The role of estrogens and estrogen-like molecules, including isoflavones, in regulating bone cell activities is essential in understanding the etiology and treatment of post-menopausal osteoporosis. Although estrogen replacement (HRT) has been the main therapy used to prevent and treat osteoporosis, there are concerns about its safety. Isoflavones have attracted attention to their potential roles in osteoporosis prevention and treatment. We have compared the effects of the isoflavone daidzein (1 nM), which has no effect on tyrosine kinases, and 17beta-estradiol (1 nM) on the development and function of cultured osteoblasts isolated from long bones of young female piglets. Daidzein increased ALP activity, osteocalcin secretion, and mineralization, while E2 increased only ALP activity. The content of ERbeta and osteoprotegerin secretion by control cells gradually increased during osteoblast differentiation, whereas the ERalpha and RANK-L content decreased. Daidzein enhanced only the nuclear ERbeta whereas estradiol increased both ERalpha and ERbeta. Daidzein and estradiol increased osteoprotegerin and RANK-L secretion. Daidzein had a more pronounced effect than did estradiol. Daidzein and estradiol increased the membrane content of RANK-L and the nuclear content of runx2/Cbfa1. Daidzein enhanced the nuclear content of progesterone and vitamin D receptors but not as much as did estradiol. All the effects of daidzein were blocked by ICI 182,780. We conclude that a low concentration of daidzein may exert its anti-resorptive action by increasing the activity of porcine mature osteoblasts via ERbeta, by regulating runx2/Cbfa1 production, and by stimulating the secretion of key proteins involved in osteoclastogenesis, such as osteoprotegerin and RANK-ligand.

Studies on calcium nutrition in appropriate large animal models can be directly relevant to humans. We have examined the effect of dietary Ca deficiency on various bone and bone-related variables, including plasma markers, histomorphometry, mineral content and breaking strength in pigs. Three groups of eight 38-d-old female pigs were fed adequate (0.9%; control), low (0.4%; LCa) or very low (0.1%; VLCa) Ca diets for 32 d. Plasma Ca significantly decreased over time only in the VLCa-deficient pigs. The concentrations of the parathyroid hormones (PTH) and calcitriol increased as Ca deficiency developed, and the plasma PTH and calcitriol levels varied inversely with dietary Ca. The total bone ash contents, bending moments, trabecular bone volume and the mineral apposition rate all decreased as the calcium intake decreased. The osteoclast surface areas were greater than those of controls in both Ca-deficient groups, whereas the osteoblast surface areas were greater only in the VLCa group. The plasma osteoblast-related markers (alkaline phosphatase, carboxy-terminal propeptide of type I procollagen and osteocalcin) were either greater or unaffected in the Ca-deficient pigs. The results indicate that deficient bone mineralization combined with an increased bone resorption led to bone loss and fragility. The differences in the changes in bone cells (number and activity) between LCa and VLCa groups might be due to differences (time and extent) of circulating PTH and calcitriol. The defective mineralization in both Ca-depleted groups resulted mainly from the lack of Ca because their osteoblast activity was either maintained or stimulated. The results also underline the progressive sensitivity of pigs to Ca supply and the usefulness of this model.

We have demonstrated that alkaline phosphatase activity and collagen synthesis are dose-dependently stimulated by ascorbic acid in differentiated pig osteoblasts. In this study we further examined the relationship between ascorbic acid and bone metabolism by feeding young pigs large amounts of ascorbic acid. Three groups of seven 47-d-old pigs were given no ascorbic acid supplement (control), 500 (500 AA) or 1000 (1000 AA) mg ascorbic acid/kg diet for 4 mo. Calcium and P absorption and retention were evaluated by a 14-d balance trial immediately before killing in control and 1000 AA groups only (n = 6). Bones were collected at death and the bone ash and bending moment (three-point bending test) determined. Various plasma and urine indices of bone metabolism, especially those reflecting collagen degradation (hydroxyproline, deoxypyridinoline) and synthesis (carboxyterminal propeptide of type I collagen) were monitored. The plasma ascorbic acid concentrations increased with time and paralleled the dietary concentrations (P < 0.01). The Ca and P balances and the bone ash and bending moments in the ascorbic acid-supplemented pigs did not differ from those of the controls. Plasma osteocalcin was elevated (P < 0.05), whereas the other bone formation markers, alkaline phosphatase and carboxy terminal propeptide of type I collagen, were not affected by ascorbic acid. The plasma concentrations of Ca, P and 1,25-dihydroxycholecalciferol did not differ among the three groups. The unaffected urinary excretion of deoxypyridinoline and hydroxyproline in the ascorbic acid-supplemented pigs indicates that ascorbic acid does not alter bone resorption. Thus, high intakes of ascorbic acid have no positive influence on bone metabolism and bone characteristics in pigs. The in vivo long-term effects do not correlate with the short-term in vitro effects previously reported.

Résumé ― Les phosphopeptides issus de la caséine sont prétendus augmenter l'absorption intestinale du calcium. Une étude de 50 jours a été effectuée sur des porcs âgés de

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