A central paradox in transforming growth factor beta (TGF-?) biology is how the same growth factor can induce such divergent responses as growth stimulation (i.e., mesenchymal cells) and growth inhibition (i.e., epithelial cells)? Considering the pivotal role TGF-? has in a number of normal and pathological conditions, addressing that issue is fundamental if we hope to develop specific intervention strategies. To that end, we have been investigating the general hypothesis that the cellular response to TGF-? is dependent upon an integrated action of the trafficking and signaling machinery. In support of that proposal, we provide evidence that (i) a unique motif in the type II TGF-? receptor (T2R) regulates basolateral membrane delivery via a non-canonical mechanism; (ii) sorting nexin 9 (SNX9) distinguishes Smad3-dependent responses from Smad2; and (iii) the induction of ErbB family members is critical to the mesenchymal cell phenotype regulated by TGF-?. In this competing renewal we will extend these concepts using a variety of biochemical, biological, and morphologic approaches. First, we will determine the role of the mammalian retromer complex in regulating TGF-?R trafficking. As a number of diseases result from defects in the ability to sort or transport proteins to defined cellular locales, characterizing the operative trans-acting factors provides potential mechanisms to alter the cellular response to TGF-?. Second, the mechanism(s) by which SNX9 controls Smad3 transcriptional activity will be characterized. Given that the majority of Smad-dependent transcription is mediated via Smad3, these studies raise the possibility that intervention strategies can be developed to enhance or diminish Smad3-specific responses. Third, the role of EGF family members in pro-fibrotic TGF-? signaling will be defined. Since TGF-? is regulated by the concerted action of both direct and indirect mediators, our finding that ErbB ligands are fundamental for the pro-fibrotic phenotype seen following addition of TGF-? suggests new paradigms to the genesis of tissue fibrosis.