The mTORC2/SGK1/NDRG1 Signaling Axis Is Critical for Mesomesenchymal Transition of Pleural Mesothelial Cells and the Progression of Pleural Fibrosis.

Progressive lung scarring due to persistent pleural organization often results in pleural fibrosis (PF). This process affects patients with complicated parapneumonic pleural effusions, empyema, and other pleural diseases prone to loculation. In PF, pleural mesothelial cells undergo mesomesenchymal transition (MesoMT) to become profibrotic, characterized by increased expression of α-smooth muscle actin (α-SMA) and matrix proteins, including collagen (Col)-1. In our previous study, we showed that blocking PI3K/Akt signaling inhibits MesoMT induction in human pleural mesothelial cells (HPMCs). However, the downstream signaling pathways leading to MesoMT induction remain obscure. Here, we investigated the role of mammalian target of rapamycin (mTOR) complexes (mTORC1/2) in MesoMT induction. Our studies show that activation of the downstream mediator mTORC1/2 complex is likewise a critical component of MesoMT. Specific targeting of mTORC1/2 complex using pharmacological inhibitors, such as INK128 and AZD8055, significantly inhibited TGF-β-induced MesoMT markers in HPMCs. We further identified mTORC2/Rictor complex as the principal contributor to MesoMT progression induced by TGF-β. Knockdown of Rictor, but not Raptor, attenuated TGF-β induced MesoMT in these cells. In these studies, we further show that concomitant activation of the SGK1/NDRG1 signaling cascade is essential for inducing MesoMT. Targeting SGK1 and NDRG1 with siRNA and small molecular inhibitors attenuated TGF-β-induced MesoMT in HPMCs. Additionally, preclinical studies in our Streptococcus pneumoniae mediated mouse model of PF showed that inhibition of mTORC1/2 with INK128 significantly attenuated the progression of PF in sub-acute and chronic injury. In conclusion, our studies demonstrate that mTORC2/Rictor-mediated activation of SGK1/NDRG1 are critical for MesoMT induction, and targeting this pathway could inhibit or even reverse the progression of MesoMT and PF.