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Advances in Brief |
Victorian Breast Cancer Research Consortium Invasion and Metastasis Unit [J. T. P., E. W. T.] and Cellular Signaling Laboratory [T. T.], St. Vincents Institute of Medical Research, Fitzroy, 3065, Australia, and Departments of Orthopaedic Surgery [A. A., E. W. T.], Cell Biology [D. D., E. W. T.], and The Vincent T. Lombardi Cancer Center [D. D., E. W. T.], Georgetown University Medical Center, Washington, D.C. 20007
| ABSTRACT |
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| Introduction |
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| Materials and Methods |
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Inhibitors.
PD153035 and AG1478, tyrosine kinase inhibitors with selectivity for the EGFR, were generous gifts from Dr. D. W. Fry (Parke-Davis Pharmaceutical Research, Ann Arbor, MI) and Dr. A. Levitzki (Jerusalem University, Jerusalem, Israel), respectively. The PI3k inhibitors wortmannin and LY294002 compound, the MEK inhibitor PD098059, and the PLC inhibitor U73122 were obtained from Calbiochem (Alexandria, Australia). The EGFR blocking antibody was purchased from Upstate Biotechnology, Inc. (Lake Placid, NY).
Chemomigration Assay.
Cell migration was determined using a 48-well microchemotaxis chamber assay (Neuroprobe, Cabin John, MD) as described previously (12)
. Cell migration was quantified by the number of cells that migrated directionally through a collagen IV (40 µg/ml; Becton Dickinson, Bedford, MA)-coated 8-µm-pore polyvinyl pyrrolidone-free polycarbonate filter (Poretics, Livermore, CA) toward the chemoattractant (12)
. Briefly, cells (1 x 106 cells/ml) were resuspended in serum-free RPMI 1640 containing 0.1% BSA (RPMI 1640/BSA) and were either untreated or pretreated at room temperature with inhibitors (15 min) or antibodies (45 min). Recombinant human EGF (Becton Dickinson) was diluted to the indicated concentrations and added to the lower wells. Chambers were incubated at 37°C in a humidified incubator in an atmosphere of 5% CO2:95% air for 4 h, after which the filters were removed, fixed, and stained with Diff-Quik (Baxer Scientific, McGaw Park, IL) and mounted on glass slides. Nonmigrated cells were removed by wiping with a cotton swab. At least four random fields of vision/well (x20 objective) were counted for quantitation of cell migration. Triplicate wells were performed in each assay, and the assay was repeated at least three times.
Proliferation Analysis.
Cells (2 x 103) were plated in triplicate wells overnight in a 96-well plate with 200 µl of RPMI 1640/FCS. Each well was gently washed twice with unsupplemented RPMI 1640, and then either positive control (RPMI 1640/FBS), negative control (RPMI 1640/BSA), or test media (various dilutions of EGF in RPMI 1640/BSA) were added. Separate plates were set aside for WST-1 analysis (Roche, Castle Hill, Australia) on days 1, 3, and 5. Reduction of the WST-1 dye, due to mitochondrial activity, is linearly proportional to the cell number and was determined by analysis of absorption at 450 nm using a Dynatech ELISA plate reader (Dynatech International, Chantilly, VA).
ERK2 Activation Analysis.
Cells were grown to 80% confluence and serum-starved in RPMI 1640/0.1% fetal bovine serum for 18 h, washed in PBS, and harvested with brief exposure to 0.05% trypsin and 0.02% EDTA. After trypsin inhibition with soybean trypsin inhibitor (0.5 mg/ml) for a period of 10 min, the cells were collected by centrifugation, washed once in RPMI 1640/BSA with soybean trypsin inhibitor and then washed once in RPMI 1640/BSA. The cells were then held in suspension in RPMI 1640/BSA during pretreatment with wortmannin or PD098059 in 0.1% DMSO (final concentration) or with vehicle alone (0.1% DMSO) for 15 min. The cells were then plated onto 6-well tissue culture plates that had been precoated overnight with collagen IV (40 µg/ml) or poly-L-lysine (100 µg/ml) in PBS at 4°C. The plates were then incubated for 30 min or 4 h at 37°C in a humidified incubator with an atmosphere of 5% CO2:95% air in either the presence or absence of EGF (10 ng/ml). After incubation, the plates were washed twice with ice-cold PBS, and the cells were lysed with 2x reducing sample buffer at 95°C, frozen on dry ice, and stored at -70°C until analysis. Samples were analyzed by 12% SDS-PAGE and immunoblotted with ERK2 monoclonal antibody IB3E9 (kindly provided by Mike Weber, University of Virginia, Charlottesville, VA).
| Results and Discussion |
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Because there was a clear dissociation between the migratory and proliferative responses to EGF in the MDA-MB-231 cell line, we used a number of standard approaches to identify the signal transduction pathways involved in the EGF-stimulated migratory response in these cells. Treatment of the cells with the PI3K inhibitor wortmannin (100 nM) resulted in a significant inhibition of both basal migration (from 100% to 44.20 ± 6.81%; P < 0.01) and EGF-induced migration (from 338.40 ± 15.15% to 117 ± 3.12%; P < 0.01; Fig. 2A
). Similar levels of inhibition were also seen with another PI3K inhibitor, LY294002 (data not shown). PI3K involvement has previously been shown in PDGF-stimulated migration (15
, 16)
; however, to our knowledge, it has yet to be demonstrated for EGF. The exact mechanism by which PI3K modulates cell migration has yet to be fully delineated. However, Azuma et al. (17)
postulated that activation of PI3K via growth factor receptors causes the activation and recruitment of the small GTPase Rac to membrane sites where ruffling and motility are to occur. Activated Rac at the membrane recruits a multiprotein complex, leading to cycles of activation and inactivation of its downstream effector gelsolin, an actin-modifying protein. The activation and inactivation of gelsolin lead to the severing and partial dissolution of actin filament networks and the creation of nucleation sites for actin reassembly (17)
. Gelsolin has been previously shown to be critical for EGFR-mediated migration in fibroblasts (5)
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In addition to PI3K and MAP kinases, PLC
is another major signaling molecule that lies downstream of EGFR signaling. Pretreatment of the MDA-MB-231 cells with the PLC inhibitor U73122 (2 µM) resulted in a significant reduction of the basal migration from 100% to 23.40 ± 3.80% (P < 0.006). In addition, the EGF-stimulated migration was also significantly inhibited from 335.60 ± 22.00% to 138.60 ± 25.20% (P < 0.01; Fig. 2C
). PLC involvement in growth factor-mediated migration has been demonstrated in relation to PDGF BB and insulin-like growth factor I-mediated migration in smooth muscle cells (19)
, as well as in the migration of fibroblasts in response to EGF stimulation (11)
. The activation of PLC by growth factor receptors leads to the hydrolysis of phosphatidylinositol-(4,5)-bisphosphate, and thus to the generation of diacylglycerol (DAG) and inositol triphosphate (IP3) which leads to the activation of protein kinase C and the release of intracellular stores of calcium, respectively. Both protein kinase C and calcium are effectors that have previously been reported to modulate processes required for cell motility (3)
. Mobilization of actin modifying proteins such as gelsolin from inactive sites to active cytoskeletal regions has also been linked to PLC activation, leading to cycles of actin dissolution and polymerisation, resulting in cell movement (20)
. Recent studies have indicated that the PI3K product phosphatidylinositol-(3,4,5)-P3 acts to directly and indirectly to enhance PLC
-catalyzed breakdown of phosphatidylinositol-(4,5)-bisphosphate (21)
. We examined whether PLC may be downstream of PI3K in the EGF-induced migration in the MDA-MB-231 cells. Simultaneous inhibition of PI3K and PLC resulted in further inhibition of cell migration, indicating that PI3K and PLC may lie along divergent pathways in the MDA-MB-231 cells (data not shown).
In conclusion, we clearly demonstrate the potent and reproducible migratory responses in the MDA-MB-231 breast cancer cell line. These responses are chemotactic rather than chemokinetic, indicating that breast cancer cells may move along EGFR ligand gradients. This would be particularly relevant to the production of such ligands by stromal cells, as has been demonstrated during pregnancy in the rat (22) . Interestingly, such migratory profiles can be seen in breast cancer cells that have superseded growth factor proliferative signals, such as activated ras, and, as such, represent viable therapeutic targets. The characterization of signaling pathways for EGFR-mediated migration, which involves PI3k and PLC, appears consistent with migratory responses to other polypeptide growth factors (e.g., PDGF and insulin-like growth factor I; Refs. 16 , 19 ) and may provide more general opportunities to target for growth factor-stimulated migration in general.
| ACKNOWLEDGMENTS |
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| FOOTNOTES |
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1 Supported in part by the Victorian Breast Cancer Research Consortium, a developmental pilot grant from the USPHS SPORE in Breast Cancer, USPHS Grant 2P50-CA58185-04 to the Lombardi Cancer Center, and the Lombardi Cancer Center Tissue Culture and Cytochemistry & Microscopy Core Facilities supported by USPHS Grant 2P30-CA-51008. T. T. is a National Health and Medical Research Council C. J. Martin Fellow. ![]()
2 To whom requests for reprints should be addressed, at VBCRC Unit, St. Vincents Institute of Medical Research, 41 Victoria Parade, Fitzroy, 3065, Australia. Phone: 61-3-9288-2569; Fax: 61-3-9416-2676; E-mail: rik{at}ariel.its.unimelb.edu.au ![]()
3 The abbreviations used are: EGFR, epidermal growth factor receptor; EGF, epidermal growth factor; PI3K, phosphatidylinositol 3'-kinase; PLC, phospholipase C; ERK, extracellular signal-regulated protein kinase; MAP, mitogen-activated protein; MEK, MAP kinase kinase; PDGF, platelet-derived growth factor. ![]()
Received 6/28/99. Accepted 9/17/99.
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