Tetraspanin CD151 is associated with laminin-binding integrins and controls tumor cell migration and invasion. By analyzing responses of breast cancer cells to various growth factors, we showed that depletion of CD151 specifically attenuates transforming growth factor beta 1 (TGF beta 1)-induced scattering and proliferation of breast cancer cells in three-dimensional Matrigel. CD151-dependent cell scattering requires its association with either alpha 3 beta 1 or alpha 6 integrins, but it is independent of the recruitment of CD151 to tetraspanin- enriched microdomains. We also found that CD151 regulates the compartmentalization of TGF-beta type I receptor (T beta RI/ALK-5) and specifically controls the TGF beta 1-induced activation of p38. In contrast, signaling leading to activation of Smad2/3, c-Akt, and Erk1/2 proteins was comparable in CD151(+) and CD151(-) cells. Attenuation of TGF beta 1-induced responses correlated with reduced retention in the lung vascular bed, inhibition of pneumocyte-induced scattering of breast cancer cells in three-dimensional Matrigel, and decrease in experimental metastasis to the lungs. These results identify CD151 as a positive regulator of TGF beta 1-initiated signaling and highlight the important role played by this tetraspanin in TGF beta 1-induced breast cancer metastasis. Cancer Res; 70(14); 6059-70. (C) 2010 AACR.