Virtually all mammalian cells secrete at least one TGF-β isoform, and most have TGF-β receptors on their surface. However, the cellular responses induced by TGF-β vary among cell types. In epithelial cells and fibroblasts, for example, TGF-β induces expression of extracellular-matrix proteins (e.g., fibronectins and collagens; see Chapter 20). It also induces expression of proteins that inhibit serum proteases, which otherwise would degrade these extracellular-matrix proteins. This inhibition stabilizes the matrix, allowing cells to form stable tissues. The protease-inhibitory proteins include plasminogen activator inhibitor 1 (PAI-1). Transcription of the PAI-1 gene requires formation of a complex of the transcription factor TFE3 with the R-Smad/co-Smad (Smad3/Smad4) complex and binding of all these proteins to specific sequences within the regulatory region of the PAI-1 gene (Figure 16-3, bottom). By partnering with other transcription factors expressed in fibroblasts, the R-Smad/co-Smad complex promotes expression of genes encoding other proteins such as p15^INK4B, which arrests the cell cycle at the G₁ stage and thus blocks cell proliferation (see Chapter 19).

More generally, in order for the Smad3/Smad4 complex to bind to a DNA regulatory region and activate a given gene, the DNA binding segment must be in an active “open” chromatin conformation. Equally important, binding of the Smad3/Smad4 complex requires binding of other transcription factors at adjacent sites in the DNA; often these transcription factors are master transcription factors that determine the identity of a cell during its development. For example, we will learn in Chapter 21 that the transcription factor Oct4 forms a complex with two other transcription factors, Sox2 and Nanog, that are essential for keeping embryonic stem (ES) cells in an undifferentiated state, and that expression of these three master transcription factors, together with one or two other transcription factors, is sufficient to reprogram a differentiated cell (e.g., a cell of the immune system) into an ES cell. TGF–β activation of the Smad3/Smad4 complex is also required to maintain ES cells in an undifferentiated state.

The chromatin immunoprecipitation–DNA sequence experiment summarized in Figure 16-4a shows that in ES cells, the Smad3/Smad4 complex binds to DNA at sites adjacent to those occupied by Oct4; similar experiments showed that Sox2 and Nanog are also bound at most of these sites. In contrast, MyoD and the related transcription factor Myf5 are the master transcription factors in development of striated muscle, and Oct4, Sox2, and Nanog are not expressed; in muscle cells, the Smad3/Smad4 complex binds to gene regulatory sites adjacent to those occupied by MyoD, which are very different DNA sites from those that bind the Smad3/Smad4 complex in ES cells. About 13 percent of the genes in muscle whose regulatory regions are bound by the Smad3/Smad4 complex in muscle cells are also bound by the Smad3/Smad4 complex in ES cells, but detailed analyses of these binding sites showed that the Smad3/Smad4 complex regulates the same gene in these different cell types by binding to different regulatory DNA sequences, most presumably enhancer regions, which are occupied by different cell-specific master transcription factors (Figure 16-4b). As we noted earlier, other types of transcription factors activated by other types of cell-surface receptors also activate or repress different sets of genes in different types of cells. We will see in this chapter that those transcription factors regulate those genes in the same manner as the Smads activated by the TGF-β receptors.

725