Hormonal Control of Bone Collagen Synthesisin Vitro: Inhibitory Effect of 1-Hydroxylated Vitamin D Metabolites*

Abstract

The effects of vitamin D metabolites on bone collagen synthesis were assessed in organ cultures of fetal rat calvaria by measuring the incorporation of a 2-h pulse of [ :) H]proline into collagenase-digestible (CDP) and non-collagen protein (NCP) by using purified bacterial collagenase. Addition of la,25-dihydroxyvitamin Da (la,25(OH)2D.i) to cultures containing 5% vitamin D-deficient serum produced a dose-related inhibition of incorporation of proline into CDP over the range of 10"" to 10~7 M at 24 h. Similar inhibition was obtained with la,25(OH)2D,) in 4-day cultures by using medium supplemented with bovine serum albumin instead of serum. The effect on the labeling of CDP was selective and in most experiments there was no effect on incorporation of proline into NCP. la,24R,25-trihydroxyvitamin D. t (la,24R,25(OH).,D.,) at K)-9 -10-7 M also inhibited the labeling of CDP, but three other compounds, 25-hydroxy-, 24R,25-dihydroxy-, and 24S,25-dihydroxyvitamin D.i, had no consistent effect. To test whether a metabolite of vitamin D in serum might affect collagen synthesis, bones were cultured with 1-10% serum from vitamin D-deficient and vitamin D-treated animals. There was no greater labeling of CDP and NCP with vitamin D-sufficient serum. We conclude that the vitamin D metabolites which are most active in stimulating bone resorption, la,25(OH) 2 D:t and la,24R,25(OH). } D.), also inhibit bone collagen synthesis in vitro. No evidence for a stimulatory effect on bone collagen synthesis was obtained by using the currently available natural metabolites of vitamin D or by comparing serum from vitamin D-deficient or -treated animals. (Endocrinology 102: 731 , 1978) V ITAMIN D appears to be required for skeletal growth and mineralization. Hence, it seems paradoxical that the active form of the vitamin, la,25-dihydroxyvitamin D3 (la,25(OH) 2 D;3), is a potent stimulator of bone resorption (1). One explanation for this apparent discrepancy is that intestinal absorption of calcium and phosphate is increased to a greater extent by the active metabolites of vitamin D than is bone resorption. This would increase the serum calcium and phosphate concentrations, thus enhancing bone mineralization, while at the same time suppressing parathyroid hormone (PTH) so that bone resorption might not be increased in vivo. It may also be important to stimulate bone resorption in order to maintain skeletal growth because, at least in bone remodeling, new bone formation generally occurs at resorption sites. In either case, vitamin D would have a permissive, rather than a direct, role in bone growth. The initial stimulus to bone formation might be from other growth-regulating hormones such as insulin and somatomedin(s) (2, 3), by local feedback effects of changes in mineralization, or by serum calcium and phosphate concentration themselves (4).An attractive alternative is that vitamin D or its metabolites have a direct stimulatory effect, either upon collagen synthesis by osteoblasts or upon bone mineralization. In vivo experiments (5) and...

Altered growth, differentiation, and responsiveness to epidermal growth factor of human embryonic mesenchymal cells of palate by persistent rubella virus infection.

“…The precipitates were solubilized with 0.5 M NaOH and their radioactivity was counted (noncollagenous protein). Percent collagen synthesized was calculated by the formula described (18). EGF receptor assay.…”

Studies of the Effects of Cholecalciferol Upon the Metabolism of Rachitic Chick Growth Cartilage

“…Cholecalciferol infusion has also been re ported to increase bone formation without affecting resorption in normal dogs [14] and bone collagen synthesis was found to be in creased in chickens [15] and rats [8] during healing of rickets. The failure of the low den sity calcified phase to be converted to the higher density hydroxyapatite phase in ra chitic chick bone appears to be related to the deficiency of vitamin D rather than to the accompanying disturbances in mineral me tabolism [16], Administration of 1,25-dihydroxycholecalciferol to chickens [17] or rats [18] can at least partly correct the inhibition of mineralization induced by treatment with ethane-1 -hydroxy-1,1 -diphosphonate but 24,25-dihydroxycholecalciferol was not found to be effective [18] and treatment with both dihydroxylated metabolites has been claimed to be necessary to prevent rachitic bone lesions in chickens maintained on a vitamin D-deficient diet [19], In vitro studies with these metabolites suggest that they can influence the synthesis of structural macro molecules; 1,25-dihydroxycholecalciferol has been reported to inhibit collagen synthesis in cultures of fetal rat calvaria [20] and in creased incorporation of 3SS 04 into proteo glycan was found in cultures of rabbit growth plate cells in the presence of either 1,25-or 24,25-dihydroxycholecalciferol [21],…”

Effects of phosphate and 1,25(OH)2D3 on in vitro bone collagen synthesis in the hypophosphatemic mouse

“…Inhibition of bone collagen synthesis by 1,25(OH)..,D3 has already been reported in another organ culture system [ 14,15]. The effect of medium Pi concentration on the response to 1,25(OH)2D3 was also investigated.…”