NIH Public Access Author Manuscript Cancer Res. Author manuscript; available in PMC 2010 April 15. Published in final edited form as: Cancer Res. 2009 April 15; 69(8): 3272–3277. doi:10.1158/0008-5472.CAN-08-4886.

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The L6 Protein TM4SF1 Is Critical for Endothelial Cell Function and Tumor Angiogenesis Shou-Ching Shih1, Andrew Zukauskas1, Dan Li1, Guanmei Liu1, Lay-Hong Ang2, Janice A. Nagy1, Lawrence F. Brown1, and Harold F. Dvorak1 1Center for Vascular Biological Research and Department of Pathology, Harvard Medical School, Boston, Massachusetts 2Imaging

Core Facility, Beth Israel Deaconess Medical Center, Harvard Medical School, Boston, Massachusetts

Abstract Transmembrane-4-L-six-family-1 (TM4SF1) was originally described as a cancer cell protein. Here, we show that it is highly expressed in the vascular endothelium of human cancers and in a banded pattern in the filopodia of cultured endothelial cells (EC). TM4SF1 knockdown prevented filopodia formation, inhibited cell mobility, blocked cytokinesis, and rendered EC senescent. Integrin-α5 and integrin-β1 subunits gave a similar staining pattern and interacted constitutively with TM4SF1, whereas integrin subunits often associated with angiogenesis (αV,β3,β5) interacted with TM4SF1 only after vascular endothelial growth factor (VEGF)-A or thrombin stimulation. TM4SF1 knockdown substantially inhibited maturation of VEGF-A164–induced angiogenesis. Thus, TM4SF1 is a key regulator of EC function in vitro and of pathologic angiogenesis in vivo and is potentially an attractive target for antiangiogenesis therapy.

Introduction Transmembrane-4-L-six-family-1 (TM4SF1), also known as L6, was first identified as a protein abundantly expressed in a variety of epithelial cancer cells (1,2) and weakly expressed in normal vascular endothelium (3,4). It has the topology of a tetraspanin (5); however, TM4SF1 and five other structurally similar proteins (TM4SF4, 5, 18, 19, and 20) lack overall sequence homology with the 33 genuine tetraspanins, and the characteristic CCG motif in the large extracellular loop (5). Therefore, these proteins have been classified separately as the L6 family (5). Experiments on tumor cells had previously shown TM4SF1 to be important for growth (1), motility (6), invasion (7), and metastasis (8). However, the role of TM4SF1 in vascular endothelium has not been investigated. We report here that TM4SF1 is overexpressed in the vascular endothelial cells (EC) of several human cancers and is critically important for their function.

©2009 American Association for Cancer Research. Requests for reprints: Shou-Ching Shih and Harold F. Dvorak, Pathology Department, Beth Israel Deaconess Medical Center, 99 Brookline Avenue, Boston, MA 02215. Phone: 617-667-8156/617-667-8529; Fax: 617-667-3591; sshih2@bidmc.harvard.edu and hdvorak@bidmc.harvard.edu. Note: Supplementary data for this article are available at

Cancer Research Online (www.mianfeiwendang.com). Disclosure of Potential Conflicts of Interest No potential conflicts of interest were disclosed.