Integrin receptors for cell adhesion to extracellular matrix have important roles in promoting tumor growth and progression. Integrin α3β1 is highly expressed in breast cancer cells where it is thought to promote invasion and metastasis; however, its roles in regulating malignant tumor cell behavior remain unclear. In the current study, we used short-hairpin RNA (shRNA) to show that suppression of α3β1 in a human breast cancer cell line, MDA-MB-231, leads to decreased tumorigenicity, reduced invasiveness, and decreased production of factors that stimulate endothelial cell migration. Real-time PCR revealed that suppression of α3β1 caused a dramatic reduction in expression of the cyclooxygenase-2 (COX-2) gene, which is frequently over-expressed in breast cancers and has been exploited as a therapeutic target. Decreased COX-2 was accompanied by reduced prostaglandin E2 (PGE2), a major prostanoid produced downstream of COX-2 and an important effector of COX-2 signaling. shRNA-mediated suppression of COX-2 showed that it has a role in tumor cell invasion and crosstalk to endothelial cells. Furthermore, treatment with PGE2 restored these functions in α3β1-deficient MDA-MB-231 cells. These findings identify a role for α3β1 in regulating two properties of tumor cells that facilitate cancer progression: invasiveness and ability to stimulate endothelial cells. They also reveal a novel role for COX-2 as a downstream effector of α3β1 in tumor cells, thereby identifying α3β1 as a potential therapeutic target to inhibit breast cancer.

The tetraspanin CD151 forms stoichiometric complexes with laminin-binding integrins (e.g., α3β1, α6β1, and α6β4) and regulates their ligand-binding and signaling functions. We have found that high expression of CD151 in breast cancers is associated with decreased overall survival (3.44-fold higher risk of death). Five-year estimated survival rates were 45.8% (95% confidence interval, 16.4-71.4%) for CD151-positive patients and 79.9% (95% confidence interval, 62.2-90.0%) for CD151-negative patients. Furthermore, CD151 was positively associated with axillary lymph node involvement. To study the biological significance of this observation, we investigated the contribution of CD151 in breast cancer tumorigenesis using MDA-MB-231 cells as a model system. Stable down-regulation of this tetraspanin by short-hairpin RNA decreased the tumorigenicity of these cells in mice. Detailed immunohistologic analysis of CD151 (+) and CD151(−) xenografts showed differences in tumor vascular pattern. Vascularization observed at the subcutaneous border of the CD151(+) tumors was less pronounced or absent in the CD151(−) xenografts. In vitro experiments have established that depletion of CD151 did not affect the inherent proliferative capacity of breast cancer cells in three-dimensional extracellular matrices, but modified their responses to endothelial cells in coculture experiments. The modulatory activity of CD151 was dependent on its association with both α3β1 and α6β4 integrins. These data point to a new role of CD151 in tumorigenesis, whereby it functions as an important regulator of communication between tumor cells and endothelial cells. These results also identify CD151 as a potentially novel prognostic marker and target for therapy in breast cancer. (Mol Cancer Res 2009;7(6):787-98)