Matrix regulation of cell shape and gene expression

Matrix

regulation

of cell shape and gene expression J.A. McDonald

Departments

of Internal

Medicine

and Biochemistry, Washington St Louis, Missouri, USA

Current Opinion in Cell Biology 1989,

School

of Medicine,

1:995-999

2nd edition, Sinauer Associate, 1988) provides an excellent overview of this topic. Formation of epithelia involves the development of ceU polarity that is linked to increased cell-cell interaction and to interactions with extraceUular matrix components [ 11. The first epithelium formed in mammals, the trophectoderm of the pre-implantation embryo arises from blastomeres following their polarization and compaction. Increased cell-cell contact mediated by the cell-cell adhesive molecule uvomorulin (also known as E-cadherin, L-cell adhesion molecule (LCAM) and cell-CAM 120/80) is followed by ceU polarization with early organization of apical and then basolateral plasma membranes [ 11. The role of basement membrane synthesis and organization during the initial formation of this epithelium is not clear [ 21. After epithelia derived from ectoderm and endoderm are formed, subsequent refinements (e.g. development of the gut, respiratory tract, nervous system and dermis) arise primarily from growth and/or branching from pre-existing epithelial sheets. This process requires the inlluence of surrounding mesenchyme or epithelial-mesenchymal interaction. In addition to inducing various structures, such as the lens, or branching in responsive epithelial tissues (salivary gland and lung), mesenchyme can also be induced to form epithelia. For example, endothelium, mesothelium, kidney tubules, and female and male reproductive tracts are all epithelia derived from mesenchyme

Introduction

The extracellular matrix of multicellular organisms is generally considered to play a critical role in cytodiIferentiation and organogenesis during embryogenesis. Once the architecture of an adult tissue is established, the extracellular matrix scaffold is required to allow orderly repair following injury (Vracko, Am J Path01 1974,77:313-346). These functions are accomplished by reciprocal instructions between cells and matrix, an interaction termed ‘dynamic reciprocity’ (Bomstein et al. In P and S Biomedical Science Symposia Series, volume 6 edited by Nosse1 and Voge. Academic Press, 1982, pp 215-228). This expression captures the essence of the relationship between cells and their surrounding extracellular matrix. Cells produce and secrete matrix molecules, often vectonally, and organize specialized matrices such as basal lamina, loose connective tissue matrices, tendons, and bone. The matrix in turn has marked effects on ceU shape, polarity and ditferentiated function. Cells communicate with various matrix components (for review, see Komblihtt and Gutman, Biol Rev 1988,63:465-507) using surface receptors, including glycosyltransferases (Runyan et al, J Cell Biol 1988, 107:1863-1871), and peripheral (Mecham et al., Biochemtitry 1989, 28:37163722; Hall et al, J Cell Biol1988, 107687-697) and integral membrane proteins that, in the case of the integrin family, contain transmembrane components which interact with cytoskeletal elements (Solowska et al, J Cell Biol 1989, 109:853861). Changes in cytoskeletal organization and, possibly, in production of other second messengers in mm modify gene expression (BisseU, J i%eor Biol1982, 33~31-68; Hay, Modem Cell Biol1983, 2:509-548; Bissell and BarceUos-Hoff, J Cell Sci 1987, 8 (suppl) 327-343; Ekblom et al., Annu Re~j Cell Bioll986, 2:27-47).

Extracellular and cell-cell

University

[21.

Extracellular

matrix

effects during

development

Although the extracellular matrix is often considered to play a critical role in epithelial-mesenchymal interaction during development, direct proof of this hypothesis has been difficult to achieve. For example, collagen deposition was considered critical for branching morphogenesis of the salivary gland and lung, but the Movl3 mouse lacking type I collagen expression in most, but not all, tissues (see Kratochwil et al, Cell 1989, 57:807-816) has normal salivary gland branching (Kratochwil et al, Dev Biol1986, 117:596-606). However, good evidence for a role of the laminin A chain in induction of renal tubular

matrix induction of polarization contacts in developing epithelia

I find it helpful to consider studies on the effects of extracellular matrix on cell phenotype in the context of embryogenesis. The text by Gilbert (Developmental Biology, Abbreviations CAM--cell

adhesion

molecule;

@

EH%Englebreth-Holm-Swarm;

Current

Science

ltd

ISSN

TCF-transforming

0955-0674

growth

factor.

995

9%

Extracellqlar

matrix

epithelium from nephrogenic mesenchym& in the kidney (Grobstein, E.a$ Cell Res 1956,10:424-440) has been obtained by Klein and co-workers [3]. After the nephrogenie mesenchyme condenses under the intluence of the ureteric epithelium, uvomorulin and the A chain of laminin are expresed, the former uniformly and the latter at the basal surface of the condensing tubular epithelium [3]. Antibodies to the carboxyterminal globular domain of the laminin A chain prevent subsequent polarization of the condensed mesenchyme. There are additional examples of mesenchymal expression of ‘basal lamina’ components. Solursh and Jensen (Development 1988,104:41-49) have found that laminin is also localized to mesenchyme in premyogenic areas in the developing chick wing bud. Using co-cultures of intestinal endodermal cells with fibroblasts and heterospecific intracoelomic grafts, Simon-Assmann et al (Development 1988, 102:339347) have shown that type lV collagen is derived from mesenchyme, and that epithelial-mesenchymal interactions are required for its expression and deposition in a basement membrane. Ceil polarization need not precede extracellular matrix secretion and deposition. For example, laminin binding mediated via either the high afiinity 67kD elastin/laminin receptor (Mecham et al, 19891, integrin receptors (Gehlsen et al, Science 1988, 241:1228-1229; Sonnenberg et al, Nature 1988,336:487-489; Tomaselli et al, J Cell Biol 1988, 107:1241-1252) or alternative laminin receptors (Hall et al, 19881, could induce receptor association (similar to patching of immunoglobulins; Roman et al, J Cell B(oll989, 108:25292543) resulting in cell polarization mediated via changes in the cytoskeleton. Matrix components also play a role in guiding migrating cells during development. For example, the subepidermal matrix of a white axolotl mutant fails to develop in time to support migration of neural crest cells destined to become pigmented cells, although the specitic matrix component(s) have not been identied (L&berg et al, Dev Biol1989, 131:168-181). The extensive literature on this subject is beyond the scope of this review (see McClay and Ettensohn, Annu Rev Cell Bioll987,3:319345).

Cells must balance adhesion to the extracellular matrix with cell-cell adhesion CeU shape is coupled to growth and metabolic activity (Folkman and Moscona, Nature 1978, 273:345-349; Gospodarowicz et aL, Cancer Res 1978, 38:4155-4171). In many cases involving cultured cells, the effects of exttaceUular matrix on cell shape and gene expression can be viewed in the context of a tussle between cell-cell interactions and cell-substrate interactions. As Ingber and Folkman [4] put it, ‘capillary endothelial cells appear to walk a line line of structural stability with extensive ceU spreading and growth at one extreme and complete loss of anchorage resulting in rounding and ceU death.’ Loss of ceU adhesion is postulated to be one signal for

keratinocytes to stop dividing (Watt, J .Cell Sci 1987, 8 (suppl): 313326). In vitro, there is an inverse correlation between ceU and nuclear size and proliferation and sheet migration in large vessel endothelial cells. This is modulated by the extracellular matrix substrate; high concentrtions of tibronectin promote spreading but retard migration (Madri et al, Am J Path1 1988, 132:X&27). Interactions of fibronectin with the ar.& integrin receptor have been shown to decrease libroblast migration in vitro (Akiyama et al, J Cell Biol1989, 109863875) and may retard endothelial repair in vivo (Madri et al, Lab Invest 1989,60:755-765X

Culturing parenchymal cells on extracellular matrix components maintains their differentiated phenotype Cells derived from epithelial parenchyma (e.g. thyroid, liver, testes, lung, gut and mammary gland) rapidly lose their ability to carry out specialized metaboloic function when removed from the parent tissue and cultured on plastic or in type I collagen-coated dishes that support extensive ceU spreading (reviewed by BisseU and BraceUosHoff, 1987). However, when type I collagen gels are detached from the substrate, forming a ‘floating gel’, gel contraction and a more rounded ceU phenotype occur, accompanied by improved expression of the differentiated phenotype (Emerman and Pitelka, In Vitro Cell Dev Biol 1977, 13:316-328). Floating gels allow changes in cell shape, contraction, migration, and improved nutrient access, and often result in formation of hollow or branching glandular patterns, mimicking those of the parent tissue (Ormerod and Rudland, In Vitro Cell Dev Biol 1988,24:17-27). Nutrient access may be just as important as the substrate in inducing the differentiated polarized phenotype (Parry et al, J Cell Bioll987, 105:20432051). Similar phenotypic effects are often obtained using ‘threedimensional’ collagen gels (Madri et al, J Cell Biol1988, 106:137%384). Substrates of basal lamina components or laminin alone also promote cell-cell adhesion and a more cuboidal ceU shape. Culturing differentiated parenchymal epithelial cells on extracellular matrix components from basement membranes, either purilied as a mixture of laminin, type IV collagen, entactin and other molecules isolated from the Englebreth-Holm-Swarm tumor (‘EHS matrix’; Klein et al, Biocbemist?y 1985,25:312-318; commercially available as Mauigel@), or isolated from tissue (Reid et al, Ann NY Acua’ Sci 1980, 349:7O-761, allows at least partial retention of the differentiated phenotype of lung epithelial type II cells (Shannon et aL, B&him Biqihys Acta 1987,931:143156; Rannels et al, Am J Pbysioll987, 253:C835-C845), hepatocytes [5,6], sertoli cells, pancreatic acinar cells and mammary epithelium [7]. Proteogtycans are also important in maintaining ceU phenotype. Chondroitin and dermatan sulfate proteoglycans or liver heparin increase the electrical coupling and expression of the major gap junction protein by hepatocytes (Spray

Matrix

et al, J Cell Biol1987, 105:541-551). Aortic endothelial cells synthesize a variety of heparan sulfate proteoglycans whose localization is dicated by the surrounding matrix (Keller et al., Am J Path01 1987, 128:286-298; Keller et al, Am J Path01 1987, 128:29!&306). Loss of the differentiated phenotype by epithelial cells in vitro is accompanied by increased expression of cytoskeletal proteins. Plating hepatocytes [5,6] or mammary eptihelium [7] on plastic or dried collagen results in a more spread morphology, accompanied by increases in expression of cytoskeletal components, including actin, tubulin, cytokeratin, vinculin and a-actin. Hepatocytes cultured on hydrated collagen gels at high cell density or on EHS matrix substrates, grow as compact cells or aggregates, respectively, and maintain expression of hepatocyte-specific products, including albumin, aIinhibitor III, at-antitrypsin, transferrin and haptoglobin [6]. Thus, in hepatocytes and mammary epithelium, cell shape, retention of the differentiated phenotype and expression of cytoskeletal components are closely linked. Although a higher level of expression of cytoskeletal components accompanies dderentiation of hepatocytes and mammary epithelial cells, organization of a basal cytoskeleton (which can be mediated by soluble matrix components as well as by adjacent basal lamina) stabilizes the phenotype of lens epithelium (reviewed by Hay, 1983). Inhibiting either the actin or microtubular components of the cytoskeleton with cytochalasin D or colchicine specifically inhibits laminin induction of milk proteins, but not of other proteins, by cultured mam mary epithelium [7]. The inhibitoty effects of colchicine may be related to the role of microtubules in maintaining eptihelial cell polarity (Achier et al, J Cell Biol 1989, 109:179-189). With optimal culture conditions and matrices, the cellular organization, hormonal responsiveness and expression of differentiation-specific proteins achieved by cells cultured in vitro often rivals that found in zlivo (Mauchamp et al, J Cell Sci 1987,8(suppl):345-358; Chen and BisseU, J Biol C&m 1987,262:17247-17250). Gene expression in cells cultured on matrix is regulated both at the transcriptional and post-transcriptional level. Levels of cognate messenger RN4.s for rat mammary epithelial cell milk proteins, although increased by culturing on EHS matrix or laminin, do not account for net increases in synthesis (Blum et al, Exp Cell Res 1987, 173:322-340). The stimulatory effects of substrate (and hormones) on a cultured mammary epithelial cell line are both transcriptional and post-transcriptional (Eisenstein and Rosen, Mol Cell Biol 1988,8:31833190). In one intriguing case, chondrocytes transcribe a poorly translatable form of the messenger RNA encoding the a2 subunit of type I collagen when cultured in suspension, and a translatable form with a different 5’ end when cultured as a monolayer (Bennett et al., J Biol Cbem 1989, 264:8402+09). Perhaps it should come as no surprise that epithelial cells often lose their differentiated function in zlitr0 under nonphysiologic conditons. Decreased cell-cell adhesion occurs naturally when epithelia are converted to mesenchyme during development. For example, neural

regulation

of cell shape and gene expression

McDonald

epitbelium and cardiac cushion endothelium are both converted to mesenchyme. When cultured in collagen gels, both lens (Greenburg and Hay, Dez~ Biol 1986, 115:363379) and MDCK epithelial cells lose polarity and become mesenchymal in nature (Zuk et al, J Cell Biol 1989, 108903919). This transformation is related to changes in cell-cell interaction. Inhibiting uvomorulin function inhibits formation of junctional elements including the apical zonula adherens, zonula occludens and desmosomes in MDCK cells (Gumbiner et al, J Cefl Biol 1988, 107:157+1587), and results in their conversion to mesenchymal cells that subsequently invade collagen matrices and embryonic heart tissue [8]. MDCK cells transformed with the oncogenic Harvey and Moloney sarcoma viruses also invade tissue and do not express uvomorulin

[81. Mesenchymal cells respond optimally to extracellular matrix in a developmentally regulated fashion. For example, inhibiting glycosaminoglycan synthesis by cultured rat vascular smooth muscle cells with P-D-xyloside inhibits their expression of other matrix components and growth. This effect is reversed by culturing the cells on their own matrix [9]. Ligamentum nuchae (the suspensory neck ligament of grazing animals) fibroblasts that have been removed from ligaments prior to the onset of elastin synthesis, initiate elastin synthesis when cultured on a nonviable extracellular matrix produced by older cells. Interestingly, elastin receptors (as assessed by chemotaxis) are also expressed by the induced fibroblasts, although it is not known if the expression of these receptors is related to induction of the elastin-synthesizing phenotype (Mecham et al., J Cell Bioll984, 98:18131816; Mecham et al., J Cell Bioll984, 98:1804-1812).

Matrix

receptors

and gene expression

Although the conclusion that components of the extracellular matrix are important in regulating ceU shape and differentiated function is inescapable, the specific mechanism(s) by which the expression of differentiation-specific genes is regulated by extracellular matrix remain largely unclear. BisseU et al. (1982; Bissel and BarceUosHoff, 1987) have suggested that occupancy of matrix receptors alters gene expression via changes in the cytoskeleton leading to association with polyribosomes and altered messenger RNA stability and translation. Communication with the nuclear matrix could also affect gene transcription. Ingber and Folkman 141 summarize many ways in which changes in cell shape, mediated through the extracellular matrix, may regulate gene expression. Many matrix components (interstitial collagens, coUagen type Iv, laminin and fibronectin) bind to integrin receptors of the p-1 class (Buck and Horwitz, Annu Rezl Cell Biol 1987, 3:179-205), so there is considerable interest in the mechanisms and consequences of integrin receptor-l&and interaction. The common p subunit of these heterodimeric receptors has a highly con-

997

998

Extracellular

matrix

served, short cytoplasmic domain that is known to interact with focal contacts (Solowska et al, 1989) and with a newly discovered molecule termed fibulin (Argraves et al, Cell 1989,58:623-629), whet-as the im.raceUular function of the cytoplasmic domain of the a subunit is unhewn. Intact integrin receptors interact with talin, and the talin-integrin complexes bind vinculin (Horwitz et al, Nature 1986,320:531-533). Cell-substrate adhesion, mediated by the a+ libronectin receptor complex, results in extensive ceU spreading and in organization of microflament bundles and decreased fibroblast mobility (Akiyama et al, 1989). Thus, integrins may directly interact with the cytoskeleton which in turn may bind and stabilize messenger RNAS for various genes or transmit information to the nucleus. Several lines of evidence suggest that matrix effects on ceU shape and gene expression are mediated via receptors. Many matrix components, including interstitial collagens, elastin and fibronectin, are chemotactic for mesenchymal and other cells in vitro. In the case of fibronectin (Albini et al, J Cell Biof 1987,105:1867-1872) and elastin (Mecham et aL, 1989) this is mediated via ceU surface receptors. Antibodies to the PI subunit of integrins that inhibit ceU attachment on collagen, laminin and fibronectin substrates prevent myoblast differentiation (Menko and Boettiger, Cell 1987, 51:51-57). The production of collagenase and stromolysin by rabbit synovial libroblasts is stimulated by a monoclonal antibody that binds to the 9 subunit of the libronectin receptor complex and prevents ligand occupancy, without obvious changes in ceU shape or cytoskeletal organization [lo]. Cultured smooth muscle cells are switched from a contractile to a ‘synthetic’ phenotype by culturing them on fibronectin or on RGDS-containing peptides that bind to the vitronectin and fibronectin receptor complex, and this effect is inhibited by antibodies to a PI integrin [ 111. Now that antibodies that block several specif? integrin receptors are available [lo] (see also Wayner et al, J Cell Biol1987, 105:18731884; Wayner et al, J Cell Bioll988, 107:1881-1891; Akiyama et al, 1989) their involvement in other models of cell differentiation may be tested.

valves) from endothelial cells is induced by myocardial tissue, and the inducing factor appears to be a member of the TGF-p family [ 121. Interestingly, TGF8 increases expression of tenascin (myotendinous antigen or hexabrachion), a large glycoprotein that inhibits fibronectin-mediated cell spreading (Chiquet-Ehrismann et al, Cell 1988, 53:383390). TGF-PI is produced by MCF-7 breast carcinoma cells and induces the synthesis of tenascin by fibroblasts. The tenascin in turn disrupts cell-cell interactions between the epitheloid MCF-7 cells [13]. Thus, TGF-PI could modulate epithelial-mesenchymal transformation, in part by stimulating tenascin expression. Based upon its temporal and spatial pattern of expression, tenascin has also been implicated in epithelial-mesenchymal interactions in mammary gland development (Inaguma et al, Dev Biol1988, 128:245-255). There are other sources for matrix-derived or -associated growth factors. Jaminin possesses endogenous mitogenic activity (Panayotou et al, Cell 1989, 56:93-101). Basic Iibroblast growth factor is released from injured endothelial cells (McNeil et al, J Cell Biol 1989, 109:811822), binds to heparan sulfate proteoglycan in basement membranes (Folkman et al, Am J Path01 1988, 130:393-‘@0) and can be released by heparinase (Vltivsky et al, Proc Nat1 Acud Sci USA 1987, 84:2292-2296). Thus a ceUular response to extracellular matrix may be mediated via growth factor receptors.

Summary

matrix

(1) Detailed knowledge of the structure and func-

effects

Some of the effects that extracellualr matrix exerts upon cells are probably mediated via growth factors or growth factor-like effects. Transforming growth factor (TGF)-p stimulates expression of matrix and integrin matrix receptor genes (Roberts et al, J Biol C%em 1988, 263:4586-4592; Heino et al, J Biol cbem 1989, 264380-388; Ignoa and Massague, J Biol C&m 1986, 261:43374345). In some cases TGF-8, can substitute for an inducing tissue. The combination of basic fibroblast growth factor and TGF-p induces mesoderm formation in [email protected] (Kimelman and Kirschner, CeU 1987, 51869877). In heart development, formation of cardiac cushion mesenchyme (the precursor of

directions

Understanding the complex relationship between cell shape, gene expression and the extracellular matrix depends upon parallel development in many areas, including the following.

(2)

Growth factors and extracellular on gene expression

and future

(3)

(4)

tion of the various classes of matrix receptors and the mechanism(s) of signal transduction from these receptors to the ceU (Ruoslahti, Annu Rev Biocbem 1988, 57:375-413). The use of novel molecular genetic methods for the deletion and reconstitution of genes for matrix and matrix receptors from cells and intact animals (Mansour et al, Nature 1988, 336:348-352; Stacey et al, Nature 1988, 332:131-136). Understanding how changes in cell-matrix interactions modify cell-cell interactions and the development of ceU polarity. For example, does the interaction of epithelial cells with matrix components affect the expression or activity of cell-cell adhesive molecules such as uvomorulin? The effect of laminin substrates on ceU differentiation may also include enhanced cell-cell interaction (Noel et al, In Vitro Cell Dev Bid 1988, 24:373380). Understanding the relationship between changes in cytoskeletal organization and the activity of various differentiation-speciUc genes.

Matrix

Acknowledgements

regulation 7. l

I am indebted to Eric Simon and Charlotte R Kennedy for reading the manuscript. This work is supported by National Institute of Health grants 9 RI GM38276-02 and 5 PO1 Hl29594@4 (PPG).

Annotated reading

references

l l e

Of interest Of outstanding

1.

RODRIGUEZ~BOUIAN

l

Reviews 2. l

Discusses molecules

concepts

McDonald

BLUM Jf, WICHA MS: Role of the cytoskeleton in laminin induced mammary gene expression. J Cd Pkysiol 1988. 135:1322.

A laminin

substrate induces synthesis of several milk-specilic proteins by cdNred mammary epithelium, and this effect is inhibited by cytochai&n D and colchicine without a&cting general protein synthesis.

8.

BEHRENS J, MAREEL MM,

l

setting tumor cell invasion: Epithelial cells acquire invasive properties alter the loss of uvomorulin-mediated cell-cell adhesion. J Cd Bill 1989, lo%24352447.

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BIRCHMEIER

W:

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Inhibiting uvomorulin-mediated adhesion of MDCK epithelial ceU.s with antibodies renders them invasive in colfagen gels and heart tissue. Oncogenic transformation has similar elfects. This demonstrates that loss of ceU-ceU adhesion can result in conversion of epithelium to mes. enchyme.

interest

Motphogenesis of the po larized epithelial cell phenotype. Science 1989, 247:718-725. current

of cell shape and gene expression

E. NELSON WJ:

of the establishment

of epithelial

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and

l

ceil polarity.

EKBLOM P: Developmentally reguiated convertion enchyme to epithelium. FASEB / 1989, 3:2141-2150. the role of extracellular in kidney development.

9.

ceULceU

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HAMATIHF, BR~-ITON El Cluw DJ: Inhibition of proteogfycan synthesis alters extraceUular matrbr deposition, poliferation, and cytoskeletal organization of rat aortic smooth muscle cells in culture. / C&II Bid 1989, lOEk249~2505.

Rat aortic smooth muscle cells, cukured with 4-methyUumbefUfety@-Dxyloside to inhibit proteoglycan synthesis, are deficient in extraceUufar matrix accumulation, mukUayemd growth and actin microfilaments. 10.

WERE 2, TREMBLE PM, BEHRENDTSEN D, Crtoww~ E, DAMXY CH,: Signal transduction through the fibroneetin receptor induces collagenase and stromefysin gene expression. / cell Bid 1989, 109~877-889. A monoclonal antibody biding to the % subunit of the integtin 6. bronectin receptor and cell adhesive fragments of fibronectin induces collagenase and stromolysii expression by libroblasts. Although the mechanism is not clear (intact fibronectin did not have this effect), this appears to be a clear example of altered gene expression mediated by an integrin matrix receptor without obvious changes in cell shape and cytoskeleton. l e

KLEIN G, ~GECCER M, T~MPL R, EKBU%! P: Role of laminin A chain in the development of epitheliai cell polarity. cell 1988, 55:331-341. The hminin A chain is newiy expressed on differentiating kidney 3.

l e

tubules and antibodies to its carboxyterminal domain inhibit the polarization of condensing nephrogenic mesenchyme. This demonstrates a role for differential expression of the laminin A chain (and presumably cognate cell surface receptors) in kidney development. INBGER E, FOU
4. l

5. l

SCHUEIZ K~HNMAN

EG, LI D, HK, Etswc~

OMIEC~NSKI CJ, MUUER-EBEIU~ARD B, GUZEUAN PS: Regulation of

U,

gene expression in adult rat hepatocytes cultured on a basement membrane matrix. / cell Ply&~/ 1988, 134:309-323.

Plating primary hepatrxytes on Mauigel@ mah-ttains their differentiated function and cuboidal morphology, whereas cells plated on type 1 collagen flatten and increase expression of cytoskeletal genes and type I collagen. 6. l

BEN-ZE’EV cell and

A, ROBINSON

GR

Bucrrxs

NLR,

FARMER -SR:

CeU-

cell-matrix interactions diRerentiaUy regulate the expression of hepatic and cytoskeletai genes in primary atkures of rat hepatocytes. Proc Nafl Acad Sci USA 1988,

85:2161-2165. Plating hepatocytes on hydtated collagen gels at high ceU density or on EHS matrix induces synthesis of liver-specific proteins, whereas at low density, expression of liver-specific proteins decreases and that of cytoskeletal proteins increases.

11.

HEDIN

l

A subtrate of the cell-attachment sequence of fibronectin (Arg-Gty-Asp&r) is suRicient to promote transition of arterial smooth muscle cell.5 from a contractile to a synthetic phenotype. Da, Bid 1989, 133:4B!+501.

U, B~~TCER

BA

Lumhw

J. JOHANQXIN

S. THYBERG J:

Fibronectin and RGDS substrates alter the differentiated properties of rat aomc smooth muscle ceils from a contractile to a synthetic phenotype, and this effect is inhibited by antibody to the g subunit of the fibronectin receptor. 12. l

JD, RUNYANRB: Epithelial-mesenchymal ceU transfo~ mation in the embryonic heart can be mediated, in part, by transforming growth factor 0. Deu Bill 1989, 134:392-4CB.

Ports

TGF-8 can replace the requirement for an inducing tissue to transform cardiac endothelium to mesenchyme. and antibodies inhibiting TGF-fl prevent normal tt-dnsfomtation. 13. l e

CHIQUET.EHRISMANN R KAUA of tenasin and transforming

P, PLU~~N CA growth factor-8

Participation in reciprocal

epithelial-mesenchymal interactions of MCR ceils and fibroblast. tin Res 1939, 494322-4325. MCF7 human mammary carcinoma c-eUsproduce TGF-g that induces fibroblasts to produce tenascin. The tenascin in turn interferes with the formation of ceU-ceU contacts by MCR cells. These observations provide a potential link between TGF-8, epithelial-mesenchymal transformation, inhibition of celLceIl interaction and matrix gene expression.

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