Distinct pathways of extracellular signal-regulated kinase activation by growth factors, fibronectin and parathyroid hormone 1-34

Modulation of MAPK signalling by immobilized adhesive peptides: Effect on stem cell response to BMP-9-derived peptides

Biomimetic materials were developed to regulate stem cell behaviour. We have analyzed the influence of polycaprolactone (PCL) films, functionalized with adhesive peptides derived from fibronectin (pFibro) or bone sialoprotein (pBSP), on the response of murine multipotent C3H10T1/2 cells to bone morphogenetic protein-9 (BMP-9) and its derived peptides (pBMP-9 and SpBMP-9). PCL-pFibro promoted better cell cytoskeleton organization and faster focal adhesion kinase activation than did PCL-pBSP. PCL-pFibro also promoted MAPK signalling to improve the cell response to BMP-9 by inactivating ERK1/2 and stimulating p38 and JNK. BMP-9, pBMP-9 and SpBMP-9 induced greater phosphorylation of Smad1/5/8 in cells attached to PCL-pFibro than in cells on PCL-pBSP. These phosphorylated Smad1/5/8 were translocated to the nucleus. BMP-9 and its derived peptides restored the phosphorylation of JNK in cells on PCL-pBSP, but it remained less phosphorylated than in cells on PCL-pFibro stimulated with pBMP-9 and SpBMP-9. Cells attached to PCL-pFibro contained more Runx2, essential for stem cell commitment to become osteoblasts, than did cells on PCL-pBSP when incubated with BMP-9 and its derived peptides. Runx2 was no longer detected when the cells were pre-treated with JNK inhibitor. Therefore pFibro plus BMP-9 and its derived peptides may be a promising strategy to develop biomimetic materials.

STATEMENT OF SIGNIFICANCE

Biomaterials functionalized with adhesive peptides to favour bone repair have generated a great interest over the past decade. However, the effect of these materials on the ability of cells to respond to growth factors remains poorly known. One major growth factor subfamily involved in bone formation is the bone morphogenetic protein (BMP). However, these BMPs are expensive. We therefore developed less costly derived molecules. We showed how adhesive peptides derived from bone matrix proteins grafted onto polymer films affect the intracellular signalling and thus the ability of stem cells to be activated by BMP and its derived molecules. We have therefore identified a combination of bioactive polymers and BMP molecules that direct the stem cells towards bone forming cells.

Adenylyl cyclase 6 mediates loading-induced bone adaptation in vivo

Primary cilia are single, nonmotile, antenna-like structures extending from the apical membrane of most mammalian cells. They may mediate mechanotransduction, the conversion of external mechanical stimuli into biochemical intracellular signals. Previously we demonstrated that adenylyl cyclase 6 (AC6), a membrane-bound enzyme enriched in primary cilia of MLO-Y4 osteocyte-like cells, may play a role in a primary cilium-dependent mechanism of osteocyte mechanotransduction in vitro. In this study, we determined whether AC6 deletion impairs loading-induced bone formation in vivo. Skeletally mature mice with a global knockout of AC6 exhibited normal bone morphology and responded to osteogenic chemical stimuli similar to wild-type mice. Following ulnar loading over 3 consecutive days, bone formation parameters were assessed using dynamic histomorphometry. Mice lacking AC6 formed significantly less bone than control animals (41% lower bone formation rate). Furthermore, there was an attenuated flow-induced increase in COX-2 mRNA expression levels in primary bone cells isolated from AC6 knockout mice compared to controls (1.3±0.1- vs. 2.6±0.2-fold increase). Collectively, these data indicate that AC6 plays a role in loading-induced bone adaptation, and these findings are consistent with our previous studies implicating primary cilia and AC6 in a novel mechanism of osteocyte mechanotransduction.

Extracellular calcium chronically induced human osteoblasts effects: specific modulation of osteocalcin and collagen type XV

Fluctuation in extracellular calcium (Ca(2+)) concentration occurs during bone remodeling. Free ionized Ca(2+) plays a critical role in regulating osteoblast functions. We analyzed the effects of different concentrations of free ionized Ca(2+) (0.5, 1.3, and 2.6 mM) on human osteoblasts and we evaluated osteoblastic phenotype (marker expression and cell morphology) and functions (osteogenic differentiation, cell proliferation, and cell signaling). Our data show human osteoblasts that chronically stimulated with 0.5, 1.3, or 2.6 mM Ca(2+) significantly increase intracellular content of alkaline phosphatase, collagen type I, osteocalcin, and bone sialoprotein, whereas collagen type XV was down-modulated and RUNX2 expression was not affected. We also found a Ca(2+) concentration-dependent increase in osteogenic differentiation and cell proliferation, associated to an increase of signaling protein PLCβ1 and p-ERK. Human osteoblast morphology was affected by Ca(2+) as seen by the presence of numerous nucleoli, cells in mitosis, cell junctions, and an increased number of vacuoles. In conclusion, our data show a clear phenotypical and functional effect of extracellular Ca(2+) on human osteoblasts and support the hypothesis of a direct role of this cation in the bone remodeling processes.

Calcium sensing receptor-dependent and receptor-independent activation of osteoblast replication and survival by strontium ranelate

Age-related osteopenia is characterized by a negative balance between bone resorption and formation. The anti-osteoporotic drug strontium ranelate was found to reduce bone resorption and to promote bone formation. Here, we investigated the implication of the calcium-sensing receptor (CaSR) in the response to strontium ranelate using osteoblasts from CaSR knockout [CaSR(-/-)] and wild-type [CaSR(+/+)] mice. We showed that calcium and strontium ranelates increased cell replication in [CaSR(-/-)] and [CaSR(+/+)] osteoblasts. Strontium ranelate rapidly increased ERK1/2 phosphorylation in [CaSR(+/+)] but not in [CaSR(-/-)] osteoblasts, indicating that strontium ranelate can act independent of the CaSR/ERK1/2 cascade to promote osteoblast replication. We also showed that strontium ranelate prevented cell apoptosis induced by serum deprivation or the pro-inflammatory cytokines IL-1beta and TNF-alpha in [CaSR(-/-)] and [CaSR(+/+)] osteoblasts, indicating that CaSR is not the only receptor involved in the protective effect of strontium ranelate on osteoblast apoptosis. Strontium ranelate activated the Akt pro-survival pathway in [CaSR(-/-)] and [CaSR(+/+)] osteoblasts, and pharmacological inhibition of Akt abrogated the anti-apoptotic effect of strontium ranelate. Furthermore, both the proliferative and anti-apoptotic effects of strontium ranelate in [CaSR(-/-)] and [CaSR(+/+)] osteoblasts were abrogated by selective inhibition of COX-2. The results provide genetic and biochemical evidence that the effects of strontium ranelate on osteoblast replication and survival involve ERK1/2 and Akt signalling and PGE2 production, independent of CaSR expression. The finding that CaSR-dependent and CaSR-independent pathways mediate the beneficial effects of strontium ranelate on osteoblasts, provides novel insight into the mechanism of action of this anti-osteoporotic agent on osteoblastogenesis.

The small GTPase RhoA is crucial for MC3T3-E1 osteoblastic cell survival

Prolongation of cell survival through prevention of apoptosis is considered to be a significant factor leading to anabolic responses in bone. The current studies were carried out to determine the role of the small GTPase, RhoA, in osteoblast apoptosis, since RhoA has been found to be critical for cell survival in other tissues. We investigated the effects of inhibitors and activators of RhoA signaling on osteoblast apoptosis. In addition, we assessed the relationship of this pathway to parathyroid hormone (PTH) effects on apoptotic signaling and cell survival. RhoA is activated by geranylgeranylation, which promotes its membrane anchoring. In serum-starved MC3T3-E1 osteoblastic cells, inhibition of geranylgeranylation with geranylgeranyl transferase I inhibitors increased activity of caspase-3, a component step in the apoptosis cascade, and increased cell death. Dominant negative RhoA and Y27632, an inhibitor of the RhoA effector Rho kinase, also increased caspase-3 activity. A geranylgeranyl group donor, geranylgeraniol, antagonized the effect of the geranylgeranyl transferase I inhibitor GGTI-2166, but could not overcome the effect of the Rho kinase inhibitor. PTH 1-34, a potent anti-apoptotic agent, completely antagonized the stimulatory effects of GGTI-2166, dominant negative RhoA, and Y27632, on caspase-3 activity. The results suggest that RhoA signaling is essential for osteoblastic cell survival but that the survival effects of PTH 1-34 are independent of this pathway.

Parathyroid hormone induces mitogen-activated kinase phosphatase 1 in murine osteoblasts primarily through cAMP-protein kinase A signaling

BACKGROUND

Parathyroid hormone (PTH) regulates osteoblast function by binding to the PTH receptor 1 (PTHR1) to activate downstream signaling to induce expression of primary response genes (PRGs), which affect various aspects of the osteoblast phenotype. We previously identified PTH-induced PRGs in MC3T3-E1 cells, including mitogen-activated protein kinase (MAPK) phosphatase 1 (mkp1), which dephosphorylates members of the MAPK family. The aim of this study was to explore the molecular mechanisms of PTH's induction of mkp1 in primary mouse osteoblasts.

METHODS

Northern and Western analyses were used to determine mkp1 mRNA and protein expression. In vivo experiments were also performed to determine PTH's effect on mkp1 in mouse calvariae and long bones.

RESULTS

A total of 10 nM PTH and PTH-related protein (PTHrP) maximally induced mkp1 mRNA levels after 1 hour in osteoblasts. PTH also increased mkp1 protein expression, and induced mkp1 mRNA independent of new protein synthesis. PTHR1 triggers protein kinase A (PKA), PKC, and calcium pathways. Although PKA and PKC agonists induced mkp1 mRNA levels, only cyclic adenosine 3':5'-monophosphate (cAMP)-PKA inhibition blocked PTH-induced mkp1 mRNA levels. These data suggest that PTH-induced mkp1 mRNA levels are primarily mediated through the cAMP-PKA pathway. Further, prostaglandin E2 (PGE2), which activates cAMP-PKA and PKC, induced mkp1 mRNA to a greater extent than PGF2alpha and fluprostenol, which activate PKC signaling only. Finally, PTH maximally induced mkp1 mRNA levels in mouse calvariae and long bones in vivo at 0.5 hours.

CONCLUSIONS

mkp1's in vitro and in vivo induction in PTH-target tissues suggests its involvement in some of the effects of PTH on osteoblast function. mkp1 may be an important target gene in the anabolic effect of PTH on osteoblasts.

PACAP support of neuronal survival requires MAPK- and activity-generated signals

Pituitary adenylate cyclase-activating polypeptide (PACAP) is expressed in the parasympathetic ciliary ganglion (CG) and modulates nicotinic acetylcholine receptor function. PACAP also provides trophic support, promoting partial survival of CG neurons in culture and full survival when accompanied by membrane depolarization. We probed the adenylate cyclase (AC) and phospholipase-C (PLC) transduction cascades stimulated by PACAP to determine their respective roles in supporting neuronal survival and examined their interaction with signals generated by membrane activity. While PLC-dependent signaling was dispensable, AC-generated signals proved critical for PACAP to support neuronal survival. Specifically, PACAP-supported survival was mimicked by 8Br-cAMP and blocked by inhibiting either PKA or the phosphorylation of mitogen-activated protein kinase (MAPK). The ability of PACAP to promote survival was additionally dependent on spontaneous activity as blocking Na+ or Ca2+ channel currents completely abrogated trophic effects. Our results underscore the importance of coordinated MAPK- and activity-generated signals in transducing neuropeptide-mediated parasympathetic neuronal survival.

Osteoblast Elastic Modulus Measured by Atomic Force Microscopy Is Substrate Dependent

The actin and microtubule cytoskeleton have been found to contribute to the elastic modulus of cells, which may be modulated by adhesion to extracellular matrix (ECM) proteins and subsequent alterations in the cytoskeleton. In this study, the apparent elastic modulus (Eapp) of osteoblast-like MC3T3-E1 cells adhered to fibronectin (FN), vitronectin (VN), type I collagen (COLI), fetal bovine serum (FBS), or poly-l-lysine (PLL), and bare glass were determined using an atomic force microscope (AFM). The E(app) of osteoblasts adhered to ECM proteins (FN, VN, COLI, and FBS) that bind cells via integrins were higher compared to cells on glass and PLL, which adhere cells through nonspecific binding. Also, osteoblasts adhered to FN, VN, COLI, and FBS had F-actin stress fiber formation, while osteoblasts on glass and PLL showed few F-actin fibers. Disruption of the actin cytoskeleton decreased E(app) of osteoblasts plated on FN to the level of osteoblasts plated on glass, while microtubule disruption had no significant effect. This suggests that the elevated modulus of osteoblasts adhered to FN was due to remodeling of the actin cytoskeleton upon adhesion to ECM proteins. Modulation of cell stiffness upon adhesion to various substrates may influence mechanosignal transduction in osteoblasts.

Defective PTH regulation of sodium-dependent phosphate transport in NHERF-1-/- renal proximal tubule cells and wild-type cells adapted to low-phosphate media

The present experiments using primary cultures from renal proximal tubule cells examine two aspects of the regulation of sodium-dependent phosphate transport and membrane sodium-dependent phosphate transporter (Npt2a) expression by parathyroid hormone (PTH). Sodium-dependent phosphate transport in proximal tubule cells from wild-type mice grown in normal-phosphate media averaged 4.4 +/- 0.5 nmol.mg protein(-1).10 min(-1) and was inhibited by 30.5 +/- 8.6% by PTH (10(-7) M). This was associated with a 32.7 +/- 5.2% decrease in Npt2a expression in the plasma membrane. Proximal tubule cells from Na(+)/H(+) exchanger regulatory factor-1 (NHERF-1)(-/-) mice had a lower rate of phosphate transport compared with wild-type cells and a significantly reduced inhibitory response to PTH. Wild-type cells incubated in low-phosphate media for 24 h had a higher rate of phosphate transport compared with wild-type cells grown in normal-phosphate media but a significantly blunted inhibitory response to PTH. These data indicate a role for NHERF-1 in mediating the membrane retrieval of Npt2a and the subsequent inhibition of phosphate transport in renal proximal tubules. These studies also suggest that there is a blunted phosphaturic effect of PTH in cells adapted to low-phosphate media.

Direct Effects of Polymyxin B on Human Dendritic Cells Maturation

Polymyxin B is a lipopolysaccharide binding antibiotic used to inactivate potential lipopolysaccharide contaminations when evaluating the activity of different agents on innate immune cells. We report that polymyxin B is able to induce directly in monocyte-derived human dendritic cells (DCs) several functional and molecular modifications characteristic of DCs undergoing a maturation process. DCs incubated with polymyxin B up-regulate the expression of HLA class I and II, the co-stimulatory CD86 molecule, and show an increase in the fraction of adherent cells at short time, which persist at 48 h of incubation. Adhesion to the plate was required for the polymyxin B-induced DCs maturation. A transient activation of IkappaB-alpha/NF-kappaB and ERK1/2 pathways at short time and a further ERK1/2 activation at long term were also detected. Neither up-regulation of the maturation marker CD83 nor activation of p38 nor induction of cytokines secretion was observed in DCs treated with polymyxin B. We demonstrated that inhibition of IkappaB-alpha/NF-kappaB pathway abolishes polymyxin B effects. ERK1/2 inhibition instead allowed DCs treated with polymyxin B to progress in their maturation process as revealed by the increased up-regulation of the CD83 co-stimulatory molecules, the activation of p38, and the reduced adhesion to culture plates at 48 h of incubation. Our results indicate that polymyxin B induces a partial maturation of human DCs through increased adhesion to a substrate and activation of the IkappaB-alpha/NF-kappaB pathway. The increased ERK1/2 activation observed, even though correlating with the initial phases of the maturation process, actually inhibits the occurrence of full maturation.

Parathyroid hormone secretion and action: evidence for discrete receptors for the carboxyl-terminal region and related biological actions of carboxyl- terminal ligands

PTH is a major systemic regulator of the concentrations of calcium, phosphate, and active vitamin D metabolites in blood and of cellular activity in bone. Intermittently administered PTH and amino-terminal PTH peptide fragments or analogs also augment bone mass and currently are being introduced into clinical practice as therapies for osteoporosis. The amino-terminal region of PTH is known to be both necessary and sufficient for full activity at PTH/PTHrP receptors (PTH1Rs), which mediate the classical biological actions of the hormone. It is well known that multiple carboxyl-terminal fragments of PTH are present in blood, where they comprise the major form(s) of circulating hormone, but these fragments have long been regarded as inert by-products of PTH metabolism because they neither bind to nor activate PTH1Rs. New in vitro and in vivo evidence, together with older observations extending over the past 20 yr, now points strongly to the existence of novel large carboxyl-terminal PTH fragments in blood and to receptors for these fragments that appear to mediate unique biological actions in bone. This review traces the development of this field in the context of the evolution of our understanding of the "classical" receptor for amino-terminal PTH and the now convincing evidence for these receptors for carboxyl-terminal PTH. The review summarizes current knowledge of the structure, secretion, and metabolism of PTH and its circulating fragments, details available information concerning the pharmacology and actions of carboxyl-terminal PTH receptors, and frames their likely biological and clinical significance. It seems likely that physiological parathyroid regulation of calcium and bone metabolism may involve receptors for circulating carboxy-terminal PTH ligands as well as the action of amino-terminal determinants within the PTH molecule on the classical PTH1R.

Impact of the mitogen-activated protein kinase pathway on parathyroid hormone-related protein actions in osteoblasts

Parathyroid hormone-related protein (PTHrP) regulates proliferation and differentiation of osteoblastic cells via binding to the parathyroid hormone receptor (PTH-1R). The cAMP-dependent protein kinase A pathway governs the majority of these effects, but recent evidence also implicates the MAPK pathway. MC3T3-E1 subclone 4 cells (MC4) were treated with the MAPK inhibitor U0126 and PTHrP. In differentiated MC4 cells, osteocalcin and bone sialoprotein gene expression were both down-regulated by PTHrP and also by inhibition of the MAPK pathway. PTHrP-mediated down-regulation of PTH-1R mRNA and up-regulation of c-fos mRNA were MAPK-independent, whereas PTHrP stimulation of fra-2 and interleukin-6 (IL-6) mRNA was MAPK-dependent. Luciferase promoter assays revealed that regulation of IL-6 involved the cAMP-dependent protein kinase A and MAPK pathways with a potential minor role of the protein kinase C pathway, and a promoter region containing an activator protein-1 site was necessary for PTHrP-induced IL-6 gene transcription. An alternative pathway, through cAMP/Epac/Rap1/MAPK, mediated ERK phosphorylation but was not sufficient for IL-6 promoter activation. Phosphorylation of the transcription factor CREB was also necessary but not sufficient for PTHrP-mediated IL-6 promoter activity. Most interesting, a bidirectional effect was found with PTHrP increasing phosphorylated ERK in undifferentiated MC4 cells but decreasing phosphorylated ERK in differentiated cells. These data indicate that inactivation of the MAPK pathway shows differential regulation of PTHrP-stimulated activator protein-1 members, blocks PTHrP-stimulated IL-6, and synergistically down-regulates certain osteoblastic markers associated with differentiation. These novel findings indicate that the MAPK pathway plays a selective but important role in the actions of PTHrP.