Insulin-like growth factor binding proteins in femoral and vertebral bone marrow stromal cells: expression and regulation by thyroid hormone and dexamethasone

Insulin-like growth factor (IGF)-I is an important regulator of bone metabolism. Clinical observations suggest that different anatomic sites within the adult skeleton respond differently to hormonal and therapeutic treatment, and recent studies on bone marrow stromal cells in culture show that there are skeletal site-dependent differences in the gene expression of IGF-I. The actions of IGF-I and -II on bone cells are known to be modulated by the IGF binding proteins (IGFBP)-1 through -6 and the Type I and Type II IGF receptors. Therefore, we compared the expression of IGFBP-1 through -6 in adult female rat bone marrow stromal cell cultures derived from two separate skeletal sites: vertebrae and femurs. The cultures were maintained simultaneously under conditions that support osteoblast differentiation from osteoprogenitors present in the femoral and vertebral marrow cell populations. We also addressed whether IGFBP messenger RNA levels are regulated by thyroid hormone (T(3)) and dexamethasone (dex) treatment in femoral vs. vertebral marrow stromal cells in vitro, since steroid hormones play an important role in skeletal function. Northern blot analyses revealed that there are distinct skeletal site differences in the gene expression of IGFBPs. The vertebral marrow cultures express IGFBP-2 through -6 mRNAs, with IGFBP-2, IGFBP-4, and IGFBP-6 mRNAs predominating. The femoral marrow stromal cell cultures express only IGFBP-4 and IGFBP-6. Importantly, vertebral marrow cultures have much higher IGFBP mRNA steady-state levels than femoral cultures for all the detected IGFBP transcripts. IGFBP-1 is not detected in either femoral or vertebral cultures. In addition to a skeletal site difference, we show that T(3) and dex regulate the expression of specific IGFBP mRNAs. T(3) treatment also upregulates IGF-I protein secretion by vertebral marrow stromal cell cultures. Interestingly, the type I receptor for IGF-I was expressed equivalently in cultures from the two skeletal sites. These findings have important implications for the anatomical site specificities of hormonal responses that are noted in the skeleton.

Clinical review 123: Anabolic therapy for osteoporosis

All currently available, approved therapies for osteoporosis inhibit bone resorption. By acting at this site in the bone remodeling cycle, estrogens, selective estrogen receptor modulators, calcitonin, and the bisphosphonates all have the capacity to increase bone mineral density and to reduce the risk of new fractures. There can be no doubt that these agents have had an enormous impact on our diagnostic and therapeutic approach to osteoporosis. Despite their great value, the antiresorptives are generally not associated with dramatic increases in bone mass, and their action to reduce fracture risk, although highly significant, is rarely more than 50% of the baseline risk. Another approach is anabolic therapy, in which bone formation is directly stimulated. In this review we will summarize the anabolic agents that have been studied and present a current view of their current standing. Fluoride, GH, insulin-like growth factor I, the statins, and PTH will be reviewed. Although still in development, approaches to combination therapy with antiresorptives and anabolic agents are also promising.

Variation in bone biomechanical properties, microstructure, and density in BXH recombinant inbred mice

To test the hypothesis that factors associated with bone strength (i.e., volumetric bone mineral density [vBMD], geometry, and microstructure) have heritable components, we exploited the 12 BXH recombinant inbred (RI) strains of mice derived from C57BL/6J (B6; low bone mass) and C3H/HeJ (C3H; high bone mass) progenitor strains. The femurs and lumbar vertebrae from each BXH RI strain were characterized for phenotypes of vBMD, microstructural, biomechanical, and geometrical properties. Methods included bending (femur) and compression (vertebra) testing, peripheral quantitative computed tomography (pQCT), and microcomputed tomography (microCT). Segregation patterns of femoral and vertebral biomechanical properties among the BXH RI strains suggested polygenic regulation. Femoral biomechanical properties were strongly associated with femoral width in the anteroposterior (AP) direction and cortical thickness--geometric properties with complex genetic regulation. Vertebral vBMD and biomechanical properties measured in BXH RI strains showed a greater variability than either B6 or C3H progenitors, suggesting both progenitor strains have independent subsets of genes that yield similar vBMD and strength. The microCT and pQCT data suggested that the distribution of vertebral mineral into cortical and trabecular compartments is regulated genetically. Although the B6 and C3H progenitors had similar vertebral strength, their vertebral structures were markedly different: B6 had good trabecular bone structure and modest cortical bone mineral content (BMC), whereas C3H had high cortical BMC combined with a deficiency in trabecular structure. These structural traits segregated independently in the BXH RI strains. Finally, vertebral strength was not correlated consistently with femoral strength among the BXH RI strains, suggesting genetic regulation of bone strength is site specific.

Postnatal and pubertal skeletal changes contribute predominantly to the differences in peak bone density between C3H/HeJ and C57BL/6J mice

Previous studies have shown that 60-70% of variance in peak bone density is determined genetically. The higher the peak bone density, the less likely an individual is to eventually develop osteoporosis. Therefore, the amount of bone accrued during postnatal and pubertal growth is an important determining factor in the development of osteoporosis. We evaluated the contribution of skeletal changes before, during, and after puberty to the development of peak bone density in C3H/HeJ (C3H) and C57BL/6J (B6) mice. Volumetric bone density and geometric parameters at the middiaphysis of femora were measured by peripheral quantitative computed tomography (pQCT) from days 7 to 56. Additionally, biochemical markers of bone remodeling in serum and bone extracts were quantified. Both B6 and C3H mice showed similar body and femoral weights. B6 mice had greater middiaphyseal total bone area and thinner cortices than did C3H mice. Within strains, males had thicker cortices than did females. C3H mice accumulated more minerals throughout the study, with the most rapid accumulation occurring postnatally (days 7-23) and during pubertal maturation (days 23-31). C3H mice had higher volumetric bone density as early as day 7, compared with B6 mice. Higher serum insulin-like growth factor I (IGF-I) was present in C3H mice postnatally at day 7 and day 14. Until day 31, B6 male and female mice had significantly higher serum osteocalcin than C3H male and female mice, respectively. Alkaline phosphatase (ALP) was found to be significantly higher in the bone extract of C3H mice compared with B6 mice at day 14. These data are consistent with and support the hypothesis that the greater amount of bone accrued during postnatal and pubertal growth in C3H mice compared with B6 mice may be caused by increased cortical thickness, increased endosteal bone formation, and decreased endosteal bone resorption.

Emerging anabolic treatments for osteoporosis

Therapy for osteoporosis is principally centered on the use of agents that block bone resorption and supplementation with vitamin D and calcium. Although these drugs are effective in reducing the risk of subsequent fractures, and modestly increasing bone density, most patients being treated for osteoporosis still have low bone mass and a greater risk of fracture. Anabolic agents stimulate bone formation, strength, and mass. In addition, there is emerging evidence that anabolic agents can reduce subsequent fracture risk. The two most promising agents, parathyroid hormone (PTH) and GH/IGF-I, act to increase osteoblast mediated bone formation. A review of the potential usefulness of PTH and GH/IGF-I is presented.

Defining the genetics of osteoporosis: using the mouse to understand man

Very low bone mineral density (BMD) is now considered as diagnostic of osteoporosis. Moreover, many women who are osteopenic eventually develop osteoporotic fractures. Hence, bone density testing has occupied center stage in the diagnosis and treatment of this disorder. In addition, over the last several years, BMD has been utilized as the phenotype of choice for defining heritable markers for osteoporotic fractures. However, genetic studies in humans have been limited to some degree by the tremendous heterogeneity among populations, as well as multiple genetic, heritable and environmental determinants of the BMD phenotype. Recent advances in technology have afforded investigators the opportunity to study acquisition and maintenance of BMD in small animals. Along with newer knockout and transgenic strategies, quantification of mouse bone mass has advanced our understanding of both the biologic and genetic determinants of bone density. In this review, we will examine the use of the mouse to map the heritable factors that regulate bone acquisition. We will also examine the role of newer technology to decompose the bone density phenotype into components that are amenable to genetic studies. This review will focus on three models: (1) healthy inbred (2) recombinant inbred, and (3) congenic strains of mice. Progress in this area with these strains has been rapid, and a summary of several quantitative trait loci (QTLs) is provided. The future of the mouse as a tool to map the genes that define the osteoporosis syndrome is extremely promising.

Treatment of postmenopausal osteoporosis: an evidence-based approach

I have outlined several therapeutic approaches to the prevention and treatment of PMO. As we enter the 21st century it is certain that even more options will become available for the physician. But, several issues are still unresolved and will require more extensive investigation. These include the following: 1. Employing intermittent (weekly or monthly) oral or intravenous bisphosphonate therapy to treat PMO. 2. Using combination therapy such as an anabolic agent and an anti-resorptive to further reduce fracture risk and enhance BMD. 3. Understanding the various factors which contribute to fracture risk reduction independent of changes in bone mineral density. 4. Defining the role of biochemical markers in predicting fracture risk reduction. 5. Designing new SERMS which combine the best properties of estrogen and estrogen-like products while avoiding significant side effects. What is certain, is that there are now cost effective treatments for this disease. It remains up to the practicing physician to select, based on the existing evidence, treatments most appropriate for the individual patient.

Spontaneous fracture (sfx): a mouse genetic model of defective peripubertal bone formation

A new mouse model of stage-specific bone growth failure and fracture has been recovered as an autosomal recessive mutation, designated spontaneous fracture (sfx). The sfx/sfx mice are phenotypically normal until shortly after weaning, when reduced mobility and impaired somatic growth are first noted. By 6 weeks of age, body, spleen, and thymus weights, as well as hematocrits and serum calcium, inorganic phosphate, total alkaline phosphatase, insulin-like growth factor-I, and osteocalcin levels are decreased. The sfx/sfx mice also show reduced femoral cortical density and diaphyseal circumference, as well as a paucity of mature osteoblasts on bone surfaces. Histological analyses of the femur and tibia in the mutants show subtle reduction of chondrocyte numbers in epiphyseal-plate columns, reduction of matrix, and near absence of osteoid below the differentiated chondrocytes. Trabeculae in proximal tibiae, iliacs, and vertebral bodies are sparse and thin. Cortical bone thickness of mutants is markedly thinned in all sites examined. By 7-8 weeks, radiographic films routinely show spontaneous impact fractures of the distal femur accompanied by callus formation, whereas complete fractures are less commonly observed. Volumetric bone mineral density (BMD) of mutant femurs is similar to +/? littermates in the center of the femoral diaphysis, but BMD declines as either end of the femoral diaphysis is approached. We have mapped the gene responsible for this phenotype to central Chromosome 14. Reduced bone mass, impaired bone formation, abnormalities of bone architecture, and a disposition to spontaneous fracture identify sfx/sfx mice as a useful model for understanding the mechanisms responsible for peripubertal bone formation.

Serum IGF-I is higher in gymnasts than runners and predicts bone and lean mass

INTRODUCTION

We examined the relationships between insulin-like growth factor I (IGF-I), its binding protein (IGFBP-3), body composition, and bone mineral density (BMD) in collegiate runners (N = 13), gymnasts (N = 10), and noncompetitive women (N = 10).

METHODS

Subjects were evaluated by dual-energy x-ray absorptiometry for body composition and BMD of the spine, hip, and whole body, fasting serum levels of IGF-I and IGFBP-3, and dietary intake. The ratio IGF-I/IGFBP-3 was calculated as a marker of IGF-I bioavailability.

RESULTS

In ANOVA, IGF-I and IGF-I/IGFBP-3 in athletes with oligomenorrhea and amenorrhea did not differ from eumenorrheic athletes; thus, values were pooled. Lean/height2 and bone mass at the hip and spine were higher in gymnasts than runners and controls. Total caloric intake was similar between groups. IGF-I and IGF-I/IGFBP-3 differed between groups with gymnasts having higher IGF-I values than runners (397+/-58 vs 288+/-73 ng x mL(-1), P < 0.001) and higher IGF-I/IGFBP-3 than controls and runners (0.065+/-0.009 vs 0.056+/-0.008 vs 0.045+/-0.009, P = 0.0001). In simple regression, IGF-I and IGF-/IGFBP-3 were related to lean/height2 and BMD of the lumbar spine and hip (P < 0.01-0.0001). IGF-I and IGF-I/IGFBP-3 were multicollinear; thus, the ratio was used in subsequent stepwise regression. Lean mass, corrected for body surface area (height2), independently predicted spine and trochanteric BMD (R2 = 0.26, 0.28, respectively), whereas IGF-I/IGFBP-3 and lean/height2 together contributed to 48% of the variance in femoral neck BMD.

CONCLUSION

We conclude that, in this group of young adult women, lower BMD in runners may be due, in part, to lower levels of IGF-I and the ratio of IGF-I-to-IGFBP-3 and that IGF-I may mediate the relationship between bone and lean mass.

Sex steroids, the insulin-like growth factor regulatory system, and aging: implications for the management of older postmenopausal women

Aging is associated with profound changes in the growth hormone/insulin-like growth factor (IGF) regulatory system. These include reductions in growth hormone, IGF-I, IGFBP3, and IGFBP-5 and an increase in IGFBP-4. These changes, coupled with rather marked declines in sex steroid production from both the ovary and adrenals may combine to have very deleterious effects on several organ systems in the postmenopausal woman. In particular, the prevalence of two very common diseases, osteoporosis and coronary artery disease, increase dramatically after the cessation of gonadal steroid production. The complex interrelationship between the IGF regulatory system and estrogens/androgens in the postmenopausal period may provide important clues as to the pathophysiology of both these disorders. In this paper, we begin to define the role of IGF-I (and its constituent IGF binding proteins) in skeletal and vascular tissue. Recent experimental data show the effects of estrogen on circulating and tissue IGFs in older individuals. Finally, estrogen replacement therapy affects the IGF regulatory system in postmenopausal women. Although conclusions from early studies remain somewhat preliminary, it is likely that the IGF regulatory system will be a prime target for future studies into the pathogenesis of several age and sex hormone related degenerative disorders.

Mapping quantitative trait loci for serum insulin-like growth factor-1 levels in mice

Serum insulin-like growth factor-1 (IGF-1) and femoral bone mineral density (BMD) differ between two inbred strains of mice, C3H/HeJ (C3H) and C57BL/6J (B6), by approximately 30% and 50%, respectively. Similarly, skeletal IGF-1 content, bone formation, mineral apposition, and marrow stromal cell numbers are higher in C3H than in B6 mice. Because IGF-1 and several bone parameters cosegregate, we hypothesize that the serum IGF-1 phenotype has a strong heritable component and that genetic determinants for serum IGF-1 are involved in the regulation of bone mass. We intercrossed (B6 x C3H)F1 hybrids and analyzed 682 F2 female offspring at 4 months of age for serum IGF-1 by radioimmunoassay and femoral BMD by peripheral quantitative computerized tomography (pQCT). Genomic DNA was assayed by polymerase chain reaction (PCR) to determine alleles for 114 Mit markers inherited in F2 mice at average distances of 14 centimorgans (cM) along each chromosome (Chr). Serum IGF-1 levels in the F2 progeny were relatively normal in distribution, but showed a greater range than either progenitor, indicating that serum IGF-1 level is a polygenic trait with an estimated heritability of 52%. Serum IGF-1 correlated with femoral length (r = 0.266, p < 0.0001) and femoral BMD (r = 0.267, p < 0.0001). Whole genome scans for main effects associated with serum IGF-1 levels revealed three significant QTLs (in order of significance) on mouse Chrs 6, 15, and 10. The QTL on Chr 6 showed a significant reduction in IGF-1 associated with increasing C3H allele number, whereas the Chr 15 and Chr 10 loci showed additive effects with increasing C3H allele number. A genome-wide search for interacting marker pairs identified a significant interaction between the Chr 6 QTL and a locus on Chr 11. This interactive effect suggested that when the Chr 11 locus was homozygous for C3H, there was no effect of the Chr 6 locus on serum IGF-1; however, the combination of C3H alleles on Chr 6 with B6 alleles on Chr 11 was associated with reduced serum IGF-1 concentrations. To test this in vivo, we tested congenic mice carrying the Chr 6 QTL region from C3H on a B6 background (B6.C3H-6). Both serum IGF-1 and femoral BMD were significantly lower in female congenic than progenitor B6 mice. In summary, we identified three major QTLs on mouse Chrs 6, 10, and 15, and noted a major locus-locus interaction between Chrs 6 and 11. We named these QTLs IGF-1 serum levels (Igf1sl1 to Igf1sl4). Functional isolation of the Igf1sl1 QTL on Chr 6 for IGF-1 in B6.C3H-6 congenic mice demonstrated effects on both the IGF-1 and BMD phenotypes. The genetic determinants of these Igf1sl QTLs will provide much insight into the regulation of IGF-1 and the subsequent acquisition of peak bone mass.

Pathophysiology of osteoporosis

The pathogenesis of osteoporosis is complex and multifactorial. Alterations in bone density almost certainly represent the final common pathway by which pathologic factors affect risk of future osteoporotic fracture. The interplay of various physiologic processes, which result in peak bone mass, and maintenance of adult bone mass are key to understanding the pathogenesis of the disease. Changes in hormonal status, and in particular estradiol, clearly are important factors in both formation and resorption of bone in men and women. Perturbations in growth hormone activity, musculoskeletal function, dietary intake of calcium and vitamin D, and genetic determinants are also important pathogenic factors. Sorting out this complex interaction will challenge investigators and clinicians well into the next century.

IGF-I and osteoporosis

Serum IGF-I is controlled by several different regulatory factors. The final adult level represents the sum of the inert circulating depot, newly derived IGF-I synthesized from various tissues including liver, heart, kidney, bone, and others, and the departure from the circulation of IGF-I through mechanisms including receptor internalization and proteolysis of several IGFBPs. Although there is a numerical relationship between measurable IGF-I and bone mass, or risk of fractures, it is not clear that it is causal. Certainly, in situations like chronic undernutrition, which can lead to musculoskeletal instability and fractures, hepatic IGF-I expression is impaired. Yet, it is uncertain whether low levels of circulating IGF-I actually cause osteoporosis. Moreover, it has not been proved that serum levels of this peptide always reflect tissue concentrations. Caution must be undertaken in deciphering the results of a low, normal, or high IGF-I in relation to osteoporosis. Future studies should help define more clearly the possible pathogenic relationship between IGF-I and bone mass.

Parathyroid hormone as a therapy for idiopathic osteoporosis in men: effects on bone mineral density and bone markers

Osteoporosis in men poses a unique therapeutic challenge. Clinical studies have focused largely on the more prevalent problem of post-menopausal osteoporosis, with few gender-specific studies exploring treatment options in men. Idiopathic osteoporosis in middle-aged men presents an additional dilemma, because in the majority of patients it is a low bone turnover state for which there are currently no available anabolic agents. We conducted an 18-month randomized, double blind, placebo-controlled trial of 23 men with idiopathic osteoporosis, 30-68 yr old (mean age +/- SEM, 50 +/- 1.9 yr). All patients received 1,500 mg calcium and 400 IU vitamin D daily. Ten patients were randomized to receive 400 IU PTH-(1-34), and 13 patients received vehicle, administered by daily sc injection. Serum and urinary biochemistries, including markers of bone turnover were measured every 3 months. Bone densitometry of the lumbar spine, hip, and radius was performed every 6 months. PTH-(1-34) was associated with a marked 13.5% increase in bone mass at the lumbar spine, whereas that in the control group did not change (P < 0.001). The mean lumbar spine T-score improved from -3.5 +/- 0.2 to 2.4 +/- 0.4. Femoral neck bone mineral density in the PTH-treated group increased 2.9% (P < 0.05). The 1/3 site of the distal radius showed no change from baseline in the PTH-treated group. There were no significant changes in serum calcium concentration, 24-h urinary calcium excretion, or 1,25-dihydroxyvitamin D in either group. All markers of bone turnover increased in the PTH-treated patients, with the greatest changes in serum osteocalcin and urinary N-telopeptide (230% and 375% above baseline by 12 months, respectively; P < 0.001). Free pyridinoline and markers of bone formation that showed little correlation with each other at baseline, became highly correlated in the PTH-treated group (r = 0.1; P = 0.29 at baseline; to r = 0.7; P < 0.0001 at 18 months), a pattern absent in the control patients. The best predictor of the lumbar spine response to PTH at 18 months was the combination of pyridinoline at baseline and osteocalcin at 3 months (70% of the variance). PTH is a potent stimulator of skeletal dynamics in men with idiopathic, low turnover osteoporosis; is associated with substantial increases in lumbar spine and hip bone density; and may prove to be an efficacious anabolic agent in men with this disorder.

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.

Genetic regulation of cortical and trabecular bone strength and microstructure in inbred strains of mice

The inbred strains of mice C57BL/6J (B6) and C3H/HeJ (C3H) have very different femoral peak bone densities and may serve as models for studying the genetic regulation of bone mass. Our objective was to further define the bone biomechanics and microstructure of these two inbred strains. Microarchitecture of the proximal femur, femoral midshaft, and lumbar vertebrae were evaluated in three dimensions using microcomputed tomography (microCT) with an isotropic voxel size of 17 microm. Mineralization of the distal femur was determined using quantitative back-scatter electron (BSE) imaging. MicroCT images suggested that C3H mice had thicker femoral and vertebral cortices compared with B6. The C3H bone tissue also was more highly mineralized. However, C3H mice had few trabeculae in the vertebral bodies, femoral neck, and greater trochanter. The trabecular number (Tb.N) in the C3H vertebral bodies was about half of that in B6 vertebrae (2.8(-1) +/- 0.1 mm(-1) vs. 5.1(-1) +/- 0.2 mm(-1); p < 0.0001). The thick, more highly mineralized femoral cortex of C3H mice resulted in greater bending strength of the femoral diaphysis (62.1 +/- 1.2N vs. 27.4 +/- 0.5N, p < 0.0001). In contrast, strengths of the lumbar vertebra were not significantly different between inbred strains (p = 0.5), presumably because the thicker cortices were combined with inferior trabecular structure in the vertebrae of C3H mice. These results indicate that C3H mice benefit from alleles that enhance femoral strength but paradoxically are deficient in trabecular bone structure in the lumbar vertebrae.

Pathogenesis of osteoporosis

Osteoporosis is a very common disorder affecting millions of post-menopausal women and men of various ages. Although the disease is manifested by painful fractures of the spine, hip or radius, the underlying pathogenesis is complex and multifactorial. One of the strongest predictors of future osteoporotic fractures is low bone mineral density. The determinants of adult bone density include the rate of bone acquisition during adolescence and the absolute loss of bone during the six decades of adult life. Recent studies have clarified how bone mass is acquired during the early teen years in both boys and girls. Genetic factors play an enormous role in defining the height of acquisition of bone mass; however, these factors also interact with environmental and hormonal determinants. Many more studies have focused on adult bone loss. Disorders in bone remodelling result in an imbalance in bone turnover, favouring resorption over formation. Systemic factors such as oestrogen deprivation and parathyroid hormone strongly activate remodelling and can, in several circumstances, lead to imbalances in the remodelling cycle. The molecular cues that couple bone formation to resorption have recently been elucidated, and those factors may themselves become therapeutic targets for future treatment regimens to prevent osteoporosis and its resultant fractures.

Enhancement of bone mass in osteoporotic women with parathyroid hormone followed by alendronate

Treatment of osteoporosis with PTH causes a marked increase in vertebral bone mineral density (BMD). However, this effect is rapidly reversed when the treatment is stopped. The purpose of the present study was to determine whether the bisphosphonate alendronate could preserve or enhance bone density in patients previously treated with PTH. Sixty-six postmenopausal osteoporotic women were treated for 1 yr with 50, 75, or 100 microg recombinant human PTH-(1-84) or placebo, and then were given 10 mg alendronate daily for an additional year. BMD was measured in the femoral neck, lumbar spine, and whole body. Markers of bone turnover included skeletal alkaline phosphatase, osteocalcin, and N-telopeptide. During the first year, changes in BMD (mean +/- SD) in women receiving PTH (all doses combined) were 7.1 +/- 5.6% (spine), 0.3 +/- 6.2% (femoral neck), and -2.3 +/- 3.3% (total body). After switching to alendronate for 1 yr in women who previously had received PTH, mean changes in BMD were 13.4 +/- 6.4% (spine), 4.4 +/- 7.2% (femoral neck), and 2.6 +/- 3.1% (whole body). In the subgroup of patients who had received the highest dose of PTH, the mean increase in vertebral BMD was 14.6 +/- 7.9%. All markers of bone turnover increased during treatment with PTH and decreased to below baseline after 1 yr of alendronate. In conclusion, sequential treatment of osteoporosis with PTH and alendronate results in an increase in vertebral bone density that is considerably more than has been reported with alendronate or estrogens alone. This combination of drugs may be a useful approach to maximizing bone density in women with vertebral osteoporosis.

Bisphosphonates: safety and efficacy in the treatment and prevention of osteoporosis

Osteoporosis affects more than 28 million Americans. With the advent of accessible and affordable diagnostic studies, awareness and recognition of this disease by patients and clinicians are growing. Osteoporotic fractures of the spine and hip are costly and associated with significant morbidity and mortality. Over the past decade, a surge of new antiosteoporotic drugs have been labeled or are awaiting labeling by the U.S. Food and Drug Administration. One class of agents used to treat osteoporosis is the bisphosphonates, which inhibit bone resorption, cause an increase in bone mineral density and reduce the risk of future fractures caused by aging, estrogen deficiency and corticosteroid use. Overall, bisphosphonates have been shown to have a strong safety and tolerability profile.

Regulation of protein metabolism in middle-aged, premenopausal women: roles of adiposity and estradiol

The age-related loss of fat-free mass (FFM) is accelerated in women during the middle-age years and continues at an increased rate throughout the postmenopausal period. Because protein is the primary structural component of fat-free tissue, changes in FFM are largely due to alterations in protein metabolism. Knowledge of the hormonal and physiological correlates of protein metabolism in middle-aged women, therefore, has important implications for understanding the mechanisms underlying changes in FFM. We measured leucine kinetics (expressed relative to FFM: micromol/kg FFM/h) in 46 middle-aged, premenopausal women (mean +/- SD, 47 +/- 3 yr) after an overnight fast (i.e. basal) and during euglycemic hyperinsulinemia (40 mU/m2/min) using a 5.5-h infusion of [1-13C]leucine. Additionally, we measured insulin-stimulated glucose disposal by euglycemic hyperinsulinemic clamp, body composition by dual energy x-ray absorptiometry, abdominal fat distribution by computed tomography, and hormone levels by RIA as possible correlates of protein metabolism. Under basal conditions, stepwise regression analysis showed that leucine appearance (i.e. protein breakdown) was related to percent body fat and serum estradiol (r2 = 40%; P < 0.01), and leucine oxidation was related to serum estradiol and percent body fat (r2 = 26%; P < 0.05). Under euglycemic hyperinsulinemic conditions, no variables correlated with the percent change in leucine appearance. The percent change in leucine oxidation was related to intraabdominal adipose tissue area and glucose disposal rate (r2 = 48%; P < 0.01). Correlates and r2 values for nonoxidative leucine disposal (i.e. protein synthesis) under basal and euglycemic hyperinsulinemic conditions were similar to those observed for leucine appearance. From these results, we conclude that adiposity and/or serum estradiol may contribute to the regulation of protein metabolism and FFM in middle-aged, premenopausal women.

Growth hormone and aging

Aging is associated with a significant decline in secretion of growth hormone. This in turn leads to reduced circulating IGF-I and changes in IGF-binding proteins. Growth hormone replacement to growth hormone-deficient individuals has been shown to improve quality of life, enhance bone and muscle mass, and reduce cardiovascular risk. However, studies with growth hormone therapy in the elderly have been somewhat disappointing with minimal changes in lean body mass, musculoskeletal function, and overall quality of life. Moreover, recent evidence suggests that high normal serum IGF-I levels may be associated with a greater risk of several neoplastic disorders. Hence, there is less enthusiasm for reversing the changes of the "somatopause" with recombinant growth factors. An overview of these issues and the prospects for the future will be discussed in this article.

Insulin-like growth factor I and bone: from mouse to man

Serum insulin-like growth factor I (IGF-I) is regulated by numerous variables, including growth hormone (GH), nutritional status, gonadal steroids and other hormones. However, the circulating IGF-I phenotype is also under heritable regulation, and several genetic determinants may be important in defining tissue-specific expression of the gene encoding this peptide. A very strong correlation has been found between serum IGF-I concentration and bone acquisition in both mice and humans. Based on previous studies as well as ongoing work with mice, it has been hypothesized that regulation of the serum IGF-I phenotype includes non-GH-dependent factors and, furthermore, that these determinants are also involved in the acquisition of bone mass. This paper reports that, by performing intercrosses between two inbred strains of mice of similar age, size and length, but with different serum levels of IGF-I, we have identified regulatory loci for serum IGF-I and established their relationship to putative quantitative trait loci for bone mineral density. Mapping these quantitative trait loci will help refine our understanding of disorders related to IGF-I.