TGF-induced PI 3 kinase-dependent Mnk-1 activation is necessary for Ser-209 phosphorylation of eIF4E and mesangial cell hypertrophy

Transforming growth factor (TGF)-induced canonical signal transduction is involved in glomerular mesangial cell hypertrophy; however, the role played by the noncanonical TGF signaling remains largely unexplored. TGF time-dependently stimulated eIF4E phosphorylation at Ser-209 concomitant with enhanced phosphorylation of Erk1/2 (extracellular signal regulated kinase1/2) and MEK (mitogen-activated and extracellular signal-regulated kinase kinase) in mesangial cells. Inhibition of Erk1/2 by MEK inhibitor or by expression of dominant negative Erk2 blocked eIF4E phosphorylation, resulting in attenuation of TGF-induced protein synthesis and mesangial cell hypertrophy. Expression of constitutively active (CA) MEK was sufficient to induce protein synthesis and hypertrophy similar to those induced by TGF. Pharmacological or dominant negative inhibition of phosphatidylinositol (PI) 3 kinase decreased MEK/Erk1/2 phosphorylation leading to suppression of eIF4E phosphorylation. Inducible phosphorylation of eIF4E at Ser-209 is mediated by Mnk-1 (mitogen-activated protein kinase signal-integrating kinase-1). Both PI 3 kinase and Erk1/2 promoted phosphorylation of Mnk-1 in response to TGF. Dominant negative Mnk-1 significantly inhibited TGF-stimulated protein synthesis and hypertrophy. Interestingly, inhibition of mTORC1 activity, which blocks dissociation of eIF4E-4EBP-1 complex, decreased TGF-stimulated phosphorylation of eIF4E without any effect on Mnk-1 phosphorylation. Furthermore, mutant eIF4E S209D, which mimics phosphorylated eIF4E, promoted protein synthesis and hypertrophy similar to TGF. These results were confirmed using phosphorylation deficient mutant of eIF4E. Together our results highlight a significant role of dissociation of 4EBP-1-eIF4E complex for Mnk-1-mediated phosphorylation of eIF4E. Moreover, we conclude that TGF-induced noncanonical signaling circuit involving PI 3 kinase-dependent Mnk-1-mediated phosphorylation of eIF4E at Ser-209 is required to facilitate mesangial cell hypertrophy. J. Cell. Physiol. 228: 16171626, 2013. (c) 2013 Wiley Periodicals, Inc.