Autocrine transforming growth factor-beta signaling mediates Smad-independent motility in human cancer cells.

Transforming growth factor-beta (TGF-beta) is a pleiotropic growth factor that plays a critical role in modulating cell growth, differentiation, and plasticity. There is increasing evidence that after cells lose their sensitivity to TGF-beta-mediated growth inhibition, autocrine TGF-beta signaling may potentially promote tumor cell motility and invasiveness. To understand the molecular mechanisms by which autocrine TGF-beta may selectively contribute to tumor cell motility, we have generated MDA-MB-231 breast cancer cells stably expressing a kinase-inactive type II TGF-beta receptor (T beta RII-K277R). Our data indicate that T beta RII-K277R is expressed, can associate with the type I TGF-beta receptor, and block both Smad-dependent and -independent signaling pathways activated by TGF-beta. In addition, wound closure and transwell migration assays indicated that the basal migratory potential of T beta RII-K277R expressing cells was impaired. The impaired motility of T beta RII-K277R cells could be restored by reconstituting TGF-beta signaling with a constitutively active TGF-beta type I receptor (ALK5(TD)) but not by reconstituting Smad signaling with Smad2/4 or Smad3/4 expression. In addition, the levels of ALK5(TD) expression sufficient to restore motility in the cells expressing T beta RII-K277R were associated with an increase in phosphorylation of Akt and extracellular signal-regulated kinase 1/2 but not Smad2. These data indicate that different signaling pathways require different thresholds of TGF-beta activation and suggest that TGF-beta promotes motility through mechanisms independent of Smad signaling, possibly involving activation of the phosphatidylinositol 3-kinase/Akt and/or mitogen-activated protein kinase pathways.