Acylation of gelatin-agarose and the enhancement by heparin of fibronectin binding

ARCHIVES OF BIOCHEMISTRY AND BIOPHYSICS Vol. 266, No. 1, October, pp. 181-1X8,1988

Acylation of Gelatin-Agarose and the Enhancement by Heparin of Fibronectin Binding

ROBERT L. SMITH’ AND CHARLES A. GRIFFIN

Department of Biochemistry and Molecular Biology, Louisiana State University School of Medicine, Shreveport, Louisiana 7’1130~3932

Received March 28,1988, and in revised form May 31,1988

The enhancement of the binding of plasma fibronectin to collagen or gelatin by heparin was previously thought to be due primarily to interaction of heparin with fibronectin. We observed, however, that the elution of purified human plasma fibronectin from heparin- treated gelatin-agarose required the same high urea concentrations regardless of whether heparin treatment preceded or followed fibronectin adsorption. Acylation of gelatin-agarose with acetic anhydride or succinic anhydride had little effect upon fibronectin binding, yet the heparin enhancement of fibronectin binding was abolished by either acylation reaction. When heparin binding to gelatin-agarose was investigated with dansyl heparin, gelatin-agarose bound substantial quantities of labeled heparin which could be readily dissociated from the matrix with 2 M NaCl. Acetylated gelatin- agarose did not bind detectable amounts of dansyl heparin. We interpret these results as evidence that the stronger binding of fibronectin to gelatin-agarose in the presence of heparin is due to heparin itself binding to gelatin, thus allowing fibronectin to bind simultaneously to both immobilized ligands through appropriate domains of the glycoprotein. 0 1988 Academic Press, Inc.

The binding of fibronectin to native and denatured collagen is one of numerous ligand interactions exhibited by this extra- cellular high-molecular-weight glycoprotein (1, 2). Each of the two subunits of plasma fibronectin contains a single do- main which binds either collagen or gelatin (3,4).

Fibronectin also binds glycosaminoglycans, especially heparin and heparan sulfate (5-7). Plasma fibronectin binds heparin in moderate ionic environments at multiple sites, one in an N-terminal do- main of the subunits (8), and another near the C-terminus (9). These heparin-binding domains are distinct from the region

1 To whom correspondence should be addressed.

which binds collagen or gelatin. Additional sites in fibronectin fragments have been described which bind heparin only at low ionic strength (10).

Heparin enhances the binding of fibronectin to gelatin and collagen (11-13). Both heparin and heparan sulfate induce conformational changes in fibronectin (14-16). Because of these effects it has been suggested that these ligand-binding events may be cooperative and mediated by conformational changes.

We report here evidence that the enhancement by heparin of the binding of human plasma fibronectin to gelatin-agarose is the result of heparin binding simultaneously to gelatin and fibronectin. We show that, although acylation of most amino groups in gelatin has little effect

181 0003-9861/88 $3.00 Copyright Q 1988 by Academic Press. Inc. All rights of reproduction in any form reserved.

182 SMITH AND GRIFFIN

upon fibronectin binding, it completely eliminates the heparin enhancement of this binding. We also show that fluores- cently labeled heparin binds to gelatin- agarose but not to acetylated gelatin- agarose.

EXPERIMENTAL PROCEDURES

Materials

Gelatin-agarose was prepared as previously described (17) by coupling Sigma type I porcine skin gelatin with CNBr-activated Sepharose CL-4B (Phar- macia). This material contained about 3 mg of gelatin per milliliter of packed medium. Fibronectin was purified from titrated frozen human plasma as previously described (17), by elution of the adsorbed plasma protein from gelatin-agarose with 50 mM, pH 5.5, sodium citrate buffer containing 100 mM NaCI. Unlabeled heparin was from Sigma (sodium salt, grade I). Dansyl’ heparin was prepared as previously described (18). 0-Methylisourea sulfate was from K&K Labs (Plainview, NY). All other reagents were of analytical quality.

Methods

Afinity chrcrmatogruphy. Experiments were per- formed at room temperature with identical 2-ml columns of acylated or nonacylated gelatin-agarose. The equilibrating buffer was 50 mM Tris-Cl, pH 7.5. Frac- tions were collected directly from the bottom of the columns which were run by gravity flow with a hydro- static head of 2-4 cm (flow rate, approx 2 ml/min). When a sample was applied or the eluant was changed, the previous solution was allowed to sink completely into the column bed before the next eluant was applied. Protein elution was monitored by absor- bance at 280 nm, while dansyl heparin was measured by fluorescence at 550 nm, with excitation at 330 nm.

Acylation of gelatin-agarose. Acylation was carried out essentially according to Means and Feeney (19). For succinylation, 21 ml of gelatin-agarose suspended in an equal volume of 1 M NaHCOa was acylated at room temperature for 2 h by adding, at lo-min inter- vals, 56-mg portions of succinic anhydride dissolved in dioxane. pH was maintained at 7.8-8.1 by dropwise addition of 2 M NaOH. Reaction conditions were the same for acetylation, except that the acylating agent, acetic anhydride, was added without solvent to the reaction mixture. Each acylated gelatin-agarose

’ Abbreviation used: dansyl, 5-(dimethylamino)- naphthalene-1-sulfonyl.

preparation was washed exhaustively with 50 mM

Tris-Cl, pH 7.5, containing 0.1 M NaCl prior to chromatography experiments.

Guanidination of gelatin and gelatin-agarose. To obtain a measure of the relative amounts of free e-amino groups in acetylated and nonacylated gelatin agarose, samples of each were reacted with 0-methylisourea according to the method of Cupo et al. (20). Excess reagent and buffer salts were removed from reacted gelatin-agarose samples by washing them on a filter funnel with distilled water, and from reacted gelatin by dialysis against distilled water. In the guanidination reaction, accessible, nonacylated lysyl residues are converted to acid-stable homoarginyl residues. The extent of conversion was subsequently ascer- tained by loss of lysine and appearance of homo- arginine upon amino acid analysis of acid hydrolysates (20).

Amino acid analysis. Protein or protein-agarose samples were hydrolyzed in 6 N HCl in evacuated tubes at 110°C for 20 h. After removal of HCl in vacua, hydrolysates were analyzed on a Beckman Model 121 M amino acid analyzer. Dark insoluble matter in the protein-agarose hydrolysates was removed prior to analysis by filtration through a 0.45-pm pore mem- brane.

RESULTS

The effects of heparin upon affinity chromatography of plasma fibronectin on gelatin-agarose are shown in Fig. 1. In the absence of heparin treatment, most of the bound fibronectin was eluted with 2 M urea. If, however, heparin was passed through the affinity matrix prior to fibronectin application, elution of the bulk of the fibronectin was delayed until treatment with 4 M urea. An almost identical effect was observed if heparin treatment followed application of fibronectin to the column as previously described by Rouslahti and Engvall(12). These results suggested to us that heparin, an acidic, highly sulfated glycosaminoglycan, might exert its fibronectin-binding enhancement effect by binding to positively charged groups on the gelatin affinity matrix.

The gelatin-agarose preparation used in the experiments shown in Fig. 1 was acylated with either succinic anhydride or acetic anhydride and then tested for affinity for fibronectin in experiments identical to those described above. The re-

ACYLATION OF GELATIN AND FIBRONECTIN BINDING 183

EFFLUENT VOLUME (ml)

FIG. 1. Effect of heparin on elution of fibronectin from gelatin-agarose. Purified human plasma flbronectin (1.5 mg in 1.2 ml 50 mM Tris-Cl buffer, pH 7.5) was applied at 0 ml to each of three identical columns (2 ml) of gelatin-agarose. (A) Equilibrated with Tris buffer alone; (B) pretreated with 5 mg heparin in 5 ml Tris buffer, followed by 8 ml of buffer; (C) fibronectin application followed, at 8 ml, with 5 mg heparin in 5 ml Tris buffer. Beginning at 21 ml, all columns were eluted sequentially with 6 ml each of 1,2,4, and 8 M urea in 50 mM Tris-Cl, pH 7.5.

sulk are shown in Fig. 2. Two major fea- tures of these experiments were evident: (i) Neither acylation treatment of the gelatin-agarose had a major effect on fibronectin binding; and (ii) heparin treatment of acylated gelatin-agarose, either before or after fibronectin application, failed to enhance fibronectin binding. Dissociation of fibronectin at lower urea concentration was somewhat greater in the acetylated compared with the succinylated medium, suggesting slightly weaker binding in response to the neutral acyl groups. Except for abolition of the enhancing effect of heparin, succinylated gelatin-agarose was in-

distinguishable in its fibronectin-binding behavior from the untreated medium.

An additional observation was that heparin application to the column following adsorption of fibronectin resulted in the dissociation of a significant amount of fibronectin from both acylated gelatin- agarose media during heparin treatment prior to elution with urea. A similar though smaller effect of heparin was seen in the corresponding experiment with nonacylated gelatin-agarose (Fig. 1C).

The direct interaction of heparin with normal and acetylated gelatin-agarose was investigated using heparin containing


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FIG. 2. Effect of heparin on elution of fibronectin from acylated gelatin-agarose. Two sets of three columns (2 ml each) containing acetylated (-) and succinylated (. . .) gelatin-agarose were treated in the same manner as corresponding columns of nonacylated medium in Fig. 1, i.e., (A) no heparin treatment; (B) columns treated with 5 mg heparin prior to fibronectin application; (C) columns treated with 5 mg of heparin following fibronectin application.

the fluorescent dansyl group (Fig. 3). Non- acetylated gelatin-agarose bound approxi- mately half of the 250 pg of dansyl heparin applied, which was subsequently eluted with 2 M NaCl. In contrast, acetylated gelatin-agarose bound no detectable heparin as dansyl heparin, nor did unsubstituted agarose (not shown). Recovery of the applied fluorescence was quantitative in these experiments.

Unlabeled and dansyl-labeled heparin enhanced the affinity of fibronectin for gelatin-agarose in a similar manner, as shown in Fig. 4. As with unlabeled heparin, the effect of dansyl heparin was to shift elution of a greater fraction of the gelatin- bound fibronectin to higher urea concen-

trations. Also, a substantial quantity of the column-bound heparin eluted along with fibronectin under the influence of 4 M

urea. When dansyl heparin application was followed by treatment of gelatin-agarose with 2 M NaCl to elute dansyl heparin before fibronectin application, the fibronectin elution profile of the affinity medium was the same as that for heparin-untreated medium. The somewhat stronger binding of fibronectin in these columns in the absence of heparin, compared to those in Fig. 1, is the result of using different preparations of gelatin-agarose in the two sets of experiments (compare Figs. 1A and 4A).


B

ZM P&Cl

I

I I 0 10 20

FRACTION NO. (1.5 ml)

FIG. 3. Interactions of dansyl heparin with gelatin- agarose (A), and with acetylated gelatin-agarose (B). Columns (2 ml) of each medium were equilibrated with 50 mM Tris-Cl, pH 7.5. A solution of 0.25 mg of dansyl heparin (1 ml) in Tris buffer was applied to each column, then eluted with 20 ml of buffer. At the arrow, each column was treated with 15 ml of 2 M NaCl in Tris buffer. Relative fluorescence of fractions (1.5 ml) was determined at 550 nm with excitation at 330 nm.

Under the conditions used for acylation of gelatin-agarose, free e-amino groups of lysyl residues are the primary sites of modification (21). To obtain a measure of the extent of acylation of accessible lysyl residues in the modified affinity medium, we subjected both acetylated and nonacetylated gelatin-agarose to guanidination with 0-methylisourea (20). The resulting conversion of lysyl to homoarginyl residues is revealed in amino acid analyses of hydrolysates as loss of lysine and a corresponding appearance of acid-stable homo- arginine. Acetylated lysyl residues are converted to free lysine by acid hydrolysis.

The lysine content of hydrolysates of these samples, shown normalized to proline content in Table I, is compared with the lysine content of nonguanidinated acetyl gelatin- agarose hydrolysate. Greater than 60% of lysyl residues was accessible to guanidination in nonacetylated gelatin-agarose, whereas almost no guanidination occurred on lysyl residues of the acetylated medium. Lysine content in a hydrolysate of acetylated, nonguanidinated gelatin-agarose was indistinguishable from that of nonacetylated, nonguanidinated medium (not shown). Other amino acids, represented here by alanine and proline, are unchanged by either acetylation or guanidination.

DISCUSSION

In this study we have reexamined the effects of heparin on the binding of fibronectin to gelatin. We have confirmed the observations of Ruoslahti and Engvall(12) that the dissociation of fibronectin from solid state gelatin requires higher urea concentration after heparin treatment of the complex than is required when no heparin is added. We also discovered, however, that pretreatment of gelatin-agarose with heparin prior to fibronectin application yielded results which were indistinguishable from those obtained when heparin was applied after fibronectin.

Whereas it has been generally accepted that the mechanism of heparin-enhancing effects on binding of fibronectin to other ligands such as gelatin involves an induction of fibronectin conformational changes by heparin, our initial observations suggested that the binding enhancement effects might be due merely to adsorption of an- ionic heparin to cationic groups on gelatin and subsequent multisite binding of fibronectin to both immobilized ligands.

To test our hypothesis that fibronectin binding is enhanced by gelatin-bound heparin, we treated gelatin-agarose with either acetic anhydride or succinic anhydride to acylate exposed amino groups on the affinity medium. The result was that, although fibronectin binding alone was only slightly affected, heparin enhance-


-60


FIG. 4. Effect of dansyl heparin upon fibronectin chromatography on gelatin-agarose. The gelatin- agarose in these experiments was different from that used in Fig. 1. Equilibration buffer was 50 mM

Tris-Cl, pH 7.5. (A) Buffer only; (B) and (C) treatment with 1.4 ml dansyl heparin (0.2 mg) followed by 10 ml of Tris buffer; (C) dansyl heparin was followed by 8 ml of 2 M NaCl in Tris buffer, then buffer alone. At 30 ml, 1.5 mg plasma fibronectin in 1.2 ml Tris buffer was applied to each column, followed by 6 ml Tris buffer, then each was treated sequentially with 6 ml each of 1, 2,4, and 8 M

urea in Tris buffer.

ment of binding was eliminated. Binding of heparin (as dansyl heparin) to gelatin- agarose was clearly demonstrated in our studies, and was shown to be eliminated by acetylation. In contrast, Ruoshlati and Engvall observed no binding of [35S]- heparin to gelatin-agarose (12). The reason for this difference is not clear, but it is un- likely that binding was significantly affected by the presence of the dansyl flu- orophore.

In this study we observed that heparin alone caused some dissociation of fibronectin from gelatin in the absence of added urea, a phenomenon which was enhanced

by gelatin acylation. This paradoxical behavior is consistent with the report by Bray et al. that extraction of fibronectin from tissues was facilitated by heparin (22).

Although precise quantitation of the extent of acylation of the gelatin-agarose used in these studies was not achieved, the guanidination experiments clearly demonstrated that, of the lysyl residues accessible to the guanidinating reagent, essentially no free amino groups remained after acetylation, or that any remaining amino groups did not contribute significantly to heparin binding by gelatin.


TABLE I

EFFECT OF GUANIDINATION UPON LYSINE CONTENT OF GELATIN-AGAROSE AND ACETYLATED

GELATIN-AGAROSE

mental results described here indicate that the stronger binding of fibronectin to gelatin-agarose in the presence of heparin is due to heparin itself binding to gelatin, allowing fibronectin to then bind to both immobilized ligands. Since simple multipoint attachment in this system adequately ex- plains the observed stronger reciprocal binding, the role of major conformational changes seen in fibronectin in response to glycosaminoglycans is not apparent. The observation made in our studies that heparin, under certain circumstances, actually weakens the interaction of fibronectin with gelatin suggests that glycosaminoglycans, and perhaps other fibronectin ligands, may alter its binding behavior in unexpected ways.

Ala/Pro Lys/Pro ALys”

Acetyl-gelatin- agarose

Guanidinated 0.81 0.138

acetyl- gelatin- agarose b

Guanidinated 0.80 0.136 -2%

gelatin- agarose* 0.81 0.053 ~624

n Change in lysine content, compared to nonguanidinated acetyl-gelatin-agarose.

* Guanidinated 3 days at 4°C in 0.5 M O-methylisourea.

Denatured collagen binds fibronectin more strongly than native collagen (2,23), and studies with defined collagen fragments have shown that a linear amino acid sequence in a region of the ~(1) chain near the site which is sensitive to mammalian collagenase interacts most strongly (23, 24). Thus, our observation that plasma fibronectin binding to gelatin is essentially unaffected by either succinylation or acetylation of accessible amino groups might have been expected since this region is not rich in lysyl residues.

It is clear that glycosaminoglycans, especially heparin and heparan sulfate, can produce substantial conformational changes in fibronectin (14-16). Also, there is abundant evidence that these glycosaminoglycans exert significant modulating effects upon the macromolecular organiza- tion and cell-interactive properties of ex- tracellular matrices, especially those containing collagen and fibronectin (25-27). It remains to be seen, however, whether such interactions among the known macromolecular ligands of fibronectin are mediated primarily by conformational changes in fibronectin or are simply the result of multiple points of bridging to otherwise inde- pendent domains. In any case the experi-

ACKNOWLEDGMENTS

We are grateful to Dr. Michael N. Blackburn for the generous gift of dansyl heparin and for helpful advice and discussions. We also thank Carol Ann Ardoin and Lucy Anderson for preparation of the manuscript.

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Acylation of gelatin-agarose and the enhancement by heparin of fibronectin binding

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