Expression of RANKL/OPG during bone remodeling in vivo.

The interaction between receptor activator of nuclear factor κB ligand (RANKL) and osteoprotegerin (OPG) plays a dominant role in osteoclastogenesis. As both proteins are produced by osteoblast lineage cells, they are considered to represent a key link between bone formation and resorption. In this study, we investigated the expression of RANKL and OPG during bone remodeling in vivo to determine the relationship between osteoclastogenic stimulation and osteoblastic differentiation. Total RNA was prepared from rat femurs after marrow ablation on days 0, 3, 6, and 9. The temporal activation patterns of osteoblast-related genes (procollagen α1 (I), alkaline phosphatase, osteopontin, and osteocalcin) were examined by Northern blot analysis. An appreciable increase in the expression of these osteoblast markers was observed on day 3. The peak increase in gene expression was observed on day 6 followed by a slight reduction by day 9. Real-time PCR analysis showed that the OPG mRNA expression was markedly upregulated on day 6 and slightly decreased on day 9. In contrast, RANKL mRNA expression was increased by more than 20-fold on day 9. The RANKL/OPG ratio, an index of osteoclastogenic stimulation, peaked on day 9. Histological analysis showed that RANKL and OPG immunoreactivity were predominantly associated with bone marrow cells. The expression of bone formation markers was activated in the bone formation phase, followed by the stimulation of RANKL/OPG expression in the bone resorption phase, which confirmed that these molecules are key factors linking bone formation to resorption during bone remodeling.

Effect of IGF-I and PDGF administered in vivo on the expression of osteoblast-related genes in old rats.

In order to establish the cellular basis for using growth factors as possible therapeutic agents for the age-dependent deficit in bone formation activity, we examined the individual and combined effects of IGF-I and/or platelet-derived growth factor (PDGF) on the gene expression of osteoblast-related markers in male rats. The expression of osteoblast markers was examined in the femurs of adult and old rats following marrow ablation, which amplifies gene expression activity. The mRNA levels of collagen(alpha1) (I) (COLI), alkaline phosphatase (AP), osteopontin (OP) and osteocalcin (OC) were significantly lower in the old as compared with the adult rats. To determine whether growth factors can abolish the age-related deficits in mRNA expression in old bone, PDGF and/or IGF-I were infused directly into the right femur for 5 days following marrow ablation. The contralateral femur was infused with vehicle only and used as a control. PDGF stimulated the expression of OP mRNA in both adult and old rats, whereas COLI, AP and OC mRNAs were not affected. IGF-I infusion did not have a significant effect on mRNA expression in adult rats. In contrast, treatment with IGF-I significantly enhanced the mRNA levels of COLI, AP and OP in old rats. To examine whether the combination of both factors could affect the expression of osteoblast markers synergistically, PDGF and IGF-I were infused together. In adult bones, the combined treatment with PDGF and IGF-I caused a slight increase in the level of OP gene expression but no change in AP, OC or COLI genes. Although neither IGF-I nor PDGF alone was effective in stimulating the expression of OC, the combined treatment in old bones enhanced OC expression significantly. The expression of COLI, AP and OP was also stimulated, but the stimulation was no different from that of IGF-I alone. In PDGF plus IGF-I treatment with a high dose, no dose-response effects were observed. Within the limits of the present study, it is suggested that IGF-I and, to a much lesser extent, PDGF may partially restore the deficit in the expression of osteoblast markers in old bones, and that the combination of both factors is slightly better than IGF-I alone in stimulating OC expression.

Local and systemic expression of insulin-like growth factor-I (IGF-I) mRNAs in rat after bone marrow ablation.

Local and systemic expression of insulin-like growth factor-I (IGF-I) during bone formation was studied using the rat bone marrow ablation model. The temporal expression pattern of IGF-I mRNA in rat femurs was examined. The IGF-I mRNA level was enhanced rapidly after ablation reaching a level threefold greater than basal by day 3 (P < 0.01) and declined to basal or below basal level by day 5. Histological analysis showed that IGF- I immunoreactivity was predominantly associated with the mesenchymal cells at the bone/connective tissue interface and osteoblastic cells at active sites of bone formation. Serum level of IGF-I increased 50 and 130%, respectively (P < 0.005), over the basal level at days 3 and 6. We also investigated the systemic expression of IGF-I in liver and kidney. In contrast, hepatic IGF-I gene expression decreased 37 and 48%, respectively, at days 3 and 6 after marrow ablation (P < 0.001). Kidney IGF-I mRNA levels also fell 13 and 27%, respectively, at days 3 and 6 (P < 0.005). The present findings suggest that locally produced IGF-I during bone formation may not only serve as an autocrine/paracrine factor but also influence systemic expression of IGF-I in other organs.

Actions of bFGF on mitogenic activity and lineage expression in rat osteoprogenitor cells: effect of age.

Rat osteoprogenitor cells were used to examine the effects of bFGF on DNA synthesis and the expression of osteoblast (OB)-related genes. bFGF, as low as 0.1 ng/ml, stimulated DNA synthesis. bFGF also increased the mRNA level of osteopontin (OP) and decreased that of type I collagen (COL I). When cultures were grown in dexamethasone (DEX) to induce OB lineage commitment, the expression of COL I, alkaline phosphatase (AP) and OP was greatly enhanced. Subsequent incubation with bFGF partially negated the stimulatory effect of DEX on AP and COL I mRNAs. bFGF also inhibited the expression of osteocalcin mRNA in cells grown in 1,25(OH)2D3 and DEX. Combined effects of bFGF with IGF-I or PDGF on DNA synthesis and OP expression were examined. bFGF + IGF-I, but not bFGF + PDGF, was more effective than PDGF alone. By comparing cells from adult and old animals, we found that bFGF-induced mitogenic activity was reduced significantly with age. In contrast, the effect of bFGF on the expression of OB genes was not significantly altered by age. These findings suggest that bFGF plays a dual role as a local positive and negative regulator on proliferation and osteogenic lineage expression, respectively, in osteoprogenitor cells, and that the mitogenic activity in response to bFGF was impaired in aging.

Treatment of osteoporosis with MMP inhibitors.

In the current study, we examined the effects of minocycline on the osteopenia of ovariectomized (OVX) aged rats using the marrow ablation model. This injury induces rapid bone formation followed by bone resorption in the marrow cavity. Old female rats were randomly divided into five groups: sham, OVX, OVX + minocycline (5-15 mg/day, orally), OVX + 17 beta-estradiol (25 micrograms/day, subcutaneously), and OVX + both agents. Rats were OVX, treated with minocycline and/or estrogen, followed by marrow ablation. Bone samples were collected 16 days post-marrow ablation. X-ray radiography of bones operated on showed that treatment of OVX old rats with minocycline increased bone mass in diaphyseal region. Diaphyseal bone mineral density (BMD) was measured by DEXA scan. Diaphyseal BMD of OVX rats was increased 17-25% by treatment with 5-15 mg of minocycline or 17 beta-estradiol. The effects of minocycline and estrogen treatments on the expression of osteoblast and osteoclast markers were also examined. Northern and dot blot analysis of RNA samples showed that treatment of OVX aged rats with minocycline increased the expression of type I collagen (COL I) (49%) and decreased that of interleukin-6 (IL-6) (31%). In contrast, estrogen treatment decreased the expression of interleukin-6 (IL-6) (39%), carbonic anhydrase II (CA II) (36%), and osteopontin (OP) (37%). Neither minocycline nor 17 beta-estradiol had an effect on the expression of osteocalcin (OC) and alkaline phosphatase (AP). To elucidate the mechanism by which minocycline prevented the loss of bone in OVX aged rats, we examined the colony-formation potential of bone marrow stromal cells in ex vivo cultures. Minocycline stimulated the colony-forming efficiency of marrow stromal cells derived from old animals. We have therefore concluded that the modest increase in BMD noted in OVX aged rats, in response to minocycline treatment, may be due to a change in bone remodeling that favors bone formation; and the anabolic effect of minocycline is likely due to its effect on the expression of COL I and/or the metabolism of osteoprogenitor cells.

Cloning of a cDNA encoding the human transforming growth factor-beta type II receptor: heterogeneity of the mRNA.

We have isolated two novel cDNAs encoding the transforming growth factor-beta (TGF-beta) type II receptor (TGF-beta IIR), termed TGF-beta IIR alpha and TGF-beta IIR beta 1 from a human fetal liver library. They have unique nucleotide (nt) sequences, compared with the reported TGF-beta IIR sequence, at the 5' end. Southern blot analysis using probes from each clone detected the specific genomic DNA fragments. RT-PCR analysis revealed a distinct pattern of expression for each isoform. These results indicated that TGF-beta IIR has heterogeneity in the structure, and the expression of TGF-beta IIR isoforms is differentially regulated. The heterogeneity of TGF-beta IIR molecules could be derived from alternative splicing and might elicit specific TGF-beta receptor functions.