PTH/PTHrP receptor is temporally regulated during osteoblast differentiation and is associated with collagen synthesis

Variable osteogenic performance of MC3T3-E1 subclones impacts their utility as models of osteoblast biology

The spontaneously immortalized murine calvarial cell line MC3T3-E1 and its derivative subclones are widely used models of osteoblast biology. Many investigators have reported conflicting data under seemingly similar experimental conditions, though the specific subclone studied is often not specified. The purpose of this study was to directly compare the commercially available MC3T3-E1 subclones 4, 14, and 24 in terms of responsiveness to osteogenic induction media and/or stimulation with rhPTH[1–34]. We assayed osteogenic gene expression, capacity to deposit and mineralize a collagenous matrix, and the expression and signaling function of PTH1R. Our data demonstrate that each subclone bears little functional resemblance to the others, or to primary calvarial osteoblasts. Specifically, whereas subclone 4 is responsive to PTH stimulation and capable of matrix mineralization, subclones 14 and 24 do not faithfully replicate these key aspects of osteoblast biology. Furthermore, little overlap was observed between the gene expression profile of subclone 4 and primary calvarial osteoblasts. Our experience working with these cell lines demonstrates that the MC3T3-E1 derived cell lines are imperfect models of osteoblast biology, and reinforce the importance of clearly articulating selection and reporting of research materials.

The effect of increased frequency of hemodialysis on vitamin C concentrations: an ancillary study of the randomized Frequent Hemodialysis Network (FHN) daily trial

BACKGROUND

Reports on vitamin C in HD patients have shown effects of vitamin C deficiency in association with scurvy symptoms. Dialyzability of water soluble vitamins is high, and substantial losses in those who are dialyzed more frequently were hypothesized. The randomized FHN Daily Trial compared the effects of in-center HD six versus three times per week. We studied baseline correlations between vitamin C and potentially associated parameters, and the effect of more frequent HD on circulating vitamin C concentrations.

METHODS

We studied vitamin C levels at baseline and months, 3, 5 and 11. Patients enrolled between 2007 and 2009 into the randomized FHN Daily trial in the East Coast consortium were approached for participation. Predialysis plasma samples were processed with metaphosphoric acid and frozen at - 70 °C for measurement with HPLC. Regression models between baseline log-transformed vitamin C and hemoglobin, CRP, eKt/V, ePCR and PTH, and a linear mixed-effects model to estimate the effect size of more frequent HD on plasma vitamin C, were constructed.

RESULTS

We studied 44 subjects enrolled in the FHN Daily trial (50 ± 12 years, 36% female, 29% Hispanics and 64% blacks, 60% anuric). Vitamin C correlated significantly with predialysis hemoglobin (r = 0.3; P = 0.03) and PTH (r = - 0.3, P = 0.04), respectively. Vitamin C did not significantly differ at baseline (6×/week, 25.8 ± 25.9 versus 3×/week, 32.6 ± 39.4 μmol/L) and no significant treatment effect on plasma vitamin C concentrations was found [- 26.2 (95%CI -57.5 to 5.1) μmol/L at Month 4 and - 2.5 (95%CI -15.6 to 10.6) μmol/L at Month 12.

CONCLUSIONS

Based on data from this large randomized-controlled trial no significant effect of the intervention on circulating plasma vitamin C concentrations was found, allaying the concerns that more frequent HD would affect the concentrations of water-soluble vitamins and adversely affect patient's well-being. Correlations between vitamin C and hemoglobin and PTH support the importance of vitamin C for normal bone and mineral metabolism, and anemia management.

Impact of the Autonomic Nervous System on the Skeleton

It is from the discovery of leptin and the central nervous system as a regulator of bone remodeling that the presence of autonomic nerves within the skeleton transitioned from a mere histological observation to the mechanism whereby neurons of the central nervous system communicate with cells of the bone microenvironment and regulate bone homeostasis. This shift in paradigm sparked new preclinical and clinical investigations aimed at defining the contribution of sympathetic, parasympathetic, and sensory nerves to the process of bone development, bone mass accrual, bone remodeling, and cancer metastasis. The aim of this article is to review the data that led to the current understanding of the interactions between the autonomic and skeletal systems and to present a critical appraisal of the literature, bringing forth a schema that can put into physiological and clinical context the main genetic and pharmacological observations pointing to the existence of an autonomic control of skeletal homeostasis. The different types of nerves found in the skeleton, their functional interactions with bone cells, their impact on bone development, bone mass accrual and remodeling, and the possible clinical or pathophysiological relevance of these findings are discussed.

Preprogrammed Long-Term Systemic Pulsatile Delivery of Parathyroid Hormone to Strengthen Bone

Parathyroid hormone (PTH) is the only US Food and Drug Administration (FDA)-approved anabolic agent for the treatment of osteoporosis. The anabolic action of PTH depends on the mode of PTH administration. Pulsatile administration promotes bone formation, however continuous PTH exposure results in bone resorption. In addition, the therapeutic effect of PTH is optimal when the dose and duration fit the therapeutic window. Current PTH treatment requires daily injection, which is neither a convenient nor a favorable choice of patients. Here, an implantable and biodegradable device capable of long-term pulsatile delivery of PTH is developed as a patient-friendly alternative. The advanced materials and fabrication techniques developed in this work enable us to preprogram a pulsatile delivery device to systemically deliver 21 daily pulses of PTH that build bone in vivo. In addition, the device is biodegradable and absorbable in vivo so that no retraction procedure is needed. Therefore, this implantable and biodegradable pulsatile device holds promise to promote bone growth and treat various conditions of bone loss without the burden of daily injections or secondary surgeries.

PTH Promotes Bone Anabolism by Stimulating Aerobic Glycolysis via IGF Signaling

Teriparatide, a recombinant peptide corresponding to amino acids 1-34 of human parathyroid hormone (PTH), has been an effective bone anabolic drug for over a decade. However, the mechanism whereby PTH stimulates bone formation remains incompletely understood. Here we report that in cultures of osteoblast-lineage cells, PTH stimulates glucose consumption and lactate production in the presence of oxygen, a hallmark of aerobic glycolysis, also known as Warburg effect. Experiments with radioactively labeled glucose demonstrate that PTH suppresses glucose entry into the tricarboxylic acid cycle (TCA cycle). Mechanistically, the increase in aerobic glycolysis is secondary to insulin-like growth factor (Igf) signaling induced by PTH, whereas the metabolic effect of Igf is dependent on activation of mammalian target of rapamycin complex 2 (mTORC2). Importantly, pharmacological perturbation of glycolysis suppresses the bone anabolic effect of intermittent PTH in the mouse. Thus, stimulation of aerobic glycolysis via Igf signaling contributes to bone anabolism in response to PTH.

The effect of vitamin C on parathyroid hormone in patients on hemodialysis with secondary hyperparathyroidism: a double blind, placebo-controlled study

BACKGROUND

Secondary hyperparathyroidism (SHPT) is a prevalent disorder in patients with chronic kidney disease. It is proffered that there is a contradictory relation between serum level of vitamin C and parathyroid hormone (PTH) in hemodialysis patients with secondary hyperparathyroidism.

OBJECTIVES

The goal of this study was to assess the effects of the supplemental vitamin C on parathyroid hormone among hemodialysis patients with secondary hyperparathyroidism.

PATIENTS AND METHODS

This randomized, placebo-controlled, double-blind and parallel-group trial was conducted on 82 hemodialysis patients with serum levels of PTH more than 200 pg/mL. In intervention group, 250 mg vitamin C was injected three times a week for 8 weeks in a row immediately at the end of each dialysis session via the intravenous route. In the control group, same term of placebo saline was injected.

RESULTS

The mean of serum PTH was 699.81 (± 318.8) and 596.03 (± 410.7) pg/mL in intervention and control groups respectively at baseline (reference range, 6 to 66 pg/mL), and at the end of study it changed to 441.4 and 424.6 in these groups. The values of serum Calcium and Phosphate did not significantly change during the study (8.4 ± 0.6 mg/dL versus 8.1 ± 0.8 mg/dL, P = 0.39; 5.89 ± 1.7 mg/dL versus 5.9 ± 1.9 mg/dL, P = 0.08, respectively).

CONCLUSIONS

This study finding does not warranted therapeutic effect of vitamin C on secondary hyperparathyroidism.

Developmental cues for bone formation from parathyroid hormone and parathyroid hormone-related protein in an ex vivo organotypic culture system of embryonic chick femora

Enhancement and application of our understanding of skeletal developmental biology is critical to developing tissue engineering approaches to bone repair. We propose that use of the developing embryonic femur as a model to further understand skeletogenesis, and the effects of key differentiation agents, will aid our understanding of the developing bone niche and inform bone reparation. We have used a three-dimensional organotypic culture system of embryonic chick femora to investigate the effects of two key skeletal differentiation agents, parathyroid hormone (PTH) and parathyroid hormone-related protein (PTHrP), on bone and cartilage development, using a combination of microcomputed tomography and histological analysis to assess tissue formation and structure, and cellular behavior. Stimulation of embryonic day 11 (E11) organotypic femur cultures with PTH and PTHrP initiated osteogenesis. Bone formation was enhanced, with increased collagen I and STRO-1 expression, and cartilage was reduced, with decreased chondrocyte proliferation, collagen II expression, and glycosaminoglycan levels. This study demonstrates the successful use of organotypic chick femur cultures as a model for bone development, evidenced by the ability of exogenous bioactive molecules to differentially modulate bone and cartilage formation. The organotypic model outlined provides a tool for analyzing key temporal stages of bone and cartilage development, providing a paradigm for translation of bone development to improve scaffolds and skeletal stem cell treatments for skeletal regenerative medicine.

Tenofovir treatment of primary osteoblasts alters gene expression profiles: implications for bone mineral density loss

There is strong clinical evidence that implicates tenofovir in the loss of bone mineral density during treatment of human immunodeficiency virus infection. In this study, we sought to test the hypothesis that tenofovir treatment of osteoblasts causes changes in the gene expression profile that would impact osteoblast function during bone formation. Primary osteoblasts were isolated and then treated with the tenofovir prodrug, tenofovir disoproxil fumarate (TDF). Total RNA from TDF-treated and untreated osteoblasts were extracted and used for microarray analysis to assess TDF-associated changes in the gene expression profile. Strikingly, the changes in gene expression profiles involved in cell signaling, cell cycle and amino acid metabolism, which would likely impact osteoblast function in bone formation. Our findings demonstrate for the first time that tenofovir treatment of primary osteoblasts results in gene expression changes that implicate loss of osteoblast function in tenofovir-associated bone mineral density loss.

Enhanced osteoclastogenesis causes osteopenia in twisted gastrulation-deficient mice through increased BMP signaling

The uncoupling of osteoblastic and osteoclastic activity is central to disorders such as osteoporosis, osteolytic malignancies, and periodontitis. Numerous studies have shown explicit functions for bone morphogenetic proteins (BMPs) in skeletogenesis. Their signaling activity has been shown in various contexts to be regulated by extracellular proteins, including Twisted gastrulation (TWSG1). However, experimental paradigms determining the effects of BMP regulators on bone remodeling are limited. In this study, we assessed the role of TWSG1 in postnatal bone homeostasis. Twsg1-deficient (Twsg1(-/-)) mice developed osteopenia that could not be explained by defective osteoblast function, because mineral apposition rate and differentiation markers were not significantly different compared with wildtype (WT) mice. Instead, we discovered a striking enhancement of osteoclastogenesis in Twsg1(-/-) mice, leading to increased bone resorption with resultant osteopenia. Enhanced osteoclastogenesis in Twsg1(-/-) mice was caused by increased cell fusion, differentiation, and function of osteoclasts. Furthermore, RANKL-mediated osteoclastogenesis and phosphorylated Smad1/5/8 levels were enhanced when WT osteoclasts were treated with recombinant BMP2, suggesting direct regulation of osteoclast differentiation by BMPs. Increase in detectable levels of phosphorylated Smad 1/5/8 was noted in osteoclasts from Twsg1(-/-) mice compared with WT mice. Furthermore, the enhanced osteoclastogenesis in Twsg1(-/-) mice was reversed in vitro in a dose-dependent manner with exposure to Noggin, a BMP antagonist, strongly suggesting that the enhanced osteoclastogenesis in Twsg1 mutants is attributable to increased BMP signaling. Thus, we present a novel and previously uncharacterized role for TWSG1 in inhibiting osteoclastogenesis through regulation of BMP activity.

Parathyroid hormone increases activating transcription factor 4 expression and activity in osteoblasts: requirement for osteocalcin gene expression

PTH is an important peptide hormone regulator of calcium homeostasis and osteoblast function. However, its mechanism of action in osteoblasts is poorly understood. Our previous study demonstrated that PTH activates mouse osteocalcin (Ocn) gene 2 promoter through the osteoblast-specific element 1 site, a recently identified activating transcription factor-4 (ATF4) -binding element. In the present study, we examined effects of PTH on ATF4 expression and activity as well as the requirement for ATF4 in the regulation of Ocn by PTH. Results show that PTH elevated levels of ATF4 mRNA and protein in a dose- and time-dependent manner. This PTH regulation requires transcriptional activity but not de novo protein synthesis. PTH also increased binding of nuclear extracts to osteoblast-specific element 1 DNA. PTH stimulated ATF4-dependent transcriptional activity mainly through protein kinase A with a lesser requirement for protein kinase C and MAPK/ERK pathways. Lastly, PTH stimulation of Ocn expression was lost by small interfering RNA down-regulation of ATF4 in MC-4 cells and Atf4(-/-) bone marrow stromal cells. Collectively, these studies for the first time demonstrate that PTH increases ATF4 expression and activity and that ATF4 is required for PTH induction of Ocn expression in osteoblasts.

In vitro and in vivo evidence for orphan nuclear receptor RORalpha function in bone metabolism

Bone is a major target site for steroid hormone action. Steroid hormones like cortisol, vitamin D, and estradiol are responsible for principal events associated with bone formation and resorption. Over the past decade, new members of the nuclear hormone gene family have been identified that lack known ligands. These orphan receptors can be used to uncover signaling molecules that regulate yet unidentified physiological networks. In the present study the function of retinoic acid receptor-related orphan receptor (ROR) alpha in bone metabolism has been examined. We showed that RORalpha and RORgamma, but not RORbeta, are expressed in mesenchymal stem cells derived from bone marrow. Interestingly, for RORalpha we observed an increased messenger signal expression between control cells and cells undergoing osteogenic differentiation. Furthermore, the direct activation of mouse bone sialoprotein by RORalpha, typically 7-fold, has been shown. In contrast, transient overexpression of RORalpha overrides the activation of the osteocalcin promoter by 1alpha,25-dihydroxyvitamin D(3). In addition, we have investigated bone mass parameters and bone geometry in the mouse mutant staggerer (sg/sg), a mouse strain that carries a deletion within the RORalpha gene. Homozygote mutants have thin long bones compared with the heterozygote animals and wild-type littermates. More interestingly, the bones of the sg/sg animals are osteopenic as indicated by the comparison of bone mineral contents of sg/sg animals to the heterozygote and wild-type animals. We conclude that these in vitro and in vivo results suggest a function for RORalpha in bone biology. RORalpha most likely acts by direct modulation of a bone matrix component.

Regulation of PTH/PTH-related protein receptor expression by endogenous PTH-related protein in the rat osteosarcoma cell line ROS 17/2.8

We have utilized clonal lines of the rat osteoblastic cell line ROS 17/2.8 stably transfected with full-length parathyroid hormone-related protein (PTHrP) cDNA in a sense or an antisense orientation to examine the effects of alteration in the production of endogenous PTHrP on expression of the PTH/PTHrP receptor. In the stably transfected clonal cell lines, changes in PTH/PTHrP receptor expression were evaluated by Northern blot analysis, whole-cell ligand binding of 125I-[Tyr36] PTHrP (1-36), and exogenous PTHrP (1-34)-stimulated cyclic adenosine monophosphate (cAMP) accumulation. Compared to control (vector-transfected) cells, PTHP-overproducing (sense-transfected) cells exhibited a marked decrease in the expression of PTH/PTHrP receptor mRNA and PTHrP ligand binding, as well as a corresponding decrease in the PTHrP (1-34)-stimulated cAMP response. By contrast, the antisense-transfected cells showed a marked increase in expression of PTH/PTHrP receptor mRNA and PTHrP (1-34) ligand binding, but a significant increase in the PTHrP (1-34)-stimulated cAMP response was not detected. Using antisense-transfected ROS cells, PTH/PTHrP receptor mRNA expression and 125I-[Tyr36] PTHrP (1-36) binding were downregulated by treatment for 24 h with exogenous PTHrP (1-36), forskolin, or dibutyryl cAMP. The findings extend those of earlier studies showing receptor downregulation by exogenous PTH by indicating that endogenous PTHrP, as well as circulating PTH, may help regulate receptor production; and suggesting that even very low concentrations of the peptide may influence receptor production.