Pathogenesis of Paget's Disease: Epidermal Heregulin-{alpha}, Motility Factor, and the HER Receptor Family

Vera R. J. Schelfhout, Elisabeth D. Coene, Bernard Delaey, Sofie Thys, David L. Page, Christian R. De Potter

Affiliations of authors: V. R. J. Schelfhout, E. D. Coene, S. Thys, C. R. De Potter, N. Goormaghtigh Institute for Pathology, University Hospital, Gent, Belgium; B. Delaey, N.V. Innogenetics, Zwijnaarde, Belgium; D. L. Page, Department of Pathology, Vanderbilt University Medical Center, Nashville, TN.

Correspondence to: Christian R. De Potter, M.D., N. Goormaghtigh Institute for Pathology, University Hospital, De Pintelaan 185, B-9000 Gent, Belgium (e-mail: Christian.DePotter{at}rug.ac.be).


    ABSTRACT
 Top
 Abstract
 Introduction
 Subjects and Methods
 Results
 Discussion
 References
 
Background and Methods: In Paget's disease of the breast, the epidermis of the nipple is infiltrated by large neoplastic cells of glandular origin. It has been hypothesized that the spread of Paget cells through the nipple epidermis is induced by a motility factor that acts via the HER2/NEU receptor. To test this hypothesis, we characterized and purified a motility factor released by keratinocytes and identified its target receptors in specimens from patients with Paget's disease and in SK-BR-3 breast adenocarcinoma cells, which overexpress HER2/NEU. Results: We isolated the motility factor from keratinocyte-conditioned medium and sequenced tryptic peptides. These sequences were used to identify the motility factor as heregulin-{alpha}, which is released by skin keratinocytes. Heregulin-{alpha} induces spreading, motility, and chemotaxis of SK-BR-3 cells, as does motility factor. Motility factor activities of heregulin-{alpha} are inhibited by monoclonal antibody AB2, directed against the extracellular domain of HER2/NEU, which blocks the binding of heregulin-{alpha}. We used in situ hybridization to show that normal epidermal cells produce heregulin-{alpha} messenger RNA and that heregulin receptors, HER3 and/or HER4, as well as their coreceptor HER2/NEU, are expressed by Paget cells. Conclusions: Heregulin-{alpha} is a motility factor that is produced and released by normal epidermal keratinocytes and thus plays a key role in the pathogenesis of Paget's disease. Paget cells express heregulin receptors HER2/NEU, as well as HER3 and/or HER4, both of which function as a co-receptor of HER2/NEU. Binding of heregulin-{alpha} to the receptor complex on Paget cells results in the chemotaxis of these breast cancer cells, which eventually migrate into the overlying nipple epidermis.



    INTRODUCTION
 Top
 Abstract
 Introduction
 Subjects and Methods
 Results
 Discussion
 References
 
Paget's disease of the breast occurs in about 1% of all patients with breast cancer. In this disease, the epidermis of the nipple is invaded by large neoplastic cells originating in the mammary gland. The overexpression of HER2/NEU protein in an unusually high percentage of patients with Paget's disease suggests that it may have an important role in the pathogenesis of this disease (14). HER2/NEU-overexpressing SK-BR-3 human breast cancer cells are attracted by a chemotactic factor released by normal epidermal cells (4) and by malignant keratinocytes (5). This chemotactic factor induced phosphorylation of HER2/NEU and motility of SK-BR-3 breast cancer cells. It was hypothesized that the chemotactic factor might act through the HER2/NEU protein because its activity could be inhibited by antibodies against the extracellular domain of HER2/NEU protein (4). Both findings suggest a chemotactic process in which HER2/NEU plays a vital role, although other members of the HER family of receptors might be involved. Indeed, it has been proposed that expression of multiple HER receptors is required for efficient ligand-induced transformation of cells (6,7), and multiple members of the HER family of receptors are expressed within normal and tumor cells (6,7).

Our aim was to further our understanding of the pathogenesis of Paget's disease by identifying and characterizing the factor(s) released by normal epidermal cells and their target receptors that are involved in the penetration and migration of breast cancer cells through the nipple epidermis.


    SUBJECTS AND METHODS
 Top
 Abstract
 Introduction
 Subjects and Methods
 Results
 Discussion
 References
 
Cell Culture

SK-BR-3 cells were grown in minimal essential medium Rega 3 that contains nonessential amino acids (BioWhittaker Inc., Walkersville, MD) supplemented with 10% fetal calf serum, 20 mM HEPES, 14.3 mM sodium bicarbonate, 50 IU/mL penicillin, 50 IU/mL streptomycin, and 2 mM L-glutamine (BioWhittaker Inc.).

Keratinocyte cultures were established from skin specimens of healthy adult donors and cultured on a feeder layer of mitomycin-C-treated Swiss 3T3 fibroblasts (8,9). After two serial passages at 1:3 split ratios, confluent cell layers were allowed to stratify for 1–2 weeks and form a multilayered epithelium. At the end of the stratification process, keratinocytes were placed in a basal medium containing Dulbecco's modified Eagle medium/Ham's F-12, 3:1 (vol/vol), for 18–24 hours; the conditioned medium, containing secreted growth factors, was then collected and stored at -20°C.

Motility and Chemotaxis

SK-BR-3 cells were detached with a solution of 0.05% trypsin and 0.5 mM EDTA in a Ca2+/Mg2+-free balanced salt solution or with 0.5 mM EDTA alone for antibody experiments. Approximately 1.5 x 104 SK-BR-3 cells were seeded per well in a 24-microwell plate in 1 mL of complete culture medium and incubated for 24 hours at 37°C in an atmosphere of 5% CO2/95% air and 100% humidity. Normal keratinocyte-conditioned medium or purified motility factor was added at various dilutions from 1:100 for conditioned medium to 1:10000 for purified factor for 1 hour at 37°C, and then cells were fixed in alcohol and stained with crystal violet. One unit of motility factor activity was defined as the amount of motility factor that causes spreading (i.e., a flat morphology and scattering of 50% of the cells within 1 hour).

To determine whether the HER2/NEU receptor was the target for the motility factor, we used the monoclonal antibody AB2 (Oncogene Research Products; Merck KGaA, Darmstadt, Germany) (10), which is directed against the extracellular domain of the HER2/NEU protein and blocks ligand binding to the receptor. We used AB2 at concentrations from 0.06 to 1 µg/mL and purified motility factor at a dilution that induced maximal spreading, scattering, and motility of SK-BR-3 cells.

Chemotaxis was analyzed in a modified Boyden chamber with the use of 24-microwell plates containing 6.5-mm polycarbonate microporous (pore size, 8 µm) cell culture inserts (TranswellTM; Corning Costar Corp., Cambridge, MA). In this assay, the lower chamber contained 600 µL of culture medium and the upper chamber contained 100 µL of culture medium and 2 x 105 SK-BR-3 cells. Normal human keratinocyte-conditioned medium or purified motility factor was added at various concentrations to the lower chambers for 24 hours. Cells on the filter were then fixed in methanol and stained with 0.1% nuclear fast red in water for 5 minutes. Cells that migrated through the filter were counted over the filter membrane and expressed as the number of cells per filter.

Chromatography

We used a total of 178 L of conditioned medium from cultured human keratinocytes. First, 10-L batches of medium were thawed, cleared by filtration over a 0.22-µm (pore size) filter, and adjusted to 10 mM sodium phosphate and 0.05% CHAPS (i.e., 3-[(3-cholamidopropyl)demethylammonio]-1-propanesulfonate) at pH 7.3. The medium was then fractionated over an 80-mL Biogel HTP hydroxyapatite column (Bio-Rad Laboratories, Hercules, CA), and the material was eluted with a phosphate buffer step gradient ranging from 50 to 400 mM. Active fractions eluted at the 400-mM step were further fractionated on a 4-mL Poros heparin column (50-µm beads; Perseptive Biosystems, Framingham, MA), and activity was eluted with a linear gradient of 150–750 mM NaCl, followed by a 1 M NaCl step. SK-BR-3-positive fractions that eluted between 700 and 750 mM NaCl were pooled, diluted 1:2 in high-pressure liquid chromatography-quality water, and adjusted to 0.2% trifluoroacetic acid. Batches of this material equivalent to 2.5 L of conditioned medium were loaded onto a 75-µL Zorbax microbore reversed-phase C8 column (5-µm bead size; LC Packings, San Francisco, CA), and material was eluted with a linear acetonitrile gradient of 6.4%–80%. SK-BR-3-positive fractions were eluted in a sharp peak at 27%–28% acetonitrile, with a minor part of the activity trailing at 28%–30% acetonitrile.

Sodium Dodecyl Sulfate–Polyacrylamide Gel Electrophoresis

Purified material from the reverse-phase C8 column step was vacuum dried, resuspended in Laemmli sample buffer, and subjected to sodium dodecyl sulfate–polyacrylamide gel electrophoresis (SDS–PAGE) on the PHASTTM system (Pharmacia LKB Biotechnology AB, Uppsala, Sweden) in 20% homogeneous polyacrylamide gels. The purified protein had an apparent molecular mass of 50 kd, as detected by silver staining. Preparative SDS–PAGE was performed on the PHAST system with an amount of purified material corresponding to 2.5 L of conditioned medium (divided over five lanes). This gel was stained with Coomassie brilliant blue R250.

Isolation, Purification, and In Situ Digestion of Motility Factor With Trypsin for Mass Spectrometry

The 50-kd protein band containing motility factor was excised from the preparative PHAST gel and destained in 50% acetonitrile in water. Gel pieces were completely dried in a vacuum centrifuge (Speed Vac; Savant Instruments, Inc., Farmingdale, NY) and immersed in 30 µL of digest buffer containing 0.08 µg of trypsin, 25 mM NH4HCO3, and 10% acetonitrile in water at pH 8.0 for 18 hours at 37°C. The resulting peptides were subsequently extracted with 50% acetonitrile and 0.1% trifluoroacetic acid in water, pooled, vacuum dried, dissolved in 20 µL of 0.1% formic acid, and analyzed by nano-liquid chromatography (NanoLC)–tandem mass spectrometry.

Identification of Motility Factor Use of NanoLC–Tandem Mass Spectrometry

NanoLC–tandem mass spectrometry was performed by on-line coupling of a FAMOSTM (LC Packings) controlled liquid chromatography system (Kontron, Milan, Italy) and Q-TOFTM (Micromass, Wytenshaw, U.K.) quadrupole-time-of-flight hybrid tandem mass spectrometer adapted with a Z-spay interface.

NanoLC separation was done on a PepMapTM (LC Packings) C18 column 150 mm long with an internal diameter of 75 µm by use of a column-switching technique. Peptides are first captured by a PepMap C18 µ-precolumn 2 mm long with an internal diameter of 0.8 mm at a flow rate of 20 µL/minute, after which the bound peptides were back flushed and gradually eluted and separated on the PepMap column with an internal diameter of 75 µm at a flow rate of 230 nL/minute. On-line mass spectrometry and tandem mass spectrometry spectra were acquired on Q-TOF mass spectrometer. Automated switching between mass spectrometry and tandem mass spectrometry was monitored by the MassLynxTM version 3.2 (Micromass) software. All obtained mass spectrometry and tandem mass spectrometry spectra were manually processed and screened for identity against the National Center for Biotechnology Information release nr 19990501 by use of the MASCOT software (http://www.matrixscience.com/).

Immunocytochemistry and Immunohistochemistry

We investigated the expression of HER1 (BioGenex Laboratories, San Ramon, CA), HER2/NEU (Calbiochem Corp., La Jolla, CA), HER3 (Novocastra Laboratories Ltd., New Castle, U.K.), and HER4 (Probio, Kent, U.K.) in SK-BR-3 cells and in biopsy specimens from 30 patients with Paget's disease by the use of specific antibodies from the suppliers indicated. Cells were grown on chamber slides and fixed in methanol at -20°C for 5 minutes, followed by acetone at 4°C for 2 minutes; immunocytochemistry was then performed. Biopsy specimens from 30 patients with Paget's disease of the nipple were formalin fixed, paraffin embedded, sectioned, and deparaffinized; immunohistochemistry was then performed. HER1, HER2/NEU, HER3, and HER4 were detected by the biotin–streptavidin–peroxidase method. For the immunochemistry of HER1, HER3, and HER4, deparaffinized sections were digested with 0.4% pepsin in 0.02 N HCl for 2 hours at 37°C.

Western Blotting

Nuclear and cytoplasmic fractions of SK-BR-3 cells were separated by use of the Pierce nuclear and cytoplasmic extraction reagent kit (Pierce Chemical Co., Rockford, IL). Cytoplasmic fractions were subjected to electrophoresis in 3%–8% Tris–acetate gels and Tris–acetate SDS running buffer (Novex, San Diego, CA) at 150 V for 2 hours. The proteins were transferred to a nitrocellulose membrane (Novex) in Tris–acetate transfer buffer (Novex) for 1 hour at 30 V. The membranes were blocked for 2–3 hours with a solution of 10% milk, phosphate-buffered saline (PBS), and 0.1% Triton X-100 at room temperature and then incubated with the primary antibodies to HER1 (1:100 dilution), HER2/NEU (1:5000 dilution), HER3 (1:20 dilution), or HER4 (1:400 dilution) overnight at 4°C. Membranes were washed in PBS and 0.1% Triton X-100 and blocked with 10% milk, PBS, and 0.1% Triton X-100. Sheep anti-mouse immunoglobulin G (IgG) peroxidase at 120 U (1:1000 dilution; Roche Molecular Biochemicals, Mannheim, Germany) was used for HER1, HER2/NEU, and HER3. Sheep anti-rabbit IgG peroxidase, 200 U (1:4000 dilution; Roche Molecular Biochemicals), was used for HER4. Proteins were detected by the biochemiluminescence technique (Roche Molecular Biochemicals) (11) and hyperfilm enhanced chemiluminescence development (Amersham Pharmacia Biotech, Uppsala, Sweden).

In Situ Hybridization

To evaluate messenger RNA (mRNA) expression, 10 randomly selected specimens were subjected to mRNA in situ hybridization. The following two DNA probes were used to visualize heregulin-{alpha} mRNA: 5`-FLUO-GCTCTCGGCGCAGGCGAGTTTGGTCCAAGGGCTCGGATCG-3` and 5`-FLUO-GCTCGGACATCTCGCCGGAGACGGAGCGCTCTACGCGGACG-3` (where FLUO is fluorescein isothiocyanate), corresponding to the predicted sequence of base pairs 62–101 and 102–142, respectively. These sequences were specific for heregulin-{alpha}, as shown by a thorough search of public sequence databases.

Five-micrometer sections were deparaffinized and incubated with 4% paraformaldehyde in PBS containing 1% diethylpyrocarbonate (DEPC) followed by 0.1 M glycine in PBS/DEPC for 5 minutes. The slides were incubated with 0.3% Triton X-100 in PBS/DEPC for 10 minutes, and proteins were digested with 10% proteinase K (Dako, Glostrup, Denmark) for 15 minutes. Slides were then postfixed in 4% paraformaldehyde. The slides were prehybridized with hybridization solution (BioGenex Laboratories) for 90 minutes at 42°C and hybridized with probe at 2.84 pg/mL preheated to 95°C for 10 minutes. The slides were then heated to 80°C for 2 minutes and incubated in a moist chamber at 37°C overnight. Between each step, the slides were rinsed with PBS and DEPC and, after hybridization, the slides were washed at 40°C in standard saline citrate. The fluorescein isothiocyanate (FITC)-coupled probe was detected with mouse anti-FITC (BioGenex Laboratories) for 20 minutes and amplified with anti-mouse biotin for 20 minutes, streptavidine peroxidase (Dako) for 20 minutes, and 3-amino-9-ethylcarbazole chromogen solution (Dako) for 10 minutes. After each step, slides were washed in PBS. The slides were counterstained with hematoxylin and mounted in Aquatex (Merck KGaA).


    RESULTS
 Top
 Abstract
 Introduction
 Subjects and Methods
 Results
 Discussion
 References
 
Motility and Chemotaxis

In the absence of motility factor, cultured SK-BR-3 breast cancer cells attached to a culture plate and remained spherical for 3 days. When human keratinocyte-conditioned medium or purified motility factor is added, cells rapidly became motile by flattening, forming long thin plasma membrane protrusions and pseudopodia, and moving apart (Fig. 1Go). Cells began to spread within 15 minutes of the addition of conditioned medium or purified factor and completed spreading 1–2 hours later.



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Fig. 1. A) The majority of SK-BR-3 breast cancer cells have a round morphology. Scale bar = 10 µm. B) The morphology of SK-BR-3 cells changes drastically after the addition of purified heregulin-{alpha}. The cells spread out and form long, thin pseudopodia. Scale bar = 10 µm.

 
When cells were preincubated with monoclonal antibody AB2, directed against the extracellular domain of HER2/NEU (data not shown), cell motility was inhibited in a concentration-dependent fashion. Spreading and cell movement were maximally inhibited at an AB2 antibody concentration of 1 µg/mL.

A modified Boyden chamber assay was used to investigate the role of the motility factor in chemotaxis. The number of SK-BR-3 cells moving through the filter increased in a concentration-dependent fashion when the concentration of purified factor in the lower compartment was increased (Fig. 2Go). After a 30-minute preincubation with monoclonal antibody AB2 (1 µg/mL), cell migration was almost completely inhibited (Fig. 2Go).



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Fig. 2. Purified motility factor from keratinocyte-conditioned medium was tested for chemotaxis on SK-BR-3 breast cancer cells in a modified Boyden chamber assay. The total number of cells migrating through a filter is indicated versus the concentration of motility factor in units. Inhibition of the chemotactic effect after the preincubation cells with monoclonal antibody AB2, to inhibit the binding of motility factor, is also indicated (*).

 
Protein Identification

Motility factor was purified from 178 L of human primary keratinocyte-conditioned medium by hydroxyapatite chromatography, heparin-affinity chromatography, and reversed-phase chromatography. Throughout purification, motility factor activity was monitored with the SK-BR-3 spreading assay. Motility factor activity in the reverse-phase C8 purified fractions corresponded to a 50-kd protein band on SDS–PAGE gels (Fig. 3Go). The total amount of purified motility factor protein (estimated at <2 µg) was too low for sequencing by Edman degradation, and thus mass spectrometry was used to identify and sequence material in the 50-kd band (12). A small amount of purified material (corresponding to 2.5 L of conditioned medium) was separated by preparative SDS–PAGE on the PHAST system (Pharmacia LKB Biotechnology AB). In total, five lanes were loaded, with a per lane amount corresponding to approximately 0.5 L of unpurified conditioned medium (or 2277 U of activity). After electrophoresis, the 50-kd motility factor bands were excised and pooled. Material in the band was digested with trypsin, and all tryptic peptides were analyzed by NanoLC–tandem mass spectrometry.



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Fig. 3. A) Polyacrylamide gel electrophoresis of motility factor. Fractions from the reverse-phase C8 column purification step (see Fig. 1Go) containing motility factor activity were pooled and loaded onto a PHASTTM gel (Pharmacia LKB Biotechnology AB, Uppsala, Sweden), subjected to electrophoresis, and silver stained. The amount of each fraction loaded on the gel corresponded to 0.125 L of unpurified conditioned medium. The total activity per lane is indicated below the lanes. B) Peptides identified by tandem mass spectrometry are in boldface type and underlined in the sequence of the heregulin-{alpha} precursor (GenBank accession No. A43273). Exons for which corresponding sequences have been identified are indicated in boldface type (exon boundaries are indicated with vertical arrows).

 
Several motility factor peptide sequences were identical to heregulin-{alpha} peptide sequences (GenBank accession No. A43273). In total, sequence information from four heregulin tryptic peptides was obtained (Table 1Go). In some cases, the peptide mass, as deduced from the mass spectrometry data, does not correspond to the theoretical peptide mass of the corresponding heregulin peptide, presumably because of secondary modifications, such as methionine oxidation, cysteine propionamide, or internal cystine-bridge formation. As shown in Fig. 3Go, all peptides identified are derived from the N-terminal part of the heregulin-{alpha} precursor and contain sequences from exons 2, 3, 4, 6, and 7 (13). From the peptide YLCKCQPGFTGAR (residues 208–220), we conclude that the heregulin isoform in this preparation is a heregulin-{alpha} isoform, which differs in this part of the epidermal growth factor-like domain from the ß isoforms (14) and the heregulin-{gamma} isoform (15). However, because additional peptide sequences in the epidermal growth factor-like domain or further downstream were not detected, it has been impossible to determine the specific {alpha} isoform.


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Table 1. Peptide masses and tandem mass spectrometry
 
Immunocytochemistry and Immunohistochemistry

To determine whether Paget cells and SK-BR-3 cells express heregulin receptors HER3 and HER4, as well as their co-receptor HER2/NEU, 30 tissue specimens from patients with Paget's disease of the breast were investigated by immunohistochemistry and SK-BR-3 cells were investigated by immunocytochemistry.

HER2/NEU, HER3, and HER4 were detected in SK-BR-3 cells (Fig. 4Go). HER4 and particularly HER2/NEU were detected mainly on small dot-like extensions of the cell, consistent with a previous report (16). HER3 was detected more diffusely throughout the cell. HER1 was not detected in SK-BR-3 cells (Fig. 4Go).



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Fig. 4. A) SK-BR-3 breast cancer cells show strong signals for HER2, HER3, and HER4 but no signal for HER1. HER2/NEU and HER4 are located on small dot-like extensions, whereas HER3 is more diffuse. Inset) Western blot showing a strong signal at 180 kd for HER4. B) Epidermis of the nipple containing scattered Paget cells. HER1 is usually not detected in Paget cells but is detected on the membrane in basal layers of the normal epidermis. HER2/NEU, HER3, and HER4 are detected on the membranes of Paget cells.

 
Of the 30 specimens tested for HER receptors, HER2/NEU was detected in the Paget cell membranes of 26 specimens (86.6%; Fig. 4Go and Table 2Go) but was not detected in the normal epidermal cells and dermis. HER3 was detected on the membranes and in the cytoplasm of Paget cells of 17 specimen (56.6%), and HER4 was detected on the membranes and in the cytoplasm of Paget cells of 23 specimens (76.6%). HER1 was detected in only four specimens (13%) but was detected weakly on membranes of the basal layers of the epidermis (Fig. 4Go). Of the 26 specimens that overexpressed HER2/NEU, 11 also expressed both HER3 and HER4, four also expressed HER3, and 11 also expressed HER4. Thus, the receptor complex consisting of HER2/NEU and HER3, HER4, or both is present in almost all cases of Paget's disease of the breast.


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Table 2. Characteristics of 30 patients with mammary Paget's disease according to HER status*
 
Specimens of underlying breast tissue from 12 patients were available, all of which contained ductal carcinoma in situ. In these specimens, the HER-expression pattern in the ductal carcinoma in situ and in the Paget cells was the same, indicating the clonal relationship of both lesions.

Western Blotting

Western blot analysis of SK-BR-3 cells showed the absence of HER1, a strong 185-kd band for HER2/NEU, a faint 170-kd band for HER3, and a strong 180-kd band for HER4 (Fig. 4Go).

In Situ Hybridization

To confirm that the normal epidermal cells surrounding the penetrating Paget cells in the nipple could express the motility factor heregulin-{alpha}, we used in situ hybridization to detect heregulin-{alpha} mRNA in sections containing Paget's disease tissue and normal tissue. In all cases, normal epidermal cells and normal ductal cells expressed heregulin-{alpha} mRNA (Fig. 5Go), but Paget cells expressed little or no heregulin-{alpha} mRNA (Fig. 5Go). These findings are consistent with the production and release of heregulin-{alpha} by cultured epidermal cells.



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Fig. 5. In situ hybridization for heregulin-{alpha} messenger RNA shows diffuse cytoplasmic staining in normal epidermal cells.

 

    DISCUSSION
 Top
 Abstract
 Introduction
 Subjects and Methods
 Results
 Discussion
 References
 
We show that normal human keratinocytes of the skin produce and secrete a motility factor that induces chemotaxis in HER2/NEU-overexpressing human breast cancer cells. We identified the motility factor as heregulin-{alpha} by the purifying and sequencing of this factor and showed, by in situ hybridization, that heregulin-{alpha} mRNA is produced by normal epidermal cells. With SK-BR-3 cells, both keratinocyte-conditioned medium and purified heregulin-{alpha} rapidly induced spreading, the formation of pseudopodia, and chemotaxis (Fig. 6Go), which was followed by rapid translocation and dispersal of cells. Plasma membrane protrusions, such as pseudopodia, are necessary in spreading, attachment, and locomotion of cells (17,18). HER2/NEU is preferentially located on pseudopodia and microvilli (16,19). Fractionation studies (20) of microvilli indicate that HER2/NEU is associated with the microfilament core through a stable interaction with a transmembrane complex that also contains actin. The role of heregulin-{alpha} in the pathogenesis of Paget's disease is consistent with the role of heregulin-{alpha} in the rabbit's ear model of excisional wound repair (i.e., stimulation of epidermal migration and proliferation). NDF-{alpha}2 (NEU-differentiating factor-{alpha}2) induces epidermal migration (21) and exerts no mitogenic effect on cultured keratinocytes (22).



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Fig. 6. Pathogenesis of Paget's disease as a consequence of the motility factor heregulin-{alpha}. Heregulin-{alpha} messenger RNA is produced by the nipple epidermal keratinocytes, which secrete heregulin-{alpha} protein. Heregulin-{alpha} then binds to a heterodimer of HER3 and HER2/NEU or of HER4 and HER2/NEU on Paget cells. HER1 is expressed on the membranes of some normal epidermal cells and does not appear to be involved in the pathogenesis of Paget's disease.

 
Because heterodimerization of HER2/NEU with HER1, HER3, or HER4 is required for transforming activity and phosphorylation of HER2/NEU (7) and because cells transfected only with HER2/NEU are unable to form colonies in agar (23), we investigated the expression of all members of the HER family. As expected from experiments at the cellular level, Paget cells also expressed HER3 and/or HER4. This finding is consistent with the requirement for the expression of multiple types of HER receptors in cells to obtain efficient phosphorylation and subsequent signal transduction (7,24,25). The expression pattern of HER4 in Paget's disease is surprising, given that the expression of HER4 is thought to be restricted to the central nervous system and the developing heart, yet this study shows by immunohistochemistry and western blotting that Paget cells (i.e., malignant epithelial cells) do overexpress HER4. Furthermore, although initially described as the ligand for the HER2/NEU protein (14,26) but later shown to be the ligand of HER3 (6,27) and HER4 (24), heregulin-{alpha} requires the expression of multiple HER receptor types for efficient signal transduction. This observation is consistent with the coexpression of HER2/NEU and at least one of the other members of the HER family in Paget cells. HER3 and HER4 are found on the cell membrane and sometimes also in the cytoplasm of Paget cells because the receptor molecules can be internalized after a ligand has bound (7). The alternative explanation, the lack of antibody specificity, seems less likely because HER3 and/or HER4 are detected in the cytoplasm of Paget cells only when HER3 and/or HER4 are detected on the cell membranes.

As in extramammary Paget's disease (3), four of 30 specimens of Paget's disease did not overexpress HER2/NEU. These cases may represent another pathogenesis that does not require overexpression of HER2/NEU and members of the HER family for migration of adenocarcinoma cells in the epidermis. Alternatively, these cases might be artifacts, reflecting false-negative tests in poorly fixed material.

We cannot exclude the possibility that other members of the NDF/heregulin family of ligands for HER, generated by alternative splicing of pro-NDF/heregulin mRNA, may be involved in the pathogenesis of mammary Paget's disease. This could occur if these ligands were much less abundant in keratinocyte-conditioned medium and not detected by SDS–PAGE. However, if this were the case, the pathogenesis of Paget's disease would remain the same (28,29). Additional experiments will be required to clarify this issue.

In conclusion, heregulin-{alpha}, a motility factor released by keratinocytes of the nipple, plays a key role in the pathogenesis of Paget's disease by attracting breast cancer cells to spread throughout the nipple epidermis. The motility factor acts on its receptors HER3 or HER4 or both, as well as on their co-receptor HER2/NEU, which are expressed by Paget cells, to induce chemotaxis and subsequent spread of Paget cells throughout the nipple epidermis.


    REFERENCES
 Top
 Abstract
 Introduction
 Subjects and Methods
 Results
 Discussion
 References
 

1 Lammie GA, Barnes DM, Millis RR, Gullick WJ. An immunohistochemical study of the presence of c-erb-2 protein in Paget's disease of the nipple. Histopathology 1989;15:505–14.[Medline]

2 Meissner K, Riviere A, Haupt G, Loning T. Study of neu-protein expression in mammary Paget's disease with and without underlying breast carcinoma and in extramammary Paget's disease. Am J Pathol 1990;137:1305–9.[Abstract]

3 Wolber RA, Dupuis BM, Wick MR. Expression of c-erbB-2 oncoprotein in mammary and extramammary Paget's disease. Am J Clin Pathol 1991;96:243–7.[Medline]

4 De Potter CR, Eeckhout I, Schelfhout AM, Geerts ML, Roels HJ. Keratinocyte induced chemotaxis in the pathogenesis of Paget's disease of the breast. Histopathology 1994;24:349–56.[Medline]

5 De Corte V, De Potter C, Vandenberghe D, Van Laerebeke N, Azam M, Roels H, et al. A 50 kDa protein present in conditioned medium of COLO-16 cells stimulates cell spreading and motility, and activates tyrosine phosphorylation of neu/HER-2 in human SK-BR-3 mammary cancer cells. J Cell Sci 1994;107:405–16.[Abstract/Free Full Text]

6 Sliwkowski MX, Schaefer G, Akita RW, Lofgren JA, Fitzpatrick VD, Nuijens A, et al. Coexpression of erbB2 and erbB3 proteins reconstitutes a high affinity receptor for heregulin. J Biol Chem 1994;269:14661–5.[Abstract/Free Full Text]

7 Cohen BD, Siegall CB, Bacus S, Foy L, Green JM, Hellstrom I, et al. Role of epidermal growth factor receptor family members in growth and differentiation of breast carcinoma. Biochem Soc Symp 1996;63:199–210.

8 Rheinwald J, Green H. Serial cultivation of strains of human epidermal keratinocytes: the formation of keratinizing colonies from single cells. Cell 1975;6:331–43.[Medline]

9 Green H, Kehinde O, Thomas J. Growth of cultured epidermal cells into multiple epithelia suitable for grafting. Proc Natl Acad Sci U S A 1979;76:5665–8.[Abstract]

10 van de Vijver MJ, Peterse JL, Mooi WJ, Wisman P, Lomans J, Dalesio O, et al. Neu-protein overexpression in breast cancer. Association with comedo-type ductal carcinoma in situ and limited prognostic value in stage II breast cancer. N Engl J Med 1988;319:1239–45.[Abstract]

11 Thorpe GH, Kricka LJ, Moseley SB, Whitehead TP. Phenols as enhancers of the chemiluminescent horseradish peroxidase–luminol–hydrogen peroxide reaction: application in luminescence-monitored enzyme immunoassays. Clin Chem 1985;31:1335–41.[Abstract/Free Full Text]

12 Mann M, Wilm M. Error-tolerant identification of peptides in sequence databases by peptide sequence tags. Anal Chem 1994;66:4390–9.[Medline]

13 Falls DL, Rosen KM, Corfas G, Lane WS, Fischbach GD. ARIA, a protein that stimulates acetylcholine receptor synthesis, is a member of the neu ligand family. Cell 1993;72:801–15.[Medline]

14 Holmes WE, Sliwkowski MX, Akita RW, Henzel WJ, Lee J, Park JW, et al. Identification of heregulin, a specific activator of p185erbB2. Science 1992;256:1205–10.[Medline]

15 Schaefer G, Fitzpatrick D, Sliwkowski M. {gamma}-Heregulin: a novel heregulin isoform that is an autocrine growth factor for the human breast cancer cell line, MDA-MB-175. Oncogene 1997;15:1385–94.[Medline]

16 De Potter CR, Quatacker J. The p185erbB2 protein is localized on cell organelles involved in cell motility. Clin Exp Metastasis 1993;11:453–61.[Medline]

17 Vasiliev JM. The role of pseudopodial reactions in the attachment of normal and transformed cells. In: Schweiger HG, editor. International cell biology 1980–1981. Berlin (Germany): Springer-Verlag; 1981. p. 774–8.

18 Wiechen K, Dietel M. c-erbB-2 anti-sense phosphorothioate oligodeoxynucleotides inhibit growth and serum-induced cell spreading of p185c-erbB-2 overexpressing ovarian carcinoma cells. Int J Cancer 1995;63:604–8.[Medline]

19 De Potter CR, Quatacker J, Maertens G, Van Daele S, Pauwels C, Verhofstede C, et al. The subcellular localization of the neu protein in human normal and neoplastic cells. Int J Cancer 1989;44:969–74.[Medline]

20 Carraway CA, Carvajal ME, Li Y, Carraway KL. Association of p185neu with microfilaments via a large glycoprotein complex in mammary carcinoma microvilli. Evidence for a microfilament-associated signal transduction particle. J Biol Chem 1993;268:5582–7.[Abstract/Free Full Text]

21 Danilenko DM, Ring BD, Lu JZ, Tarpley JE, Chang D, Liu N, et al. Neu differentiation factor upregulates epidermal migration and integrin expression in excisional wounds. J Clin Invest 1995;95;842–51.[Medline]

22 Marikovsky M, Lavi S, Pinkas-Kramarski R, Karunagaran D, Liu N, Wen D, et al. ErbB-3 mediates differential mitogenic effects of NDF/heregulin isoforms on mouse keratinocytes. Oncogene 1995;10:1403–11.[Medline]

23 Cohen BD, Green JM, Foy L, Fell HP. HER4-mediated biological and biochemical properties in NIH 3T3 cells. Evidence for HER1-HER4 heterodimers. J Biol Chem 1996;271:4813–8.[Abstract/Free Full Text]

24 Plowman GD, Green JM, Culouscou JM, Carlton GW, Rothwell VM, Buckley S. Heregulin induces tyrosine phosphorylation of HER4/p180erbB4. Nature 1993;366:473–5.[Medline]

25 Plowman GD, Culouscou JM, Whitney GS, Green JM, Carlton GW, Foy L, et al. Ligand-specific activation of HER4/p180erbB4, a fourth member of the epidermal growth factor receptor family. Proc Natl Acad Sci U S A 1993;90:1746–50.[Abstract]

26 Peles E, Bacus SS, Raymond A, Koski RA, Lu HS, Wen D, Ogden SG, et al. Isolation of the neu/HER-2 stimulatory ligand: a 44 kd glycoprotein that induces differentiation of mammary tumor cells. Cell 1992;69:205–16.[Medline]

27 Carraway KL 3rd, Sliwkowski MX, Akita R, Platko JV, Guy PM, Nuijens A, et al. The erbB3 gene product is a receptor for heregulin. J Biol Chem 1994;269:14303–6.[Abstract/Free Full Text]

28 Wen D, Suggs SV, Karunagaran D, Liu N, Cupples RL, Luo Y, et al. Structural and functional aspects of the multiplicity of Neu differentiation factors. Mol Cell Biol 1994;14:1909–19.[Abstract]

29 Weiss FU, Wallasch C, Campiglio M, Issing W, Ullrich A. Distinct characteristics of heregulin signals mediated by HER3 or HER4. J Cell Physiol 1997;173:187–95.[Medline]

Manuscript received September 21, 1999; revised January 12, 2000; accepted January 28, 2000.


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