obesity , an energy balance disorder represented as excessive accumulation of white adipose tissue ( wat ) , is a major public health problem in developed countries . the cellular composition of wat includes primarily adipocytes and preadipocytes , as well as endothelial cells and macrophages . obesity is the result of increases in cell size ( adipocyte hypertrophy ) and cell number . adipocytes are not only the storage depots of energy , but also the source of various cytokines and hormones . these so - called adipokines , e.g. , tumor necrosis factor- , leptin , adiponectin , and resistin , target the central nervous system and peripheral tissues ( wat , liver and muscle ) to modulate energy metabolism ( gregoire et al . the first phase of adipogenesis is fate determination , which involves the conversion of mscs to preadipocytes . the second phase , known as terminal differentiation , is the process by which preadipocytes differentiate into adipocytes . many studies have defined mechanisms by which transcription factors and hormones regulate terminal differentiation in cultured 3t3-l1/f442 cells . the first step is induction of the expression and activation of caat / enhancer - binding protein  ( c / ebp ) and c / ebp , which , in turn , induce the expression of c / ebp and peroxisome proliferator - activated receptor  ( ppar ) , the central transcriptional regulators for adipogenesis . a variety of signaling pathways regulate adipogenesis , including transforming growth factor ( tgf)- , tumor necrosis factor- , wnt , insulin/ insulin - like growth factor-1 , and other growth factors . extracellular matrix ( ecm ) is crucial for tissue and organ development ; however , it has not received sufficient attention in adipose tissue because of its apparent paucity ( due to the minimal space ecm could occupy ) and the difficulties associated with analysis of ecm components . the main constituents of ecm in wat are collagen ( types i , iv , and vi ) , laminin ( ln-1 , 8) , fibronectin ( fn ) , hyaluronan , and proteoglycans ( khan et al . adipogenesis involves extensive ecm remodeling , changes in cell - ecm adhesion , and cytoskeletal rearrangement . cell - associated ecm switches from a fn - enriched matrix into basement membrane ( bm ) . for example , fn inhibits adipogenesis , whereas ln and type iv collagen enhance adipogenesis , in two - dimensional cell culture ( oconnor et al . 2003 ; rodriguez fernandez and ben - zeev 1989 ; spiegelman and ginty 1983 ) . matricellular proteins regulate cell - cell and cell - ecm interactions , and they are involved in developmental and disease / stress - induced remodeling ( bornstein and sage 2002 ) . these proteins interact with ecm components , as well as with cell surface receptors and growth factors , to modulate cell signaling . secreted protein acidic and rich in cysteine ( sparc ) is the first known matricellular protein linked to the  balance of wats . there are increases in both cell size and cell number in wat of sparc - null mice compared with that of wt mice ( bradshaw et al . sparc inhibits differentiation of preadipocytes ( or mscs ) into adipocytes but favors that of osteoblastocytes ( delany et al . sparc is enriched in wat , and both subcutaneous and mesenteric wats show high levels of sparc mrna ( takahashi et al . expressed in bone marrow mscs , preadipocytes , and mature adipocytes , sparc is enhanced at certain stages of adipogenesis ( chavey et al . 2003 ; nie and sage 2009 ) . the ecm remodeling accompanying adipogenesis requires matricellular proteins , especially sparc , to stabilize the tissue and maintain the balance of lipogenesis and lipolysis . indeed , sparc is significantly enhanced in obese mouse wat ( tartare - deckert et al . the plasma concentration of sparc was correlated positively with body mass index ( takahashi et al . ecm remodeling requires sparc to facilitate the development of mature collagen fibrils and other post - translational modifications of collagen . however , other adipokines and metabolic regulation may override the effects of tgf , sparc , and collagen . ecm remodeling requires sparc , whereas sparc , to some extent , can prevent hypertrophy of adipocytes and hyperplasia of adipocyte progenitors . according to the when cell size reaches a certain threshold , signaling favors adipocyte differentiation ; alternatively , preadipocyte proliferation is activated and cell number will increase . the disorganized collagen matrix in sparc - null wat might allow adipocytes to grow to a greater size and would eventually lead to hyperplastic wat . in addition , sparc appears to regulate appetite control such that total wat accumulation is enhanced . adipogenesis is characterized by conversion of a fn - rich stromal ecm to a ln - rich bm ; consistently , integrin expression switches from 5 ( fn ) to 6 ( ln ) ( liu et al . 2005 ) . ln and matrigel enhance adipogenesis ; fn and poly - l - lysine exert inhibitory effects on adipogenesis ( gregoire et al . 1998 ; liu et al . the inhibitory activity of fn requires cell spreading , and cytochalasin d , via its disruption of actin filaments , can overcome the inhibitory effects of fn ( spiegelman and ginty 1983 ) . fn is linked to actin stress fibers , whereas ln polymerization on the cell surface promotes the formation of a cortical actin network ( colognato et al . , sparc enhances the deposition of fn and the expression of its receptor , 5 integrin ( nie and sage 2009 ) . in lung fibroblasts , sparc enhanced cell - mediated partial unfolding of fn molecules and fn - induced stress fiber formation through ilk - dependent contractile signaling ( barker et al . sparc might also enhance this process through ilk signaling and contribute to the increased deposition of fn . injections of bm proteins , in combination with fgf-2 , can induce de novo adipogenesis at the site of the injection ( kawaguchi et al . sparc inhibits the expression of the ln 1 chain and one of its major receptors , 6 integrin , as well as the deposition of ln itself during adipogenesis ( nie and sage 2009 ) . sparc regulates the secretion and deposition of ln in lens cells ( weaver et al . the expression of 6 integrin correlates with the growth - arrest of preadipocytes during adipogenesis , and it favors differentiation over proliferation of preadipocytes ( liu et al . the interaction of ln and 71 integrin is required for basal lamina formation and actin reorganization in muscle cells ( colognato et al . similarly , 6 integrin might be required for receptor - facilitated ln self - assembly , basal lamina formation , actin reorganization , and other adipogenic signaling events . moreover , sparc interacts with type iv collagen ( maurer et al . sparc is required for collagen - iv - dependent stability of basal lamina in drosophila embryos ( martinek et al . therefore , it is likely that sparc inhibits adipogenesis in part by interference with the formation of bm . collagen contributes to the support of tissues and can thereby regulate adipogenesis . in c. elegans , diminished expression of collagen by rnai causes substantial reduction in fat , whereas rnai targeting of collagen triple helical repeats increases fat content ( ashrafi et al . the predominantly - expressed collagen mrnas in wat encode types i , iv , and vi ( khan et al . type i collagen forms collagen fibrils , found in the interstitial space ; type iv collagen contributes to the bm , whereas type vi collagen interacts with both type i and iv collagen ( kuo et al . 1997 ) . in 2-dimensional cell culture , the effect of collagen fibril structure on adipocyte differentiation is minimal ( oconnor et al . 2003 ) . however , in a 3-dimensional collagen matrix and in vivo , adipogenesis is significantly reduced ( > 80% ) in membrane - type 1 matrix metalloproteinase ( mt1-mmp ) null preadipocytes due to diminished collagen remodeling ( chun et al . there is less collagen degradation and a more dense collagen fibrillar network in wat of mt1-mmp - null mice compared with that of wt mice ( chun et al . the absence of mmp-19 , which cleaves type iv collagen , enhances diet - induced obesity ( > 2fold ) and increases adipocyte size ( pendas et al . consistently , mice receiving a high fat diet and treated with mmp inhibitors exhibited less wat and increased numbers of adipocytes . these data indicate that the collagen network is a barrier for adipogenesis and that collagen remodeling is required for adipocytes to grow and divide . 1995 ) , regulates fibrillogenesis of type i , and is required for the development of mature cross - linked collagen fibrils . in the skin of sparc - null mice , hydroxyproline content is reduced , and the collagen fibers are smaller , immature , and less crosslinked ( bradshaw et al . 2002 ; bradshaw et al . there is also less hydroxyproline in the wat of these animals ( bradshaw et al . the decreased collagen content and disorganized collagen network in sparc - null mice might be more susceptible to protease degradation and remodeling , properties allowing adipocytes to grow to a greater size and contributing to hyperplastic wat . type vi collagen modulates collagen fibrillogenesis , affects fn assembly , and is anchored to bm through its interaction with type iv collagen ( kuo et al . 2001 ) . because wats from collagen vi - null mice exhibit a substantially increased interstitial space among mature adipocytes , type vi collagen might serve as a link between the bm network and collagen fibrils ( types i and iii ) ( khan et al . 2009 ) . collagen vi - null mice show increased adipocyte size , although the total weight of wat is lower than that of wild - type mice ( khan et al . 2009 ) . because sparc - null mice have increased levels of soluble type vi collagen in their dermis ( bradshaw et al . 2002 ) , sparc might affect the expression , folding , post - translational modification , and/or secretion of type vi collagen in wat . preadipocytes enter a state of growth arrest in the g1 phase of the cell cycle prior to their terminal differentiation . sparc inhibits preadipocyte adhesion by its interaction with integrin 51 and enhances cell migration . in melanoma cells if sparc diminishes e - cadherin in preadipocytes , this change would lead to nuclear translocation and accumulation of -catenin . thus , preadipocytes could exit from the state of growth arrest but not be able to terminally - differentiate into adipocytes . msc can differentiate into a variety of tissues , including muscle , bone , cartilage , and fat . wnt signaling , one of the key factors controlling msc fate , inhibits adipogenesis and enhances myogenesis or osteoblastogenesis ( arango et al . constitutively - activated frizzled ( fz ) increases the stability of -catenin , inhibits apoptosis , inhibits adipogenesis , and induces osteoblastogenesis ( kennell and macdougald 2005 ) . disruption of wnt/-catenin signaling leads to spontaneous adipocyte differentiation ( longo et al . 2004 ; ross et al . endogenous wnts , especially wnt10b , restrain preadipocyte differentiation ( longo et al . 2004 ) . activation of this pathway is sufficient to inhibit the differentiation and apoptosis of preadipocytes through an inhibition of c / ebp and ppar ( farmer 2005 ; ross et al . wnt proteins bind to fz receptors and low - density lipoprotein receptor - related protein ( lrp ) coreceptors 5/6 to activate several signaling pathways . importantly , the inhibition of glycogen synthase kinase 3 ( gsk3 ) via wnt results in the stabilization of -catenin in the cytoplasm , and reduces its proteosomal degradation . after translocation to the nucleus , -catenin binds to and coactivates transcription factors that include members of the t - cell factor / lymphoid - enhancing factor ( tcf / lef ) family ( christodoulides et al . sparc - null mice exhibit increased accumulation of fat and a decrease in bone mass . recently we have shown that sparc inhibits adipogenesis and enhances osteoblastogenesis by its enhancement of the accumulation of -catenin ( nie and sage 2009 ) . both exogenous and endogenous the decrease in the abundance of -catenin coincides with the accumulation of ppar ( moldes et al . ppar can suppress wnt signaling by its targeting of phosphorylated -catenin to the proteasome ( girnun et al . conversely , -catenin not only inhibits expression of ppar , but also inhibits its transcriptional activity through a direct interaction involving its tcf / lef binding domain ( liu et al . in addition , wnt/-catenin enhances osteoblastogenesis through repression of the expression of c / ebp and ppar ( kang et al . consistently , sparc inhibited the expression of ppar and c / ebp at later stages of differentiation . these data provide further evidence that sparc inhibits the key transcriptional cascades of adipogenesis through the wnt/-catenin pathway . inactivation of -catenin in mscs blocks osteoblast differentiation ( hartmann 2006 ) . during postnatal development , -catenin is also involved in osteoblast proliferation and maturation , and in osteoclastogenesis ( glass et al . conditional inactivation of -catenin in skeletal progenitors , or inactivation of wnt10b or lrp5 ( a wnt receptor ) , results in low bone mass ( osteopenia ) ( hartmann 2006 ) . the -catenin - tcf / lef complex regulates the expression of the osteogenesis genes , runx2 , rank ligand , osteoprotegerin , and osteocalcin ( dong et al . similarly , sparc - null mice develop osteopenia that becomes more severe as the animals age ( delany et al . consistently , both mscs from sparc - null bone marrow and preadipocytes from sparc - null wat , show an increased tendency to form adipocytes and a decreased tendency to form osteoblasts ( delany et al . 2003 ) . therefore , sparc could regulate osteoblast formation , maturation , and survival through the wnt/-catenin pathway . as shown in fig .  1 , we also demonstrated that sparc activates integrin - linked kinase ( ilk ) , which in turn phosphorylates and inhibits gsk3 activity and leads to the accumulation of -catenin in both the cytosol and the nucleus ( nie and sage 2009 ) . which protein and/or protein complex is downstream of sparc that affects the activation of ilk ? sparc interacts not only with ecm components , but also with integrins and growth factors ; therefore , a possible pathway is the activation of ilk through an interaction of sparc with integrins . sparc interacts with 1 integrin and subsequently enhances the activation of ilk in lens epithelial cells ( weaver et al . 2008 ) . in preadipocytes , sparc - mimicking peptides bind to 51 integrin ( nie et al . sparc enhances the activity of ilk at an early stage of adipocyte differentiation , and an ilk - specific inhibitor partially blocks the inhibitory effects of sparc ( nie and sage 2009 ) . ilk can be activated by an interaction with the cytoplasmic tail of 1/3 integrins ( oloumi et al . moreover , sparc partially preserves cell adhesion to ecm during an early stage of adipogenesis ( nie and sage 2009 ) . in the absence of sparc , preadipocytes partially detach , focal adhesion complexes are lost , focal adhesion kinase ( fak ) and ilk activity are decreased , and the ecm is changed from a fn - enriched matrix to a ln - enriched matrix . the partial detachment from fn - enriched ecm is crucial to allow cells to retract and for lipid to accumulate . sparc might interfere with the mechanosensor function of integrin(s ) and inhibit both outside - in and inside - out signaling of this receptor . in addition , the fn - ecm matrix is reorganized , and possibly cleaved , during an early stage of adipogenesis ( selvarajan et al . the changes in the interaction of fn with its cognate integrin might lead to conformational changes of the receptor and cell rounding or contraction . sparc could interact with integrin to maintain the activity of ilk and fak in preadipocytes ( fig .  2 , state 4 ) . consequently , ilk activity would preserve the linkage among ecm components , integrins , and the actin cytoskeleton , with subsequent preservation of focal adhesion complexes . consistent with this proposal , sparc also induces the phosphorylation and activation of fak in glioma cells ( shi et al . 1sparc ( s , in red star ) inhibits adipogenesis by its enhancement of -catenin signaling . inhibition of gsk-3 activity results in the accumulation of -catenin , which can translocate to the nucleus . in the nucleus , -catenin can inhibit the activity of ppar , activate tcf / lef transcripotion factors , and regulate the expressions of many genes including those encoding growth factors and ecm proteins . sparc could activate ilk through an interaction with integrins or with receptor tyrosine kinases ( receptors of growth factors ) . s , sparc ; gf , growth factors ; rtk , receptor tyrosine kinase ; pi3k , phosphoinositide-3 kinase ;  and  ,  and  subunits of integrin ; tcf / lef , t - cell factor / lymphoid - enhancing factorfig . 2a model showing how sparc ( s , in red star ) can interfere with integrin signaling . 2 ) during the adhesion process , sparc competes with an integrin ligand ( l ) for its binding site , and prevents the conformational change of the integrin to active state ( dashed arrow ) . signaling and subsequent deadhesion process , sparc interacts with integrin and prevent the conformational change of integrin to state 1 ( dashed arrow ) . we currently can not predict the binding sites of sparc on integrin in either state 2 or 4 . l , ligand of integrin ; s , sparc ;  and  ,  and  subunits of integrin ; pink circle , -propeller domain of integrin  subunit ; yellow circle , i domain of integrin  subunit ; green circle , i - like domain of integrin  subunit sparc ( s , in red star ) inhibits adipogenesis by its enhancement of -catenin signaling . inhibition of gsk-3 activity results in the accumulation of -catenin , which can translocate to the nucleus . in the nucleus , -catenin can inhibit the activity of ppar , activate tcf / lef transcripotion factors , and regulate the expressions of many genes including those encoding growth factors and ecm proteins . sparc could activate ilk through an interaction with integrins or with receptor tyrosine kinases ( receptors of growth factors ) . s , sparc ; gf , growth factors ; rtk , receptor tyrosine kinase ; pi3k , phosphoinositide-3 kinase ;  and  ,  and  subunits of integrin ; tcf / lef , t - cell factor / lymphoid - enhancing factor a model showing how sparc ( s , in red star ) can interfere with integrin signaling . 2 ) during the adhesion process , sparc competes with an integrin ligand ( l ) for its binding site , and prevents the conformational change of the integrin to active state ( dashed arrow ) . signaling and subsequent deadhesion process , sparc interacts with integrin and prevent the conformational change of integrin to state 1 ( dashed arrow ) . we currently can not predict the binding sites of sparc on integrin in either state 2 or 4 . l , ligand of integrin ; s , sparc ;  and  ,  and  subunits of integrin ; pink circle , -propeller domain of integrin  subunit ; yellow circle , i domain of integrin  subunit ; green circle , i - like domain of integrin  subunit sparc might retard the deadhesion process during an early stage of adipocyte differentiation by its interference with integrin - mediated signaling . the adhesive effect of sparc in adipogenesis is not inconsistent with the anti - adhesive function of sparc in endothelial and glioma cells ( rempel et al . , sparc might compete with certain ecm proteins for their binding sites on integrins ( fig .  2 , alternatively , sparc could activate ilk by its interference with growth - factor signaling ( fig .  1 ) . insulin , platelet - derived growth factor ( pdgf ) , and other growth factors can stimulate ilk activity through phosphoinositide 3-kinases ( dedhar et al . sparc is involved in growth factor signaling , e.g. , it binds to pdgf and vascular endothelial growth factor ( vegf ) , and inhibits the binding of these growth factors to their cognate receptors ( battegay et al . however , further investigation is required to ascertain whether sparc activates ilk by signaling via growth - factor . during adipogenesis , wnt/-catenin signaling leads to the remodeling of ecm and changes in the expression of integrin and ecm components . although it is not clear whether wnt/-catenin directly regulates the expression of integrins or any ecm components , our evidence indicates that -catenin - mediated signaling is involved in the regulation of integrin expression . licl , a strong inducer of -catenin signaling , significantly inhibited the expression of 6 integrin . ilk activity , which also enhances -catenin accumulation , is required for sparc to inhibit the expression of 6 integrin . because wnt/-catenin has been shown to regulate ecm remodeling and expression through ppar ( liu et al . arteriolar development precedes adipocyte differentiation in internal wat depots ( hausman and richardson 2004 ) . interestingly , adipocytes and endothelial cells might share a common progenitor ( planat - benard et al . adipose progenitor cells reside in the adipose vasculature as demonstrated in ppar-tet transactivator knock - in mice ( tang et al . 2008 ; traktuev et al . 2008 ) . moreover , blockade of vegf signaling can inhibit wat formation in vivo ( fukumura et al . 2003 ) , whereas depletion of adipose vasculature results in wat resorption ( kolonin et al . 2004 ) . proteolytic fragments of sparc have different effects on angiogenesis , and the function of sparc is cell / tissue / condition - specific and concentration - dependent . the effect of sparc depends on the exposed epitope and its conformation , and on the availability of binding partners / targets . sparc peptides 2.1 ( in the follistatin domain ) and 4.2 ( in the c - terminal ca binding ef - hand ) exert inhibitory effects on cell proliferation ( jendraschak and sage 1996 ) . sparc also interacts with pdgf and interferes with pdgf signaling that indirectly promotes angiogenesis ( raines et al . sparc contains a copper - binding motif ( k)ghk that stimulates endothelial cell proliferation and angiogenesis in vivo and in vitro ( lane et al . there is a heightened neovascular response in sponge implants in sparc - null , compared to wild - type mice . ( bradshaw et al . 2001 ) in wats , the driving forces in angiogenesis are hypoxia and signals from the sympathetic nervous system ( cao 2007 ) . vegf - a signaling is balanced by vegfr-1 and -2 : vegfr-1 mediates an inhibitory signal , and vegfr-2 , a stimulatory signal . sparc binds to vegf - a and inhibits the activation of vegfr-1 but not that of vegfr-2 in vitro ( kupprion et al . 1998 ) . consistent with these data , sparc competes with vegf - a for the binding and activation of vegfr-1 and thereby enhances choroidal neovascularization ( nozaki et al . sparc might also limit the activation of vegfr-1 by vegf - a , and thereby promote angiogenesis , during adipogenesis . 1 ) sparc inhibits adipogenesis and enhances osteoblastogenesis by its enhancement of the wnt/-catenin pathway ; 2 ) sparc also inhibits adipogenesis through its regulation of collagen expression and deposition ; 3 ) sparc appears to drive preadipocytes from the status of growth arrest and therefore prevents terminal differentiation ; 4 ) sparc might inhibit adipogenesis through its regulation of adipose tissue angiogenesis . further investigation is needed to uncover the mechanisms underlying the inhibitory effects of sparc on adipogenesis in vivo . we also can not exclude the possibility that sparc affects lipid metabolism in other organs , e.g. , brain , liver , and pancreas , and thereby affects lipid accumulation in wat .
adipogenesis , a key step in the pathogenesis of obesity , involves extensive ecm remodeling , changes in cell - ecm interactions , and cytoskeletal rearrangement . 
 matricellular proteins regulate cell - cell and cell - ecm interactions . 
 evidence in vivo and in vitro indicates that the prototypic matricellular protein , sparc , inhibits adipogenesis and promotes osteoblastogenesis . 
 herein we discuss mechanisms underlying the inhibitory effect of sparc on adipogenesis . 
 sparc enhances the wnt/-catenin signaling pathway and regulates the expression and posttranslational modification of collagen . 
 sparc might drive preadipocytes away from the status of growth arrest and therefore prevent terminal differentiation . 
 sparc could also decrease wat deposition through its negative effects on angiogenesis . 
 therefore , several stages of white adipose tissue accumulation are sensitive to the inhibitory effects of sparc .