JOAN MASSAGUI~ CELL GROWTH FACTORS A helping hand from proteoglycans Cell-surface proteoglycans help present some polypeptide growth factors to their receptors and may act as a reservoir for others. Growth factors are secreted polypeptides that guide cell proliferation, differentiation and organization in de- veloping tissues. The central event in the action of growth factors is their binding to signal-transducing re- ceptors on the plasma membrane of target cells. This interaction is highly regulated and diversified; a single growth factor may exist in various isoforms that bind to multiple receptor subtypes each with distinctive affin- ity and signaling properties. Context-specific and cell- specific expression patterns of factors and receptors determine their availability to each other. The system is further regulated by the ability of growth factors to interact with certain binding molecules that are func- tionally distinct from the signaling receptors. Some of these molecules are circulating proteins that participate in growth factor transport or clearance. Others include a newly identified series of cell-surface and pericellu- lar molecules that bind growth factors, such as basic fibroblast growth factor (bFGF) and transforming growth factor-]3 (TGF-[3), and apparently regulate their access to signaling receptors. The particular interest of these binding components lies not only in their biological properties but also in the fact that they are proteo- glycans, which have not usually ranked high in the peck- ing order of molecules of interest to cell biologists. Proteoglycans are highly glycosylated proteins that carry sulfated glycosaminoglycans (GAGs) as the principal car- bohydrate chains and are abundant components of car- tilage, bone and interstitial tissue matrices where they play important structural roles. Much less is known about the structure and role of various low-abundance proteo- glycans that are found in cell membranes and in pericellu- lar matrices. A recent report byYayon et al., [1], however, represents significant progress in this area in demonstrat- ing that certain cell-surface proteoglycans are indispens- able accessories for presentation of bFGF to its signaling receptor. A potent stimulator of migration, proliferation and dif- ferentiation of cells of mesenchymal and neuroecto- dermal origin [2], bFGF binds to specific high-affin- ity receptors that possess intrinsic protein tyrosine ki- nase activity in their cytoplasmic domain and are di- rectly involved in bFGF signal transduction [3]. Like other FGF-related factors, collectively known as heparin-bind- ing growth factors, bFGF also binds to GAGs, specifi- cally to the kind known as heparan sulfate GAGs [4]. These are highly sulfated polymers of acetylglucosamine and uronic acids, which are present in certain proteo- glycans and are the main components of tissue extracts known as heparin. The affinity of bFGF for heparan sul- fate (Kd ~10-9M) is lower than for the signaling re- ceptor (Kd "5 x 10-11M). However, Yayon et al. [1] have now found that binding of bFGF to heparan sul- fate is required before bFGF binds to the signaling re- ceptor. Their experiments show that bFGF cannot bind to the high-affinity bFGF receptor expressed in mutant Chinese hamster ovary cells that have a defect in GAG synthesis and, therefore, lack cell-surface heparan sulfate binding sites for bFGF. High-affinity binding can be re- stored by addition of exogenous heparan sulfate. That is, cell-surface heparan sulfate proteoglycans act as an acces- sory in presenting bFGF to the signaling receptor (Fig. 1). Unlike growth factor receptors that readily bind their lig- ands in solution, bFGF receptors seem not to recognize the solution conformation of bFGF but only the confor- mation of bFGF bound to heparan sulfate. As bFGF binds to heparan sulfate but not to other types of GAGs, the binding must involve more than simple ionic interactions with negatively charged GAGs. With re- gard to the specificity of this interaction, it will be impor- Fig. 1. Basic fibroblast growth factor binds first to the GAG chains of proteoglycans and is then presented to its cell surface receptor. Volume 1 Number 2 1991 117
Transcript
JOAN MASSAGUI~ CELL GROWTH FACTORS
A helping hand from proteoglycans Cell-surface proteoglycans help present some polypeptide
growth factors to their receptors and may act as a reservoir for others.
Growth factors are secreted polypeptides that guide cell proliferation, differentiation and organization in de- veloping tissues. The central event in the action of growth factors is their binding to signal-transducing re- ceptors on the plasma membrane of target cells. This interaction is highly regulated and diversified; a single growth factor may exist in various isoforms that bind to multiple receptor subtypes each with distinctive affin- ity and signaling properties. Context-specific and cell- specific expression patterns of factors and receptors determine their availability to each other. The system is further regulated by the ability of growth factors to interact with certain binding molecules that are func- tionally distinct from the signaling receptors. Some of these molecules are circulating proteins that participate in growth factor transport or clearance. Others include a newly identified series of cell-surface and pericellu- lar molecules that bind growth factors, such as basic fibroblast growth factor (bFGF) and transforming growth factor-]3 (TGF-[3), and apparently regulate their access to signaling receptors. The particular interest of these binding components lies not only in their biological properties but also in the fact that they are proteo- glycans, which have not usually ranked high in the peck- ing order of molecules of interest to cell biologists.
Proteoglycans are highly glycosylated proteins that carry sulfated glycosaminoglycans (GAGs) as the principal car- bohydrate chains and are abundant components of car- tilage, bone and interstitial tissue matrices where they play important structural roles. Much less is known about the structure and role of various low-abundance proteo- glycans that are found in cell membranes and in pericellu- lar matrices. A recent report byYayon et al., [ 1], however, represents significant progress in this area in demonstrat- ing that certain cell-surface proteoglycans are indispens-
able accessories for presentation of bFGF to its signaling receptor.
A potent stimulator of migration, proliferation and dif- ferentiation of cells of mesenchymal and neuroecto- dermal origin [2], bFGF binds to specific high-affin- ity receptors that possess intrinsic protein tyrosine ki- nase activity in their cytoplasmic domain and are di- rectly involved in bFGF signal transduction [3]. Like other FGF-related factors, collectively known as heparin-bind- ing growth factors, bFGF also binds to GAGs, specifi- cally to the kind known as heparan sulfate GAGs [4]. These are highly sulfated polymers of acetylglucosamine and uronic acids, which are present in certain proteo- glycans and are the main components of tissue extracts known as heparin. The affinity of bFGF for heparan sul- fate (K d ~10 -9M) is lower than for the signaling re- ceptor (K d " 5 x 10-11M). However, Yayon et al. [1] have now found that binding of bFGF to heparan sul- fate is required before bFGF binds to the signaling re- ceptor. Their experiments show that bFGF cannot bind to the high-affinity bFGF receptor expressed in mutant Chinese hamster ovary cells that have a defect in GAG synthesis and, therefore, lack cell-surface heparan sulfate binding sites for bFGF. High-affinity binding can be re- stored by addition of exogenous heparan sulfate. That is, cell-surface heparan sulfate proteoglycans act as an acces- sory in presenting bFGF to the signaling receptor (Fig. 1). Unlike growth factor receptors that readily bind their lig- ands in solution, bFGF receptors seem not to recognize the solution conformation of bFGF but only the confor- mation of bFGF bound to heparan sulfate.
As bFGF binds to heparan sulfate but not to other types of GAGs, the binding must involve more than simple ionic interactions with negatively charged GAGs. With re- gard to the specificity of this interaction, it will be impor-
Fig. 1. Basic fibroblast growth factor binds first to the GAG chains of proteoglycans and is then presented to its cell surface receptor.
Volume 1 Number 2 1991 117
tant to determine whether all or only some heparan sul- fate GAGs can bind bFGF, whether individual members of the FGF family prefer distinct subtypes of heparan sul- fate GAGs, and the identity of the specific proteoglycan core protein(s) that carry these GAGs.
Progress towards the last goal has recently been achieved with the identification of FGF-HSPG, the core protein of a membrane-anchored heparan sulfate proteoglycan from hamster that was cloned on the basis of the abil- ity of its GAG chains to bind bFGF [5]. By deduction from its cDNA, FGF-HSPG is a 33 kD integral membrane protein consisting of an extracellular region with six Ser-Gly amino acid sequences as potential GAG attach- ment sites, a hydrophobic transmembrane domain, and a short cytoplasmic region with no currently recognisable special features. FGF-HSPG is likely to be the hamster homologue of murine syndecan, a previously described membrane proteoglycan initially implicated in mediating the adhesion of cells to extracellular matrix [6]. The properties of FGF-HSPG/syndecan suggest the possibility that proteoglycans that can bind growth factors may serve multiple functions. These functions might be individually regulated by cell-specific and stage-specific differences in the GAG chain composition of the proteoglycan, or by release of the extracellular domain of the proteoglycan into the medium, both of which have been documented in the case of syndecan [6].
TGF-[3 is another growth factor that binds to mem- brane proteoglycans. A widely distributed polypeptide that inhibits cell proliferation, regulates differentiation and modulates cell adhesion, extracellular matrix pro- duction and other cellular activities [7], TGF-[~ binds betaglycan, a proteoglycan that exists in both membrane- anchored and released forms in various tissues and in many cultured cell lines. Like FGF-HSPG/syndecan, betaglycan contains heparan sulfate and chondroitin sul- fate GAG chains, and has lower affinity (K d ~ 10-10 M) for TGF-IB than do the signaling TGF-IB receptors (K d ,-, 10 -11 M) [7]. Betaglycan, however, differs func tionally from bFGF-binding proteoglycans in two im- portant respects. First, it does not appear to be re- quired for TGF-[3 binding to signaling receptors as these receptors bind TGF-[3 with high affinity in sev- eral cell lines that lack detectable betaglycan. Perhaps betaglycan serves as a cell surface reservoir of TGF-[3
by capturing this factor and preventing its rapid clear- ance after a local burst of TGF-[3 production or re- lease from blood platelets (Fig. 2). Second, beta- glycan binds TGF-[3 via its core protein [8,9]. Exper- iments with the mutant Chinese hamster ovary cells mentioned above have shown that the GAG chains in betaglycan are dispensable for TGF-[3 binding [9]. What, then, is the role of GAGs in betaglycan? One pos- sibility is that, by binding to extracellular matrices via the GAG chains, membrane-anchored and released forms of betaglycan might participate in cell adhesion or as TGF-[3 reservoirs in the extracellular matrix, respectively [7]. It would be interesting to determine whether betaglycan can simultaneously bind TGF-[3 via its core and bFGF via its heparan sulfate chains.
Other proteoglycans that may bind TGF-[3 are decorin and biglycan [10]. These are major chondroitin sulfate proteoglycans at the extracellular matrices. Unlike FGF- HSPG/syndecan or betaglycan, decorin and biglycan are not membrane proteoglycans. Evidence obtained with preparations rich in decorin core protein suggests that this proteoglycan binds TGF-]3 (Kd, 2 x 10-9M). It will be important to test whether TGF-[3 binds directly to the core protein of decorin and with what stoichiometry. As the expression of decorin and biglycan is elevated by TGF-.I3[7], it has been proposed that these two proteo- glycans may act as negative feed-back regulators of the level of free TGF-[3 in tissues [10].
The notion that growth factor receptors may require ac- cessory molecules to bind ligand or to store ligand near the cell surface is only beginning to emerge. It came as a surprise that some of the first examples of acces sory molecules are membrane proteoglycans. Additional examples may soon emerge because unique membrane proteoglycans are being identified at a rapid pace and various growth factors are being found that bind to hep- aran sulfate. These include various hematopoietic colony- stimulating factors [11], the TGF [3-like bone morpho- genetic proteins [12], and members of the int-1 growth factor family [13]. It is remarkable that these proteo- glycans are anchored in the cell membrane, whether they bind factor via the GAG chains as in the case of FGF-HSPG or via the core protein as in the case of betaglycan. Evidence accumulated thus far does not sup- port the possibility that these proteoglycans are directly
TGF -[3
[] []
Fig. 2. Proteoglycan cores may act as a cell-surface reservoir for transforming growth factor-~.
118 @ 1991 Current Bio logy
involved in signal transduction. The reason for their membrane anchored nature, if there is one, is unclear. Our knowledge of these molecules, however, seems ready to undergo major progress in the near future, now that proteoglycans are rising in the pecking order.
References 1. YAYON a~ KLAGSBRUN M, ESKO JD, LEDER P, ORNITZ DM: Cell
surface, heparin-like molecules are required for binding of basic fibroblast growth factor to its high aflimty receptor. Cell 1991, 64:841-849.
3. LEE PL, JOHNSON DE, COUSENS LS, FRIED VA, WILLIAMS LT: Pu- rllication and complementary DNA cloning of a receptor for basic fibroblast growth factor. Science 1989, 245:57-60.
4. MOSCATEUa D: Metabolism of receptor-bound and matrix- bound basic fibroblast growth factor by bovine capillary en- dothelial cells. J Cell Biol 1988, 107:753~759.
5. KaEFER MC, STEPHANS JC, CRAWFORD K, OKINO K, BARN PJ: Ligand-affmity cloning and structure of a cell surface hep- aran sulfate proteoglycan that binds basic fibroblast growth factor. Proc Natl Acad Sci USA 1990, 87:6985~5989.
6. SAUNDERS S, JALKANEN M, O'FARRELL S, BEe~',~mm M: Molecular cloning of syndecan, an integral membrane proteo#ycan. J Cell Biol 1989. 108:1547-1556.
7. MASSAGUE J: The transforming growth factor-~ family. Ann Rev Cell Biol 1990, 6:597-641.
8. SEGARINI P, SEYEDIN S: The high molecular weight receptor to transforming growth factor-~ contains glycosaminoglycan chains. J Biol Chem 1988, 263:836643370.
9. CHEIEETZ S, MASSAGUg J: The TGF-~ receptor proteoglycan. cell sSurface expression and ligand binding in the absence of glycosaminoglycan chains. J Biol Chem 1989 264:12025 12028.
10. YAMAGUCI~ Y, MANN DM, RUOSLAH~ E: Negative regulation of transforming growth factor-~ by the proteoglycan decorin. Nature 1990, 346:281-283.
11. ROBERTS R, GALLAGHER J, SPOONER E, ALLEN TD, BLOOMFIELD F, DEXTER TM: Heparan sulphate bound growth factors: a mechanism for stromal cell haemopoiesis. Nature 1988. 332:376-378.
12. WOZNEY JM, ROSEN V, CELESTE aJ, MITSOCK LM, WHITTERS MJ, KRIZ RV, HEVaCK RM, WANG EA: Novel regulators of bone formation: molecular clones and activities. Science 1988, 242:1528 -11534.
13. BRADLEY RS, BROWN AMC: The proto-oncogene int-I encodes a secreted protein associated with the extracellular matrix. EMBO J 1990, 9:1569-1575.
Joan Massague, Memorial Sloan-Kettering Cancer Center. 1275 York Avenue, New York, NY 10021, USA.
THE NEXT ISSUE OF CURRENT OPINION IN CELL BIOLOGY
reviews the past year's developments in cell regulation and multiplication. In the section on Cell Regulation edi ted by Michael Hanley and Robert Michell the reviews are:
C y t o k i n e s a n d t h e i r r e c e p t o r s by H Benton C e l l u l a r a g e i n g a n d s e n e s c e n c e by J Campisi
T h e Ras s u p e r f a m i l i e s : r e g u l a t o r y p r o t e i n s a n d p o s t - t r a n s l a t i o n a l m o d i f i c a t i o n s by T Evans, M Han and RA Cer ione
T h e r o l e o f p h o s p h o l i p a s e s a n d p h o s p h o l i p i d - d e r i v e d s i g n a l s i n c e l l a c t i v a t i o n by J Ferguson and M Hanley
C y c l i c AMP s e c o n d m e s s e n g e r s y s t e m s by GS McKnight T r a n s g e n i c a p p r o a c h e s t o m o d i f i c a t i o n o f c e l l a n d t i s s u e f u n c t i o n by J Mullins
T h e c e l l u l a r m e a s u r e m e n t o f t i m e by A Groves, O Bogler, P Jan and M Noble C u r r e n t d e v e l o p m e n t s i n G - p r o t e i n c o u p l e d r e c e p t o r s by F Libert, G Vassart and M Parmentier
P r o l i n e - d i r e c t e d p h o s p h o r y l a t i o n a n d c e l l c y c l e r e g u l a t i o n by FL Hail and PR Vulliet R e g u l a t e d c e l l d e a t h by A WyUie; C a l c i u m r e g u l a t i o n a n d h o m e o s t a s i s by T Cheek
In the section on Cell Multiplication edited by John Pringle the reviews are:
C o n t r o l o f t h e b a c t e r i a l c e l l c y c l e by A Jacq and B Holland F r o m f u n g i t o f l i e s : p a t t e r n a n d f o r m i n t h e c e l l c y c l e by K Gull
T e m p o r a l e v e n t s r e g u l a t i n g t h e e a r l y p h a s e s o f t h e m a m m a l i a n c e l l c y c l e by G Nacre, A Sharma and A Lee
S p a t i a l a s p e c t s o f c y t o k i n e s i s i n p l a n t c e l l s by S Wick M i t o t i c C o n t r o l by J Mailer; T h e y e a s t c e l l c y c l e by M Goeb l and M Winey
T h e c o m p l e x i t y o f c e l l p r o l i f e r a t i o n c o n t r o l i n m a m m a l i a n c e l l s by C Schneider, S Gustincich and G Del Sai
T h e r o l e o f p 5 3 i n t h e n o r m a l c o n t r o l o f c e l l p r o l i f e r a t i o n b y J Milner
