CA2238257A1 - Endocytosis of amf-r and uses thereof in cancer therapy - Google Patents

Abstract

The present invention relates to the use of internalization of autocrine motility factor receptor (AMF-R) to target motile cells, which comprises at least one molecule which binds to autocrine motility factor receptor (AMF-R) and is internalized by motile cells.

Description

ENDOCYTOSIS OF AMF-R AND USES THEREOF IN CANCER THERAPY
BACKGROUND OF THE INVENTION
(a) Field of the Invention The invention relates to the use of the endocytosis or internalization of autocrine motility factor receptor (AMF-R) as a means to target motile cells, such as metastatic tumor cells.
(b) Description of Prior Art Expression of autocrine motility factor receptor (AMF-R) is associated with the acquisition of motile and metastatic properties by tumor cells (Nabi et al., 1992; Silletti and Raz, 1996). AMF-R is a cell surface receptor which mediates motility stimulation by its 55 kD polypeptide ligand, AMF, recently shown to be homologous to phosphohexose isomerase.
Transduction of the AMF motility signal occurs via receptor phosphorylation, a pertussis-toxin sensitive G-protein, inositol phosphate production, tyrosine kinase and protein kinase C activation and production of the lipoxygenase metabolite 12-HETE
(Silletti and Raz, 1996) . AMF-R is localized not only to the plasma membrane but also to an intracellular tubular organelle, the AMF-R tubule (Nabi et al., 1992;
Benlimame et al., 1995). AMF-R tubules are distinct from endosomes and lysosomes; by post-embedding immunoelectron microscopy AMF-R is present primarily in smooth tubules which extend from ribosome-studded cisternae however AMF-R tubules do not colocalize with ERGIC-53, a marker for the ER-Golgi intermediate compartment (Benlimame et al., 1995; Wang et al., 1997). Following treatment with ilimaquinone, AMF-R
tubules acquire a fenestrated morphology typical of smooth ER suggesting that the AMF-R tubule is a distinct smooth subdomain of the endoplasmic reticulum

- 2 -(Wang et al., 1997). The intracellular distribution of this cell surface receptor to smooth ER implicates AMF-R recycling in its function in cell motility and tumor cell metastasis.
It would be highly desirable to be provided with a means to target motile cells, such as metastatic tumor cells.
SUI~1ARY OF THE INVENTION
One aim of the present invention is to provide a means to target motile cells, such as metastatic tumor cells.
Another aim of the present invention is to provide the use of the endocytosis or internalization of autocrine motility factor receptor (AMF-R) as a means to target motile cells, such as metastatic tumor cells.
In accordance with the present invention, it is demonstrated that AMF-R is concentrated at the cell surface within smooth plasmalemmal vesicles or caveolae and that AMF is internalized via a non-clathrin pathway to intracellular smooth ER tubules. The results of the present invention identify an ANRt-R-mediated clathrin-independent internalization pathway to the endoplasmic reticulum which may be implicated in AMF-R function in tumor cell motility and metastasis.
In accordance with the present invention there is provided the use of internalization of autocrine motility factor receptor (AMF-R) to target motile cells, which comprises at least one molecule which binds to autocrine motility factor receptor (AMF-R) and is internalized by motile cells, such as metastatic tumor cells.
In accordance with the present invention there is also provided a therapeutical conjugate target to specifically kill motile cells, which comprises a

- 3 -molecule which binds to autocrine motility factor receptor (AMF-R) attached to a drug suitable to kill said motile cells, such as metastatic tumor cells.
In accordance with the present invention there is also provided a method to specifically kill cancer cells in vitro and/or in vi vo, which comprises administering an effective amount of the conjugate of the present invention.
The cells in vitro may be leukemias purging cells whereas the cells in vi vo may be metastatic tumor cells.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 illustrates the electron microscopic localization of AMF-R in NIH-3T3 fibroblasts and HeLa cells;
Fig. 2 illustrates the colocalization of AMF-R
and caveolin by confocal microscopy;
Fig. 3 illustrates bAMF and anti-AMF-R mAb colocalize on the cell surface;
Fig. 4 illustrates the internalization of bAMF
to AMF-R tubules;
Fig. 5 illustrates the localization of internalized bAMF to AMF-R tubules by confocal microscopy; and Fig. 6 illustrates the electron microscopy of the internalization pathway of bAMF.
DETAILED DESCRIPTION OF THE INVENTION
Autocrine motility factor receptor (AMF-R) is a cell surface receptor which is also localized to a smooth subdomain of the endoplasmic reticulum (ER), the AMF-R tubule. By post-embedding immunoelectron microscopy, AMF-R concentrates within smooth plasmalemmal vesicles or caveolae in both NIH-3T3 fibroblasts and HeLa cells. By confocal microscopy,

- 4 -cell surface AMF-R labeled by the addition of anti-AMF-R antibody to viable cells at 4°C exhibits partial colocalization with caveolin confirming the localization of cell surface AMF-R to caveolae.
Labeling of cell surface AMF-R by either anti-AMF-R
antibody or biotinylated AMF (bAMF) exhibits extensive colocalization and after a pulse of 1-2 hours at 37°C, bAMF accumulates in densely labeled perinuclear structures as well as fainter tubular structures which colocalize with AMF-R tubules. After a subsequent 2-4 hour chase bAMF is localized predominantly to AMF-R
tubules. Cytoplasmic acidification, blocking clathrin-mediated endocytosis, results in the essentially exclusive distribution of internalized bAMF to AMF-R
tubules. By confocal microscopy, the tubular structures labeled by internalized bAMF show complete colocalization with AMF-R tubules. bAMF internalized in the presence of a 10-fold excess of unlabeled AMF
labels perinuclear punctate structures, which are therefore the product of fluid phase endocytosis, but not AMF-R tubules demonstrating that bAMF targeting to AMF-R tubules occurs via a receptor-mediated pathway.
By electron microscopy, bAMF internalized for 10 minutes is located to cell surface caveolae and after 30 minutes is present within smooth and rough ER
tubules. AMF is therefore internalized via a receptor-mediated clathrin-independent pathway to smooth endoplasmic reticulum. The steady state localization of AMF-R to caveolae implicates these cell surface invaginations in AMF-R endocytosis.
MATERIALS AND METHODS
Cells and Cell Culture NIH-3T3 fibroblasts obtained from the ATCC were cloned and a highly spread clone was used for these studies. HeLa and NIH-3T3 cells were grown in an air-5~

- 5 -C02 incubator at constant humidity in Dulbecco's minimum essential medium (DMEM) containing non-essential amino acids, vitamins, glutamine and a penicillin-streptomycin antibiotic mixture (Gibco, Burlington, Ontario) supplemented with 5o fetal calf serum (Immunocorp, Montreal, Quebec) for HeLa or 10~
calf serum (Gibco, Burlington, Ontario) for NIH-3T3 cells.
Antibodies and chemicals Monoclonal antibody against AMF-R was used in the form of concentrated hybridoma supernatant (Nabi et al., 1990). Rabbit anti-caveolin polyclonal antibody was purchased from Transduction Laboratories (Lexington, KY), rabbit anti-biotin antibody from Sigma (St. Louis, Missouri), and rat anti-LAMP-1 from the Developmental Studies Hybridoma Bank (University of Iowa, Iowa City). Secondary antibodies conjugated to either fluorescein, Texas Red or 12 nm gold particles and streptavidin conjugated to fluorescein or Texas Red were purchased from Jackson Immunoresearch Laboratories (West Grove, PA). Texas Red conjugated human diferric transferrin was kindly provided by Dr. Tim McGraw (Columbia University, New York, NY). Streptavidin conjugated to 10 nm gold particles was purchased from Sigma. The secondary antibodies were designed for use in multiple labeling studies and no interspecies cross-reactivity was detected. To detect antibodies to AMF-R, secondary antibodies specific for the a chain of rat IgM were used.
Rabbit phosphohexose isomerase was purchased from Sigma and biotinylated with NHS-LC-biotin (Pierce, Rockford, Illinois) according to the manufacturer's instructions. To assess its purity, biotinylated

- 6 -phosphohexose isomerase was separated by SDS-PAGE, transferred to nitrocellulose, probed with horseradish peroxidase conjugated streptavidin (Jackson Immunoresearch Laboratories) and revealed by chemiluminescence.
Immunofluorescence Cells were plated on glass cover slips 2 days prior to each experiment at a concentration of 30,000 cells/35 mm dish. For AMF-R surface labeling, the cells were incubated in DMEM minus bicarbonate supplemented with 25 mM Hepes pH 7.2 and 2.5~ serum for 15 min at 4°C prior to labeling with anti-AMF-R primary antibody or biotinylated AMF at 4°C for 30 min. The cells were washed at 4°C and then fixed with 3$ paraformaldehyde in phosphate buffered saline (pH 7.4) supplemented with 0.1 mM Ca++ and 1 mM Mg++ (PBS/CM) for 15 min at room temperature. For caveolin labeling, after AMF-R surface labeling at 4°C and fixation as above, the cells were permeabilized with 0.2o Triton X-100 for 10 min, then extensively washed with PBS/CM containing to BSA. The cells were incubated with rabbit anti-caveolin polyclonal antibodies, washed, and then incubated with FITC goat anti-rat IgM to reveal anti-AMF-R and Texas Red donkey anti-rabbit IgG to reveal anti-caveolin.
Cell surface labeling with biotinylated AMF was revealed with rabbit anti-biotin antibody and fluorescent anti-rabbit secondary antibody.
For the AMF internalization studies, NIH-3T3 cells were pulsed with biotinylated AMF 0250-500 ug/ml) and chased at 37°C for the indicated periods of time prior to fixation by the addition of precooled (-80°C) methanol/acetone directly to the cells. After fixation, internalized bAMF was revealed with Texas Red

- 7 _ streptavidin and lysosomes and AMF-R tubules by anti-T-A_M_P-1 and anti-AMF-R antibodies, respectively, followed by the corresponding FITC-conjugated secondary antibodies. Disruption of clathrin coated pits and vesicles by cytoplasmic acidification was performed essentially as previously described (Heuser, 1989).
NIH-3T3 cells were pretreated with acidification medium (DMEM containing 5o calf serum and 50mM MES pH 5.5) for minutes at 37°C prior to addition of bAMF in 10 acidification medium for one hour at 37°C. To ensure that cellular acidification blocked clathrin-mediated endocytosis, Texas Red transferrin (50 ug/ml) was added to cells in regular or acidification medium for 30 minutes at 37°C after which the cells were fixed with 15 3o paraformaldehyde.
After labeling the coverslips were mounted in Airvol (Air Products and Chemicals Inc., Allentown, PA) and viewed in a Zeiss Axioskop fluorescent microscope equipped with a 63X Plan Apochromat objective and selective filters. Confocal microscopy was performed with the 60X Nikon Plan Apochromat objective of a dual channel BioRad 600 laser scanning confocal microscope equipped with a krypton/argon laser and the corresponding dichroic reflectors to distinguish fluorescein and Texas Red labeling. Confocal images were printed using a Polaroid TX 1500 video printer.
Electron microscopy Post-embedding immunolabeling for AMF-R was performed as previously described (Benlimame et al., 1995). Cells grown on petri dishes were rinsed and incubated at 37°C in Ringer's solution for 15 minutes before fixing in Ringer's solution containing 2~
paraformaldehyde and 0.2o glutaraldehyde for 30 minutes - g _ at 37°C. The fixed cells were rinsed in PBS/CM, scraped from the petri dish and collected by centrifugation.
The cell pellet was post-fixed for 30 minutes with 1$
osmium tetroxide in PBS/CM containing 1.5~ potassium ferrocyanide (reduced osmium), dehydrated and embedded in LR-White resin. Ultra-thin sections (80 nm) were blocked with 2$ BSA, 0.2 o gelatin in PBS/CM for 1 hour, and then incubated at room temperature with anti-AMF-R antibody for 1 hour followed by 12 nm gold conjugated goat anti-rat antibodies for 1 hour. The sections were then stained with 5o uranyl acetate and examined in a Philips 300 electron microscope. The numerical density of gold particles associated with plasma membrane, caveolae, clathrin coated pits and vesicles, smooth tubules and vesicles, and rough ER was determined. The length of the limiting membrane of the indicated organelles was measured using a Sigma-Scan measurement system and the gold particles associated with these organelles counted. Rough ER was defined by the presence of a linear array of membrane-associated ribosomes. Smooth vesicles attached to the plasma membrane or within 100 nm of the plasma membrane were considered to be caveolae. Control labeling with non-immune rat IgM antibodies was analyzed similarly.
To follow the endocytic pathway of AMF by electron microscopy, biotinylated AMF was internalized as described for the fluorescence studies and detected by postembedding labeling with streptavidin conjugated to 10 nm gold as described above. No labeling was observed in the absence of biotinylated AMF.

RESULTS
Localization of AMF-R to cell surface caveolae By post-embedding immunoelectron microscopy in NIH-3T3 and HeLa cells, AMF-R is primarily localized to smooth intracellular membranous tubules (Figs. lA,D), similar in morphology to those previously described in MDCK cells (Benlimame et al., 1995). HeLa (Figs. lA,B,C) and NIH-3T3 (Figs. 1D,E,F) cells were post-embedding immunolabeled with anti-AMF-R and 12-nm gold-conjugated anti-rat IgM secondary antibodies.
Typical AMF-R labeling of smooth tubules (Figs. lA,D, arrows) and cell surface caveolae (Figs. 1B,C,E,F, arrowheads) is shown. PM: plasma membrane. Bar = 0.2~un.
At the cell surface, AMF-R label localizes to smooth invaginations of the plasma membrane morphologically equivalent to caveolae (Figs.
1B,C,E,F). Quantification of the labeling revealed that the predominant AMF-R label is localized to smooth tubules and vesicles, flat regions of the plasma membrane and caveolae (Table 1).

Table 1 Localization of AMF-R in HeLa and NIH-3T3 cells by immunoelectron microscopy Smooth Rough Flat Caveolae Clathrin-tubules endoplasmicplasma coated and reticulummembrane pits vesicles and vesicles HeLa AMF-R

# gold particles660 34 147 52 3 Nm membrane 328.5 187.5 245.6 14.6 19.3 gold particIes/Nm201 0.150.18 0.60 3.56 + 0.18+
+ 0.04 + 0.08 0.53 0.13 Control # gold particles83 6 25 3 1 Nm membrane 307.5 95.2 211.0 16.1 5.0 gold particles/Nm0-27 0.06 0.12 0.19 + 0.20 + 0.08 + 0.03 + 0.05 0.10 + 0.21 NIH~T3 AMF-R

# gold particles640 74 296 44 2 Nm membrane 432.6 308.9 308.8 28.8 34.7 gold particIes/Nm1.48 0.24 0.96 1.53 + 0.06 + 0.10 + 0.06 + 0.10 0.30 + 0.06 Control # gold particles33 7 10 4 2 Nm membrane 303.1 109.8 138.1 31.7 6.2 gold particleslNm0.11 0.06 0.07 0.13 + 0.32 0.02 + 0.03 + 0.03 0.06 + 0.18 Gold particles associated with the indicated membrane organelles were counted and the density per dun membrane length determined. Control labeling was determined using a nonimmune rat IgM antibody (Benlimame et al., 1995).
While specific label was previously detected in the rough ER of MDCK cells (Benlimame et al., 1995), the density of labeling of rough ER tubules in NIH-3T3 and HeLa cells is reduced and at control levels. The density of AMF-R labeling of caveolae is equal to that of intracellular smooth tubules and vesicles in NIH-3T3 cells and greater than that of intracellular smooth tubules and vesicles in HeLa cells and essentially no AMF-R label is found within clathrin coated pits and vesicles. The density of AMF-R labeling in caveolae is increased relative to flat regions of the plasma membrane. However, based on the total number of gold particles at the plasma membrane, only 13 0 of cell surface AMF-R in NIH-3T3 and 26 o in HeLa cells is found within caveolae.
To assess whether cell surface AMF-R
colocalizes with caveolin, viable NIH-3T3 cells were surface labeled for AMF-R by the addition of anti-AMF-R
antibodies to viable cells at 4°C (Nabi et al., 1992) and then double immunofluorescently labeled after fixation and permeabilization with antibodies to caveolin (Fig. 2). Viable NIH-3T3 cells were labeled for cell surface AMF-R at 4°C (Fig. 2A) and for caveolin after fixation and permeabilization (Fig. 2B).
To demonstrate the colocalization of AMF-R and caveolin, confocal images from both fluorescent channels were superimposed (Fig. 2C, AMF-R in green and caveolin in red) and colocalization appears in yellow.
Bar = 20 dun.
While the punctate AMF-R surface label (Fig. 2A) did not completely colocalize with the finer caveolin labeling (Fig. 2B), confocal microscopy clearly revealed distinct points and patterns labeled for both cell surface AMF-R and caveolin (Fig. 2C, yellow). Peripheral regions densely labeled for both AMF-R and caveolin were frequently observed. The partial colocalization of cell surface AMF-R with caveolin is consistent with the fact that, based on the EM data, only 13 o of cell surface AMF-R was localized within the caveolae of NIH-3T3 cells.

Internalization of AMF
The ligand for AMF-R, AMF, is homologous to phosphohexose isomerase (Watanabe et al., 1996).
Phosphohexose isomerase (referred to here as AMF) was biotinylated and after separation by SDS-PAGE revealed a single major band after revelation of the blots with streptavidin-HRP (Fig. 3A). bAMF (phosphohexose isomerase) migrated as a single band in protein blots revealed with HRP-streptavidin (Fig. 3A). Confocal imaging of cell surface labeling of viable NIH-3T3 cells at 4°C with bAMF (Fig. 3B) or anti-AMF-R antibody (Fig. 3C). Confocal images from both fluorescent channels were superimposed (Fig. 3D; bAMF in green and AMF-R in red) and revealed a significant degree of colocalization in yellow. Bar = 20 um.
Cell surface labeling of NIH-3T3 cells by the addition of both biotinylated AMF (bAMF) (Fig. 3B) and anti-AMF-R at 4°C (Fig. 3C) revealed a high degree of colocalization (Fig. 3D, yellow) demonstrating that AMF
and antibodies to AMF-R recognize the same receptor.
The presence of spots labeled exclusively with either bAMF or anti-AMF-R may be due to the fact that the two were added together and may compete for the same site.
Pulse labeling of NIH-3T3 cells with bAMF for one or two hours resulted in the ability to detect both punctate structures as well as fainter tubular structures which colocalized with AMF-R tubules (Figs. 4A, B). NIH-3T3 cells were pulse labeled with bAMF at 37°C for one hour (Figs. 4A, B), for two hours and chased for 4 hours (Figs. 4C, D) or for one hour in medium acidified to pH 5.5 to disrupt clathrin-mediated endocytosis (Figs. 4E, F). After fixation with methanol/acetone, cells were double labeled with Texas Red-streptavidin to reveal bAMF (Figs. 4A, C, E) and anti-AMF-R mAb and FITC-conjugated anti-rat secondary antibody to reveal AMF-R labeling (Figs. 4B, D, F). To ensure that cellular acidification disrupted clathrin-mediated endocytosis of transferrin receptor, NIH-3T3 cells were incubated at 37°C with Texas Red transferrin for 30 minutes in regular medium (Fig. 4G) or in medium acidified to pH 5.5 (Fig. 4H) . Bar = 20 dun.
Under these conditions, the extent of punctate and tubular labeling varied between cells. Fibrillar labeling of bAMF was also observed and has been determined to be localized to the cell surface (not shown) . An extended chase of 2 or 4 hours after a two hour pulse resulted in decreased punctate labeling and the accumulation of bAMF labeling in tubular structures which colocalized with AMF-R tubules (Figs. 4C,D). The vast majority of the cells exhibited predominantly intracellular tubular labeling as well as cell surface fibrillar labeling. Following treatment of cells with acidified medium (pH 5.5) and disruption of clathrin coated pits and vesicles (Heuser, 1989), bAMF
internalized for one hour is localized to intracellular AMF-R tubules (Figs. 4E, F). In the acidified medium, internalized transferrin did not cluster in the perinuclear recycling compartment demonstrating that the acidification procedure did indeed disrupt clathrin-mediated endocytosis (Figs. 4G,H). bAMF is therefore internalized via a clathrin-independent endocytic pathway to the smooth ER.
The colocalization of bAMF labeled tubules with AMF-R tubules was confirmed by confocal microscopy (Fig. 5). NIH-3T3 cells were pulse labeled with bAMF at 37°C for one hour in regular medium (Figs. 5A-F), for one hour in medium acidified to pH 5.5 to disrupt clathrin-mediated endocytosis (Figs. 5G-I), or in regular medium in the presence of 10-fold excess unlabeled AMF (Figs. 5J-L) prior to fixation with methanol/acetone. bAMF was revealed with Texas Red-streptavidin (Figs. 5A, D, G, J), and AMF-R (Figs. 5B, H, K) or LAMP-1 (Fig. 5E) labeled with the appropriate primary antibodies and FITC-conjugated secondary antibodies. Confocal images from both fluorescent channels were superimposed (Figs. 5C, I, L: bAMF in red and AMF-R in green; F: bAMF in red and LAMP-1 in green) and colocalization appears in yellow. Bar = 10 dun.
Following a 1 hour bAMF internalization, internalized bAMF is localized to tubular structures which colocalize with AMF-R tubules (Figs. 5A-C) as well as to punctate structures which exhibit partial colocalization with LAMP-1 positive lysosomes (Figs. 5D-F). As seen here, the intense punctate labeling can hide the fainter tubular labeling of bAMF
in some cells (Figs. 4A, 5D). In acidification medium, the vast majority of bAMF labeling, aside from cell surface fibrils, is localized to tubules which colocalize with AMF-R tubules (Figs. 5G-I). bAMF
internalized for 1 hour in the presence of 10-fold excess unlabeled AMF is localized only to punctate structures and no labeling of AMF-R tubules can be detected (Figs. 5J-L). While the extent of tubular labeling of bAMF varies between cells under control conditions (Figs. 5A, D), in the presence of excess unlabeled AMF the localization of bAMF to AMF-R tubules is never observed (Figs. 5J-L). bAMF internalization to intracellular AMF-R tubules therefore occurs via a receptor-mediated process. The inability of excess AMF
to block bAMF internalization to punctate perinuclear structures, which exhibit partial colocalization with LAMP-1 labeled lysosomes, demonstrates that this labeling is not saturable and corresponds to non-specific fluid phase uptake. The disappearance of lysosomal labeling following extended chase times (Fig. 4C, D) is therefore most likely due to lysosomal degradation of fluid phase internalized bAMF.
The location of biotinylated AMF internalized at 37°C was determined by post-embedding electron microscopy with streptavidin-10 nm gold. Following a 10 minute pulse biotinylated AMF could be detected in caveolae (Figs. 6A, B). NIH-3T3 cells were pulsed with bAMF at 37°C for 10 (Figs. 6A,B,H) or 30 minutes (Figs. 6C,D,E,F,G,I). The localization of bAMF was revealed by postembedding labeling with 10 nm gold-conjugated streptavidin. After 10 minutes, bAMF is localized to cell surface caveolae (Figs. 6A, B). After a 30 minute pulse, bAMF is localized to caveolae and smooth vesicles (Figs. 6C,D) and also appears in intracellular membranous tubules (Figs. 6E,F,G) including distinctive smooth (Fig. 6E) and rough (Fig. 6F) ER elements. bAMF labeling of dense lysosomal structures is also detected (Fig. 6H,I). Bar = 0.1 um.
Following a 30 minute pulse, both caveolae and intracellular smooth and rough ER elements were labeled (Figs. 6C-G). Dense structures morphologically equivalent to lysosomes are also labeled and presumably correspond to the perinuclear structures densely labeled for internalized bAMF by immunofluorescence (Figs. 6H, I) .
DISCUSSION
AMF-R localization to caveolae By post-embedding immunoelectron microscopy AMF-R is localized to morphologically identifiable caveolae as well as to smooth ER tubules (Fig. l;
Tabletl). In contrast to polarized epithelial MDCK
cells (Benlimame et al., 1995), labeling of rough ER
tubules was not above background in either NIH-3T3 or HeLa cells indicating that AMF-R is a specific marker for smooth ER in these two cell types. The localization of AMF-R to caveolae was confirmed by the colocalization of cell surface AMF-R, labeled by the addition of anti-AMF-R to viable cells at 4°C, with caveolin by confocal fluorescence microscopy (Fig. 2).
By both postembedding immunoelectron microscopy and confocal double labeling with caveolin, only a minor portion of cell surface AMF-R actually distributes to caveolae identified either morphologically or by the presence of caveolin. Based on the labeling of AMF-R by immunoelectron microscopy, only about 50 of total cellular AMF-R is actually localized to caveolae (Table 1) .
Transduction of the AMF motility signal is mediated by a pertussis-toxin sensitive G protein, phosphorylation of AMF-R and both protein kinase C and tyrosine kinase activities (Silletti et al., 1996).
Caveolar cell surface domains have been proposed to be plasma membrane regions which serve to assemble molecules involved in receptor-mediated signal transduction, including heterodimeric G-proteins, protein kinase C and tyrosine kinases (Lisanti et al., 1994). The involvement of heterotrimeric G proteins as well as tyrosine kinase and protein kinase C activities in transduction of the AMF motility signal is consistent with the localization of AMF-R to cell surface caveolae.
Clathrin-independent internalization of AMF-R to smooth ER tubules The presence of AMF-R both at the cell surface and within an intracellular ER-associated organelle suggested that the receptor recycles between these two cellular sites (Nabi et al., 1992; Benlimame et al., 1995). The fact that the density of AMF-R labeling within caveolae is equivalent to (NIH-3T3) or greater than (HeLa) that of smooth vesicles and tubules is consistent with the concentration of AMF-R within caveolae prior to vesicle budding and fusion with AMF-R
tubules (Table 1).
AMF exhibits sequence identity to phosphohexose isomerase (Watanabe et al., 1996). Biotinylated phosphohexose isomerase or bAMF colocalizes with cell surface AMF-R labeled with antibodies to AMF-R at 4°C
and is endocytosed by cells at 37°C to tubules which colocalize with smooth ER AMF-R tubules by confocal microscopy and to morphologically identifiable ER
tubules by electron microscopy. bAMF internalization to smooth ER tubules is a receptor-mediated process as it can be blocked by the presence of excess unlabeled AMF.
Cellular acidification, which specifically blocks clathrin mediated endocytosis, disrupts the internalization of transferrin, but not that of bAMF to AMF-R tubules, demonstrating that AMF-R is internalized to AMF-R tubules via a non-clathrin endocytic pathway.
Under the conditions used in these experiments, internalization of bAMF to lysosomal structures is also observed. This lysosomal labeling is observed even in the presence of excess unlabeled AMF indicating that it is not receptor-mediated and due to fluid phase uptake.
We have therefore identified a clathrin-independent AMF-R-mediated endocytic pathway which targets bAMF to the endoplasmic reticulum. The localization of AMF-R
and internalized bAMF to cell surface caveolae by electron microscopy implicates these smooth invaginations of the plasma membrane in the endocytosis of AMF-R to the smooth ER subdomain for which it is a marker (Benlimame et al., 1995; Wang et al., 1997).
Role of caveolae in AMF-R internalization Whether caveolae are involved in endocytic processes in non-endothelial cells remains a controversial subject and whether clathrin-independent internalization routes involve caveolae or clathrin-coated pits without the clathrin is not clear (van Deurs et al., 1993).
The receptor-mediated endocytosis of bAMF not to endosomes and lysosomes but to the ER certainly suggests that bAMF endocytosis is not mediated by uncoated clathrin vesicles. SV40 virus associates with caveolae and is internalized via smooth plasmalemmal vesicles to smooth tubules which are extensions of the ER (Kartenbeck et al., 1989). The internalization pathway of SV40 to the ER (Kartenbeck et al., 1989) is therefore remarkably similar to that of AMF-R described here and the localization of both AMF-R and SV40 to cell surface caveolae certainly implicates caveolae in this ER directed endocytic pathway. AMF activation of AMF-R may stimulate both transduction of the AMF
motility stimulating signal and internalization of AMF-R, perhaps within the same cell surface caveolar domain.
AMF recycling and cell motility The established role of AMF-R in cell motility and metastasis implicates AMF-R internalization, and subsequent recycling to the cell surface, in the motile process (Nabi et al., 1992; Silletti and Raz, 1996).
This recycling pathway stimulated by the cytokine, AMF, may represent a motility specific membrane targeting pathway.
The present invention will be more readily un derstood by referring to the following examples which are given to illustrate the invention rather than to limit its scope.
EXAMPLE I
The internalization of cancer specific ligands as a means to target metastatic tumor cells has previously been demonstrated for the BR96 antigen. The monoclonal antibody BR96 is specific for the Le"

polylactosamine carbohydrate antigen expressed abundantly on numerous carcinomas (Hellstrom et al., 1990). The BR96 antibody has been shown to be internalized via coated pits to multivesicular bodies, endosomes and finally to lysosomes where it is degraded (Garrigues et al., 1993) Toxin conjugates of this internalizing monoclonal antibody, such as BR96 doxorubicin immunoconjugates or Pseudomonas PE40 exotoxin fusion proteins, effected complete regression of xenografted human carcinomas in athymic mice (Friedman et al., 1993; Trail et al., 1993).
While the invention has been described in con-nection with specific embodiments thereof, it will be understood that it is capable of further modifications and this application is intended to cover any varia-tions, uses, or adaptations of the invention following, in general, the principles of the invention and including such departures from the present disclosure as come within known or customary practice within the art to which the invention pertains and as may be applied to the essential features hereinbefore set forth, and as follows in the scope of the appended claims.

REFERENCES
Benlimame, N. et al. (1995) J. Cell Biol. 129, 459-471 Friedman, P.N. et al. (1993) J. Immunol. 150: 3054-3061 Garrigues, J. et al. (1993) Am. J. Pathol. 142: 607-622 Hellstrom, I. et al. (1990) Cancer Res. 50:2183-2190 Heuser, J. (1989) J. Cell Biol. 108, 401-411 Kartenbeck, J. et al. (1989) J. Cell Biol. 109, 2721-Lisanti, M.P. et al. (1994) Trends Cell Biol. 4:231-235 Nabi, I.R. et al. (1992) Cancer Met. Rev. 11, 5-20 Silletti, S. et al. (1996) Am. J. Pathol. 148, 1649-1660.
Trail, P.A. et al. (1993) Science 261:212-215 van Deurs, B. et al. (1993) Trends Cell Biol. 3, 249-251.
Wang, H.-J. et al. (1997) J. Cell Sci. 110, 3043-3053.
Watanabe, H. et al. (1996) Cancer Res. 56, 2960-2963.

Claims (7)

WHAT IS CLAIMED IS:

1. The use of internalization of autocrine motility factor receptor (AMF-R) to target motile cells, which comprises at least one molecule which binds to autocrine motility factor receptor (AMF-R) and is internalized by motile cells.

2. The use of claim 1, wherein said cells are metastatic tumor cells.

3. A therapeutical conjugate target to specifically kill motile cells, which comprises a molecule which binds to autocrine motility factor receptor (AMF-R) attached to a drug suitable to kill said motile cells.

4. The conjugate of claim 3, wherein said cells are metastatic tumor cells.

5. A method to specifically kill cancer cells in vitro and/or in vivo, which comprises administering an effective amount of the conjugate of claim 3.

6. The method of claim 5, wherein said cells in vitro are leukemias purging cells.

7. The method of claim 5, wherein said cells in vivo are metastatic tumor cells.