Intravital Imaging Illuminates Transforming Growth Factor β Signaling Switches during Metastasis

Transforming growth factor beta (TGF beta) has seemingly contradictory roles in tumor progression: it can promote metastatic invasion but also act as a tumor suppressor. Recently, two studies have used intravital imaging to unravel the role of TGF beta at different stages of the metastatic process. TGF beta promotes single cell motility, which enables invasion into blood vessels. However the activation of TGF beta signaling is a transient event and is not maintained at distant sites. The downregulation of TGF beta signaling at secondary sites then permits growth of secondary tumors. In the absence of TGF beta, cells are restricted to collective movement and lymphatic spread. Here, we discuss these findings and their potential implications. Cancer Res; 70(9); 3435-9. (C) 2010 AACR.