Generating robust human valvular interstitial cell cultures: Protocol and considerations

Research in heart valve biology is a growing field that has yet to elucidate the fundamentals of valve disease. Human valvular interstitial cells (hVICs) are the best option for studying the cellular mechanisms behind valvular pathologies. However, there is a wide range of isolation procedures for these cells published in the literature. To what extent various isolation methods, patient pathologies, and seeding densities influence the behaviour of hVICs remains unclear. Here, we present an optimised method of hVIC isolation from diseased human valves donated at the time of surgery. We show that two rounds of 1000 U/mL collagenase digestion for not >2 h results in a phenotypically stable cell culture with a near complete absence of endothelial cell contamination. We also suggest that cells should be seeded at 10,000 cells/cm² for experimentation. We found that patient pathology does not affect the success of the isolation procedure, and that instead, successful cultures are predicted by ensuring >500 mg valve tissue as starting material.

Fibroblast growth factor 2 inhibits myofibroblastic activation of valvular interstitial cells

Heart valve disease is a growing problem worldwide. Though very common in older adults, the mechanisms behind the development of the disease aren't well understood, and at present the only therapeutic option is valve replacement. Valvular interstitial cells (VICs) may hold the answer. These cells can undergo pathological differentiation into contractile myofibroblasts or osteoblasts, leading to thickening and calcification of the valve tissue. Our study aimed to characterise the effect of fibroblast growth factor 2 (FGF-2) on the differentiation potential of VICs. We isolated VICs from diseased human valves and treated these cells with FGF-2 and TGF-β to elucidate effect of these growth factors on several myofibroblastic outcomes, in particular immunocytochemistry and gene expression. We used TGF-β as a positive control for myofibroblastic differentiation. We found that FGF-2 promotes a 'quiescent-type' morphology and inhibits the formation of α-smooth muscle actin positive myofibroblasts. FGF-2 reduced the calcification potential of VICs, with a marked reduction in the number of calcific nodules. FGF-2 interrupted the 'canonical' TGF-β signalling pathway, reducing the nuclear translocation of the SMAD2/3 complex. The panel of genes assayed revealed that FGF-2 promoted a quiescent-type pattern of gene expression, with significant downregulations in typical myofibroblast markers α smooth muscle actin, extracellular matrix proteins, and scleraxis. We did not see evidence of osteoblast differentiation: neither matrix-type calcification nor changes in osteoblast associated gene expression were observed. Our findings show that FGF-2 can reverse the myofibroblastic phenotype of VICs isolated from diseased valves and inhibit the calcification potential of these cells.

Novel Growth Factor Combination for Improving Rotator Cuff Repair: A Rat In Vivo Study

BACKGROUND

The lack of healing at the repaired tendon-bone interface is an important cause of failure after rotator cuff repair. While augmentation with growth factors (GFs) has demonstrated promise, the ideal combination must target all 3 tissue types at the tendon-bone interface.

HYPOTHESIS

The GF combination of transforming growth factor beta 1, Insulin-like growth factor 1, and parathyroid hormone will promote tenocyte proliferation and differentiation and improve the biomechanical and histological quality of the repaired tendon-bone interface.

STUDY DESIGN

Controlled laboratory study.

METHODS

In vitro, human tenocytes were cultured in the presence of the GF combination for 72 hours, and cell growth assays and the expression of genes specific to tendon, cartilage, and bone were analyzed. In vivo, adult rats (N = 46) underwent detachment and repair of the left supraspinatus tendon. A PVA-tyramine gel was used to deliver the GF combination to the tendon-bone interface. Histological, biomechanical, and RNA microarray analysis was performed at 6 and 12 weeks after surgery. Immunohistochemistry for type II and X collagen was performed at 12 weeks.

RESULTS

When treated with the GF combination in vitro, human tenocytes proliferated 1.5 times more than control (P = .04). The expression of scleraxis increased 65-fold (P = .013). The expression of Sox-9 (P = .011), type I collagen (P = .021), fibromodulin (P = .0075), and biglycan (P = .010) was also significantly increased, while the expression of PPARγ was decreased (P = .007). At 6 and 12 weeks postoperatively, the quality of healing on histology was significantly higher in the GF group, with the formation of a more mature tendon-bone interface, as confirmed by immunohistochemistry for type II and X collagen. The GF group achieved a load at failure and Young modulus >1.5 times higher at both time points. Microarrays at 6 weeks demonstrated upregulation of genes involved in leukocyte aggregation (S100A8, S100A9) and tissue mineralization (Bglap, serglycin, Fam20c).

CONCLUSION

The GF combination promoted protendon and cartilage responses in human tenocytes in vitro; it also improved the histological appearance and mechanical properties of the repair in vivo. Microarrays of the tendon-bone interface identified inflammatory and mineralization pathways affected by the GF combination, providing novel therapeutic targets for further research.

CLINICAL RELEVANCE

The use of this GF combination is translatable to patients and may improve healing after rotator cuff repair.

Fibroblast growth factor 23 levels decline following sleeve gastrectomy

OBJECTIVE

Levels of fibroblast growth factor 23 (FGF23) have been positively associated with measures of adiposity, cardiovascular disease and mortality. It is unclear whether the relationship of FGF23 with cardiovascular disease and mortality is confounded by obesity. We aimed to determine whether FGF23 concentrations decline following a reduction in adiposity after sleeve gastrectomy (SG).

DESIGN

The effect of SG on FGF23 was evaluated in 22 obese adults (59% male) with type 2 diabetes. Fat mass, weight, BMI, plasma intact FGF23, parathyroid hormone (PTH) and leptin were determined at baseline and at 12 months following SG.

RESULTS

At baseline, median (IQR) age was 51 (43-54) years, fat mass 47.8 (41.0-59.4) kg, BMI 40.9 (36.9-46.9) kg/m² and FGF23 66.2 (55.3-82.9) pg/mL. Significant changes in median BMI (-10.8 kg/m² , 95% CI: -12.9 to -7.2, P < 0.0001), fat mass (-20.0 kg, 95% CI: -26.7 to -12.4, P < 0.0001) and weight (-34.7 kg, 95% CI -40.0 to -23.1, P < 0.0001) were observed after SG. FGF23 (-12.4 pg/mL, 95% CI: -19.5 to 2.0, P = 0.005), PTH (-1.1 pmol/L, 95% CI: -1.7 to 0.2, P = 0.009) and leptin (-1687 pg/mL, 95% CI -4524 to -563, P = 0.01) declined following SG. Change in FGF23 was not significantly associated with change in measures of adiposity, PTH or leptin.

CONCLUSIONS

FGF23 concentrations decline in the setting of significant weight loss following SG, implying that increased FGF23 concentrations are a downstream consequence of obesity, which may confound its association with cardiometabolic dysfunction. Mediators of the relationship between adiposity and FGF23 require further elucidation.

Tyrosine Kinase Inhibitors Regulate OPG through Inhibition of PDGFRβ

Nilotinib and imatinib are tyrosine kinase inhibitors (TKIs) used in the treatment of chronic myeloid leukemia (CML) and gastrointestinal stromal tumors (GIST). In vitro, imatinib and nilotinib inhibit osteoclastogenesis, and in patients they reduce levels of bone resorption. One of the mechanisms that might underlie these effects is an increase in the production of osteoprotegerin (OPG). In the current work we report that platelet-derived growth factor receptor beta (PDGFRβ) signaling regulates OPG production in vitro. In addition, we have shown that TKIs have effects on RANKL signaling through inhibition of the PDGFRβ and other target receptors. These findings have implications for our understanding of the mechanisms by which TKIs affect osteoclastogenesis, and the role of PDGFRβ signaling in regulating osteoclastogenesis. Further studies are indicated to confirm the clinical effects of PDGFRβ-inhibitors and to elaborate the intracellular pathways that underpin these effects.

Low-dose fluoride in postmenopausal women: a randomized controlled trial

CONTEXT

Trials of high-dose fluoride have reported increased bone formation and bone mineral density (BMD), but impaired bone mineralization and either adverse or neutral effects on fracture risk. Meta-analysis of a heterogeneous dataset of small trials suggests that daily doses of <20 mg fluoride might reduce fracture risk, but it is not known whether low doses of fluoride are safely anabolic to bone.

OBJECTIVE

We set out to investigate the skeletal effects of low doses of fluoride.

DESIGN, SETTING, AND PARTICIPANTS

We conducted a double-blind, placebo-controlled randomized trial over 1 year at an academic research center, in 180 postmenopausal women with osteopenia.

INTERVENTION

Participants received daily treatment with tablets containing placebo, 2.5 mg fluoride, 5 mg fluoride, or 10 mg fluoride.

MAIN OUTCOME MEASURES

The primary endpoint was a change in lumbar spine BMD at 1 year; secondary endpoints were hip and forearm BMD, and markers of bone turnover. Safety was assessed by histomorphometric analysis of transiliac bone biopsies from a subset of participants.

RESULTS

Compared to placebo, none of the doses of fluoride altered BMD at any site. The bone formation marker, procollagen type I N-terminal propeptide, increased significantly in the 5 mg and 10 mg fluoride groups compared to placebo (P = .04 and .005, respectively). No differences were observed between placebo and any of the fluoride groups in levels of β-C-terminal telopeptide of type I collagen.

CONCLUSIONS

Low-dose fluoride does not induce substantial effects on surrogates of skeletal health and is unlikely to be an effective therapy for osteoporosis.

The skeletal effects of the tyrosine kinase inhibitor nilotinib

Nilotinib is a tyrosine kinase inhibitor (TKI) developed to manage imatinib-resistance in patients with chronic myeloid leukemia (CML). It inhibits similar molecular targets to imatinib, but is a significantly more potent inhibitor of Bcr-Abl. Nilotinib exhibits off-target effects in other tissues, and of relevance to bone metabolism, hypophosphataemia has been reported in up to 30% of patients receiving nilotinib. We have assessed the effects of nilotinib on bone cells in vitro and on bone metabolism in patients receiving nilotinib for treatment of CML. We firstly investigated the effects of nilotinib on proliferating and differentiating osteoblastic cells, and on osteoclastogenesis in murine bone marrow cultures and RAW264.7 cells. Nilotinib potently inhibited osteoblast proliferation (0.01-1uM), through inhibition of the platelet-derived growth factor (PDGFR). There was a biphasic effect on osteoblast differentiation such that it was reduced by lower concentrations of nilotinib (0.1-0.5uM), with no effect at higher concentrations (1uM). Nilotinib also potently inhibited osteoclastogenesis, predominantly by stromal-cell dependent mechanisms. Thus, nilotinib decreased osteoclast development in murine bone marrow cultures, but did not affect osteoclastogenesis in RAW264.7 cells. Nilotinib treatment of osteoblastic cells increased expression and secretion of OPG and decreased expression of RANKL. In 10 patients receiving nilotinib, levels of bone turnover markers were in the low-normal range, despite secondary hyperparathyroidism, findings that are similar to those in patients treated with imatinib. Bone density tended to be higher than age and gender-matched normal values. These data suggest that nilotinib may have important effects on bone metabolism. Prospective studies should be conducted to determine the long-term effects of nilotinib on bone density and calcium metabolism.

The atypical anti-psychotic clozapine decreases bone mass in rats in vivo

BACKGROUND

Fracture risk is increased in patients with schizophrenia, who often receive long-term therapy with anti-psychotic drugs. The mechanisms by which skeletal fragility is increased in patients with psychosis include increased risk of falling, but direct skeletal toxicity of anti-psychotic drugs is a possibility that has not been investigated.

METHODS

We examined the skeletal effects, in vivo and in vitro, of a typical anti-psychotic drug, haloperidol, which primarily inhibits dopaminergic signaling, and an atypical anti-psychotic drug, clozapine, which predominantly inhibits serotonergic signaling.

RESULTS

In growing rats, 42 days of clozapine treatment reduced whole body bone mineral density by 15% (P<0.01 vs vehicle), and trabecular and cortical bone volume, as assessed by microcomputed tomography, by 29% and 15%, respectively (P<0.05 vs vehicle for each). Treatment with haloperidol did not affect bone density. Clozapine, but not haloperidol, transiently increased levels of serum corticosterone, and decreased levels of serum testosterone. In vitro, clozapine dose-dependently decreased osteoblast mitogenesis, osteoblast differentiation and osteoclastogenesis, while haloperidol did not affect any of these parameters.

CONCLUSIONS

These data demonstrate that clozapine, but not haloperidol, exerts adverse skeletal effects in rodents, and that this effect may be attributable to direct actions to reduce osteoblast growth and function. Long-term administration of clozapine may therefore negatively affect bone health, and clinical studies to investigate this possibility are warranted.

Identification of dairy fractions with anti-inflammatory properties in models of acute gout

AIMS

Large epidemiological studies have shown that low-fat dairy intake reduces the risk of developing gout. It was hypothesised that factors within dairy fractions inhibit the inflammatory response to monosodium urate monohydrate (MSU) crystals.

METHODS

Dairy fractions were tested in MSU crystal-stimulated THP-1 cell assays. Fractions with inhibitory effects were then tested in the murine urate peritonitis model.

RESULTS

Two dairy fractions were found to have consistent inhibitory effects. Glycomacropeptide (GMP) and G600 milk fat extract both inhibited interleukin-1beta (IL1beta) gene and protein expression in the THP-1 cell assay. Conversely, standard milk fat increased IL8 protein expression in the THP-1 cell assay. Oral administration of GMP and G600 milk fat extract inhibited cellular influx in the urate peritonitis model.

CONCLUSIONS

Both protein and lipid fractions within dairy products are capable of modulating the inflammatory response to MSU crystals.

Decreased bone turnover despite persistent secondary hyperparathyroidism during prolonged treatment with imatinib

CONTEXT

The tyrosine kinase inhibitor imatinib mesylate has an established role in the management of a number of malignant and proliferative conditions. Cross-sectional and short-term prospective studies have demonstrated secondary hyperparathyroidism during imatinib therapy, and variable changes in markers of bone turnover.

OBJECTIVE

Our objective was to determine the biochemical and skeletal effects of imatinib during long-term therapy.

DESIGN

This was a 2-yr prospective study.

SETTING

The study was performed at an academic clinical research center.

PATIENTS OR OTHER PARTICIPANTS

Nine patients with bcr-abl positive chronic myeloid leukemia were included in the study.

INTERVENTIONS

Patients received Imatinib mesylate 400 mg/d.

MAIN OUTCOME MEASURES

Serum and urine biochemistry, markers of bone turnover, and bone mineral density were measured.

RESULTS

Participants developed mild secondary hyperparathyroidism, with significant decreases in serum calcium and phosphate (P < 0.05 and P < 0.0001 vs. baseline, respectively) and an increase in PTH (P < 0.0001 vs. baseline). Biochemical markers of bone turnover demonstrated a biphasic response, with an initial increase in markers of bone formation being followed by a decrease in markers of both formation and resorption. Bone density at the lumbar spine increased [mean (95% confidence interval) change from baseline 3.6% (1.6, 5.5); P = 0.003] as did that at the total body [1.4% (0.2, 2.5); P = 0.065], whereas that at the proximal femur did not change [-0.12% (-3.0, 2.7); P = 0.93]. Body weight and fat mass increased significantly (P < 0.0001 vs. baseline).

CONCLUSIONS

Long-term treatment with imatinib leads to persistent mild secondary hyperparathyroidism. Despite this, bone turnover is decreased, and bone density is stable or increased. Evaluation of the skeletal actions and safety of imatinib during longer-term therapy is warranted.

Imatinib promotes osteoblast differentiation by inhibiting PDGFR signaling and inhibits osteoclastogenesis by both direct and stromal cell-dependent mechanisms

Several lines of evidence suggest that imatinib may affect skeletal tissue. We show that inhibition by imatinib of PDGFR signaling in osteoblasts activates osteoblast differentiation and inhibits osteoblast proliferation and that imatinib inhibits osteoclastogenesis by both stromal cell-dependent and direct effects on osteoclast precursors.

INTRODUCTION

Imatinib mesylate, an orally active inhibitor of the c-abl, c-kit, and platelet-derived growth factor receptor (PDGFR) tyrosine kinases, is in clinical use for the treatment of chronic myeloid leukemia (CML) and gastrointestinal stromal cell tumors. Interruption of both c-kit and c-abl signaling in mice induces osteopenia, suggesting that imatinib might have adverse effects on the skeleton. However, biochemical markers of bone formation increase in patients with CML starting imatinib therapy, whereas bone resorption is unchanged, despite secondary hyperparathyroidism. We assessed the actions of imatinib on bone cells in vitro to study the cellular and molecular mechanism(s) underlying the skeletal effects we observed in imatinib-treated patients.

MATERIALS AND METHODS

Osteoblast differentiation was assessed using a mineralization assay, proliferation by [(3)H]thymidine incorporation, and apoptosis by a TUNEL assay. Osteoclastogenesis was assessed using murine bone marrow cultures and RAW 264.7 cells. RT and multiplex PCR were performed on RNA prepared from human bone marrow samples, osteoblastic cells, and murine bone marrow cultures. Osteoprotegerin was measured by ELISA.

RESULTS

The molecular targets of imatinib are expressed in bone cells. In vitro, imatinib increases osteoblast differentiation and prevents PDGF-induced inhibition of this process. Imatinib inhibits proliferation of osteoblast-like cells induced by serum and PDGF. In murine bone marrow cultures, imatinib inhibits osteoclastogenesis stimulated by 1,25-dihydroxyvitamin D(3) and partially inhibits osteoclastogenesis induced by RANKL and macrophage-colony stimulating factor. Imatinib partially inhibited osteoclastogenesis in RANKL-stimulated RAW-264.7 cells. Treatment with imatinib increases the expression of osteoprotegerin in bone marrow from patients with CML and osteoblastic cells.

CONCLUSIONS

Taken together with recent in vivo data, these results suggest a role for the molecular targets of imatinib in bone cell function, that inhibition by imatinib of PDGFR signaling in osteoblasts activates bone formation, and that the antiresorptive actions of imatinib are mediated by both stromal cell-dependent and direct effects on osteoclast precursors.

Lactoferrin potently inhibits osteoblast apoptosis, via an LRP1-independent pathway

Lactoferrin induces osteoblast proliferation in vitro and is anabolic to bone in vivo. We recently reported that the low-density lipoprotein-receptor-related protein 1 (LRP1), a multifunctional member of the LDL receptor family, transduces the mitogenic signal activated by lactoferrin. Here we investigate the effects of lactoferrin on osteoblast survival. At periphysiological concentrations (1-10mug/ml), lactoferrin protects both primary rat osteoblastic cells and SaOS2 cells from apoptosis induced by serum withdrawal. Surprisingly, this effect was not sensitive to the LRP1/2 inhibitor receptor-associated protein (RAP). Neither did lactoferrin selectively prevent apoptosis in fibroblastic cells expressing wild-type LRP1 compared to LRP1-null fibroblasts. Lactoferrin activates PI3 kinase-dependent Akt signaling in osteoblasts but this effect is neither LRP1-dependent nor required for lactoferrin-induced cell survival. Lactoferrin activates p42/44 MAPK signaling, but inhibiting this process does not abrogate its pro-survival actions. These results demonstrate that lactoferrin promotes osteoblast survival, an effect that may contribute to its anabolic skeletal actions in vivo. Our data also suggest that the molecular mechanisms that underpin the ability of lactoferrin to promote cell survival differ fundamentally from those which subserve its mitogenic actions, in particular being mediated by a distinct cell-membrane-based receptor.

Deletion of aspartate 182 in OPG causes juvenile Paget's disease by impairing both protein secretion and binding to RANKL

Mutations in the OPG gene cause idiopathic hyperphosphatasia. We characterized the effects of one such mutation and found that the mutant OPG is poorly secreted and has reduced biological activity compared with the wildtype protein. Therefore, correct structure and cellular processing of OPG is essential for normal bone remodeling.

INTRODUCTION

Inactivating mutations in osteoprotegerin (OPG) cause juvenile Paget's disease (JPD). We recently reported a family with JPD in which affected members were homozygous for an in-frame mutation resulting in the deletion of aspartate 182 in OPG. Here we report the structural and functional characterization of the OPGdeltaD182 mutant protein.

MATERIALS AND METHODS

Inhibition of osteoclastogenesis by the recombinant OPG proteins was studied in a murine bone marrow culture. Binding of wildtype and mutant OPG to RANKL was measured in two experimental systems: glutathione-S-transferase (GST) pull-down assay and surface plasmon resonance. Site-directed mutagenesis was used to study the glycosylation of OPGdeltaD182 in two potential glycosylation sites adjacent to the deleted aspartate residue at position 182. ELISA and Western blots were used to determine OPG concentrations in cell lysates and conditioned media from transiently transfected cells.

RESULTS

OPGdeltaD182 inhibited the generation of osteoclasts less effectively than the wildtype protein and had a reduced ability to bind to RANKL. The apparent higher molecular weight of OPGdeltaD182 compared with the wildtype is a result of hyperglycosylation of asparagine residues at positions 178 and 183. Glycosylation at N183 has the potential to disrupt OPG structure by interfering with disulphide bond formation and correct protein folding. Transient transfection experiments in SaOS2 cells suggest that OPGdeltaD182 is retained within the cell, a typical response to unstable or incorrect protein folding.

CONCLUSIONS

Taken together, these data suggest that the deletion of aspartate 182 impairs both the secretion and activity of OPG, which in turn provides an explanation for the increased osteoclastogenesis and high bone turnover observed in JPD patients with this mutation.

The Low-Density Lipoprotein Receptor-Related Protein 1 Is a Mitogenic Receptor for Lactoferrin in Osteoblastic Cells

Lactoferrin induces osteoblast proliferation and survival in vitro and is anabolic to bone in vivo. The molecular mechanisms by which lactoferrin exerts these biological actions are not known, but lactoferrin is known to bind to two members of the low-density lipoprotein receptor family, low- density lipoprotein receptor-related proteins 1 (LRP1) and 2 (LRP2). We have examined the role(s) of these receptors in the actions of lactoferrin on osteoblasts. We show that lactoferrin binds to cultured osteoblastic cells, and that LRP1 and LRP2 are expressed in several osteoblastic cell types. In primary rat osteoblastic cells, the LRP1/2 inhibitor receptor associated protein blocks endocytosis of lactoferrin and abrogates lactoferrin-induced p42/44 MAPK signaling and mitogenesis. Lactoferrin-induced mitogenesis is also inhibited by an antibody to LRP1. Lactoferrin also induces receptor associated protein-sensitive activation of p42/44 MAPK signaling and proliferation in osteoblastic human SaOS-2 cells, which express LRP1 but not LRP2. The mitogenic response of LRP1-null fibroblastic cells to lactoferrin is substantially reduced compared with that of cells expressing wild-type LRP1. The endocytic and signaling functions of LRP1 are independent of each other, because lactoferrin can activate mitogenic signaling in conditions in which endocytosis is inhibited. Taken together, these results 1) suggest that mitogenic signaling through LRP1 to p42/44 MAPKs contributes to the anabolic skeletal actions of lactoferrin; 2) demonstrate growth-promoting actions of a third LRP family member in osteoblasts; and 3) provide further evidence that LRP1 functions as a signaling receptor in addition to its recognized role in ligand endocytosis.

Osteoblastic cells express phospholipid receptors and phosphatases and proliferate in response to sphingosine-1-phosphate

Sphingosine-1-phosphate (S1P) is a naturally occurring polar phospholipid that exerts growth-factor--like effects in a number of cell types, potentially by endocrine, paracrine, and intracrine mechanisms. The biological actions of S1P are mediated in part by its specific binding to at least four members of the edg/lp family of G protein-coupled receptors (edg 1, 3, 6, and 8), and further regulated by the actions of specific cell membrane-bound lipid phosphate phosphatases (LPPs). Recent evidence has suggested that S1P may act as an osteoblast growth factor. In the current study, we demonstrate expression in osteoblastic cells of the S1P receptors edg 1, edg 5 and edg 8; the lipid phosphate phosphatases LPP-1, LPP-2, and LPP-3, and the S1P phosphatase mSPP1. S1P exerts dose-dependent mitogenic effects in cultures of primary rat osteoblastic cells and SaOS-2 cells. S1P also activates p42/44 mitogen-activated protein (MAP) kinases in osteoblastic cells, and the proliferative effects of S1P in these cultures are partially abrogated by PD-98059 and U-0126, specific inhibitors of p42/44 MAP kinase signaling. S1P-induced p42/44 MAP kinase activation in osteoblastic cells is dependent on functional Gi proteins and intracellular calcium fluxes, but not on protein kinase A, phospha-tidylinositol-3 kinase, or protein Kinase C. These data demonstrate (1) the expression by osteoblastic cells of several cell membrane-bound components of phospholipid signaling and (2) that S1P is a potent osteoblast mitogen, the proliferative action of which is mediated by a signaling pathway that involves Gi proteins, intracellular calcium, and p42/44 MAP kinases.

Parallel phosphatidylinositol-3 kinase and p42/44 mitogen-activated protein kinase signaling pathways subserve the mitogenic and antiapoptotic actions of insulin-like growth factor I in osteoblastic cells

IGF-I is an endocrine and paracrine regulator of skeletal homeostasis, principally by virtue of its anabolic effects on osteoblastic cells. In the current study, we examined the intracellular signaling pathways by which IGF-I promotes proliferation and survival in SaOS-2 human osteoblastic cells. Inhibition of each of the phosphatidylinositol-3 kinase (PI-3 kinase), p42/44 MAPK, and p70s6 kinase pathways partially inhibited the ability of IGF-I to stimulate osteoblast proliferation and survival. Because activation of p70s6 kinase is downstream of both PI-3 kinase and p42/44 MAPK activation in osteoblasts treated with IGF-I, this ribosomal kinase represents a convergence point for IGF-I-induced PI-3 kinase and p42/44 MAPK signaling in osteoblastic cells. In addition, abrogation of PI-3 kinase-dependent Akt signaling, which does not inhibit IGF-I-induced p70s6 kinase phosphorylation, also inhibited the antiapoptotic effects of IGF-I in osteoblasts. Finally, interruption of G beta gamma signaling partially abrogated the ability of IGF-I to promote osteoblast survival, without inhibiting signaling through PI-3 kinase/Akt, p42/44 MAPKs, or p70s6 kinase. These data suggest that IGF-I signals osteoblast mitogenesis and survival through parallel, partly overlapping intracellular pathways involving PI-3 kinase, p42/44 MAPKs, and G beta gamma subunits.

The phospholipids sphingosine-1-phosphate and lysophosphatidic acid prevent apoptosis in osteoblastic cells via a signaling pathway involving G(i) proteins and phosphatidylinositol-3 kinase

The naturally occurring phospholipids lysophosphatidic acid (LPA) and sphingosine-1-phosphate (S1P) have recently emerged as bioactive compounds that exert mitogenic effects in many cell types, including osteoblasts. In the current study, we examined the ability of each of these compounds to influence osteoblast survival. Using terminal deoxynucleotidyl transferase-mediated deoxyuridine 5'-triphosphate nick-end labeling and DNA fragmentation assays, we found that both LPA and S1P dose-dependently inhibited (by at least 50% and 40%, respectively) the apoptosis induced by serum withdrawal in cultures of primary calvarial rat osteoblasts and SaOS-2 cells. The antiapoptotic effects were inhibited by pertussis toxin, wortmannin, and LY294002, implicating G(i) proteins and phosphatidylinositol-3 kinase (PI-3 kinase) in the signaling pathway that mediates phospholipid-induced osteoblast survival. Specific inhibitors of p42/44 MAPK signaling did not block LPA- or S1P-induced osteoblast survival. LPA and S1P induced PI-3 kinase-dependent activation of p70 S6 kinase, but rapamycin, a specific inhibitor of p70 S6 kinase activation, did not prevent phospholipid-induced osteoblast survival. LPA and S1P also inhibited apoptosis in Swiss 3T3 fibroblastic cells in a G(i) protein-dependent fashion. In fibroblastic cells, however, the antiapoptotic effects of S1P were sensitive to inhibition of both PI-3 kinase and p42/44 MAPK signaling, whereas those of LPA were partially abrogated by inhibitors of p42/44 MAPK signaling but not by PI-3 kinase inhibitors. These data demonstrate that LPA and S1P potently promote osteoblast survival in vitro, and that cell-type specificity exists in the antiapoptotic signaling pathways activated by phospholipids.

Lysophosphatidic acid is an osteoblast mitogen whose proliferative actions involve G(i) proteins and protein kinase C, but not P42/44 mitogen-activated protein kinases

The simple glycerophospholipid lysophosphatidic acid (LPA) acts both as an intermediary in phospholipid metabolism and as an intercellular signaling molecule in its own right. In various cell types, LPA signals through its membrane-bound, G protein-coupled receptors to influence cellular processes such as proliferation, survival, and cytoskeletal function. Its actions in bone cells have not been studied. Here we show that the LPA receptor, LP(A1)/edg-2/vzg-1, is expressed in primary rat osteoblasts and the UMR 106-01 osteoblastic cell line. LPA potently induces DNA synthesis and an increase in cell number in cultures of osteoblastic cells. LPA rapidly (within 10 min) stimulates phosphorylation of p42/44 mitogen-activated protein (MAP) kinases in osteoblastic cells, an effect that is sensitive to inhibition of G(i) proteins, inhibition of influx of extracellular calcium, and inhibition of protein kinase C. LPA-induced DNA synthesis is partially inhibited by either pertussis toxin or calphostin C, but is insensitive to specific inhibitors of MEK, the kinase upstream of p42/44 MAP kinases, or of phosphatidylinositol-3 kinases. These data demonstrate that LPA is an osteoblast mitogen whose signaling effects in osteoblastic cells include activation of p42/44 MAP kinases. However, the LPA mitogenic signal in osteoblastic cells, while requiring G(i) proteins and protein kinase C, is independent of the activity of p42/44 MAP kinases.

The effect of leukemia inhibitory factor on bone in vivo

The local effects of leukemia inhibitory factor (LIF) on bone turnover in vivo have been examined. Recombinant murine LIF (0.2 micrograms) or vehicle was injected daily for 5 days over the right hemicalvaria, and the mice were killed on day 6 or 13. Effects on calvarial bone morphology were assessed using quantitative histomorphometry of nondecalcified bone tissue. Increased bone resorption was present in LIF-treated hemicalvaria compared with that in the noninjected hemicalvaria or calvaria from mice injected with vehicle alone at both 6 and 13 days. Significant increases in LIF-treated animals were as follows. Eroded surface increased 10-fold (P = 0.022), osteoclast surface increased 5-fold (P = 0.003), osteoclast numbers increased 3-fold (P = 0.002), and the number of osteoclast nuclei increased 3-fold (P = 0.009). Fibrotic tissue was laid down in the resorption defects, and there was an accompanying thickening of the periosteum (3 times greater in LIF-injected animals; P = 0.003), causing the overall thickness of the treated bones to be almost doubled (P = 0.045). Indices of bone formation were increased in animals treated with LIF. Osteoblast numbers, osteoblast surface, and osteoid area were doubled (P = 0.012, 0.016, and 0.058, respectively). Similar effects of LIF were seen in indomethacin-treated animals. Small but statistically significant morphological changes were also seen in the left noninjected hemicalvariae when LIF-treated animals were compared to controls. LIF increased periosteal area (P = 0.01) and total mineralized bone area (P = 0.002). In conclusion, LIF accelerated bone turnover locally in a prostaglandin-independent manner in normal mice, demonstrating its potential to modify in vivo bone cell function dramatically.

Regulation of osteoblast proliferation by leukemia inhibitory factor

We recently showed that leukemia inhibitory factor (LIF) stimulates 45Ca release from neonatal mouse calvariae in vitro and that it increases DNA and protein synthesis in this model. To elucidate further the actions of LIF on bone we now report the effects of this cytokine on DNA synthesis and cell proliferation in isolated fetal rat osteoblasts and in the osteogenic sarcoma cell line, UMR-106. In both actively growing and growth-arrested rat osteoblasts, LIF stimulated [3H]thymidine incorporation in a dose-dependent manner. The increase in DNA synthesis was time dependent, was associated with an increase in the number of osteoblasts, and was not blocked by indomethacin. LIF-treated cells showed reduced [3H]thymidine incorporation in comparison with control, as they approached confluence, possibly because of the increased cell density in the LIF-treated cultures. In UMR-106 cells, treatment with LIF inhibited [3H]thymidine incorporation in both actively growing and growth-arrested cultures. The effect was dose dependent and sustained with time. There was a corresponding decrease in cell numbers. It is concluded that although LIF causes an early stimulation of proliferation in isolated osteoblasts, it has opposing effects on UMR-106 cells. It is not possible to determine which of these effects is more relevant to the actions of LIF in vivo. The demonstration of a LIF effect on both these cell types, however, provides further evidence that this cytokine acts directly on osteoblasts.