LSC - 2017 - mTOR-mediated cap-dependent translation is essential for human lung myofibroblast differentiation and collagen I synthesis

Introduction: Aberrant control of translation has been recently described as a key regulator of the idiopathic pulmonary fibrosis (IPF) myofibroblast phenotype. Mammalian target of rapamycin (mTOR) regulates mRNA translation via inhibition of the repressor 4E-BP1, permitting formation of the eIF4F translation initiation complex that binds the 5’ mRNA cap. In this study, we investigate the mechanisms by which mTOR exerts translational control of myofibroblast differentiation and collagen I production. Methods & Results: TGFs1-induced collagen I and a-SMA protein produced by human lung fibroblasts, determined by high-content imaging under macromolecular crowding conditions, is potently inhibited to baseline levels in the presence of structurally distinct potent ATP-competitive mTOR inhibitors, but not rapamycin. In contrast, maximum reduction in TGFs1-inducedCOL1A1andACTA2mRNA levels achieved by mTOR inhibition, but not rapamycin, is only ~50%. Immunoprecipitation of the 5’ mRNA cap and eIF4F components eIF4E and eIF4G reveals that mTOR inhibition prevents TGFs1-induced recruitment of eIF4G and enhances binding of 4E-BP1. Similarly, 4EGI-1, an inhibitor of cap-dependent translation, inhibited cap complex formation whereas rapamycin had no effect. Furthermore, 4EGI-1 attenuated TGFs1-inducedCOL1A1andACTA2mRNA levels by ~80% and ~90% respectively. Conclusions: mTOR-mediated cap-dependent translation, via modulation of the 4E-BP1 axis, is a critical regulator of TGFs1-induced collagen I and a-SMA synthesis. Direct ATP-competitive mTOR inhibitors may represent a potential novel therapeutic strategy for targeting pro-fibrotic TGFs1signalling in the context of IPF.