The effects of ascorbic acid deficiency on calcium and collagen metabolism in cultured fetal rat bones

Spontaneous fractures in the mouse mutant sfx are caused by deletion of the gulonolactone oxidase gene, causing vitamin C deficiency

Using a mouse mutant that fractures spontaneously and dies at a very young age, we identified that a deletion of the GULO gene, which is involved in the synthesis of vitamin C, is the cause of impaired osteoblast differentiation, reduced bone formation, and development of spontaneous fractures.

INTRODUCTION

A major public health problem worldwide, osteoporosis is a disease characterized by inadequate bone mass necessary for mechanical support, resulting in bone fracture. To identify the genetic basis for osteoporotic fractures, we used a mouse model that develops spontaneous fractures (sfx) at a very early age.

MATERIALS AND METHODS

Skeletal phenotype of the sfx phenotype was evaluated by DXA using PIXImus instrumentation and by dynamic histomorphometry. The sfx gene was identified using various molecular genetic approaches, including fine mapping and sequencing of candidate genes, whole genome microarray, and PCR amplification of candidate genes using cDNA and genomic DNA as templates. Gene expression of selected candidate genes was performed using real-time PCR analysis. Osteoblast differentiation was measured by bone marrow stromal cell nodule assay.

RESULTS

Femur and tibial BMD were reduced by 27% and 36%, respectively, in sfx mice at 5 weeks of age. Histomorphometric analyses of bones from sfx mice revealed that bone formation rate is reduced by >90% and is caused by impairment of differentiated functions of osteoblasts. The sfx gene was fine mapped to a 2 MB region containing approximately 30 genes in chromosome 14. By using various molecular genetic approaches, we identified that deletion of the gulonolactone oxidase (GULO) gene, which is involved in the synthesis of ascorbic acid, is responsible for the sfx phenotype. We established that ascorbic acid deficiency caused by deletion of the GULO gene (38,146-bp region) contributes to fractures and premature death because the sfx phenotype can be corrected in vivo by treating sfx mice with ascorbic acid and because osteoblasts derived from sfx mice are only able to form mineralized nodules when treated with ascorbic acid. Treatment of bone marrow stromal cells derived from sfx/sfx mice in vitro with ascorbic acid increased expression levels of type I collagen, alkaline phosphatase, and osteocalcin several-fold.

CONCLUSION

The sfx is a mutation of the GULO gene, which leads to ascorbic acid deficiency, impaired osteoblast cell function, and fractures in affected mice. Based on these and other findings, we propose that ascorbic acid is essential for the maintenance of differentiated functions of osteoblasts and other cell types.

Guidelines for using in vitro methods to study the effects of phyto-oestrogens on bone

These guidelines review the relevant literature on the way plant phyto-oestrogens act on bone and the responsiveness of different bone cell systems to phyto-oestrogenic compounds. The primary emphasis is on the experimental conditions used, the markers available for assessing osteoblast and osteoclast function, and their expected sensitivity. Finally, we assess the published results to derive some general recommendations for in vitro experiments in this area of research.

Dexamethasone induces sodium-dependant vitamin C transporter in a mouse osteoblastic cell line MC3T3-E1

The regulation of intracellular ascorbic acid (AsA) levels may be under the control of an AsA-specific membrane transporter. The present study investigates AsA uptake and expression of Na-dependent vitamin C transporter (SVCT) mRNA in the mouse osteoblastic cell line, MC3T3-E1. Among eight compounds tested, dexamethasone (Dex) all-trans retinoic acid, transforming growth factor beta, prostaglandin E2 and transferrin significantly and respectively) stimulated the update of AsA into MC3T3-E1 cells. Among these five, Dex was the most active, inducing mSVCT2 mRNA and the uptake of AsA in a time- and concentration-dependant manner. Dex did not induce mSVCT1 mRNA. These results suggest that the Dex-induced stimulation of AsA incorporation into osteoblastic cells is mediated by the induction of mSVCT2. Since Dex reduced alkaline phosphatase activity in MC3T3-E1 cells in our culture conditions, Dex-induced stimulation of AsA incorporation might not be the result of differentiation. Hormone-regulated changes of SVCT expression may have an important role in cell functions.

Effect of ascorbic acid deficiency on primary and reparative dentinogenesis in non-ascorbate-synthesizing ODS rats

Ascorbic acid is essential to the biosynthesis of collagen, the major organic matrix component of dentine. The ODS rat is a mutant strain of Wistar rat characterized by hereditary lack of L-gulono-gamma-lactone oxidase and thus is unable to synthesize ascorbic acid. ODS rats were given an ascorbic acid-free diet to investigate how ascorbic acid deficiency affects dentine formation in vivo. Histomorphometric analysis on their growing molars and incisors showed a significant reduction in both size and mineral apposition rate of dentine, as revealed by contact microradiography and fluorescent time-marking, respectively. A similar reduction in bone formation was simultaneously demonstrated in the mandible, confirming the previously reported osteopathic effects of ascorbic acid deficiency. When pulp inflammation was induced in lower first molars by making unsealed pulp exposures, specimens from control animals showed continuous deposition of an osteodentine-like tissue in the radicular pulp chamber; this type of mineralized tissue formation was greatly reduced in ascorbic acid-deprived animals. These results indicate that ascorbic acid deficiency hampers dentine formation under both physiological and pathological conditions of the dentine/pulp complex. ODS rats could be useful in investigating in vivo effects of ascorbic acid deficiency on the formation of dentine and other dental mineralized tissues.

Ascorbic acid alters collagen integrins in bone culture

The effects of ascorbic acid on collagen synthesis, mineralization, and integrins were investigated in a mineralizing organ culture system derived from 20-day fetal rat parietal bones. A significant dose-dependent decrease in calcification at 96 h was demonstrated with decreasing concentrations of ascorbic acid (100-0 microg/ml). No effect on DNA content, [3H]thymidine incorporation, or dry weight was found in control (100 microg/ml ascorbic acid) bones compared with bones treated with decreased ascorbic acid concentrations (10, 1, and 0 microg/ml). Collagen synthesis, measured by [3H]proline incorporation, and alpha1(I) procollagen messenger RNA levels were also unaffected. However, ascorbic acid produced a dose-dependent decrease in the hydroxyproline content, with a maximal 76.8% decrease in bones without ascorbic acid compared with the control bones with 100 microg/ml ascorbic acid. Light microscopy of the ascorbic acid-deficient bones revealed a disruption of the osteoblast layer with misshapen osteoblasts and a decrease in the osteoid seam. The loss of osteoblast organization was also confirmed by analyzing the integrins for collagen by Northern and Western blot and immunofluorescence microscopy. A dose-dependent decrease in alpha2 and beta1 integrin messenger RNA levels and in alpha1, alpha2, and beta1 protein were found in 96-h bone cultures deficient in ascorbic acid. These integrin subunits mediate the binding of osteoblasts to collagen. Immunofluorescence microscopy also demonstrated a dose-dependent decrease in alpha2 and beta1 staining of the osteoblast layer. However, the protein levels of alpha3 and alpha5 subunits were not affected. No beta5 was detected, whereas only bones cultured without ascorbic acid demonstrated a small decrease in alpha(v) and beta3 protein levels. The alpha3, alpha5, alpha(v), and beta3 subunits are involved in cell binding to extracellular matrix proteins other than collagen. Thus, the integrins for collagen are down-regulated, probably in response to the underhydroxylated collagen fibrils, which causes a disruption of osteoblast organization leading to a decrease in mineralization of bone. Integrin assays for specific extracellular proteins may be useful tools in detecting matrix defects in various metabolic bone diseases.

Vitamin C supplementation does not modify bone mineral content or mineral absorption in growing pigs

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.

Adaptive regulation of ascorbate transport in osteoblastic cells

Osteoblasts possess a concentrative L-ascorbate (vitamin C) uptake mechanism involving a Na(+)-dependent ascorbate transporter located in the plasma membrane. The transporter is specific for ascorbate and stereoselective for L-ascorbate over D-isoascorbate. The present study examined the effects of ascorbate supplementation and deprivation on the activity of this transport system. L-ascorbate transport activity was determined by measuring uptake of the vitamin by ROS 17/2.8 osteosarcoma cells during 1 minute incubations with 5 microM L-[14C]ascorbate. The initial rate of L-[14C]ascorbate uptake by ROS 17/2.8 cells grown for 18 h in L-ascorbate-replete medium was 89 +/- 8 nmol/g protein per minute. Following removal of L-ascorbate from the growth medium, the initial rate of uptake increased within 6 h to 126 +/- 13 nmol/g protein per minute. Conversely, the initial rate of uptake by cells grown in ascorbate-free medium decreased following the addition of L-ascorbate, but not D-isoascorbate, to the medium. The effect of ascorbate pretreatment was specific for ascorbate transport in that preincubation of cultures with L-ascorbate did not affect uptake of 2-deoxy-D-glucose. Kinetic analysis revealed that modulation of ascorbate transport arose from changes in the apparent maximum rate of transport (Vmax) without changes in the affinity of the transport system for L-ascorbate. These experiments are the first to show that ascorbate transport by osteoblastic cells responds to vitamin C deprivation and supplementation. Adaptation of transport activity to substrate availability may play an important role in the physiological regulation of intracellular ascorbate levels.

Ascorbate uptake by ROS 17/2.8 osteoblast-like cells: substrate specificity and sensitivity to transport inhibitors

Ascorbate (reduced vitamin C) is required for bone formation. We have shown previously that both the osteoblast-like cell line ROS 17/2.8 and primary cultures of rat calvarial cells possess a saturable, Na(+)-dependent uptake system for L-ascorbate (J Membr Biol 111:83-91, 1989). The purpose of the present study was to investigate the specificity of this transport system for organic anions and its sensitivity to transport inhibitors. Initial rates of ascorbate uptake were measured by incubating ROS 17/2.8 cells with [L-14C]ascorbate at 37 degrees C. Uptake of [L-14C]ascorbate (5 microM) was inhibited 98 +/- 1% by coincubation with unlabeled L-ascorbate (3 mM) and 48 +/- 4% by salicylate (3 mM), but it was not affected by 3 mM formate, lactate, pyruvate, gluconate, oxalate, malonate, or succinate. Uptake of the radiolabeled vitamin also was not affected by acute (1 minute) exposure of the cells to the Na+ transport inhibitors amiloride and ouabain or the glucose transport inhibitor cytochalasin B. In contrast, anion transport inhibitors rapidly (less than 1 minute) and reversibly blocked [L-14C]ascorbate uptake. In order of potency, these drugs were 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS) approximately equal to sulfinpyrazone greater than furosemide approximately equal to 4-acetamido-4'-isothiocyanostilbene-2,2'-disulfonic acid (SITS). These findings indicate that the ascorbate transporter is relatively specific for the ascorbate anion, since other organic anions (with the exception of salicylate) did not compete with ascorbate for uptake. Rapid and reversible inhibition by the impermeant antagonists DIDS and SITS suggests that they interact directly with the ascorbate transporter, consistent with location of the transport system in the plasma membrane.

Bone histomorphometric analysis for the cause of osteopenia in vitamin C-deficient rat (ODS rat)

A particular strain of rat, the osteogenic disorder rat (ODS rat), was established in 1973. Phenotypic expression of od/od in ODS rat develops signs characteristics of a vitamin C-deficient animal, with bleeding tendencies and limb fractures. We investigated the bone histomorphometry to clarify the pathogenesis of osteopathy found in ODS rat. Bone histomorphometry revealed that static parameters reflecting bone formation were found to be remarkably decreased in od/od rats. These observations were more prominent in the metaphysis of distal femurs of od/od rats than those in the tail vertebrae. Parameters reflecting bone resorption in od/od rats were reduced in the distal femoral metaphysis, but were similar to those of controls in the tail vertebrae. These parameters were restored to control levels after ascorbic acid supplementation to pair-fed od/od rats. The mineral appositional rate in od/od rats was not significantly different from that in controls. Although body weight gain in pair-fed controls was significantly reduced compared to those fed ad libitum, histomorphometric parameters, on the contrary, were unaltered between these groups. Our present study provides evidence that the cause of osteopenia found in od/od rat is attributable to an imbalance between the total amounts of resorption and formation, and the pathogenesis of osteopathy could be due to ascorbic acid deficiency itself rather than malnutrition.

High-affinity sodium-dependent uptake of ascorbic acid by rat osteoblasts

Ascorbic acid is essential for the formation of bone by osteoblasts, but the mechanism by which osteoblasts transport ascorbate has not been investigated previously. We examined the uptake of L-[14C]ascorbate by a rat osteoblast-like cell line (ROS 17/2.8) and by primary cultures of rat calvaria cells. In both systems, cells accumulated L-[14C]ascorbate during incubations of 1-30 min at 37 degrees C. Unlike propionic acid, which diffuses across membranes in protonated form, ascorbic acid did not markedly alter cytosolic pH. Initial ascorbate uptake rate saturated with increasing substrate concentration, reflecting a high-affinity interaction that could be described by Michaelis-Menten kinetics (apparent Km = 30 +/- 2 microM and Vmax = 1460 +/- 140 nmol ascorbate/g protein/min in ROS 17/2.8 cells incubated with 138 mM extracellular Na+). Consistent with a stereoselective carrier-mediated mechanism, unlabeled L-ascorbate was a more potent inhibitor (IC50 = 30 +/- 5 microM) of L-[14C]ascorbate transport than was D-isoascorbate (IC50 = 380 +/- 55 microM). Uptake was dependent on both temperature and Na+, since it was inhibited by cooling to 4 degrees C and by substitution of K+, Li+ or N-methyl-D-glucamine for extracellular Na+. Decreasing the external Na+ concentration lowered both the affinity of the transporter for ascorbate and the apparent maximum velocity of transport. We conclude that osteoblasts possess a stereoselective, high-affinity, Na+-dependent transport system for ascorbate. This system may play a role in the regulation of bone formation.

In vitro mineralization of fetal rat parietal bones in defined serum-free medium: effect of beta-glycerol phosphate

We have developed a bone organ culture system that mineralizes in vitro. Fetal rat parietal bones (20 days old) were cultured in a chemically defined serum-free medium containing physiological 3 mM phosphate. During 5 days in culture, calcium content increased from 26 to 55 micrograms and dry weight increased from 137 to 194 micrograms. After 2 days in vivo, the calcium content of the parietal bone showed a comparable increase to 49 micrograms and dry weight increased to 183 micrograms. During culture, the mineralized bone area in thick sections increased from 11 to 23%, which paralleled the doubling in calcium content. Fluorescent calcein labeling during the 5 day culture period demonstrated that calcification occurs in an ordered pattern. Protein synthesis was assessed by measuring incorporation of [3H]proline into collagenase-digestible protein (CDP) and noncollagen protein (NCP). The percentage collagen synthesis decreased from 17.5% at 0 time to 5.0% at 2 days and then increased to 9.4% at 5 days of culture. Varying the inorganic phosphate concentration in the medium or adding beta-glycerol phosphate was found to affect mineralization. After 5 days in culture, bones treated with 1 mM phosphate exhibited a large region of unmineralized osteoid with only a 23% increase in calcium content compared with 112% in control (3 mM phosphate) bones and a 28% increase in dry weight compared with a 40% increase in control. Treatment for 5 days with 6 mM phosphate or 1, 3, or 10 mM beta-glycerol phosphate had no significant effect on dry weight compared to control bones. However, bone calcium content increased significantly from 55 +/- 5 micrograms in control cultures to 105 +/- 7 with 6 mM phosphate, 74 +/- 6 with 3 mM beta-glycerol phosphate, and 75 +/- 5 micrograms with 10 mM beta-glycerol phosphate. Calcified area measured by histomorphometry was also significantly greater than in control bones, but this was mainly due to ectopic calcification in the periosteum, representing from 23 to 74% of the total increase in calcified matrix in bones cultured with 6 mM phosphate or 1-10 mM beta-glycerol phosphate. Ultrastructural analysis demonstrated that ectopic calcification was associated with cell death and debris. Therefore, calcification with beta-glycerol phosphate and high concentrations of inorganic phosphate differed from mineralization in vivo or in bones cultured with a physiologically concentration of phosphate.

The effect of ascorbic acid on the metabolism of rat calvarial bone cells in vitro

Addition of 50 micrograms/ml sodium ascorbate to confluent cultures of isolated rat calvarium bone cells resulted in a 21% increase in DNA production, a 50-60% increase in incorporation of [14C]proline into collagenous and noncollagenous proteins, and a 200% increase in alkaline phosphatase activity; under identical conditions, [35S]sulfate incorporation into proteoglycans (glycosaminoglycans) was not affected. These results suggest that ascorbate may be important in maintaining or stimulating the osteogenic phenotype of normal bone cells.

A technique for the in vitro incubation of deer antler tissue

1. A procedure for the in vitro incubation of velvet deer antler tissue was developed. Biopsy samples were collected in June with a trephine from 2 adult white-tailed deer and incubated in modified BGJb medium up to 48 hr. Calcium (Ca) and hydroxyproline (OH-proline) concentrations in the tissue were determined. 2. A significant increase (P less than 0.05) in Ca was exhibited at 4 and 8 hr of incubation, and, after replenishment of media, at 48 hr. 3. Hydroxyproline concentrations continued to rise throughout the duration of the incubation period and were significantly higher than controls (P less than 0.05) at 16, 24, and 48 hr. 4. Results suggest antler tissue can be incubated in vitro with the protocol described, although length of incubation may vary with parameter measured.

Effects of ascorbic acid on alkaline phosphatase activity and hormone responsiveness in the osteoblastic osteosarcoma cell line UMR-106

L-ascorbic acid at physiological concentrations (10 micrograms/ml) increased alkaline phosphatase activity in the osteoblastlike rat osteosarcoma cell line, UMR-106. The increase was dose-dependent and detectable at 6 hours after the addition of 100 micrograms/ml ascorbic acid to the medium. Treatment of the cells with 100 micrograms/ml ascorbic acid potentiated the response of cAMP to both PTH and PGE1, while cell growth was inhibited. Furthermore, the number of colonies formed by the cells grown in the soft agar was significantly reduced by increasing concentrations of ascorbic acid. These results indicate that ascorbic acid might play some role in the differentiation of osteoblasts.

Hormonal responses to 1,25-dihydroxyvitamin D3 in cultured mouse osteoblast-like cells--modulation by changes in receptor level

The level of 1,25(OH)2D3 receptors in cultured mouse osteoblast-like (OB) cells is modulated by the rate of cell proliferation. We have studied two 1,25(OH)2D3-induced bioresponses to ascertain whether the changes in receptor levels during growth in culture alter cell responsiveness. Nuclear receptor levels were high (127 fmol/100 micrograms DNA) in rapidly dividing (log) cells and low (25 fmol/100 micrograms DNA) in quiescent (confluent) cells. The bioresponses we studied were induction of 25(OH)D3-24-hydroxylase activity (24-hydroxylase) and inhibition of collagen synthesis. The basal levels of 24-hydroxylase were low and similar in cells at log growth phase and confluence. At a maximal induction dose of 13 nM, 1,25(OH)2D3 induced a three-fold rise in enzyme activity at long growth phase, but only caused less than two-fold rise at confluence. The half-maximal dose (ED50) was slightly shifted from 0.6 nM to 0.8 nM. Daily measurement of 1,25(OH)2D3 receptor levels and maximal induction of 24-hydroxylase activity throughout the culture cycle showed a strong correlation between receptor abundance and enzyme induction. The basal level of collagen synthesized by cells in log growth phase was approximately 5% and increased to approximately 8% at confluence. Maximal inhibition of collagen synthesis by 1,25(OH)2D3 reached 80% of control levels in log cells, but was only 40% of control in confluent cells. The ED50 was approximately 0.1 nM in the log cells and increased to approximately 1 nM at confluence. Daily assay of 1,25(OH)2D3 receptor levels and 1,25(OH)2D3 responses during the culture cycle indicated a correlation between changes in receptor level and the extent of inhibition of collagen synthesis. These changes in bioresponse at various growth phases did not occur in rat OB cells where the 1,25(OH)2D3 receptor levels were independent of cell proliferation. The results indicate that cell proliferation rate, via change in receptor levels, determines the magnitude and sensitivity of the cellular responses to 1,25(OH)2D3.

Specifically decreased collagen biosynthesis in scurvy dissociated from an effect on proline hydroxylation and correlated with body weight loss. In vitro studies in guinea pig calvarial bones

The question whether ascorbate regulates collagen production solely through its direct role in proline hydroxylation was investigated. Proteins in calvarial bones from control and scorbutic weanling guinea pigs were labeled in short-term cultures with radioactive proline. Proteins were digested with purified bacterial collagenase to distinguish between effects on collagen polypeptide production and hydroxyproline formation. There was a preferential decrease in the absolute rate of collagen biosynthesis beginning after 2 wk of ascorbate deficiency, and this effect was temporally dissociated from decreased proline hydroxylation. There were no significant changes in the absolute rates of collagen degradation or noncollagen protein production. In vitro inhibition of proline hydroxylation in normal bone with alpha, alpha'-dipyridyl did not affect the relative rate of collagen synthesis, further dissociating these functions. Ascorbate added to scorbutic bone cultures reversed defective proline hydroxylation but not defective collagen synthesis, suggesting that the latter was an indirect effect of scurvy. There was a linear correlation between the extent of body weight lost during the 3rd and 4th wk of scurvy and the rate of collagen synthesis in scorbutic bone. This correlation also applied to control animals receiving ascorbate, but with weight loss induced by food restriction. These studies establish for the first time that ascorbate deficiency in guinea pigs leads to a specific decrease in collagen polypeptide synthesis and suggest that this decrease results from the reduced food intake and/or weight-loss characteristic of scurvy.

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

Calvarial bones from hypophosphatemic (Hyp) mice and normal littermates were cultured in a chemically defined medium to determine: (a) the effect of medium phosphate (Pi) concentration (1, 2, and 3 mM) on collagen synthesis; (b) the effect of 1,25-dihydroxycholecalciferol [1,25(OH)2D3] (10(-12)M-10(-7)M) on collagen synthesis; and (c) whether bone responsiveness to 1,25(OH)2D3 was affected by changes in medium Pi concentration. Bone collagen synthesis was evaluated by measuring [ 3H ]hydroxyproline formation. The distribution of labeled hydroxyproline between bone explant and culture medium (total and dialyzable fraction) was studied. These experiments confirm that 1,25(OH)2D3 inhibits specifically bone collagen synthesis in vitro. We did not detect any effect of medium Pi concentration on basal collagen synthesis but were able to demonstrate that lowering medium Pi concentration increased the 1,25(OH)2D3-induced inhibition of collagen synthesis. Bones from both genotypes responded to 1,25(OH)2D3, but modulation of this response by changes in Pi concentration was altered in Hyp bone as, in contrast to normal bone, its response to 1,25(OH)2D3 was unaffected when medium Pi concentration was decreased from 3 to 2 mM. These findings support the hypothesis of an altered response of bone to 1,25(OH)2D3 in the Hyp mouse.

A histological and biochemical study of the effect of vitamin C-deficiency on induction of amelogenesis in hamster molars in vitro

Second maxillary molars of hamsters were cultured in the presence or absence of 250 micrograms/ml vitamin C for periods up to 12 days. At various days, cultured explants were studied with histological and biochemical methods to investigate effects of vitamin C-deficiency on matrix production and mineralization in vitro. As biochemical parameters for protein synthesis and mineralization, uptake and incorporation of [3H]-proline, 45Ca and 32PO4 were used. To discriminate between synthesis of collagenous and non-collagenous proteins digestion of the [3H]-proline-labelled material by purified collagenase and its degree of hydroxylation were measured. Histologically, in control explants cultured with vitamin C, normal dentinogenesis, amelogenesis and mineralization in vitro were observed. In the vitamin C-deficient explants, odontoblasts produced an abnormal predentine matrix and de-differentiated. Eventually, this matrix mineralized aspecifically. In the presence of this abnormal matrix, ameloblasts failed to differentiate, which suggests that a cell-matrix type of interaction is involved in the differentiation of the pre-ameloblasts. Biochemically, in the vitamin C-deficient explants protein synthesis and collagen synthesis were reduced by about the same extent; the in-vitro produced collagens appeared under-hydroxylated and could, in degraded form, easily be extracted in formic acid. An increase of [3H]-proline solubility in formic acid in the control explants, however, paralleled enamel matrix production and was attributed to the solubilization of proline-rich enamel matrix proteins. The production of acid insoluble phosphate was not affected by vitamin C-deficiency. The uptakes of 45Ca and 32PO4 were retarded and the molar Ca:PO4 uptake ratio was lower, reflecting the histologically observed aspecific mineralization.

Effect of ascorbic acid on protein synthesis and collagen hydroxylation in continuous flow organ cultures of adult mouse periodontal tissues

A continuous flow organ culture system (CFCS) was used to determine the effect of ascorbic acid on the synthesis of collagen and noncollagenous protein by bone of the alveolar process and periodontal ligament in organ cultures of adult mouse periodontium. For the last 24 h of 2 day cultures, 5 microCi/ml 3H-proline was added to the medium. Highly purified collagenase was used to separate the collagenous and noncollagenous proteins and the incorporation of isotope into each fraction measured. Collage synthesized in the presence of less than 10 micrograms/ml ascorbic acid was found to be highly under-hydroxylated (pro:hypro ap. acts. 2.3-3.1) in both tissues. When the ascorbic acid levels were between 25 and 100 micrograms/ml, the synthesis of collagenous proteins was selectively stimulated and hydroxylation significantly improved (pro:hypro sp. acts 1.72-1.89). The effect of ascorbic acid was not related to tissue viability since tissues cultured initially in the absence of ascorbic acid were able to recover completely when compared to controls given ascorbic acid continuously. The proportion of radioactivity in collagen and noncollagenous protein, collagen hydroxylation, and percentage of collagen synthesized as type III (av.23%) in bone of the alveolar process was similar to that found in vivo. However, in the periodontal ligament in vitro the proportion of noncollagenous protein synthesized was increased from 70% to 87% and the percentage of type III collagen increased from 14% to 26% compared to in vivo results.

Parathyroid hormone stimulates bone formation and resorption in organ culture: evidence for a coupling mechanism

We have developed an in vitro system, using embryonic chicken tibiae grown in a serum-free medium, which exhibits simultaneous bone formation and resorption. Tibiae from 8-day embryos increased in mean (+/- SD) length (4.0 +/- 0.4 to 11.0 +/- 0.3 mm) and dry weight (0.30 +/- 0.04 to 0.84 +/- 0.04 mg) during 12 days in vitro. There was increased incorporation of [3H]proline into hydroxyproline (120 +/- 20 to 340 +/- 20 cpm/mg of bone per 24 hr) as a measure of collagen synthesis, as well as a 62 +/- 5% increase in total calcium and 45Ca taken up as an indication of active mineralization. A physiologic concentration (1 pM) of parathyroid hormone was found to stimulate bone resorption over control levels in this system. Parathyroid hormone stimulated the release of [3H]hydroxyproline from the bone shafts but not from the cartilage ends, indicating the specificity of the response. With 1 pM parathyroid hormone we observed an acute inhibition of bone formation, followed (after 12-16 hr) by a chronic stimulation of bone formation during the 12-day incubation. Both mineral uptake and matrix formation were enhanced at approximately the same rate during the 12-day incubation. The chronic enhancement of formation required parathyroid hormone only for the initial 8-10 hr of incubation. These results could be explained by the production or release of a factor from bone to stimulate formation in response to the acute increase in resorption--a "coupling factor." Indeed, dialyzed culture medium conditioned by actively resorbing bones stimulated bone formation over controls when added to organ cultures at a 1:20 dilution. The factor is larger than 12,000 daltons as determined by dialysis. The factor is specific for the bone shaft and did not affect the cartilage ends.

Some aspects of the modulation and regulation of collagen synthesis in vitro

We reviewed here a number of publications containing data on the quantitative aspects of collagen synthesis in vitro. In one section we discussed the factors which modulate the amount of collagen synthesized in various culture systems and in another section we presented experimental evidence for regulatory mechanisms operating in collagen synthesis on the transcriptional and/or translational levels. We believe that growing knowledge of the mechanisms controlling collagen synthesis will help us to understand and deal with fibrotic processes better.

Inhibitory effect of cadmium on collagenous peptide synthesis of embryonic chick bone in tissue culture

The effect of cadmium (Cd) on bone collagen synthesis was assessed in organ cultures of embryonic femur by measuring the incorporation of [3H]-proline(Pro) into collagenous-digestible protein (CDP) using purified bacterial collagenase. Cd produced a decrease when [3H]Pro ws incorporated in CDP. There was little alteration in the hydroxylation of [3H]Pro to [3H]hydroxyproline(Hyp) in CDP of Cd-treated bone. An accumulation of underhydroxylated collagen and a decrese in the activity of prolyl hydroxylase (EC 1.14.11.2) in Cd-treated bone was not observed. These reslts indicated that the inhibitory effect of Cd on collagen synthesis was largely due to inhibition of collagenous peptide synthesis without inhibition of its hydroxylation.

Fetal rat bone in organ culture: effect of bone growth and bone atrophy on the comparative losses of 45Ca and 3H-tetracycline

Fetal rat bones were cultured in either growth-inducing or resorption-inducing media to study mineral losses during bone growth and atrophy in vitro. Whole radii and ulnae from 19-day-old fetal rats, prelabeled with 45Ca and/or 3H-tetracycline, were cultured intact or cut, and then digested by collagenase to obtain the calcified portion of the bones. Three- to five-fold more 3H-tetracycline than 45Ca was lost from the calcified portion when the bones were cultured for 4 days in growth-inducing media. Similar small amounts of 45Ca were lost from live and killed bones, but more 3H-tetracycline was lost from live bones than from killed bones. More 3H-tetracycline was released into the growth medium with a low concentration of calcium (0.5 mM) than when the calcium concentration was high (1.0 mM); no significant difference was seen in the release of 45Ca into the medium at different calcium concentrations. Larger amounts of both isotopes were lost when the prelabeled bones were cultured in resorption-inducing media than in growth-inducing media. When parathyroid hormone stimulated bone resorption in a resorption-inducing medium, equal proportions of both isotopes and bone collagen were lost. Greater losses of 3H-tetracycline than of 45Ca suggest that 45Ca was conserved locally during the resorption that accompanies bone growth, but not during resorption that accompanies bone atrophy.

Leucocyte ascorbic acid and pregnancy

1. Leucocyte ascorbic acid concentrations have been measured in 1147 females during early pregnancy and in smaller numbers of women before conception, throughout pregnancy and at 6 months post partum. 2. The leucocyte concentration in the 1st trimester was found to be affected by season, social class and smoking. Selecting individuals by extremes of social class, season and smoking produced two small populations with almost separate ascorbic acid distributions and mean concentrations of 21.7 and 45.1 microgram/10(8) leucocytes. 3. Early pregnancy had little effect on leucocyte ascorbic acid concentrations but values decreased in the second trimester. However, this was associated with a leucocytosis so that the total leucocyte ascorbic acid content of blood was unchanged. 4. Low ascorbic acid concentrations during the 1st trimester were not associated with subsequent spontaneous abortions, still-births or neonatal deaths, but there was an increased frequency of low values in women who gave birth to infants smaller than 3250 g. 5. The adequacy of ascorbic acid reserves in early pregnancy is discussed.