Evidence for a diminished maturation of preosteoblasts into osteoblasts during aging in rats: an ultrastructural analysis

The effect of aging on distraction osteogenesis in the rat

The effect of age on bone formation in the limb lengthening model of distraction osteogenesis (DO) was investigated in two studies using Sprague-Dawley (SD) rats from two colonies at various ages (CAMM: 9 vs 24 months, Harlan: 4 vs 24 months). External fixators were placed on the right tibiae of 30 male SD rats (20 CAMM, 10 Harlan) and mid-diaphyseal osteotomies were performed. Distraction was performed at 0.2 mm bid for 20 days (CAMM) or 14 days (Harlan). The experimental (DO) and control (contra-lateral) tibiae were removed for high-resolution radiography and decalcified histology. Videomicroscopy was used to quantitate radiodensity, histology (matrix type) and relative areas of cell proliferation, which was identified by proliferating cell nuclear antigen (PCNA) immunochemistry. Both studies demonstrated an age-related decrease in the percent mineralized bone (radiodensity) in the distraction gap (CAMM 9 vs 24 months: 68% vs 51%, P < 0.003; Harlan 4 vs 24 months: 95% vs 36%, P < 0.001) and no significant colony or distraction time-specific difference was seen between the two colonies of 24-month-old rats. Histology was performed on the Harlan rats. The DO gaps in the 24-month-old rats demonstrated less endosteal new bone compared to the 4-month-old rats (P < 0.01), but equivalent periosteal new bone. In 4-month-old rats, PCNA-immunostained cells were organized along the primary matrix front (where the first deposition of osteoid occurs) extending across both periosteal and endosteal surfaces. In 24-month-old rats, PCNA+ cells were organized in zones along the periosteal new bone fronts only and irregularly scattered throughout the endosteal gap within a fibrovascular non-ossifying matrix. These results indicate that 24-month-old rats have a relative deficit in endosteal bone formation which may not be related to cell proliferation but rather to cell organization. This model reflects the clinical situation where radiographic findings in older patients demonstrate significant delays in mineralization during DO. We believe this model of DO in aged rats presents unique in vivo opportunities to test hypotheses concerning (1) the effects of aging on bone repair, (2) the effects of pharmacological agents on bone repair in a geriatric setting, and (3) to study the mechanisms underlying DO.

Lasofoxifene (CP-336,156) protects against the age-related changes in bone mass, bone strength, and total serum cholesterol in intact aged male rats

The purpose of this study was to evaluate if long-term (6 months) treatment with lasofoxifene (LAS), a new selective estrogen receptor modulator (SERM), can protect against age-related changes in bone mass and bone strength in intact aged male rats. Sprague-Dawley male rats at 15 months of age were treated (daily oral gavage) with either vehicle (n = 12) or LAS at 0.01 mg/kg per day (n = 12) or 0.1 mg/kg per day (n = 11) for 6 months. A group of 15 rats was necropsied at 15 months of age and served as basal controls. No significant change was found in body weight between basal and vehicle controls. However, an age-related increase in fat body mass (+42%) and decrease in lean body mass (-8.5%) was observed in controls. Compared with vehicle controls, LAS at both doses significantly decreased body weight and fat body mass but did not affect lean body mass. No significant difference was found in prostate wet weight among all groups. Total serum cholesterol was significantly decreased in all LAS-treated rats compared with both the basal and the vehicle controls. Both doses of LAS treatment completely prevented the age-related increase in serum osteocalcin. Peripheral quantitative computerized tomography (pQCT) analysis at the distal femoral metaphysis indicated that the age-related decrease in total density, trabecular density, and cortical thickness was completely prevented by treatment with LAS at 0.01 mg/kg per day or 0.1 mg/kg per day. Histomorphometric analysis of proximal tibial cancellous bone showed an age-related decrease in trabecular bone volume (TBV; -46%), trabecular number (Tb.N), wall thickness (W.Th), mineral apposition rate, and bone formation rate-tissue area referent. Moreover, an age-related increase in trabecular separation (Tb.Sp) and eroded surface was observed. LAS at 0.01 mg/kg per day or 0.1 mg/kg per day completely prevented these age-related changes in bone mass, bone structure, and bone turnover. Similarly, the age-related decrease in TBV and trabecular thickness (Tb.Th) and the age-related increase in osteoclast number (Oc.N) and osteoclast surface (Oc.S) in the third lumbar vertebral cancellous bone were completely prevented by treatment with LAS at both doses. Further, LAS at both doses completely prevented the age-related decrease in ultimate strength (-47%) and stiffness (-37%) of the fifth lumbar vertebral body. These results show that treatment with LAS for 6 months in male rats completely prevents the age-related decreases in bone mass and bone strength by inhibiting the increased bone resorption and bone turnover associated with aging. Further, LAS reduced total serum cholesterol and did not affect the prostate weight in these rats. Our data support the potential use of a SERM for protecting against the age-related changes in bone and serum cholesterol in elderly men.

Age- and gender-related changes in the cellularity of human bone marrow and the prevalence of osteoblastic progenitors

Bone marrow harvested by aspiration contains connective tissue progenitor cells which can be induced to express a bone phenotype in vitro. The number of osteoblastic progenitors can be estimated by counting the colony-forming units which express alkaline phosphatase (CFU-APs). This study was undertaken to test the hypothesis that human aging is associated with a significant change in the number or prevalence of osteoblastic progenitors in the bone marrow. Four 2-ml bone marrow aspirates were harvested bilaterally from the anterior iliac crest of 57 patients, 31 men (age 15-83) and 26 women (age 13-79). A mean of 64 million nucleated cells was harvested per aspirate. The mean prevalence of CFU-APs was found to be 55 per million nucleated cells. These data revealed a significant age-related decline in the number of nucleated cells harvested per aspirate for both men and women (P = 0.002). The number of CFU-APs harvested per aspirate also decreased significantly with age for women (P = 0.02), but not for men (P = 0.3). These findings are relevant to the harvest of bone marrow derived connective tissue progenitors for bone grafting and other tissue engineering applications, and may also be relevant to the pathophysiology of age-related bone loss and post-menopausal osteoporosis.

Effect of maturation on the osteogenic response of cultured stromal bone marrow cells to basic fibroblast growth factor

Formation of bone-like tissue in culture by stromal bone marrow cells (SBMC) derived from young growing rats is dependent on dexamethasone (Dex) (Cell Tissue Res 254:317; 1988) and is significantly enhanced by basic fibroblast growth factor (bFGF) (J Bone Miner Res 8:919; 1993). The aim of this study was to examine the effect of maturation on the osteogenic potential and the response to Dex and bFGF of SBMC by using cultures derived from young growing (6 weeks old) and adult (9 months old) rats. SBMC cultures were grown in the presence of Dex (10(-8) or 10(-7) mol/L) at both P(0) and P(1) and either in the presence or absence of bFGF. The effect of Dex and bFGF on mineralized bone-like tissue (MBT) formation was assessed at P(1). The highest levels of mineralized tissue formation in P(1) subcultures in the absence of bFGF were obtained when cultures derived from young rats (6 weeks old) were treated with Dex 10(-7) and 10(-8) mol/L at P(0) and P(1), respectively, and when cultures derived from adult rats were exposed to Dex 10(-8) mol/L both at P(0) and P(1). Under these optimal Dex concentrations, the amount of MBT formed by adult rat-derived cultures was 15-fold lower than that of young rat-derived ones. The addition of bFGF to P(0) cultures or to P(1) cultures grown under optimal Dex conditions enhanced MBT formation in P(1) cultures derived from both young and adult rats, but this effect was considerably more pronounced in the adult rat-derived cultures. The maximal levels of MBT formation were produced by cultures derived from adult rats treated with bFGF at both P(0) and P(1), whereas in cultures derived from young rats, the addition of bFGF at P(0) was not necessary for maximal MBT production. This stimulating effect of bFGF on MBT formation by adult rat-derived cultures was accompanied by a 2.2-, 1.8-, and 4.3-fold increase in proliferation, alkaline phosphatase activity, and Ca(2+) deposition rate, respectively. bFGF increased the level of glucocorticoid receptor by approximately 2. 3-fold in Dex-treated cultures derived from young animals. These results indicate that maturation is associated with a decrease in the proportion of osteoprogenitor cells in the stromal bone marrow and in their capacity to express the osteogenic phenotype. They further point to the significant role of bFGF in stimulating proliferation and osteogenic expression of stromal bone marrow osteoprogenitors derived from adult rats.

Identification of osteoblast/osteocyte factor 45 (OF45), a bone-specific cDNA encoding an RGD-containing protein that is highly expressed in osteoblasts and osteocytes

We describe the cloning and characterization of a novel bone-specific cDNA predicted to encode an extracellular matrix protein. This cDNA was identified by subtractive hybridization based upon its high expression in bone marrow-derived osteoblasts. By Northern blot analysis, we detected a single 2-kilobase mRNA transcript in bone, whereas no expression was detected in other tissues. Immunohistochemistry revealed that the protein was expressed highly in osteocytes within trabecular and cortical bone. RNA and protein expression analysis using in vivo marrow ablation as a model of bone remodeling demonstrated that this gene was expressed only in cells that were embedded within bone matrix in contrast to the earlier expression of known osteoblast markers. The cDNA was predicted to encode a serine/glycine-rich secreted peptide containing numerous potential phosphorylation sites and one RGD sequence motif. The interaction of RGD domain-containing peptides with integrins has been shown previously to regulate bone remodeling by promoting recruitment, attachment, and differentiation of osteoblasts and osteoclasts. Secretion of this RGD-containing protein from osteocytes has the potential to regulate cellular activities within the bone environment and thereby may impact bone homeostasis. We propose the name OF45 (osteoblast/osteocyte factor of 45 kDa) for this novel cDNA.

Decreased cellular activity and replicative capacity of osteoblastic cells isolated from the periarticular bone of rheumatoid arthritis patients compared with osteoarthritis patients

OBJECTIVE

Periarticular osteopenia is frequently observed in rheumatoid arthritis (RA). Bone loss has been considered to be at least partly due to inadequate bone formation, which in turn, is largely dependent on the number of osteoblasts and the osteoblastic activity. Normal human somatic cells undergo a finite number of cell divisions and ultimately enter a nondividing state called replicative senescence. It has been proposed that the telomere, the terminal sequence of chromosomes, is the mitotic clock that triggers senescence. In the present study, we sought to clarify the relationship between periarticular osteopenia and osteoblast replicative senescence in RA.

METHODS

We examined age-related changes in cellular activity (alkaline phosphatase activity, osteocalcin and C-terminal type I procollagen secretion, and cAMP response to parathyroid hormone), replicative capacity, and senescent cell expression in osteoblasts from periarticular bone samples obtained from 15 patients with RA and 15 age-matched patients with osteoarthritis (OA). Cellular replicative capacity was analyzed by the mean telomere length and in vitro remaining replicative lifespan of the cells.

RESULTS

In both OA and RA groups, the cell proliferation rate, the levels of osteoblastic markers, mean telomere length, and replicative lifespan in osteoblastic cells gradually decreased with the increasing age of the donor. The percentage of senescent osteoblastic cells in the periarticular bone increased with age in both groups, and the rate of expression of senescent cells was higher in RA patients than in age-matched OA patients. The osteoblastic activities and replicative capacity of osteoblastic cells from RA patients were lower than those from OA patients at any donor age. The age-related decreases in the osteoblastic activity and replicative capacity of osteoblastic cells from periarticular bone were greater in RA patients than in OA patients.

CONCLUSION

Our results suggest that osteoblast replicative senescence in periarticular bones occurs more rapidly with aging in RA than in OA patients and contributes to periarticular osteopenia in RA.

Anabolic effect of prostaglandin E2 on cortical bone of aged male rats comes mainly from modeling-dependent bone gain

In this study, prostaglandin E2 (3 mg/kg per day) was administered to 20-month-old male Wistar rats for 10 and 30 days. Histomorphometric analyses were performed on double-fluorescent-labeled undecalcified tibial shaft sections. Thirty days of prostaglandin E2 (PGE2) administration increased bone formation rate/total bone surface from undetectable levels to 0.6 microm/day at the periosteal surface and from 0.5 to 2.1 microm/day at the endocortical surface. Endocortical osteoid surface area increased from 2% to 67% at day 10 and decreased to 6% at day 30; woven and lamellar bone formation started at day 0, but was most obvious at day 30, resulting in a 12% increase of total bone mass. The red to yellow marrow ratio was 0.2 in pretreatment controls, and increased to 1.6 by day 10 and 2.4 by day 30 with PGE2 administration. Intracortical cavity number and area increased after 10 days of PGE2 treatment, but with forming osteon number and area far exceeding those of resorption cavities at day 30. Endocortical modeling surface/endocortical surface was only 1.5%, and remodeling was 11.1% in pretreatment controls. PGE2 treatment increased modeling to 24.5% in the 10 day group and 93.7% in the 30 day group, whereas remodeling remained unchanged at 10 days, and decreased to 6.2% at 30 days. Osteoprogenitor cells and osteoblasts could not be detected in pretreatment controls, but increased by day 10, and returned almost to control levels by 30 days. Our data indicate that PGE2 induced periosteal and endocortical bone formation mainly by modeling-dependent bone gain, accompanied by increases in intracortical remodeling and red bone marrow, and a transient increase in the osteoprogenitor cells adjacent to the endocortical surface. These findings suggest that 20-month-old male Wistar rats were very responsive to the anabolic action of PGE2 in the tibial shaft, a site consisting mainly of cortical bone and yellow marrow.

Age-related inhibitory activity of rat bone marrow supernatant on osteoblast proliferation

Because histomorphometric indices of bone formation (osteoblastic index, tetracyclin-labeled perimeter) are deeply depressed in aged rats, while in vitro proliferation of trabecular bone cells was found increased, we hypothesized that a signal to proliferate, correctly induced by increased strains on scarce bone, could be opposed in vivo by an inhibitor present in the bone marrow extracellular medium. Thus, we tested the effect of bone marrow extracellular fluid (BM supernatant) of rat femoral diaphysis on cultures of primary osteoblasts and osteoblastic cell lines and found that it inhibited bone cell proliferation. In a group of 69 female rats aged 4, 12, and 15/21 months, there was a stepwise increase in the inhibitory activity of the BM supernatant. The double reciprocal plots relating inhibition power of the medium to BM supernatant dilution suggest that we deal with a simple system and that the kinetics of the phenomenon are the same in older and younger animals. Moreover, proliferation inhibition by BM supernatant and trabecular bone surface measured by histomorphometry in the distal femoral metaphysis were inversely correlated. Because the extracellular fluid of bone marrow is also the medium surrounding the osteoblasts and their precursor cells, our results suggest that the bone marrow negatively regulates osteogenic cells and that this inhibition could contribute to the inability of older animals to supply osteoblasts to bone in proportion to the demand. Preliminary biochemical characterization of the inhibitor suggests it to be a protein of 30-40 kDa with an isoelectric point (pI) of about 6.5.

Age-associated changes in the stimulatory effect of transforming growth factor beta on human osteogenic colony formation

Previous studies have indicated that the mitogenic responsiveness of human bone cells may change with age. In the present study, we examined whether aging affects the capacity of transforming growth factor beta (TGF-beta) to stimulate the colony formation of human osteoprogenitor cells. Outgrowths of bone cells from 98 iliac crest biopsies were plated at a density of 25 cells/cm2 and cultured for 3 weeks in the presence of 10% fetal calf serum. Approximately 5% of the plated cells gave rise to clonal colonies. TGF-beta (10(-11) M) significantly increased the estimated number of cells per colony. However, the stimulatory effect of TGF-beta significantly declined with donor age (r = -0.26, P = 0.01). Whereas TGF-beta raised the average number of cells per colony in cultures from donors below the age of 50 years by 136+/-50%, the average increase was only 43+/-16% in donors older than 60 years. These data raise the possibility that aging may be associated with a declining capacity of TGF-beta to enlarge the pool of bone cells that can be generated from a single human osteoblast progenitor cell.

Age-related osteogenic potential of mesenchymal stromal stem cells from human vertebral bone marrow

Mesenchymal stem cells (MSCs) residing in bone marrow (BM) are the progenitors for osteoblasts and for several other cell types. In humans, the age-related decrease in bone mass could reflect decreased osteoblasts secondary to an age-related loss of osteoprogenitors. To test this hypothesis, BM cells were isolated from vertebral bodies of thoracic and lumbar spine (T1-L5) from 41 donors (16 women and 25 men) of various ages (3-70 years old) after death from traumatic injury. Primary cultures were grown in alpha modified essential medium with fetal bovine serum for 13 days until adherent cells formed colonies (CFU-Fs). Colonies that stained positive for alkaline phosphatase activity (CFU-F/ALP+) were considered to have osteogenic potential. BM nucleated cells were plated (0.5, 1, 2.5, 5, or 10 x 106 cells/10-cm dish) and grown in dexamethasone (Dex), which promotes osteoblastic differentiation. The optimal plating efficiency using BM-derived cells from donors of various ages was 5 x 106 cells/10-cm dish. BM-derived cells were also grown in the absence of Dex at this plating density. At the optimal plating density, in the presence of Dex, the number of CFU-F/ALP+ present in the BM of the younger donors (3-36 years old) was 66.2 +/- 9.6 per 106 cells (mean +/- SEM), but only 14.7 +/- 2.6 per 106 cells in the older donors (41-70 years old). With longer-term culture (4-5 weeks) of these BM cells in medium containing 10 mM beta-glycerophosphate and 100 microg/ml ascorbic acid, the extracellular matrix mineralized, a result consistent with mature osteoblastic function. These results demonstrate that the number of MSCs with osteogenic potential (CFU-F/ALP+) decreases early during aging in humans and may be responsible for the age-related reduction in osteoblast number. Our results are particularly important in that the vertebrae are a site of high turnover osteoporosis and, possibly, the earliest site of bone loss in age-related osteoporosis.

Influence of skeletal site of origin and donor age on 1,25(OH)2D3-induced response of various osteoblastic markers in human osteoblastic cells

Age-related bone loss may be a consequence of a lack of osteoblastic formation and/or function. In vitro, the osteoblastic response to 1,25(OH)2D3, an important regulator of osteoblastic function, appears to depend on the stage of osteoblastic maturation. In this study, we examined the response to 1,25(OH)2D3 of C-terminal type I procollagen (PICP), alkaline phosphatase (ALP), and osteocalcin (OC) secretion in primary cultures of osteoblastic cells from human trabecular bone (hOB). Forty-four bone samples were obtained from subjects undergoing knee arthroplastia, 20 aged 50-70 (64 +/- 5), and 24 >70 (73 +/- 2) years. Another 33 bone samples were obtained from subjects undergoing hip arthroplastia, 21 were aged 50-70 (64 +/- 4) and 12 >70 (75 +/- 5) years. Pooling knee and hip hOB cell cultures, we found that PICP secretion decreased after 1,25(OH)2D3 in hOB cells from the older group (>70 years). Treatment with 1,25(OH)2D3 increased ALP secretion in these cells only in the younger group (50-70 years), whereas it increased OC secretion in hOB cells in both age groups. By pooling hOB cell cultures from both age groups we found that knee hOB cells increased OC secretion, and decreased PICP secretion, after 1,25(OH)2D3. This metabolite also increased OC secretion in hip hOB cells. Considering the influence of donor age at the same skeletal site, 1,25(OH)2D3 was found to stimulate ALP secretion only in knee hOB cells in the younger group. In contrast, this metabolite decreased ALP secretion in hip hOB cells in the older group. PICP secretion decreased after 1,25(OH)2D3 only in hOB cells in the older group, at both skeletal sites. In age-matched cultures, OC secretion was lower in hip hOB cells compared with those from the knee in the older group, but was similar in these cell cultures from both skeletal sites in the younger group. OC secretion after 1,25(OH)2D3 stimulation did not show age differences in knee hOB cells, but was lower in hip hOB in the older group. In summary, our results demonstrate that the response of various osteoblastic markers to 1,25(OH)2D3 in primary cultures of hOB cells depends on the donor age and skeletal site of origin.

Bone loss (osteopenia) in old male mice results from diminished activity and availability of TGF-beta

One of the universal characteristics of the long bones and spines of middle-age and older mammals is a loss in bone mass (osteopenia). In humans, if this bone loss is severe enough, it results in osteoporosis, a skeletal disorder characterized by a markedly increased incidence of fractures with sequelae that may include pain, loss of mobility, and in the event of hip fracture, even death within a relatively few months of injury. An important contributing factor to the development of osteoporosis appears to be a diminution in the number and activity of osteoblasts responsible for synthesizing new bone matrix. The findings in the present and other similar studies suggest that this reduction in osteoblast number and activity is due to an age-related diminution in the size and osteogenic potential of the bone marrow osteoblast progenitor cell (OPC or CFU-f) compartment. We previously postulated that these regressive changes in the OPC/CFU-f compartment occurred in old animals because of a reduction in the amount and/or activity of TGF-beta1, an autocrine growth factor important in the promotion of OPC/CFU-f proliferation and differentiation. In support of this hypothesis, we now report that (1) the osteogenic capacity of the bone marrow of 24-month-old BALB/c mice, as assessed in vivo, is markedly reduced relative to that of 3-4-month-old animals, (2) that the matrix of the long bones of old mice contains significantly less TGF-beta than that of young mice, (3) that OPC's/CFU-f's isolated from old mice produce less TGF-beta in vitro than those recovered from young mice, and (4) that OPC's/CFU-f's from old mice express significantly more TGF-beta receptor (Types I, II, and III) than those of young animals and that such cells are more responsive in vitro to exogenous recombinant TGF-beta1. We also find that colony number and proliferative activity of OPC's/CFU-f's of young mice and old mice, respectively, are significantly reduced when incubated in the presence of neutralizing TGF-beta1 antibody. Collectively, these data are consistent with the hypothesis that in old male mice the reduction in the synthesis and, perhaps, availability from the bone matrix of TGF-beta1 contributes to a diminution in the size and development potential of the bone marrow osteoprogenitor pool.

Effect of aging on functional changes of periodontal tissue cells

Although the severity of periodontal disease is known to be affected by age, functional changes of periodontal tissue cells during the aging process are not well characterized. It is important to define how cellular aging affects the progression of periodontal diseases associated with the aging process. In vitro aging of human gingival fibroblast (HGF) and periodontal ligament fibroblast (HPLF) cells was prepared by sequential subcultivations (5 to 6 passages as young, 18 to 20 passages as old). GFs were also prepared from gingiva of Down's syndrome patients and 60-week-old rats. Fetal rat calvarial osteoblasts were prepared by sequential digestion with collagenase. HGF and HPLF cells were treated with lipopolysaccharide (LPS) and cyclic tension force, respectively. Amounts of PGE2, interleukin (IL)-1 beta, IL-6, and plasminogen activator (PA) in conditioned media were measured. Total RNA was extracted, and mRNA expression was analyzed by reverse transcription polymerase chain reaction (RT-PCR). LPS-stimulated PGE2, IL-1 beta, IL-6, and PA production was increased in "old" HGF compared to younger cells. According to RT-PCR analysis, gene expression of COX-2, IL-1 beta, IL-6, and tissue type (t) PA was higher in old cells than in young cells. Cyclic tension force to HPLF also stimulated phenotypic and gene expression of IL-1 beta, PGE2 (COX-2 gene) and tPA. These findings suggest that aging in both HGF and HPLF may be an important factor in the severity of periodontal disease through higher production of inflammatory mediators in response to both LPS and mechanical stress. In addition, oxygen radical-treated fibronectin (FN) as substratum diminished bone nodule formation by osteoblasts when compared with intact FN. This finding suggests that FN plays an important role in Osteoblast activity and that FN damaged by oxygen radicals during the aging process may be related to less bone formation.

Withdrawal of parathyroid hormone treatment causes rapid resorption of newly formed vertebral cancellous and endocortical bone in old rats

When administered intermittently, parathyroid hormone (PTH) is a strong anabolic agent, increasing both bone mass and bone mechanical strength and competence. This study evaluates the fate of PTH-induced bone in vertebral bodies after withdrawal of PTH treatment in normal old rats. Sixty-seven 21-month-old male rats were treated with 62 microg/kg/day PTH(1-34) for 8 weeks, followed by saline or bisphosphonate (risedronate, 5 microg/kg twice a week) for another 8 weeks. The rats were scanned by dual-energy X-ray absorptiometry at intervals. The bone mineral content (BMC) of L2-5 increased by 33% during the PTH treatment. The BMC started decreasing shortly after withdrawal of PTH and continued to decline during the 8 weeks after withdrawal of PTH. Risedronate, however, prevented this decrease in BMC. All rats were labeled with tetracycline and calcein 3 weeks and 1 week before the cessation of PTH therapy. In the cancellous bone, PTH increased the mineralized surface: 32.9% +/- 2.8% (mean +/- standard error of the mean) vs. controls 12.0% +/- 1.5%, the mineral appositional rate (0.65 +/- 0.02 to 0.88 +/- 0.06 microm/day), and the cancellous bone volume (BV/TV: 14.5% +/- 0.7% to 27.5% +/- 1.7%). Withdrawal of PTH induced a fast and pronounced bone resorption, decreasing both the extent of the fluorochrome labels and the cancellous bone volume to control values. Risedronate prevented this resorption. In the cortical bone of the vertebral shell, PTH induced large increases in the endocortical mineralized surface, mineral appositional rate, and cortical area. The endocortical fluorochrome labels were, however, resorbed after withdrawal of PTH. Risedronate maintained both the fluorochrome labels and the cortical area. At the periosteum, the response to PTH was less evident, however, and hardly any labeling was seen at the periosteum facing the vertebral canal either in the controls or in the PTH-treated rats. The compressive strength of the vertebral body specimens increased with PTH treatment whether measured in newtons (317 +/- 23 to 623 +/- 54 N), normalized to cross-sectional area (23.0 +/- 1.4 to 44.7 +/- 2.5 N/mm2), or to ash content per millimeter height (58 +/- 2 to 76 +/- 2 N x mm/mg). Withdrawal of PTH decreased the compressive strength and competence to control values. Risedronate, however, maintained the PTH-induced mechanical strength and competence. The study discloses that even in very old rats withdrawal of PTH treatment causes a rapid and pronounced decline in the bone mass deposited during PTH treatment; treatment with risedronate can, however, maintain the PTH-induced bone properties in the axial skeleton of old rats.

Abnormal cancellous bone collagen metabolism in osteoarthritis

Biochemical investigations into the pathogenesis of osteoarthritis have, for the last two decades, concentrated on the mechanisms involved in the destruction of the articular cartilage. Although bone changes are known to occur, the biochemistry of the collagenous matrix within osteoarthritic bone has received scant attention. We report that bone collagen metabolism is increased within osteoarthritic femoral heads, with the greatest changes occurring within the subchondral zone. Collagen synthesis and its potential to mineralize were determined by the carboxy-terminal propeptide content and alkaline phosphatase activity, respectively. These data supported elevated new matrix formation. Our finding of a three- to fourfold increase in TGF-beta in osteoarthritic bone indicates that this might represent a stimulus for the increased collagen synthesis observed. Of additional significance is the hypomineralization of deposited collagen in the subchondral zone of osteoarthritic femoral heads, supporting a greater proportion of osteoid in the diseased tissue. The cross-linking of collagen was similar to that observed for controls. In addition, the degradative potential of osteoarthritic bone was considerably higher as demonstrated by increased matrix metalloproteinase 2 activity, and again the greater activity was associated with the subchondral bone tissue. The polarization exhibited in the metabolism of bone collagen from osteoarthritic hips might exacerbate the processes involved in joint deterioration by altering joint morphology. This in turn may alter the distribution of mechanical forces to the various tissues, to which bone is a sensitive responder. Bone collagen metabolism is clearly an important factor in the pathogenesis of osteoarthritis and certainly warrants further biochemical study.

The decrease in bone mass associated with aging and menopause

The human skeleton accumulates bone up to approximately age 30, after which bone is gradually lost. Although estrogen replacement therapy prevents postmenopausal bone loss, it is not certain that estrogen deficiency alone is responsible for the decrease in bone mass. Progesterone deficiency could also be a factor, and progesterone replacement therapy has been shown to prevent postmenopausal bone loss associated with ovarian dysfunction. This article reviews what is known about bone remodeling and bone loss as a function of age and gender, discusses evidence from studies in rats that progesterone plays an important role in regulating bone formation, and suggests directions for future studies in predicting the success or failure of implant therapy based on the number and kinds of osteoprogenitor cells present.

H2O2-derived free radicals treated fibronectin substratum reduces the bone nodule formation of rat calvarial osteoblast

Fibronectin (FN) is involved in various cellular activities such as adhesion, proliferation and migration as a substratum. Since the metabolic turnover of FN is much slower than other cellular components, it may be affected by the oxygen free radicals produced in the aging process. However, the effect of oxygen free radicals on FN as substratum in bone formation has not been well characterized. The objective of this study was to examine the effect on the bone forming activity of osteoblasts using an oxygen free radical treated FN substratum in vitro (H2O2-Cu2+system). SDS-PAGE, Western blotting and immuno-blotting analysis revealed that FN was degradated and/or modified by H2O2-Cu2+ (.OH) treatment. Bone nodule formation per well was examined for total number, total area and area per nodule, which data were then compared between non-coated and FN-coated, and between FN-coated and .OH treated FN-coated. Bone nodule formation in the FN-coated was significantly greater than in the non-coated. Furthermore, bone nodule formation in .OH treated FN-coated was significantly less than that of FN-coated. These findings suggested that FN plays important roles in osteoblast activity and that FN substratum damaged by the oxygen free radicals produced by the aging process may cause decline of bone nodule formation through inhibition of the proliferation, differentiation and calcification processes.

Age-related decreases in stimulatory G protein-coupled adenylate cyclase activity in osteoblastic cells

In this study we examined parathyroid hormone (PTH)-, forskolin (FSK)-, and cholera toxin (CTX)-stimulated adenosine 3',5'-cyclic monophosphate (cAMP) accumulation in rat osteoblastic cells (ROB) isolated from young (4 mo), mature (12 mo), and old (24-28 mo) male rats. Exposure to PTH increased cAMP accumulation in a concentration-dependent manner in all ROB cells examined. However, the maximum response in ROB from young rats was threefold greater than the maximum response in those from mature and old rats. Exposure to FSK also stimulated cAMP accumulation in a concentration-dependent manner, but there were no significant differences in responsiveness among ROB isolated from young, mature, and old rats. Exposure to CTX resulted in a dramatic concentration-dependent increase in cAMP in ROB from young rats but only a modest increase in ROB from mature and old rats. PTH binding kinetics were similar in ROB from rats in each age group. These data suggest an age-related defect in stimulatory G protein coupling to adenylate cyclase, which contributes to decreased osteoblastic responsiveness to PTH.

Progesterone-mediated stimulation of osteoprogenitor proliferation and differentiation in cell populations derived from adult or fetal rat bone tissue depends on the serum component of the culture media

We have shown previously that progesterone (Prog) and dexamethasone (Dex) stimulate osteoprogenitor proliferation and differentiation in cell populations derived from adult rat vertebrae and in primary cultures of fetal rat calvariae. In these two in vitro systems, osteoprogenitors can be identified by the appearance of colonies of differentiated osteoblasts producing bone (bone nodule formation). Culture conditions supporting proliferation and differentiation of osteoprogenitors include a requirement for the presence of serum in the culture media. Our major interest in the present study was to investigate whether Prog- and Dex-mediated osteoprogenitor proliferation and differentiation was observed to the same degree in different lots of fetal bovine serum (FBS). In addition, we wanted to investigate whether osteoprogenitors present in cell populations derived from fetal calvarial bone and those present in populations derived from adult vertebral bone would respond similarly under the different culture conditions. We found that, in populations derived from adult rat vertebrae, the effects of the serum component of the culture medium on the number of bone nodules induced by Prog and on the dose-dependency of the Prog effect were striking: in culture media containing the most effective serum the number of bone nodules was 22-fold higher than that in the least effective serum. In addition, Prog responses were detectable at 10(-5) M only in some sera but were significant at 10(-7) M in others. The effect of Dex in the adult rat vertebrae-derived populations was much less dependent on the serum used: the number of bone nodules in culture media containing the most effective serum was only 1.3 times greater than that in media containing the least effective serum. In cell populations derived from fetal calvariae, the serum dependence of the Prog response was less pronounced: a 4.3-fold increase over control was observed in the most effective serum, and a 2.4-fold increase in the least effective serum. No effects of the serum component of the culture medium on the Dex response were detectable. Thus, Prog-induced bone nodule formation appears to be strongly dependent on the particular type of FBS used for osteoprogenitors present in bone cell populations derived from adult rat vertebrae but much less so in populations obtained from fetal rat calvariae. Preliminary experiments suggest that the estrogen content of the culture media may be one of the determinants regulating Prog responsiveness of the osteoprogenitors. Dex-induced proliferation and differentiation of osteoprogenitors in bone cell populations derived from both adult rat vertebrae and fetal rat calvariae, on the other hand, did not appear to be strongly dependent on factor(s) present in the FBS component of the culture medium.

Age-related changes in osteogenic stem cells in mice

Osteoblasts arise from partially differentiated osteogenic progenitor cells (OPCs) which in turn arise from undifferentiated marrow stromal mesenchymal stem cells (MSCs). It has been postulated that age-related defects in osteoblast number and function may be due to quantitative and qualitative stem cell defects. To examine this possibility, we compared osteogenic stem cell number and in vitro function in marrow cells from 4-month-old and 24-month-old male BALB/c mice. Histologic studies demonstrated that these mice undergo age-related bone loss resembling that seen in humans. In primary MSC cultures grown in media supplemented with 10 nM dexamethasone, cultures from older animals yielded an average of 41% fewer OPC colonies per given number of marrow cells plated (p < 0.001). This implies that for a given number of marrow cells there are fewer stem cells with osteogenic potential in older animals than there are in younger animals. The basal proliferative rate in cultures from older animals, as measured by 3H-thymidine uptake, was more than three times that observed in cultures from young animals (p < 0.005). However, the increase in proliferative response to serum stimulation was 10-fold in the younger cultures (p <0.001) and insignificant (p <0.4) in the older cultures. Colonies in both age groups became alkaline phosphatase positive at the same rate, and virtually all colonies were positive after 12 days of culture. Cultures from both age groups produced abundant type I collagen. These studies suggest that defects in the number and proliferative potential of MSCs may underlie age-related defects in osteoblast number and function.

Effects of androgens on subpopulations of the human osteosarcoma cell line SaOS2

Previously, we showed that androgens stimulate murine and human osteoblast-like cell proliferation and differentiation by mechanisms involving increased responses to mitogenic growth factors (GF) and increased GF production. To explain this dual action of androgens on primary osteoblastic cell populations we advanced the hypothesis that androgens exert differential effects on osteoblastic subpopulations. We subcloned a human osteosarcoma cell line (SaOS2) into subpopulations expressing high (HAS) and low (LAS) levels of alkaline phosphatase (ALP). The obtained subclones differed significantly in their ALP production and expressed a high and low ALP phenotype, respectively, for the entire experimental period. Dihydrotestosterone (DHT) increased specific ALP activity and type-I procollagen peptide secretion in both HAS and LAS. DHT pretreatment enhanced the mitogenic action of basic fibroblast growth factor (bFGF) and insulinlike growth factor 2 (IGF2) only in HAS. The enhanced mitogenic effect of IGF2 in HAS after DHT pretreatment was associated with increased IGF2-receptor mRNA levels. Therefore, we conclude that androgens exert their osteoanabolic action (1) by stimulating differentiated functions of osteoblastic cells with a high and a low ALP phenotype, and (2) via increased growth factor receptor expression and thereby enhancing mitogenic growth factor responses only in HAS.

Parathyroid hormone (1-34) increases vertebral bone mass, compressive strength, and quality in old rats

Human parathyroid hormone 1-34 (PTH) exerts an anabolic effect on bone in younger rats. The aim of the present study was to examine the effect of PTH on vertebral bone in 2-year-old male rats. The rats were treated with daily injections of 15 nmol/kg PTH or vehicle (V) for 56 days. Tetracycline and calcein were injected on day 15 and day 40 of the treatment period, respectively. The PTH treatment did not influence the body weights of the rats, the volumes of whole vertebra, or the vertebral body heights. However, the PTH treatment induced profound changes in the bone structure. Histomorphometric analyses of the vertebral bodies (L-6) revealed an approximate doubling of the cancellous bone volume after PTH treatment from 24.6 +/- 1.3% to 54.9 +/- 2.0% (p < 0.001) as well as a doubling of the trabecular thickness while the bone surface/bone volume decreased by 60%. PTH treatment also increased bone formation as indicated by an increase in mineral apposition rate (from 0.42 +/- 0.01 to 0.89 +/- 0.01 microns/day, p < 0.01), increased mineralizing surface (from 7.8 +/- 1.4 to 43.8 +/- 1.9%, p < 0.01) and an increase in both volume-related and surface-related bone formation rates (5 and 11 times, respectively). The biomechanical properties were analyzed using standardized bone specimens from the vertebral bodies of L-4 by applying cranial-caudal compression in a materials testing machine. The PTH treatment induced a substantial increase in the strength of the vertebral body: ultimate load increased by 66%, ultimate stiffness by 47%, and energy absorption by 98%. The increase in vertebral body strength was also evident after normalizing the parameters to the cross sectional area and the ash content of the vertebral body specimens. PTH treatment increased ultimate stress from 26 +/- 3 to 44 +/- 3 N per mm2 (p < 0.01) and increased ultimate load normalized to ash content per mm specimen height from 59 +/- 4 to 72 +/- 4 N (mm/mg) (p < 0.05). The PTH treatment induced an increase in dry defatted bone density and ash density of both the vertebral body specimen (L-4) and the whole vertebra (L-5). In conclusion, PTH showed a remarkable ability to stimulate bone formation in the vertebral body of old rats. Furthermore, the biomechanical analysis revealed an enhanced compressive bone strength, even after correction for the increased bone mass, indicating an improved bone quality after the PTH treatment.

Identification and behavior of epithelial stem cells in the Drosophila ovary

Throughout their lives, adult Drosophila females continuously produce oocytes, each surrounded by an epithelial monolayer of follicle cells. To characterize the somatic stem cells that give rise to ovarian follicle cells, we marked dividing cells using FLP-catalyzed mitotic recombination and analyzed the resulting clones. Each ovariole in young females contains, on average, two somatic stem cells located near the border of germarium regions 2a and 2b. The somatic stem cells do not coordinate their divisions either with each other or with the germline stem cells. As females age, initially mosaic ovarioles become monoclonal, indicating that functional somatic stem cells have a finite life span. Analysis of agametic flies revealed that somatic cells continue to divide in the absence of a germline. Under these conditions, the somatic stem cells develop near the tip of the ovariole (the normal site of the germline stem cells), and a subpopulation of somatic cells that normally separates the germline and somatic stem cells is missing.

Age-dependent expression of osteoblastic phenotypic markers in normal human osteoblasts cultured long-term in the presence of dexamethasone

We have previously shown that osteoblasts derived from trabecular bone explants and cultured long term in 10 nM dexamethasone ((HOB + DEX) cells) exhibited properties consistent with a more differentiated phenotype compared with those grown in the absence of dexamethasone ((HOB-DEX) cells). To characterize these two cell models further, we measured the steady-state mRNA levels of the phenotypic markers alkaline phosphatase (ALP), collagen type I (COLL) and osteocalcin (OC), OC production, and the activities of ALP and parathyroid hormone (PTH)-stimulated adenylate cyclase. These findings were then correlated with the age and sex of the bone donors. Long-term culture in dexamethasone significantly increased ALP and OC mRNA levels and the activities of ALP and PTH-stimulated adenylate cyclase but not OC production, in (HOB + DEX) compared with (HOB-DEX) cells (p < 0.05). When the data were examined with respect to the age of the bone donor, age-dependent differences in the expression and responses to dexamethasone were apparent. ALP and PTH-stimulated adenylate cyclase activities decreased with increasing age of the bone donor in (HOB-DEX) and (HOB + DEX) cells (p < 0.05). There were no significant correlations between phenotypic marker mRNA levels and bone donor age in (HOB-DEX) and ((HOB + DEX) cells. All age-dependent decreases in ALP and PTH-stimulated cyclase activities were enhanced in the (HOB + DEX) cells. However, when the data were examined according to the sex of the bone donor, there were no differences in mRNA levels, OC production, or ALP and cyclase activities between cells from male and female donors. These results indicate an age dependence in the expression of osteoblastic markers in human bone cells at different stages of differentiation: thus osteoblastic cultures derived from older donors are likely to contain fewer osteoprogenitor cells, lower levels of glucocorticoid receptors or represent more differentiated osteoblasts compared with those derived from younger donors.