AMPK Inhibits the Stimulatory Effects of TGF-β on Smad2/3 Activity, Cell Migration, and Epithelial-to-Mesenchymal Transition

AMP-activated protein kinase (AMPK), an important downstream effector of the tumor suppressor liver kinase 1 (LKB1) and pharmacologic target of metformin, is well known to exert a preventive and inhibitory effect on tumorigenesis; however, its role in cancer progression and metastasis has not been well characterized. The present study investigates the potential roles of AMPK in inhibiting cancer-cell migration and epithelial-to-mesenchymal transition (EMT) by regulating the canonical transforming growth factor beta (TGF-beta) signaling pathway, an important promoting factor for cancer progression. Our results showed that activation of AMPK by metformin inhibited TGF-beta-induced Smad2/3 phosphorylation in cancer cells in a dose-dependent manner. The effect of metformin is dependent on the presence of LKB1. A similar effect was obtained by expressing a constitutive active mutant of AMPK alpha 1 subunit, whereas the expression of a dominant negative mutant of AMPK alpha 1 or ablation of AMPK alpha subunits greatly enhanced TGF-beta stimulation of Smad2/3 phosphorylation. As a consequence, expression of genes downstream of Smad2/3, including plasminogen activator inhibitor-1, fibronectin, and connective tissue growth factor, was suppressed by metformin in a LKB1-dependent fashion. In addition, metformin blocked TGF-beta-induced inteleukin-6 expression through both LKB1-dependent and - independent mechanisms. Our results also indicate that activation of LKB1/AMPK inhibits TGF-beta-stimulated cancer cell migration. Finally, TGF-beta induction of EMT was inhibited by phenformin and enhanced by knockdown of LKB1 expression with shRNA. Together, our data suggest that AMPK could be a drug target for controlling cancer progression and metastasis.