Transforming growth factor-β (TGF-β) can induce epithelial to mesenchymal transdifferentiation (EMT) in mammary epithelial cells. TGF-β-meditated EMT involves the stimulation of a number of signaling pathways by the sequential binding of the type II and type I serine/threonine kinase receptors, respectively. Integrins comprise a family of heterodimeric extracellular matrix receptors that mediate cell adhesion and intracellular signaling, hence making them crucial for EMT progression. In light of substantial evidence indicating TGF-β regulation of various β1integrins and their extracellular matrix ligands, we examined the cross-talk between the TGF-β and integrin signal transduction pathways. Using an inducible system for the expression of a cytoplasmically truncated dominant negative TGF-β type II receptor, we blocked TGF-β-mediated growth inhibition, transcriptional activation, and EMT progression. Dominant negative TGF-β type II receptor expression inhibited TGF-β signaling to the SMAD and AKT pathways, but did not block TGF-β-mediated p38MAPK activation. Interestingly, blocking integrin β1function inhibited TGF-β-mediated p38MAPK activation and EMT progression. Limiting p38MAPK activity through the expression of a dominant negative-p38MAPK also blocked TGF-β-mediated EMT. In summary, TGF-β-mediated p38MAPK activation is dependent on functional integrin β1, and p38MAPK activity is required but is not sufficient to induce EMT.
Neonatal and Adult Dermal Fibroblasts Show Differences in Transforming Growth Factor (TGF-β) Secretion and TGF-β Type II Receptor Expression at Baseline and Under Constant Stretch Conditions
