ABSTRACT
Tumor endothelial marker 7 (TEM7) was recently identified as an mRNA transcript overexpressed in the blood vessels of human solid tumors. Here, we identify several new variants of TEM7, derived by alternative splicing, that are predicted to be intracellular (TEM7-I), secreted (TEM7-S), or on the cell surface membrane (TEM7-M) of tumor endothelium. Using new antibodies against the TEM7 protein, we confirmed the predicted expression of TEM7 on the cell surface and demonstrated that TEM7-M protein, like its mRNA, is overexpressed on the endothelium of various tumor types. We then used an affinity purification strategy to search for TEM7-binding proteins and identified cortactin as a protein capable of binding to the extracellular region of both TEM7 and its closest homologue, TEM7-related (TEM7R), which is also expressed in tumor endothelium. The binding domain of cortactin was mapped to a unique nine-amino acid region in its plexin-like domain. These studies establish the overexpression of TEM7 protein in tumor endothelium and provide new opportunities for the delivery of therapeutic and imaging agents to the vessels of solid tumors.
Subject(s)
Biomarkers, Tumor/metabolism , Endothelium, Vascular/metabolism , Membrane Proteins/metabolism , Neoplasms/blood supply , Alternative Splicing , Amino Acid Sequence , Animals , Biomarkers, Tumor/biosynthesis , Biomarkers, Tumor/genetics , Chromatography, Affinity , Colorectal Neoplasms/blood supply , Colorectal Neoplasms/metabolism , Esophageal Neoplasms/blood supply , Esophageal Neoplasms/metabolism , Humans , Lung Neoplasms/blood supply , Lung Neoplasms/metabolism , Membrane Proteins/biosynthesis , Membrane Proteins/genetics , Mice , Molecular Sequence Data , Neoplasms/genetics , Neoplasms/metabolism , Protein Isoforms , RNA, Messenger/biosynthesis , RNA, Messenger/geneticsABSTRACT
Tumor endothelial marker (TEM)8 was uncovered as a gene expressed predominantly in tumor endothelium, and its protein product was recently identified as the receptor for anthrax toxin. Here, we demonstrate that TEM8 protein is preferentially expressed in endothelial cells of neoplastic tissue. We used the extracellular domain of TEM8 to search for ligands and identified the alpha 3 subunit of collagen VI as an interacting partner. The TEM8-interacting region on collagen alpha 3(VI) was mapped to its COOH-terminal C5 domain. Remarkably, collagen alpha 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.