Cellular and Molecular Effects of Growth Hormone and Estrogen on Human Bone Cells

Bone metabolism during the perimenopausal transition: a prospective study

OBJECTIVE

Changes in biochemical markers of bone formation and resorption were followed over the course of 1 year in premenopausal, perimenopausal and early postmenopausal women.

METHODS

Sixty-four subjects were analyzed, grouped according to their menstrual pattern, menopausal complaints and endocrinological parameters to be premenopausal (n=20), perimenopausal (n=24) or early postmenopausal (n=20). The parameters studied at four visits during the 12-month study period were the urinary pyridinium cross-links pyridinoline (PYD) and deoxypyridinoline (DPD), and N-terminal telopeptide (NTX) as bone resorption markers, as well as osteocalcin (OC) and bone-specific alkaline phosphatase (BAP) in serum, representing bone formation. The longitudinal changes over time as well as intergroup differences were analyzed using generalized estimating equations (GEE) in connection with Wald statistics.

RESULTS

Over the course of 1 year BAP levels decreased in the late premenopausal group (P<0.05). The perimenopausal group exhibited significant changes of PYD, DPD and OC (P<0.01), NTX levels were higher than in premenopause. Postmenopausal subjects had elevated NTX values, while PYD and DPD levels remained close to the perimenopausal range. Only for OC a time effect was seen during postmenopause.

CONCLUSIONS

Changes in bone turnover already begin in late premenopause, when decreased bone formation may precede increased bone resorption. The rise of NTX from late premenopause through early postmenopause indicates diagnostic sensitivity of this parameter to changes in bone metabolism induced by estrogen withdrawal. PYD and DPD do not follow this pattern, but change significantly with time during perimenopause to then remain largely unchanged in early postmenopause.

The IGFBP-3 mRNA and protein levels are IGF-I-dependent and GH-independent in MG-63 human osteosarcoma cells

Growth Hormone (GH), Insulin-like Growth Factors (IGFs) and IGF-Binding Proteins which modulate the IGFs' bioavailability (e.g. IGFBP-3, -4, -5), are essential regulators of bone remodeling. In this study, MG-63 human osteosarcoma cells were used as a model system to investigate the mechanism(s) whereby IGF-I and GH control IGFBP-3 gene expression. Physiological concentrations of IGF-I (1-20 nM) induced a dose-dependent increase in the steady-state amount of IGFBP-3 mRNA (maximal stimulation: approximately 9-10-fold). This increase was detectable 3 h after the onset of IGF-I treatment, was enhanced over a 24 h period, then plateaued until at least 30 h. Consistently, a dose-dependent increase in IGFBP-3 secretion ( approximately 40-50-fold at IGF-I concentrations>/=16 nM) was observed by western ligand- and immuno-blot analysis of MG-63 cells conditioned medium, and its time course was similar to that observed for IGFBP-3 transcripts. IGFBP-3 mRNA stability (t(1/2) approximately 20 h) was identical in the presence or absence of IGF-I treatment. By contrast, human (h) GH treatment (24-72 h) of MG-63 cells did not increase IGFBP-3 secretion in the conditioned medium. Ectopic expression of recombinant rat GH-R resulted in hGH-enhanced expression of GH-responsive reporter gene constructs, but did not increase endogenous IGFBP-3 gene expression, suggesting that the GH unresponsiveness was not only due to the very low level of GH binding sites at the plasma membrane level. Altogether, these results support the conclusions that in MG-63 cells (i) transcriptional rather post-transcriptional mechanisms are involved in the IGF-I-induced increase of IGFBP-3; (ii) the abundance of GH-R is very low at the plasma membrane level; (iii) the dowstream GH-signaling cascade is fully functional in this human osteosarcoma cell line; and (iv) the endogenous IGFBP-3 gene is not responsive to hGH in human MG-63 osteosarcoma cells.

Isolation and characterization of osteoblast cultures from normal and osteopenic sheep for biomaterials evaluation

Being very useful in the analysis of bone cell differentiation and activity, osteoblast cultures are also used in the in vitro biocompatibility study of new materials. The aim of this work was to evaluate sheep osteoblast cultures derived from normal and ovariectomized animals, and then to assess the in vitro biomaterial behavior on these cultures, taking into account the quality of bone where orthopedic devices are clinically used. For this purpose, we characterized sheep osteoblast cultures, isolated from iliac crest bone of normal (NB osteoblast culture) and osteopenic after ovariectomy (OB osteoblast culture) sheep. Moreover, we studied cell behavior when cultured on different biomaterials (titanium and two biological glasses, RKKP and AP40). Cell characterization at baseline demonstrated that both cultures (NB and OB) showed normal osteoblastic behavior. On the contrary, osteoblasts derived from osteopenic bone and cultivated on AP40 for 6 days revealed a different behavior in terms of both cell morphology and metabolic activity. Statistical analysis (one-way analysis of variance and Scheffé's post hoc multiple-comparison tests) revealed significant differences in Ca level (p<0.0005), MTT test (p<0.0005) and OC production (p<0.05). These in vitro tests demonstrated that sheep osteoblast cultures can be useful when determining biocompatibility and osteointegration of orthopedic materials, and also when evaluating for the presence of osteoporosis.