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Advances in Brief |
1 Department of Anatomy and Cellular Biology, Tufts University School of Medicine, Boston, Massachusetts, and 2 Departments of Research and Medicine, Veterans Affairs Medical Center, Northport, New York
| ABSTRACT |
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| Introduction |
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Emmprin was originally identified as a factor on the surface of tumor cells that induces matrix metalloproteinase (MMP) production in fibroblasts and endothelial cells (1 , 4, 5, 6, 7) . More recently it has been shown that increased production of emmprin also stimulates MMP-2 production and increased invasiveness in tumor cells themselves (3 , 6 , 8 , 9) , most likely due to homophilic interaction between emmprin molecules on adjacent cells (9) . However, in vivo, one of the most marked responses to overexpression of emmprin is enhanced tumor growth (3) . Consequently we have investigated whether emmprin alters the growth properties of tumor cells in vitro. We chose MDA MB-436 and MCF-7 human mammary carcinoma cells for this study because they are relatively benign and express low levels of emmprin compared with more aggressive tumor cell lines. Using these cells, we find that increasing emmprin expression induces their ability to grow in an anchorage-independent manner, most likely due to increased activity of cell survival signaling pathways.
Hyaluronan is a very high molecular weight polysaccharide composed of repeating disaccharide units, (ß 1,4-GlcUA- ß 1,3-GlcNAc-)n. It serves a structural role in many connective tissues but is also associated with the pericellular matrix surrounding proliferating and motile cells in normal and pathological systems, where it serves both structural and signaling functions (10 , 11) . In particular, recent work has shown that interaction of hyaluronan with receptors on the surface of tumor cells induces anchorage-independent growth and plays a critical role in cell survival (11, 12, 13, 14) . Thus, we determined whether emmprin stimulates hyaluronan production and its effects on cell survival, and found that this is indeed the case.
| Materials and Methods |
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Soft Agar Assay.
Soft agar assays were performed as published previously (15)
. Cells were harvested, washed, and resuspended in 2x growth medium. Five thousand cells in 1 ml of 2x growth medium, with or without 100 µg/ml hyaluronan oligomers (Seikagaku Corp.), 100 µg/ml chitin oligomers (Seikagaku Corp.), 50 nM wortmannin (Sigma), or 20 µM LY294002 (Sigma), were mixed with 1 ml of 0.4% agarose (Life Technologies, Inc.) for plating, as described previously (15)
. After incubation for 1314 days, the cultures were processed for staining. Colonies >0.15 mm in size were counted under x10 magnification using an ocular scale lens.
Hyaluronan Assay.
The hyaluronan content of conditioned medium was determined from cultures of 1 x 104 cells/well collected over a 24-h incubation period (after preincubation for 24 h and washing), using a competitive ELISA-like method (16)
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Reverse Transcription-PCR.
Semiquantitative reverse transcription-PCR was carried out by published methods (17)
. The primers used were identical to those used previously, except for Has1, in which case the forward primer was 5' GTCTGTGACTCGGACAC 3', and the reverse primer was: 5' CTAGAGGACCGCTGATG 3'.
Western Blot Analyses.
Similar numbers of each type of cell were grown under routine monolayer conditions and harvested at 8090% confluence. Because vector- and emmprin-transfected cells grew at approximately the same rates in monolayer culture, the numbers of cells harvested from each were similar. The cells were lysed and processed for SDS-PAGE and Western blot analyses by standard methods as described previously (15)
. Equal amounts of protein (15 µg) were used for the Western blot analyses.
| Results |
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We then tested the effect of increased emmprin expression on growth of colonies in soft agar. This is a measure of the ability of cells to grow in an anchorage-independent manner, a characteristic of transformed cells (18
, 19)
. We found that wild-type and vector-transfected cells formed few colonies, whereas the emmprin-transfected cells formed numerous large colonies in soft agar (Fig. 1)
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2-fold increase in hyaluronan levels over controls (Fig. 2A)
3-fold (Fig. 2B)
25% increase; data not shown).
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4-fold stimulation of Has1,
10-fold stimulation of Has2, and
3.5-fold stimulation of Has3 expression. Emmprin caused
2-fold stimulation of Has1,
17-fold stimulation of Has2, and
3-fold stimulation of Has3 in MCF-7 cells (Fig. 2C)
Hyaluronan Oligosaccharides Inhibit Emmprin-Enhanced Anchorage-Independent Growth.
We have shown above that emmprin stimulates both anchorage-independent growth and hyaluronan production. Next we tested whether the effect of emmprin on anchorage-independent growth is dependent on hyaluronan. We have shown previously that perturbation of endogenous hyaluronan interactions by treatment with hyaluronan oligosaccharides (oligomers) inhibits anchorage-independent growth of several tumor cell types (15
, 21)
. These oligomers compete for interaction of endogenous hyaluronan polymer with cell surface receptors and consequently attenuate hyaluronan-dependent signaling (15)
. Therefore, we tested the effect of the hyaluronan oligomers on emmprin-stimulated formation of colonies in soft agar. We conducted the assay with emmprin-transfected MDA MB-436 and MCF-7 cells in the presence and absence of 100 µg/ml hyaluronan oligomers, determined previously as a reliably effective concentration (15)
. Chitin oligomers were used as a control because they are very similar in structure to hyaluronan oligomers but do not affect soft agar colony formation (15)
. Treatment with the hyaluronan oligomers reversed the effect of emmprin on colony formation in both MDA MB-436 and MCF-7 cells, whereas the chitin oligomers did not have a significant effect (Fig. 3)
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We then measured the effect of emmprin on phosphorylation of downstream components of the PI3k cell survival pathway, e.g., Akt and BAD. Increased expression of emmprin caused elevated phosphorylation of both Akt and BAD (Fig. 4A)
. The latter was measured at amino acid position ser136, the site of Akt phosphorylation (22)
. Cotreatment with hyaluronan oligomers reversed this effect (Fig. 4A)
. No effect on total Akt levels was observed (Fig. 4A)
. As noted previously (13)
, the level of phosphorylation of BAD at ser136 was low in MCF-7 cells. Consequently we also examined phosphorylation of extracellular signal-regulated kinase (Erk) and of BAD at ser112, the site of phosphorylation by Erk, because Erk-mediated phosphorylation of BAD also promotes cell survival (22)
. In both cases, stimulation due to increased emmprin expression and reversal by cotreatment with hyaluronan oligomers were clearly observed (Fig. 4B)
. Unlike MCF-7 cells, pBAD136 levels were high in MDA MB-436 cells and, as noted above, were inhibited by hyaluronan oligomers (Fig. 4A)
. Nevertheless, we also examined pErk levels in MDA MB-436 cells but found only modest stimulation as a result of increased emmprin expression (data not shown). Thus, emmprin stimulation of the Akt pathway is more prominent in MDA MB-436 cells, whereas stimulation of the Erk pathway is more prominent in MCF-7 cells, and both are reversed by hyaluronan oligomers. Finally, because anchorage-independent growth is associated with altered integrin signaling, we also examined the effect of emmprin and cotreatment with hyaluronan oligomers on phosphorylation of focal adhesion kinase (FAK), a central component in integrin-mediated cell survival signaling (23)
. As expected we found that emmprin stimulates FAK phosphorylation, and hyaluronan oligomers reverse this effect (Fig. 4C)
. No effects on total FAK expression were observed (data not shown).
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| Discussion |
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A consequence of emmprin-induced stimulation of hyaluronan production in the less malignant human mammary carcinoma cells used herein is enhancement of their ability to grow in an anchorage-independent manner, one of the major hallmarks of transformed cells (18 , 19) . Our previous studies have demonstrated the importance of interactions between hyaluronan and tumor cell surface receptors, especially CD44, in regulating cell survival signaling (13, 14, 15 , 21) . Accordingly, we found that emmprin stimulates cell survival signaling in a hyaluronan-dependent fashion. Cell survival signaling via the PI3k and Erk pathways is critical to resistance of malignant tumor cells to apoptosis. In accord with this dependence on cell survival pathways, we found recently that emmprin enhances drug resistance and that this effect is dependent on hyaluronan (13) . In addition it has been found that drug resistance in malignant cancer cells correlates with emmprin expression (8) .
Our data suggest that the emmprin-induced increase in hyaluronan production is due to elevated expression of hyaluronan synthases. However, it is possible that other factors, such as decreased degradation, may contribute to the increase in hyaluronan. Future studies will focus on the precise mechanisms whereby emmprin stimulates hyaluronan synthase expression and on whether emmprin also affects hyaluronan turnover. In preliminary studies (data not shown) we also examined the influence of emmprin on organization of hyaluronan at the cell surface. In several cell types, hyaluronan forms a cell-associated template for formation of a highly hydrated pericellular matrix (10) . We found that increased emmprin causes enhancement of pericellular matrix assembly, but this phenomenon could only be observed at cell densities approaching confluence, not in sparse cultures. The relationship of this pericellular matrix to malignant cell properties clearly warrants further study.
It is apparent that emmprin is a major controlling factor of activities that induce malignancy in cancers, both with respect to tumor cell properties and to tumor cell-stromal cell interactions. This conclusion is supported by the observation that experimental enhancement of emmprin expression in less malignant human breast cancer cells stimulates both growth and invasiveness in vivo (3) .
| ACKNOWLEDGMENTS |
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| FOOTNOTES |
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The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked advertisement in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
Note: S. Misra, S. Ghatak, and B. Toole are currently at the Department of Cell Biology and Anatomy, Medical University of South Carolina, 173 Ashley Avenue, Charleston, SC 29425.
Requests for reprints: Bryan Toole, Department of Cell Biology and Anatomy, Medical University of South Carolina, 173 Ashley Ave, Charleston, SC 29425. E-mail: toolebp{at}musc.edu
Received 9/ 8/03. Revised 12/22/03. Accepted 1/ 7/04.
| REFERENCES |
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