SEARCH

SEARCH BY CITATION

Keywords:

  • insulin-like growth factors;
  • insulin-like growth factor binding protein-2;
  • bone resorption;
  • bone formation;
  • bone turnover markers;
  • BMD

Abstract

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. MATERIALS AND METHODS
  5. RESULTS
  6. DISCUSSION
  7. Acknowledgements
  8. REFERENCES

Elevated serum IGFBP-2 is associated with lower BMD in men and women. It is unknown whether IGFBP-2 serves as a negative regulator of bone metabolism by decreasing bone formation or increasing bone resorption. Studying an age-stratified community-based sample of 344 men and 276 women, IGFBP-2 was the strongest predictor of increased bone resorption among the IGF/IGFBPs studied.

Introduction: Serum insulin-like growth factor binding protein-2 (IGFBP-2), which increases with age, is a predictor of low BMD among aging men and women. However, it is unknown whether IGFBP-2 negatively influences bone metabolism by decreasing bone formation or increasing bone resorption. Few have examined the relation between the insulin-like growth factors (IGFs) and their binding proteins (IGFBPs) with bone turnover markers.

Materials and Methods: In an age-stratified, random sample of the community, we examined the association between serum IGF-I, IGF-II, IGFBP-1, −2, and −3, and bone turnover markers before and after adjustment for potential confounders (age, body mass index, bioavailable estradiol and testosterone, and sex hormone binding globulin). Analyses were stratified by sex and menopausal status.

Results: We studied 344 men (age range, 23–90 yr) and 276 women (age range, 21–93 yr; 166 postmenopausal) not on oral contraceptives or hormone replacement. Among the IGF/IGFBPs assessed, IGFBP-2 was the strongest and most consistent predictor of bone turnover in men and women. After adjustment for potential confounders, IGFBP-2 was positively associated with osteocalcin (OC) and urine and serum N-teleopeptide (NTX) in men (r = 0.20, 0.26, and 0.23, respectively; p < 0.001), serum C-telopeptide (CTX) in premenopausal women (r = 0.28; p < 0.01), and OC, urine NTX, and serum CTX in postmenopausal women (r = 0.24, 0.33, and 0.19, respectively; p < 0.05).

Conclusions: Higher serum IGFBP-2, which is predictive of lower BMD, is associated with increased markers of bone resorption, independent of age, body mass, and sex hormones. The association between IGFBP-2 and markers of bone formation may reflect coupling with increased bone resorption, which is not adequate to maintain BMD.


INTRODUCTION

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. MATERIALS AND METHODS
  5. RESULTS
  6. DISCUSSION
  7. Acknowledgements
  8. REFERENCES

The insulin-like growth factor (IGF) system, a growth promoting regulatory system that includes IGF-I, IGF-II, and six IGF binding proteins (IGFBPs), is known to play an important role in skeletal modeling and growth during pubertal development.(1–3) Serum IGF/IGFBPs levels are known to change with aging, and thus, may also contribute to age-related bone loss,(4,5) although their role is not well understood. Whereas the decline in sex steroid levels are key factors in the pathogenesis of bone loss associated with aging, further understanding on other potential risk factors, including the IGF system, may improve our ability to assess risk and manage this condition.

Studying serum IGF-I, IGF-II, IGFBP-1, IGFBP-2, and IGFBP-3 in a community-based sample of adult men and women, we previously showed that high IGFBP-2 levels were the strongest predictor of low BMD, particularly among men and postmenopausal women, independent of age and bioavailable sex steroids.(6) Although some have found no association between IGFBP-2 and BMD,(7–9) others have shown an inverse association between IGFBP-2 and BMD in both men(10,11) and women.(12,13) Furthermore, elevated serum IGFBP-2 levels were reported to be a predictor of osteoporotic fractures in men.(14)

These findings suggest that elevated serum IGFBP-2 may have a negative effect on bone metabolism in aging men and women. However, it is unknown whether the deleterious effects of IGFBP-2 on bone metabolism reflect a decrease in bone formation or an increase in bone resorption. IGFBP-2 has been shown to have an inhibitory effect on anabolic IGF-I and IGF-II action on bone,(15–17) so elevated levels could lead to a decrease in bone formation by interfering with IGF action. On the other hand, IGFBP-2 is recognized to have IGF-independent actions in some tissues.(18–20) Although such effects have not been well evaluated in bone tissue, it does remain possible that elevated IGFBP-2 could be negatively influencing bone metabolism independent of IGF action, by either decreasing bone formation or increasing bone resorption. Regardless, it remains unknown how IGFBP-2 may be influencing bone turnover in aging men and women.

Relatively few studies have examined the association between the IGF/IGFBPs and markers of bone turnover in both young and old adult men and women. We therefore studied the relationship of serum IGF-I, IGF-II, and the binding proteins IGFBP-1, IGFBP-2, and IGFBP-3 with markers of bone formation and bone resorption among an age-stratified, population-based sample of adult men and women in whom we have previously characterized the associations between these IGF/IGFBPs and BMD.(6) Exploring the role of IGFBP-2, in particular, on markers of bone turnover would help to improve our understanding on the mechanism whereby IGFBP-2 may exert its adverse effects on bone metabolism of aging men and women.

MATERIALS AND METHODS

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. MATERIALS AND METHODS
  5. RESULTS
  6. DISCUSSION
  7. Acknowledgements
  8. REFERENCES

Study subjects

After approval by Mayo Clinic's Institutional Review Board, subjects were recruited from age-stratified random samples of Rochester, MN, men and women that were selected using the medical records linkage system of the Rochester Epidemiology Project.(21) More than one half of the Rochester population is identified annually in this system, and the majority is seen in any 3-yr period. Thus, the enumerated population (Rochester women seen in 1990 ± 1 yr and men seen in 1991 ± 1 yr) approximates the underlying population of the community, including both free-living and institutionalized individuals. Equal numbers of male and female subjects over the age of 20 yr were recruited to enroll 100 subjects per decade of age stratum. Altogether, 348 men and 351 women were recruited, as described in detail previously.(22) All but 13 men and 2 women were white, reflecting the ethnic composition of the population (96% white in 1990).

Because exogenous sex steroids have been shown to influence serum IGF/IGFBP levels,(23–27) we excluded one man receiving testosterone replacement; another three were excluded because of inexplicably high serum bioavailable estradiol levels (>60 pg/ml). Of the 351 women who participated, 28 premenopausal women on oral contraceptive pills (OCPs) and 47 postmenopausal women on hormone replacement therapy (HRT) were excluded from these analyses.

Study protocol

After providing written informed consent, subjects were interviewed in accordance with a standard protocol to collect clinical, demographic, and lifestyle data. BMD and body composition measurements were performed, and blood samples were drawn for biochemical analyses. Fasting state serum samples were obtained between 8:00 and 9:00 a.m., and for premenopausal women, were collected during the follicular phase of the menstrual cycle. All samples were stored at −70°C until analyzed. In addition, each subject underwent anthropometric assessment at the time of initial interview, which included measurements of height to the nearest 0.1 cm and weight in light clothes without shoes to the nearest 0.1 kg. Body mass index (BMI; kg/m2) was calculated from the height and weight of each subject.

Bone turnover markers:

Bone formation was assessed by measurement of fasting serum levels of bone alkaline phosphatase (BALP) isoenzyme, carboxy-terminal propeptide of type I collagen (PICP), and osteocalcin (OC). Serum BALP was measured by ELISA (Metra Biosystems, Mountain View, CA, USA; interassay CV < 11%).(28) Serum PICP was also measured by ELISA (Prolagen C; Metra Biosystems; interassay CV < 7%). Serum OC was measured by radioimmunoassay (RIA) using antibody G12 (CIS Biointernational, Bedford, MA, USA; interassay CV < 6%).(29) Bone resorption was evaluated through 24-h urine levels of cross-linked N-telopeptides (NTX) of type I collagen, assessed by ELISA (Osteomark; Ostex International, Seattle, WA, USA; interassay CV, 10%). The urine NTX was expressed in units of nanomoles per liter of glomerular filtrate (nM/liter GF), because this represents the more precise correction for alterations in renal function. The glomerular filtration rate was determined from the creatinine clearance and urine creatinine. Results for urine NTX expressed as nM/mM Cr are also provided for comparison; the correlation between these two variables is high (r = 0.91). We also measured fasting serum levels of NTX by an ELISA kit (Osteomark NTX Serum; Ostex International; interassay CV < 17%). For women only, bone resorption was also assessed using fasting serum cross-linked C-telopeptide of type I collagen (CTX; Osteometer BioTech; CV < 14%).

IGF/IGFBP measurements:

Total IGF-I and IGF-II were each measured by a two-site immunoradiometric assay (IRMA), after separation from their binding proteins with a simple organic solvent extraction (Diagnostic Systems Laboratories, Webster, TX, USA; interassay CVs, 6% for each).(30) IGFBP-1 and IGFBP-3 were also each measured by a two-site IRMA (Diagnostic Systems Laboratories; interassay CV, 7% for IGFBP-1 and 14% for IGFBP-3). IGFBP-2 was measured by a double antibody RIA (Diagnostic Systems Laboratories; interassay CV, 16%).

Sex steroids:

Fasting serum samples were assayed by RIA for total estradiol and total testosterone (Diagnostic Products Corp., Los Angeles, CA, USA; interassay CVs, 11% for each), and sex hormone binding globulin (SHBG; Wien Laboratories, Succasunna, NJ, USA; interassay CV, 7%). Non–SHBG-bound (bioavailable) fractions of estradiol (BioE2) and testosterone (BioT) were measured using a modification of the techniques of O'Connor et al.(31) and Tremblay and Dube(32) as described previously.(22)

Statistical analysis

Analyses were stratified by sex. In women, analyses were further stratified by menopausal status. Menopause was defined as either documented bilateral oophorectomy or >6 mo without a menstrual period. The Wilcoxon rank sum test was used to compare differences in medians for all variables by sex and menopausal status. To determine which of the IGF/IGFBPs were more strongly associated with each of the bone turnover markers, we performed age-adjusted stepwise regression analyses, where all IGF/IGFBPs were included as potential predictors. We also used Pearson correlations to examine the relationship between each of the IGF/IGFBPs and bone turnover markers, both unadjusted and adjusted for age. We examined the correlation between each of IGF/IGFBP measures and each of the markers of bone turnover after further adjustment for potential confounders: age, BMI, Bio E2, Bio T, and SHBG. We adjusted for bioavailable sex steroids in our analyses, not total E2 or T, based on work done previously in this cohort.(22) A significance level of p < 0.05 was used in all analyses. IGFBP-1, NTX, and CTX were log-transformed for analyses. Bio E2 was log-transformed for analyses involving postmenopausal women. All analyses were performed using SAS (SAS Institute, Cary, NC, USA).

RESULTS

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. MATERIALS AND METHODS
  5. RESULTS
  6. DISCUSSION
  7. Acknowledgements
  8. REFERENCES

Our study population for our analyses consisted of 344 men (age range, 23–90 yr; mean, 55.3 ± 19.6 [SD] yr) and 276 women (age range, 21–93 yr; mean, 56.6 ± 19.9 yr; 166 postmenopausal). The characteristics of subjects are outlined in Table 1.

Table Table 1.. Baseline Characteristics of an Age-Stratified Sample of Rochester, MN, Men and Women Not on OCPs or HRT
Thumbnail image of

As previously shown in this population, in both men and women, IGF-I and IGFBP-3 levels fell with advancing age, IGFBP-2 levels tended to rise with older age, whereas there was no association between age and either IGF-II or IGFBP-1.(6) In women, but not in men, OC, urine NTX, and serum NTX levels all tended to be higher with older age (Figs. 1 and 2). Serum CTX levels also tended to be higher with increasing age in older women (Fig. 2). As noted in the figures, a few outlier data were noted among the markers of bone turnover in men and women. We have presented our results with outlier data included, but our findings remained similar when outlier data were excluded.

thumbnail image

Figure Figure 1. (A) BALP, (B) PICP, (C) OC, (D) urine NTX, and (E) serum NTX levels as a function of age in an age-stratified sample of Rochester, MN, men.

Download figure to PowerPoint

thumbnail image

Figure Figure 2. (A) BALP, (B) PICP, (C) OC, (D) urine NTX, (E) serum NTX, and (F) serum CTX levels as a function of age in an age-stratified sample of Rochester, MN, women (open circles and solid line, premenopausal women; filled circles and dashed line, postmenopausal women).

Download figure to PowerPoint

IGF/IGFBPs as predictors of bone turnover markers

Bone formation markers:

In age-adjusted step-wise regression models, where all IGF/IGFBPs were included as potential predictors of each of the bone formation markers, there were differences in the significant (p < 0.05) predictors identified for men and women. In men, high serum IGF-I was a predictor of elevated BALP, high serum IGFBP-2 was a predictor of elevated PICP, whereas high serum levels of both were predictors of elevated levels of OC. In premenopausal women, high serum IGFBP-1 and IGFBP-2 were both predictors of elevated OC, whereas none of the IGF/IGFBPs were predictors of either BALP or PICP. In postmenopausal women, low serum IGFBP-3 was a predictor of both elevated levels of BALP and PICP, whereas both high serum IGFBP-2 and IGFBP-1 were predictors elevated OC.

Bone resoption markers:

In similar analyses examining the bone resorption markers, high serum IGF-I and IGFBP-2 were both predictors of elevated urine and serum NTX in men. In premenopausal women, high serum IGFBP-2 was the only predictor of elevated urine NTX, serum NTX and CTX. In postmenopausal women, high serum IGF-I and IGFBP-2, but low IGFBP-3, were predictors of elevated levels of urine NTX, whereas high serum IGFBP-2 was the only predictor of both increased serum NTX and CTX. Overall, among the IGF/IGFBPs studied, IGFBP-2 emerged as the strongest predictor of all bone resorption markers in men and premenopausal and postmenopausal women.

When we explored for an interaction between IGF-I and IGFBP-2 in these regression models, we found no relevant associations for any of the bone turnover markers, in either men or women.

Multivariate-adjusted correlations for IGF/IGFBPs and bone turnover markers

Unadjusted and adjusted correlations between IGF/IGFBPs and bone turnover markers for all 344 men, 110 premenopausal women, and 166 postmenopausal women are presented in Tables 2, 3, and 4, respectively.

Table Table 2.. Pearson Correlation Coefficients Between the IGF/IGFBPs and Bone Turnover Markers Among an Age-Stratified Sample of Rochester, MN, Men (N = 344)
Thumbnail image of
Table Table 3.. Pearson Correlation Coefficients Between the IGF/IGFBPs and Bone Turnover Markers Among an Age-Stratified Sample of Premenopausal, Rochester, MN, Women Not on OCPs (N = 110)
Thumbnail image of
Table Table 4.. Pearson Correlation Coefficients Between the IGF/IGFBPs and Bone Turnover Markers Among an Age-Stratified Sample of Postmenopausal, Rochester, MN, Women Not on HRT (N = 166)
Thumbnail image of

IGF-I

Adjusted for age, high serum IGF-I was associated with increased levels of BALP, OC, urine NTX, and serum NTX in men (r = 0.16, 0.13, 0.14, and 0.12, respectively; all p < 0.05; Table 2), but low levels of BALP in postmenopausal women (r = −0.16, p < 0.05; Table 4). After additional adjustment for BMI, Bio E2, Bio T, and SHBG, high serum IGF-I remained associated with increased BALP, OC, urine NTX, and serum NTX in men only (r = 0.22, 0.15, 0.20, and 0.19, respectively; p < 0.01) (Tables 2–4).

IGF-II

No associations were seen between IGF-II and any of the markers of bone turnover in multivariate adjusted models for either men or women (Tables 2–4).

IGFBP-1

In multivariate adjusted models, high serum IGFBP-1 was associated with increased levels of OC in men (r = 0.12, p < 0.05; Table 2) and increased levels of both BALP and OC in premenopausal women (r = 0.22 and 0.20, respectively; p < 0.05; Table 3). In multivariate adjusted models, no association was seen between IGFBP-1 and any of the bone turnover markers in postmenopausal women (Table 4).

IGFBP-2

In age-adjusted models, high serum IGFBP-2 was associated with elevated levels of several markers of bone turnover in men and women (Tables 2–4). In multivariate adjusted models, high serum IGFBP-2 levels remained associated with increased serum OC, urine NTX, and serum NTX in men (r = 0.20, 0.26, and 0.23, respectively; p < 0.001), with increased serum CTX in premenopausal women (r = 0.28; p < 0.01), and with increased serum OC, urine NTX, and serum CTX in postmenopausal women (r = 0.24, 0.33, and 0.19, respectively; p < 0.05) (Tables 2–4).

IGFBP-3

In multivariate adjusted models, high serum IGFBP-3 was associated with elevated urine and serum NTX in men (r = 0.15 and 0.14, respectively; p < 0.05; Table 2), but low urine NTX in postmenopausal women (r = −0.17; p < 0.05; Table 4).

DISCUSSION

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. MATERIALS AND METHODS
  5. RESULTS
  6. DISCUSSION
  7. Acknowledgements
  8. REFERENCES

In this population-based study of adult men and women, we found serum IGFBP-2 to be the strongest and most consistent predictor of bone turnover among the IGF/IGFBPs studied. High serum IGFBP-2 levels were more consistently associated with elevated levels of bone resorption markers, particularly in men and postmenopausal women, independent of age, body mass index, and sex steroid levels.

Although osteocalcin, a bone formation marker, was increased with higher levels of serum IGFBP-2 despite adjustment for confounders, including age, this may be the result from coupling of bone formation with increased bone resorption. However, we did not identify an association between IGFBP-2 and other markers of bone formation after adjustment for potential confounders (age, BMI, and sex steroids). Bone formation and bone resorption markers were correlated with each other and ranged from 0.29 to 0.58 for men, 0.25 to 0.56 for premenopausal women, and 0.39 to 0.64 for postmenopausal women. In men and both groups of women, the weakest correlations observed were between PICP and bone resorption markers, whereas the strongest correlations were seen between OC and bone resorption markers. When we explored whether IGFBP-2 remained an independent predictor of osteocalcin after adjustment for markers of bone resorption, we found that the association was attenuated in men, whereas no further association was identified in women (data not shown). These results would suggest that the association between IGFBP-2 and OC may be confounded by the strong correlation between OC and bone resorption markers. Nevertheless, given the fact that we previously showed that higher serum IGFBP-2 levels are associated with lower BMD in this population,(6) it would seem that any apparent increase in bone formation reflects increased bone turnover, yet is not adequate to maintain BMD.

Whereas IGF-I, IGF-II, and IGFBP-3 are all considered anabolic to bone,(2,19,33) only among men did we find a consistent association between higher serum IGF-I levels and greater markers of bone formation. In our age-adjusted, stepwise regression model where we examined all the IGF/IGFBPs together as potential predictors, both IGF-I and IGFBP-2 were independent predictors of bone turnover in men. The increase in bone resorption markers observed with higher IGF-I levels, again, likely reflects coupling from enhanced bone formation. Unlike in men, we did not identify an association between IGF-I and bone formation markers in women. Men are known to have greater periosteal apposition of bone with aging than women, and we speculate that this may be, in part, under IGF-I control. We found no relevant interactions between IGF-I and IGFBP-2 on markers of bone turnover. Interestingly, we identified a positive association between IGFBP-1 and some bone formation markers among men and premenopausal women. In this same population of men and women, however, we found no association between IGFBP-1 and BMD.(6) Contrary to our expectations, in postmenopausal women, there was a tendency for an inverse association between IGFBP-3 and some bone turnover markers. Although generally considered a potentiator of IGF action, IGFBP-3 has also been shown to have inhibitory effects on osteoblasts.(19) Again, however, we did not find strong associations between the IGF/IGFBPs and BMD in this same study population, with the exception of IGFBP-2,(6) which is consistent with our overall findings on the relation between the IGF/IGFBPs and bone turnover markers.

Our results add further evidence that elevated IGFBP-2 may serve as a negative regulator of bone metabolism.(10–15,34,35) How this occurs is uncertain but open to speculation. Although, in animal and in vitro studies, high IGFBP-2 levels have a negative effect on developing bone by interfering with the anabolic effects of IGF-I and IGF-II,(15–17) this would be an unlikely explanation for our results; we would have anticipated an inverse association with bone formation markers, which we did not find, not a positive association with bone resorption. The mechanism by which elevated IGFBP-2 exerts its negative effect on bone may differ for mature bones. It is possible that IGFBP-2 may be stimulating bone resorption through IGF-independent effects. IGFBP-2 has been shown to independently stimulate the proliferation and activation of peripheral blood mononuclear cells(20); cytokines released from activated peripheral blood mononuclear cells have been implicated in increasing bone resorption.(36–39)

However, in contrast to the evidence for a negative impact of IGFBP-2 on bone, some studies have suggested an anabolic action of IGFBP-2 on bone. Exogenous IGFBP-2 stimulates IGF-II–mediated osteoblast activity in vitro,(40) whereas systemic administration of an IGF-II/IGFBP-2 complex prevented bone loss in rats after sciatic neurectomy.(41) Concomitant elevations in an IGF-II precursor molecule (IGF-IIE) and IGFBP-2 were noted in patients with the rare syndrome of hepatitis C–associated osteosclerosis.(42) These studies would suggest that IGFBP-2 might actually have beneficial effects to bone under certain circumstances. Nonetheless, elevated serum IGFBP-2 may play a different role on bone metabolism in the hormonal milieu of aging men and women.

Our study has limitations. Bone turnover markers were collected at a single point in time; the biologic variation in these different markers could account for some of the lack of associations observed. It is not known whether the observed associations based on serum levels of the IGF/IGFBPs reflect their action at the level of bone tissue. The IGF/IGFBPs are produced by osteoblasts and are known to have autocrine and paracrine effects.(33) Furthermore, we do not have measures of three of the six known IGFBPs (IGFBP-4, IGFBP-5, and IGFBP-6), of which IGFBP-4 and IGFBP-5 have been shown to be important regulators of bone metabolism at the level of bone tissue.(33,43) How IGFBP-2 may interact with IGFBP-4 or IGFBP-5 is unknown. Nevertheless, our study is the first to examine the association between serum IGFBP-2 and bone turnover markers in a relatively large number of both young and older, men and women. We have also been able to evaluate the associations between the IGF/IGFBPs and bone turnover markers, while taking into account other important covariates including sex steroid levels. Although we do not have measures of free estradiol and testosterone, we have previously shown important associations between bioavailable sex steroids and bone metabolism in this same cohort,(22) so adjustment for bioavailable sex steroids were performed in analyses. We can not exclude the fact that residual confounding could still be influencing our associations between the IGF/IGFBPs and bone turnover markers caused by our lack of free estradiol and testosterone measures. Although not all potential confounders could be addressed, we did explore whether results were influenced by season or vitamin D or PTH levels, but they were not (data not shown). Although our study population consisted of a large sample of men and women spanning a wide age range, those excluded from participation were primarily men and women who were cognitively impaired and therefore unable to provide informed consent.(22) Our results, therefore, may not be generalizable to those individuals. Finally, our results are from a cross-sectional, epidemiologic study. Whereas our findings are hypothesis generating, we can not infer causality, and additional studies are needed to clarify the relation between IGF/IGFBPs, including IGFBP-2, on bone metabolism of aging men and women. Our data would suggest the need for further exploring the role of the IGF system on age-related bone loss.

In summary, we identified higher serum IGFBP-2 levels to be associated with increased bone turnover, particularly increased bone resorption, in aging men and women. Our results add further evidence to suggest that elevated serum IGFBP-2 levels may serve as a negative regulator of bone metabolism in older men and women. These findings provide additional support on the likely role of the IGF system in age-related bone loss. Moreover, the association between higher serum IGFBP-2 and enhanced bone resorption would also suggest that IGFBP-2 may have a greater negative influence on trabecular bone than cortical bone. Further evaluation on the influence of IGFBP-2 on aspects on bone microarchitecture in aging men and women would be of interest.

Acknowledgements

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. MATERIALS AND METHODS
  5. RESULTS
  6. DISCUSSION
  7. Acknowledgements
  8. REFERENCES

The authors thank Vicki Gathje and Joan Muhs for help in recruiting subjects, Margaret Holets for BMD measurements, Roberta Soderberg for sample handling, and Jim Peterson for assistance with figures. This work was supported by Research Grants AR27065 and RR00585 from the National Institutes of Health, U.S. Public Health Service.

REFERENCES

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. MATERIALS AND METHODS
  5. RESULTS
  6. DISCUSSION
  7. Acknowledgements
  8. REFERENCES
  • 1
    Rogol AD 1992 Growth and growth hormone secretion at puberty: The role of gonadal steroid hormones. Acta Paediatr Suppl 383: 1520.
  • 2
    Rajaram S, Baylink DJ, Mohan S 1997 Insulin-like growth factor-binding proteins in serum and other biological fluids: Regulation and functions. Endocr Rev 18: 801831.
  • 3
    Van Wyk JJ, Smith EP 1999 Insulin-like growth factors and skeletal growth: Possibilities for therapeutic interventions. J Clin Endocrinol Metab 84: 43494354.
  • 4
    Bennett AE, Wahner HW, Riggs BL, Hintz RL 1984 Insulin-like growth factors I and II: Aging and bone density in women. J Clin Endocrinol Metab 59: 701704.
  • 5
    Kveiborg M, Flyvbjerg A, Rattan SI, Kassem M 2000 Changes in the insulin-like growth factor-system may contribute to in vitro age-related impaired osteoblast functions. Exp Gerontol 35: 10611074.
  • 6
    Amin S, Riggs BL, Atkinson EJ, Oberg AL, Melton LJ III, Khosla S 2004 A potentially deleterious role of IGFBP-2 on bone density in aging men and women. J Bone Miner Res 19: 10751083.
  • 7
    Kim JG, Shin CS, Choi YM, Moon SY, Kim SY, Lee JY 1999 The relationship among circulating insulin-like growth factor components, biochemical markers of bone turnover and bone mineral density in postmenopausal women under the age of 60. Clin Endocrinol (Oxf) 51: 301307.
  • 8
    Nasu M, Sugimoto T, Chihara M, Hiraumi M, Kurimoto F, Chihara K 1997 Effect of natural menopause on serum levels of IGF-I and IGF-binding proteins: Relationship with bone mineral density and lipid metabolism in perimenopausal women. Eur J Endocrinol 136: 608616.
  • 9
    Johansson AG, Eriksen EF, Lindh E, Langdahl B, Blum WF, Lindahl A, Ljunggren O, Ljunghall S 1997 Reduced serum levels of the growth hormone-dependent insulin-like growth factor binding protein and a negative bone balance at the level of individual remodeling units in idiopathic osteoporosis in men. J Clin Endocrinol Metab 82: 27952798.
  • 10
    van den Beld AW, Blum WF, Pols HA, Grobbee DE, Lamberts SW 2003 Serum insulin-like growth factor binding protein-2 levels as an indicator of functional ability in elderly men. Eur J Endocrinol 148: 627634.
  • 11
    Gillberg P, Olofsson H, Mallmin H, Blum WF, Ljunghall S, Nilsson AG 2002 Bone mineral density in femoral neck is positively correlated to circulating insulin-like growth factor (IGF)-I and IGF-binding protein (IGFBP)-3 in Swedish men. Calcif Tissue Int 70: 2229.
  • 12
    Sugimoto T, Nishiyama K, Kuribayashi F, Chihara K 1997 Serum levels of insulin-like growth factor (IGF) I, IGF-binding protein (IGFBP)-2, and IGFBP-3 in osteoporotic patients with and without spinal fractures. J Bone Miner Res 12: 12721279.
  • 13
    Kim JG, Lee JY 1996 Serum insulin-like growth factor binding protein profiles in postmenopausal women: Their correlation with bone mineral density. Am J Obstet Gynecol 174: 15111517.
  • 14
    Meier C, Nguyen TV, Center JR, Seibel MJ, Eisman JA 2005 High IGFBP-2 levels are associated with an increased risk of osteoporotic fractures in elderly men. J Bone Miner Res 20: S1; S36.
  • 15
    Fisher MC, Meyer C, Garber G, Dealy CN 2005 Role of IGFBP2, IGF-I and IGF-II in regulating long bone growth. Bone 37: 741750.
  • 16
    Kream BE, Tetradis S, Lafrancis D, Fall PM, Feyen JH, Raisz LG 1997 Modulation of the effects of glucocorticoids on collagen synthesis in fetal rat calvariae by insulin-like growth factor binding protein-2. J Bone Miner Res 12: 889895.
  • 17
    Raisz LG, Fall PM, Gabbitas BY, McCarthy TL, Kream BE, Canalis E 1993 Effects of prostaglandin E2 on bone formation in cultured fetal rat calvariae: Role of insulin-like growth factor-I. Endocrinology 133: 15041510.
  • 18
    Firth SM, Baxter RC 2002 Cellular actions of the insulin-like growth factor binding proteins. Endocr Rev 23: 824854.
  • 19
    Jones JI, Clemmons DR 1995 Insulin-like growth factors and their binding proteins: Biological actions. Endocr Rev 16: 334.
  • 20
    Hettmer S, Dannecker L, Foell J, Elmlinger MW, Dannecker GE 2005 Effects of insulin-like growth factors and insulin-like growth factor binding protein-2 on the in vitro proliferation of peripheral blood mononuclear cells. Hum Immunol 66: 95103.
  • 21
    Melton LJ III 1996 History of the Rochester Epidemiologic Project. Mayo Clin Proc 71: 266274.
  • 22
    Khosla S, Melton LJ III, Atkinson EJ, O'Fallon WM, Klee GG, Riggs BL 1998 Relationship of serum sex steroid levels and bone turnover markers with bone mineral density in men and women: A key role for bioavailable estrogen. J Clin Endocrinol Metab 83: 22662274.
  • 23
    Gori F, Hofbauer LC, Conover CA, Khosla S 1999 Effects of androgens on the insulin-like growth factor system in an androgen-responsive human osteoblastic cell line. Endocrinology 140: 55795586.
  • 24
    Campagnoli C, Biglia N, Cantamessa C, Lesca L, Lotano MR, Sismondi P 1998 Insulin-like growth factor I (IGF-I) serum level modifications during transdermal estradiol treatment in postmenopausal women: A possible bimodal effect depending on basal IGF-I values. Gynecol Endocrinol 12: 259266.
  • 25
    Ashton WS, Degnan BM, Daniel A, Francis GL 1995 Testosterone increases insulin-like growth factor-1 and insulin-like growth factor-binding protein. Ann Clin Lab Sci 25: 381388.
  • 26
    Kassem M, Okazaki R, Harris SA, Spelsberg TC, Conover CA, Riggs BL 1998 Estrogen effects on insulin-like growth factor gene expression in a human osteoblastic cell line with high levels of estrogen receptor. Calcif Tissue Int 62: 6066.
  • 27
    Munzer T, Rosen CJ, Harman SM, Pabst KM, St Clair C, Sorkin JD, Blackman MR 2006 Effects of GH and/or sex steroids on circulating IGF-I and IGFBPs in healthy, aged women and men. Am J Physiol Endocrinol Metab 290: E1006E1013.
  • 28
    Duda RJ Jr, O'Brien JF, Katzmann JA, Peterson JM, Mann KG, Riggs BL 1988 Concurrent assays of circulating bone Gla-protein and bone alkaline phosphatase: Effects of sex, age, and metabolic bone disease. J Clin Endocrinol Metab 66: 951957.
  • 29
    Delmas PD, Stenner D, Wahner HW, Mann KG, Riggs BL 1983 Increase in serum bone gamma-carboxyglutamic acid protein with aging in women. Implications for the mechanism of age-related bone loss. J Clin Invest 71: 13161321.
  • 30
    Daughaday WH, Mariz IK, Blethen SL 1980 Inhibition of access of bound somatomedin to membrane receptor and immunobinding sites: A comparison of radioreceptor and radioimmunoassay of somatomedin in native and acid-ethanol-extracted serum. J Clin Endocrinol Metab 51: 781788.
  • 31
    O'Connor S, Baker HW, Dulmanis A, Hudson B 1973 The measurement of sex steroid binding globulin by differential ammonium sulphate precipitation. J Steroid Biochem 4: 331339.
  • 32
    Tremblay RR, Dube JY 1974 Plasma concentrations of free and non-TeBG bound testosterone in women on oral contraceptives. Contraception 10: 599605.
  • 33
    Conover CA, Rosen C 2002 The role of insulin-like growth factors and binding proteins in bone cell biology. BilezikianJP, RaiszLG, RodanGA (eds.) Principles of Bone Biology, 2nd ed., vol. 1. Academia Press, San Diego, CA, USA, 801815.
  • 34
    Grinspoon S, Miller K, Herzog D, Clemmons D, Klibanski A 2003 Effects of recombinant human insulin-like growth factor (IGF)-I and estrogen administration on IGF-I, IGF binding protein (IGFBP)-2, and IGFBP-3 in anorexia nervosa: A randomized-controlled study. J Clin Endocrinol Metab 88: 11421149.
  • 35
    Eckstein F, Pavicic T, Nedbal S, Schmidt C, Wehr U, Rambeck W, Wolf E, Hoeflich A 2002 Insulin-like growth factor-binding protein-2 (IGFBP-2) overexpression negatively regulates bone size and mass, but not density, in the absence and presence of growth hormone/IGF-I excess in transgenic mice. Anat Embryol (Berl) 206: 139148.
  • 36
    Fagiolo U, Cossarizza A, Scala E, Fanales-Belasio E, Ortolani C, Cozzi E, Monti D, Franceschi C, Paganelli R 1993 Increased cytokine production in mononuclear cells of healthy elderly people. Eur J Immunol 23: 23752378.
  • 37
    D'Amelio P, Grimaldi A, Pescarmona GP, Tamone C, Roato I, Isaia G 2005 Spontaneous osteoclast formation from peripheral blood mononuclear cells in postmenopausal osteoporosis. FASEB J 19: 410412.
  • 38
    Clowes JA, Riggs BL, Khosla S 2005 The role of the immune system in the pathophysiology of osteoporosis. Immunol Rev 208: 207227.
  • 39
    Weitzmann MN, Pacifici R 2005 The role of T lymphocytes in bone metabolism. Immunol Rev 208: 154168.
  • 40
    Palermo C, Manduca P, Gazzerro E, Foppiani L, Segat D, Barreca A 2004 Potentiating role of IGFBP-2 on IGF-II-stimulated alkaline phosphatase activity in differentiating osteoblasts. Am J Physiol Endocrinol Metab 286: E648E657.
  • 41
    Conover CA, Johnstone EW, Turner RT, Evans GL, John Ballard FJ, Doran PM, Khosla S 2002 Subcutaneous administration of insulin-like growth factor (IGF)-II/IGF binding protein-2 complex stimulates bone formation and prevents loss of bone mineral density in a rat model of disuse osteoporosis. Growth Horm IGF Res 12: 178183.
  • 42
    Khosla S, Hassoun AA, Baker BK, Liu F, Zein NN, Whyte MP, Reasner CA, Nippoldt TB, Tiegs RD, Hintz RL, Conover CA 1998 Insulin-like growth factor system abnormalities in hepatitis C-associated osteosclerosis. Potential insights into increasing bone mass in adults. J Clin Invest 101: 21652173.
  • 43
    Mohan S, Farley JR, Baylink DJ 1995 Age-related changes in IGFBP-4 and IGFBP-5 levels in human serum and bone: Implications for bone loss with aging. Prog Growth Factor Res 6: 465473.