Binding of TGF-13 to its receptors can elicit a variety of responses in different cell types. For example, TGF-13 induces plasminogen activator inhibitor in epithelial cells and specific immunoglobulins in B cells. In both cell types, Smad3 is activated. Given the conservation of the signaling pathway, what accounts for the diversity of the response to TGF-13 in various cell types?