Release of insulin-like growth factor carrier proteins by osteoblasts: stimulation by estradiol and growth hormone

Endocrine control of body composition in infancy, childhood, and puberty

Body composition exhibits marked variations across the early human lifetime. The precise physiological mechanisms that drive such developmental adaptations are difficult to establish. This clinical challenge reflects an array of potentially confounding factors, such as marked intersubject differences in tissue compartments; the incremental nature of longitudinal intrasubject variations in body composition; technical limitations in quantitating the unobserved mass of mineral, fat, water, and muscle ad seriatim; and the multifold contributions of genetic, dietary, environmental, hormonal, nutritional, and behavioral signals to physical and sexual maturation. From an endocrine perspective (reviewed here), gonadal sex steroids and GH/IGF-I constitute prime determinants of evolving body composition. The present critical review examines hormonal regulation of body composition in infancy, childhood, and puberty.

Analyses of hind leg skeletons in human growth hormone transgenic rats

Growth hormone (GH) is essential in the development and growth of the skeleton and for the maintenance of bone mass and density, and its secretion is known to decline with aging. We have previously produced transgenic rats with low circulating GH that represent several age-associated phenotypes such as obesity, insulin-resistance and leptin-resistance. In the present study, the cross-sectional area, bone mineral density, and strength indexes of the hind leg skeletons of the transgenic rats were examined by an X-ray computed tomography scanning. The mean cross-sectional area of the transgenic rats showed no increase after 2 months old up to 8 months old and the strength indexes were significantly lower than their non-transgenic siblings at all ages examined. The trabecular bone mineral density in the transgenic rats drastically decreased at 8 months old, while the cortical bone mineral density was comparable to the non-transgenic rats, suggesting the onset of osteoporosis at this period. The results obtained in this study indicate that the transgenic rats could be useful model to gain insight into the complex mechanism leading to osteoporosis with aging.

Changes in proinflammatory cytokine activity after menopause

There is now a large body of evidence suggesting that the decline in ovarian function with menopause is associated with spontaneous increases in proinflammatory cytokines. The cytokines that have obtained the most attention are IL-1, IL-6, and TNF-alpha. The exact mechanisms by which estrogen interferes with cytokine activity are still incompletely known but may potentially include interactions of the ER with other transcription factors, modulation of nitric oxide activity, antioxidative effects, plasma membrane actions, and changes in immune cell function. Experimental and clinical studies strongly support a link between the increased state of proinflammatory cytokine activity and postmenopausal bone loss. Preliminary evidence suggests that these changes also might be relevant to vascular homeostasis and the development of atherosclerosis. Better knowledge of the mechanisms and the time course of these interactions may open new avenues for the prevention and treatment of some of the most prevalent and important disorders in postmenopausal women.

Growth hormone therapy and fracture risk in the growth hormone-deficient adult

Adults with childhood-onset growth hormone deficiency (GHD) and younger adults with adult-onset GHD have a reduced bone mineral content (BMC). Recent trials with prolonged GH replacement therapy have demonstrated increased BMC in such patients. GH treatment in animals increases the amount of bone and the total strength while the density (BMC per unit volume) and the quality of the bone is not increased. A sensitive non-invasive parameter for the detection of effects of GH on bone in clinical studies is therefore to use the BMC from dual-energy X-ray absorption (DEXA) analysis. Bone density is strongly related to fracture risk in women. A number of other risk factors for fractures can be identified in adult GHD patients which, collectively, might explain the increased fracture frequency observed in these patients. The increase in BMC in response to long-term GH replacement therapy is promising. Whether more prolonged treatment will result in a normalization of the bone mass and reduced fracture frequency remains to be established.

Estrogen modulation of osteoblastic cell-to-cell communication

Two osteoblastic cell populations, calvarial and marrow stromal cells, were exposed to estrogen derivatives in vitro. The hormonal effect was monitored by following intracellular Ca+2 levels [Ca+2]i and gap-junction communication. We measured fast changes in intracellular Ca+2 levels in response, of these cells, to the steroid hormones. The changes were dose dependent revealing maximal activity at 100 pM by 17-beta-Estradiol and 1 nM by estradiol-CMO. Additionally, the effect of estrogen, on functional coupling of the cells, was measured using fluorescence dye migration and counting the number of neighboring cells coupled by gap junctions. An uncoupling effect was demonstrated in response of these cells to estrogen treatment. The quick stereospecific effect was achieved in the presence of 17-beta-estradiol but not in the presence of 17-alpha-estradiol. These results suggest the involvement of plasma membrane receptors in addition to the already known nuclear receptors in transducing the hormone effects in the osteoblastic cells.

Estrogen enhances differentiation of osteoblasts in mouse bone marrow culture

The effects of estrogen on bone are possibly mediated by several cell types. In the present study, the effect of 17beta-estradiol (E2) on osteoblast-like cells was investigated by using mouse bone marrow cultures. Bone marrow cells were harvested from the shafts of femurs of 10-week-old NMRI mice and cultured. On day 6, confluent primary cultures were trypsinized and subcultured. Under the conditions used (Keila, S., Pitaru, S., Grosskopf, A., and Wernreb, M. Bone marrow from mechanically unloaded rat bones expresses reduced osteogenic capacity in vitro. J Bone Miner Res 9:321-327; 1994), the bone marrow cultures showed differentiation towards the osteoblastic phenotype. This was demonstrated by the appearance of osteoblastic markers such as alpha1(I) collagen (COL1), alkaline phosphatase (ALP), osteocalcin (OCN), osteopontin (OP), and transforming growth factor-beta1 (TGFbeta1), which were detected by using reverse transcriptase polymerase chain reaction (RT-PCR). Bone nodule formation, including deposition of collagen fibers and matrix mineralization, was also studied at several time points of the 3-week culture period. The effect of E2 on the appearance of osteoblastic markers was studied by incubating cultures in the presence or absence of the hormone. The messenger ribonucleic acid (mRNA) for the estrogen receptor (ER) was found to be expressed at all time points as demonstrated by RT-PCR. When grown with E2, the rate of cell proliferation was increased in the early phase of cultures, but not after day 6. The addition of E2 in subcultures resulted in an increase of levels of mRNA for COL1, ALP, OCN, OP, and TGF-beta1. ALP activity was also increased. Bone nodule formation, as well as calcium contents, were significantly increased in the cultures grown in the presence of E2. All E2 concentrations used (0.01-10 nmol/L) were effective but the maximum response was obtained with 0.1 nmol/L E2. Addition of the antiestrogen ICI 182,780 abolished the E2-induced stimulation of proliferation and later an increase in ALP activity. Addition of ICI 182,780 without the hormone did not cause any changes when compared to control cultures. In conclusion, our results demonstrate that E2 stimulates sequential differentiation of osteoblasts and increases deposition and mineralization of matrix in mouse bone marrow cultures in an estrogen receptor-dependent manner.

Estrogen regulation of a transforming growth factor-beta inducible early gene that inhibits deoxyribonucleic acid synthesis in human osteoblasts

This laboratory reported the identification and characterization of a unique three zinc finger, transcription factor-like, transforming growth factor-beta inducible early gene (TIEG) (see Ref. 35). TIEG expression has been shown to be tissue- and cell type specific, enhanced by specific growth factors, and to decrease with advancing stages of breast cancer. Recent studies involving TIEG overexpression in pancreatic carcinoma cells indicate that TIEG expression inhibits DNA synthesis, similar to a tumor suppressor-like gene, and plays a role in apoptosis (see Ref. 37). This paper describes the rapid, but transient, induction of TIEG steady-state messenger RNA (mRNA) levels by 17beta-estradiol (E2) in estrogen receptor (ER)-positive, human fetal osteoblastic (hFOB/ER) cells. This rapid induction is shown to be ER- and steroid dose-dependent but protein synthesis independent. An antagonism between E2 and PTH, which occurs in skeletal metabolism, is shown to concur rapidly with TIEG mRNA expression. Scanning confocal microscopy (using polarized, laser-based immunofluorescence) shows that TIEG protein is localized in the nucleus of hFOB/ER cells, with the levels rapidly increasing after E2 treatment. The rapid E2-induced increase in TIEG expression is followed by an E2-induced inhibition of DNA synthesis in the hFOB/ER cells. Antiestrogens block not only the induction of TIEG mRNA levels but also the inhibition of cell proliferation. Lastly, hFOB cells, stably transfected with a TIEG expression vector, display markedly reduced DNA synthesis/cell proliferation, compared with nontransfected cells. These results support the finding that TIEG is an early responding regulatory gene for E2 in human osteoblast cells that inhibits DNA synthesis. It is speculated that TIEG may play a role in the signaling pathway for E2 in inhibiting cell proliferation.

Growth hormone and bone

It is well known that GH is important in the regulation of longitudinal bone growth. Its role in the regulation of bone metabolism in man has not been understood until recently. Several in vivo and in vitro studies have demonstrated that GH is important in the regulation of both bone formation and bone resorption. In Figure 9 a simplified model for the cellular effects of GH in the regulation of bone remodeling is presented (Fig. 9). GH increases bone formation in two ways: via a direct interaction with GHRs on osteoblasts and via an induction of endocrine and autocrine/paracrine IGF-I. It is difficult to say how much of the GH effect is mediated by IGFs and how much is IGF-independent. GH treatment also results in increased bone resorption. It is still unknown whether osteoclasts express functional GHRs, but recent in vitro studies indicate that GH regulates osteoclast formation in bone marrow cultures. Possible modulations of the GH/IGF axis by glucocorticoids and estrogens are also included in Fig. 9. GH deficiency results in a decreased bone mass in both man and experimental animals. Long-term treatment (> 18 months) of GHD patients with GH results in an increased bone mass. GH treatment also increases bone mass and the total mechanical strength of bones in rats with a normal GH secretion. Recent clinical studies demonstrate that GH treatment of patients with normal GH secretion increases biochemical markers for both bone formation and bone resorption. Because of the short duration of GH treatment in man with normal GH secretion, the effect on bone mass is still inconclusive. Interestingly, GH treatment to GHD adults initially results in increased bone resorption with an increased number of bone-remodeling units and more newly produced unmineralized bone, resulting in an apparent low or unchanged bone mass. However, GH treatment for more than 18 months gives increased bone formation and bone mineralization of newly produced bone and a concomitant increase in bone mass as determined with DEXA. Thus, the action of GH on bone metabolism in GHD adults is 2-fold: it stimulates both bone resorption and bone formation. We therefore propose "the biphasic model" of GH action in bone remodeling (Fig. 10). According to this model, GH initially increases bone resorption with a concomitant bone loss that is followed by a phase of increased bone formation. After the moment when bone formation is stimulated more than bone resorption (transition point), bone mass is increased. However, a net gain of bone mass caused by GH may take some time as the initial decrease in bone mass must first be replaced (Fig. 10). When all clinical studies of GH treatment of GHD adults are taken into account, it appears that the "transition point" occurs after approximately 6 months and that a net increase of bone mass will be seen after 12-18 months of GH treatment. It should be emphasized that the biphasic model of GH action in bone remodeling is based on findings in GHD adults. It remains to be clarified whether or not it is valid for subjects with normal GH secretion. A treatment intended to increase the effects of GH/IGF-I axis on bone metabolism might include: 1) GH, 2) IGF, 3) other hormones/factors increasing the local IGF-I production in bone, and 4) GH-releasing factors. Other hormones/growth factors increasing local IGF may be important but are not discussed in this article. IGF-I has been shown to increase bone mass in animal models and biochemical markers in humans. However, no effect on bone mass has yet been presented in humans. Because the financial cost for GH treatment is high it has been suggested that GH-releasing factors might be used to stimulate the GH/IGF-I axis. The advantage of GH-releasing factors over GH is that some of them can be administered orally and that they may induce a more physiological GH secretion. (ABSTRACT TRUNCATED)

Estrogen regulation of human osteoblastic cell proliferation and differentiation

Estrogen (E2) has been shown to prevent bone loss among postmenopausal women. The molecular mechanism(s) by which this is accomplished is not clear. The discovery of E2 receptor (ER) in osteoblasts and osteoclasts has implicated these cells as direct targets for E2. Previous studies on the effects of E2 on osteoblastic cells in vitro or in organ culture present conflicting results, possibly due to heterogeneity in cell types, stage of differentiation, ER levels, and/or species differences. The effects of E2 on gene expression during various stages of human osteoblast cell differentiation has not been investigated extensively. In this study we employed a newly developed human fetal osteoblastic cell line (hFOB/ER9) that contains high levels of ER to examine the effects of E2 on osteoblast proliferation and differentiation. The basal levels and E2 effects on the expression of various extracellular matrix proteins were also characterized throughout different stages of differentiation. These stages include a proliferative/relatively undifferentiated stage (day 6), a matrix maturation stage (days 10-14), and a mineralization/calcified nodule stage (day 18). During the stage of rapid cell proliferation, E2 treatment of hFOB/ER9 cells resulted in a dose-dependent decrease in [3H]thymidine incorporation to a maximum of 72% compared to the vehicle control value. Treatment of hFOB/ER9 cells with 10(-9) M E2 for 48 h resulted in an increase in alkaline phosphatase (AP) activity throughout cell differentiation. The magnitude of AP induction varied from approximately 200-500%. In contrast, E2 decreased osteocalcin protein levels to a minimum of 54% compared to the vehicle control value. The steady state messenger RNA levels for AP increased and osteocalcin decreased after E2 treatment, similar to the responses observed at the protein level. At all stages, there was little or no effect of E2 on type I collagen protein levels or osteonectin steady state messenger RNA levels. The E2 responses on hFOB/ER9 cell matrix protein expression and cell proliferation were mediated through the ER, as cultures cotreated with a 100-fold molar excess of a type II anti-E2 (ICI 182,780) abrogated these effects. These results support the hypothesis that E2 does have an effect on osteoblastic differentiation by decreasing hFOB/ER9 cell proliferation and differentially regulating extracellular matrix expression.

Evidence for insulin-like growth factor (IGF)-independent transcriptional regulation of IGF binding protein-3 by growth hormone in SKHEP-1 human hepatocarcinoma cells

Insulin-like growth factor (IGF)-binding protein-3 (IGFBP-3) is a polypeptide that forms a ternary complex with IGFs and an acid-labile subunit. The hormonal regulation of the components of this complex is highly controversial, and both IGF-I and GH have been shown to mediate the expression/synthesis of IGFBP-3. This study investigates the regulation of IGFBP-3 protein, measured by RIA and Western ligand blot, and messenger RNA (mRNA) expression, measured by Northern analysis and reverse transcriptase-PCR, in SKHEP-1 human hepatocarcinoma cells. SKHEP-1 cells significantly increased the IGFBP-3 concentrations in conditioned medium (CM) when treated with GH (0.1-10 ng/ml), IGF-I (1-100 ng/ml), or Des(1-3)-IGF-I (1-100 ng/ml) in a dose-dependent manner (>3-fold). The increase in IGFBP-3 protein concentrations in CM was accompanied by a corresponding increase in IGFBP-3 mRNA levels. Interestingly, time-course studies showed that the GH-induced increase in IGFBP-3 mRNA preceded the IGF-I-induced increase (6 h for GH-induced IGFBP-3 mRNA; 12 h for IGF-I-induced IGFBP-3 mRNA). The half-life of IGFBP-3 mRNA was evaluated after transcriptional arrest by treatment with a RNA polymerase II inhibitor (5,6-dichloro-1beta-D-ribofuranosylbenzimidazole), and was found to be 14-18 h and unaltered by GH or IGF-I treatment. The induction of IGFBP-3 by GH was not due to the indirect action of locally synthesized IGF-I, because 1) no immunoreactive IGF-I was detected in the CM of control or GH-treated cells; 2) Northern blots revealed no IGF-I mRNA expression in SKHEP-1 cells; 3) reverse transcriptase-PCR did not detect any expression of the IGF-I gene; and 4) time-course studies showed an earlier increase in IGFBP-3 mRNA after GH treatment than after IGF-I treatment. Thus, the results obtained in this study are consistent with an IGF-I-independent regulation of IGFBP-3 gene expression by GH.

Insulin-like growth factor-binding proteins (IGFBPs) and their regulatory dynamics

The IGFBPs are a family of homologous proteins that have co-evolved with the IGFs and that confer upon the IGF regulatory system both functional and tissue specificity. IGFBPs are not merely carrier proteins for IGFs, but hold a central position in IGF ligand-receptor interactions through influences on both the bioavailability and distribution of IGFs in the extracellular environment. In addition, IGFBPs appear to have intrinsic biological activity independent of IGFs. The current status of research on IGFBPs is reviewed herein. Following a brief introduction to the entire IGF/IGFBP system, separate sections for each of the six cloned mammalian IGFBPs, the most extensive for IGFBP3, cover selected topics that emphasize the dynamics of IGFBPs--that is, their regulation in cells, their functionally important post-translational modifications, and their interactions in the cellular microenvironment--and how these dynamics influence physiological function.

Evidence that human bone cells in culture secrete insulin-like growth factor (IGF)-II and IGF binding protein-3 but not acid-labile subunit both under basal and regulated conditions

Insulin-like growth factors (IGFs) are found in human circulation predominantly as part of a growth hormone (GH)-dependent complex of 125-150 kD, which is composed of three subunits: IGF-I or IGF-II, an acid stable IGF binding protein (IGFBP)-3, and an acid labile subunit (ALS). Although recent studies demonstrate that a number of cell types in culture secrete IGFs and IGFBP-3, very little is known with regard to the origin of circulating ALS. To test the hypothesis that human bone cells (HBCs), which produce abundant amounts of IGF-II and IGFBP-3, also produce ALS, we measured the IGF-I, IGF-II, IGFBP-3, and ALS levels using specific radioimmunoassays (RIAs) in the conditioned medium (CM) of untransformed normal HBCs and SaOS-2 osteosarcoma cells treated with various effectors (IGF-II, osteogenic protein-1 [OP-1, bone morphogenetic protein-7] and human GH) for 48 h. No detectable levels (< 3 ng/ml) of ALS were found in the CM of various HBC types under basal conditions. In contrast, CM collected from liver explants in culture contained significant amount of ALS prepared and assayed under identical conditions. The IGF-I level was also undetectable in the CM of various HBC types. In the IGF-II (3, 30 ng/ml)-treated HBC CM, the IGFBP-3 level was increased in a dose-dependent manner but neither IGF-I nor ALS could be detected. In the SaOS-2 cell culture, OP-1 (1, 100 ng/ml) increased both IGF-II and IGFBP-3 secretion but neither ALS nor IGF-I secretion. Treatment of HBCs with GH (1, 10, 100 ng/ml) had no significant effect on the secretion of either IGF-I, IGF-II, IGFBP-3, or ALS.(ABSTRACT TRUNCATED AT 250 WORDS)

The impact of gender on the progression of chronic renal disease

Observations in experimental animals and in humans have shown that the rate of progression of renal disease is influenced by gender. Deterioration of renal function in patients with chronic renal disease is more rapid in men than in women, independent of differences in blood pressure or serum cholesterol levels. In addition to genetically determined differences between the sexes in renal structure and function, sex hormones may directly influence many of the processes implicated in the pathogenesis of renal disease progression. Potential mechanisms include receptor-mediated effects of sex hormones on glomerular hemodynamics and mesangial cell proliferation and matrix accumulation as well as effects on the synthesis and release of cytokines, vasoactive agents, and growth factors. In addition, estrogens may exert potent antioxidant actions in the mesangial microenvironment, which may contribute to the protective effect of female gender.

Comparison of the action of 17 beta-estradiol and progestins with insulin-like growth factors-I/-II and transforming growth factor-beta 1 on the growth of normal adult human bone-forming cells

Endogenous growth factors may be involved in the prevention of bone loss by estrogen and progestins in postmenopausal women. The present study was performed to compare the action of estrogen/progestins on bone-derived cells with the effects of exogenously added purified growth factors. Human osteoblast-like (HOB) cells were incubated with 17 beta-estradiol (E2), progesterone (P), dydrogesterone (DD), 20 alpha-dihydroxydydrogesterone (DHD), with and without the growth factors, insulin-like growth factors-I/-II (IGF-I/-II) or transforming growth factor-beta type 1 (TGF-beta 1) for 24 h under serum-free conditions. Cell growth and DNA synthesis were assessed by spectophotometrical analysis of total cell number and immunochemical detection of BrdU incorporation, respectively. Compared with the sex steroids, incubation of the cells with IGF-I or TGF-beta 1 resulted in at least a two-fold increase of total HOB cell numbers. No difference in stimulating HOB growth was observed between IGF-II and the female sex steroids E2 and P. Combining IGF-I/-II or TGF-beta 1 with either E2 or P did not result in a significantly further increase in the human osteoblast-like cell growth. In conclusion, the bone anabolic growth factors, IGF-I and TGF-beta 1, may be more important regulators of osteoblast proliferation than the female sex steroids. An interaction of estrogen/gestagens with the growth factors IGF-I/-II or TGF-beta 1 was not evident from the growth of human bone-forming cells in short-term cultures.

Thrombospondin co-localises with TGF beta and IGF-I in the extracellular matrix of human osteoblast-like cells and is modulated by 17 beta estradiol

Thrombospondin (TSP) is a multifunctional glycoprotein which is synthesised by several cell types including osteoblasts, and incorporated into the extracellular matrix (ECM) of these cells. The function and regulation of TSP in bone is not clear. In this study, using a long term culture model of human osteoblast-like cells, we examined the distribution of TSP in the ECM and its modulation by added estradiol. In this model the osteoblast-like cells form a regular multilayer which continues to increase in depth up to 50 days post confluence. In the ECM of these cultures and in 19-week fetal bone, the bone markers osteocalcin and alkaline phosphatase were diffusely distributed in the matrix. In contrast, labelling for TSP was concentrated, confined to the banded collagen and its immediately adjacent ECM. This pattern of labelling resembled that of the growth factors transforming growth factor beta-I (TGF beta), and insulin-like growth factor-I (IGF-I), with which TSP label co-localised. Labelling intensities were comparable between fetal bone and the in vitro material for TSP, TGF beta and IGF-I. TSP label was present by 10 days post confluence, reached a maximum by 20 days, and declined slowly thereafter, a time course which was similar to that of IGF-I. Incubation of osteoblast-like cell cultures with 17 beta estradiol resulted in an increase in multilayer depth and a maximal 3-fold increase in TSP labeling at 30 days as well as approximately 2-fold increases for TGF beta and IGF-I. The dose-response relationship for these responses to estradiol treatment was biphasic with maximal increases at 10(-10) M-10(-11) M of added estradiol. Treatment with 17 alpha estradiol produced labelling intensities that were not significantly different from controls. Studies with other cell types have suggested that TSP may be involved in modulation of growth factor activity. The similarities between TSP, TGF beta and IGF-I, in terms of their distribution and regulation by 17 beta estradiol treatment, may indicate a role for TSP in modulating bone cell proliferation and function through interaction with local growth factors.

The contribution of hypogonadism to the development of osteoporosis in thalassaemia major: new therapeutic approaches

OBJECTIVE

The osteoporosis seen in thalassaemia major is of multifactorial origin. The aim of the study was to evaluate the contribution of hypogonadism to the development of this osteoporosis and to assess the efficacy of new sex hormone replacement therapy regimens.

DESIGN AND PATIENTS

Sixty-seven patients were studied: 12 were hypogonadal, 32 had been on previous hormone replacement therapy (conjugated oestrogens plus medroxyprogesterone for females, depot testosterone esters for males); 10 had received continuous courses of treatment and 22 3-monthly on/off courses, and 22 were eugonadal without previous replacement therapy. Twenty-seven of the above patients were evaluated prospectively at 16 and 32 months during different therapeutic approaches (12 without treatment, 7 on continuous replacement and 8 on/off schemes followed by continuous therapy during the second observation period). The continuous schemes comprised either transdermal oestradiol (100 micrograms) plus medroxyprogesterone for females or hCG to produce serum testosterone concentrations within normal range, for males.

MEASUREMENTS

Bone mineral density (BMD) and bone mineral content (BMC) of lumbar spine and distal end of radius were measured by dual-energy X-ray absorptiometry.

RESULTS

Spinal BMD was found to be more than 30% lower than that of controls matched for sex and age with no difference between sexes. Radial BMD was less impaired and showed significantly (P < 0.01) higher levels in males (decrease of 5.8% +/- 2.3, mean +/- SD) than in females (-14.5 +/- 3.4%, mean +/- SD). In the retrospective evaluation it was found that the hypogonadal group had the lowest (P < 0.0001) BMD levels (0.62 +/- 0.01, mean +/- SE) and the highest were observed on the continuous replacement group (0.83 +/- 0.04), whereas the values of the other groups were similar. In a multiple regression analysis model it was found that only sex steroid levels were related to the BMD measurements (for oestradiol t = 2.6, P = 0.01 and for testosterone t = 6.5, P = 0.0001), whereas parameters related to haemolytic anaemia and desferrioxamine treatment were not. In the prospective study the continuous replacement group increased BMD and BMC values more than the on/off treatment courses (P = 0.01).

CONCLUSIONS

Hypogonadism seems to play an important role in the development of osteopenia-osteoporosis in thalassaemia major; continuous hormone replacement therapy with transdermal oestrogen for females or hCG for responding males best improves the bone density parameters.

Basic fibroblast growth factor regulates IGF-I binding proteins in the clonal osteoblastic cell line MC3T3-E1

In previous studies, we reported that basic fibroblast growth factor (bFGF) regulates insulin-like growth factor messenger RNAs and protein levels in the osteoblastic MC3T3-E1 cells. In the present study, we examined the expression of insulin-like growth factor binding proteins (IGFBPs) in MC3T3-E1 cells and determined whether bFGF altered IGFBP mRNAs and protein levels. Since previous studies suggested that IGFBPs can inhibit DNA synthesis stimulated by IGF-I, we wondered whether the mitogenic effect of bFGF was altered by exogenous IGFBP-3. Confluent MC3T3-E1 cells were serum-deprived for 24 h and then treated with bFGF for 6-24 h. In control cultures, MC3T3-E1 cells expressed the mRNAs for IGF-I, IGF-II, and IGFBP-2, 4, 5, and 6 but not IGFBP-1 or 3. A 24 h treatment with bFGF at 10(-8) M decreased IGF-I mRNA by 97%, IGF-II mRNA by 73%, IGFBP-2 by 64%, IGFBP-4 by 73%, IGFBP-5 by 95%, and IGFBP-6 by 65%. The inhibitory effect of bFGF on IGF-I and IGFBP mRNA levels was not altered by aphidicolin, an inhibitor of cell replication. bFGF 10 nM decreased IGF-I levels determined by radioimmunoassay after acidification by 45% and 72% at 24 and 48 h, respectively. Western ligand blot for IGF binding proteins revealed that MC3T3-E1 cells expressed IGFBPs of 24, 30, and 34 kD. Treatment with bFGF 10(-8) M decreased the levels of the 24 and 30 kD band at 24 h but increased the 34 kD band.(ABSTRACT TRUNCATED AT 250 WORDS)

Circulating levels of insulin-like growth factor binding protein 3 (IGFBP-3) and insulin-like growth factor I (IGF-I) in perimenopausal women

The study investigated possible menopause-related changes in circulating insulin-like growth factor binding protein 3 (IGFBP-3) levels and their relationship with insulin-like growth factor I (IGF-I) plasma levels. Forty-three healthy women, aged 45-55 years, were studied (22 premenopausal and 21 postmenopausal, matched for age and body mass index); in all subjects plasma IGF-I and IGFBP-3 levels were measured by radioimmunoassay. No difference was found between mean IGFBP-3 plasma levels in the two groups studied (premenopausal 3.42 +/- 0.49 v postmenopausal 3.46 +/- 0.58 mg/l), while mean IGF-I levels were significantly lower in postmenopausal as compared with premenopausal women (136.7 +/- 37.86 v 175.7 +/- 51.91 ng/ml, p < 0.02). Multiple regression analysis showed no significant effect of age, body mass index and years since menopause on IGFBP-3 levels; however, considering the IGF-I/IGFBP-3 ratio as a possible parameter of circulating free somatomedin C, an inverse correlation was found with years since menopause (n = 43, r = -0.499, p < 0.001). We conclude that lack of oestrogen induces different effects on circulating IGF-I and IGFBP-3, possibly reflecting a real decrease in IGF-I activity.

Pregnancy: a clue to normal regulation of B lymphopoiesis

The large-scale production of lymphocytes in the bone marrow reflects a delicate balance between positive and negative regulatory signals. For instance, interleukin 7 (IL-7) provides a positive signal, and appears to be both essential and limiting in the mouse. However, much less is understood concerning the negative molecular signals that may limit the output of lymphocytes. Here, Paul Kincade and colleagues discuss how a chance observation with pregnant mice revealed that sex steroids can act as negative regulators of B lymphopoiesis, and may do so under normal steady-state conditions.

Growth hormone and parathyroid hormone stimulate IGFBP-3 in rat osteoblasts

Osteoblast-like cells prepared from calvaria of newborn rats produce insulin-like growth factor (IGF) I and several insulin-like growth factor binding proteins (IGFBPs) in vitro. Among the IGFBPs found in conditioned cell culture medium, IGFBP-3 is the most abundant. Intact IGFBP-3, as assessed by 125I-labeled IGF-II ligand blot analysis, is more abundant in culture media of cells exposed to growth hormone (GH) or to parathyroid hormone (PTH), both at 5 x 10(-9) mol/l, for 24 h. At the same time, concentrations of IGF-I are increased in media of cells exposed to PTH but not to GH, compared with hormone-free control cultures. IGFBP-3 mRNA is increased in osteoblasts exposed to PTH or to GH but not in response to 5 x 10(-9) mol/l IGF-I. PTH exerts a rapid (within 2 h) stimulatory effect on IGF-I and IGFBP-3 production, both at the message and peptide levels, whereas GH increases only IGFBP-3, both at the message and peptide levels (after 24 h). We conclude that IGF-I does not mediate increased IGFBP-3 production by rat osteoblasts in response to GH and PTH.

Studies on the regulation of insulin-like growth factor binding protein 3 secretion in human osteosarcoma cells in vitro

Previous studies demonstrated that insulin-like growth factors (IGFs) are important autocrine and paracrine mitogens for human bone cells in vitro and that IGF binding proteins (IGFBPs) are important regulators of the biologic actions of IGFs. Thus, the actions of IGFs may be determined not only by their concentrations but also by the type and amount of IGFBPs produced by human bone cells at a local site in bone. In this study, we sought to determine the effects of dexamethasone, 1,25-(OH)2D3, and parathyroid hormone (PTH) on the secretion of IGFBP-3 in human osteosarcoma cell lines. Serum-free cultures of low- and high-alkaline phosphatase (ALP) SaOS-2, MG-63, and TE89 human osteosarcoma cells were treated for 24 or 48 h with the effectors and the conditioned media used for determination of IGFBP-3 using a radioimmunoassay. We report that (1) the basal rate of IGFBP-3 secretion (ng/mg cellular protein) was dependent upon cell type, with TE89 > low-ALP Saos-2 > MG-63 > high-ALP SaOS-2 cells, and did not correlate with either basal cell proliferation or basal cellular ALP activity; (2) dexamethasone (10(-12)-10(-7) M) inhibited IGFBP-3 secretion in a dose-dependent manner in low-ALP SaOS-2, MG-63, and TE89 cells but not in high-ALP SaOS-2 cells; (3) 1,25-(OH)2D3 (10(-11)-10(-8) M) stimulated IGFBP-3 secretion in a dose-dependent manner in MG-63, low-ALP SaOS-2, and high-ALP SaOS-2 cells, and the coaddition of TGF-beta and 1,25-(OH)2D3 increased synergistically IGFBP-3 secretion and cellular ALP activity in MG-63 cells; and (4) human PTH-(1-34) (0.1-100 ng/ml) had no significant effect on IGFBP-3 secretion in MG-63, low-ALP SaOS-2, or high-ALP SaOS-2 cells. We conclude that such agents as dexamethasone, 1,25-(OH)2D3, and PTH differentially regulate IGFBP-3 secretion in human osteosarcoma cells in vitro.

A comparison of the action of progestins and estrogen on the growth and differentiation of normal adult human osteoblast-like cells in vitro

Estrogen/gestagen replacement therapy prevents excess bone loss in postmenopausal women. The mode of action by which these sex steroids exert their anabolic effects on bone has not been completely clarified yet. In this study, 17 beta-estradiol (E2), as well as progestins progesterone (P), dydrogesterone (DD), 20 alpha-dihydroxydydrogesterone (DHD), medroxyprogesterone acetate (MPA), and cyproterone acetate (CPA) were able to stimulate the mitogenesis and differentiation of normal adult human osteoblast-like (HOB) cells harvested from female trabecular bone explants. The different progestins exerted a more pronounced stimulatory effect on HOB proliferation than E2 did. The combination of E2 with P, DD, or DHD did not result in a statistically significant further increase of HOB proliferation, as compared with the progestins alone. In general, E2 showed a stronger differentiation-inducing effect than the progestins, as measured by histochemical staining of the HOB cells for alkaline phosphatase activity. Combining E2 and the progestins did not result in a further increase of the number of alkaline phosphatase positive cells, compared with E2 alone. The different progestins proved to be equally potent in stimulating HOB proliferation and differentiation. In conclusion, progestins as well as E2 exerted anabolic but differential effects on normal adult human osteoblasts in vitro.

Characterization, regulation, and function of specific cell membrane receptors for insulin-like growth factor I on bone endothelial cells

It is now widely accepted that insulin-like growth factor-I (IGF-I) has a local regulatory role in bone remodeling. IGF-I has also been demonstrated to regulate proliferation of bone-derived endothelial cells. Such studies suggest a role of IGF-I in skeletal angiogenesis. Using BBE cells, a bovine bone endothelial cell line, we characterized the kinetics and chemical properties of IGF-I receptors and examined the effect of IGF-I on bone endothelium migration. Two classes of binding sites with high affinity for IGF-I were detected by binding experiments on bone endothelial cells. Both competition analyses and cross-linking studies revealed the presence of type I IGF receptor in bone endothelial cells. Moreover, these cells produced and released authentic IGF-I into the medium, as evidenced by radioimmunoassay analyses of gel-filtered conditioned media. Both IGF-I binding capacity and release decreased either with increases in cell number or after treatment with 17 beta-estradiol (17 beta E2) and parathyroid hormone (PTH). Both hormones also inhibited chemotactic responses of bone endothelial cells to IGF-I. Taken together, these results strongly suggest that IGF-I, a growth factor that promotes the proliferation of various bone cell types, also induces growth and chemotactic responses in bone endothelium acting through the type I IGF receptor. This may be part of a generalized response of bone cells to IGF-I that facilitates cell migration.

Absence of a direct anabolic effect of 17 beta-estradiol on normal human bone marrow stromal cells

The effects of 17 beta-estradiol on the proliferation and differentiation of cultured normal human bone marrow stromal cells were investigated. Treatment of 17 beta-estradiol at the concentration of 10(-6) approximately 10(-10) M for either 48 hours or 7 days did not affect [3H] thymidine incorporation. 17 beta-estradiol (10(-8)M) treatment for 4 or 7 days also failed to stimulate alkaline phosphatase activity. Similarly, incubation with 17 beta-estradiol (10(-8) M) for 48 hours did not increase the incorporation of [3H] proline into collagenase digestible protein and noncollagen proteins and secretion of IGF-I and IGF binding proteins in human bone marrow stromal cells. Present data indicate that 17 beta-estradiol does not have a direct effect on cultured normal human bone marrow stromal cells. With previous findings that estradiol elicits few effects on normal human osteoblasts, our results strongly suggest that estrogen does not have a direct anabolic effect on normal human osteoblast lineage. Therefore, the in vivo estrogen effects may be entirely through an antiresorptive mechanism or, if any anabolic role of estrogen is present, it must be indirect and mediated by other hormones or local factors.

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.

Regulation of insulin-like growth factor binding protein 4 by a specific insulin-like growth factor binding protein 4 proteinase in normal human osteoblast-like cells: implications in bone cell physiology

Insulin-like growth factor binding protein 4 (IGFBP-4) is secreted by normal human osteoblast-like cells (hOB) and is a potent inhibitor of insulin-like growth factor (IGF) action in vitro. In previous studies, IGF treatment of hOB in culture led to markedly reduced medium levels of IGFBP-4 as detected by western ligand blotting. In the present study, incubation of hOB-conditioned medium (hOB-CM) with IGF under cell-free conditions resulted in a similar loss of IGFBP-4. Both IGF-I and IGF-II were capable of inducing a decrease in IGFBP-4; however, IGF-II was more effective. When the six characterized IGFBP were added to hOB-CM, only IGFBP-4 disappeared in response to IGF-II addition. This IGF-regulated loss of IGFBP-4 was inhibited by metalloproteinase inhibitors and appeared to be due to a proteinase that cleaved IGFBP-4 in 18 and 14 kD fragments identified by western immunoblotting. Conditioned media from eight of eight different donor hOB lines tested exhibited IGFBP-4 proteinase activity. To assess the biologic consequences of IGF-II-induced IGFBP-4 proteolysis, we treated hOB with IGF-II for 5 h, which decreased medium IGF-BP-4 by 70%, and then measured IGF-I and insulin stimulation of [3H]thymidine incorporation. IGF-II itself was not mitogenic and had no effect on insulin-stimulated [3H]thymidine incorporation. However, pretreatment of cultured hOB with IGF-II enhanced IGF-I-stimulated [3H]thymidine incorporation threefold.(ABSTRACT TRUNCATED AT 250 WORDS)

Decreased serum levels of insulin-like growth factors and IGF binding protein 3 in osteoporosis

OBJECTIVES

The aim of the study was to investigate endogenous growth hormone (GH) secretion in patients with osteoporosis and in patients with degenerative bone diseases or no spinal disease by measuring serum insulin-like growth factors 1 and 2 (IGFs) and their major binding protein 3 (BP-3) as an indirect parameter of GH secretion.

DESIGN

A cross-sectional study.

SETTING

All patients were seen as out-patients of the Endocrinology Department of the University of Heidelberg where all bone parameters were measured. IGFs and BP-3 serum levels were measured at the Children's Hospital of the University of Tübingen.

SUBJECTS

A total number of 310 patients were studied. The group with primary osteoporosis and vertebral fractures (OPO) consisted of 141 patients (98 females, 43 males). Spinal degenerative bone disease or osteoarthritis (DEG) was present in 108 patients (91 females, 17 males). Sixty-one control patients (56 females, 5 males) had no spinal disease on X-ray, but presented with lower back pain.

MAIN OUTCOME MEASURES

Serum levels of IGFs, BP-3, PTH and 25-vitamin D3 were measured by radioimmunoassay. Bone mineral density (BMD) was determined using absorptiometry; anthropometric parameters and menopausal status were recorded.

RESULTS

There was no difference in age and years after menopause between OPO and DEG, but control individuals were younger. Mean IGFs and BP-3 serum levels in patients with OPO were lower (P < 0.001) than those in patients with DEG or in controls. Patients with DEG had significantly higher BP-3 levels than controls (P < 0.001). There was a significant (P < 0.05) negative correlation of BP-3 with age in females with OPO, but not in controls or in DEG patients. The IGFs did not decrease with age in any of the three groups. Binding protein 3 was positively correlated (P < 0.05) with BMD in postmenopausal women with OPO but not in controls or DEG patients.

CONCLUSION

We conclude that systemic IGFs and IGF binding protein 3 are decreased in patients with osteoporosis. Further studies are needed to investigate whether this is as a result of diminished secretion of endogenous GH and whether this reflects the local circumstances of IGFs and IGF binding proteins in bone.

Effects of estrogen replacement on insulin-like growth factor I concentrations in serum and bone tissue and on interleukin 1 secretion from spleen macrophages in oophorectomized rats

Oophorectomy (OOX) has been known to increase bone turnover, but its precise mechanism is not fully understood. In order to further investigate the mechanism, we determined insulinlike growth factor I (IGF-I) concentrations in serum and bone tissue and interleukin 1 (IL-1) release from spleen macrophages in oophorectomized rats because it has been demonstrated that IGF-I stimulates bone formation and IL-1 stimulates bone resorption. Female 8-week-old Wistar rats were divided into four groups: (1) control, (2) OOX, (3) OOX given estradiol, and (4) control given estradiol. Ten micrograms/kg of 17 beta-estradiol was given daily by subcutaneous injection. After 5 weeks of treatment, IGF-I concentrations in the extract from right femur and in serum were determined by specific radioimmunoassay. IL-1 activity released from lipopolysaccharide (LPS)-stimulated spleen macrophages was determined by bioassay. IGF-I contents in the femur and IGF-I concentrations in serum in oophorectomized rats were significantly higher than those in control rats. Treatment by estradiol inhibited the increase in IGF-I concentrations both in femur and in serum. IL-1 release from LPS-stimulated spleen macrophages in oophorectomized rats was increased, and treatment by estradiol also inhibited the stimulated IL-1 release. The ash weights and the calcium contents of left femur in oophorectomized rats were lower than those in control rats. These results suggest that both IGF-I and IL-1 may be involved in the mechanism of the regulation of bone turnover in oophorectomized rats.

Insulinlike growth factor gene expression in human fracture callus

The effects of insulinlike growth factors on bone and cartilage-derived cells in culture have been extensively investigated, but there is little information on their role in vivo in bone, especially in fracture healing. This study investigated insulinlike growth factor (IGF) I and II mRNA expression in normally healing human fractures by in situ hybridization. Endothelial and mesenchymal cells at the granulation tissue stage expressed IGF-II mRNA. At the stage of bone and cartilage formation, osteoblasts and non-hypertrophic chondrocytes expressed mRNA for both IGF-I and II. Some osteoclasts were positive for IGF-II mRNA at the stage of bone remodeling. The greater time span of IGF-II expression relative to IGF-I reflects the predominance of IGF-II in human bone matrix. Taken together with the known effects of IGFs on bone and cartilage cells in vitro, these findings support a role for IGFs in local cellular regulation in human fracture healing.

Effect of estrogen deficiency on IGF-I plasma levels: relationship with bone mineral density in perimenopausal women

Bone tissue is a source of growth factors; among them, insulinlike growth factor I (IGF-I) is probably an important local regulator of bone formation. This study has been carried out in order to assess the effects of natural menopause on plasma concentrations of IGF-I in the first 6 years after the cessation of gonadal function independent of age. We also examined the relationship between plasma IGF-I levels and bone mineral density (BMD) measured at the lumbar spine (LS), at the ultradistal radius (UDR), and at the junction of the distal and middle thirds of the radius (MR). Sixty-seven healthy nonobese women, aged 45-55, were studied (premenopausal n = 21; postmenopausal n = 46, from 1 to 6 years since menopause). Plasma IGF-I levels were measured by RIA, after acid-ethanol extraction. BMD of the forearm was measured by dual-photon densitometer and BMD of the LS was assessed by quantitative digital radiography. Mean values of IGF-I plasma levels were significantly reduced in postmenopausal women compared to the premenopausal group. Menopausal duration did not influence IGF-I plasma levels in postmenopausal women. We also found a positive correlation between IGF-I levels and BMD measured at MR both in pre- and postmenopausal women, while a correlation with LS and UDR-BMD was found only in fertile subjects. The results show that IGF-I plasma levels decrease immediately after menopause, since significantly lower levels are reached in the first years. The correlations found between plasma IGF-I levels and BMD suggest a possible role of reduced IGF-I in bone loss at particular skeletal sites.

Immunoelectron microscopic detection of estrogen target cells in the bone marrow of estrogen-treated male Japanese quail

The localization of estrogen receptors (ERs) in bone marrow cells was studied immunoelectron microscopically in the femurs of estrogen-treated male Japanese quail. The electron-dense reaction product showing ER localization was not detected in the bone marrow cells of the male quail. After 48 h of estrogen treatment, nuclear reaction product was observed in many mononuclear cells in the bone marrow. Most were located in the marrow stroma, and they were divided into undifferentiated and differentiated type cells, characterized by poorly and well-developed cytoplasm, respectively. Some ER-positive cells were situated in the hematopoietic tissue and had moderately developed cytoplasm. Thus, estrogen target cells may be present in the stroma and hematopoietic tissue of bone marrow.

Activation of protein kinase C inhibits 45Ca-accumulation in cultures of osteoblast-like cells: possible involvement of insulin-like growth factor-I

In a previous report, we have demonstrated that exogenous insulin-like growth factor-I (IGF-I) stimulates 45Ca-accumulation into extracellular matrix in long-term cultures of osteoblast-like MC3T3-E1 cells and that 45Ca-accumulation occurs even in the cultures without exogenous IGF-I. In this study, effects of protein kinase C (PKC) on IGF-I secretion and 45Ca-accumulation into extracellular matrix were examined in 6-week cultured MC3T3-E1 cells. The MC3T3-E1 cells secreted IGF-I spontaneously. The PKC activator, 12-O-tetradecanoylphorbol-13-acetate (TPA) suppressed IGF-I secretion in a dose-dependent manner. 4 alpha-Phorbol 12,13-didecanoate (4 alpha-PDD), which is inactive for PKC, had little effect on the secretion. 1-Oleoyl-2-acetylglycerol, a specific activator for PKC, also suppressed the IGF-I secretion dose dependently. H-7, a PKC inhibitor, recovered the inhibitory effect of TPA. On the other hand, TPA inhibited the 45Ca-accumulation into extracellular matrix in cultures of these cells dose dependently, whereas 4 alpha-PDD was ineffective in this capacity. The TPA-induced inhibition of 45Ca-accumulation was recovered almost to the control level by H-7. Exogenous IGF-I recovered the inhibitory effect of TPA on 45Ca-accumulation. In spite of the inhibitory effects of TPA as above, TPA had little effect on DNA synthesis in these cells. These results suggest that the activation of PKC inhibits calcification via suppression of IGF-I secretion in osteoblast-like cells.

Bone and cartilage responsiveness to sex steroid hormones

Gonadal steroids influence the skeletal growth and metabolism both during the pubertal growth spurt and in adulthood with aging. It is now generally agreed that sex steroid effect on skeletal tissues is due to indirect and direct actions. In this presentation, in vitro effects of sex steroids on cartilage cells are reported by comparison with those observed on bone cells.

Prostaglandin E2 stimulates synthesis of insulin-like growth factor binding protein-3 in rat bone cells in vitro

Prostaglandin E2 is produced by bone cells and increases cyclic AMP in these cells. Like PTH and dibutyryl cyclic AMP, PGE2 is a potent stimulator of IGF-I synthesis in cultured rat osteoblasts and inhibits DNA synthesis and type I procollagen gene expression. In addition, PGE2 inhibits the response of the cells toward IGF-I after 1 day but not after 4 days of incubation. Rat calvaria osteoblasts constitutively release IGFBPs into the culture medium, in particular IGFBP-2 and IGFBP-3. Like growth hormone, PGE2 stimulates the accumulation of IGFBP-3. PGE2 rapidly increases IGF-I and IGFBP-3 mRNA expression in calvaria cells, with a time course clearly different from that observed in response to growth hormone. Thus, PGE2 modifies not only the synthesis of IGF-I but also that of IGFBP-3 in skeletal tissue.

Stimulatory effects of estrogen and progesterone on proliferation and differentiation of normal human osteoblast-like cells in vitro

Here we report that osteoblast-like cells derived from female and male adult human trabecular bone are able to directly respond to 17 beta-estradiol (E2) and progesterone (P). In short-term (1 day) cultures using serum-free and phenol red-free medium, both steroid hormones were found to stimulate DNA synthesis and growth of the human osteoblast-like cells. P was more potent in stimulating osteoblast growth compared to E2. On the other hand, E2 showed a stronger differentiation-inducing effect as determined by analysis of the number of cells displaying cytochemical alkaline phosphatase (AP) activity, a marker for the mature osteoblast phenotype. Combination of E2 and P resulted in a further increase in DNA synthesis, but did not further affect the number of cells expressing AP activity. In conclusion, female sex steroids may be involved in regulating bone mass in human adults via a direct anabolic action on the bone forming cells.

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.

Testosterone increases serum 1,25-dihydroxyvitamin D and insulin-like growth factor-I in hypogonadal men

The short-term metabolic effects of testosterone treatment on circulating levels of 1,25-dihydroxyvitamin D and insulin-like growth factor-I (IGF-I) were studied in 13 hypogonadal men. The study group included 11 men with Klinefelter's syndrome, with varying degree of androgen deficiency, and two men with secondary hypogonadism. Pretreatment levels of 1,25-dihydroxyvitamin D, vitamin D-binding protein and IGF-binding protein-I were all within the normal range. The levels of IGF-I were lower than normal in 5/11 of the Klinefelter patients and in one patient with GH-deficiency. Testosterone treatment increased circulating total 1,25-dihydroxyvitamin D significantly from 75 +/- 4 pmol l-1 (mean +/- SEM) to 86 +/- 4 (P less than 0.01) and the free 1,25-dihydroxyvitamin D-index from 1.95 +/- 0.11 to 2.39 +/- 0.12 (P less than 0.01). Serum levels of IGF-I increased from 117 +/- 22 micrograms/l to 143 +/- 23 (P less than 0.01) during androgen treatment. No significant effects on levels of IGF-binding protein-I were seen. It is concluded that androgen therapy increases the availability of 1,25-dihydroxyvitamin D and the level of IGF-I, which may be important links in the action of testosterone.

Insulin-like growth factor binding protein (IGFBP) inhibits IGF action on human osteosarcoma cells

The influence of a human insulin-like growth factor binding protein, hIGFBP-1, on the action of IGFs on human osteosarcoma cells was examined. hIGFBP-1 was found to block binding of IGFs to their receptors on MG-63 cells and subsequent IGF stimulation of DNA synthesis. Concurrent incubation of hIGFBP-1 with either 125I-IGF-I or 125I-IGF-II prevented the binding of both 125I-IGFs to cells in a dose-dependent manner. hIGFBP-1 inhibition of IGF binding occurred similarly under both 4 degrees and 37 degrees C conditions. Additionally, hIGFBP-1 facilitated the dissociation of IGFs bound to cells. The inhibitory effect of hIGFBP-1 on IGF-1 mediated 3H-thymidine incorporation into DNA was dose dependent. hIGFBP-1 did not inhibit binding to or stimulation of growth in MG-63 cells by des3-IGF-1, an IGF-I analog with a 100-fold less affinity for hIGFBP-I. This confirmed that hIGFBP-1 competed for IGF receptor binding sites on MG-63. Since hIGFBP-1 did not bind to cells, inhibition of IGF action was indirect, presumably through the formation of extracellular soluble bioinactive IGF-BP complexes.

Effect of protein deprivation on insulin-like growth factor-binding proteins in rats

The effect of protein deprivation on plasma concentration of insulin-like growth factor-binding proteins (IGFBP) was studied in rats. A significant decrease in the concentration of IGFBP of molecular weight (mass) approximately 40 kDa was observed in protein-deprived rats. There was no prominent effect of protein deprivation on the concentration of IGFBP with molecular weights of about 30 kDa or 22-24 kDa. The binding capacity to plasma IGFBP of exogenously-added 125I-labelled insulin-like growth factor-1 (125I-IGF-1) was also studied. IGFBP of molecular weight about 30 and 22-24 kDa (the native form of this protein is presumed to be 29 kDa) in protein-deprived rat plasma bound more 125I-IGF-1 than those in protein-fed rat plasma. This suggested that these IGFBP in protein-deprived rat plasma are relatively unsaturated by endogenous IGF-1. The response of IGFBP to protein deprivation which was elucidated in the present investigations add further evidence to our previous assumption that IGFBP play an important role in protein nutrition.

Immunoelectron microscopic demonstration of estrogen receptors in osteogenic cells of Japanese quail

The localization of estrogen receptors (ERs) in osteogenic cells was immunoelectron microscopically examined in the femurs of female and estrogen-treated male Japanese quail. An electron dense reaction product showing ER localization was observed in the nuclei of osteoblasts and immature osteocytes in the medullary bone of the female quail. However, reaction product was not seen in the osteoclasts. On the endosteal bone surface of male quail, nuclear reaction product was detected in bone lining cells. After 24 h of estrogen treatment, reaction product was observed in the nuclei of preosteoblasts on the endosteal bone surface. After 48 h, the medullary bone partly appeared along the endosteal surface. Nuclear reaction product was seen in osteoblasts on the medullary bone surface.

Lack of a direct effect of estrogen on proliferation and differentiation of normal human osteoblast-like cells

Although osteoblasts contain estrogen receptors, it is unclear whether estrogen has direct effects on osteoblast proliferation and differentiation. We evaluated the effects of 17 beta-estradiol treatment (1 pM to 10 nM) on the proliferation and differentiation of cultured normal adult human cells that expressed many of the phenotypic characteristics and hormonal sensitivities of mature osteoblasts (hOB cells). Treatment of hOB cells with estradiol for as long as 144 h did not affect the rate of DNA synthesis and had minimal, if any, effects on differentiated function. Whereas alkaline phosphatase activity was increased by nearly twofold (P less than 0.01) when the hOB cells were treated with 1 nM 1,25-dihydroxyvitamin D3 [1,25-(OH)2D3], treatment with estradiol had no effect when given alone and did not affect the cells' response to 1,25-(OH)2D3. Similarly, the release of bone gla protein (BGP, osteocalcin) was induced by treatment with 1,25-(OH)2D3 (P less than 0.05), but estradiol treatment did not affect this response. Cellular levels of mRNA for alkaline phosphatase and BGP were not altered by estradiol treatment. We conclude that estradiol treatment does not have major effects on the growth or differentiation of cultured hOB cells. These results are consistent with previous observations in vivo that indicate estrogen acts principally to decrease bone resorption, not to modulate its formation.

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)

Effects and interactions of 17 beta-estradiol, T3 and 1,25(OH)2D3 on cultured osteoblasts from mature rats

Osteoporosis being frequently associated with hyperthyroidism and, mostly after menopause, with deficiency in estrogens, we tried to elucidate the interactions of estrogens and triiodothyronine (T3) with calcitriol by using cultured osteoblast-like cells obtained from mature rat bone. The tested parameters included [3H]thymidine incorporation, evaluation of the alkaline phosphatase activity of the cell layer and osteocalcin production in the culture medium. At physiological concentrations, 17 beta-estradiol and T3 stimulated alkaline phosphatase activity, did not enhance osteocalcin production and slightly inhibited [3H]thymidine incorporation. At higher concentrations, 17 beta-estradiol decreased the alkaline phosphatase and osteocalcin response to calcitriol whereas T3, although decreasing alkaline phosphatase activity, markedly increased the osteocalcin secretion elicited by calcitriol. These observations emphasize the complex physiology of osteoblasts and confirm different behaviors of alkaline phosphatase and of osteocalcin as markers of bone turnover.