Peroxisome proliferator-activated receptor γ promotes epithelial to mesenchymal transformation by Rho GTPase-dependent activation of ERK1/2

Peroxisome proliferator-activated receptor γ (PPARγ) causes epithelial to mesenchymal transformation (EMT) in intestinal epithelial cells, as evidenced by reorganization of the actin cytoskeleton, acquisition of a polarized, mesenchymal cellular morphology, increased cellular motility, and colony scattering. This response is due to activation of Cdc42, resulting in p21-activated kinase-dependent phosphorylation and activation of MEK1 Ser ²⁹⁸ and activation of ERK1/2. Dominant negative MEK1, MEK2, and ERK2 block PPARγ-induced EMT, whereas constitutively active MEK1 and MEK2 induce a mesenchymal phenotype similar to that evoked by PPARγ. PPARγ also stimulates ERK1/2 phosphorylation in the intestinal epithelium in vivo. PPARγ induces the p110α subunit of phosphoinositide 3-kinase (PI3K), and inhibition of PI3K blocks PPARγ-dependent phosphorylation of MEK1 Ser²⁹⁸, activation of ERK1/2, and EMT. We conclude that PPARγ regulates the motility of intestinal epithelial cells through a mitogen-activated protein kinase cascade that involves PI3K, Cdc42, p21-activated kinase, MEK1, and ERK1/2. Regulation of cellular motility through Rho family GTPases has not been previously reported for nuclear receptors, and elucidation of the mechanism that accounts for the role of PPARγ in regulating motility of intestinal epithelial cells provides fundamental new insight into the function of this receptor during renewal of the intestinal epithelium.