Progesterone and dexamethasone stimulate proliferation and differentiation of osteoprogenitors and progenitors for adipocytes and macrophages in cell populations derived from adult rat vertebrae

Evidence linking exposure of fish primary macrophages to antibiotics activates the NF-kB pathway

Low doses of antibiotics are ubiquitous in the marine environment and may exert negative effects on non-target aquatic organisms. Using primary macrophages of common carp, we investigated the mechanisms of action following exposure to several common antibiotics; cefotaxime, enrofloxacin, tetracycline, sulfamonomethoxine, and their mixtures, and explored the immunomodulatory effects associated with the nuclear factor-κB (NF-κB) signaling pathway. A KEGG pathway analysis was conducted using the sixty-six differentially expressed genes found in all treatments, and showed that exposure to 100 μg/L of antibiotics could affect regulation of the NF-κB signaling pathway, suggesting that activation of NF-κB is a common response in all four classes of antibiotics. In addition, the four antibiotics induced nf-κb and NF-κB-associated cytokines expression, as verified by qPCR, however, these induction responses by four antibiotics were minor when compared to the same concentration of LPS treatment (100 μg/L). Antagonists of NF-κB blocked many of the immune effects of the antibiotics, providing evidence that NF-κB pathways mediate the actions of all four antibiotics. Moreover, exposure to environmentally relevant, low levels (0.01-100 μg/L) of antibiotics induced a NF-κB-mediated immune response, including endogenous generation of ROS, activity of antioxidant enzymes, as well as expression of cytokine and apoptosis. Moreover, exposure to mixtures of antibiotics presented greater effects on most tested immunological parameters than exposure to a single antibiotic, suggesting additive effects from multiple antibiotics in the environment. This study demonstrates that exposure of fish primary macrophages to low doses of antibiotics activates the NF-kB pathway.

Low-dose dexamethasone affects osteoblast viability by inducing autophagy via intracellular ROS

Glucocorticoids (GCs) are closely associated with the progression of GC-induced osteoporosis (GIOP) by inhibiting osteoblast viability. However, endogenous GCs are important for bone development. In addition, previous studies have demonstrated that GCs could induce autophagy, a cytoprotective process that is protective against various stressors. In the present study, the aim is to explore whether osteoblasts exhibited dose-dependent viability in the presence of GCs due to autophagy. hFOB 1.19 osteoblasts were treated with various doses of dexamethasone (DEX; 10⁻⁸-10⁻⁴ M) for 0, 24, 48 and 72 h. The results revealed a biphasic effect of DEX on the viability of hFOB 1.19 cells; a high dose of DEX (≥10⁻⁶ M) accelerated cell apoptosis, while a low dose of DEX (10⁻⁸ M) increased cell viability. Furthermore, significantly increased autophagy was observed in the low dose DEX treatment group, as indicated by the expression of the autophagy-associated proteins beclin 1 and microtubule-associated protein light chain 3, and the detection of autophagosomes. Another finding was that DEX upregulated intracellular reactive oxygen species (ROS), which was decreased by the autophagy agonist rapamycin. The increase in autophagy and cell viability associated with low-dose DEX (10⁻⁸ M) was suppressed by the ROS scavenger catalase and the autophagy inhibitor 3-methyladenine. In conclusion, the results revealed that GCs affected osteoblast viability in a dose-dependent manner. A low dose of GCs increased osteoblast viability by inducing autophagy via intracellular ROS. The results indicate that autophagy may be a novel mechanism by which osteoblasts survive GC exposure and provide a potential therapeutic target for treating GIOP.

The in vitro immune modulatory effect of bisphenol A on fish macrophages via estrogen receptor α and nuclear factor-κB signaling

Bisphenol A (BPA) is a well-known environmental endocrine-disrupting chemical. Employing primary macrophages from head kidney of red common carp (Cyprinus carpio), the present study aimed to evaluate the immune modulatory effect of BPA and to explore its potential action mechanism associated with estrogen receptor (ER) and nuclear factor-κB (NF-κB) pathways. A dynamic response process was observed in macrophages upon various concentrations of BPA exposure, which significantly enhanced the antibacterial activity of macrophages at 0.1, 1, or 10 μg/L, but instead induced the apoptosis at 100, 1000, and 10,000 μg/L. A potential pro-inflammatory effect of BPA exposure was suggested, judging from the increased production of nitrite oxide and reactive oxygen species (ROS), the induction of interleukin-1β mRNA and protein, as well as NF-κB and other NF-κB-associated immune gene expression. Following BPA coexposure with the ER or NF-κB antagonist, the induction of ROS, ERα, and NF-κB-associated immune gene expression was significantly inhibited, implying interaction between those two pathways. This study thus indicated that low doses of BPA exposure alone could significantly disturb the immune response of fish primary macrophages in vitro, and for the first time revealed the synergistic action of ERα and NF-κB transcription factors in the BPA effect.

The influence of aging and estradiol to progesterone ratio on rat macrophage phenotypic profile and NO and TNF-α production

The phenotype and function of tissue macrophages substantially depend on the cellular milieu and biological effector molecules, such as steroid hormones, to which they are exposed. Furthermore, in female rats, aging is associated with the altered macrophage functioning and the increased estrogen level is followed by a decrease in that of progesterone. Therefore, the present study aimed to investigate the influence of estradiol/progesterone balance on rat macrophage function and phenotype throughout whole adult lifespan. We ovariectomized rats at the late prepubertal age or at the very end of reproductive lifespan, and examined the expression of ED2 (CD163, a marker of mature resident macrophages related to secretion of inflammatory mediators) on peritoneal macrophages and their ability to produce TNF-α and NO upon LPS-stimulation at different age points. In addition, to delineate direct and indirect effects of estrogen, we assessed the in vitro influence of different concentrations of 17β-estradiol on LPS-induced macrophage TNF-α and NO production. Results showed that: (a) the low frequency of ED2(high) cells amongst peritoneal macrophages of aged rats was accompanied with the reduced TNF-α, but not NO production; (b) estradiol level gradually increased following ovariectomy; (c) macrophage ED2 expression and TNF-α production were dependent on estradiol/progesterone balance and they changed in the same direction; (d) changes in estradiol/progesterone balance differentially affected macrophages TNF-α and NO production; and (e) estradiol exerted pro-inflammatory and anti-inflammatory effects on macrophages in vivo and in vitro, respectively. Overall, our study discloses that estradiol/progesterone balance contributes to the fine-tuning of rat macrophage secretory capacity, and adds to a better understanding of the ovarian steroid hormone role in the regulation of macrophage function, and its significance for the age-associated changes in innate immunity.

Inhibition of the progesterone nuclear receptor during the bone linear growth phase increases peak bone mass in female mice

Augmentation of the peak bone mass (PBM) may be one of the most effective interventions to reduce the risk of developing osteoporosis later in life; however treatments to augment PBM are currently limited. Our study evaluated whether a greater PBM could be achieved either in the progesterone nuclear receptor knockout mice (PRKO) or by using a nuclear progesterone receptor (nPR) antagonist, RU486 in mice. Compared to their wild type (WT) littermates the female PRKO mice developed significantly higher cancellous and cortical mass in the distal femurs, and this was associated with increased bone formation. The high bone mass phenotype was partially reproduced by administering RU486 in female WT mice from 1–3 months of age. Our results suggest that the inhibition of the nPR during the rapid bone growth period (1–3 months) increases osteogenesis, which results in acquisition of higher bone mass. Our findings suggest a crucial role for progesterone signaling in bone acquisition and inhibition of the nPR as a novel approach to augment bone mass, which may have the potential to reduce the burden of osteoporosis.

Cord blood--an alternative source for bone regeneration

Bone regeneration is one of the best investigated pathways in mesenchymal stromal cell (MSC) biology. Therefore strong efforts have been made to introduce tissue engineering and cell therapeutics as an alternative treatment option for patients with bone defects. This review of the literature gives an overview of MSC biology aiming for clinical application including advantages but also specific challenges and problems which are associated with cord blood derived stromal cell (CB-MSC) as a source for bone regeneration. The use of postnatal CB-MSC is ethically uncomplicated and requires no invasive harvesting procedure. Moreover, most data document a high osteogenic potential of CB-MCS and also low immunoreactivity compared with other MSC types. The expression profile of CB-MSC during osteogenic differentiation shows similarities to that of other MSC types. Within the umbilical cord different MSC types have been characterized which are potent to differentiate into osteoblasts. In contrast to a large number of in vitro investigations there are only few in vivo studies available so far.

Nuclear receptor profile in calvarial bone cells undergoing osteogenic versus adipogenic differentiation

Nuclear receptors (NRs) are key regulators of cell function and differentiation. We examined NR expression during osteogenic versus adipogenic differentiation of primary mouse calvarial osteoblasts (MOBs). MOBs were cultured for 21 days in osteogenic or adipogenic differentiation media. von Kossa and Oil Red O staining, and qRT-PCR of marker genes and 49 NRs were performed. PCR amplicons were subcloned to establish correct sequences and absolute standard curves. Forty-three NRs were detected at days 0-21. Uncentered average linkage hierarchical clustering identified four expression clusters: NRs (1) upregulated during osteogenic, but not adipogenic, differentiation, (2) upregulated in both conditions, with greater upregulation during adipogenic differentiation, (3) upregulated equally in both conditions, (4) downregulated during adipogenic, but not osteogenic, differentiation. One-way ANOVA with contrast revealed 20 NRs upregulated during osteogenic differentiation and 12 NRs upregulated during adipogenic differentiation. Two-way ANOVA demonstrated that 18 NRs were higher in osteogenic media, while 9 NRs were higher in adipogenic media. The time effect revealed 16 upregulated NRs. The interaction of condition with time revealed 6 NRs with higher expression rate during adipogenic differentiation and 3 NRs with higher expression rate during osteogenic differentiation. Relative NR abundance at days 0 and 21 were ranked. Basal ranking changed at least 5 positions for 13 NRs in osteogenic media and 9 NRs in adipogenic media. Osteogenic and adipogenic differentiation significantly altered NR expression in MOBs. These differences offer a fingerprint of cellular commitment and may provide clues to the underlying mechanisms of osteogenic versus adipogenic differentiation.

Short-term effect of ovariectomy on osteoprogenitors in the healing rat mandibular incisor extraction socket

OVX increased the percentage of AP-positive CFU-F in healing rat mandible. The increase of the number of osteoprogenitors was not significant in rat mandible-derived cultures but was in femur-derived ones. This suggests that the effect of OVX on osteoprogenitors is either smaller or develops later in mandible relative to femur.

INTRODUCTION

Osteoprogenitors play an essential role in the regeneration process that leads to the successful integration of dental implants. However, it is unclear how systemic osteoporosis affects osteoprogenitors in oral bone. The present study was designed to determine the short-term effects of ovariectomy (OVX) on osteoprogenitors from the healing extraction socket in rat mandible.

METHODS

Six-month-old rats were ovariectomized (n=8) and control rats were left intact (n=8). Two weeks post-OVX, the right mandibular incisor was extracted. Four weeks post-extraction, the basal mandibular bone between the 1st and 3rd molar in the healing extraction socket was used to determine the number of fibroblastic progenitors (CFU-F), alkaline phosphatase-positive fibroblastic progenitors (AP-positive CFU-F), Dex-dependent osteoprogenitors (CFU-O Dex) and Prog-dependent osteoprogenitors (CFU-O Prog) using colony assays (n=5). Osteocalcin mRNA expression was evaluated using in situ hybridization (n=3). Data were analyzed using two-way ANOVA or Student's t-test.

RESULTS

OVX increased the percentage of AP-positive CFU-F in both mandible and femur. The number of CFU-O was increased only in femur. Osteocalcin mRNA expression in regenerating mandible was not statistically different between control and OVX animals.

CONCLUSION

Our results suggest that the effect of OVX on osteoprogenitors is either smaller or develops later in mandible relative to femur.

Ovariectomy of 12-month-old rats: effects on osteoprogenitor numbers in bone cell populations isolated from femur and on histomorphometric parameters of bone turnover in corresponding tibia

Ovariectomy (OVX) in rats results in increased bone turnover and decreased bone volume and bone mineral density when measured in the metaphyses of long bones. We have investigated the effects of OVX on changes in the number of progenitors in cell populations derived from the metaphyseal bone of femurs of ovariectomized rats at 12 months of age, by using colony assays, bone nodule assays, and limiting dilution analysis at 1.5 and 9 months post-OVX. We have also measured histomorphometric parameters of bone formation and resorption in the corresponding tibia at the same time-points. A significant increase, as shown by bone nodule assays and limiting dilution analysis, occurs in the number of progesterone- and dexamethasone-responsive osteoprogenitors in cell populations isolated from ovariectomized rats at the 9-month post-OVX time-point. Progesterone-responsive osteoprogenitors are also increased at 1.5 months post-OVX. The number of fibroblast colony-forming units does not change. Histomorphometry has shown that OVX causes an increase in osteoblast surfaces, mineralizing surfaces, and bone formation rate at both 1.5 and 9 months post-OVX. The mineral apposition rate is increased at 1.5 months post-OVX. OVX also increases parameters of bone resorption at both time-points, the net result being a decrease in bone mineral density and cancellous bone volume at 9 months post-OVX. Thus, OVX in rats at 12 months of age is associated with an increase in the number of both progesterone- and dexamethasone-responsive osteoprogenitors 9 months post-OVX; this corresponds with increases in the histomorphometric parameters of bone formation.

Dydrogesterone and norethisterone regulate expression of lipoprotein lipase and hormone-sensitive lipase in human subcutaneous abdominal adipocytes

AIM

In premenopausal women, hyperandrogenicity is associated with central obesity and an increased cardiovascular risk. The aim of this study was to investigate the effects of dydrogesterone (DYD) (a non-androgenic progestogen) and norethisterone (NET) (an androgenic progestogen) on lipoprotein lipase (LPL), hormone-sensitive lipase (HSL) and glycerol release in adipocytes isolated from subcutaneous abdominal adipose tissue.

METHODS

Adipose tissue was obtained from 12 non-diabetic women, mean age 51 years (range 37-78) and mean body mass index 25.4 kg/m(2) (range 20.3-26.4). Adipocytes were treated with increasing doses of DYD and NET for 48 h prior to protein extraction. Effects on lipogenesis and lipolysis were assessed using western blotting to determine the expression of key enzymes, LPL (56 kDa) and HSL (84 kDa) respectively. Measurement of glycerol release into the medium provided an assessment of lipolytic activity.

RESULTS

Expression of LPL was increased by DYD and NET (mean protein expression relative to control +/- s.e.), with greatest effect at 10(-8) M for DYD: 2.32 +/- 0.51 (p < 0.01) and 10(-8) M for NET: 2.06 +/- 0.19 (p < 0.01). In contrast, HSL expression was reduced by all concentrations of DYD, with maximal effect at 10(-9) M : 0.49 +/- 0.02 (p < 0.001). NET reduced HSL expression at all concentrations from 10(-9) M : 0.62 +/- 0.06 (p < 0.001) to 10(-7) M : 0.69 +/- 0.08 (p < 0.001). Glycerol measurements supported the HSL expression studies although they were not statistically significant (p > 0.05).

CONCLUSIONS

DYD and NET significantly increased LPL expression relative to control, while significantly reducing HSL expression. At the concentrations studied, similar effects were observed with the androgenic NET and the non-androgenic DYD despite differing effects on the lipid profile when taken orally in combination with oestrogen. Further work examining the effects of different progestogens on body fat distribution may enable progestogen use to be tailored to maximize benefits and minimize potential harm.

Different effects of insulin and insulin-like growth factors I and II on osteoprogenitors and adipocyte progenitors in fetal rat bone cell populations

We investigated the effects of insulin (1-1,000 nM), insulin-like growth factor (IGF)-I, and IGF-II (3-100 nM each) alone or together with 10 nM dexamethasone (DEX) or 10 nM 1,25-dihydroxyvitamin D(3) (1,25[OH](2)D(3)) on proliferation and differentiation of adipocyte and osteoblast progenitors in bone cell populations derived from fetal rat calvaria. The effects on differentiation were evaluated by counting the number of bone or osteoid nodules and adipocyte colonies and the effects on proliferation, by measuring their size by image analysis. The types of cells studied were 1,25(OH)(2)D(3)- and DEX-responsive adipocyte progenitors and DEX-dependent and independent osteoprogenitors. Both IGF-I and IGF-II stimulated osteoprogenitor differentiation both alone and in the presence of DEX, while insulin stimulated osteoprogenitor differentiation only in the absence of DEX. Neither IGF-I/-II nor insulin affected proliferation of osteoprogenitors. Insulin had little effect on adipocyte differentiation by itself but strongly stimulated differentiation in the presence of either 1,25(OH)(2)D(3) or DEX, while IGF-II stimulated adipocyte differentiation in both the absence and presence of 1,25(OH)(2)D(3) or DEX. IGF-I by itself or in the presence of DEX strongly stimulated adipocyte cell differentiation but had little effect in the presence of 1,25(OH)(2)D(3). Our results demonstrate that insulin, IGF-II, and IGF-I have specific and different effects on the differentiation and proliferation of different groups of progenitor cells.

Efeito da atividade física no osso normal e na prevenção e tratamento da osteoporose

A osteoporose é uma doença cada vez mais diagnosticada em mulheres e homens de todo o mundo. Embora os esteróides sexuais sejam importantes na gênese da osteoporose, a inatividade física constitui um fator de risco. O exercício físico atua no osso por efeito direto, via força mecânica, ou indireto, mediado por fatores hormonais. Mas os mecanismos pelos quais a atividade física melhora a massa óssea ainda não são totalmente conhecidos. Baseando-se nos resultados que demonstram os efeitos benéficos da atividade física no tecido ósseo, a prática de esportes vem sendo cada vez mais indicada na prevenção e até mesmo no tratamento da osteoporose. O objetivo desta revisão é descrever os efeitos da atividade física no tecido ósseo normal e na prevenção e tratamento da osteoporose.

Role of osterix in endothelin-1-induced downregulation of vascular endothelial growth factor in osteoblastic cells

Endothelin-1 (ET-1) is produced by vascular endothelial cells to play an important role during bone development, remodeling and repair. ET-1 promotes osteoblastic cell proliferation and differentiation, but has the unique effect of downregulating vascular endothelial growth factor (VEGF) and may thereby control angiogenesis during bone production. Our objectives were to identify the intracellular mechanisms by which ET-1 controls VEGF expression during osteoblastic proliferation and differentiation. ET-1 induced osteoblastic differentiation in rat SBMC-D8 osteoblastic cells, but downregulated expression of VEGF mRNA isoforms (VEGF120, 164 and 188) as demonstrated by semi-quantitative reverse transcription polymerase chain reaction (RT-PCR) and by use of a luciferase reporter construct containing the promoter region of the VEGF gene. Co-transfection with the endothelin receptor A (ETRA) had the same effect. ET-1 and ETRA both upregulated the transcription factor osterix (Osx). RNA silencing of Osx resulted in an upregulation of VEGF. This study supports the novel inhibitory role for ET-1, via Osx, on VEGF synthesis in osteoblastic cells as a possible mechanism in the temporal and spatial feedback of angiogenesis to bone formation and resorption.

The effect of reloading on bone volume, osteoblast number, and osteoprogenitor characteristics: studies in hind limb unloaded rats

Skeletal unloading during space flight results in bone loss. In astronauts the extent to which bone is lost varies greatly between different bones of the skeleton as well as between different individuals. Following return to earth, recovery of bone mass during reloading also varies between different bones and different individuals. Due to this variability and the limited number of subjects it is difficult to study the effects of unloading/reloading on bone in humans. A viable alternative is to use the rat model of hind limb unloading developed at NASA. We have previously demonstrated that, in 6-week-old male rats, 14 days of unloading result in a decrease in osteoprogenitor number in cell populations isolated from the proximal femur. The goal of the present study was to determine the number of osteoprogenitor cells present in cell populations derived from the proximal femur of young rats after 14 days of unloading followed by 14 days of reloading and to characterize their proliferative capacity. To do this, we determined the number of alkaline phosphatase-positive colony forming units (CFU-AP) and osteoblast CFU (CFU-O). To establish whether the effects of unloading and reloading were specific for cells of the osteoblast lineage, we also determined the number of fibroblastic CFU (CFU-F). Effects on proliferation were evaluated by measuring the size of CFU-O. Unloading resulted in a 66% reduction in CFU-AP. CFU-O numbers were decreased by 76% and mean colony size was 33% less than controls. The decrease in osteogenic and osteoprogenitor cells in vitro paralleled the decrease in bone volume (- 50%), osteoblast number (- 35%), and bone formation rate (- 46%) observed in the proximal tibial metaphysis of unloaded rats. Unloading had no effect on osteoclast number or surface. Subsequent reloading for 14 days restored CFU-AP. CFU-O numbers were only partially restored at 14 days (83% of controls) but nodule size was 1.2-fold greater than controls. Neither unloading nor reloading had an effect on the total number of progenitors (CFU-F). Reloading restored bone volume back to control values, but osteoblast number and bone formation rate were still lower than those in corresponding controls. Both osteoclast number and surface were lower in reloaded animals than in age-matched controls. Our results indicate that 14 days of unloading result in a decrease in osteoprogenitor number and that reloading for 14 subsequent days completely restores CFU-AP and bone volume to control levels, while the number of CFU-O in vitro and osteoblasts in vivo were partially recovered but still lower than corresponding controls. Strikingly, osteoclastic bone resorption after 14 days of reloading was greatly reduced compared to controls, suggesting a significant contribution of this to the recovery process.

Mechanical loading modulates glutamate receptor subunit expression in bone

The cellular mechanisms coupling mechanical loading with bone remodeling remain unclear. In the CNS, the excitatory amino acid glutamate (Glu) serves as a potent neurotransmitter exerting its effects via various membrane Glu receptors (GluR). Nerves containing Glu exist close to bone cells expressing functional GluRs. Demonstration of a mechanically sensitive glutamate/aspartate transporter protein and the ability of glutamate to stimulate bone resorption in vitro suggest a role for glutamate linking mechanical load and bone remodeling. We used immunohistochemical techniques to identify the expression of N-methyl-d-aspartate acid (NMDA) and non-NMDA (AMPA or kainate) ionotropic GluR subunits on bone cells in vivo. In bone sections from young adult rats, osteoclasts expressed numerous GluR subunits including AMPA (GluR2/3 and GluR4), kainic acid (GluR567) and NMDA (NMDAR2A, NMDAR2B and NMDAR2C) receptor subtypes. Bone lining cells demonstrated immunoexpression for NMDAR2A, NMDAR2B, NMDAR2C, GluR567, GluR23, GluR2 and GluR4 subunits. Immunoexpression was not evident on osteocytes, chondrocytes or vascular channels. To investigate the effects of mechanical loading on GluR expression, we used a Materials Testing System (MTS) to apply 10 N sinusoidal axial compressive loads percutaneously to the right limbs (radius/ulna, tibia/fibula) of rats. Each limb underwent 300-load cycles/day (cycle rate, 1 Hz) for 4 consecutive days. Contralateral, non-loaded limbs served as controls. Mechanically loaded limbs revealed a load-induced loss of immunoexpression for GluR2/3, GluR4, GluR567 and NMDAR2A on osteoclasts and NMDAR2A, NMDAR2B, GluR2/3 and GluR4 on bone lining cells. Both neonatal rabbit and rat osteoclasts were cultured on bone slices to investigate the effect of the NMDA receptor antagonist, MK801, and the AMPA/kainic acid receptor antagonist, NBQX, on osteoclast resorptive activity in vitro. The inhibition of resorptive function seen suggested that both NMDAR and kainic acid receptor function are required for normal osteoclast function. While the exact role of ionotropic GluRs in skeletal tissue remains unclear, the modulation of GluR subunit expression by mechanical loading lends further support for participation of Glu as a mechanical loading effector. These ionotropic receptors appear to be functionally relevant to normal osteoclast resorptive activity.

Osteoprogenitor viability in cell populations isolated from rat femora is not affected by 24 h storage at 4 degrees C

This study was initiated to determine whether partially dissected bones of rats could be refrigerated for 24 h in saline without losing viability of progenitor cells, specifically osteoprogenitors. This is directly applicable to studies involving bone tissue requiring overnight shipment, for example, studies involving space flown animals, grafting experiments, or transplantation. We evaluated cell populations isolated from the proximal femur of 6-week-old male Fisher 344 rats. Explants from the left femur were prepared and placed into culture immediately following dissection, while the right femur was cleaned, fragmented, and stored in saline at 4 degrees C for 24 h, after which explant cultures were initiated. After 11 days of explant culture, cells were collected from outgrowths, counted, and plated to initiate experiments. Plated cells were grown for either 15 or 21 days. To determine if storage affected the total number of colony forming progenitors, alkaline phosphatase positive colonies, or the number of osteoprogenitors, were counted. There was no significant difference in any of the types of colony forming units examined between cell populations derived from freshly prepared samples or those stored for 24 h, indicating that storage at 4 degrees C of bone tissue for 24 h in saline does not affect the osteogenic potential or the number of osteoprogenitors of the cell populations isolated.

Pregnancy-associated plasma protein-A proteolytic activity in rat vertebral cell cultures: Stimulation by dexamethasone-a potential mechanism for glucocorticoid regulation of osteoprogenitor proliferation and differentiation

Glucocorticoids (GCs) at physiological concentrations stimulate osteoprogenitor proliferation and differentiation in rat bone cell populations, and this is mediated in part by an increased response to insulin-like growth factors (IGFs). Since IGF binding proteins (IGFBPs) modulate IGF actions, we evaluated whether the increased IGF responsiveness might be associated with decreased inhibitory IGFBP-4 peptide levels. Rat vertebral cells were cultured for up to 20 days with or without dexamethasone (Dex). Cell layer proteins were extracted at day 6, 8, 14, and 20, conditioned media (CM) collected at day 8, 14, and 20, and total RNA isolated at day 14 and 20 of culture. Western blotting showed that cell layer IGFBP-4 levels were lower, while IGFBP-4 protease activity in CM was higher, in Dex-treated cultures. Addition of pregnancy-associated plasma protein-A (PAPP-A) antibody to CM abrogated IGFBP-4 proteolysis. PAPP-A mRNA levels were the same in control and Dex-treated cultures as evaluated by RT-PCR. Our data demonstrate that activity of the IGFBP-4 protease, PAPP-A, in rat bone cell cultures is increased by Dex via post-transcriptional mechanisms. Since IGFBP-4 mRNA levels in Dex-treated cultures were the same as in controls at day 8, slightly lower than in controls at day 14, and higher than in controls at day 20 as shown previously, the decreased IGFBP-4 peptide levels in Dex-treated cultures likely result from increased IGFBP-4 proteolysis by the elevated PAPP-A enzymatic activity. Our findings underscore a novel mechanism whereby GCs increase IGF responses in rat bone cells via PAPP-A-induced IGFBP-4 proteolysis.

Effect of simulated weightlessness on osteoprogenitor cell number and proliferation in young and adult rats

Experiments with rats flown in space or hind limb unloaded (HU) indicate that bone loss in both conditions is associated with a decrease in bone volume and osteoblast surface in cancellous and cortical bone. We hypothesize that the decrease in osteoblastic bone formation and osteoblast surface is related to a decrease in the number of osteoprogenitors and/or decreased proliferation of their progeny. We tested this hypothesis by evaluating the effect of 14 days of HU on the number of osteoprogenitors (osteoblast colony forming units; CFU-O), fibroblastic colony forming units (CFU-F), and alkaline phosphatase-positive CFU (CFU-AP) in cell populations derived from the proximal femur (unloaded) and the proximal humerus (normally loaded) in 6-week-old and 6-month-old rats. To confirm the effect of unloading on bone volume and structure, static histomorphometric parameters were measured in the proximal tibial metaphysis. Effects of HU on proliferation of osteoprogenitors were evaluated by measuring the size of CFU-O. HU did not affect the total number of progenitors (CFU-F) in young or adult rats in any of the cell populations. In femoral populations of young rats, HU decreased CFU-O by 71.0% and mean colony size was reduced by 20%. HU decreased CFU-AP by 31.3%. As expected, no changes in CFU-O or CFU-AP were seen in cell populations from the humerus. In femoral cell populations of adult rats, HU decreased CFU-O and CFU-AP by 16.6% and 36.6%, respectively. Again, no effects were seen in cell populations from the humerus. In 6-week-old rats, there was a greater decrease in bone volume, osteoblast number, and osteoblast surface in the proximal tibial metaphysis than that observed in adult rats. Both trabecular thickness and trabecular number were decreased in young rats but remained unaffected in adults. Neither osteoclast number nor surface was affected by unloading. Our results show that the HU-induced decrease in the number of osteoprogenitors observed in vitro parallels the effects of HU on bone volume and osteoblast number in young and old rats in vivo, suggesting that the two may be interdependent. HU also reduced CFU-O colony size in femoral populations indicating a diminished proliferative capacity of osteoblastic colonies.

Efeito da associação hipertireoidismo-castração no osso de ratas adultas

Foi estudada a relação tireóide-gônadas e sua influência sobre a morfologia óssea de ratas Wistar, com cinco meses de idade, castradas e induzidas ao hipertireoidismo ou mantidas em eutireoidismo por período de 30, 60 e 90 dias. Ratas não castradas foram mantidas nas mesmas condições e serviram como controle. Ao final de cada período, foram determinadas as concentrações plasmáticas de T4 livre, progesterona e estradiol. Os ossos de cada grupo foram submetidos às análises radiológica e histológica. O hipertireoidismo nas ratas não castradas levou à alteração da morfologia do osso, variável ao longo do período experimental, conduzindo, aos 60 dias, à perda de osso trabecular por aumento da reabsorção óssea. Aos 90 dias não houve perda óssea porque o aumento da reabsorção foi acompanhado por maior aposição óssea. Nas ratas eutireóideas castradas, a diminuição dos níveis de progesterona inibiu a aposição óssea, causando, aos 30 dias, pequena perda do osso trabecular das vértebras lombares e do osso alveolar, que se intensificou aos 60 e 90 dias, atingindo também o osso cortical. A administração de tiroxina nas ratas castradas reduziu a osteopenia decorrente da castração aos 60 dias, mas não aos 90 dias, quando a perda óssea foi mais extensa. Conclui-se que o hipoprogesteronismo e o hipoestrogenismo alteram o metabolismo ósseo e que a resposta do osso ao hipertireoidismo depende do perfil plasmático dos esteróides sexuais, do tempo de exposição e da configuração do tecido ósseo.

Number, frequency, self-renewal, and expansion of osteoprogenitor cells (CFU-O) in subcultured female rat vertebral cell populations

The present study was undertaken to determine whether the frequency and/or number of dexamethasone- and progesterone-responsive osteoprogenitors in cell populations derived from vertebrae of 6-week-old female rats could be increased relative to that of other progenitors. Frequencies and numbers of both progenitor types were determined for up to six subcultures using continuous subculturing, limiting dilution analysis, and colony assays. In dexamethasone-containing medium, subculturing resulted in an eightfold increase in the total number of dexamethasone-responsive osteoprogenitors and a 14-fold increase in progesterone-responsive osteoprogenitors in second subculture cells over first subculture cells without a significant increase in the frequency of these progenitors. From the third subculture onward, the frequency of both classes of osteoprogenitors decreased in a linear manner and none were observed after six subcultures. Similar results were obtained in progesterone-containing medium. Limiting dilution analysis in the presence of dexamethasone indicated that 2.61 % of cells represented a colony forming unit-fibroblast and 0.28 % represented an osteoprogenitor in first subculture cells, while in second subculture cells, these frequencies increased to 5.56 % and 0.40 %, respectively. Results show that while the frequency of colony forming unit-osteoprogenitor is not increased in the second subculture over the first, the total number of osteoprogenitors is greatly increased because of expansion of the total progenitor cell pool.

A osteoporose e os distúrbios endócrinos da tireóide e das gônadas

Apesar da dedicação incessante dos pesquisadores no estudo da osteoporose, muito ainda necessita ser elucidado. A deficiência dos esteróides sexuais, principalmente a de estrógeno, é considerada a principal causa de osteoporose, embora existam inúmeros outros fatores envolvidos. O hipertireoidismo, por exemplo, é considerado um dos fatores de risco para indução ou agravamento da osteoporose e tem despertado o interesse para o estudo dos efeitos de T3 e T4 sobre o metabolismo ósseo. Embora o hipotireoidismo e a afuncionalidade das gônadas seja uma associação freqüente na mulher, a hipofunção da tireóide não é considerada fator de risco para a osteoporose da menopausa. Assim, o estudo da inter-relação entre os distúrbios endócrinos, tão comuns na idade avançada, e a osteoporose é fundamental, pois deste conhecimento poderão advir meios de controle e tratamento adequados, bem como a definição da real natureza do distúrbio ósseo. O objetivo desta revisão é apresentar e discutir alguns aspectos da osteoporose e sua inter-relação com os distúrbios endócrinos da tireóide e das gônadas.

Proliferation, differentiation and self-renewal of osteoprogenitors in vertebral cell populations from aged and young female rats

A significant contribution to the bone loss associated with aging is likely to be a decline in bone formation. We have characterized and compared the number, capacity for proliferation and differentiation and the self-renewal ability of osteoprogenitors of aged (17-26-month-old) and young (1.5-month-old) female Wistar rats using limiting dilution analyses and continuous subculture experiments. Cells were obtained from outgrowths of explants of lumbar vertebrae (L1-L6) and grown in alpha-minimal essential medium (alpha-MEM), 10% FBS and 50 microg/ml ascorbic acid with or without dexamethasone (Dex; 0.3-300 nM) or progesterone (Prog; 0.01-10 microM). Growth curves for cell populations of both age groups were similar with population doubling times of 27.1 and 26.7 h for the aged and young animals, respectively. Osteoprogenitors from both age groups formed bone nodules when cultured in the presence of either Dex or Prog. Limiting dilution analysis in the presence of 10 nM Dex showed no difference between the aged and young rats in the number of colony forming units-fibroblast (CFU-F), alkaline phosphatase-positive colony forming units-fibroblast (AP+ CFU-F) or colony forming units-osteoblast (CFU-O). No differences were also found for any progenitor within the aged group. Limiting dilution analysis in the presence of 3 microM Prog showed no differences in the numbers of CFU-F, AP+ CFU-F or CFU-O between the aged and young groups or within the aged group. Continuous subculture of cells in the presence of 10 nM Dex revealed that the number of nodules per 10(4) plated cells increased in second subculture over first subculture cells in the young group but decreased in the aged group. Also, in third to fifth subculture cells, the number of nodules was lower in the aged group than in the young group. A similar pattern was observed in the presence of 3 microM Prog. Results indicate that the cell population doubling times, growth characteristics, and the number of CFU-F and osteoprogenitors in vertebral bone cell populations from aged rats and young rats are similar. This suggests that the bone loss associated with aging is not caused by a decrease in osteoprogenitor cell number. However, cell populations from the aged rats showed a reduced capacity for self-renewal in vitro, which would ultimately translate into a reduced number of osteoblasts and might be partly responsible for a decrease in bone formation in aged animals.

Expression of insulin-like growth factor system constituents in differentiating rat osteoblastic cell populations

The synthetic glucocorticoid dexamethasone (Dex) and insulin-like growth factor-I and -II (IGF-I and -II) stimulate osteoprogenitor proliferation and differentiation in bone cell populations isolated from adult rat vertebrae. Since glucocorticoids have been shown to regulate gene expression of IGFs and IGF binding proteins (IGFBPs) in several experimental models, we investigated whether Dex-stimulated osteoprogenitor proliferation and differentiation was associated with changes in mRNA levels of the IGF system components (i.e., IGF-I and -II, the type 1 and 2 IGF receptor, the insulin receptor and six IGFBPs). Osteoprogenitor-containing bone cell populations were isolated from the outgrowth of vertebral explant cultures of 3-month-old female rats and cultured for 20 days. Total RNA was extracted at day 8, 14, and 20, and mRNA levels of the IGF system constituents were compared between differentiating (Dex-treated) and non-differentiating (control) cultures. Northern hybridization data from 8- and 20-day cultures showed that mRNA levels of IGF-I were markedly lower in Dex-treated cultures than in control cultures at day 8 and 20. At day 20, mRNA levels of IGFBP-3 were also lower in Dex-treated cultures. Signals of IGFBP-5 mRNA were undetectable. To increase the sensitivity of our detection methods and therefore evaluate mRNA levels of all the components of the IGF system, we performed reverse transcription-polymerase chain reaction (RT-PCR) analysis of RNA extracted at day 8, 14, and 20 of culture. In agreement with the Northern data, IGF-I mRNA levels in Dex-treated cultures were lower than in control cultures at all three time points, and IGFBP-3 levels were lower in Dex-treated cultures at day 20 of culture. However, at day 8 and 14, IGFBP-3 mRNA levels were higher in Dex-treated cultures than in controls. Levels of the 2 IGF receptor mRNA and the insulin receptor mRNA were lower in Dex-treated cultures. Dex-treated cultures also had decreased levels of IGFBP-1 mRNA but increased levels of IGFBP-2 mRNA at all three time points. IGFBP-4 levels were lower at day 14 in Dex-treated cultures than in controls but higher at day 20. IGF-II and IGFBP-5 mRNA levels in control and Dex-treated cultures were similar. Signals for IGFBP-6 were undetectable. Our findings show that glucocorticoid-induced osteoprogenitor proliferation and differentiation in adult rat bone cell populations are associated with significant changes in the mRNA levels of virtually all components of the IGF system. Some of these changes are dependent on the stages of development (e.g., regulation of IGFBP-3 and -4) and some remain similar trends at all stages (e.g., regulation of IGF-I and the three receptors).

Androgen effects on bone and muscle

Bone health and strength are dependent on the coupling of cone resorption and bone formation. This process is governed by the interaction of osteoclasts and osteoblasts plus the modulating influence of the bone mechanicosensory cells-the osteocytes. Both sex steroids-estrogen (E) and testosterone (T)- have receptors on all bone cells, with androgen dominance on osteoblasts and osteocytes. Specific receptors for the weaker androgens, such as DHEA have also been identified. The activity of the sex steroids, influenced by various enzymes found in bone, is reflective of the hormone ligand before its binding to the bone cells. As a result, T acts both directly and via its aromatization to estradiol. The activity of the androgens also varies with the bone surface; periosteal cells, for example, do not have 5alpha-reductase activity, indicating that T is the active metabolite at this clinically important site. Androgens influence bone cell function via local and systemic growth factors and cytokines. By enhancing osteoblast differentiation, androgens regulate bone matrix production, organization, and mineralization. Androgens also regulate osteoclast recruitment and activity. Endogenous androgens increase bone mineral density (BMD) in both adolescent and adult premenopausal women. Women with excess endogenous androgen-for example, those with hirsutism and polycystic ovary syndrome (PCOS)-have increased BMD compared with normal young women. E and androgen therapy increases BMD to a greater degree than does E therapy alone. This is true for both oral combinations of esterified E and methyltestosterone and for subcutaneous T implants. Androgenic progestins have an additive effect on BMD when combined with E therapy and have the further advantage of being protective to the endometrium in E-treated women. Androgens increase muscle mass and strength. The resulting improvement in physical activity leads to the activation of bone-forming sites and the stimulation of the bone formation-modulating cells, the osteocytes. Mechanical loading, when combined with hormone therapy, results in greater osteogenic response than does either alone.

Exposure of KS483 cells to estrogen enhances osteogenesis and inhibits adipogenesis

Osteoblasts and adipocytes arise from a common progenitor cell in bone marrow. Whether estrogen directly regulates the progenitor cells differentiating into osteoblasts or adipocytes remains unknown. Using a mouse clonal cell line KS483 cultured in charcoal-stripped fetal bovine serum (FBS), we showed that 17beta-estradiol (E2) stimulates the differentiation of progenitor cells into osteoblasts and concurrently inhibits adipocyte formation in an estrogen receptor (ER)-dependent way. E2 increased alkaline phosphate (ALP) activity and nodule formation and stimulated messenger RNA (mRNA) expression of core-binding factor alpha-1 (Cbfa1), parathyroid hormone/parathyroid hormone-related protein receptors (PTH/PTHrP-Rs), and osteocalcin. In contrast, E2 decreased adipocyte numbers and down-regulated mRNA expression of peroxisome proliferator-activated receptor-gamma (PPARgamma)2, adipocyte protein 2 (aP2), and lipoprotein lipase (LPL). Furthermore, the reciprocal control of osteoblast and adipocyte differentiation by E2 was observed also in the presence of the adipogenic mixture of isobutylmethylxanthine, dexamethasone, and insulin. Immunohistochemical staining showed that ERalpha and ERbeta were present in osteoblasts and adipocytes. A new mouse splice variant ERbeta2 was identified, which differed in two amino acid residues from the rat isoform. E2 down-regulated mRNA expression of ERalpha, ERbeta1, and ERbeta2. The effects of E2 are not restricted to the KS483 cell line because similar results were obtained in mouse bone marrow cell cultures. Our results indicate that estrogen, in addition to stimulation of osteogenesis, inhibits adipogenesis, which might explain the clinical observations that estrogen-deficiency leads to an increase in adipocytes.

Estrogen regulation of growth and alkaline phosphatase expression by cultured human bone marrow stromal cells

Estrogen has been reported to regulate the growth and differentiation of cultured murine osteoprogenitor cells in bone marrow stroma. This study tested the ability of 17beta-estradiol (E2) to regulate growth and expression of alkaline phosphatase (ALP), an osteoblastic differentiation marker, in strains of normal human bone marrow stromal cells derived from different donors. In eight strains examined, E2 at 1 and 10 nM produced at most modest effectxs on growth and ALP activity. Growth inhibition, seen in 4 of the 8 strains, was more common than stimulation (2 of the 8 strains); the greatest observed E2 effect was an inhibition of ca. 50%. E2 altered ALP activity less dramatically than cell growth. Differences from control in total ALP per culture were seen in only two strains: one was a reduction, one an increase. Colony forming assays were used to determine if E2 changed the proportion of ALP-expressing cells in marrow stromal cell cultures. In contrast to growth experiments, ALP expression under colony forming conditions (200 cells per 35 mm-diameter well) was dependent on the type of serum supplementation used. Under permissive conditions using medium supplemented with 10% charcoal-treated fetal bovine serum, 10 nM E2 increased the number of ALP-positive colonies (cfu-ap) but not the total number of colonies formed (cfu-f). When cells cultured in the presence or absence of 10 nM E2 were replated at colony forming densities, significantly higher proportions of cfu-ap were found in 2 of 6 strains examined, while pretreatment with E2 affected the number of cfu-f in only 1 of the 6 strains. Similar results were obtained when colony formation was carried out in the presence of dexamethasone and ascorbate, although these agents themselves increased the formation of both cfu-f and cfu-ap. These results show that the direct effects of E2 on human marrow stromal cells are small and vary depending on the cell strain and on the experimental conditions; however, the E2 actions observed in this study were consistent with reports that E2 exerts direct actions on osteoblasts and osteoblast progenitor cells that favor rather than suppress their phenotypic expression.

Extracellular matrix mineralization and osteoblast gene expression by human adipose tissue-derived stromal cells

Human adipose tissue represents an abundant reservoir of stromal cells with potential utility for tissue engineering. The current study demonstrates the ability of human adipose tissue-derived stromal cells to display some of the hallmarks of osteoblast differentiation in vitro. Following treatment with ascorbate, beta-glycerophosphate, dexamethasone, and 1,25 dihydroxy vitamin D(3), adipose tissue-derived stromal cells mineralize their extracellular matrix based on detection of calcium phosphate deposits using Alizarin Red and von Kossa histochemical stains. Fourier transform infrared analysis demonstrates the apatitic nature of these crystals. Mineralization is accompanied by increased expression or activity of the osteoblast-associated proteins osteocalcin and alkaline phosphatase. These and other osteoblast-associated gene markers are detected based on polymerase chain reaction. In contrast, the adipocyte gene markers--leptin, lipoprotein lipase, and peroxisome proliferator activated receptor gamma2--are reduced under mineralization conditions, consistent with the reciprocal relationship postulated to exist between adipocytes and osteoblasts. The current work supports the presence of a multipotent stromal cell population within human extramedullary adipose tissue. These findings have potential implications for human bone tissue bioengineering.

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.

High concentrations of dexamethasone suppress the proliferation but not the differentiation or further maturation of human osteoblast precursors in vitro: relevance to glucocorticoid-induced osteoporosis

OBJECTIVE

The use of glucocorticoids (GCs) in the treatment of RA is a frequent cause of bone loss. In vitro, however, this same class of steroids has been shown to promote the recruitment and/or maturation of primitive osteogenic precursors present in the colony forming unit-fibroblastic (CFU-F) fraction of human bone and marrow. In an effort to reconcile these conflicting observations, we investigated the effects of the synthetic GC dexamethasone (Dx) on parameters of growth and osteogenic differentiation in cultures of bone marrow stromal cells derived from a large cohort of adult human donors (n=30).

METHODS

Marrow suspensions were cultured in the absence and presence of Dx at concentrations between 10 pm and 1 microm. After 28 days we determined the number and diameter of colonies formed, the total number of cells, the surface expression of receptors for selected growth factors and extracellular matrix proteins and, based on the expression of the developmental markers alkaline phosphatase (AP) and the antigen recognized by the STRO-1 monoclonal antibody, the proportion of cells undergoing osteogenic differentiation and their extent of maturation.

RESULTS

At a physiologically equivalent concentration, Dx had no effect on the adhesion of CFU-F or on their subsequent proliferation, but did promote their osteogenic differentiation and further maturation. These effects were independent of changes in the expression of the receptors for fibroblast growth factors, insulin-like growth factor 1, nerve growth factor, platelet-derived growth factors and parathyroid hormone/parathyroid hormone-related protein, but were associated with changes in the number of cells expressing the alpha(2) and alpha(4), but not beta(1), integrin subunits. At supraphysiological concentrations, the effects of Dx on the osteogenic recruitment and maturation of CFU-F and their progeny were maintained but at the expense of a decrease in cell number.

CONCLUSIONS

A decrease in the proliferation of osteogenic precursors, but not in their differentiation or maturation, is likely to be a key factor in the genesis of GC-induced bone loss.

Insulin-like growth factor-1 and -2 stimulate osteoprogenitor proliferation and differentiation and adipocyte formation in cell populations derived from adult rat bone

In the present study, we test the hypothesis that the stimulation of osteoprogenitor differentiation by the glucocorticoid dexamethasone (Dex) is mediated, at least in part, through components of the insulin-like growth factor (IGF) system. Because it has been suggested that osteoprogenitors and adipocyte progenitors originate from the same precursor cells, and that their differentiation in many systems is reciprocally regulated, the effects of Dex and IGF on adipocyte formation were also evaluated in the same cultures. In view of the presence of IGFs and their binding proteins in serum, we also evaluated to what degree the effects of IGF-1 and IGF-2 on differentiation of osteoblasts and adipocytes were affected by the serum concentration of the culture media. Bone cell populations were isolated from vertebrae of 3-month-old female Wistar rats using an explant culture technique. Osteoprogenitor differentiation was evaluated by a colony assay: Bone-forming osteoblastic colonies (bone nodules) derived from single osteoprogenitors were identified by alkaline phosphatase (ALP) staining and/or staining for mineralized matrix according to the von Kossa technique. Unmineralized nodules and osteoblastic colonies were subsequently identified by their distinctive color and morphology after methylene blue counterstaining. Differentiated adipocytes were identified by Sudan IV staining. IGF-1 and IGF-2 stimulated both osteoprogenitor and adipocyte progenitor differentiation in a dose-dependent pattern. The stimulation of osteoprogenitor differentiation by IGF was not dependent on Dex, but differentiation of adipocytes was. The stimulatory effects of IGF-1 and IGF-2 on osteoprogenitor differentiation were greater in media containing 2.5% fetal bovine serum (FBS) than in media containing 5% or 10% FBS, whereas stimulation of adipocyte formation was greater in media containing 10% FBS. Neutralizing antibody against the type 1 IGF receptor (IGF-1R) partially blocked IGF- and Dex-induced osteoprogenitor differentiation, but did not affect IGF-induced adipocyte formation. This suggests that IGF-stimulated osteoprogenitor differentiation is mediated through IGF-1R, that the stimulation of adipocyte formation by IGF is not, that the stimulatory effects of Dex on osteoprogenitor differentiation are partially mediated through IGF-1R, and that the effects on adipocyte differentiation appear to be mediated through signaling pathways other than the IGF-1R.

Frequency of stromal lineage colony forming units in bone marrow of peroxisome proliferator-activated receptor-alpha-null mice

The bone marrow stroma, consisting of adipocytes, fibroblasts, and osteoblasts, develops from a multipotent mesenchymal progenitor. The recently described nuclear hormone receptors, known as peroxisome proliferator-activated receptors (PPARs), regulate transcription of genes involved in adipogenesis. Consistent with this is the observation that PPARalpha-null mice exhibit greater extramedullary adipose stores compared with their wild-type controls. To determine if the status of the PPARalpha protein also influenced bone marrow stromal cell differentiation, this study compared the frequency of colony forming units for bone marrow adipocytes (CFU-A), alkaline phosphatase-positive fibroblasts (CFU-F/ALP+), and osteoblasts (CFU-O) between wild-type and PPARalpha-null mice. The CFU frequencies for all lineages were not significantly different in either gender at age 3 weeks, independent of the PPARalpha background. However, histologic analysis showed that the cross-sectional area of the femur in male PPARalpha null mice was significantly greater than that of PPARalpha-null female mice and of both wild-type genders. This was due to an increased marrow cavity space rather than an increased cortical bone area. In addition, while the percentage area of cortical bone occupied by lacunae was equivalent in the PPARalpha and wild-type males, this value was significantly greater in PPARalpha-null female mice compared with wild-type females. At age 3-6 months, no significant difference was observed in the CFU-A frequencies, based on either PPARalpha status or gender. The wild-type male CFU-F/ALP+ frequency was significantly greater than the CFU-F/ALP+ in all other groups. Although the PPARalpha status had no influence on the CFU-O frequency, the number of CFU-O was greater in male than in female mice. Sequential incubation of stromal cells in either adipogenic- or osteoblastic-inducing media did not alter the number of CFU-A or CFU-O. These results indicate that the PPARalpha-null genotype does not influence bone marrow stromal cell numbers.

Progesterone stimulation of human insulin-like growth factor-binding protein-5 gene transcription in human osteoblasts is mediated by a CACCC sequence in the proximal promoter

Insulin-like growth factor-binding protein-5 (IGFBP-5) is produced by osteoblasts and potentiates insulin-like growth factor mitogenic stimulation in osteoblast cell cultures. Progesterone (PG) increased IGFBP-5 expression in normal human osteoblasts and increased IGFBP-5 transcription in U2 human osteosarcoma cells. We developed a chloramphenicol acetyltransferase reporter construct containing the human IGFBP-5 proximal promoter sequence, which includes TATA and CAAT boxes, and five putative PG response element half-sites. 10(-8) M PG increased promoter activity of this construct in U2 cells co-transfected with a PG receptor isoform A (PR(A)) expression vector. Analysis of 5' deletion constructs indicates that PG transactivation of IGFBP-5 promoter activity does not require the PG response element half-sites but does require the region -162 to -124 containing two tandem CACCC box sequences. Mutation of the proximal CACCC box at -139 eliminated PG transactivation. Gel shift assays using a -162 to -124 DNA fragment, U2 cell nuclear extracts, and purified PR(A) protein indicate that nuclear factors bind to a CACCC sequence at -139 and that PR(A) alters the pattern of transcription factor interaction with the CACCC sequence. Using a luciferase reporter construct containing base pairs -252 to +24 of the IGFBP-5 promoter, we found that both PR(A) and PR(B) isoforms mediated PG stimulation of promoter activity. These results suggest that PG may stimulate IGFBP-5 gene transcription via a novel mechanism involving PR and CACCC-binding factors.

Progesterone- and dexamethasone-dependent osteoprogenitors in bone cell populations derived from rat vertebrae are different and distinct

Previous experiments have demonstrated that bone cell populations derived from explants of lumbar vertebral bone of adult female rats contain osteoprogenitors that require dexamethasone (Dex) or progesterone (Prog) to proliferate and differentiate into fully differentiated bone-forming osteoblasts. We now show that the Prog-dependent population cannot be detected in male rats after sexual maturation, but is present in prepubertal rats of both sexes and can be induced in adult male-derived populations by culturing the explants in medium containing 17beta-estradiol (10(-9)-10(-8) M). This suggested that the Prog- and Dex-dependent osteoprogenitors in adult female-derived populations were probably distinct populations and that the survival of the Prog-dependent osteoprogenitors and/or their ability to proliferate are dependent on the presence of estrogen. We then proceeded to prove this by using replica plating. When one of the paired colonies duplicated was cultured in medium containing Dex (10(-8) M) and the other in medium containing Prog (10(-5) M), 5.0% of duplicates formed bone in Prog only, 11.1% formed bone in Dex only, and 3.4% formed bone in both Prog and Dex. In all cases the size of the bone-forming colonies in Dex-treated cultures was larger than that in Prog-treated cultures, indicating that the effects of Dex on osteoprogenitor proliferation are greater than those of Prog. The results demonstrate the existence of three classes ofosteoprogenitors in adult female rat-derived bone cell populations: a class responding to Dex only, a class responding to Prog only, and a class responding to both Dex and Prog. The results also indicate that the effects of Prog are not mediated by Prog binding to the glucocorticoid receptor and imply that Prog plays an important role in maintaining bone mass through regulating the class of osteoprogenitors responsive to Prog.

Glucocorticoids regulate the expression of the human osteoblastic endothelin A receptor gene

The endothelial cell-derived peptide endothelin 1 (ET1) stimulates cell proliferation and differentiated functions of human osteoblastic cells (HOC), and HOC constitutively express the endothelin A receptor (ETRA). Therefore, ET1 may play an important role in the regulation of bone cell metabolism. As glucocorticoids (GC) exert a profound influence on bone metabolism and increase the effects of ET1 on bone cell metabolism in vitro, the effects of GC on ETRA expression in HOC were investigated. Dexamethasone (DEX) increased ETRA mRNA levels in a dose- and time-dependent fashion. The effects of dexamethasone, prednisolone, and deflazacort on the increase of ETRA mRNA levels correlate positively with their binding affinity to the GC receptor. Scatchard analysis of ET1 binding data to HOC revealed that DEX increased the binding capacity for ET1 from 25,300 to 62,800 binding sites per osteoblastic cell, leading to an enhanced mitogenic effect of ET1 on HOC after preincubation with DEX. Transiently transfected primary HOC with a reporter gene construct, containing the 5'-flanking region of the ETRA gene fused to luciferase gene, showed a promoter-dependent expression of the reporter gene and the induction of reporter gene expression by DEX treatment. Total RNA extracts of femoral head biopsies with osteonecrotic lesions from GC-treated patients showed threefold higher ETRA mRNA levels compared with extracts of bone biopsies from patients with traumatically induced osteonecrosis and coxarthrosis. Furthermore, GC treatment increased plasma ET1 levels by 50% compared with pretreatment values. These findings suggest that GC induced upregulation of ETRA, and ET1 plasma levels enhance ET1's anabolic action on bone cell metabolism. Increased ET1 concentrations may also impair bone perfusion by vasoconstriction in a metabolically activated skeletal region.

Glucocorticoid stimulation of Na+-dependent ascorbic acid transport in osteoblast-like cells

Ascorbic acid (AA) is an essential cofactor for osteoblast differentiation both in vivo and in vitro. Before it can function, this vitamin must be transported into cells via a specific Na+-dependent AA transporter. In this study, we examine the regulation of this transport activity by glucocorticoids, a class of steroid hormones known to stimulate in vitro osteoblast differentiation. Dexamethasone stimulated Na+-dependent AA transport activity approximately twofold in primary rat calvarial osteoblasts. Effects of hormone on ascorbic acid transport were rapid (detected within 24 h) and were maximally stimulated by 25-50 nM dexamethasone. Similar effects of dexamethasone on transport activity were also observed in murine MC3T3-E1 cells. This preosteoblast cell line was used for a more detailed characterization of the glucocorticoid response. Transport activity was stimulated selectively by glucocorticoids (dexamethasone > corticosterone) relative to other steroid hormones (progesterone and 17-beta-estradiol) and was blocked when cells were cultured in the presence of cycloheximide, a protein synthesis inhibitor. Kinetic analysis of AA transporter activity in control and dexamethasone-treated cells indicated a Km of approximately 17 microM for both groups. In contrast, dexamethasone increased Vmax by approximately 2.5-fold. Cells also contained an Na+-independent glucose transport activity that has been reported in other systems to transport vitamin C as oxidized dehydroascorbic acid. In marked contrast to Na+-dependent AA transport, this activity was inhibited by dexamethasone. Thus, glucocorticoids increase Na+-dependent AA transport in osteoblasts, possibly via up-regulation of transporter synthesis, and this response can be resolved from actions of glucocorticoids on glucose transport.

Thyroid hormone excess increases insulin-like growth factor I transcripts in bone marrow cell cultures: divergent effects on vertebral and femoral cell cultures

Thyroid hormones (T3 and T4) regulate bone development, growth, and turnover. Studies have suggested that different skeletal sites respond differently to thyroid hormones. Therefore, we examined the in vitro T3 responsiveness of cells committed to the osteoblast lineage as a function of skeletal location. Bone marrow cells derived from female rat femurs and vertebrae were cultured using conditions that induce osteogenic differentiation. Cells from both sites formed mineralized bone nodules in primary and secondary culture. In femoral cultures, collagen type I (coll I) and osteocalcin (OC) messenger RNA (mRNA) levels increased from the earliest time point examined (day 3) to a maximum on day 12 and thereafter declined to undetectable levels. T3 increased both OC and coll I mRNA, resulting in a continuous expression throughout the culture period. Insulin-like growth factor I (IGF-I) gene expression was detected at very low levels by Northern analysis of femoral total RNA, and T3 only marginally enhanced IGF-I mRNA levels. In vertebral cultures, OC and coll I mRNA levels also increased with time in culture, but remained expressed throughout the culture period. OC and coll I mRNA levels were not markedly altered in response to T3. In contrast to femoral cells, IGF-I gene expression was easily visualized in Northern blots from untreated vertebral cultures and was markedly increased by the addition of T3. The continuous presence of T3 (10(-7) M) in the medium for 18 days caused a marked decrease in the number of alkaline phosphatase-positive colonies formed in femoral secondary cultures, but only a slight decrease in the number in vertebral cultures. In addition, short term (6 days) exposure to T3 (10(-7) M) at the beginning of the culture period decreased alkaline phosphatase activity in femoral cultures, but not in vertebral cultures. These findings indicate that there are skeletal site-dependent differences in the in vitro responses of cells of the osteoblastic lineage to thyroid hormone.

Inorganic polyphosphate in human osteoblast-like cells

Significant amounts of inorganic polyphosphates and of polyphosphate-degrading exopolyphosphatase activity were detected in human mandibular-derived osteoblast-like cells. The amount of both soluble and insoluble long-chain polyphosphate in unstimulated osteoblast-like cells was higher than in human gingival cells, erythrocytes, peripheral blood mononuclear cells, and human blood plasma. The cellular content of polyphosphate in osteoblast-like cells strongly decreased after a combined treatment of the cells with the stimulators of osteoblast proliferation and differentiation, dexamethasone, beta-glycerophosphate, epidermal growth factor, and ascorbic acid. The amount of soluble long-chain polyphosphate, but not the amount of insoluble long-chain polyphosphate, further decreased after an additional treatment with 1alpha,25-dihydroxyvitamin D3 (1,25(OH)2D3). The decrease in polyphosphate content during treatment with dexamethasone, beta-glycerophosphate, epidermal growth factor, and ascorbic acid was accompanied by a decrease in exopolyphosphatase, pyrophosphatase, and alkaline phosphatase activity. However, additional treatment with 1,25(OH)2D3 resulted in an increase in these enzyme activities. Osteoblast-like cell exopolyphosphatase activity and exopolyphosphatase activity in yeast, rat tissues, and human leukemia cell line HL60 were inhibited by the bisphosphonates etidronate and, to a lesser extent, clodronate and pamidronate. From our results, we assume that inorganic polyphosphate may be involved in modulation of the mineralization process in bone tissue.

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.

Estrogen stimulation of creatine kinase B specific activity in 3T3L1 adipocytes after their differentiation in culture: dependence on estrogen receptor

We have demonstrated previously that rat adipose tissue showed sex and depot-specific responses to gonadal steroids. The epididymal fat pad in males responded exclusively to androgens by increased specific activity of the brain type isozyme of creatine kinase (CK). In females, the parametrial adipose tissue responded exclusively to estrogens. The present study was undertaken to follow the responsiveness to steroid hormones, and the presence of estrogen receptors (ER), in 3T3L1 cells during their differentiation from pre-adipocytes to adipocytes. In pre-adipocytes in which the basal CK specific activity is low, there was no CK response to 17beta estradiol (E2) or dihydrotestosterone (DHT). Differentiation of the cells into adipocytes was accompanied by increased basal CK activity which was stimulated by E2, but not by DHT. Responsiveness to E2 began 5 days after switching pre-adipocytes to differentiation medium. Upon differentiation, ER became demonstrable in the cell nuclei by staining with FITC labeled anti-idiotypic antibody (clone 1D5) directed against the steroid binding domain of ER. The response to E2 was time-dependent and blocked completely by cycloheximide or actinomycin D. 1D5 itself, which has an estrogen mimetic effect, stimulated CK activity in the cells similarly to E2. The antiestrogen tamoxifen which also stimulated CK activity in the adipocytes, completely blocked E2 action. The 'pure' antagonist of E2, ICI 164,384 and the tissue-selective antiestrogens, raloxifene or tamoxifen methiodide were also complete antagonists with no agonistic effects. The response of the 3T3L1 adipocytes to E2 was upregulated by 1,25(OH)2D3. Moreover, IGF1 was also a potent stimulator of CK in these cells, and therefore may mediate partially the stimulation by E2. Transient transfection of the pre-adipocytes with ER permitted E2 induction of CK. Thus, the appearance of ER and concomitant responsiveness to E2 is another hormone-related change occurring in 3T3L1 cells during differentiation, in addition to changes such as development of insulin responsiveness. The interactions in this system provide a useful in vitro model for investigating the development of responsiveness to E2.

Progesterone stimulates proliferation and differentiation of osteoprogenitor cells in bone cell populations derived from adult female but not from adult male rats

We examined the effects of the synthetic glucocorticoid dexamethasone (Dex) and of the sex steroids progesterone (Prog) and testosterone (Testo) on proliferation and differentiation of progenitors for osteoblasts, adipocytes, and macrophages in cell populations derived from lumbar vertebrae of adult male and female rats. To assay for these progenitors, we used a previously described colony assay, where progenitors are identified by the appearance of colonies of the differentiated phenotype in long term cultures of cell populations containing these progenitors. In cell populations derived from both males and females, Dex (10(-9)-10(-6) mol/L) induced a similar dose-dependent increase in the number of osteoblast colonies (bone nodules), colonies of alkaline phosphatase (AP)-positive cells, adipocyte colonies and macrophage colonies (ED2-positive cells). Prog (10(-8)-10(-5) mol/L), on the other hand, increased bone nodule formation in female-derived populations but not in male-derived populations. Maximal stimulation was seen at 10(-5) mol/L. 17 beta-Estradiol (E2) enhanced the Prog-induced increase in the number of bone nodules in a dose-related fashion. Maximal stimulation was seen at 10(-8) mol/L E2. E2 (10(-9)-10(-6) mol/L) had no effect on Dex-induced bone nodule formation, indicating that the effect is specific for Prog-induced stimulation of bone nodule formation. Prog also caused a dose-dependent increase in the number of colonies of AP-positive cells. Interestingly, the effect of Prog on the number of AP-positive colonies was the same in populations derived from both sexes. Prog-induced stimulation of adipocyte and macrophage development in female-derived populations was significantly greater than that in male-derived populations. Testo (10(-9)-10(-5) mol/L) had no effect on any of the parameters evaluated in populations derived from either males or females. These observations demonstrate that the effect of Prog on proliferation and differentiation of progenitors for osteoblasts, adipocytes, and macrophages in cell populations derived from lumbar vertebrae of adult male or female rats is sex-dependent and is seen either only (bone nodule formation) or more pronounced (adipocyte and macrophage development) in female-derived populations.

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.