Individual and combined effects of calciotropic hormones and growth factors on mineral metabolism in embryonic chick tibiae

Abstract

We have investigated single and combined effects of calciotropic hormones and growth factors on the regulation of alkaline phosphatase (ALP) activity and calcium metabolism in an optimized serum-free bone organ culture system of embryonic chick tibiae. Parathyroid hormone PTH(1-34) alone mobilized calcium from bone tissue time- and dose-dependently and inhibited ALP activity. Both the bisphosphonate (BM 21.0955) and to a lesser extent salmon calcitonin alone slightly increased calcium uptake and inhibited the stimulation of bone resorption by PTH(1-34). 1,25(OH)2D3 mobilized calcium and inhibited ALP activity in contrast to 24,25(OH)2D3 which inhibited ALP activity but had no significant effect on calcium metabolism. Interestingly the combination of PTH(1-34) with 1,25(OH)2D3 but not 24,25(OH)2D3 reduced calcium mobilization. The combination of the midregional fragment PTH(28-48), which by itself has no effect on calcium metabolism, with 1,25(OH)2D3 reduced calcium mobilization more efficiently. Several PTH-regulated mediators have been assayed in this system. Of the tested growth factors, IGF-I at high concentrations caused bone resorption with no effect on ALP activity. TGF-beta 1 (transforming growth factor beta) and BMP-2 had no significant effect on calcium metabolism; however, ALP activity was inhibited by TGF-beta 1 and induced dose dependently by BMP-2. Of the other factors known to be present in bone, platelet-derived growth factor (PDGFA/B) and epidermal growth factor (EGF) had a small effect on calcium mobilization but had no effect on ALP activity. bFGF reduced ALP activity slightly without an effect on calcium metabolism. Our results show that this in vitro system can mimic some interactions of calciotropic hormones in vivo and allows the assaying of mediators in terms of regulation of ALP activity and of calcium metabolism.

hPTH‐fragments (53–84) and (28–48) antagonize the stimulation of calcium release and repression of alkaline phosphatase activity by hPTH‐(1–34) in vitro

“…In this study we have used organ cultures of embryonic chick tibiae [13] to determine the effects of PTH fragments hPTH-(53-84) and (28-48) on calcium level in the medium and ALP activity in the tissue. When 10 nM hPTH-(53-84), hPTH-(28-48) and hPTH-(3-34) were added to the cultures, no significant effect on calcium was observed whereas both hPTH-(1-34) and hPTH-(1-84) at a concentration of 10 nM induced a significant release of calcium into the medium.…”

“…In isolation, neither hPTH-(53-84) nor hPTH-(28-48) had any affect calcium release however, both peptides significantly induced ALP activity [13,14]. Calcium levels in the culture system were optimised such that calcium release induced by PTH-(1-34) were maximised.…”

“…In this investigation we have demonstrated that the PTH fragments, hPTH-(53-84) and hPTH-(28-48), antagonised the effects of hPTH-(1-34) on calcium release and ALP activity, which are known to be cAMP mediated. In isolation, neither hPTH-(53-84) nor hPTH-(28-48) had any affect calcium release however, both peptides significantly induced ALP activity [13,14]. Calcium levels in the culture system were optimised such that calcium release induced by PTH-(1-34) were maximised.…”

“…The C-terminal fragment PTH-(53-84) induces also ALP activity in bone cells and regulates calcium influx in chondrocytes [10]. Simultaneous exposure of cells and tissues to these PTH-fragments has been shown to modulate the changes in proliferation and differentiation induced by PTH-(1-34) [11][12][13]. From these studies it is apparent that the N-, C-and mid-regional PTH-fragments are biologically active on bone cells and are involved in the regulation of bone metabolism.…”

The role of Vitamin D3 metabolism in prostate cancer

“…The synthetic less calcemic analogs of Vitamin D appear to be more potential in the treatment of prostate cancer when compared with 1␣,25(OH) 2 D 3 [53,54]. This suggests that the Vitamin D-derived compounds may be good candidates of human clinical trials in prostate cancer.…”