Review Article Deregulation of Bone Forming Cells in Bone Diseases and Anabolic Effects of ...

13 Pages·2015·1.3 MB·English

Hindawi Publishing Corporation
BioMed Research International
Volume 2014, Article ID 814057, 12 pages
http://dx.doi.org/10.1155/2014/814057
Review Article
Deregulation of Bone Forming Cells in Bone
Diseases and Anabolic Effects of Strontium-Containing
Agents and Biomaterials
ShuangTan,1BinbinZhang,2XiaomeiZhu,3PingAo,3HuajieGuo,2
WeihongYi,4andGuang-QianZhou1,2
1KeyStateLaboratoryforBiotherapy,SichuanUniversity,Chengdu610065,China
2Anti-AgeingandRegenerativeMedicineCentre,ShenzhenUniversity,3688NanhaiAvenue,Shenzhen,Guangdong518060,China
3InstituteforSystemsBiomedicine,ShanghaiJiaotongUniversity,Shanghai200240,China
4DepartmentofSpinalSurgery,NanshanPeople’sHospital,Shenzhen518000,China
CorrespondenceshouldbeaddressedtoWeihongYi;[email protected];[email protected]
Received15November2013;Revised20February2014;Accepted3March2014;Published31March2014
AcademicEditor:MichaelGelinsky
Copyright©2014ShuangTanetal.ThisisanopenaccessarticledistributedundertheCreativeCommonsAttributionLicense,
whichpermitsunrestricteduse,distribution,andreproductioninanymedium,providedtheoriginalworkisproperlycited.
Age-relatedbonelossandosteoporosisareassociatedwithboneremodelingchangesthatarefeaturedwithdecreasedtrabecular
and periosteal bone formation relative to bone resorption. Current anticatabolic therapies focusing on the inhibition of bone
resorptionmaynotbesufficientinthepreventionorreversalofage-relatedbonedeteriorationandthereisabigneedinpromoting
osteoblastogenesisandboneformation.Enhancedunderstandingofthenetworkformedbykeysignalingpathwaysandmolecules
regulating bone forming cells in health and diseases has therefore become highly significant. The successful development of
agonist/antagonistofthePTHandWntsignalingpathwaysareprofitsoftheunderstandingofthesekeypathways.Asthecore
componentofanapprovedantiosteoporosisagent,strontiumtakesitseffectonosteoblastsatmultilevelthroughmultiplepathways,
representingagoodexampleinrevealingandexploringanabolicmechanisms.Therecognitionofstrontiumeffectsonbonehas
ledtoitsexpectedapplicationinavarietyofbiomaterialscaffoldsusedintissueengineeringstrategiesaimingatbonerepairing
andregeneration.Whilesummarizingtherecentprogressintheserespects,thisreviewalsoproposesthenewapproachessuchas
systemsbiologyinordertorevealnewinsightsinthepathologyofosteoporosisaswellaspossiblediscoveryofnewtherapies.
1.Introduction [1, 2] due to decreased number and activity of individual
osteoblastic cells. Such dysfunctions of osteoblasts may be
Bone remodeling is a physiological process that maintains caused by extrinsic mechanisms, such as changes in levels
the integrity of the skeleton by removing old bone and of systemic hormones and growth factors of bone tissues,
replacingitwithyoungmatrix.Animbalancebetweenbone and intrinsic mechanisms such as cellular apoptosis and
resorption and bone formation with ageing will result in senescence [2–4]. As a consequence, both trabecular and
the increased rate of bone turnover rate and bone loss. The periosteal bone formation decline [5]. Most of the cur-
age-related progressive bone loss is exaggerated in patients rentlyavailabletherapiesforosteoporosis,includingamino-
withosteoporosis,adiseasecharacterizedbydecreasedbone bisphosphonates, estrogens and selective estrogen receptor
mass,increasedbonefragility,andincreasedriskoffractures modulators (SERMS), and inhibitors for the receptor acti-
[1].Astheelderpopulationinthesocietyrapidlyincreases, vator of nuclear factor 𝜅B ligand (RANKL), take effect
osteoporosis has become one of the most common public mainly by reducing bone resorption. However, these thera-
healthproblems. piesfrequentlyexhibitsecondaryeffectsduetothecoupling
In the case of the age-related bone loss or osteoporosis, phenomenon of bone formation by osteoblasts and bone
theosteoblast-mediatedboneformationisseverelyimpaired resorption by osteoclasts [6]. Therefore, anabolic drugs are
2 BioMedResearchInternational
hopedtotargetosteoblasticcellstoincreaseboneformation progenitorstomatureosteoblastsandthereforeare,bydefi-
andbonestrength[7],thoughanticatabolicsmaybeefficient nition,heterogeneous.However,theyareusuallyconsidered
instabilizingbonemass. toexpressthetranscriptionfactorRunx2,atamoreadvanced
Inthispaper,accordingly,wefocusonthefunctionalreg- stageofdifferentiation,bothRunx2andosterix(OSX),which
bothcontroltheosteodifferentiation[11].Thedifferentiation
ulationofosteoblastlineagecellsinhealthandosteoporosis,
stageofosteoblastsinfluencestheirfunctionalrolesinbone
the currently proposed anabolic agents such as teriparatide
remodeling. Mice deficient in osteoblasts are deficient in
(PTH(1–34)), sclerostin or DKK1 inhibitors, and strontium
osteoclasts[11].Incontrast,conditionaldepletionofmature
that target specific signaling mechanisms underlying the
osteoblasts in vivo only ablates bone formation and osteo-
osteoblastfunctionandosteoporosis.Thewideuseofstron-
clastic bone resorption persists [12]. Therefore, immature
tium in various orthopaedic scaffolds is also summarized.
osteoblastsalsoinfluenceosteoclastogenesiswhereasmature
Through the analysis of current knowledge, some insights
osteoblasts perform the matrix production and mineraliza-
intofurtherstudiesofosteoblastregulationandtherapeutic
tionfunctions.
explorationareprovided.
Duringboneformation,asubsetofosteoblastsundergoes
terminal differentiation and becomes engulfed by unmin-
eralized osteoid [13]. Following mineralization of the bone
2. Osteoprogenitors,Osteoblasts,
matrix,theseentombedcellsarecalledosteocytes.Osteocytes
andOsteocytes are cocooned in fluid-filled cavities (lacunae) within the
mineralized bone and are highly abundant, accounting for
Boneformationisdependentontherecruitmentofsufficient
90–95%ofallbonecells[13].Osteocyteshavelongdendrite-
numberofosteoblastsandactivityofindividualosteoblasts.
like processes extending throughout canaliculi (tunnels)
Osteoblastic cells are recruited to bone forming surfaces
withinthemineralizedmatrix.Thesedendrite-likeprocesses
mainly from a group of skeletal stem cells with osteogenic
formanetworkandinteractwithotherosteocytesandwith
differentiationpotential.Bonemarrowcontainsasmallpop-
osteoblastsonthebonesurface[14].Theprimaryfunctionof
ulation of mesenchymal stem cells (MSCs) that are capable
the interaction between the osteocyte-osteoblast/lining cell
of giving rise to bone, cartilage, fat, or fibrous connective
syncytium is mechanosensation [15]. Osteocytes transduce
tissue[8].Cellpopulationswithpropertiescharacteristicof
stresssignalsfrombendingorstretchingofboneintobiologic
bone marrow MSC have been isolated from many other
activity and respond to mechanical load. The network is
tissues such as adult peripheral blood, dental pulp, adipose
thoughttobeintegralinthedetectionofmechanicalstrain
tissue, fetal cord blood, and liver. These self-renewing mul-
and associated bone microscopic cracks/fractures within
tipotent stem cells can give rise to osteoprogenitor cells in
the mineralized bone that accumulates as a result of nor-
various tissues under the right environmental conditions.
mal skeletal loading and fatigue [16]. Signaling molecules
Osteoprogenitor cells in turn give rise to and maintain
involved in mechanotransduction include prostaglandin
the osteoblasts that synthesize new bone matrix on bone
E2, cyclooxygenase 2, various kinases, Runx2, and nitrous
forming surfaces, the osteocytes within bone matrix that
oxide. Therefore, osteocytes initiate and direct the subse-
support bone structure, and the protective lining cells that
quent remodeling process and support bone structure and
cover the surface of quiescent bone. Therefore, osteoblastic
metabolism.
lineage cells comprise a diverse population of cells, includ-
Osteocytes express osteocalcin, galectin 3, CD44, and
ingimmature,differentiating,andmaturematrix-producing
severalotherbonematrixproteinsthatsupportintercellular
osteoblasts.Differentosteoblastsmayexpressdifferentsetsof
adhesion and regulate exchange of mineral in the bone
genes, which may represent the heterogeneity of trabecular
fluidwithinlacunaeandthecanalicularnetwork.Osteocytes
microarchitectureatdifferentanatomicsites,thesite-specific
regulate phosphate metabolism and matrix mineralization
differences in the response to different signals and disease
throughthesecretionofphosphate-regulatingfactorssuchas
states, and regional variation in the ability to response to
antiosteoporoticagents[9]. FGF23, Phex, Dmp1, and expression of sclerostin (encoded
bygeneSOST)andDKK1thatnegativelyregulatesWntand
Particularly,flattenedbone-liningcellsarethoughttobe
BMPssignaling[17].Osteocytesarelinkedmetabolicallyand
quiescentosteoblaststhatformtheendosteumontrabecular
electrically through gap junctions composed primarily of
and endosteal surfaces and underlie the periosteum on the
mineralized surface. More primitive osteoblasts that are connexin 43, which are required for osteocyte maturation,
found near functioning osteoblasts in the bone remodeling function,andsurvival[18].
unit can be identified based on their expression of alkaline
phosphatase. Active and mature osteoblasts synthesizing
bonematrixhavelargenuclei,enlargedGolgistructures,and 3.TheMolecularRegulationofOsteoblast
extensive endoplasmic reticulum. These osteoblasts secrete DifferentiationandFunction
typeIcollagenandothermatrixproteinsvectoriallytoward
theboneformationsurface.Besides,osteoblastshaveseveral Differentiationofmesenchymalstemcellsintotheosteoblast
other important roles in bone remodeling, including pro- lineageisundertightregulationorchestratedthroughmulti-
ductionofosteoclastogenicfactorsandbonemineralization plesignalingpathways.Amongthewell-characterizedarethe
[10]. Preosteoblasts encompass all cells transitioning from fibroblast growth factor (FGF), transforming growth factor
BioMedResearchInternational 3
𝛽 (TGF𝛽), hedgehog families, Wnt signaling, and notch ofbonehomeostasis,withactivationofthepathwayleading
pathways[19]. to increased, and inhibition to decreased, bone mass and
strength [37] and the Wnt signaling is now known as a key
mechanism regulating bone metabolism [38]. Activation of
3.1.TheFGFSignaling. TheFGFpathwayconsistsof23lig-
the Wnt signaling results in a generalized increase in bone
andsthattransducetheirsignalthroughoneofthefourFGF
massthroughouttheskeleton.Wntligandsactivatenumerous
receptors(FGFRs),functioningininitiatingcondensationof
intracellularpathwaysupontargetingonvariousmembrane
themesenchymeandproliferationofprogenitorcells.Upon
receptors,whichareeitherdependentorindependenton𝛽-
binding to cell surface receptors, FGFs lead to activation
catenin,anintracellulartransducer.𝛽-cateninisexpressedin
of multiple signaling modules, including mitogen-activated
mesenchymal precursor cells and its inactivation promotes
protein kinase (MAPK), phosphoinositide 3-kinase (PI3K),
their differentiation into chondrocytes instead of osteoblast
STAT1,andPKC.TemporalexpressionandactivityofFGFR
[34]. During the 𝛽-catenin-dependent Wnt signaling, 𝛽-
are critical for membranous bone formation and regulate
cateninisstabilizedfollowingbindingtoitsreceptorsFrizzled
theproliferation,differentiation,andapoptosisofosteoblasts
(FZD) and lipoprotein receptor-related protein 5 (LRP5) or
[20]. FGFRs contribute to normal skeletal development,
LRP6,whichinturnleadtothetranscriptionoftargetgenes
as gene mutation or deletions are associated with severe
suchasRunx2.Onthebasisofextensivestudiesconducted
dwarfism [21]. FGF18 plays a critical role in maturation
thus far on Wnt signaling in bone, the pathway has now
of osteoblast and FGF2 increases RUNX2 phosphorylation
becomethetargetfortherapeuticinterventiontorestorebone
and functional activity [22]. Similarly, the activation of
strengthinmillionsofpatientsatriskforfracture[37].
FGFR2signalingresultsinincreasedRunx2expressionand
enhancedosteoblastdifferentiation[22].FGFR1signalingat
an early developmental stage promotes osteoblast differen- 3.4. Hedgehog. The hedgehog signaling through Indian
tiation without affecting Runx2 expression. Finally, FGFR3 hedgehog(Ihh),asecretedmoleculeofthehedgehogfamily,
is involved in regulation of osteoblast number and osteoid isrequiredforosteoblastdifferentiationthroughendochon-
mineralization.Therefore,FGFsignalinghasdiverserolesin dral bone formation[39]. Ihh binds to the receptor patched
the regulation of preosteoblast proliferation and osteoblast homologue 1 (PTCH1) through the transmembrane protein
differentiationthroughdifferentreceptors. smoothened (SMO), consequently regulating target gene
transcription. Ihh controls osteoblast differentiation firstly
by inducing Runx2 in mesenchymal cells. Secondly, Ihh
3.2. BMPs. Bone morphogenetic proteins (BMP) are mem-
bers of the TGF𝛽 superfamily. This group of proteins has enhances Runx2 action through an interaction between
signal transducer Gli2 and Runx2 in osteoblast [40]. Ihh is
a number of diverse functions in multiple developmental
alsoneededforosteoblastproliferationandsurvivalandmice
processes ranged from embryogenesis, organogenesis, bone
deficientIhhgenelackosteoblastprogenitorcells[39].
formation, cell proliferation, and stem cell differentiation
[23–28]. BMPs signal through homomeric or heteromeric
type I and type II receptors, which are expressed in all 3.5.TheNotchSignaling. Thenotchsignalingmediatesbroad
cell types. Specific BMP receptors influence specific lineage cell-cellcommunications.Oncetheirligandsarebindingto
direction.BMP2signalingisrequiredforthestimulationof the neighbouring cell surface, notch receptors are cleaved
mesenchymalprogenitorcellsbyinducingexpressionofboth by the 𝛾-secretase complex. Consequently, the intracellular
Runx2andOsterix,leadingtoosteoblastdifferentiation[29– domainofnotchisreleasedfromtheplasmamembraneand
31].InductionofRunx2andOsterixbyBMP2andsubsequent translocatestothenucleus,interactingwithRBPJ𝜅/CBF1to
upregulationofosteoblast-specificgenesinvolvesDlx5,Smad activate downstream target transcription factors. By physi-
transducers,andtheMAPKpathway.TGF𝛽itselfplaysmore callyassociatingwithRunx2andinterferingwithfunctional
complex role during bone remodeling, with the inhibition activity of Runx2, notch inhibits osteoblast differentiation.
of Runx2 and osteoblast differentiation in vitro but mainly Mutations in the notch signaling cause skeletal patterning
promotingboneformationinvivo[29,32]. defectsinhumanandnotchdeficiencyleadstosevereosteo-
porosis in mice [41]. Interestingly, through its expression
in osteoblasts, notch exerts dimorphic effect during bone
3.3. The Wnt Signaling. During skeletal development, the
remodeling; notch also inhibits osteoclast differentiation
Wnt signaling is implicated in multiple steps and pro-
throughcontrollingproductionof“decoy”receptorOPGby
cesses, including proximal-distal outgrowth and limb pat-
osteoblast [41]. Thus far, the role of the notch signaling in
terning, and in MSC lineage commitment for chondroge-
bonediseasesremainstobefurtherelucidated.
nesis, osteogenesis, myogenesis, and adipogenesis [33–36].
Consequently,theWntsignalingaffectsallaspectsofskeletal
development. The importance of Wnt signaling in bone 3.6.Hormones. Besideslocalgrowthfactors,anumberofsys-
diseases has been recognized since the rare human muta- temichormonesregulatebonemassbyregulatingosteoblast
tions affecting bone negatively (osteoporosis-pseudoglioma differentiation and influencing bone formation. Many of
syndrome)orpositively(high-bonemassphenotype)wereall themactonosteoblaststoexpressM-CSFandRANKLthat
identifiedtoresideincomponentsofthecanonicalWntsig- in turn regulate osteoclast differentiation. We herein only
nalingmachineryadecadeago.Mousegeneticstudieshave discusssomeasexamples,particularlythoseunderstoodwell
further confirmedthe role of the pathway in the regulation thusfar.
4 BioMedResearchInternational
Parathyroid hormone (PTH) produced by the chief Thisisasystemic,skeletaldisordercharacterizedbylowbone
cells of parathyroid gland plays a primary role in calcium mass with a high susceptibility to fractures. Osteoporosis
homeostasis through its action on bone and kidney and is characterized by reduced bone mass and deterioration of
through enhanced synthesis of another hypercalcemic hor- bone microarchitecture, resulting in bone fragility. Primary
mone, 1,25(OH)2 vitamin D3 [42]. The anabolic effects osteoporosisiseitherapostmenopausalorage-relateddisease
of PTH on bone formation are mediated through PTH ofelderlypeople,essentiallyoccuringbecausetheproduction
receptor-dependentmechanisms.PTHenhancesosteoblastic of bone by means of osteoblasts cannot compensate for
cell proliferation and function, extends mature osteoblast
boneresorptionbyosteoclasts[55].Sexhormones,including
life span through antiapoptotic effects, enhances the Wnt
both estrogen and androgen, act on osteoblasts for their
signaling through inhibition of Wnt antagonist sclerostin,
survival and, at the same time, induce osteoclast apoptosis
andpromotesthelocalproductionofboneanabolicgrowth
throughactivationoftheFASL/FASpathway[56].Therefore,
factors such as insulin-like growth factor 1 (IGF1) [43]. In
withdrawal or decline of sex hormones is the principal
addition, PTH promotes bone formation partially through
determinant of primary osteoporosis. Moreover, estrogens
phosphorylationandactivationofRunx2,resultinginexpres-
suppress the production of proosteoclastogenic cytokines
sionofosteoblastgenes[44].PTHalsoinhibitsproteasome
(such as IL-1, IL-6, TNF-𝛼, and RANKL) and stimulate the
mediated degradation of Runx2 and increases expression
secretionofOPGbyosteoblasts[57,58].
ofosterixtoenhanceosteoprogenitorlineagedetermination
[45].Astheonlyapprovedanabolicagent,intermittentPTH
therapyhasbeendemonstratedtohavebeneficialeffectson 4.1. The Activation of the Inflammatory Pathways. Many
increasing bone mass and diminishing bone fragility asso- chronic diseases have a local or a systemic inflammatory
ciatedwithosteoporosisresultingfromaging,sexhormone basis, which has overall deleterious effects on bone mass
deficiency, and glucocorticoids use [46]. The other major [59,60],leadingtothesecondaryosteoporosiswithanonset
hypercalcemic hormone is 1,25(OH)2 vitamin D3, a steroid atanyage.ThestimulatoryactionoftheNF-𝜅Bsignaltrans-
hormone that favors intestinal absorption of calcium [47]. ductioninosteoclastdevelopmentandfunctionalactivityis
DeletionorinactivationofthevitaminDreceptor(VDR)in
widelyrecognized[61];recentlyithasalsobeendemonstrated
miceandinhumansleadstorickets,aphenotypecompletely that NF-𝜅B activation is potently inhibitory to osteoblast
reversible in both organisms by treatment with calcium.
commitment,differentiation,andmineralizationinvivoand
VitaminD3positivelyregulatestheexpressionofosteoblas- in vitro [62]. Tumor necrosis factor (TNF-𝛼) is a potent
tic phenotype markers. However, continuous exposure to NF-𝜅B inducer and activation of p65 (a NF-𝜅B subunit) by
parathyroid hormone (PTH), parathyroid hormone-related
TNF-𝛼 has been shown to suppress transcription of osteo-
protein(PTHrP),andlowdosesof1,25-dihydroxyvitaminD3
calcininosteoblasticcells[63].Pharmacologicalsuppression
also stimulates osteoblasts to express M-CSF and RANKL,
of TNF-𝛼 is reported to reverse age-related defects in bone
leadingtoincreasedosteoclastproductionandboneresorp-
formationinamousefracturehealingmodelandsynergizes
tion[48,49].
TGF𝛽-andBMP-2-inducedSmadsignalingindifferentiating
Animportantnonsteroidalregulatorofbonemassislep-
osteoblasts[64].Inaddition,TNF-𝛼upregulatesSmurf1,an
tin,ahormonethatfunctionsthroughaninhibitoryactionof
E3 ligase that promotes proteasomal degradation of bone
thehypothalamusonboneformation[50,51].Leptinismade
morphogenetic signaling proteins [65]. NF-𝜅B signaling in
by fat cells and functions to suppress appetite and inhibit
osteoblasts has been shown to upregulate Smad7, a general
boneformationbybindingtoreceptorsinthehypothalamus.
inhibitorofSmadpathway.Finally,adirectinhibitoryaction
Mice and humans deficient in leptin or its hypothalamic
of NF-𝜅B on bone formation was demonstrated in vivo to
receptor are obese and have a higher than normal bone
showtime-andstage-specificinhibitionofIBkinase(IKK)
mass.Theeffectorpathwayfromhypothalamustoboneisthe
indifferentiatedosteoblasts,increasedtrabecularbonemass,
sympathetic nervous system and sympathetic neurons pro-
ducenoradrenaline,whichbindsto𝛽2-adrenergicreceptors andamelioratedovariectomy-inducedboneloss[66].
(𝛽2-AR)onosteoblasts[51,52].Mutantmicelacking𝛽2-AR
have increased bone mass but do not respond to leptin by 4.2.OtherSignalingAlterationsandTherapeuticsOpportuni-
reduction in bone mass [53], while ovariectomized 𝛽2-AR ties. AlterationsintheWntsignalinghaveprofoundimpact
nullmicefailtolosebonemass,suggestingthatmaintenance on age-related bone loss in mice [67]. Mechanical loading
of the sympathetic nervous system in bone may require upregulates the Wnt signaling in MSC [68], suggesting
estrogen,whichisessentialinregulatingboneremodelingvia that the combination of reduced 𝛽-catenin signaling and
tworelatedreceptors,ER𝛼andER𝛽expressedbyosteoblasts. decreasedmechanicalstimulationwithagemaycontributeto
theage-relateddeclineinboneformation.Basedontheseand
otherstudies,theimportantroleoftheWntsignalinginthe
4.DeregulationofBoneForming control of bone formation has been well recognized as this
CellsinDiseases pathwayissuggestedtobeapotentialtherapeutictarget[69].
TheWntsignalingalterationisrelatedtoincreasedmarrow
Thecorrectbalancebetweenbonedepositionandresorption adipogenesis.Withtheaimofincreasingosteoblastogenesis
iscrucialforthepropermaintenanceofthebonemassandthe andtheboneformation,severalpharmacologicalagentshave
lossofthiscouplingisthestartingpointforosteoporosis[54]. been developed that act on bone marrow MSC to favor
BioMedResearchInternational 5
osteoblastogenesisanddecreaseadipogenesis[70].Nonphar- differentiation markers, such as alkaline phosphatase, type-
macological means to enhance MSC differentiation toward 1 collagen, bone sialoprotein and osteocalcin in murine
osteoblastsincludelow-magnitudemechanicalsignals[71]. bone marrow MSC, osteoprogenitor cells, and immature
Mostdrugscurrentlyavailableforthetreatmentorpre- osteoblasts[80,87,88].Furthermore,theagentalsopromoted
vention of osteoporosis are antiresorptive, including estro- theultimatedifferentiationofhumanosteoblastsintoosteo-
gensandselectiveestrogenreceptormodulators,bisphospho- cytes,asindicatedbytheincreasedexpressionofosteocyte-
restrictive markers such as dentin matrix protein 1 [85].
nates,anddenosumabblockingtheRANKL/RANKpathway
Overall, the available in vitro data indicate that strontium
[72]. It is desirable to identify novel agents that can exhibit
promotestheosteogenicdifferentiationprogramandreduces
an anabolic function in order to improve and restore bone
osteoblast apoptosis, thereby promoting osteoblastogenesis.
mass.Teriparatide(PTH1-34)iscurrentlytheonlyUSFood
The positive effects on preosteoblast eventually result in
andDrugAdministration-(FDA-)approvedanabolicagent
increased bone nodule formation, a hallmark of in vitro
for the treatment of osteoporosis [37]. Although the effect
osteogenesis[80,85,88].
of PTH can be anabolic or catabolic depending on the
Age-related bone loss is generally associated with
dose, intermittent administration increases trabecular bone
osteoblast insufficiency relative to the adipogenesis,
formation [73, 74]. Systemic administration of antagonists
which is responsible for the progressive adiposity often
toDKK1orsclerostinmaypossiblyaffectonlytheskeleton,
observedinosteoporosis.Thenumberofmatureosteoblasts
favoringtheendogenousWntsignalingandincreasingbone
and adipocytes in bone marrow is influenced by the
formation without affecting the Wnt signaling in other
differentiationofthecommonmesenchymalprogenitorcell
organs.
towardsonephenotypeandawayfromtheother.Incontrast
Currently, only strontium ranelate proves to have roles to the promoting role in osteoblast, strontium exhibits
in both the osteoclast inhibition and bone formation pro- inhibitoryroleinadipogenesisofMSC[89].InmurineMSC
motion.Therefore,furtherresearchinthetopicmayprovide cultures, strontium increased Runx2 expression and matrix
insightsnotonlyintoimprovingtheeffectsofSr-containing mineralization and decreased peroxisome proliferator-
agents but also into discovering novel and more effective activated receptor gamma 2 (PPARc2) expression and
therapies. adipogenesis. This effect was associated with enhanced
expression of the Wnt noncanonical representative Wnt5a
and adipogenic modulatorMaf and was abrogated by Wnt-
5.AnabolicEffectsofStrontium-Containing andnuclearfactorofactivatedT-cells(NFATc)antagonists,
AgentsonOsteoblasts indicating a critical role for the Wnt and NFATc/Maf
signalingintheswitchinadipogenesistoosteoblastogenesis
Severalstrontium-containingagents,suchasSrfructose,1,6- inducedbystrontium[89].
diphosphate, and strontium citrate [75], have been exper-
imentally demonstrated to have antiosteoporotic effects,
6.MolecularBasisoftheRoleofStrontiumin
amongwhichstrontiumranelateisanapproveddrug[76–78].
At the cellular level, it was shown that bone marrow BoneFormingCells
MSC culture, when exposed to strontium (Sr), displayed
a significant increase in the expression of the master gene 6.1.CalciumSensingReceptor(CaSR). TheCaSRbelongsto
and Runx2, as well as bone sialoprotein (BSP), and this subfamily 3 of G-protein coupled receptor family (GPCR)
was associated with a significant increase in the formation [90,91]andcanactivateG𝛼iandG𝛼q/11G-proteins,conse-
ofcolony-formingunitosteoblasts(CFU-obs).Interestingly, quentlyresultingindecreasedintracellularcyclicadenosine
the activation of gene expression by Sr varies with the monophosphate (cAMP) level, the stimulation of phos-
differentiation stage of MSC: Runx2 and BSP in bone mar- pholipase C𝛽,inositol1,4,5-triphosphate, and the release of
2+
row MSC; Runx2 and osteocalcin in preosteoblasts; BSP intracellularCa [90].TheCaSRisphysiologicallyexpressed
and osteocalcin in mature osteoblasts. Strontium ranelate- athighlevelintheparathyroidchiefcellsandsenseschanges
2+
treated ovariectomised (OVX) animals exhibited increased in extracellular Ca level, leading to an adjustment in the
bone formation and decreased bone resorption, leading to release of the PTH [90]. Similarly, CaSR can sense other
2+
preventionoftrabecularbonelossandimprovementofbone divalent and trivalent cations, including Sr because of its
microarchitecture and strength [79–81]. Clinical data also similaratomicandionicpropertiestoCa2+.
revealed that strontium ranelate treatment increased bone Experimental evidences show that strontium acts on
mineralappositionrateandimprovedtrabecularmicroarchi- osteoblasts through the CaSR, leading to the activation
tectureinpostmenopausalosteoporoticwomen[82],which of MAPK signaling and consequently cell replication. In
wasassociatedfurtherwithreducedfracturerisk[76,78]. addition, strontium increases OPG and decreases RANKL
InvitroexperimentsshowedthatSrranelatehadpositive expression inosteoblastic cellsviatheCaSR [92]. However,
effects on osteoblastogenesis and activity of primary rat CaSR does not appear to be the only receptor involved
and human osteoblasts [83]. On the one hand, strontium in the effects of strontium on osteoblasts as increased cell
enhanced the replication of preosteoblastic cells [84, 85] replication and decreased apoptosis were still observed in
and reduced osteoblast apoptosis [86, 87]. On the other osteoblasts from mice deficient for CaSR when exposed
hand, strontium was found to activate many osteoblast to strontium, indicating that other cation-sensing receptors
6 BioMedResearchInternational
are functioning to sense extracellular Sr [93]. Among these can produce OPG, which functions as a decoy receptor
receptors are GPRC6A, a GPCR that is closely related to for the RANKL and can thereby reduce the production
the CaSR and senses extracellular divalent cations [94]. of osteoclasts by inhibiting the differentiation of osteoclast
Thefunctionalinvolvementofthisandothercation-sensing precursors. A positive effect of Sr ranelate was observed on
receptorsintheresponsetostrontiumremainstobefurther the OPG/RANKL ratio, causing an increased secretion of
determined. OPG and a simultaneous reduction of RANKL expression,
leading to the suppression of osteoclastogenesis [101]. The
6.2. Fibroblast Growth Factor Receptor (FGFR). As men- increase in the number of OPG-producing cells and the
tionedabove,strontiumalsostimulatesosteoblastcellgrowth decreaseinRANKLexpressionthusconstitutetheimportant
through the CaSR-independent molecular mechanism. In mechanismbywhichstrontiumexhibitsadissociatingeffect
this regard, a selective inhibitor of FGFR was able to onthecouplingbetweenboneformationandresorption.
slow down cell growth induced by strontium ranelate in
osteoblasticcells,suggestingthattheactivationofFGFRisa
6.6.TheNFATcPathway. Theinvolvementofthecalcineurin
newpotentialmechanismbywhichstrontiumcanstimulate
(Cn)/nuclearfactorofactivatedTc(NFATc)pathwayinbone
osteoblasticcellgrowth.ActivationofFGFR-dependentcell
developmentandboneremodelingwasfoundinpatientswho
growthisalsoobservedinresponsetoothercations,suggest-
were taking Cn inhibitors such as cyclosporine (CsA) and
ing that activation of FGFRs may be a new cation-sensing
mechanisminosteoblasts[95]. FK506,developedosteopenia,andhadahigherincidenceof
fractures[102].ItisknownnowthatNFATc1playsanimpor-
tant role in both osteoblasts and osteoclasts [103]. NFAT
6.3. Ras/MAPK Pathway and Ras/Akt Pathway. Our own
includes five transcription factors, NFATc1 to NFATc4 and
recent work demonstrated that rat sarcoma viral oncogene
NFAT5,whichareallinvolvedinthedifferentiationofvarious
homolog (RAS), an upstream regulator of MAPK and Akt,
was activated by strontium treatment and siRNA-mediated cell types. Highly phosphorylated NFATc transcription fac-
Ras knockdown inhibited strontium-stimulated expression torsarenormallylocalizedinthecytoplasm.Increasedintra-
ofosteogenicmarkers[96].Mitogen-activatedproteinkinase cellularcalciumlevelsactivatecalcineurin(Cn)anddephos-
(MAPK) can directly enhance osteogenesis through the phorylate NFATc1, leading to its nuclear translocation to
phosphorylation and subsequent activation of Runx2 [97]. regulatetargetgenes[104].NFATc1isexpressedintheprocess
Thesestudiessuggestthatstrontiumcanpromoteosteogenic ofosteoblastdifferentiationandboneformation[105]andhas
differentiation of MSCs through activating the Ras/MAPK beenshowntobeanimportantsignalingpathwayinvolvedin
signaling pathway and the downstream transcription factor theroleofstrontium.Increasingstrontiumconcentrationsin
Runx2 [96]. Akt kinase plays certain roles in cellular pro- theosteoblastenvironmentactivateCnandsubsequentlythe
cesses, including glucose metabolism and cell proliferation NFATc/Wntsignalingpathwaysinosteoblasts,leadingtothe
andapoptosis.Studiesonmicehavedemonstratedthatstron- increasedreplicationandsuppressedadipogenesis[89].
tiumincreasedtheproliferationandreducedtheapoptosisof
osteoblastwithactivationoftheAktkinase-relatedpathway
6.7. The Roleof WntSignaling. Itisreportedthatstrontium
[98].
promoted the translocation of 𝛽-catenin into the nucleus
throughtheactivationoftheCaSRandsubsequentactivation
6.4. Prostaglandins (PGE2). It has been confirmed that the of Akt-signaling in human osteoblasts [98]. In addition,
effectofstrontiumontheproliferationandreducedapoptosis strontiumdecreasedtheexpressionofsclerostin,aninhibitor
ofosteoblastscouldbeneutralizedbytheselectiveinhibition of the canonical Wnt signaling that acts as a negative regu-
ofcyclooxygenase-2(COX-2).Theresultsalsoindicatedthat latorofboneformation[98],providinganothermechanism
the positive effects of Sr ranelate on osteoblasts depend on by which strontium modulates the Wnt signaling. Further
the PGE2 production. Strontium induces murine MSCs to evidences on the involvement of strontium in the Wnt
expressCOX-2thatleadstotheincreasedPGE2production, pathway include that the treatment of murine osteoblasts
thuscontributingtotheincreaseddifferentiationofMSCinto withstrontiumincreasedtheexpressionofWnt3aandWnt5a
osteoblasts[99].Thismaythereforebeanewpathwaythatis and𝛽-catenintranscriptionalactivity.InhibitorsoftheWnt
activatedbystrontiuminbonecells. signaling,DKK1,andsFRP1,asolubleproteinwithhomology
totheWnt-bindingsiteofFrizzledproteins,decreasedtheSr-
6.5. The Role of the OPG/RANKL System. The primary inducedexpressionofosteoblasticgenessuchasRunx2,ALP,
mechanism by which strontium reduces osteoclast number andtype1collagen[93].TheinhibitionoftheWnt5areceptor
is to regulate the production of osteoprotegerin (OPG) by RykandthesignalingtransducerRhoAwereabletopartially
osteoblasts and the receptor activator of nuclear factor 𝜅-B abrogate Sr-induced cell proliferation and osteoblastic gene
ligand(RANKL)[100],thetwomoleculesthatplayessential expression [93]. Another study showed that Wnt5a was
roles in osteoclast differentiation. Osteoblast progenitors expressed at higher levels in the bone marrow of Sr-treated
and osteoblasts express RANKL, a molecule that binds senescent SAMP6 mice compared to vehicle-treated mice
to the receptor activator of nuclear factor 𝜅-B (RANK) andthatitplayedaroleinthedifferentiationofMSCupon
on osteoclast precursors and thereby activates intracellular Sr-treatment [89]. Therefore, strontium definitely produces
signaling,resultinginosteoclast differentiation.Osteoblasts positiveeffectsinboneviatheWntsignaling.
BioMedResearchInternational 7
6.8.Insulin-LikeGrowthFactor. Arecentstudyshowedthat scaffoldsintheformsofnanopowdersormicrosphereswere
anincreaseoftheinsulin-likegrowthfactor(IGF-1)concen- alsoreportedtopossessosteoconductiveandosteoinductive
tration was observed after six-month administration of Sr properties and have the potential to repair bone defects
ranelate at a daily dose of 2.0g [106]. This result suggests causedbyosteoporoticfractures[110].
that IGF deficit plays a definite role in the development of The most important property of bone cement as inor-
postmenopausalosteoporosisandstrontiumadministration ganic filler in load bearing orthopaedic implants is good
mayexertanadvantageousinfluenceonBMDincrease. integration with host bone with reduced bone resorption
Insummary,multiplemechanismsareinvolvedinstron- and increased bone regeneration at the implant interface.
tium effects on bone forming cells. Once exposed to stron- Similar to integration into bioglass, strontium has been
tium, multiple intracellular signaling pathways and key introducedintobonecementandthemechanicalproperties,
molecules are activated and orchestrated to promote their crystallinicproperties,andSrionreleaseactivitieshavebeen
survival, proliferation, and differentiation of osteoblasts, well evaluated. Sr-containing bone cement has been shown
whichinthemeantimeproduceaseriesofinflammatoryand to promote early bone formation by prolonging the release
osteoclast-regulatoryfactorsthatmayeventuallyfavorbone durationofstrontiumwhileremainingthestrength[111].
formation via coupling of both bone resorption and bone Biomimetic apatites could also be cosubstituted with Sr
formation. as well as other elements. In a study by Iafisco et al. [112],
hydroxyapatites were cosubstituted with foreign ions such
2+ 2− 2+
7.Strontium-IntegratedorSubstituted as Mg , CO3 , and Sr for starting materials for the
development of nanostructured biodevices for regeneration
Biomaterialsfor BoneRegeneration
of osteoporotic bone. Biological-like amounts of Mg and
Tissue engineering is extensively applied in both research CO3 ions were inserted in the apatite structure to mimic
the composition of bone apatite by the addition of Sr ions
and clinical practice of skeletal disease and trauma. For
as antiosteoporotic agent. Based on the hypothesis that the
tissueengineering,bioactivematerials,withthepropertiesof
combinationofSiandSrmayhavesynergeticeffectsonosteo-
stimulatingosteoblastsandcalcimimeticsinphysiochemical
poroticboneregeneration,theporousSr-substitutedcalcium
behaviors,arepreferentialasanimplanttofacilitatehealing
silicate(SrCS)ceramicscaffoldscombiningthefunctionsof
or to compensate for bone defects, particularly for osteo-
SrandSielementsweredevelopedwiththegoalstopromote
poroticfractureswheretheconventionalmetallicimplantis
osteoporoticbonedefectrepair[113].Boronisknowntoplay
not applicable because of bone fragility and extremely low
important roles in bone growth and maintenance, immune
bone density. The key of a bioactive material is to form a
function, and psychomotor skills. Pan et al. reported that
continuousandhighlyreactiveinterfacewiththesurround-
the incorporation of strontium significantly decreased the
ingbonetissuetoinduceabundantboneformationandthe
cytotoxicity that arises with the rapid dissolution of borate
ability to be structurally and mechanically compatible with
glass[114].Inaddition,withthedegradationofglass,itwill
bone tissue and eventually to be replaced by new bones. In
not only render boron as a nutritional element, but also
somecircumstances,itisalsodemandedthatgrowthfactors,
deliverstrontiumfornewboneformation.
drug,andionsareincorporatedintosuchamaterial,which
caninturnbeslowlyreleasedasthematerialisbiodegraded
7.2. Strontium Substitution in Implant Coating Materials.
invivo.Astheroleinstimulatingosteoblastproliferationand
Implants undergoing early instability or even subsidence
differentiationbecomeswellrecognized,strontiumiswidely
correlate with an increased risk of aseptic loosening, sub-
applied in many bone regeneration biomaterials in various
sequently requiring revision. A load-bearing orthopaedic
forms,whicharebrieflypresentedasbelow.
implant therefore needs to possess the properties of well
integration with host bone tissue and early fixation by
7.1. Strontium Integration into Bone-Supporting or Regen-
osseointegrationoftheimplantisindeeddemandedduring
erating Biomaterials. In a study by Zhang et al. [107],
surgical practices. This is an even greater challenge during
strontium-containingmesoporousbioactiveglass(Sr-MBG)
revision replacement surgery using the allograft implant
scaffoldswithcontrolledarchitectureandenhancedmechan-
because resorption of the allograft may exceed new bone
ical strength were fabricated using a three-dimensional (3-
formation and result in the instability of the prosthesis.
D)printingtechnique.TheSr-MBGscaffoldscouldcombine
Implantationofmetal-basedjointreplacementsoftenresults
the advantages of Sr-MBG such as good bone forming
incorrosionandparticlerelease,initiatingchronicinflamma-
bioactivity, controlled ion release, and enhanced mechan-
tionleadingontoosteoporosisofhostbone.Acompensative
ical strength and thus has potential application in bone
regeneration. polycaprolactone (PCL) is a resorbable poly- solutionisthecoatingofmetalimplantswithhydroxyapatite
mer extensively used in bone tissue engineering owing to (HA)ortheuseofbulkbioglass.Forthispurpose,strontium
goodstructuralpropertiesandprocessability.Severalstudies couldbeincorporatedintothematerialsurfaceorappliedin
reported that strontium-substituted bioactive glass (SrBG) dopingsurfaceofanimplant.Sabareeswaranetal.therefore
wasincorporatedintoPCLandfabricatedinto3Dbioactive tested the in vivo biocompatibility and bone healing of the
composite scaffolds that exhibited better ability to promote strontium-(Sr-)stabilizedbulkglassceramicsforshortterm
osteogenesis than the bioactive glass without strontium implantationofupto12weeksinrabbitmodelandobserved
[108,109].Strontium-(Sr-)substitutedhydroxyapatite(HAP) excellenthealing,whichiscomparabletothatseenduringthe
8 BioMedResearchInternational
useofacommerciallyavailableimplantofHA-basedbioglass moleculesplayimportantrolesfromdifferentrespects,some
alone[115].Astrontium-substitutednanohydroxyapatite(Sr- of which are emerging as therapeutic targets. Apart from
HA) coating, deposited onto porous implant surfaces, has currently proposed or trialed approaches to focusing on
thepotentialtoenhanceimplantosseointegration[116].Ina agonists/antagonists of certain osteoblastic signaling path-
cementless,experimentalgapmodelincanine,Vestemarket ways, such as hPTH1-34 (teriparatide) and antisclerostin or
al.compareda5%strontium-dopedHAbonegraftextender anti-DKK1antibodies,otherstrategiesaimingatmobilizing
with a nondoped HA extender and demonstrated that the skeletal stem cells and activating osteoblastic cell functions
areanticipatedtobeafieldworthofmuchexploring.Stron-
extender with strontium doping could protect the allograft
tium,thoughcurrentlythoughtasarelativelymildanabolic
from fast resorption and increase gap healing, leading to
agent,isconsideredasacandidateinthisrespectduetoits
the improved fixation of the implant, though the results of
dualroleinregulatingosteoblastogenesis/osteoclastogenesis
mechanicaltestwereinconclusive,suggestingthatstrontium
andosteogenesis/adipogenesisviaitsinvolvementinmultiple
couldcontributetoreversingtheimbalanceoffastresorption
pathways.Besidesbeingabletobesuppliedsystemically,the
of allograft and slower formation of new bone because of
applicationofstrontiumisbeingmuchextendedintovarious
its anabolic and anticatabolic effects. In addition, it has
biomaterialsandtherebytissueengineeringstrategiesforthe
been reported that a film of strontianite was formed on
localbonelesionsanddefects.
a bioactive surface of sodium titanate when exposed to a
Further, on the mounting evidences and rapid progress
strontiumacetatesolution.Thisstrontianitefilmenablesthe
in revealing key molecular factors and signaling pathways
localreleaseofstrontiumionsfromimplantsurfacesandthus
regulatingboneformingcellsandosteoporoticconditions,a
stimulatesboneformationinvivo[117].
systemsbiologyapproachtocoherentlyrelatingthesefactors
withmathematicalmodelingwouldbeparticularlyhelpfulto
7.3.StrontiuminMembraneMaterialsandHydrogels. Mem- betterunderstandandevaluatetherapeutics.Futureprogress
brane materials are particularly useful in guided bone inthisfieldwillhopefullyprovideopportunitiesforexploring
regeneration. In this regard, the effects of a strontium drugdiscovery.
hydroxyapatite- (SrHA-) containing membrane have exhib-
ited higher elasticity and strength than the collagen mem-
ConflictofInterests
brane[110].Meanwhile,slowstrontiumionreleasewasalso
confirmedtostimulatenewboneformation.
The authors declare that there is no conflict of interests
Strontiumhasalsobeenintroducedintodifferenthydro-
regardingthepublicationofthispaper.
gelstobeusedinbonerepair.Astudyreportedthatabonetis-
sueengineeringapproachinwhicharginine-glycine-aspartic
Authors’Contribution
acid- (RGD-)modified alginate hydrogels are crosslinked
withbioactivestrontium,zinc,andcalcium[118].Itwasfur-
Shuang Tan and Binbin Zhang contribute equally to this
thershownthatstrontiumgelsmadewithahighpercentage
work.
of guluronic acid residues (high G) were degraded more
slowlythanthosemadewithalginaterichinmannuronicacid
(highM)andsupportedproliferationofosteoblast-likecells. Acknowledgments
Afteraninitialburst,strontiumreleasefromalginategelswas
The study was supported by Natural Science Foundation of
steadyandsustained,andthemagnitudeofreleasefromhigh
China (NSFC81130034, 81171746). The content is solely the
Mgelswasbiologicallyrelevant[118].
responsibilityoftheauthors.
An amidated carboxymethylcellulosehydrogelenriched
withSrionswasevaluatedforitseffectsofstrontiumreleased
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