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3(VI)
1 Program in Human Genetics and Molecular Biology and the Sidney Kimmel Comprehensive Cancer Center and 2 Howard Hughes Medical Institute, Johns Hopkins School of Medicine, Baltimore, Maryland; 3 Department of Neurological Surgery, University of Pittsburgh, Pittsburgh, Pennsylvania; 4 Imgenex Corp., San Diego, California; and 5 Tumor Angiogenesis Section, Mouse Cancer Genetics Program, National Cancer Institute at Frederick, Frederick, Maryland
| ABSTRACT |
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3 subunit of collagen VI as an interacting partner. The TEM8-interacting region on collagen
3(VI) was mapped to its COOH-terminal C5 domain. Remarkably, collagen
3(VI) is also preferentially expressed in tumor endothelium in a pattern concordant with that of TEM8. These results suggest that the TEM8/C5 interaction may play an important biological role in tumor angiogenesis. | Introduction |
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| Materials and Methods |
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Immunohistochemistry.
Paraffin sections were deparaffinized, incubated with proteinase K (Invitrogen), heated at 95°C for 20 min in citrate buffer (pH 6.0), and treated with peroxidase blocking reagent (DAKO). Sections were incubated with a polyclonal antibody against vWF (DAKO) or a monoclonal antibody against TEM8 (clone SB12) followed by a biotin-conjugated secondary antibody (Pierce). A third layer consisting of horseradish peroxidase-conjugated antibiotin (DAKO) was then followed by diaminobenzidine (Sigma) staining. Sections were counterstained with hematoxylin.
Immunofluorescence.
Dual-color immunofluorescence was performed on fresh-frozen sections fixed in Leukoperm (Serotec) and stained with anti-TEM8 clone SB12 and polyclonal anti-vWF (DAKO). vWF was detected with a FITC-conjugated antirabbit antibody (Jackson ImmunoResearch Laboratories), and TEM8 was detected using a biotin antimouse antibody (Jackson) followed by rhodaminestreptavidin (Vector). Sections were visualized by confocal microscopy.
In Situ Hybridization.
The in situ hybridization protocol described previously (3)
was adapted for use on paraffin sections. A detailed protocol can be obtained from the authors on request.
Western Blotting.
Samples were separated by SDS-PAGE and transferred to a polyvinylidene difluoride membrane (Millipore). Western blots were probed with an anti-TEM8 primary antibody, followed by a horseradish peroxidase-conjugated antimouse secondary antibody (Jackson), and visualized using the enhanced chemiluminescence plus system (Amersham) according to the suppliers instructions.
Immunoprecipitation.
The TEM8 extracellular region (amino acids 28320) and C5 deletion constructs (C5A, C5B, and C5C) were cloned into the myc-tagged AP-Tag5 vector (Genhunter) and sequence verified. Secreted fusion proteins from cotransfected 293 cells were incubated overnight with an anti-TEM8 antibody (clone SB5) or a nonspecific IgG control antibody. Precipitated proteins were eluted from protein G-agarose beads (Roche) and detected by Western blotting using an anti-myc monoclonal antibody (Clontech).
Two-Hybrid Analysis.
The two-hybrid screen was performed using a fetal brain cDNA library and the Matchmaker II system (Clontech), according to the manufacturers suggested procedure.
| Results and Discussion |
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3(VI). To confirm the interaction between TEM8 and collagen
3(VI), we tested the ability of TEM8 and collagen
3(VI) to coprecipitate when expressed in mammalian cells. Soluble extracellular TEM8 containing its I- domain (a.k.a. vWF-A domain) was expressed in 293 cells, along with myc-tagged proteins containing either the longest collagen
3(VI) insert from the two-hybrid experiment, a subfragment of this insert containing the entire C5 domain, or a truncated C5 domain. TEM8 antibodies readily coimmunoprecipitated those collagen
3(VI) proteins containing an intact C5 domain, whereas control IgG did not (Fig. 3)
3(VI).
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3(VI) and TEM8, we performed in situ hybridization analyses to localize expression of these genes in normal and malignant colonic tissues. Staining for collagen
3(VI) was strong in the endothelium of each colorectal tumor analyzed but was weak or undetectable in the corresponding matched normal colonic mucosa, a pattern of expression strikingly similar to that of TEM8 (Fig. 4)
3(VI) in these tissues (Fig. 4
3(VI) in tumor endothelium is coordinately regulated.
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3(VI) is important for angiogenesis: (a) the closest known homologue of TEM8, capillary morphogenic protein 2, has been shown to interact with the extracellular matrix via its I-domain (8)
; (b) collagen
3(VI) has been reported to be up-regulated in healing wounds, which are known to be rich in neovasculature (9)
; and (c) collagen
3(VI) was identified in our previous unbiased screen as one of a limited number of transcripts preferentially expressed in tumor endothelium among 32,500 total transcripts analyzed (2)
. Indeed, in the same experiments wherein TEM8 was identified as the 19th most preferentially expressed transcript, collagen
3(VI) was identified as the 12th. Although the cells producing collagen
3(VI) in tumors have not been well characterized previously, the serial analysis of gene expression data along with the in situ hybridization data strongly suggest that endothelial cells are a primary source (Fig. 4)
The studies described here implicate collagen
3(VI) as a ligand for TEM8. The interaction of these proteins and their coordinate expression in tumor endothelial cells suggests a carefully orchestrated role in angiogenesis. In this regard, recent studies of the dynamics of the C5 domain of collagen
3(VI) are of interest. The C5 domain of collagen
3(VI) is initially incorporated into the newly forming type VI collagen fibrils but immediately after secretion is cut off and not present in mature collagen VI-containing matrix (10)
. Antibodies specific to the C5 domain have shown reactivity with both the cytoplasm and immediate pericellular region of cells actively producing collagen
3(VI) (10
, 11)
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The C5 domain provides a new reagent for the selective targeting of tumor vasculature through TEM8. The validity of TEM8 as a target is supported by several recent studies using anthrax toxin as an antitumor agent. The expression pattern of TEM8 may help to explain the tumor regressions observed when anthrax toxin is injected into tumor-bearing mice at nontoxic doses (12, 13, 14) . Before the identification of TEM8 as the anthrax toxin receptor, inhibition of angiogenesis was postulated to mediate the antineoplastic effects of anthrax toxin, because the treated tumors appeared "white" and were found to be deficient in CD31-positive blood vessels (12) . Given the TEM8 protein expression patterns in tumor endothelium described here, it seems likely that TEM8 is responsible for this effect.
| FOOTNOTES |
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Note: A. Nanda and E. B. Carson-Walter contributed equally to this work.
Requests for reprints: Brad St. Croix, Tumor Angiogenesis Section, Mouse Cancer Genetics Program, National Cancer Institute at Frederick, Frederick, MD 21702; E-mail: stcroix{at}ncifcrf.gov
6 A. Nanda and B. St. Croix, unpublished data. ![]()
Received 8/ 4/03. Revised 11/14/03. Accepted 12/ 4/03.
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