Parathyroid hormone potentiates the effect of insulin-like growth factor-I on bone formation

Graded Doses of Recombinant Interleukin-1β Induce Generalized Osteopenia in Rats without Altering Skeletal Growth and Joint Integrity

BACKGROUND

Whereas a primary role of interleukin-1beta (IL-1beta) in local bone remodelling and articular inflammation has been well established, the effect of prolonged systemic administration of this cytokine on total skeletal Ca, somatic growth and joint tissue has not yet been investigated.

METHODS

Five groups of 14 rats each, aged 7-8 weeks, had miniosmotic pumps (Alzet 200 microl) implanted and primed to release 100, 200, 500, 1,000 and 2,000 ng/kg/24 h of human recombinant IL-1beta (rIL-1beta) daily for 14 days. On days 0 and 14 total skeletal mineral content (BMC) was assessed by means of X-ray absorptiometry and vertebral and tibial growth was measured by computer-assisted radiometry. On the same days, blood was drawn and analyzed for rat growth hormone (rGH), insulin-like growth factor (IGF-1), and osteocalcin. Also 24-hour urine was collected for d-pyridinoline (dpd) determinations. Hind- and forepaw diameter as a parameter of joint inflammation was assessed using a micrometric calliper. Subsequently the animals were sacrificed and one tibia dissected for measurement of trabecular volume by computerized histomorphometry.

RESULTS

BMC decreased in a dose-dependent manner reaching significance at 1,000 and 2,000 ng/kg (p < 0.03 and 0.04) in close correlation with tibial trabecular volumes (r = 0.84; p < 0.02). Normal vertebral and tibial growth was recorded at all dosages. There was no evidence of joint involvement. Blood rGH and IGF-1 remained normal as did osteocalcin, the latter reflecting lack of osteoblast activation. In contrast dpd increased in a dose-dependent manner indicating enhanced bone matrix turnover.

CONCLUSION

It is concluded that graded infusions of supraphysiological doses of rIL-1beta capable of inducing osteopenia did not affect skeletal growth in the absence of articular reaction. This is in contrast with the experience recorded in experimental arthritis in which growth retardation, in addition to osteopenia, may be caused by factors other than circulating IL-1beta.

Parathyroid hormone--a drug for orthopedic surgery?

Whereas continuous exposure to PTH results in bone resorption, administration at intermittent doses results in bone formation by increasing osteoblast number and activity. The anabolic action of PTH has also been demonstrated in clinical trials, in which PTH increased the bone mass and reduced fracture rate in patients with osteoporosis. In animal models of fracture healing and fixation of orthopedic implants, PTH increases the bone density in a dose-dependent manner, leading to faster repair and better fixation. The effect appears to be stronger on the new forming bone than on pre-existing bone. Based on these preclinical studies, we suggest that intermittent PTH treatment may also benefit fracture healing and implant fixation in patients.

The influence of combined parathyroid hormone and growth hormone treatment on cortical bone in aged ovariectomized rats

The influence of combined parathyroid hormone (PTH) and growth hormone (GH) treatment on bone formation and mechanical strength was investigated in femoral middiaphysial cortical bone from 20-month-old ovariectomized (OVX) rats. The animals were OVX at 10 months of age, and at 18 months they were treated daily for 56 days with PTH(1-34) alone (60 microg/kg), recombinant human GH (rhGH) alone (2.7 mg/kg), or a combination of PTH(1-34) plus rhGH. Vehicle was given to OVX control rats. All animals were labeled at day 28 (calcein) and at day 49 (tetracycline) of the treatment period. PTH(1-34) alone gave rise to formation of a new zone of bone at the endocortical surface. rhGH alone caused substantial bone deposition at the periosteal surface without influencing the endocortical surface. Combined PTH(1-34) plus rhGH administration enhanced bone deposition at the periosteal surface to the same extent as that of rhGH alone. However, the combined treatment resulted in a more pronounced formation of new bone at the endocortical surface than was induced by PTH(1-34) alone. Both PTH(1-34) alone and rhGH alone increased the mechanical strength of the femoral diaphysis, and further increase in mechanical strength resulted from combined PTH(1-34) plus rhGH treatment. OVX by itself induced the characteristic increase in medullary cavity cross-sectional area and a minor decrease in the mechanical quality of the osseous tissue.

Skeletal unloading causes resistance of osteoprogenitor cells to parathyroid hormone and to insulin-like growth factor-I

Skeletal unloading decreases bone formation and osteoblast number in vivo and decreases the number and proliferation of bone marrow osteoprogenitor (BMOp) cells in vitro. We tested the ability of parathyroid hormone (PTH) to stimulate BMOp cells in vivo by treating Sprague Dawley rats (n = 32) with intermittent PTH(1-34) (1 h/day at 8 microg/100 g of body weight), or with vehicle via osmotic minipumps during 7 days of normal weight bearing or hind limb unloading. Marrow cells were flushed from the femur and cultured at the same initial density for up to 21 days. PTH treatment of normally loaded rats caused a 2.5-fold increase in the number of BMOp cells, with similar increases in alkaline phosphatase (ALP) activity and mineralization, compared with cultures from vehicle-treated rats. PTH treatment of hind limb unloaded rats failed to stimulate BMOp cell number, ALP activity, or mineralization. Hind limb unloading had no significant effect on PTH receptor mRNA or protein levels in the tibia. Direct in vitro PTH challenge of BMOp cells isolated from normally loaded bone failed to stimulate their proliferation and inhibited their differentiation, suggesting that the in vivo anabolic effect of intermittent PTH on BMOp cells was mediated indirectly by a PTH-induced factor. We hypothesize that this factor is insulin-like growth factor-I (IGF-I), which stimulated the in vitro proliferation and differentiation of BMOp cells isolated from normally loaded bone, but not from unloaded bone. These results suggest that IGF-I mediates the ability of PTH to stimulate BMOp cell proliferation in normally loaded bone, and that BMOp cells in unloaded bone are resistant to the anabolic effect of intermittent PTH therapy due to their resistance to IGF-I.

Growth factors: possible new clinical tools. A review

Bone tissue contains numerous cell-to-cell signaling peptides called growth factors with potent effects on bone cell metabolism. In vivo studies over the last 5 years have demonstrated that growth factors can stimulate bone formation and bone healing and these results have made them candidates for use in orthopedic surgery. In numerous clinical conditions enhanced bone formation and bone healing could improve the results of surgery; clinical trials using growth factors to stimulate bone formation in spinal surgery, and to stimulate healing of bone defects, have been initiated. Growth factors for clinical use will become commercially available in the near future. This review describes the main growth factors and their actions in vitro and in vivo in relation to bone tissue and bone healing. Possible areas for clinical use are also discussed.

Anabolic effect of parathyroid hormone is not modified by supplementation with insulinlike growth factor I (IGF-I) or growth hormone in aged female rats fed an energy-restricted or ad libitum diet

PTH increases bone mass by stimulating bone formation in vivo, and IGF-I partially mediates the stimulatory effect of PTH on collagen synthesis in vitro. The objectives of this study were to determine if the anabolic effect of PTH in aged rats was diminished by an energy-restricted (ER) diet in which IGF-I levels have been shown to be decreased, and if GH or IGF-I modified the bone response to PTH. Two strains of aged female rats, 18-24 months, fed either ad libitum (AL) or 60% ER diets since weaning, were treated subcutaneously (SC) with 8 micrograms/100 g body weight hPTH 1-34 alone or in combination with GH given SC as 0.1 mg/100 g body weight twice daily for 24 or 32 days. Additional ER-fed rats were supplemented SC with 60 micrograms/100 g body weight rhIGF-I twice daily and treated with PTH and/or GH for 12 days. Histomorphometric indices of bone volume (BV/TV), mineral apposition rate (MAR) and bone formation rate (BFR/BS) in lumbar vertebrae and bone mineral content (BMC) of tibia increased in all PTH-treated rats. BV significantly increased after 32 (p < 0.01) or 24 (p < 0.05) days, but not 12 days (NS). Significant increases in mineralizing surfaces (MS/BS) and MAR resulted in 2.9-3.6-fold increases in BFR associated with PTH treatment alone or with GH (p < 0.01). These increases occurred regardless of an ER diet, and were not modified by IGF-I or GH. Although ER rats weighed less, femur length was not affected and serum chemistry values were within the physiologic range for rats. In summary, PTH increased bone mass by stimulating bone formation in aged rats irrespective of diet, and supplementation with GH or IGF-I did not modify the response of bone to PTH.

Stimulation of trabecular bone formation by insulin-like growth factor I in adult ovariectomized rats

Although in vitro experiments indicate that insulin-like growth factor I (IGF-I) is an anabolic hormone in bone cell metabolism, the effects of IGF-I in vivo on bone formation are unclear. We thus investigated whether IGF-I is able to stimulate bone formation in adult rats with established osteopenia induced by ovariectomy (OVX). IGF-I was administered at daily doses of 0.05, 0.2, and 0.8 mg/kg for 3 wk. OVX induced a marked osteopenia in femur and tibia. Administration of IGF-I increased trabecular bone mass with a maximal effect at 0.2 mg/kg. The same dose stimulated bone formation, as revealed by an increase in osteoid surface, osteoblast surface, triple tetracycline-labeled surface, and bone formation rate. The mineral apposition rate was equally stimulated at all doses. At the highest dose, IGF-I increased osteoclast surface and osteoclast number. These data indicate that, in the adult OVX rat, IGF-I stimulates bone formation and increases trabecular bone volume at medium doses and enhances the histological indexes of bone resorption at high doses.

Evidence for a diminished maturation of preosteoblasts into osteoblasts during aging in rats: an ultrastructural analysis

Bone is subject to continuous remodeling throughout life. The age-related loss of (trabecular) bone, leading to senile osteopenia, is mainly due to impaired bone formation. Osteoblasts (OB) and osteoclasts (OC) have been identified as playing a crucial role in the process of bone turnover, but the contribution made by their precursors is not well documented. We analyzed the cells of the osteoblast and osteoclast cell lineage along the trabecular bone of tibiae and the stromal cells in the marrow of aging BN/Bi Rij rats using electron microscopy. It appeared possible to distinguish preosteoblasts (pre-OB), OB, preosteoclasts (pre-OC), OC, and inactive bone-lining cells. Periods of increase, the maximal peak, and the decrease in trabecular bone volume were defined by means of morphometric measurements of trabecular bone volume. We found a decrease of more than 10-fold in the number of OB with age, but the numbers of pre-OB, pre-OC, and OC expressed per unit bone length, although variable, were age independent. The relative bone resorption and formation surface, expressed as a percentage of the total bone surface, decreased 2- and 15-fold, respectively. In 2-year-old animals the total volume of stromal cells, part of which constitutes the stem cell compartment of the osteogenic lineage, was a quarter of that found in 1-month-old animals and a third of that found in 6-month-old animals. The loss of trabecular bone is concomitant with a sharp increase in the ratio of pre-OB/OB, the ratio of OC/OB, and in the ratio of resorption to formation surfaces. There was no relation between the ratio of pre-OC/OC with age. These data lead to the conclusion that the main factor causing bone loss with age is a diminished maturation of pre-OB into OB.

Effect of growth hormone administration and treadmill exercise on serum and skeletal IGF-I in rats

Growth factors may be mediators of local and systemic factors that enhance bone formation. This study examined the effect of treadmill exercise and ovine growth hormone administration on levels of insulin-like growth factor I (IGF-I) in serum (ng/ml), long bone, and vertebrae and on bone formation rate. Forty female rats were divided into four groups: control; exercise (17 m/min, 1 h/day); growth hormone (0.05 mg.100 g-1.day-1); growth hormone plus exercise. After 9 wk of study, the serum levels of IGF-I were higher in the intervention groups than in the control group; however, the IGF-I concentration and the periosteal bone formation rate in the long bone were significantly higher only in the exercised rats. The IGF-I concentration and the cancellous bone formation rate in the vertebrae did not differ among the experimental groups. The vertebral and long bone formation rate were correlated with bone concentrations of IGF-I. Serum levels of IGF-I were also correlated with serum osteocalcin and the long bone formation but not with the vertebral bone formation. The association of bone formation with serum and bone IGF-I supports the suggestion that IGF-I is one of the growth factors that regulate bone formation, in particular as a mediator of the response of bone to exercise.

Southwestern internal medicine conference: growth hormone--aging and osteoporosis

Until recently, the use of growth hormone (GH) has been confined to the treatment of GH-deficient children. The advent of GH produced by recombinant DNA technology has increased the availability of GH. The increased availability of GH has made possible studies of the physiology and the possible therapeutic role of this hormone and its mediator insulin-like growth factor. One area where GH may play a therapeutic role is in the treatment of osteoporosis. This review will briefly summarize normal GH physiology and discuss age-related changes in GH and insulin-like growth factor 1 (IGF-1) axis and how they may relate to age-related physiologic changes. Evidence for and against a possible therapeutic role for GH/IGF-1 in the treatment of age-related (senile) osteoporosis will be discussed.

Contrasting effects of parathyroid hormone and insulin-like growth factor I in an aged ovariectomized rat model of postmenopausal osteoporosis

Agents that exert anabolic effects on bone have generally been tested in young or estrogen-replete animals. It is unclear whether these agents exert similar effects in older ovariectomized (Ovx) animals. In this single study we examined the effects of intermittent (daily) human PTH-(1-34) and continuous infusion of human recombinant IGF-I alone and in combination on bone resorption and formation over a 14 day period in an aged Ovx rat model of postmenopausal osteoporosis (2-year-old rats, Ovx at 1 year). Compared to Ovx controls, PTH treatment increased bone mineral content (BMC) and bone volume and stimulated bone formation but had no effect on bone resorption. In contrast, IGF-I treatment reduced BMC and stimulated resorptive activity as assessed by increases in marrow volume, cortical porosity, osteoclast-positive eroded surfaces, and urinary hydroxyproline excretion. IGF-I had no effect on bone formation, but when combined with PTH, IGF-I blunted the response to PTH on the periosteal and endocortical surfaces. In summary, PTH stimulated bone formation in a manner similar to that observed in younger animals and IGF-I stimulated bone resorption rather than formation and blunted the bone-forming response to PTH. The effects of IGF-I in older Ovx rats may differ from those observed in younger estrogen-replete animals.

Physiological and pharmacological regulation of biological calcification

Biological calcification is a highly regulated process which occurs in diverse species of microorganisms, plants, and animals. Calcification provides tissues with structural rigidity to function in support and protection, supplies the organism with a reservoir for physiologically important ions, and also serves in a variety of specialized functions. In the vertebrate skeleton, hydroxyapatite crystals are laid down on a backbone of type I collagen, with the process being controlled by a wide range of noncollagenous proteins present in the local surroundings. In bone, cells of the osteoblast lineage are responsible for the synthesis of the bone matrix and many of these regulatory proteins. Osteoclasts, on the other hand, are continually resorbing bone to both produce changes in bone shape and maintain skeletal integrity, and to establish the ionic environment needed by the organism. The proliferation, differentiation, and activity of these cells is regulated by a number of growth factors and hormones. While much has already been discovered over the past few years about the involvement of various regulators in the process of mineralization, the identification and functional characterization of these factors remains an area of intense investigation. As with any complex, biological system that is in a finely tuned equilibrium under normal conditions, problems can occur. An imbalance in the processes of formation and resorption can lead to calcification disorders, and the resultant diseases of the skeletal system have a major impact on human health. A number of pharmacological agents have been, and are being, investigated for their therapeutic potential to correct these defects.(ABSTRACT TRUNCATED AT 250 WORDS)