Regulation of bone formation

Serum vitamin D metabolites are not related to growth rate, bone mineral content, or serum alkaline phosphatase in male puberty

Twenty boys were followed during their puberty for about 2 years with examinations every third month. At each examination we determined serum concentrations of 25OHD3, 1,25(OH)2D3, 24,25(OH)2D3, 25.26(OH)2D3, alkaline phosphatase (AP) and testosterone together with bone mineral content (BMC) at the distal forearm. Highly significant increases in both BMC (P less than 0.001), serum AP (P less than 0.001), and peak height velocity (PHV) followed the increase in serum testosterone. The boys were grouped according to time of maximal increase in BMC, AP, and PHV. The serum levels of the vitamin D metabolites were related to these points. No significant changes in any of the serum vitamin D metabolites were found. Thus vitamin D metabolism does not seem to be significantly influenced during the period of life when both the linear growth and bone mineralization is maximal.

Simultaneous measurement of bone resorption and collagen synthesis in neonatal mouse calvaria

A tissue culture technique which permits the simultaneous measurement of collagen synthesis and bone resorption has been developed. Cultured neonatal mouse calvaria undergo resorption when stimulated by a number of agents including parathyroid hormone, vitamin D, and prostaglandin E2. Mouse calvaria are of sufficient size to measure the extent of proline incorporation into collagenase-digestible protein. Four chemically diverse stimulators of bone resorption were tested for their effect on collagen synthesis. For each stimulator tested, the dose-response relationships for the stimulation of resorption and the inhibition of collagen synthesis were found to coincide.

Modulation of bone metabolism by two chemically distinct lipopolysaccharide fractions from Bacteroides gingivalis

Two separate species of lipopolysaccharide (LPS) from Bacteroides gingivalis 381 have been isolated. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis demonstrated not only the heterogeneity of each species, but also that they represented high- and low-molecular-weight LPS entities. Although both contained the same carbohydrate and fatty acid components, the proportions of these differed between the LPS species. The direct effects of these two species in modulation of bone resorption and bone collagen and noncollagen protein synthesis have been examined. In a bone resorption assay, these two species stimulated 45Ca release from prelabeled fetal rat bones in a concentration range of 0.5 to 3.0 micrograms/ml. The two LPS species also elicited a 30 to 40% reduction in collagen protein formation at 10 micrograms/ml. Responses of the same order of magnitude were observed with LPS from Salmonella minnesota at 10 micrograms/ml. The higher-molecular-weight LPS species also significantly inhibited noncollagen protein formation. This is the first report that LPS from B. gingivalis 381, a suspected periodontal pathogen, inhibits bone collagen formation and, in conjunction with the bone resorption potency, further implicates LPS in alveolar bone loss associated with periodontal disease.

Effects of exogenous prostanoids on the proliferation of osteoblast-like cells in vitro

The effects of several prostaglandins on the proliferation of secondary cultures of osteoblast-like cells, as measured by the incorporation of [3H]-thymidine into DNA and total DNA content of the cultures, were studied. PGE2 in the concentration range of 10(-8) to 10(-5) M caused a direct, dose-related stimulation of proliferation, while PGF2 alpha and PGD2 were less effective. PGA2 and 6-keto-PGF1 alpha were inactive in the osteoblasts in concentrations of 10(-7) to 10(-6) M. A similar stimulation profile was observed for the induction of ornithine decarboxylase (ODC, L-ornithine decarboxy-lyase, EC 4.1.1.17): the order of potency of the different prostaglandins in the induction of the ODC activity was PGE2 greater than PGF2 alpha = PGD2; again, PGA2 and 6-keto-PGF1 alpha were without effect in concentrations up to 10(-6) M. These results show that the primary prostaglandins, in order of potency PGE2 greater than PGF2 alpha = PGD2, can have a direct, stimulatory effect on the proliferation of osteoblasts, which is closely related to the induction of ODC activity.

Retinoic acid modulation of 1,25(OH)2 vitamin D3 receptors and bioresponse in bone cells: species differences between rat and mouse

Retinoic acid (RA) caused a reduction in the level of 1,25(OH)2D3 receptors to 1/3 of control in rat osteoblast-like cells (ROB) while increasing the receptor level to 3-fold the control in mouse osteoblast-like cells (MOB). Scatchard analysis of receptor binding indicated that there was no change in affinity for 1,25(OH)2D3. The changes in receptor levels required time to develop and were dose-dependent. RA also modulated the ability of cells to respond to 1,25(OH)2D3 as measured by the induction of the enzyme 25(OH)D3-24 hydroxylase. Induction of enzyme activity by 1,25(OH)2D3 closely paralleled receptor level established by RA pretreatment. In MOB, the up-regulation of the receptor occurred despite the action of RA to inhibit DNA, RNA and protein synthesis. However, RA stimulation of 1,25(OH)2D3 receptor levels was blocked by the addition of cycloheximide or actinomycin D, indicating that the up-regulation required protein and RNA synthesis. The opposite effect of RA on mouse and rat cells suggests that important species-dependent factors modulate the action of retinoids on mammalian cells.

Direct effect of parathyroid hormone on the proliferation of osteoblast-like cells; a possible involvement of cyclic AMP

Serum-starved chick osteoblast-like cells (OB cells) and periosteal fibroblasts (PF cells) were used to study the proliferative effects of parathyroid hormone (PTH) and prostaglandin E2 (PGE2). Both PTH (10(-11) to 10(-8) M) and PGE2 (10(-9) to 10(-5) M) had a direct, dose-related effect on the de novo synthesis of DNA in OB cells. The PF cells only showed a dose-dependent effect in the presence of PGE2 (10(-9) to 10(-5) M). The hormonally induced proliferation of these cells was shown to be dependent on cell density and stimulation time. An optimal response for both cell types was observed in the cell density range 1.5 to 3.5 micrograms DNA/2 cm2, when stimulated for 18 hours. As cAMP-enhancing substances (N6-dBcAMP, forskolin and IBMX) could mimic the PTH- and PGE2-induced proliferation in OB cells, the increased DNA synthesis was concluded to be mainly caused by enhanced cAMP concentrations.

Vitamin A and bone formation. Effect of an excess of retinol on bone collagen synthesis in vitro

The influence of an excess of retinol on bone formation was studied by using cultures of embryonic-chick calvaria. Retinol decreased collagen synthesis in a dose-dependent manner, non-collagenous protein synthesis being relatively unaffected. Collagen synthesis was significantly inhibited after 24 h of culture with retinol and was progressively decreased, compared with control cultures containing no retinol, as the period of culture was increased. The effect of retinol on collagen synthesis could be reversed by incubation of calvaria for further periods in retinol-free medium. Incorporation of [3H]thymidine and [3H]uridine into DNA and RNA respectively was not altered by culturing calvaria with retinol for 22 h. These latter findings, and the selectivity for collagen synthesis, all suggested that the effect observed was not a cell-toxicity phenomenon. The effect of retinol on collagen synthesis by chick calvarial osteoblasts was probably direct and not mediated by osteoclasts, since a negligible number of the latter cells is present in chick calvaria. In cultures of neonatal murine calvaria, which contain many osteoclasts, retinol similarly inhibited synthesis of collagen, but not of non-collagenous protein; the concentrations of retinol necessary to produce the response were similar to those required to stimulate bone resorption in vitro.

Mononuclear cell-conditioned medium containing mononuclear cell factor (MCF), homologous with interleukin 1, stimulates collagen and fibronectin synthesis by adherent rheumatoid synovial cells: effects of prostaglandin E2 and indomethacin

Adherent rheumatoid synovial cells produce and release into supernatant culture medium latent collagenase and PGE2. The levels of collagenase and PGE2 can be increased by a soluble factor present in mononuclear cell-conditioned medium, partially purified by gel-filtration, which has homologies with interleukin 1, and is produced by monocyte/macrophages. The synovial cell cultures produce collagens (procollagens) and fibronectin as well. The factor(s) present in the mononuclear cell conditioned medium which increases medium levels of collagenase PGE2 also stimulates synthesis of total protein as well as types I and III procollagen by the synovial cells. This stimulation by the monocyte factor is augmented in the presence of indomethacin, which blocks endogenous PGE2 production. Medium levels of fibronectin parallel those of procollagen. The addition of exogenous PGE2 abolishes the effect of indomethacin on collagen and fibronectin synthesis. These observations of mononuclear cell-mediated increases in fibronectin synthesis may account for the high levels of fibronectin found by others in rheumatoid synovium and synovial fluids as the increases in collagen synthesis might also explain the fibrosis observed in some rheumatoid joints.

Immune interferon inhibits collagen synthesis by rheumatoid synovial cells associated with decreased levels of the procollagen mRNAs

Recombinant immune interferon, (interferon-gamma, IFN-gamma) inhibits types I and III collagen synthesis by rheumatoid synovial fibroblast-like cells in culture. This decrease is associated with a decrease in the levels of types I and III procollagen mRNAs in these cells as measured by dot blot hybridization. In the control synovial cells the level of alpha 2(I) mRNA is disproportionately high compared with that of alpha 1(I) or alpha 1(III) mRNA, and IFN-gamma suppresses the level of alpha 1(I) and alpha 1(III) mRNA to a greater extent than that of alpha 2(I) mRNA. The lymphokine, IFN-gamma, may thus have a role in the regulation of collagen synthesis in inflammatory joint disease and other conditions.

Patterns and mechanisms of localized bone invasion by tumors: studies with squamous carcinomas of the head and neck

Squamous carcinomas of the head and neck provide a useful model for analyzing patterns and mechanisms of tumor-associated bone destruction. Morphological studies show that a major part of the invasive process is mediated by local osteoclasts which erode bone in front of the advancing tumor. Functional studies indicate that both fresh tumors and tumor cell lines resorb calvarial bone in an in vitro test system, again by stimulating local osteoclasts. Prostaglandins of the E2 type are regularly released by the tumors, together with indomethacin-resistant, nonprostaglandin osteolysins. Control (nonneoplastic) tissues will resorb bone and release osteolytic factors, usually at lower levels of activity--such properties are thus tumor-associated rather than tumor-specific. Xenografts of squamous carcinomas resorb bone in vitro and synthesize osteolysins. They do not invade local bone in their hosts but some grafts regularly produce a systemic hypercalcemia. General implications are discussed, particularly for other human tumors which more frequently metastasize to bone. Possible pointers to the (partial) control of the destructive process are noted.

Absence of menopausal effect on hip fracture occurrence in white females

The rate of hip fracture among White females rises sharply between ages 40 and 44 and then continues at a constant rate of acceleration doubling every five to six years throughout life with no deviation during, or in the years immediately following, menopause. We suggest that the important role of sex hormones and other factors in osteoporosis commences prior to menopause. A premenopause prevention strategy which postpones the onset of the osteoporotic process by five or six years would be expected to reduce the risk of hip fracture by 50 per cent throughout the remainder of a woman's life.

Practice characteristics of male and female physician assistants

In a national survey of 3,294 non-military responding physician assistants, sociodemographic and practice characteristics of male and female physician assistants were compared. Differences by specialty, practice setting, size of community, and hours of work were generally similar in nature, but smaller in magnitude, than those reported for physicians, and may not be of practical significance.

Metamorphosis at the sternal rib end: a new method to estimate age at death in white males

While the pubic symphysis and intracortical morphometry have provided successful results in estimating age at death, other methods and sites in the skeleton are needed to improve the accuracy of age estimation. This research is an attempt to develop a new age-determination technique by using the sternal extremity of the rib. The right fourth rib was collected at autopsy from 93 white males. The sternal extremity of each rib was analyzed in relation to the pit depth (component I), pit shape (component II), and rim and wall configurations (component III), each of which was divided into six stages. Results indicated that the age at death can be estimated from a rib within about 2 years in the second decade to about 7 years in the fifth and sixth decades of life. Pit shape and rim and wall configurations yielded better results than absolute pit depth alone. While this method has a potentially important contribution to skeletal anthropology, factors such as sex differences and biomechanical variation between individuals may affect the determination of age from the rib.

Insulin-like growth factor I supports differentiation of cultured osteoblast-like cells

Rat calvaria cells grown in culture for one week had properties of osteoblasts: a high content in alkaline phosphatase and a marked cyclic AMP response to parathyroid hormone (PTH). In short-term experiments, insulin-like growth factor I (IGF I) stimulated the incorporation of [14C] glucose into glycogen. When IGF I was present in the medium during 6 days the cell number increased slightly and there was a substantial, disproportionate rise in alkaline phosphatase activity of the cultures. Thus, IGF I stimulates growth, and in addition, and in contrast to other growth factors, mainly enhances differentiation of osteoblasts.

Autoantibodies against parathyroid hormone in a patient with terminal renal insufficiency

An autoantibody against mid-C-regional, possibly mid-regional, parathyroid hormone was detected in the plasma of a patient with terminal renal insufficiency who was on intermittent haemodialysis. 43-68(Tyr)hPTH and 42-55(Tyr)hPTH were selectively bound by his IgG fraction. 43-68(Tyr)hPTH had the highest affinity for the autoantibody; intact parathyroid hormone did not displace radiolabelled 42-55(Tyr)hPTH or 43-68(Tyr)hPTH. As concentrations of circulating antibodies against mid-C/mid-regional parathyroid hormone fell rises in intact and mid-C-regional parathyroid hormone levels and in alkaline phosphatase activity were observed. This autoantibody directed against the middle portion of the parathyroid hormone molecule seemed to have some protective properties against the osteoblast-stimulating activity of the hormone, implying that the mid-C-region or middle region of the molecule has some biological importance.

Stimulation of bone formation by prostaglandin E2

We examined the effect of prostaglandin E2 (PGE2), in the presence or absence of cortisol, on bone formation in 21-day fetal rat calvaria maintained in organ culture for 24 to 96 h. [3H]Thymidine and [3H] proline incorporation were used to assess DNA and collagen synthesis, respectively. Changes in dry weight and DNA content were assessed after 96 h. PGE2 (10(-7) M) stimulated both DNA and collagen synthesis in calvaria. The effect on DNA synthesis was early (24 h), transient and limited to the periosteum. Collagen synthesis was stimulated at a later time (96 h), predominantly in the central bone. Cortisol (10(-7) M) inhibited DNA and collagen synthesis. The addition of PGE2 reversed the inhibitory effects of cortisol on DNA synthesis and content and increased collagen synthesis in central bone to levels above control untreated cultures. We conclude that PGE2 has stimulatory effects on bone formation and can reverse the inhibitory effects of cortisol. Hence the effects of cortisol may be mediated in part by their ability to reduce the endogenous production of prostaglandins.

The cellular basis of bone remodeling: the quantum concept reexamined in light of recent advances in the cell biology of bone

The cellular basis of the normal bone remodeling sequence in the human adult is discussed in relation to a cycle of five stages--quiescence, activation, resorption, reversal, formation, and return to quiescence. Normally, 80% or more of free bone surfaces are quiescent with respect to remodeling. The structure of the quiescent surface comprises 5 layers; listed in order out toward the bone marrow these are: the lamina limitans (the electron dense outer edge of the mineralized bone matrix), unmineralized connective tissue that may be confused with osteoid by light microscopy, flattened lining cells of osteoblast lineage separated by narrow gaps, more unmineralized connective tissue, and finally either the squamous sac cells of red marrow or the cytoplasm of fat cells of yellow marrow. Activation requires the recruitment of new osteoclasts derived from precursor cells of the mononuclear phagocyte system (and so ultimately from the hematopoietic stem cell), a method for precursor cells to penetrate the cellular and connective tissue barrier of the quiescent surface, and so gain access to the bone mineral, and mechanisms for their attraction and binding to the mineralized surface, possibly in response to chemotactic signals released from bone matrix or mineral. Each of these three steps is probably mediated in some way by lining cells. Resorption is carried out by osteoclasts, most of which are multinucleated. The mean life span of individual nuclei is about 12.5 days; the additional nuclei needed to sustain resorption may be derived from local as well as blood-bone precursors, but nothing is known of their fate.(ABSTRACT TRUNCATED AT 250 WORDS)