MUC1 Expressed in PanCi Cells Decreases Adhesion ... - Europe PMC

Amercan Journal of Pathology, Vol. 148, No. 3, March 1996 Copyight © American Society for Investigative Pathology

MUC1 Expressed in PanCi Cells Decreases Adhesion to Type 1 Collagen but Increases Contraction in Collagen Lattices

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domain.1-3 The molecule consists of a protein core containing 20 to 125 highly 0-glycosylated tandem repeating units, each of which are 20 amino acids long, rich in serine, threonine, glycine, alanine, and proline. Genetic polymorphism gives rise to molecules of variable molecular mass, ranging from 300 to 750 kd.45 In normal tissue the initially proposed function of MUC1 was as a protective barrier, which also serves as an epithelial lubricant, whereas its up-regulated tumorigenic counterpart may protect malignant cells from cytotoxic cell interactions as well as promoting metastasis.6-9 Breast, pancreatic, and other adenocarcinomas have been shown to overexpress aberrantly glycosylated forms of the MUC1 mucin,10 11 distinguishable from normal MUC1 by molecular weight and immunohistochemical analysis.12 This increased expression of MUC1 mucin is accompanied by a loss of apical polarization in tumor cells with ensuing circumferential expression. 13 Studies in vitro also show decreased cellcell and cell-extracellular matrix (cell-ECM) adhesion,14 properties that are theoretically advantageous for the invasive metastatic phenotype.15 To date, nine human MUC genes have been partially or fully cloned and sequenced. Each gene differs from its family members by the size and composition of the tandem repeat unit sequence. MUC216,17 and MUC318 have been detected in the small and large intestine, respectively; MUC419 -5a, -5b, and -5c20-22 are confined to the lung; MUC623 is present on the gastric mucosa; and MUC7 is a low molecular weight human salivary mucin.24 Although all of the MUC genes encode for secreted forms of the mucin, MUC1 is the only member of the mucin family to contain a transmembrane region capable of alternative splicing,25 gener-

The human MUC1 mucin is unique among the mucin family in being a transmembrane glycoprotein that is not only present on the apical surfaces of normal breast, pancreatic, and other glandular epithelia but is also secreted by cleavage of the extracellular

Supported by a grant (G108/147) from The Medical Research Council, UK. Accepted for publication November 24, 1995. Address reprint requests to Dr. El-Nasir Lalani, Department of Histopathology, Royal Postgraduate Medical School, Du Cane Road, London W12 ONN, United Kingdom.

Mark J. H. Hudson,* Gordon W. H. Stamp,*t Michael A. Hollingsworth,A Massimo Pignatelli,*t and El-Nasir Lalani*t From the Department of Histopathology,* Royal

Postgraduate Medical School, Hammersmith Hospital, and the Imperial Cancer Research Fund/Royal Collage of Surgeons,t Histopathology Unit, London, United Kingdom; and the Th-e Eppley Institute,t University of Nebraska Medical Centre, Nebraska

A subline of human pancreatic ceUls (PanCI) that expresses low levels of cytokeratins 8 and 18 but not MUCI mucin was transfected witb botb 3.5-kb and 3.9-kb full-lengtb MUCI cDNA. The MUCI-positive clone expressing the larger mucin was sbown to express increased levels of cytokeratins 8 and 18 compared witb the parental line or vector controls. Growth of these MUCI-transfected cells in type I collagen gels produced marked gel contraction that could be signiflcantly reduced by the synthetic peptide SRGDTG or by growth in serum and fibronectin-depleted media. Cellular binding to type I coUlagen wasfound to be reduced by two- tofourfold in cells expressing the MUCI mucin, for which the greatest inhibition was observed in cels expressing the largerform. No difference in cellular binding to fibronectin was observed. From these data we conclude that the human MUCI mucin modifies the differentiated state of human pancreatic cells by altering cytokeratin expression and reducing adhesion to type I collagen but paradoxically enhancing the ceUlular contractile phenotype, effects that appear to be mediated by integrin expression and/or function. (Am J Pathol 1996

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ating both membrane-associated and secreted forms of the mucin. Cytological organization and polarization depend on complex systems of structural and biochemical organization. Cell shape alterations are mediated by intracellular cytoskeletal assemblies composed of cytokeratins, microtubules, and actin filaments. Interestingly, the cytoplasmic tail of the MUC1 mucin has been shown to interact with the intracellular actin cytoskeleton,26 although the significance of such interactions are still unclear. There are at least 20 different genes encoding epithelial cytokeratin polypeptides.27'28 Differentiated epithelial cell types have been shown to be distinguishable from each other by their selective expression of cytokeratin polypeptides. During development, cytokeratins are the earliest intermediate filaments expressed, and both ecto- and endoderm are characterized by typical polarized epithelial cells with extended arrays of intermediate filaments containing cytokeratins 8 and 18.2930 Cellular attachment to the extracellular matrix is vital in the determination of cell morphology and the maintenance of cellular function and tissue integrity. Such binding not only anchors cells to the extracellular matrix but also provides positional signals to direct cellular migration and differentiation. The two principal types of cellular interactions are cell-cell and cell-ECM adhesion. Integrins appear to be the primary mediators of cell-ECM adhesion and also serve as one of the many families of molecules active in cell-cell attachment, which may act by modulating cytoskeletal components. In vitro studies trying to unravel cellular mechanisms of underlying wound healing and morphogenesis31-35 have extensively utilized three-dimensional collagen gel lattice contraction. Gel contraction by fibroblasts has been reported by Bell and colleagues,36 and recently epithelial colorectal cell lines have been shown to induce contraction that can be blocked by monoclonal antibodies to the a2f1 integrin.37 The aims of this study were to identify the cytoskeletal alterations after MUC1 expression, to analyze the effect of MUC1 on cell-ECM adhesion, and to determine whether MUC1-expressing human pancreatic transfectant cell lines contributed to extracellular matrix remodeling by matrix contraction.

3.5-kb or 3.9-kb full-length MUCi cDNA under the control of the 3-actin promoter, to generate PanCE2 (vector control), PanCE3 (30 tandem repeats), and PanCF5 (42 tandem repeats), respectively. The MUC1 cDNA used to transfect the PanCE3 and PanCF5 cell lines differed only in the number of genomic tandem repeating units as indicated.

Materials and Methods Cell Lines

Immunoblotting

PanCl, a human pancreatic adenocarcinoma cell line was transfected by Batra et a132 with the eukaryotic expression vector pHl3 APr-1-neo alone or containing a

Cell Culture PanCl cells were maintained as monolayers on plastic in Dulbecco's modified Eagle's medium (DMEM) supplemented with 10% fetal bovine serum (FBS; Gibco, Renfrewshire, UK). The same batch of serum and medium was used throughout this study. Transfected clones were grown in the above medium containing the antibiotic G-418 sulfate (Gibco) at 600 Ag/ml. All cell lines were maintained in a humidified cell incubator with 10% CO2 in air at 370C. The antibiotic G-418 sulfate was removed from culture medium for 48 hours before any experimental work was performed.

Immunocytochemistry Normal and transfected cells were grown on sterile, four-well glass slides overnight or until 70 to 80% confluent. Slides were washed once in phosphate-buffered saline (PBS; pH 7.2 to 7.4) and fixed for 5 minutes in a solution of methanol/acetone (50:50, -20°C). After fixation, slides were washed three times with PBS and immersed in 70% methanol containing 1% (v/v) hydrogen peroxide for 30 minutes to block endogenous peroxidase. Slides were incubated with 4% (v/v) normal rabbit serum (Dako, Wycombe, UK) in PBS for 15 minutes in a humidity tray, excess serum was removed, and sections were incubated for 45 minutes with HMFG-238 antibody (30 ,ul). After incubation, slides were washed three times with PBS and then incubated for an additional 45 minutes with 4% (v/v) horseradishperoxide-labeled rabbit anti-mouse IgG (Dako). Slides were washed three times with PBS, and peroxidase was detected with diaminobenzidine tetrahydrochloride (0.25 mg/ml) in PBS containing 0.1% (v/v) hydrogen peroxide for 5 minutes. Cells were counterstained in hematoxylin.

Cell lysates were prepared by adding 1 ml of cold lysis buffer (710 mmol/L f3-mercaptoethanol, 10% (v/v) glycerol, 345 mmol/L sodium dodecyl sulfate (SDS), 6.25% (v/v) 1 mol/L Tris base, pH 6.8, and 1 mmol/L EDTA) to 107 cells on ice. Lysates were immediately boiled for

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10 minutes, placed on ice for 5 minutes, and then centrifuged at 6000 rpm (Heraeus omnifuge 2.0 RS) at 4°C for 5 minutes. Samples were placed on ice for 5 minutes, and supernatants were spun at 13,000 rpm in a microcentrifuge and stored at -20°C until required. Equivalent amounts of protein from different lysate samples were resolved by SDS-polyacrylamide gel electrophoresis (3.76 p,g/well) in either 10 or 7.5% acrylamide gels, as described by Laemmli,39 and transferred to nitrocellulose in a Western blot chamber (Hoefer Scientific Instruments, Newcastle-under-Lyme, UK) at 250 mA for 2 hours in transfer buffer (25 mmol/L Tris base, 192 mmol/L glycine, and 20% (v/v) methanol). Equivalent lane loading was assessed after transfer using Ponceau S (Sigma, Poole, UK) staining. Nitrocellulose membrane was blocked for 1 hour in Trisbuffered saline (TBS; 10 mmol/L Tris base and 154 mmol/L NaCI, pH 7.4) containing 5% nonfat dry milk powder. The membrane was then washed briefly with TBS containing 0.05% (v/v) Tween (TBST) followed by incubation with either HMFG-2, LE41 40 or LE6140 antibody supernatants for 2 hours. The membrane was washed three times (5 minutes each wash) with TBST, followed by incubation with TBST containing 5% (w/v) nonfat dry milk powder and 0.3% (v/v) horseradishperoxide-labeled rabbit anti-mouse IgG for 1 hour. After three additional washes in TBST, the protein bands were visualized by incubation with 0.3% (w/v) 4-chloro1-napthol (Sigma, Poole, UK) in absolute methanol containing 0.02% (v/v) hydrogen peroxide. Protein band analysis was determined using a personal densitometer Si (Molecular Dynamics, Sunnyvale, CA) linked to a Power Macintosh 8100. Gel images were analyzed with the software program IP Lab Gel (Signal Analytics Corp., Vienna, VA). Briefly, five separate readings were taken from each band; each reading is the sum of pixel intensity for a fixed pixel area (276 pixels). For each band, pixel intensities were averaged and background readings were subtracted (measurements taken as above) to give an average band intensity for each lane.

Collagen Gel Preparation Collagen gels were prepared using Vitrogen 100 (Celtrix, Santa Clara, CA) according to the manufacturer's instructions. A single-cell suspension (2 x 104 cells/0.5 ml) obtained after trypsinization and syringing three times through a 25-gauge needle were mixed with 5 ml of neutralized Vitrogen 100 collagen solution (pH 7.4 ± 0.2) and aliquotted into prewarmed (37°C) 1-cm organ culture dishes (Gibco). Collagen gelation was then initiated by warming the collagen solution at 370C for 60 minutes in a cell incubator. The gels were

then overlaid with 2 ml of DMEM supplemented with 10% FBS and returned to the incubator. Spent medium was aspirated and gels re-fed every 48 to 72 hours. Gels were fixed in 10% formol saline, processed, embedded in paraffin wax, and sectioned for immunocytochemistry.

Preparation of Collagen Gels for Contraction Assay Contraction assays were performed on cellular suspensions in DMEM in the presence and absence of 10% FBS or fibronectin. Cells were grown until 95% confluent and removed using trypsin/EDTA as before. Single-cell suspensions were obtained by passing cells three times through a 25-gauge needle and were then serially diluted to 1 x 106 to 1 x 103 cells/ml in appropriate medium. Aliquots (1 ml) of cell suspensions were added to 0.58 ml of neutralized Vitrogen 100 collagen solution (1 mg/ml) previously stored on ice, mixed (pH 7.4 + 0.2), and pipetted (0.5 ml) onto 24-well plates (Costar 3524) previously coated with 0.66% (w/v) agarose. Cells were then returned to the incubator for 24 hours. Fibronectin removal from serum was achieved by incubating 10 ml of 4% beaded agarose containing 3 to 6 mg/ml covalently bound gelatin (Sigma) with 5 ml of FBS overnight at 4°C. Solution was then centrifuged at 500 rpm (Heraeus omnifuge 2.0 RS) for 5 minutes and FBS filtered first through a 0.45-gm then a 0.2-,um hydrophobic filter (Sartorius, Gottingen, Germany). For blocking studies, cells were preincubated for 10 minutes with either the SRGDTG (0 to 10 ,ug/ml) or SRGETG (0 to 10 ,ug/ml) peptides and then treated as above.

Measurement of Ge/ Size After 24 hours in culture, three separate measurements of gel diameter were taken at 0,120, and 240 degrees, which were averaged to obtain a mean gel radius used to determine the final gel area after 24 hours in culture. The value deduced was used to calculate the percentage decrease in gel area using: (wr02 - 1rr242/rrr2) x 100, where ro and r24 are the mean values for gel radii at 0 and 24 hours, respectively.

Cell Adhesion Assays Bacteriological microtiter plates (Flow Laboratories, Oxfordshire, UK) were incubated with a solution (100 ,l) of either collagen type (0 to 80 gg/ml) or fibronectin (0 to 80 gg/ml, from human placenta; Sigma) overnight at 4°C. Wells were then washed

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three times with PBS and blocked with 100 ,lI of bovine serum albumin (0.5 mg/ml) for 1 hour at 250C. After blocking, wells were washed once with PBS and then incubated with 104 cells in 100 ,lI of DMEM lacking FBS for 3 hours. After incubation, wells were washed three times in PBS and incubated for 3 hours with 60 ,ul of 0.05 mol/L citrate (pH 5.0), containing 0.25% Triton X-100 and 3.75 mmol/L p-nitrophenylN-acetyl-p-D-glucosaminide. The reaction was stopped by the addition of 90 ,ul of 50 mmol/L glycine (pH 10.4), containing 5 mmol/L EDTA. Cellular adherence was determined by measuring the level of hexosaminidase activity of each well compared with a cellular standard by reading the absorbances for each well at 405 nm on a Titertek Multiskan Plus.

Results Detection of MUC 1 and Cytokeratin Expression All four pancreatic lines were analyzed for the expression of MUC1 mucin using immunocytochemistry and Western blotting. Data presented are for cell lines PanCE3 (transfected with 3.5-kb MUC1 cDNA), PanCF5 (transfected with 3.9-kb MUC1 cDNA), and PanCE2 (vector control). For clarity, details for the parental line PanCl have been omitted as they were identical for the results obtained with the vector control

PanCE2.

Immunocytochemical Detection of MUC1 Immunoreactivity for MUC1 in cells grown on glass was demonstrated in 95 to 100% of the PanCE3 and PanCF5 cells (Figure 1, B-C), whereas no expression was detected in the control line PanCE2 (Figure 1A). There is heterogeneity of staining intensity with predominant membrane staining, accompanied by diffuse cytoplasmic staining in a minority of the cells. Both transfectant lines embedded in collagen (Figure 1, E-F) were also strongly immunoreactive for MUCI in the great majority of the cells. The staining is darker than the monolayers partly due to fixation differences and the thickness of the gel sections (15 to 18 ,tmol/L). No expression was detected in the control line PanCE2 (Figure 1 D).

Western Blotting Western blot analysis of each cell line grown as a monolayer on plastic clearly shows the expression of MUC1 using the primary antibody HMFG-2 in the trans-

Table 1.

Densitometnic Analysis of Western Blots

Cell Line

Cytokeratin 8

Cytokeratin 18

PanCE2 PanCF5 PanCE3

19,350 ± 6,818 82,130 ± 22,700 22,540 ± 5,610

6,150 ± 1,540 39,750 + 7,570 5,960 ± 1,510

Each data point represents the mean intensity of five readings (minus background) taken at random along each band present in Figure 2B and C.

fected PanCE3 and PanCF5 lines. No detectable expression was found in the PanCE2 control (Figure 2A). The PanCE3 cell line produces three clear bands, a broad band at approximately 250 kd and two bands at 130 and 120 kd. The PanCF5 line, due to the additional 0.4-kb sequence coding for 12 tandem repeats, synthesizes a larger MUC1 molecule with a broad band at 280 kd and two bands at 170 and 160 kd. The largest band present in both lines is within the molecular weight range predicted for the expressed glycosylated MUC1 mucin. The smaller bands approximate to the calculated sizes for the whole and cleaved forms of the unglycosylated MUC1 protein core

molecule.4142 Assessment of several Western blots by densitometric analysis showed that the PanCF5 cell line had a fourfold and sixfold greater band intensity for cytokeratins 8 and 18, respectively, whereas the PanCE3 line approximated that of the vector control PanCE2 (Table 1). In addition, MUC1 expression had no effect on the cellular expression of vimentin, actin, and the integrins aV, aV/f33, and ,B1 (data not shown) as determined by similar densitometric analysis.

MUC1-Transfected Cells Generate Increased Tractional Forces within Type I Collagen Matrices The data in Figure 3 clearly show that MUC1 transfectants cause significant collagen gel contraction compared with the vector control. Gel contraction was found to be highly dependent on the presence of FBS (Figure 4) and fibronectin (Figure 5), which could be blocked by preincubation of the cells with the synthetic peptide SRGDTG (RGD) but not SRGETG (RGE) (Figure 6). This result suggests that the cells at least in part bind to the collagen through fibronectin; however, the partial removal of vitronectin and other serum proteins resulting from the incubation of FBS with adsorbed gelatin may mean other integrins are involved in the contraction process. Hall et a143 have shown that PanCl cells express a number of RGD-dependent integrin receptors, most im-

Gel Contraction and Increased Keratin Expression by MUC1 955 AJP March 1996, Vol. 148, No. 3

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Figure 1. MUCi mucin expression in cell lines grown on glass using the mouse MAb HMFG-2. A: PanCE2 (vector control) shows no staining. B: PanCE3 (transfected with 3.5-kb MUC1). C: PanCE5 (transfected with 3.9-kb MUC1). Both PanCE3 and PanCF5 show clear membrane and cytoplasmic expression. Similar staining was repeated on cell lines grown in three-dimensional type I collagen gels. D: PanCE2 (vector control) shows no staining. E and F: PanCE3 and PanCF5, respectively, are both strongly immunoreactiveforMUC1 on membranes and in the cytoplasm. Original magnification, X 400.

portantly a3f31 binding to fibronectin and avf3l and av35 binding to vitronectin. Therefore, the exact type of integrin interaction has not yet been determined. However, no expression of the a2-integrin was detected (data not shown), indicating the contraction by the transfectants is not mediated by VLA-2.7

Cellular Binding to Type I Collagen and Fibronectin Analysis of cellular binding to type collagen revealed that both MUCO transfectants showed between a two- and fourfold decrease in binding

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Figure 2. Western blot for the expression ofMUC1 mucin and cytokeratins 8 and 18. A: Detection of MUC1 using the MAb HMFG-2. B: Detection of cytokeratin 8 using the MAb LE41. C: Detection of cytokeratin 18 using MAb LE61. For all panels: lane 1, PanCE2; lane 2, PanCF5; and lane 3, PanCE3.

compared with the vector control (Figure 7A), in which PanCF5, expressing the larger of the mucins, resulted in the greatest inhibition. Similar binding studies to fibronectin (Figure 7B) showed no significant differences in cellular binding, except for the PanCF5 clone, which showed slight inhibition of binding at higher concentrations of surface-adsorbed protein.

Discussion A late passage PanCl cell line expressing no MUC1 mucin and low levels of cytokeratins was transfected by Batra et al32 with human MUC1 cDNA of different sizes. Transfectants PanCE2 (vector control) and PanCE3 and PanCF5 (MUC1 transfectants) were characterized at the molecular level and by electron microscopy.32 44 The resulting transfectants (PanCE3 and PanCF5) showed an increase in the level of cellular endoplasmic reticulum and Golgi and secretory vesicles compared with the parental cell line (PanCl) and vector control. These observations were attributed to either cellular differentiation or the increased production of cellular organelles for the synthesis and transport of MUC1 mucin. We chose to further analyze these transfectants and examine the effect of MUC1 on type 11 and Ill intermediate filament expression and collagen gel contraction. Our results consistently demonstrate that MUC1 expression resulted in increased expression of cytokeratins 8 and 18, but only in the cell line transfected with the 3.9-kb MUC1 insert (PanCF5). No demonstrable effect was observed on vimentin, actin, and the integrins aV, aV/433, and (31 expression (data not shown). The function(s) of the intermediate filaments is still unclear, but it is considered that cytokeratins are relatively rigid structural com-

ponents that help maintain epithelial integrity.45 It is at present unclear why the additional tandem repeats in the PanCF5 cells should so radically alter

levels of expression of cytokeratins 8 and 18. Our results also indicate that cell surface expression of MUC1 mucin significantly reduces cell-ECM interaction with type collagen. Ligtenberg et al14 demonstrated that MUC1 decreased cell-cell interaction. This reduced affinity could theoretically be an effect of MUC1 forming a large hydrated physical barrier, extending 200 to 500 nm from the membrane surface, masking interactions between small extracellular surface receptors and extracellular matrix molecules.46 In addition, the variation in binding affinities between the transfectants PanCF5 and PanCE3 appears to correlate well with the differing sizes of the MUC1 mucin expressed by each of the 100 do

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Figure 3. Collagen gel contraction in the presence of FBS. The mean percentage decrease in the area (n = 3) of type I collagen gels in response to incubation with increasing numbers ofcells in thepresence of FBS after 24 hours of incubation at 3 70C, 10% C02. *, PanCE2; *, PanCE3; EL, PanCF5.

Gel Contraction and Increased Keratin Expression by MUC1 957 AJP March 1996, Vol. 148, No. 3

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transfectants. In contrast, only minimal inhibition of binding to fibronectin was observed in the clone expressing the larger mucin (PanCF5), with no inhibition in the clone expressing the smaller mucin form (PanCE3). This minimal inhibition of binding to fibronectin together with the maximal inhibition of binding to collagen type suggests that the size of the MUCI mucin is important in determining the degree of cell-ECM interaction. Tractional forces generated by cells exert a pivotal role in embryogenesis and wound healing,33'47 and we have demonstrated that transfectants expressing

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MUC1 induce significant collagen gel contraction. Several integrins, particularly those containing a2, av, and a5 subunits, bind to ligands at sites containing the amino acid sequence Arg-Gly-Asp (RGD).48 The synthetic peptide RGD blocked the contractile effect of the transfectants, suggesting that the effect is mediated via the RGD-related integrins. We further examined the effect of MUC1 on the expression of the integrins a2, aV, aV/f33, and (1 and found no induction or down-regulation at the protein level (data not shown). It is possible that the expression of the MUC1 mucin may result in either integrin receptor aggregation and/or conformational change whereby integrin function is possibly enhanced without a change in expression level.49 Thus it appears at least in this cell line that collagen gel contraction and adhesion to type collagen are independent mechanisms not mediated by the a2f1 integrin, unlike the case in colorectal and breast carcinoma cell

lines.37 In addition to this, growth in FBS-depleted media greatly reduced the degree of contraction to a level comparable to the vector control, which agrees with observations described by Chan.50 FBS is essential for maximal gel contraction, suggesting that one or more soluble factors are required. The removal of fibronectin from FBS by binding to gelatin produced identical results in gel contraction to those observed with FBS-depleted medium. The possibility of complete or partial removal of other serum proteins, like vitronectin, has meant that the exact type of integrin interaction is not yet clear. However, it is likely that the interaction observed is mediated via one or more of the a3f31, avg3l, or av135 integrin receptors.

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Figure 5. Collagen gel contraction in normal orfibronectin-depleted medium. The mean percentage decrease in the area (n = 3) of type I collagen gels after 24 hours of incubation at 370C, 10% C02 in the presence of increasing numbers of cells in normal or fibronectindepleted medium. E, PanCE3 (normal medium); *, PanCE3 without fibronectin; E, PanCF5 (normal medium); 0, PanCF5 without fibronectin.

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Peptide Concentration (gg/ml) Figure 6. Inhibition of collagen gel contraction (n = 3) by the cell line PanCE3 after the addition of the synthetic peptides SRGDTG (RGD) and SRGETG (RGE) for 24 hours at 3 7°C, 10% CO2

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Acknowledgments The authors thank Mr. T. Oates for technical assistance.

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References

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We have shown that there are alterations in cytokeratin levels in a pancreatic cell line after MUC1 expression, which, together with functional alterations demonstrated by enhanced contraction in collagen gels but paradoxical decreased adhesion, has significant implications for the role of MUC1 in the mechanisms of tumor differentiation and progression in vivo.

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hence motility. These mechanisms may be exerted by affecting the functional integrity of integrins expressed in a motile phenotype (eg, a6 and av). Thus the well to moderately differentiated MUCl-expressing pancreatic cell lines SW1990 and CAPAN-2 readily metastasize, unlike the less differentiated pancreatic cell lines MIA PaCa-2 and PanCl, which have been shown to express lower levels of the

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Figure 7. Cellular adhesion to type I collagen (A) orfibronectin (B). Adhesion was determined for the cell lines PanCE2, PanCE3, and PanCF5 by measuring cellular retention after the addition of lo' cells to microtiter wells containing increasing levels of surface-adsorbed protein.

Our results clearly show that alterations in cytokeratin expression are independent of the contraction/retraction mechanism as PanCE3 also showed this effect but had minimal alterations in cytokeratin levels. It has been previously shown that the cytoplasmic tail of MUC1 is associated with intracellular actin filaments as their depolymerization by cytochalasin D treatment results in the disruption of apical MUC1 expression.51 It is conceivable that MUC1 may alter the intracellular organization of actin, generating a more contractile phenotype, as observed in collagen gels. Our combined collagen gel contraction and cellbinding studies suggest that these changes could facilitate tumor invasion and metastasis, first, by decreasing cell-matrix adhesion allowing cells to begin migration and, second, by enhancing traction and

1. Baeckstrom D, Nilsson 0, Price MR, Lindholm L, Hansson GC: Discrimination of MUC1 mucins from other sialyl-Le(a)-carrying glycoproteins produced by colon carcinoma cells using a novel monoclonal antibody. Cancer Res 1993, 53:755-761 2. Peat N, Gendler SJ, Lalani EN, Duhig T, Taylor PJ: Tissue-specific expression of a human polymorphic epithelial mucin (MUC1) in transgenic mice. Cancer Res 1992, 52:1954-1960 3. Gendler SJ, Spicer AP, Lalani EN, Duhig T, Peat N, Burchell J, Pemberton L, Boshell M, Taylor PJ: Structure and biology of a carcinoma-associated mucin, MUC1. Am Rev Respir Dis 1991, 144: S42-S47 4. Swallow DM, Gendler S, Griffiths B, Corney G, Taylor PJ, Bramwell, ME: The human tumour-associated epithelial mucins are coded by an expressed hypervariable gene locus PUM. Nature 1987, 328:82-84 5. Gendler SJ, Lancaster CA, Taylor PJ, Duhig T, Peat N, Burchell J, Pemberton L, Lalani EN, Wilson D: Molecular cloning and expression of human tumor-associated polymorphic epithelial mucin. J Biol Chem 1990, 265: 15286-15293 6. Lalani EN, Berdichevsky F, Boshell M, Shearer M, Wilson D, Stauss H, Gendler SJ, Taylor PJ: Expression of the gene coding for a human mucin in mouse mammary tumor cells can affect their tumorigenicity. J Biol Chem 1991, 266:15420-15426 7. Bresalier RS, Niv Y, Byrd JC, Duh QY, Toribara NW, Rockwell RW, Dahiya R, Kim YS: Mucin production by human colonic carcinoma cells correlates with their

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