Decreased AP-1 activity and interleukin-11 expression by bone marrow stromal cells may be associated with impaired bone formation in aged mice

Low-intensity pulsed ultrasound regulates bone marrow mesenchymal stromal cells differentiation and inhibits bone loss by activating the IL-11-Wnt/β-catenin signaling pathway

BACKGROUND

Osteoporosis (OP) is a common metabolic bone disease. Low-intensity pulsed ultrasound (LIPUS) can effectively promote bone formation and fracture healing. The Wnt/β-catenin signaling pathway is crucial for regulating bone homeostasis and bone diseases, and its downregulation is one of the main mechanisms of osteoporosis pathogenesis. Interleukin-11 (IL-11), which is regulated by mechanical stress, is a key factor in bone remodeling. Here, we investigated the optimal intervention parameters for LIPUS, the relationships among LIPUS, IL-11, and the Wnt/β-catenin signaling pathway, and the effects of LIPUS on bone loss and potential molecular mechanisms in ovariectomized (OVX) mice.

METHODS

Bone marrow mesenchymal stromal cells (BMSCs) were subjected to LIPUS intervention for 0, 10, or 20 min to determine the optimal intervention time. The mediating role of IL-11 in LIPUS intervention was explored through IL-11 knockdown and overexpression. Finally, animal experiments were conducted to investigate the in vivo therapeutic effects of LIPUS.

RESULTS

The optimal intervention time for LIPUS was 20 min. LIPUS promoted IL-11 expression and upregulated the Wnt/β-catenin signaling pathway, thereby promoting osteogenic differentiation and inhibiting adipogenic differentiation of BMSCs. IL-11 mediates the regulation of the Wnt/β-catenin signaling pathway by LIPUS. Additionally, LIPUS effectively improved the bone microstructure in ovariectomized mice, inhibited bone loss, promoted IL-11 expression in bone tissue, and activated the Wnt/β-catenin signaling pathway in the femur.

CONCLUSION

Low-intensity pulsed ultrasound can regulate BMSCs differentiation and inhibit bone loss by promoting IL-11 expression and activating the Wnt/β-catenin signaling pathway.

Bone-fat linkage via interleukin-11 in response to mechanical loading

Positive regulators of bone formation, such as mechanical loading and PTH, stimulate and negative regulators, such as aging and glucocorticoid excess, suppress IL-11 gene transcription in osteoblastic cells. Signal transduction from mechanical loading and PTH stimulation involves two pathways: one is Ca2+-ERK-CREB pathway which facilitates binding of ∆FosB/JunD to the AP-1 site to enhance IL-11 gene transcription, and the other is Smad1/5 phosphorylation that promotes IL-11 gene transcription via SBE binding and complex formation with ∆FosB/JunD. The increased IL-11 suppresses Sost expression via IL-11Rα-STAT1/3-HDAC4/5 pathway and enhances Wnt signaling in the bone to stimulate bone formation. Thus, IL-11 mediates stimulatory and inhibitory signals of bone formation by affecting Wnt signaling. Physiologically important stimulation of bone formation is exercise-induced mechanical loading, but exercise simultaneously requires energy source for muscle contraction. Exercise-induced stimulation of IL-11 expression in the bone increases the secretion of IL-11 from the bone. The increased circulating IL-11 acts like a hormone to enhance adipolysis as an energy source with a reduction in adipogenic differentiation via a suppression of Dkk1/2 expression in the adipose tissue. Such bone-fat linkage can be a mechanism whereby exercise increases bone mass and, at the same time, maintains energy supply from the adipose tissue.

Roles of IL-11 in the regulation of bone metabolism

Bone metabolism is the basis for maintaining the normal physiological state of bone, and imbalance of bone metabolism can lead to a series of metabolic bone diseases. As a member of the IL-6 family, IL-11 acts primarily through the classical signaling pathway IL-11/Receptors, IL-11 (IL-11R)/Glycoprotein 130 (gp130). The regulatory role of IL-11 in bone metabolism has been found earlier, but mainly focuses on the effects on osteogenesis and osteoclasis. In recent years, more studies have focused on IL-11's roles and related mechanisms in different bone metabolism activities. IL-11 regulates osteoblasts, osteoclasts, BM stromal cells, adipose tissue-derived mesenchymal stem cells, and chondrocytes. It's involved in bone homeostasis, including osteogenesis, osteolysis, bone marrow (BM) hematopoiesis, BM adipogenesis, and bone metastasis. This review exams IL-11's role in pathology and bone tissue, the cytokines and pathways that regulate IL-11 expression, and the feedback regulations of these pathways.

The Emerging Role of Interleukin-(IL)-11/IL-11R in Bone Metabolism and Homeostasis: From Cytokine to Osteokine

Interleukin-(IL)-11 is a cytokine involved in hematopoiesis, cancer metastasis, and inflammation. IL-11 belongs to the IL-6 cytokine family, binding to the complex of receptors glycoprotein gp130 and the ligand-specific-receptor subunits (IL-11Rα or their soluble counterpart sIL-11R). IL-11/IL-11R signaling enhances osteoblast differentiation and bone formation and mitigates osteoclast-induced bone resorption and cancer bone metastasis. Recent studies have shown that systemic and osteoblast/osteocyte-specific IL-11 deficiency leads to reduced bone mass and formation, but also adiposity, glucose intolerance, and insulin resistance. In humans, mutations of IL-11 and the receptor IL-11RA genes are associated with height reduction, osteoarthritis, and craniosynostosis. In this review, we describe the emerging role of IL-11/IL-11R signaling in bone metabolism by targeting osteoblasts, osteoclasts, osteocytes, and bone mineralization. Furthermore, IL-11 promotes osteogenesis and suppresses adipogenesis, thereby influencing the fate of osteoblast/adipocyte differentiation derived from pluripotent mesenchymal stem cells. We have newly identified IL-11 as a bone-derived cytokine that regulates bone metabolism and the link between bone and other organs. Thus, IL-11 is vital in bone homeostasis and could be considered a potential therapeutic strategy.

Tethering not required: the glucocorticoid receptor binds directly to activator protein-1 recognition motifs to repress inflammatory genes

The glucocorticoid receptor (GR) is a ligand-regulated transcription factor that controls the expression of extensive gene networks, driving both up- and down-regulation. GR utilizes multiple DNA-binding-dependent and -independent mechanisms to achieve context-specific transcriptional outcomes. The DNA-binding-independent mechanism involves tethering of GR to the pro-inflammatory transcription factor activator protein-1 (AP-1) through protein-protein interactions. This mechanism has served as the predominant model of GR-mediated transrepression of inflammatory genes. However, ChIP-seq data have consistently shown GR to occupy AP-1 response elements (TREs), even in the absence of AP-1. Therefore, the current model is insufficient to explain GR action at these sites. Here, we show that GR regulates a subset of inflammatory genes in a DNA-binding-dependent manner. Using structural biology and biochemical approaches, we show that GR binds directly to TREs via sequence-specific contacts to a GR-binding sequence (GBS) half-site found embedded within the TRE motif. Furthermore, we show that GR-mediated transrepression observed at TRE sites to be DNA-binding-dependent. This represents a paradigm shift in the field, showing that GR uses multiple mechanisms to suppress inflammatory gene expression. This work further expands our understanding of this complex multifaceted transcription factor.

Expression of the IL-11 Gene in Metastatic Cells Is Supported by Runx2-Smad and Runx2-cJun Complexes Induced by TGFβ1

In tumor cells, two factors are abnormally increased that contribute to metastatic bone disease: Runx2, a transcription factor that promotes expression of metastasis related and osteolytic genes; and IL-11, a secreted osteolytic cytokine. Here, we addressed a compelling question: Does Runx2 regulate IL-11 gene expression? We find a positive correlation between Runx2, IL-11 and TGFβ1, a driver of the vicious cycle of metastatic bone disease, in prostate cancer (PC) cell lines representing early (LNCaP) and late (PC3) stage disease. Further, like Runx2 knockdown, IL-11 knockdown significantly reduced expression of several osteolytic factors. Modulation of Runx2 expression results in corresponding changes in IL-11 expression. The IL-11 gene has Runx2, AP-1 sites and Smad binding elements located on the IL-11 promoter. Here, we demonstrated that Runx2-c-Jun as well as Runx2-Smad complexes upregulate IL-11 expression. Functional studies identified a significant loss of IL-11 expression in PC3 cells in the presence of the Runx2-HTY mutant protein, a mutation that disrupts Runx2-Smad signaling. In response to TGFβ1 and in the presence of Runx2, we observed a 30-fold induction of IL-11 expression, accompanied by increased c-Jun binding to the IL-11 promoter. Immunoprecipitation and in situ co-localization studies demonstrated that Runx2 and c-Jun form nuclear complexes in PC3 cells. Thus, TGFβ1 signaling induces two independent transcriptional pathways - AP-1 and Runx2. These transcriptional activators converge on IL-11 as a result of Runx2-Smad and Runx2-c-Jun interactions to amplify IL-11 gene expression that, together with Runx2, supports the osteolytic pathology of cancer induced bone disease.

Donor age negatively affects the immunoregulatory properties of both adipose and bone marrow derived mesenchymal stem cells

PURPOSE

Age negatively impacts the biologic features of mesenchymal stem cells (MSCs), including decreased expansion kinetics and differentiation potential. Clinically, donor-age may be within a wide spectrum; therefore, investigation of the role of donor's age on immunoregulatory potential is of critical importance to translate stem cell therapies from bench to bedside.

METHODS

Adipose and bone marrow derived MSCs (ASCs and BMSCs) were isolated in parallel from Lewis and Brown Norway rats of young (less than 4-week old) and senior groups (older than 15-month). The presentation of cells and time required for growth to 90% confluence was recorded. FACS sorting based on the expression of CD90 and CD29 double positive and CD45 CD11 double negative quantified the proportions of MSCs. After expansion, ASCs and BMSCs from different age groups were co-cultured in mixed lymphocyte reaction (MLR; Lewis vs. Brown Norway) assays. The suppression of CD3(+)CD4(+) and CD3(+)CD8(+) T cell populations by different sources of MSCs were compared.

RESULTS

The kinetics of cell growth was slower in old animals (17.3±2days) compared with young animals (8.8±3days), and cell morphology was irregular and enlarged in the senior groups. The yield of MSCs by FACS sorting was significantly higher in young groups compared to senior groups (p<0.02). With regard to immunoregulatory potential, senior ASCs failed to induce any CD3(+)CD4(+) T cell suppression (p>0.05). In addition, young BMSCs-induced suppression was more prominent than seniors (p<0.05).

CONCLUSIONS

Donor age should be taken into consideration when using recipient MSC of either bone marrow or adipose origin in clinical applications.

The interrelationship between bone and fat: from cellular see-saw to endocrine reciprocity

The number of mature osteoblasts and marrow adipocytes in bone is influenced by the differentiation of the common mesenchymal progenitor cell towards one phenotype and away from the other. Consequently, factors which promote adipogenesis not only lead to fatty marrow but also inhibit osteoblastogenesis, resulting in decreased osteoblast numbers, diminished bone formation and, potentially, inadequate bone mass and osteoporosis. In addition to osteoblast and bone adipocyte numbers being influenced by this skewing of progenitor cell differentiation towards one phenotype, mature osteoblasts and adipocytes secrete factors which may evoke changes in the cell fate and function of each other. This review examines the endogenous factors, such as PPAR-γ2, Wnt, IGF-1, GH, FGF-2, oestrogen, the GP130 signalling cytokines, vitamin D and glucocorticoids, which regulate the selection between osteoblastogenesis and adipogenesis and the interrelationship between fat and bone. The role of adipokines on bone, such as adiponectin and leptin, as well as adipose-derived oestrogen, is reviewed and the role of bone as an energy regulating endocrine organ is discussed.

Neopterin, inflammation-associated product, prolongs erythropoiesis suppression in aged SAMP1 mice due to senescent stromal-cell impairment

Anemia induced by inflammation is well known to be more serious in the elderly than in non-elderly adults; however, the reason why this is so remains unclear. Neopterin produced by monocytes during inflammation promotes myelopoiesis but suppresses B-lymphopoiesis and erythropoiesis, by activating stromal cells in mice. Here, age-related changes in the erythropoietic response to neopterin were determined using senescence accelerated mice (SAMP1) with senescence stromal-cell impairment. Intravenous injection of neopterin into young mice (8-12 weeks old) resulted in a decrease in erythroid progenitor cell number in the bone marrow (BM), concomitant with an increase in myeloid progenitor cell number over one week. Intravenous injection of neopterin into aged mice (30-36 weeks old) resulted in a prolonged decrease in erythroid progenitor cell number in the BM over three weeks and a limited increase in myeloid progenitor cell number over one day. Neopterin treatment induced a decrease in serum erythropoietin concentrations in young mice but not in aged mice. The gene expression of tumor necrosis factor α (TNF-α), a negative regulator of erythropoiesis, was up-regulated in the BM of both young and aged mice, and the degree of TNF-α up-regulation was the same in both groups. The gene expression of interleukin (IL)-11, a positive regulator of erythropoiesis, was also up-regulated over one day in both young and aged mice. However, IL-11 gene expression remained up-regulated thereafter in young mice, whereas it was rapidly down-regulated in aged mice. These data suggest that prolonged suppression of erythropoiesis in aged mice may be due to a decrease in the production of positive regulators rather than to an increase in the production of negative regulators. Our combined data suggest that age-related impairment of stromal cells induces serious anemia in the elderly during inflammation.

Regulation of osteoblast differentiation by interleukin-11 via AP-1 and Smad signaling

Mechanical stress and parathyroid hormone (PTH) are major stimulators, and aging and glucocorticoids excess are important suppressors of osteoblast differentiation. Mechanical stress and PTH stimulate interleukin (IL)-11 expression in cells of osteoblast lineage by enhancing transcription of IL-11 gene via an increase in intracellular Ca²⁺. The elevated Ca²⁺ activates extracellular signal-regulated kinase (ERK) to enhance phosphorylation of cyclic AMP response element-binding protein (CREB), which binds to the fosB gene promoter and enhances ΔFosB expression. ΔFosB dimerizes with JunD on the IL-11 gene promoter to enhance its transcription. Both mechanical stress and PTH also stimulate phosphorylation of Smad1 via an activation of protein kinase Cδ (PKCδ). Phosphorylated Smad1 binds to the IL-11 gene promoter and forms complex with ΔFosB/JunD to further enhance IL-11 gene transcription. The increased IL-11 then suppresses expression of Wnt inhibitors, including Dickkopf 1 (Dkk1) and 2, and enhances Wnt signaling to stimulate osteoblast differentiation and inhibit adipocyte differentiation. The suppression of osteoblast differentiation by aging involves a decrease in IL-11 gene transcription by a reduction in JunD binding to the activator protein (AP)-1 site of the IL-11 gene promoter. Glucocorticoids inhibit transcriptional activation of IL-11 gene by an interaction of glucocorticoid-glucocorticoid receptor (GR) complex with ΔFosB/JunD heterodimer. Thus, factors that enhance osteoblast differentiation stimulate, and those which suppress osteoblast differentiation inhibit IL-11 gene transcription, and IL-11 enhances Wnt signaling by suppressing expression of its inhibitors. These observations are consistent with the notion that IL-11 mediates stimulatory and inhibitory signals of osteoblast differentiation by affecting Wnt signaling.

An anti-inflammatory selective glucocorticoid receptor modulator preserves osteoblast differentiation

Glucocorticoids (GCs) are in widespread use to treat inflammatory bone diseases, such as rheumatoid arthritis (RA). Their anti-inflammatory efficacy, however, is accompanied by deleterious effects on bone, leading to GC-induced osteoporosis (GIO). These effects include up-regulation of the receptor activator of NF-κB ligand/osteoprotegerin (RANKL/OPG) ratio to promote bone-resorbing osteoclasts and include inhibition of bone-forming osteoblasts. We previously identified suppression of osteoblast differentiation by the monomer glucocorticoid receptor (GR) via the inhibition of Il11 expression as a crucial mechanism for GIO. Here we show that the GR-modulating substance compound A (CpdA), which does not induce GR dimerization, still suppresses proinflammatory cytokines in fibroblast-like synovial cells from patients with RA and in osteoblasts. In contrast to the full GR agonist dexamethasone, it does not unfavorably alter the RANKL/OPG ratio and does not affect Il11 expression and subsequent STAT3 phosphorylation in these cells. Notably, while dexamethasone inhibits osteoblast differentiation, CpdA does not affect osteoblast differentiation in vitro and in vivo. We describe here for the first time that selective GR modulators can act against inflammation, while not impairing osteoblast differentiation.

Integrative physiology of the aging bone: insights from animal and cellular models

Age-related bone loss is a common worldwide phenomenon in the aging population, placing them at an increased risk of fractures. Fortunately, basic and translational studies have been pivotal in providing us with a mechanistic understanding of the cellular and molecular pathophysiology of this condition. This review focuses on the current concepts and paradigms of age-related bone loss and how various animal and cellular models have broadened our understanding in this fascinating but complex area. Changes in hormonal, neuronal, and biochemical cues with age and their effect on bone have been discussed. This review also outlines recent studies on the relationship between bone and fat in the marrow, as well as the fate of the marrow mesenchymal stromal cell population, which can give rise to either bone-forming osteoblasts or fat-forming adipocytic cells as a function of age.

Mechanical stress activates Smad pathway through PKCδ to enhance interleukin-11 gene transcription in osteoblasts

BACKGROUND

Mechanical stress rapidly induces ΔFosB expression in osteoblasts, which binds to interleukin (IL)-11 gene promoter to enhance IL-11 expression, and IL-11 enhances osteoblast differentiation. Because bone morphogenetic proteins (BMPs) also stimulate IL-11 expression in osteoblasts, there is a possibility that BMP-Smad signaling is involved in the enhancement of osteoblast differentiation by mechanical stress. The present study was undertaken to clarify whether mechanical stress affects BMP-Smad signaling, and if so, to elucidate the role of Smad signaling in mechanical stress-induced enhancement of IL-11 gene transcription.

METHODOLOGY/PRINCIPAL FINDINGS

Mechanical loading by fluid shear stress (FSS) induced phosphorylation of BMP-specific receptor-regulated Smads (BR-Smads), Smad1/5, in murine primary osteoblasts (mPOBs). FSS rapidly phosphorylated Y311 of protein kinase C (PKC)δ, and phosphorylated PKCδ interacted with BR-Smads to phosphorylate BR-Smads. Transfection of PKCδ siRNA or Y311F mutant PKCδ abrogated BR-Smads phosphorylation and suppressed IL-11 gene transcription enhanced by FSS. Activated BR-Smads bound to the Smad-binding element (SBE) of IL-11 gene promoter and formed complex with ΔFosB/JunD heterodimer via binding to the C-terminal region of JunD. Site-directed mutagenesis in the SBE and the AP-1 site revealed that both SBE and AP-1 sites were required for full activation of IL-11 gene promoter by FSS.

CONCLUSIONS/SIGNIFICANCE

These results demonstrate that PKCδ-BR-Smads pathway plays an important role in the intracellular signaling in response to mechanical stress, and that a cross-talk between PKCδ-BR-Smads and ΔFosB/JunD pathways synergistically stimulates IL-11 gene transcription in response to mechanical stress.

Glucocorticoids suppress bone formation by attenuating osteoblast differentiation via the monomeric glucocorticoid receptor

Development of osteoporosis severely complicates long-term glucocorticoid (GC) therapy. Using a Cre-transgenic mouse line, we now demonstrate that GCs are unable to repress bone formation in the absence of glucocorticoid receptor (GR) expression in osteoblasts as they become refractory to hormone-induced apoptosis, inhibition of proliferation, and differentiation. In contrast, GC treatment still reduces bone formation in mice carrying a mutation that only disrupts GR dimerization, resulting in bone loss in vivo, enhanced apoptosis, and suppressed differentiation in vitro. The inhibitory GC effects on osteoblasts can be explained by a mechanism involving suppression of cytokines, such as interleukin 11, via interaction of the monomeric GR with AP-1, but not NF-kappaB. Thus, GCs inhibit cytokines independent of GR dimerization and thereby attenuate osteoblast differentiation, which accounts, in part, for bone loss during GC therapy.

Mechanical stress induces Interleukin-11 expression to stimulate osteoblast differentiation

Molecular mechanism of mechanical stress-induced bone formation remains unclear. We demonstrate that mechanical unloading suppresses and reloading enhances Interleukin (IL)-11 gene expression in the hindlimb of mice in vivo. Mechanical stress to osteoblasts by fluid shear stress (FSS) in vitro rapidly and transiently enhances fosB gene transcription, stimulates binding of DeltaFosB/JunD complex to activator protein (AP)-1 site of the IL-11 gene promoter, and enhances IL-11 gene transcription. Anti-IL-11 antibody blocks mechanical stress-induced enhancement of osteoblastogenesis and suppression of adipogenesis, suggesting the requirement of IL-11 for the stimulation of osteoblast differentiation by mechanical stress. Down-regulation of DeltaFosB/JunD by small interfering RNA (siRNA) suppresses and overexpression of DeltaFosB/JunD enhances IL-11 gene promoter activity. Consistent with our previous observations that up-regulation of DeltaFosB depends upon activation of cyclic AMP response element-binding protein (CREB) via Ca(2+)-dependent activation of extracellular signal-regulated kinase (ERK) to phosphorylate CREB, mechanical stress-induced activation of IL-11 gene transcription is dependent upon Ca(2+)-ERK pathway. Present results also demonstrated that FSS to osteoblasts enhances canonical Wnt signaling in vitro, and that mechanical unloading induces and reloading suppresses the expression of a canonical Wnt signal inhibitor, dickkopf2 (Dkk2), in vivo. In addition, IL-11 siRNA enhances Dkk2 expression suppressed by FSS, and osteoblasts from IL-11 transgenic mice show reduced Dkk2 mRNA expression than those from wild-type mice. These observations are consistent with the notion that mechanical stress stimulates IL-11 gene transcription via an enhanced DeltaFosB/JunD binding to the IL-11 gene promoter, and that increased IL-11 enhances canonical Wnt signal at least in part via a reduction in Dkk2 expression to stimulate osteoblast differentiation.

Static tensional forces increase osteogenic gene expression in three-dimensional periodontal ligament cell culture

Orthodontic tooth movement results from the combinational process of both bone resorption and formation in the compressive and tension sides, respectively. However, the genes responsible for new bone formation in tension sides have not been determined. In this study, we used DNA microarray and real-time RT-PCR to identify genes in human periodontal ligament (PDL) cells that undergo significant changes in expression in response to static tensional forces (2 or 12 hours). The genes found were alkaline phospatase (ALP), matrix metalloproteinases (MMPs), vascular endothelial growth factor (VEGF), and several collagen genes. Furthermore, an ELISA evaluating the expression of VEGF, type IV collagen and MMP-2 found levels significantly increased after 24 and 72 hours (P 0.05). ALP activity was also increased after 24 hours (P 0.05). Collectively, we found the genes up-regulated in our study by the static tensional force are related to osteogenic processes such as matrix synthesis and angiogenesis.

Mechanical stimulation of bone formation is normal in the SAMP6 mouse

With aging, the skeleton may have diminished responsiveness to mechanical stimulation. The senescence-accelerated mouse SAMP6 has many features of senile osteoporosis and is thus a useful model to examine how the osteoporotic skeleton responds to mechanical loading. We performed in vivo tibial bending on 4-month-old SAMP6 (osteoporotic) and SAMR1 (control) mice. Loading was applied daily (60 cycles/day, 5 days/week) for 2 weeks at peak force levels that produced estimated endocortical strains of 1,000 and 2,000 microepsilon In a separate group of mice, sham bending was applied. Comparisons were made between right (loaded) and left (nonloaded) tibiae. Tibial bone marrow cells were cultured under osteogenic conditions and stained for alkaline phosphatase (ALP) and alizarin red (ALIZ) at 14 and 28 days, respectively. Tibiae were then embedded in plastic and sectioned, and endocortical bone formation was assessed based on calcein labels. Tibial bending did not alter the osteogenic potential of the marrow as there were no significant differences in ALP or ALIZ staining between loaded and nonloaded bones. Tibial bending activated the formation of endocortical bone in both SAMP6 and SAMR1 mice, whereas sham bending did not elicit an endocortical response. Both groups of mice exhibited bending strain-dependent increases in bone formation rate. We found little evidence of diminished responsiveness to loading in the SAMP6 skeleton. In conclusion, the ability of the SAMP6 mouse to respond normally to an anabolic mechanical stimulus distinguishes it from chronologically aged animals. This finding highlights a limitation of the SAMP6 mouse as a model of senile osteoporosis.

Basic fibroblast growth factor-induced neuronal differentiation of mouse bone marrow stromal cells requires FGFR-1, MAPK/ERK, and transcription factor AP-1

It has been reported recently that bone marrow stromal cells (BMSCs) are able to differentiate into various neural cells both in vivo and in vitro (Egusa, H., Schweizer, F. E., Wang, C. C., Matsuka, Y., and Nishimura, I. (2005) J. Biol. Chem. 280, 23691-23697). However, the underlying mechanisms remain largely unknown. In this report, we have demonstrated that basic fibroblast growth factor (bFGF) alone effectively induces mouse BMSC neuronal differentiation. These differentiated neuronal cells exhibit characteristic electrophysiological properties and elevated levels of the neuronal differentiation marker, growth-associated protein-43 (GAP-43). To explore possible signaling pathways, we first analyzed the expression of various FGF receptors in mouse BMSCs. FGF receptor-1, -2, and -3 were detected, but only FGFR-1 was shown to be activated by bFGF. Small interfering RNA knock down of FGFR-1 in BMSCs significantly inhibited neuronal differentiation. Moreover, we have shown that the mitogen-activated protein kinase (ERK1/2) is persistently activated and blockage of ERK activity with the ERK-specific inhibitor U0126 prevents neuronal differentiation. It appears that activation of ERK cascade and neuronal differentiation of BMSCs induced by bFGF are independent of Ras activity but require functions of phospholipase C-gamma pathway. Lastly, we examined the role of the immediate-early transcription factors AP-1 and NF-kappaB and have found that phospholipase C-gamma-dependent c-Jun and ERK-dependent c-fos, but not the NF-kappaB, are strongly activated by bFGF, which in turn regulates the neuronal differentiation of BMSCs.

Induction of Senile Osteoporosis in Normal Mice by Intra-Bone Marrow-Bone Marrow Transplantation from Osteoporosis-Prone Mice

A P6 substrain of the senescence accelerated mouse (SAMP6) spontaneously develops osteoporosis early in life. These mice show the clinical signs of osteoporosis, such as elevated levels of urinary deoxypyridinoline (Dpd), decreased bone mineral density (BMD), and a significant loss of trabecular and cortical bone thickness at 12 months of age. Here, we describe the transfer of osteoporosis to a normal strain by the injection of bone marrow cells from SAMP6 donors directly into the bone marrow cavity (intra-bone marrow-bone marrow transplantation [IBM-BMT]). More than 1 month after IBM-BMT, hematolymphoid cells were completely reconstituted by donor-derived cells, and bone marrow stromal cells that could differentiate into osteocytes were also found to be of donor origin. In addition, the recipient C57BL/6 mouse showed the features of osteoporosis in the trabecular bone. Decreases in BMD and increases in urinary Dpd were also observed. When the message levels of cytokines (interleukin [IL]-11, IL-6, receptor activator of NF-kappaB ligand [RANKL], osteoprotegerin, macrophage-colony-stimulating factor, and insulin-like growth factor-1) were examined by reverse transcription-polymerase chain reaction (RT-PCR) and real-time RT-PCR analysis, IL-6 and IL-11 were reduced to a level similar to that in SAMP6 mice, whereas that of RANKL was increased. These findings indicate that not only the hemopoietic system but also the bone marrow microenvironment are reconstituted as a result of IBM-BMT, and suggest that the development of senile osteoporosis might be attributable to "stem cell disorders." Disclosure of potential conflicts of interest is found at the end of this article.

Heme oxygenase-1 and interleukin-11 are overexpressed in stress-induced premature senescence of human WI-38 fibroblasts induced by tert-butylhydroperoxide and ethanol

Acute repeated exposures to subcytotoxic concentrations of tert-butylhydroperoxide and ethanol trigger premature senescence of human diploid fibroblasts. In the present work we found an increased mRNA and protein level of interleukin-11 and heme oxygenase-1 in premature senescence of WI-38 human diploid foetal lung fibroblasts induced by both tert-butylhydroperoxide and ethanol. We tested whether interleukin-11 and heme oxygenase-1 could protect against tert-butylhydroperoxide- or ethanol-induced premature senescence when stable overexpression was established using a retroviral vector-based transduction. No protective effect was found against the decrease of the proliferative potential, the increase of the proportion of senescence-associated ss-galactosidase positive cells and the increase of the mRNA levels of six senescence-associated genes.

Treatment of senile osteoporosis in SAMP6 mice by intra-bone marrow injection of allogeneic bone marrow cells

A substrain of the senescence-accelerated mouse, SAMP6 (senescence-accelerated mouse prone 6), spontaneously develops osteoporosis early in life. Therefore, this strain is a useful animal model for developing new strategies for the treatment of osteoporosis in humans. We succeeded in treating osteoporosis in SAMP6 mice after the onset of this disease, using a newly developed method of bone marrow transplantation (BMT): Allogeneic bone marrow cells obtained from normal mouse strains were directly injected into the bone marrow cavity of irradiated SAMP6 mice (intra-bone marrow BMT [IBM-BMT]). After the treatment with IBM-BMT, hematolymphoid cells were completely reconstituted by donor-derived cells, and bone marrow stromal cells were also found to be of donor origin. The treated SAMP6 mice showed histologically-normal trabecular bone. In addition, bone mineral density and urinary deoxypiridinoline, a hallmark of bone destruction, were normalized. When the message levels of cytokines (tumor necrosis factor alpha, interleukin-6 [IL-6], IL-11, and receptor activator of nuclear factor-kappa B ligand [RANKL]) were examined, IL-11, RANKL (from bone marrow stromal cells), and IL-6 (from osteoclasts), which regulate bone remodeling, were restored to levels similar to those in normal B6 mice. These findings indicate that not only the hemopoietic system but also the bone marrow microenvironment were normalized after IBM-BMT, resulting in an amelioration of the imbalance between bone absorption and formation.

Epidermal growth factor (EGF)-mediated DNA-binding activity of AP-1 is attenuated in senescent human epidermal keratinocytes

The proliferative responses of cells to mitogens decrease during aging, and this may result from age-related defects in signal transduction in response to mitogens. In this study, we have investigated the age-related alteration of responses to epidermal growth factor (EGF) in cultured human keratinocytes that were senesced in vitro by repeated passage. The stimulation with EGF increased the DNA-binding activity of activator protein 1 (AP-1), an important transcription factor for cell proliferation, in young keratinocytes, whereas the binding activity showed little or slight change in the senescent cells. The induced DNA-binding activity of AP-1 in young cells was inhibited by PD 98059, an inhibitor of MEK, and partially inhibited by GF 109203X, an inhibitor of protein kinase C. Western blot analysis demonstrated that EGF induced dramatic increase in the phosphorylation of EGF receptor (EGFR) and extracellular signal-regulated kinases (ERK) in young cells, while this phosphorylation was much less profound in senescent cells. Finally, the application of EGF to young cells resulted in increased phosphorylation of Fra-2, a Fos protein component of the Jun/Fos heterodimer AP-1 complex. This EGF-induced Fra-2 phosphorylation was attenuated in senescent cells. Taken together, our study suggests that the signal transduction mediated by EGF/ERK pathway is altered in senescent human keratinocytes, and this change may be attributed, in part, to the decreased AP-1 transcription activity observed in senescent keratinocytes.

Purification and Characterization of Recombinant Truncated Human Interleukin-11 Expressed as Fusion Protein in Escherichia coli

Mature human interleukin-11 (HuIL-11) is a cytokine consisting of 178 amino acid residues that results from scission of the N-terminal signal peptide, consisting of 21 amino acid residaues, from the corresponding nascent polypeptide. A DNA fragment encoding a truncated HuIL-11 (trHuIL-11), with an additional 5 amino acid residues removed from the N-terminus, was cloned into vector pGEX-2T between the BamHI site and the EcoRI site. Upon transformation with Escherichia coli BL21, the construct over-produced a glutathione S-transferase (GST)-fused protein in a soluble form after IPTG induction. The fusion protein was initially fractionated with butyl-Sepharose 4 fast flow column and by affinity chromatography using a GSH-Sepharose 4B column. On-site enzymatic release with thrombin gave the target protein at 96% purity as judged by SDS-PAGE and HPLC. Expression of the interleukin as a GST-fused protein thus greatly improved downstream processing. Subsequent biological activity assay suggested that trHuIL-11 had similar activity profile to the naturally produced sample and may be a promising candidate for further development as biopharmaceutical.

Menin suppresses osteoblast differentiation by antagonizing the AP-1 factor, JunD

Mice null for menin, the product of the multiple endocrine neoplasia type 1 (MEN1) gene, exhibit cranial and facial hypoplasia suggesting a role for menin in bone formation. We have shown previously that menin is required for the commitment of multipotential mesenchymal stem cells into the osteoblast lineage in part by interacting with the bone morphogenetic protein (BMP)-2 signaling molecules Smad1/5, and the key osteoblast transcriptional regulator, Runx2 (Sowa H., Kaji, H., Hendy, G. N., Canaff, L., Komori, T., Sugimoto, T., and Chihara, K. (2004) J. Biol. Chem. 279, 40267-40275). However, menin inhibits the later differentiation of committed osteoblasts. The activator protein-1 (AP-1) transcription factor, JunD, is expressed in osteoblasts and has been shown to interact with menin in other cell types. Here, we examined the consequences of menin-JunD interaction on osteoblast differentiation in mouse osteoblastic MC3T3-E1 cells. JunD expression, assessed by immunoblot, gradually increased during osteoblast differentiation. Stable expression of JunD enhanced expression of the differentiation markers, Runx2, type 1 collagen (COL1), and osteocalcin (OCN) and alkaline phosphatase (ALP) activity and mineralization. Hence, JunD promotes osteoblast differentiation. In MC3T3-E1 cells in which menin expression was reduced by stable menin antisense DNA transfection, JunD levels were increased. When JunD and menin were co-transfected in MC3T3-E1 cells, they co-immunoprecipitated. JunD overexpression increased the transcriptional activity of an AP-1 luciferase reporter construct, and this activity was reduced by co-transfection of menin. Therefore, JunD and menin interact both physically and functionally in osteoblasts. Furthermore, menin overexpression inhibited the ALP activity induced by JunD. In conclusion, the data suggest that menin suppresses osteoblast maturation, in part, by inhibiting the differentiation actions of JunD.