) with respect to its antagonist activity but has no appreciable agonist activity in vivo.

Calcium, vitamin D, and parathyroid hormone status in young white and black women: association with racial differences in bone mass

The etiology of the racial disparity in bone mass and fracture rate is unknown. Since the PTH-vitamin D endocrine system is a major regulator of calcium metabolism and bone turnover, this cross-sectional study examined the relationship of radial and lumbar bone density to vitamin D metabolite and PTH concentrations and to calcium intake and excretion in 67 white and 70 black highly comparable, healthy, premenopausal women. Bone density at both radial and lumbar sites was higher in blacks than in whites. Serum 25-hydroxyvitamin D was slightly but not statistically significantly (P = 0.08), lower in blacks than in whites, but there were no racial differences in 1,25-dihydroxyvitamin D, PTH, or renal tubular maximum for reabsorption of phosphate. The mean 25-hydroxyvitamin D concentration in blacks was well within the normal range and was not associated with evidence of secondary hyperparathyroidism. There were no correlations of bone density to vitamin D or PTH concentrations. Although there were no racial differences in dietary intake of calcium and vitamin D or in sodium excretion, 24-h urinary calcium excretion was significantly lower in blacks than in whites, and calcium excretion was inversely associated with radial bone density. In contrast to previous reports, in healthy, normal weight, premenopausal black women there is no evidence of vitamin D deficiency or secondary hyperparathyroidism, suggesting that factors other than the vitamin D-PTH axis are responsible for racial differences in bone mass.

Constitutive expression of a vitamin D 1-hydroxylase in a myelomonocytic cell line: a model for studying 1,25-dihydroxyvitamin D production in vitro

The capacity of the v-myc-transformed, chicken myelomonocytic cell line HD-11 to metabolize 25-hydroxyvitamin D3 (25-OHD3) was examined. HD-11 cells produced and secreted a metabolite of 25-OHD3 that was bound with high affinity by receptor for 1,25-dihydroxyvitamin D3 [1,25-(OH)2D3]. On normal-phase HPLC, this metabolite cochromatographed with authentic 1,25-(OH)2D3 in both hexane- and methylene chloride-based solvent systems. The 25-OHD3 1-hydroxylation reaction was substrate saturable with a Km of 73 nM 25-OHD3 and a maximal velocity of 167 fmol per 10(6) cells per h. This reaction was inhibited by ketoconazole, a recognized inhibitor of cytochrome P450 mixed-function oxidases including the authentic, renal 25-OHD3 1-hydroxylase. On the other hand, HD-11 cell 1,25-(OH)2D3 production was not affected by the antioxidant DPPD, a known inhibitor of free radical-generated 1,25-(OH)2D3. In addition to synthesizing 1,25-(OH)2D3, this monocyte-macrophage cell line also has the potential to be a target for the hormone; HD-11 cells express high-affinity receptor for 1,25-(OH)2D3 (Kin = 0.06 nM).

Vitamin D supplementation in elderly nursing home residents increases 25(OH)D but not 1,25(OH)2D

Vitamin D metabolism in elderly individuals can be compromised by several mechanisms. We previously described reduced concentrations of 1,25-dihydroxyvitamin D [1,25(OH)2D] in 30% of elderly nursing home residents. The present study assesses the effect of vitamin D supplementation on 25-hydroxyvitamin D [25(OH)D] and 1,25(OH)2D. We performed a double-blind study in which 30 elderly nursing home residents were randomly given either 50 micrograms vitamin D or a placebo daily for 6 wk. Vitamin D metabolites, immunometrically assayed parathyroid hormone (IRMA-PTH), ionized calcium, and bone Gla hormone (BGP) were measured in serum at baseline and biweekly for 6 wk. Serum 25(OH)D concentrations increased significantly (P less than 0.0001) over the 6 wk in the treatment group but were unchanged in the placebo group. Serum 1,25(OH)2D, ionized calcium, BGP, and PTH were not significantly altered by the supplement. We conclude that vitamin D supplementation results in an increase in circulating 25(OH)D but not 1,25(OH)2D; however, the long-term effect on bone mineral metabolism remains unclear.

Nephrolithiasis and bone involvement in primary hyperparathyroidism

PURPOSE

The purpose of this study was to compare patients with primary hyperparathyroidism with and without nephrolithiasis with regard to (1) biochemical profile, and (2) presence and extent of bone involvement.

PATIENTS AND METHODS

Of 70 unselected patients enrolled in a longitudinal study on the natural history of primary hyperparathyroidism, 62 who underwent complete bone densitometry evaluation were considered. The patients had mild hypercalcemia (2.77 +/- 0.02 mmol/L), as well as elevated parathyroid hormone levels by mid-molecule, N-terminal, and immunoradiometric assays. Bone densitometry was assessed by dual-photon absorptiometry of the lumbar spine and femoral neck, and single-photon absorptiometry of the forearm.

RESULTS

Eleven of the 62 patients (18%) had nephrolithiasis. There was no difference in serum parathyroid hormone levels, calcium, phosphorus, serum 25-hydroxyvitamin D, and 1,25-dihydroxyvitamin D3 between those with and without kidney stones. Total daily urinary calcium excretion was higher among those who formed stones (8.2 +/- 1.0 mmol versus 6.1 +/- 0.4 mmol, p less than 0.05), but not when expressed per mmol of creatinine (0.72 +/- 0.07 versus 0.69 +/- 0.04). Urinary hydroxyproline was also higher in patients who formed stones (58 +/- 11 mg/24 hours versus 37 +/- 2 mg/24 hours; p less than 0.05). Hypercalciuria occurred in 39% of the entire cohort (n = 24), and in 33% (n = 17) of those without stones. Only 29% (n = 7) of those with hypercalciuria had nephrolithiasis. Calcium excretion correlated positively with serum 1,25-dihydroxyvitamin D3 (r = +0.32, p less than 0.05), and negatively with forearm bone mineral density (all patients: r = -0.34, p less than 0.05; hypercalciuric patients: r = -0.53, p less than 0.05). Circulating 1,25-dihydroxyvitamin D3 levels were elevated in a similar proportion of (1) all patients (31%, n = 19); (2) those with nephrolithiasis (27%); and (3) those without stones (31%). Bone mineral density was less than 80% of normal in 61% of patients, but forearm, femoral neck, and lumbar spine density were indistinguishable among those with and without stones.

CONCLUSIONS

Cortical bone demineralization occurs to the same extent and frequency in patients with and without nephrolithiasis, and these two subgroups share similar biochemical and bone densitometric profiles. The pathophysiologic events leading to renal and skeletal involvement in primary hyperparathyroidism may be less selective than previously believed, as evidenced by: (1) increased urinary hydroxyproline in patients with nephrolithiasis, and (2) documentation that urinary calcium excretion reflects not only vitamin D status, but bone resorption was well.

The synthetic human parathyroid hormone-related protein is inhibited by a parathyroid hormone antagonist in rats in vivo

The structure of a novel protein, parathyroid hormone-related protein (PTHrP), secreted by human tumors associated with hypercalcemia has recently been determined. Administration of a synthetic fragment of this protein in vivo reproduces features of the clinical paraneoplastic syndrome of humoral hypercalcemia of malignancy and produces biologic responses closely similar to those obtained with parathyroid hormone (PTH). A PTH antagonist designed to reversibly occupy PTH receptors inhibited major actions of the tumor peptide in vivo, including phosphaturia, urinary cAMP excretion, and increased serum ionized calcium. These studies indicate that PTHrP and PTH mediate their bioactivities through shared receptors in vivo and establish a potential specific mechanism-based approach utilizing PTH antagonists for the therapy of tumor-associated hypercalcemia.

Suppression of parathyroid hormone secretion with oral calcium in normal subjects and patients with primary hyperparathyroidism

Exquisite sensitivity of normal parathyroid glands to small changes in ambient calcium concentrations and impaired sensitivity in primary hyperparathyroidism have been shown in vitro. Using an assay for PTH that detects rapid changes in PTH secretion (N-terminal-specific RIA; normal range, less than 3-33 pg/mL), we determined PTH suppressibility in response to a standardized dose of oral calcium in normal subjects and patients with primary hyperparathyroidism. Nine normal subjects were given oral calcium (25 mg/kg), and blood was analyzed half-hourly for 3 h for calcium and N-terminal PTH (N-PTH). Serum calcium rose by 0.34 +/- 0.06 mg/dL (0.085 +/- 0.015 mmol/L), and N-PTH levels declined rapidly from 15.3 +/- 1.4 to 4.2 +/- 1.1 pg/mL (-73 +/- 6%; P less than 0.01). In six subjects N-PTH concentrations became undetectable. Nine patients with primary hyperparathyroidism were tested in the same manner. Serum calcium rose by 0.53 +/- 0.1 mg/dL (0.13 +/- 0.025 mmol/L), and N-PTH levels declined less, from 66 +/- 14 to 52 +/- 12 pg/mL (-21 +/- 4%; P less than 0.05). In none of the patients was the PTH reduced to less than 20 pg/mL. These results illustrate in vivo that the PTH response to oral calcium in primary hyperparathyroidism is markedly different from that in normal subjects.

Useful clinical assays for vitamin D metabolites

Advances in our understanding of the physiology of vitamin D and its importance in health and disease have depended on the accurate measurement of its metabolites in blood. Assays that were once cumbersome and insensitive are now performed easily, are highly sensitive, reproducible, and relatively inexpensive. The availability of these modern techniques has facilitated the clinical diagnosis and treatment of patients.

Extracellular calcium modulates vitamin D-dependent calbindin-D28K gene expression in chick kidney cells

The effect of extracellular calcium ion (Ca2+) concentration on 1,25-dihydroxyvitamin D3 (1,25-(OH)2D3)-induction of vitamin D-dependent calcium-binding protein (calbindin-D28K) and its mRNA levels was examined in primary chick kidney cells in vitro. When exposed to normal medium Ca2+ (1.0 mM), 1,25-(OH)2D3 increased calbindin-D28K mRNA, as measured by Northern analysis, by 4-10 fold over basal levels by 12 to 24 h after addition of hormone. In the presence of 0.5 mM Ca2+, 1,25-(OH)2D3 induced calbindin-D28K mRNA by only 2 fold, whereas, when cells were exposed to 2 mM Ca2+, the induction was 10-15 fold. This calcium modulation of 1,25-(OH)2D3 induction was also observed at the level of calbindin-D28K protein concentrations as measured by radioimmunoassay. The alterations in medium Ca2+ were not associated with any change in the rate of total RNA or protein synthesis. These studies suggest that both Ca2+ and 1,25-(OH)2D3 participate in the regulation of calbindin-D28K gene expression in the kidney.

Abnormalities in parathyroid hormone secretion and 1,25-dihydroxyvitamin D3 formation in women with osteoporosis

We investigated the parathyroid hormone-1,25-dihydroxyvitamin D3 (1,25(OH)2D) axis in osteoporosis by administering phosphate to 8 postmenopausal women with osteoporosis (49 to 78 years old) and to 10 normal women matched for age (50 to 74 years). All subjects responded with a similar increase in the serum phosphorus concentration (women with osteoporosis, 1.15 +/- 0.06 to 1.79 +/- 0.09 mmol per liter; controls, 1.14 +/- 0.05 to 1.73 +/- 0.08 mmol per liter) and a fall in the ionized calcium concentration (women with osteoporosis, 1.12 +/- 0.03 to 1.06 +/- 0.03 mmol per liter; controls, 1.17 +/- 0.01 to 1.11 +/- 0.02 mmol per liter). Parathyroid hormone levels rose 2.5-fold in the control group (15.4 +/- 2.2 to 37.9 +/- 6.1 pg per milliliter) but increased by only 43 percent in the group with osteoporosis (14.8 +/- 2.8 to 21.2 +/- 4.1 pg per milliliter), an increase similar to that previously reported in young normal subjects (53 percent). In healthy older and younger subjects, the levels of 1,25(OH)2D did not change; in the subjects with osteoporosis, however, they decreased significantly (50 percent). We conclude that older women require a greater parathyroid hormone stimulus than younger women to maintain vitamin D homeostasis, because of an age-related decline in the formation of 1,25(OH)2D in response to parathyroid hormone, and that in osteoporosis the age-appropriate parathyroid hormone response to the same hypocalcemic signal is diminished. Our results are consistent with the presence of an abnormality in parathyroid hormone secretory function in osteoporosis in addition to the universal decline in 1,25(OH)2D responsiveness associated with aging.

Bone mineral density in women with type II diabetes mellitus

Bone mineral density (BMD) was assessed in 28 women with type II diabetes mellitus and compared to 207 age-matched nondiabetic women. Mean BMD, as measured by dual-photon absorptiometry, 1.12 +/- 0.3 g/cm2 (+/- SEM), was similar to the mean BMD of control subjects, 1.06 +/- 0.1 g/cm2. Only 1 of the 28 diabetic patients had a BMD less than 0.95 g/cm2 ("fracture threshold"), whereas 25% of the control subjects had a BMD below that level. When diabetic and control subjects were matched for weight as well as age, the data continued to show similar BMD among both groups. Moreover, the disparity between the proportion of weight-matched controls (25%) and diabetic subjects (1 of 28) with a BMD below the fracture threshold persisted. Among the group of 17 diabetic subjects receiving insulin, there was a positive relationship between BMD and insulin dose. There was no significant relationship between BMD, duration of diabetes, or hemoglobin Alc. Thus, women with type II diabetes are not at increased risk for diminished BMD and may be protected against bone loss.

Tissue-specific regulation of avian vitamin D-dependent calcium-binding protein 28-kDa mRNA by 1,25-dihydroxyvitamin D3

We have studied the regulation, by 1,25-dihydroxyvitamin D3 (1,25-(OH)2D3), of vitamin D-dependent calcium-binding protein (28-kDa CaBP) mRNA in chick tissues in vivo. Northern analysis of poly(A)+ RNA was carried out using, as hybridization probes, synthetic oligonucleotides complementary to chick 28-kDa CaBP mRNA. In vitamin D-deficient chicks, 28-kDa CaBP mRNA was virtually undetectable in intestine, was clearly detectable in kidney, and present at the highest levels in cerebellum. After a single intravenous dose of 500 ng of 1,25-(OH)2D3, intestinal 28-kDa CaBP mRNA levels were increased 50-fold, kidney levels were increased 4-fold, and cerebellum levels were unchanged. Increased levels of 28-kDa CaBP mRNA were appreciated 2 h after induction and were maximal at 12 h. Pretreatment of vitamin D-deficient chicks with actinomycin D had little effect on the acute phase of the 1,25-(OH)2D3 induction of 28-kDa CaBP mRNA in intestine but blunted the induction in kidney. Pretreatment with cycloheximide caused a delayed response to 1,25-(OH)2D3 in the intestine, although control (noninhibition) levels of 28-kDa CaBP mRNA were present 12 h after hormone administration. By contrast, in the kidney, cycloheximide pretreatment resulted in an increased steady-state (vitamin D-deficient) level of 28-kDa CaBP mRNA, but completely abolished the induction of 1,25-(OH)2D3. Our studies indicate that, whereas 1,25-(OH)2D3 does not regulate 28-kDa CaBP mRNA levels in the brain, the hormone modulates 28-kDa CaBP gene expression in intestine and kidney in a tissue-specific manner, by acting through both transcriptional and post-transcriptional mechanisms.

Immunocytochemical localization of the 1,25-dihydroxyvitamin D3 receptor in target cells

We have used a monoclonal antibody (9A7) against the purified avian 1,25-dihydroxyvitamin D3 receptor to develop an immunocytochemical technique for visualization of the protein in fixed tissues and cultured cells. In Bouin's-fixed, chick intestine, 1,25-dihydroxyvitamin D3 receptor-like immunoreactivity was localized mainly in nuclei of epithelial cells and was more abundant in the crypt than in the villar cells. Receptor staining was low or undetectable in liver hepatocytes but was present in nuclei of cells lining the hepatic sinusoids. In rat brain, receptor-like immunoreactivity was abundant and widely distributed, but did not always coincide with the presence of vitamin D-dependent calcium-binding protein; 1,25-dihydroxyvitamin D3 receptor was absent from cerebellar Purkinje cells that contained abundant calcium-binding protein. In disaggregated rat bone cells, receptor immunoreactivity was present in mononuclear cells including osteoblasts and fibroblasts but was absent from osteoclasts. Two separate clones of osteoblast-like, rat osteosarcoma cells, shown in previous studies to be either receptor positive (17/2.8) or negative (24.1), demonstrated nuclear immunoreactivity in exact concordance with receptor levels as determined by ligand binding. The phenomenon of hormone-induced up-regulation of receptor was visualized in receptor-positive 3T6 fibroblasts by demonstration of markedly enhanced nuclear reactivity in cells treated with 10(-7) M 1,25-dihydroxyvitamin D3 for 48 h. Our studies demonstrate the feasibility of the immunocytochemical approach to visualize the 1,25-dihydroxyvitamin D3 receptor in target tissues and show that it is predominantly a nuclear protein in the relatively unoccupied and fully activated states. Moreover, the vitamin D-dependent calcium binding is not a universal marker for 1,25-dihydroxyvitamin D3 action. Rather, our observations suggest that the expression of the 1,25-dihydroxyvitamin D3 receptor may be connected with the state of cellular differentiation.

Animal model of primary hyperparathyroidism

An experimental model of hyperparathyroidism was developed in the rat to simulate primary hyperparathyroidism in humans. In this model thyroparathyroidectomized (TPTX) or parathyroidectomized (PTX) animals were infused for 6 days with an amount of bovine synthetic parathyroid hormone (PTH)-(1-34) fragment to restore plasma calcium levels to normal (0.7 U X h-1) or with PTH at twofold (1.4 U X h-1) or threefold (2.1 U X h-1) this basal level. Animals infused with 2.1 U X h-1 of bovine PTH-(1-34) exhibited hypercalcemia, hypophosphatemia, a reduction in theoretical renal threshold for phosphate and an increase in 1,25-dihydroxyvitamin D plasma levels that were approximately threefold the control value. In addition, these animals demonstrated nephrocalcinosis and changes of bone histology that were typical of the findings in patients with primary hyperparathyroidism. In contrast, in animals infused at 1.4 U X h-1, plasma calcium, phosphate, and theoretical renal threshold for phosphate remained within normal limits, but plasma 1,25-dihydroxyvitamin D was increased above control, suggesting that increased activity of 1 alpha-hydroxylase may be the most sensitive index of increased PTH levels. This animal model permits sustained elevation of PTH plasma levels at basal or pathologically elevated levels and should provide an effective means by which to evaluate the consequences of chronic hyperparathyroidism on epithelial function, bone, and other organ systems.