Hormonal Control of Bone Collagen Synthesis<i>in Vitro</i>: 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 [3H]proline into collagenase-digestible (CDP) and non-collagen protein (NCP) by using purified bacterial collagenase. Addition of lα,25-dihydroxyvitamin D3 (lα,25(OH)2D3) to cultures containing 5% vitamin D-deficient serum produced a dose-related inhibition of incorporation of proline into CDP over the range of 10-11 to 10-7 M at 24 h. Similar inhibition was obtained with lα,25(OH)2D3) 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. lα,24R,25-trihydroxyvitamin D3 (lα,24R,25(OH)3D3) at 10-9–10-7 M also inhibited the labeling of CDP, but three other compounds, 25-hydroxy-, 24R,25-dihydroxy-, and 24S,25-dihydroxyvitamin D3, 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, lα,25(OH)2D3) and lα,24R,25(OH)3D3), 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. (Endocrinology102: 731, 1978)