Bone matrix studies. Influences of parathyroid extract, calcitonin, and cholecalciferol and of rickets and its treatment

Elevated bone phosphorus/hydroxyproline ratio following jejunoileal bypass surgery

The degree of bone mineralization, estimated as the bone phosphorus/hydroxyproline ratio (P/Hypro), was studied in 33 patients who had undergone jejunoileal bypass surgery for massive obesity. Low values of bone P/Hypro are expected in osteomalacia. We found an elevated mean bone P/Hypro (p < 0.001) with the highest values in patients with the longest postoperative periods (rS = 0.65, p < 0.001). The study indicates that bypass surgery, in spite of vitamin D deficiency, is associated with an increased average degree of bone mineralization or a defect in bone collagen synthesis.

Vitamin D deficiency causes a selective reduction in deposition of transforming growth factor beta in rat bone: possible mechanism for impaired osteoinduction

We demonstrated previously that implants of bone matrix prepared from vitamin D-deficient (-D) rats were less osteoinductive and contained less extractable mitogenic activity compared with control implants prepared from vitamin D-replete (+D) rats and proposed that bone from -D rats is deficient in one or more specific growth factors. To test this hypothesis, bones from rats that were fed either +D or -D diets and kept in the dark for 8 wk were extracted and assayed for insulin-like growth factors I and II (IGF-I and IGF-II) and transforming growth factor beta (TGF-beta), the three most abundant growth factors in rat bone, and osteocalcin. Serum calcium, 25-hydroxyvitamin D3 and 1,25-dihydroxyvitamin D3 [1,25(OH)2D3] were determined at sacrifice. In -D rats, there were significant reductions in serum calcium, 25-hydroxyvitamin D3, and 1,25(OH)2D3 and skeletal TGF-beta but no differences in extractable skeletal protein, IGF-I, IGF-II, or osteocalcin compared with +D rats. To determine whether 1,25(OH)2D3 increased TGF-beta production by bone cells, we treated mouse calvaria for 6 days and mouse osteoblasts for 2 days with 10 nM 1,25(OH)2D3. Production of TGF-beta was increased almost 100% by 1,25(OH)2D3. We conclude that vitamin D deficiency reduces deposition of TGF-beta in rat bone and that diminished skeletal TGF-beta could contribute to the previously observed decrease in osteoinduction in implants from -D rat bone. The findings support the possibility that vitamin D and bone-derived TGF-beta are required for normal repair of the skeleton.

Demonstration of reduced mitogenic and osteoinductive activities in demineralized allogeneic bone matrix from vitamin D-deficient rats

Osteoinduction is the formation of ectopic bone that follows implantation of demineralized allogeneic bone matrix (DABM) and is believed to be secondary to the release of associated inductive factors from bone matrix. To clarify the role of vitamin D in osteoinduction, we implanted DABM from vitamin D-deficient rats (-D rats) into normal rats (+D rats). Because mitogens and osteocalcin might be involved in osteoinduction, these were measured. Mitogenic activity in extracts from mineralized allogeneic bone matrix (ABM) and DABM from both +D and -D rats was determined with an assay that utilizes monolayer cultures of embryonic chick calvarial cells. Osteocalcin in serum and DABM was measured by radioimmunoassay. DABM from -D rats did not promote osteoinduction as effectively as DABM from +D rats. Resorption of implant matrix from -D rats was diminished compared with resorption of matrix from +D rats (P less than 0.01), and the decrease was attributed to a corresponding decrease in the number of osteoclasts in the implants (P less than 0.02). Bone formation (P less than 0.01) and total implant mineralization (P less than 0.001) were significantly reduced in implants from -D rats, and the reductions corresponded with a decline in the number of osteoblasts (P less than 0.05). Mitogenic activity in DABM from +D rats was only slightly decreased as compared with activity in ABM, but DABM from -D rats contained significantly less activity (P less than 0.001). No mitogenic activity was identified in implants of DABM from either +D or -D rats 3 wk after implantation. Serum osteocalcin was significantly higher in -D as compared with +D animals. In contrast, the concentrations of osteocalcin in DABM from the two groups of animals were not significantly different from each other. These findings indicate that the diminished osteoinductive activity of DABM from -D rats results from deficiency of one or more mitogenic factors that are essential for inducing the proliferation and differentiation of bone cells at the implant site and that osteocalcin does not play a role in this regard.

Extracellular matrix proteins involved in bone induction are vitamin D dependent

Subcutaneous implantation of demineralized diaphyseal bone matrix into allogeneic rats results in local formation of cartilage and bone. However, implantation of demineralized bone matrix obtained from rachitic rats did not induce bone. Rachitic bone matrix was therefore dissociatively extracted with 4 M guanidine HCl and then reconstituted with an inactive collagenous residue of control as carrier. Such reconstituted materials also lacked bone inductive potential. On the other hand, reconstitution of guanidine HCl extracts of control bone matrix with inactive vitamin D deficient matrix did result in bone induction. Partial purification (fractions containing proteins (less than 50,000 daltons) of the guanidine HCl extract from rachitic rats on Sepharose CL-6B followed by reconstitution with inactive collagenous residues resulted in a weak (25% of control) inductive response. These observations imply that bone inductive proteins are vitamin D dependent and are reduced in matrix obtained from rachitic rats.

Effect of vitamin D deficiency on urinary excretion of connective tissue derivatives (hydroxyproline and glycosaminoglycans) in rats

The urinary excretion of two connective tissue metabolites was studied in both control and vitamin D deficient rats. Hydroxyproline (HyPRO) excretion was determined after 2, 13 and 22 months (experiment I). It decreased with aging in animals receiving the control diet. On the contrary, this excretion increased as a function of age in vitamin D deficient animals. At the age of 22 months, HyPRO excretion was respectively 31 and 1708 micrograms a day in control and deficient animals. HyPRO and glycosaminoglycans (GAG) excretion was measured on a group of both control and vitamin D deficient rats at the age of 21 months (experiment II). These results confirm the high excretion of HyPRO in deficient animals. On the contrary, the GAG excretion was higher in control animals than in deficient ones, the mean excretion being respectively 412 and 234 micrograms a day.