Investigating SMAD involvement in transforming growth factor-beta signal transduction: Identification, characterization and transcriptional regulation of Gadd45beta

Normal epithelial cells are in constant communication with their surrounding environment, largely through the detection, interpretation, and response to extracellular signaling molecules. The Transforming Growth Factor-beta (TGFbeta) superfamily of growth factors comprises over 30 such signaling molecules in humans, many of which figure prominently in development, tissue homeostasis and disease. Cells interpret the TGFbeta signal by integrating a multitude of signals into a cell type and context-dependent transcriptional profile. The transcripts comprising this profile are functionally responsible for the resulting phenotypic response, and consequently their identification and characterization is central to understanding TGFbeta in normal and diseased states. Therefore, we employed a global transcriptional profiling strategy to identify TGFbeta regulated genes in normal and cancerous tissues. This led to the cloning and characterization of Gadd45beta. We found that Gadd45beta transcriptional responsiveness to TGFbeta required the expression of the SMAD3 and SMAD4 transcription factors and the presence of a highly conserved 3-prime enhancer within the Gadd45beta genomic locus. Using mammalian cell culture and zebrafish model systems, we discovered an essential role for Gadd45beta in the TGFbeta cytostatic program and metazoan development, respectively. Additionally, our expression screen led to the identification of the Kindlerin gene, a novel TGFbeta regulated gene involved in cell adhesion and spreading. In sum, our findings provide important advancements towards understanding how normal and transformed cells interpret and respond to members of the TGFbeta family of cell signaling molecules.