Cortisol regulates the expression of Notch in osteoblasts

Glucocorticosteroid-induced spinal osteoporosis: scientific update on pathophysiology and treatment

Glucocorticosteroid-induced spinal osteoporosis (GIOP) is the most frequent of all secondary types of osteoporosis. The understanding of the pathophysiology of glucocorticoid (GC) induced bone loss is of crucial importance for appropriate treatment and prevention of debilitating fractures that occur predominantly in the spine. GIOP results from depressed bone formation due to lower activity and higher death rate of osteoblasts on the one hand, and from increase bone resorption due to prolonged lifespan of osteoclasts on the other. In addition, calcium/phosphate metabolism may be disturbed through GC effects on gut, kidney, parathyroid glands and gonads. Therefore, therapeutic agents aim at restoring balanced bone cell activity by directly decreasing apoptosis rate of osteoblasts (e.g., cyclical parathyroid hormone) or by increasing apoptosis rate of osteoclasts (e.g., bisphosphonates). Other therapeutical efforts aim at maintaining/restoring calcium/phosphate homeostasis: improving intestinal calcium absorption (using calcium supplementation, vitamin D and derivates) and avoiding increased urinary calcium loss (using thiazides) prevent or counteract a secondary hyperparthyroidism. Bisphosphonates, particularly the aminobisphosphonates risedronate and alendronate, have been shown to protect patients on GCs from (further) bone loss to reduce vertebral fracture risk. Calcitonin may be of interest in situation where bisphosphonates are contraindicated or not applicable and in cases where acute pain due to vertebral fracture has to be manage. The intermittent administration of 1-34-parathormone may be an appealing treatment alternative, based on its documented anabolic effects on bone resulting from the reduction of osteoblastic apoptosis. Calcium and vitamin D should be a systematic adjunctive measure to any drug treatment for GIOP. Based on currently available evidence, fluoride, androgens, estrogens (opposed or unopposed) cannot be recommended for the prevention and treatment of GIOP. However, substitution of gonadal hormones may be indicated if GC-induced hypogonadism is present and leads to clinical symptoms. Data using the SERM raloxifene to treat or prevent GIOP are lacking, as are data using the promising bone anabolic agent strontium ranelate. Kyphoplasty performed in appropriately selected osteoporotic patients with painful vertebral fractures is a promising addition to current medical treatment.

Parathyroid hormone stimulates expression of the Notch ligand Jagged1 in osteoblastic cells

We previously demonstrated that activation of the Parathyroid Hormone Receptor (PTH1R) in osteoblastic cells increases the Notch ligand Jagged1 and expands hematopoietic stem cells (HSC) through Notch signaling. However, regulation of Jagged1 by PTH in osteoblasts is poorly understood. The present study demonstrates that PTH treatment increases Jagged1 levels in a subpopulation of osteoblastic cells in vivo and in UMR106 osteoblastic cells in vitro. Since PTH(1-34) activates both Adenylate Cyclase/Protein Kinase A (AC/PKA) and Protein Kinase C (PKC) downstream of the PTH1R in osteoblastic cells, we independently determined the effect of either pathway on Jagged1. Activation of AC with Forskolin or PKA with PTH(1-31) or cell-permeable cAMP analogues increased osteoblastic Jagged1. This PTH-dependent Jagged1 increase was blocked by H89 and PKI, specific PKA inhibitors. In contrast, PKC activation with phorbol ester (PMA) or PTH(13-34) did not stimulate Jagged1 expression, and PTH-dependent Jagged1 stimulation was not blocked by Gö6976, a conventional PKC inhibitor. Therefore, PTH treatment stimulates osteoblastic Jagged1 mainly through the AC/PKA signaling pathway downstream of the PTH1R. Since Jagged1/Notch signaling has been implicated not only in stromal-HSC interactions but also in osteoblastic differentiation, Jagged1 may play a critical role in mediating the PTH-dependent expansion of HSC, as well as the anabolic effect of PTH in bone.

Role of the RAM domain and ankyrin repeats on notch signaling and activity in cells of osteoblastic lineage

Notch proteins belong to a family of single pass transmembrane receptors that are activated after interactions with the membrane-bound ligands Delta and Jagged/Serrate. We determined the pathways responsible for the inhibitory effects of Notch on osteoblastogenesis and the contributions of the RAM domain and ankyrin repeats to this process in cells of the osteoblastic lineage.

INTRODUCTION

Notch receptors play a role in osteoblast differentiation. Activation of Notch results in its cleavage and the release of its intracellular domain (NICD), which interacts with the CBF1/RBP-Jkappa, Suppressor of Hairless, Lag-1 (CSL) family of transcription factors. The interaction is presumably mediated by the RBP-Jkappa-associated module (RAM) of NICD, although the role of the ankyrin repeats is uncertain.

MATERIALS AND METHODS

To determine the contributions of the RAM domain and ankyrin repeats to the inhibitory effects of Notch on osteoblastogenesis, ST-2 and MC3T3-E1 cells were transfected or transduced with vectors expressing NICD, RAM (NICD DeltaRAM), and ankyrin (NICD DeltaANK) deletion mutants.

RESULTS

Notch increased the transactivation of transiently transfected 12xCSL-Luc constructs, containing 12 repeats of an RBP-Jkappa/CSL binding site, and of the hairy and E (spl) (HES)-1 promoter. Deletion of the ankyrin repeats resulted in the loss of 12xCSL-Luc and HES-1 promoter transactivation, whereas deletion of the RAM domain caused a partial loss of 12xCSL-Luc and sustained HES-1 promoter transactivation. NICD overexpression inhibited osteocalcin mRNA levels and alkaline phosphatase activity in ST-2 cells, and deletion of the ankyrin repeats, and to a lesser extent of the RAM domain, resulted in loss of the NICD inhibitory effect. NICD inhibited Wnt signaling and deletion of ankyrin repeats or the RAM domain restored Wnt signaling activity.

CONCLUSIONS

The RAM domain and ankyrin repeats are required for Notch signaling and activity, and the CSL pathway is central to the inhibitory effect of Notch on osteoblastogenesis.

Notch1 signals through Jagged2 to regulate apoptosis in the apical ectodermal ridge of the developing limb bud

The Notch family of receptors is involved in a wide variety of developmental processes, including cell fate specification, cell proliferation, and cell survival decisions during cell differentiation and tissue morphogenesis. Notch1 and Notch ligands are expressed in the developing limbs, and Notch signalling has been implicated in the formation of a variety of tissues that comprise the limb, such as the skeleton, musculature, and vasculature. Notch signalling has also been implicated in regulating overall limb size. We have used a conditional allele of Notch1 in combination with two different Cre transgenic lines to delete Notch1 function either in the limb mesenchyme or in the apical ectodermal ridge (AER) and limb ectoderm. We demonstrate that Notch signalling, involving Notch1 and Jagged2, is required to regulate the number of Fgf8-expressing cells that comprise the AER and that regulation of the levels of fibroblast growth factor signalling is important for the freeing of the digits during normal limb formation. Regulation of the extent of the AER is achieved by Notch signalling positively regulating apoptosis in the AER. We also demonstrate that Notch1 is not required for proper formation of all the derivatives of the limb mesenchyme.

HES1 cooperates with pRb to activate RUNX2-dependent transcription

The retinoblastoma protein, pRb, can activate the transcription factor RUNX2, an essential regulator of osteogenic differentiation, but the mechanism of this activation is unknown. Here we studied the interaction of pRb and RUNX2 with HES1, previously reported to augment RUNX2 activity. PRb can act to promote RUNX2/HES1 association with concomitant promoter occupancy and transcriptional activation in bone cells.

INTRODUCTION

RUNX2 (also known as OSF2/CBFA1) is a transcription factor required for osteoblast differentiation and bone formation. We have reported that RUNX2 can associate with the retinoblastoma protein pRb, a common tumor suppressor in bone, and the resultant complex can bind and activate transcription from bone-specific promoters. This activity of the pRb/RUNX2 complex may thus link differentiation control with tumor suppressor activity. However, the mechanism through which pRb can activate RUNX2 is unknown. HES1 is a reported co-activator of RUNX2 that shares a binding site on RUNX2 with pRb. Thus, we have tested the cooperativity among these factors in activating transcription from bone specific promoters.

MATERIALS AND METHODS

Coimmunoprecipitation, chromatin immunoprecipitation, and EMSA experiments were used to study the interaction of RUNX2, HES1, and pRb in cell lysates and on DNA. Transcriptional reporter assays were used to analyze the activity of RUNX2 in the presence and absence of HES1 and pRb.

RESULTS

We showed that pRb can associate with HES1, a previously described RUNX2 interactor that can itself augment RUNX2-dependent transcription. The association of HES1 with RUNX2 is augmented by pRb. Furthermore, both pRb and HES1 increase the amount of RUNX2 bound to promoter sites in vivo, pRb and HES1 synergistically activate a RUNX2-dependent reporter gene, and depletion of HES1 reduces RUNX2/pRb activity.

CONCLUSIONS

These data indicate that pRb acts as a RUNX2 co-activator at least in part by recruiting HES1 into the pRb/RUNX2 complex and further elucidate a novel role for pRb as a transcriptional co-activator in osteogenesis.

Osteoblastic Activation in the Hematopoietic Stem Cell Niche

Hematopoietic stem cells (HSC) are rare primitive cells capable of reconstituting all blood cell lineages throughout the life of an individual. The microenvironment in which stem cells reside is essential for their survival, self-renewal, and differentiation. This microenvironment, or HSC niche, has been difficult to define in bone and bone marrow, but recent studies from our laboratory and others have shown that osteoblasts, the bone-forming cells, are an essential regulatory component of this complex cellular network. We established that parathyroid hormone (PTH), through activation of the PTH/PTHrP receptor (PTH1R) in osteoblastic cells, could alter the HSC niche resulting in HSC expansion in vivo and in vitro and improving dramatically the survival of mice receiving bone marrow transplants. These findings are of great clinical appeal, because they suggest that a strategy aimed at modifying supportive cells in a stem cell niche can expand HSC. While a number of molecules have been found to be important for hematopoietic/osteoblastic interactions, we have focused on the Jagged1/Notch signaling pathway, which was necessary for the PTH-dependent HSC expansion. Since the Jagged1/Notch signaling pathway has been implicated in the microenvironmental control of stem cell self-renewal in several organ systems, definition of Jagged1 modulation, which is currently poorly understood, should provide additional molecular targets for stem cell regulation and advance the understanding of stem cell-microenvironmental interactions.

Notch 1 overexpression inhibits osteoblastogenesis by suppressing Wnt/beta-catenin but not bone morphogenetic protein signaling

Notch proteins are transmembrane receptors that control cell-fate decisions. Upon ligand binding, Notch receptors undergo proteolytic cleavage leading to the release of their intracellular domain (NICD). Overexpression of NICD impairs osteoblastogenesis, but the mechanisms are not understood. We examined consequences of the constitutive activation of Notch 1 in ST-2 cells. Notch opposed the effects of bone morphogenetic protein (BMP)-2 and Wnt 3a on alkaline phosphatase activity (APA). BMP-2 induced the phosphorylation of Smad 1/5/8 and the transactivation of a BMP/Smad-responsive construct (12xSBE-Oc-pGL3), but the effect was not modified by Notch. BMP-2 had minimal effects on the phosphorylation of the mitogen-activated protein kinases ERK, p38, and JNK, in the absence or presence of NICD. Notch overexpression decreased the transactivating effect of Wnt 3a, cytoplasmic beta-catenin levels, and Wnt-dependent gene expression. Transfection of a mutant beta-catenin expression construct, or the use of a glycogen synthase kinase 3beta inhibitor to stabilize beta-catenin, partially blocked the inhibitory effect of NICD on Wnt signaling and on APA. HES-1 or Groucho1/TLE1 RNA interference enhanced basal and induced Wnt/beta-catenin signaling opposing NICD effects, but only HES-1 silencing enhanced Wnt 3a effects on APA. In conclusion, NICD overexpression prevents BMP-2 and Wnt biological effects by suppressing Wnt but not BMP signaling. HES-1 appears to mediate effects of Notch on osteoblastogenesis.

Effect of systemic glucocorticoid therapy on bone metabolism: an update

Glucocorticoids are widely used for a range of inflammatory conditions. However, their use is complicated by significant side effects. The most important of these from a clinical point of view is on bone, where glucocorticoids substantially increase the risk of osteoporosis and fracture. This review will give an overview of the pathophysiologic basis and epidemiology of glucocorticoid-induced osteoporosis, examine diagnostic and therapeutic approaches currently available, and suggest the likely impact of the most recent scientific, clinical and pharmaceutical advances. Glucocorticoids impact on both bone formation and bone resorption, a combination that leads to rapid bone loss and increase in fracture risk. Epidemiologic studies indicate that these risks are substantial, especially at the spine, increase with age and independently of bone density, and are maintained during glucocorticoid use. The best available treatments are bisphosphonates that preserve bone density and reduce the risk of fracture at the spine. Future areas that need to be addressed are the relationship between inflammation and the action of glucocorticoids on the skeleton, and the development of anabolic therapies for glucocorticoid-induced osteoporosis.

Bone ultrasonometric features and growth hormone secretion in asthmatic patients during chronic inhaled corticosteroid therapy

BACKGROUND

Quantitative ultrasound bone densitometry (QUBD) is a new method to assess bone mineral density and bone microarchitecture. Corticosteroid (CS) therapy may diminish bone mass, alter bone quality and may influence growth hormone (GH) secretion and bone metabolism markers. Therefore, the aim of this study was to evaluate the effects of long-term therapy with inhaled CSs (ICSs) on structural bone characteristics and their correlations with GH secretion and bone markers in asthmatic patients.

METHODS

In a cross-sectional study, we enrolled 60 adult patients with mild to moderate persistent asthma: 22 on chronic (>1 year) ICS therapy, 10 naive to ICSs treatment and 28 healthy control subjects. The groups were matched for age and BMI. Each subject underwent to QUBD at the phalanxes to assess bone microarchitecture by ultrasound bone profile index (UBPI), bone density by amplitude-dependent speed of sound (AdSos); test with GH-releasing hormone (GHRH) injection with calculation of peak GH and the Delta GH (peak GH-basal GH); and hormonal and bone markers measurements.

RESULTS

Asthmatics treated with long-term ICS therapy showed a lower UBPI (P < 0.01) compared to controls (49.8 +/- 19.3 vs. 77.0 +/- 10.1, respectively) and to asthmatics never taking ICSs (73.2 +/- 9.6). In ICS-treated asthmatics, DeltaGH and GH-peak showed a significant correlation with UBPI. A significant difference was observed comparing asthmatics treated with ICSs to controls and asthmatics naive to ICSs in GH response to GHRH iv bolus. Serum osteocalcin was significantly reduced in asthmatic patients treated with ICSs.

CONCLUSIONS

In asthmatic patients, long-term ICSs treatment produces negative effects on bone quality assessed by QUBD, and such effects are associated to an impaired GH secretion.

Restoration of the coupling process and normalization of bone mass following successful treatment of endogenous Cushing's syndrome: a prospective, long-term study

OBJECTIVE

Endogenous Cushing's syndrome (CS) is associated with bone loss and an increased risk of fractures. However, the long-term outcome of treatment on bone health has not been adequately clarified.

DESIGN

We followed 33 patients with active CS prospectively before and twice after treatment (mean follow-up 33 (n = 25) and 71 months (n = 18), respectively). The patients were compared to age-, sex- and body mass index (BMI)-matched controls, also followed longitudinally.

METHODS

Bone mineral indices (bone mineral density (BMD), bone mineral content (BMC) and bone area) were evaluated in the lumbar spine (LS), femoral neck (FN), and total body (TB) by dual-energy X-ray absorptiometry (DXA). Biochemical markers of bone turnover were assessed by serum levels of osteocalcin and C-terminal telopeptides of Type-1 collagen (CTX-1).

RESULTS

Mann-Whitney rank sum tests showed that BMD of the LS, FN and TB was reduced by 14.8% (P < 0.001), 15.7% (P < 0.001), and 9.2% (P < 0.001) in CS vs. controls at baseline, with markedly reduced serum osteocalcin (P = 0.014) and increased CTX-1 (P = 0.012) levels, but no correlation between markers. At first follow-up, BMD was increased in LS (7.9%, P < 0.001) and FN (3.5%, P = 0.003) compared to baseline. The time-dependent rise in BMD (LS (r = 0.59; P = 0.002) and FN (r = 0.52; P = 0.007); Spearman's rank correlation), in CS was paralleled by increased osteocalcin (275%, P < 0.001) and correlation between biochemical markers (r = 0.92, P < 0.001; Pearson's correlation). TB BMD did not increase significantly before the second follow-up, when BMD Z-scores were normalized in all three compartments.

CONCLUSION

Our observations demonstrate restoration of coupled bone remodeling and normalization of bone mineral density in all measured skeletal compartments of treated CS patients after prolonged recovery, first significant in predominantly trabecular bone (i.e. lumbar spine).

Biological Principles for Ex Vivo Adult Stem Cell Expansion

Adult stem cells (ASCs) are the engines that drive the renewal of adult mammalian tissues. They divide continuously, throughout life, to produce new progeny cells that undergo a robust development program of differentiation and maturation to replace older expired tissue cells. The same cell turnover program may function to provide limited repair and regeneration of adult tissues in some cases. The regenerative potential of ASCs drives the current intense interest in adapting them for applications in cell replacement therapy. However, research to explore this potential has been blunted by an unyielding biological problem. ASCs have proven highly refractory to expansion of their numbers and long-term propagation in culture. A review of reported strategies to overcome this problem reveals that many studies focus on traditional cell culture factors that may not apply to ASCs and overlook a special property of ASCs that may be universally critical for successful expansion, asymmetric cell kinetics (ACK). This property is reflected by the different kinetics fate of the two sister cells resulting from an ASC division: one cell remains an ASC and keeps the potential to divide for the entire life span of the tissue, while the other cell's progeny eventually differentiates and undergoes terminal division arrest. This unique property of ASCs may prove to be the obligatory factor that must be breached by any method that will succeed in accomplishing routine expansion of ASCs of diverse tissue origin.

Mechanisms of glucocorticoid action in bone

Glucocorticoids induce rapid bone loss and increase the risk for osteoporotic fractures. The mechanisms include a phase of increased bone resorption, probably a result of the increased expression of receptor activator of nuclear factor-k-B ligand and colony stimulating factor-1, followed-up by a decrease in bone formation. This effect is central to the actions of glucocorticoids in bone and it is secondary to the loss of bone forming cells, caused by an inhibition of cell differentiation and an increase in the apoptosis of mature osteoblasts and osteocytes. Glucocorticoids also inhibit the function of mature osteoblasts and suppress the synthesis of insulin-like growth factor-I, an agent that enhances bone formation. Glucocorticoids alter the growth hormone/insulin-like growth factor axis in cartilage and, as a consequence, suppress linear growth.

Bone marrow subendosteal microenvironment harbours functionally distinct haemosupportive stromal cell populations

In adult animals, bone marrow is the major site of blood cell production, which is controlled by interactions between the local stroma and blood cell progenitors. The endosteal/subendosteal environment comprises bone-lining and adjacent reticular cells and sustains haemopoietic stem cell (HSC) self-renewal, proliferation and differentiation. We have questioned the specific role of each of these stroma cells in controlling HSC fate. We have isolated two distinct stroma-cell populations containing subendosteal reticulocytes (F-RET) and osteoblasts (F-OST) from periosteum-free fragments of murine femurs by a two-step collagenase-digestion procedure. Both populations produce similar extracellular matrix (collagen I, laminin, fibronectin, decorin), except for collagen IV, which is low in F-OST. They also express osteogenic markers: osteopontin, osteonectin, bone sialoprotein and alkaline phosphatase (ALP). The quantity and activity of ALP are however higher in F-OST. When co-cultured with bone marrow mononuclear cells or lineage-negative haemopoietic progenitors, F-OST stroma induces low proliferation and high maintenance of early haemopoietic progenitors, whereas F-RET stroma induces high short-term proliferation and differentiation. Analysis by reverse transcription/polymerase chain reaction has revealed higher levels of Jagged-1 expression by F-OST cells than by the F-RET population. Thus, two adjacent stroma cells (subendosteal and endosteal) play distinct roles in controlling the stem-cell capacity and fate of HSC and probably contribute distinctly to HSC niche formation.

Sensitivity of bone to glucocorticoids

Glucocorticoids are used widely in a range of medical specialities, but their main limitation is an adverse impact on bone. Although physicians are increasingly aware of these deleterious effects, the marked variation in susceptibility between individuals makes it difficult to predict who will develop skeletal complications with these drugs. Although the mechanisms underlying the adverse effects on bone remain unclear, the most important effect appears to be a rapid and substantial decrease in bone formation. This review will examine recent studies that quantify the risk of fracture with glucocorticoids, the mechanisms that underlie this increase in risk and the potential basis for differences in individual sensitivity. An important determinant of glucocorticoid sensitivity appears to be the presence of glucocorticoid-metabolizing enzymes within osteoblasts and this may enable improved estimates of risk and generate new approaches to the development of bone-sparing anti-inflammatory drugs.

Notch 1 impairs osteoblastic cell differentiation

Notch receptors are single pass transmembrane receptors activated by membrane-bound ligands with a role in cell proliferation and differentiation. As Notch 1 and 2 mRNAs are expressed by osteoblasts and induced by cortisol, we postulated that Notch could regulate osteoblastogenesis. We investigated the effects of retroviral vectors directing the constitutive expression of the Notch 1 intracellular domain (NotchIC) in murine ST-2 stromal and in MC3T3 cells. NotchIC overexpression was documented by increased Notch 1 transcripts and activity of the Notch-dependent Hairy Enhancer of Split promoter. In the presence of bone morphogenetic protein-2 (BMP-2), ST-2 cells differentiated toward osteoblasts forming mineralized nodules, and Notch 1 opposed this effect and decreased the expression of osteocalcin, type I collagen, and alkaline phosphatase transcripts and Delta2Delta FosB protein. Further, NotchIC decreased Wnt/beta-catenin signaling. As cells differentiated in the presence of BMP-2, they underwent apoptosis, and Notch opposed this event. In the presence of cortisol, NotchIC induced the formation of mature adipocytes and enhanced the effect of cortisol on adipsin, peroxisome proliferator-activated receptor-gamma2 and CCAAT enhancer binding protein alpha and delta mRNA levels. NotchIC also opposed MC3T3 cell differentiation and the expression of a mature osteoblastic phenotype. In conclusion, NotchIC impairs osteoblast differentiation and enhances adipogenesis in stromal cell cultures.

Osteoblastic cells regulate the haematopoietic stem cell niche

Stem cell fate is influenced by specialized microenvironments that remain poorly defined in mammals. To explore the possibility that haematopoietic stem cells derive regulatory information from bone, accounting for the localization of haematopoiesis in bone marrow, we assessed mice that were genetically altered to produce osteoblast-specific, activated PTH/PTHrP receptors (PPRs). Here we show that PPR-stimulated osteoblastic cells that are increased in number produce high levels of the Notch ligand jagged 1 and support an increase in the number of haematopoietic stem cells with evidence of Notch1 activation in vivo. Furthermore, ligand-dependent activation of PPR with parathyroid hormone (PTH) increased the number of osteoblasts in stromal cultures, and augmented ex vivo primitive haematopoietic cell growth that was abrogated by gamma-secretase inhibition of Notch activation. An increase in the number of stem cells was observed in wild-type animals after PTH injection, and survival after bone marrow transplantation was markedly improved. Therefore, osteoblastic cells are a regulatory component of the haematopoietic stem cell niche in vivo that influences stem cell function through Notch activation. Niche constituent cells or signalling pathways provide pharmacological targets with therapeutic potential for stem-cell-based therapies.