The anabolic effect of human PTH (1–34) on bone formation is blunted when bone resorption is inhibited by the bisphosphonate tiludronate—is activated resorption a prerequisite for the in vivo effect of PTH on formation in a remodeling system?

Parathyroid hormone (PTH) and its (1-34) fragment are stimulators of bone turnover that have an anabolic effect increasing trabecular bone mass when administered intermittently by daily subcutaneous injections. Its clinical use in osteoporosis, however, has been limited by the concomitant increased bone resorption and deleterious effect on cortical bone. To evaluate if a treatment combining PTH and a potent inhibitor of bone resorption would retain the anabolic effect of PTH without increasing bone resorption, we analyzed the effects of PTH (1–34) (500 IU/d) with or without the bisphosphonate tiludronate (1 mg/kg per day) for 3 months on biochemical and histological indices of bone turnover in old female sheep, an animal model which has a slow bone remodeling activity that resembles the one of elderly women. As expected, PTH (1-34) induced a significant increase of urinary pyridinoline and hydroxyproline (reflecting bone resorption), and of serum osteocalcin and alkaline phosphatase (reflecting bone formation), that were consistent with an increase of resorption and tetracycline-based formation of bone measured on iliac crest biopsy. In contrast, all biochemical and histological indices of bone turnover were decreased in sheep receiving tiludronate, a potent inhibitor of bone resorption. Surprisingly, in the combined therapy group, biochemical and histological indices of both resorption and formation did not differ from the control groups. Thus, the model of old sheep, which closely resembles the situation in old human, shows that the anabolic effect of PTH on bone is not maintained when PTH is coadministered with a bisphosphonate, in marked contrast to results noted in the growing rat. Because bisphosphonates are selective inhibitors of osteoclastic bone resorption that do not directly affect osteoblastic bone formation in vivo, these data suggest that the activation of bone resorption may be a prerequisite for the anabolic effect of PTH. Although tiludronate was the only bisphosphonate tested, our data also suggest that a combined PTH-bisphosphonate therapy is not a valid strategy for osteoporotic patients. Combination regimens of anabolic and antiresorptive agents may not be effective and should be tested in an appropriate animal model before clinical trials in osteoporotic patients are undertaken.     

Role of parathyroid hormone in mediating age-related changes in bone resorption in men

Osteoporosis in men is an important and growing public health problem. While there has been extensive work done on defining the mechanism(s) of the age-related increase in bone resorption in women, our knowledge regarding the pathogenesis of bone loss in elderly men is still incomplete. We previously demonstrated that the age-related increase in serum PTH contributes substantially to the increased bone resorption in elderly women, since suppression of PTH levels by an intravenous calcium infusion decreased bone resorption markers to a greater extent in elderly compared to premenopausal women. In the present study, we tested the hypothesis that the comparable increase in PTH levels in elderly men (age 70–78 years) was driving bone resorption to a greater extent in these men than in younger men (age 40–50 years). PTH secretion was suppressed by an intravenous calcium infusion and the corresponding changes in the bone resorption marker, urine N-telopeptide of type I collagen (NTx) were assessed. In contrast to our previous findings in pre- versus postmenopausal women, suppression of PTH secretion in elderly men did not result in a greater decrease in urine NTx excretion than in the younger men (change in NTx excretion in the elderly men, -2.79±1.99 nmol/mmol Cr, versus that in the younger men, –5.07±1.39 nmol/mmol Cr, P=0.356). Collectively, these data suggest that the relationship between the age-related increase in serum PTH levels and bone resorption differs between elderly men and women. Since both estrogen and testosterone can attenuate the bone resorbing effects of PTH, it is possible that this difference may be due to the much milder degree of sex steroid deficiency in elderly men as compared to postmenopausal women.     

双膦酸盐对骨形成蛋白诱导骨骨吸收抑制作用的研究

目的研究双膦酸盐(bisphosphonate YM 175)对骨形成蛋白(bone morphogenetic protein．简称BMP)诱导骨骨吸收的抑制作用。方法 42只大白鼠背部植入BMP,诱导出异位骨后,将大白鼠分成2组,即投药组和对照组。投药组在BMP植入后第3w至第7w,双膦酸盐(YM 175)每周投药3次,剂量1(μg／kg．d)。对照组按同样的方式给予等量的生理盐水。在BMP植入第3w、 4w、7w和10w,将BMP诱导骨取出,采用TRAP和cathepsin K染色方法来观察双膦酸盐对破骨细胞的作用。结果 BMP诱导骨3w时(即未投双膦酸盐药前),在BMP植入体周边形成编织骨 (woven bone),大量破骨细胞出现在新生骨组织表面。在4 w时,双膦酸盐投药组和对照组,新生骨组织均向BMP植入体内生长,但双膦酸盐投药组的破骨细胞较对照组有减少。在10 w时,双膦酸盐投药组和对照组均可观察到骨细胞变小并且有规律地排列的板层状骨(lamellar bone)的特征。而双膦酸盐投药组,破骨细胞死亡,与对照组比较,破骨细胞数目明显减少。结论双膦酸盐对破骨细胞性骨吸收有明显的抑制作用。     

Effects of leukemia inhibitory factor on bone resorption and DNA synthesis in neonatal mouse calvaria

Summary Leukemia inhibitory factor (LIF) is a recently characterized cytokine which has been shown to regulate cell growth and differentiation in a variety of tissues. We have shown that LIF stimulates bone resorption and DNA synthesis in bone organ culture and,in vivo, LIF has been shown to have marked effects on bone remodeling. The present study examines further the dose-response, time course and mechanisms of action of LIF in neonatal mouse calvaria.⁴⁵Ca release was significantly increased by LIF at concentrations of 10–5,000 U/ml, and its stimulation of bone resorption increased with time from 24 to 96 hours. These concentrations of LIF also increased DNA synthesis at 24 hours. At 72 hours, low concentrations of LIF produced less marked stimulation of [³H]-thymidine incorporation, and 5,000 u/ml actually inhibited DNA synthesis at both this time point and at 96 hours. The effect of LIF on⁴⁵Ca release was partially inhibited when DNA synthesis was blocked by hydroxyurea (50 μM). The resorptive effect of supramaximal concentrations of LIF was not additive to that of parathyroid hormone, 1,25-dihydroxyvitamin D₃, prostaglandin E₂, or transforming growth factor-β. Although LIF-stimulated resorption is at least partially dependent on DNA synthesis, these results suggest that there are different mechanisms involved in mediating LIF's effects on bone resorption and DNA synthesis. The demonstration of effects of LIF at low concentrations indicates that this cytokine may be involved in the physiological regulation of bone metabolismin vivo.     

Responsiveness of parathyroid hormone to bone resorption in 13-day-old embryonic chick calvaria cultures

To clarify what kind of process participates in bone resorption, time course of indices of bone resorption was investigated using 13-day-old embryonic chick calvaria. When calvariae were cultured with parathyroid hormone (PTH) at 0.01 U/ml for 8 days, hydroxyproline (Hyp) release was already stimulated by PTH in cultures by 1 to 2 days but stimulation of⁴⁵Ca release was not observed even in cultures by 6 to 8 days. Furthermore, stimulation of collagenase release by PTH was observed prior to that of Hyp release. These results indicate that collagenase release precedes Hyp release, which is followed by⁴⁵Ca release. The release of tartrate-resistant acid phosphatase (TrACP), a marker enzyme of osteoclast, was stimulated by PTH at 0.1 U/ml and above to a greater extent in cultures by 5 to 8 days (phase 11) than in cultures by 0 to 4 days (phase 1). E-64, an inhibitor of cysteine proteinase, inhibited PTH-stimulated⁴⁵Ca release strongly in phase 11 but showed a slight decrease in Hyp release in the same phase. These results suggest that first PTH stimulates collagenase production from osteoblasts, secondly collagenase degrades uncalcified collagen and lastly osteoclasts resorb mineralized bones.     

Differential effects of parathyroid hormone on protein phosphorylation in two osteoblastlike cell populations isolated from neonatal mouse calvaria

Summary To further understand the mechanism of PTH effects on bone and bone cells, we have analyzed the effect of PTH on specific protein phosphorylation in cells isolated from neonatal mouse calvaria. Four populations of cells (I–IV), isolated by sequential digestion with chromatographically purified bacterial collagenase isozymes and neutral proteinase, were cultured overnight. Alkaline phosphatase activity was greater than acid phosphatase activity in all four populations. PTH stimulated cyclic AMP production in all four populations, although the effect was greatest in populations II and III. Cultured cells were treated with PTH for up to 15 minutes. Cytosolic and membrane fractions were obtained and assayed forin vitro protein phosphorylation. No hormonal effects were found in membrane fractions. In cytosol fractions, treatment of the population II cells for 10–15 minutes with 0.1 μM PTH decreased the subsequent protein phosphorylation of an 85,000 M_r protein. In contrast, PTH treatment increasedin vitro phosphorylation of both the 85,000 and 35,000 M_r proteins in population III cells. Phosphorylation of the 35,000 M_r protein was cyclic AMP-dependent. All of the phosphoproteins appeared to be phosphorylated solely on serine or threonine residues except the 85,000 M_r protein which may also contain significant amounts of phosphotyrosine. Therefore, some of the effects of PTH are cyclic AMP-mediated and other effects may be mediated through tyrosine phosphorylation. These data indicate that PTH has differential effects onin vitro protein phosphorylation in two separable populations of isolated neonatal mouse calvarial cells and support a hypothesis that multiple osteoblastlike cells existin vivo.     

Phosphorus deficiency,parathyroid hormone and bone resorption in the growing rat

Sham-operated and parathyroidectomized (PTX) rats were divided into two pair-fed groups, one on a normal mineral intake (0.5% Ca, 0.3% P), the other on a regimen low in phosphorus (0.5% Ca, 0.03% P). P depletion led to a drop in plasma P and urine P, a rise in plasma Ca and a marked rise in urine Ca, a drop in serum magnesium and a rise in urine Mg. The changes were more pronounced in the PTX animals, but final values were the same in both groups. Parallel bone-seeking isotope (⁸⁵Sr,¹⁷⁷Lu,²³⁷Np) studies in nonablated animals revealed an increased in the urinary nuclide output and in the urine/tibia ratio in P-deficient animals. Normal and primary bone osteocytes decreased and enlarged osteocytes increased as a result of P deficiency; osteoclasts and osteoblasts also increased. Bone composition showed a drop in ash content and a rise in water, with a light decrease in both Ca and P, and a corresponding rise in hydroxyproline and nitrogen in the P-deficient animals. The results are interpreted to mean that P-deficiency in the young growing rat leads to an increase in bone resorption which occurs also in the absence of parathyroid hormone (PTH). The fact that final values were similar in the control and PTX P-deficient animals suggests that steady-state regulation can also occur without PTH. Because P-deficiency leads to rapid hypercalcemia and rapid marked hypercalciuria, there may exist a mechanism for phosphate regulation which would then supersede Ca homeostasis. The change in serum and urine Mg levels may reflect a decrease in tubular Ca and Mg reabsorption associated with P-deficiency.     

Establishment of a rapid bone resorption in vitro assay using previiously frozen mouse unfractionated bone cells pretreated with parathyroid hormone

We established a useful assay system for evaluating osteoclast-mediated bone resorption based on the use of unfractionated bone cells obtained from 10- to 11-day-old mice. When cells from 10 to 11 mice were treated for 7 days with rat parathyroid hormone (rPTH, 10⁻⁸ M), a total of 4 to 5 × 10⁷ cells could be obtained from the culture by treatment with 0.05% trypsin and 0.02% EDTA in PBS. These harvested cells contained about 20% tartrate-resistant acid phosphatase (TRAP)-positive mononuclear cells. When the harvested cells were cultured on dentine slices without rPTH, after 1 day, they formed TRAP-positive multinucleate cells that were active in bone rsorption. Eel calcitonin (eCT) decreased the number of pits in a dose-dependent manner, and its half maximal inhibition dose (ID₅₀) was 1.08 × 10⁻¹¹ M.Even after having been frozen in liquid nitrogen for 5 months, upon thawing, these cells were capable of forming pits; and this pit formation was inhibited by eCT. Since no appropriate osteoclaslic cell line for evaluating bone resorption is available at present, this system can provide a useful, practical means for assaying osteoclastic bone-resorbing activity.     

Effects of parathyroid hormone on the osteoclastic pool,bone resorption and formation in rat alveolar bone

Summary The osteoclast number and its relation to parathyroid hormone have been studied in rat alveolar bone by quantitative histology and fluorescent labeling. The osteoclast number decreases 60 h after parathyroidectomy and remains constant for the next 132 h. Parathyroid hormone administration to parathyroidectomized animals 96 h after the operation induces an increase in osteoclast number within 12 h to some-what above those of control animals. The elevated osteoclast counts remain constant for 60 h then rapidly fall over the next 24 h to the level seen in untreated parathyroidectomized animals. As determined by fluorescent labeling, normal alveolar bone resorption and formation were disturbed by parathyroidectomy, such that significant bone formation occurred for only 6 days after surgery, after which a quiescent state followed.     

辛伐他汀对甲状旁腺素相关肽(PTHrP)诱导小鼠头盖骨骨吸收的影响

[目的]探讨辛伐他汀对体内破骨细胞功能及对局部骨吸收的影响。[方法]采用甲状旁腺素相关肽(parathyroid hormone related peptide,PTHrP)诱导的小鼠头盖骨骨吸收的动物模型体系,分别采用皮下注射辛伐他汀(0、5、10、20 mg.kg-1.d-1)的方法,检测头盖骨X线片的骨吸收面积,组织学用抗酒石炭酸染色(TRAP),检测单位面积破骨细胞的数量。[结果]辛伐他汀在皮下注射(10、20 mg.kg-1.d-1)均可抑制头盖骨的骨吸收和破骨细胞的形成,皮下注射(0、5 mg.kg-1.d-1)则无明显的抑制作用。[结论]辛伐他汀对小鼠局部骨吸收有着明显的抑制作用,对局部骨吸收的治疗有着重要的意义。     

Apposition and resorption of bone during oral treatment with (3-amino-1-hydroxypropylidene)-1,1-bisphosphonate (APD)

Summary The effects of 1.5–2 years oral administration of disodium (3-amino-1-hydroxypropylidene)-1,1-bisphosphonate (APD) on bone metabolism were studied in male and female rats. APD was mixed in the food at levels of 500, 2,000 and 10,000 ppm. A dose-dependent increase in metaphyseal bone was found, indicative of continued inhibition of bone and cartilage resorption. APD did not affect mineralization of bone and cartilage, primary bone formation, or periosteal apposition. A short-term metabolic balance study was performed to compare the effects of oral with subcutaneous APD. Absorption of APD was in the order of 0.2%. Oral APD increased absorption of phosphate, probably by complexation of calcium with APD. The excess absorbed phosphate increased phosphaturia and decreased urinary calcium. The long-term toxicology study was performed at the Henkel toxicology department, Düsseldorf, West Germany. The metabolic balance study was performed in Leiden.     

Effects of a fluorinated bisphosphonate on bone remodeling in vivo

A new fluorinated bisphosphonate, difluoromethylene bisphosphonate (F₂MBP), was studied for its effects on physiologic bone remodeling in the actively growing rat tibia. Young male Sprague-Dawley rats were given daily subcutaneous injections of either saline (control) or 30 mg/kg per day of F₂MBP, dichloromethylene bisphosphonate (Cl₂MBP), or 1-hydroxyethylidene-1,1-bisphosphonate (HEBP) for 30 days. Microradiographs of the epiphysealmetaphyseal region of tibiae from animals treated with either F₂MBP or Cl₂MBP demonstrated increased radiodensity but, unlike those treated with HEBP, without an increase in the width of the epiphyseal growth plate. Quantitative analyses of these microradiographs showed that there was an increase of calcified tissue in the metaphyseal region in all diphosphonate groups when compared with controls. However, the HEBP-treated animals exhibited significantly less increase than either F₂MBP-treated or Cl₂MBP-treated rats. In addition, the total area of calcified tissue in the diaphyseal transverse sections was greater in the F₂MBP-treated animals than in controls. Elemental calcium, phosphorus, and fluorine, as detected by the electron probe, also increased in the metaphyseal region of the F₂MBP-treated animals, but no significant differences in the calcium: phosphorus ratio were found among the control, F₂MBP, and Cl₂MBP treatment groups, indicating no alterations in the chemical composition of bone. The greater amount of fluorine in the tibiae of F₂MBP-treated animals reflected the presence of the F₂MBP molecule. Thus, this study has demonstrated that this new fluorinated bisphosphonate, like Cl₂MBP, inhibits physiologic bone remodeling without disturbing mineralization. Furthermore, the presence of fluorine in F₂MBP allows the precise localization of the incorporation of the bisphosphonate within the bone.     

Effect of hydrocortisone on osteoclasts generated in cat bone marrow cultures

Summary The generation of osteoclasts in cultures of cat bone marrow was completely inhibited for 4 weeks with 10⁻⁶M hydrocortisone (HC) and partially inhibited with 10⁻⁷ to 10⁻⁹M in a dose-dependent fashion. This effect was completely reversible when cultures were exposed for only 2 weeks to 10⁻⁹ or 10⁻⁸M HC. However, cultures in which higher concentrations (10⁻⁷ to 10⁻⁵M) were maintained for the same period did not show complete recovery in terms of numbers of osteoclasts and number of nuclei per cell after withdrawal of HC, suggesting that precursor cells of osteoclasts were also damaged by HC. To study the effects of HC on osteoclasts already present in the cultures, 10⁻⁶M was added to 4-week-old untreated cultures. The number of osteoclasts decreased rapidly and a gross morphological response was also apparent (rounding of the cells leading to detachment from the substratum and inhibition of cell fusion), indicating that the generation as well as the survival of osteoclasts in vitro are sensitive to HC. The morphological changes observed under optical and electron microscopy correspond to those of the reported inactive form of osteoclasts, and suggest that their function may also be altered by HC.     

双膦酸盐的长期治疗增加了骨单位的矿化程度

目的评估双膦酸盐的长期应用对骨单位的次级矿化程度的影响。方法30只1岁龄猎犬按体重随机分成三组(每组雌雄各5只):对照组犬每天口服乳糖,低剂量组和高剂量组犬分别每天口服因卡膦酸钠incadronate0.3mgkg和0.6mgkg。所有犬持续给药3a。处死前进行四环素双标,处死后取左侧第9肋骨进行组织形态计测和次级矿化程度评估。结果组织形态计测表明两个双膦酸盐治疗组的骨激活频率(Ac.f)都明显低于对照组,分别降低了40%和82%。矿化程度的测定表明,低剂量和高剂量双膦酸盐组骨单位内的平均矿化程度(MDMB)都明显高于对照组,分别增加了22%和30%。双膦酸盐组MDMB分布曲线与对照组保持了相同的峰值,并且随治疗剂量的增大逐渐向高矿化端漂移。结论双膦酸盐长期应用明显抑制了骨转换,增加了骨单位的矿化程度。     

Activin release from bone coupled to bone resorption in organ culture of neonatal mouse calvaria

Activin, a member of the transforming growth factor-beta (TGF-beta) superfamily, is present in the bone matrix and assumed to be involved in the regulation of bone formation. In the present study, we investigated whether the release of activin from bone is coupled with bone resorption. Neonatal mouse calvaria were cultured in the presence of various stimulators of bone resorption (parathyroid hormone [PTH], interleukin-1beta, prostaglandin E2) for up to 72 h, and the activin activity in the medium was measured using a specific bioassay for activin. Activin activity was accumulated in proportion to the time- and dose-dependent increase in calcium release from bone into the medium (bone resorption). An inhibition of PTH-dependent bone resorption by a bisphosphonate, disodium dichlormethane-1,1-bisphosphonic acid (Cl2MBP), completely blocked release of activin activity from bone into the medium. In primary culture of calvarial cells, however, neither PTH nor Cl2MBP affected activin production. These findings indicate that release of activin activity from bone tissue is strongly coupled to bone resorption. Because activin possesses osteogenic activities, activin released locally from bone might be involved in the regulation of bone formation in the physiological process of bone remodeling, as has been suggested for TGF-beta.     

Effects of forskolin on bone resorption in the absence and presence of parathyroid hormone and calcitonin

Summary Release of previously incorporated⁴⁵Ca from fetal rat long bones was determined with the diterpene forskolin, both in the absence and presence of parathyroid hormone (PTH) and calcitonin (CT). In the absence of hormone, increased bone resorption was observed with 10⁻⁷M forskolin, but biphasic responses, consisting of initial decreases in⁴⁵Ca release that were followed by increased calcium mobilization, were produced with 10⁻⁶M and 10⁻⁵M forskolin. Inhibition of⁴⁵Ca release was pronounced and delayed more with 10⁻⁵M forskolin while the greatest stimulation of bone resorption was elicited by 10⁻⁶M forskolin, a response that was inhibited by 100 mU/ml CT. In the presence of 250 ng/ml PTH, a synergistic enhancement of⁴⁵Ca release occurred with 10⁻⁷M forskolin treatment while, in contrast, calcium mobilization was inhibited by 10⁻⁶M and 10⁻⁵M forskolin. Inhibition by 10⁻⁶M forskolin was characterized by “escape” while that of 10⁻⁵M forskolin was continuous over a 5 day interval. Inhibition throughout the experimental period also was noted when 10⁻⁵M forskolin was combined with 2.5 ng/ml PTH, but no effect on calcium mobilization was observed upon addition of 10⁻⁷M forskolin and, rather than inhibition, an enhancement of⁴⁵Ca release occurred when 10⁻⁶M forskolin was combined with 2.5 ng/ml PTH. Inhibition of 250 ng/ml PTH, but lack of inhibition of 2.5 ng/ml PTH by 10⁻⁶M forskolin suggests a 10⁻⁶M forskolin-sensitive portion of PTH-mediated calcium efflux. Absence of “escape” when 10⁻⁵M forskolin is combined with 250 ng/ml PTH suggests that heterologous desensitization may not play a major role in the “escape” which occurs with 10⁻⁶M forskolin.     

Effect of ACTH on PTH-stimulated bone resorption in vitro

Summary In vitro demonstration of PTH effects requires hormone concentrations greater than the “physiological” concentrations reported by radio-immunoassay or cytochemical assays. This discrepancy could be the result of binding or destruction of PTH at nonbiologically active sites. In the present study, ACTH was found to have no effect by itself on bone resorption, but addition of ACTH to bone cultures together with low concentrations of PTH resulted in a specific enhancement of PTH-stimulated bone resorption. This effect was not observed when bone resorption was stimulated by PGE₂ and 1,25(OH)₂D₃, and it was blocked by human serum. The effect of ACTH is similar to the enhancement in PTH-stimulated bone resorption by poly-l-lysine [7]. We suggest that the amplification of PTH stimulation was the result of displacement of PTH from nonbiologically active sites, making more PTH available for binding to its biologically active receptor. An alternative explanation for our results was that ACTH prevented degradation of PTH by bone-derived proteolytic enzymes. Thus the sensitivity of bioassays for PTH could be improved by adding ACTH.     

老年糖尿病并骨质疏松血胰岛素与骨钙素及PTH研究

目的对老年2型糖尿病（type 2 diabetes mellitus,T2DM）合并骨质疏松（osteoprosis,OP）患者血胰岛素（insulin,INS）与骨钙素（osteocalcin,OC或Bone Gla Protein,BGP）及甲状旁腺素（parathyroid hormone,PTH）等进行研究,探讨它们在老年T2DM合并OP发病机理中的作用。方法选择128例老年患者,根据骨密度（bone mineral density,BMD）及血糖检测结果分成3组：T2DM合并OP组（A组,43例）,T2DM无合并OP组（B组,45例）,正常对照组（C组,40例）。对3组患者的年龄、性别、体重指数（body mass index,BMI）、空腹血糖（fasting plasma glucose,FPG）、餐后2h血糖（2 hour postpradial glucose,2hPG）、糖化血红蛋白（hemoglobin A1c,HbA1c）、空腹胰岛素（FINS）、空腹C肽（FC-P）、餐后2h胰岛素（2hINS）、餐后2hC肽（2hC-P）、BGP、PTH、BMD进行比较。结果①A组FINS、FC-P、2hINS、2hC-P、BMI、BGP、BMD较B组及C组低,差异有显著性（P〈0.05）;②A组HbA1c、FPG、2hPG、PTH较B组及C组高,差异有显著性（P〈0.05）;③B组HbA1c、FPG、2hPG、PTH较C组高（P〈0.05）,其余指标B组与C组差异无显著性（P〉0.05）。结论老年T2DM合并OP患者胰岛素、C肽明显缺乏,BGP降低,BMI偏低,BMD偏低;血糖偏高,PTH升高。联合检测胰岛素、C肽及BGP,PTH、BMD有助T2DM合并OP诊断。控制血糖有助T2DM合并OP治疗。     

Contributions of parathyroid hormone (PTH)/PTH-related peptide receptor signaling pathways to the anabolic effect of PTH on bone

PTH regulates osteoblastic function by activating PTH/PTHrP receptors (PTH1Rs), which trigger several signaling pathways in parallel, including cAMP/protein kinase A (PKA) and, via both phospholipase-C (PLC)-dependent and PLC-independent mechanisms, protein kinase C (PKC). These signaling functions have been mapped to distinct domains within PTH(1–34), but their roles in mediating the anabolic effect of intermittent PTH in vivo are unclear. We compared the anabolic effects in mice of hPTH(1–34) with those of two analogs having restricted patterns of PTH1R signaling. [G¹,R¹⁹]hPTH(1–28) lacks the 29–34 domain of hPTH(1–34) needed for PLC-independent PKC activation, incorporates a Gly¹ mutation that prevents PLC activation, and stimulates only cAMP/PKA signaling. [G¹,R¹⁹]hPTH(1–34) retains the 29–34 domain and activates both cAMP/PKA and PLC-independent PKC.

Human PTH(1–34) (40 μg/kg), [G¹,R¹⁹]hPTH(1–34) (120 μg/kg), and [G¹,R¹⁹]hPTH(1–28) (800 μg/kg), at doses equipotent in elevating blood cAMP at 10 min and cAMP-dependent gene expression in bone at 6 h after s.c. injection, were administered to 10-week-old female C57BL/6J mice 5 days/week for 4 weeks. Acute blood cAMP responses, retested after 4 weeks, were not reduced by the preceding PTH treatment. The three PTH peptides induced equivalent increases in distal femoral bone mineral density (BMD), and, by microCT analysis, distal femoral and vertebral bone volume and trabecular thickness and mid-femoral cortical endosteal apposition. [G¹,R¹⁹]hPTH(1–34) and hPTH(1–34) increased distal femoral BMD more rapidly and augmented total-body BMD and bone volume of proximal tibial trabeculi to a greater extent than did [G¹,R¹⁹]hPTH(1–28).

We conclude that cAMP/PKA signaling is the dominant mechanism for the anabolic actions of PTH in trabecular bone and that PLC-independent PKC signaling, attributable to the PTH(29–34) sequence, appears to accelerate the trabecular response and augment BMD at some skeletal sites. PTH1R PLC signaling pathway is not required for an anabolic effect of intermittent PTH(1–34) on bone.     

Effect of omeprazole,an inhibitor of H+, K+-ATPase,on bone resorption in humans

Summary Though some evidence suggests that bisphosphonates (BPs) act directly on osteoclasts to inhibit bone resorption, other evidence suggests that they inhibit the development of the osteoclast. We found an increase in osteoclast recruitment in 2-day-old mice given (3-amino-1-hydroxypropylidene)-1,1-bisphosphonate (APD). A threefold increase in 5-bromo-2-deoxyuridine (BrdU)-labeled osteoclast nuclei was observed on mouse parietal bones 3 days after APD injection. This suggests that inhibition of osteoclast development is not an action of APD in mice of this age. The mechanism of the increased recruitment was investigated. As osteoclast progenitors were not detected on parietal bonesin vitro, we looked for an increase in circulating monocytes to account for the recruitment. No such increase was found, but when⁵¹Cr-labeled bone marrow was injected intraperitoneally into mice given APD there was an increase in accumulation of⁵¹Cr in calvaria and in femur and tibia over controls. This increase did not occur when⁵¹Cr-labeled erythrocytes or free⁵¹Cr was injected. We conclude that APD causes increased recruitment of osteoclast precursors by increasing the avidity of bone for hematopoietically derived cells.