The tumor suppressor Smad4/DPC 4 as a central mediator of Smad function

The invertebrate and vertebrate Smad proteins have recently been identified as important mediators of the responses to transforming growth factor beta (TGF-beta) and related factors. We have previously shown that Smad3 and Smad4 (the product of the tumor suppressor gene DPC 4) strongly synergize as mediators of TGF-beta signaling, and that inactive carboxy-terminally truncated mutants of either Smad act as dominant-negative inhibitors of the natural TGF-beta response. The finding that Smad4, unlike Smad3, does not interact with the TGF-beta receptor, coupled with the distinct structural features of Smad4, raises the possibility that Smad4 cooperates not only with Smad3, but also with Smad1 and Smad2 to mediate signaling by TGF-beta family members.

Overexpression of Smad4 in 'animal caps' taken from Xenopus embryos induced both ventral and dorsal mesoderm, thereby mimicking the effects of TGF-beta family members - bone morphogenetic protein-2 (BMP-2) or BMP-4 and activin, respectively. Low levels of Smad4 mRNA coinjected with Smad1 or Smad2 mRNA also synergized to induce ventral or dorsal mesoderm, respectively. In addition, Smad4 synergized Smad2, as it does with Smad3, to induce gene expression from the promoter for plasminogen activator inhibitor-1. The carboxy-terminal domains of both Smad3 and Smad4 were required for this synergy. Finally, a short carboxy-terminal truncation of Smad4, previously identified as a mutation of DPC 4 in tumors, blocked nuclear translocation of wild-type Smads 1, 2, 3 and 4, consistent with our observation of a physical interaction between truncated Smad4 and the other Smads.

Our observations indicate that Smad4 cooperates with Smad1, Smad2 and Smad3 to act as a common mediator of signaling by TGF-beta-related factors, and provide a mechanism that explains the dominant-negative interference with receptor signaling that results from expression of the naturally occurring Smad4/DPC 4 truncation mutant.