mTOR Inhibition Promotes Pneumonitis through Inducing Endothelial Contraction and Hyperpermeability

Compromised endothelial-cell (EC) barrier function is a hallmark of inflammatory diseases. mTOR inhibitors, widely applied as clinical therapies, cause pneumonitis through mechanisms that are not yet fully understood. This study aimed to elucidate the EC mechanisms underlying the pathogenesis of pneumonitis caused by mTOR inhibition (mTORi). Mice with EC-specific deletion of mTOR complex components (Mtor, Rptor or Rictor) were administered LPS to induce pulmonary injury. Cultured ECs were treated with pharmacologic inhibitors, siRNA, or overexpression plasmids. EC barrier function was evaluated in vivo with Evans blue assay and in vitro by measurement of transendothelial electrical resistance and albumin flux. mTORi increased basal and TNF alpha- induced EC permeability, which was caused by myosin light chain (MLC) phosphorylation-dependent cell contraction. Inactivation of mTOR kinase activity by mTORi triggered PKC delta/p38/NF-kappa B signaling that significantly upregulated TNF alpha-induced MLCK (MLC kinase) expression, whereas Raptor promoted the phosphorylation of PKC alpha/MYPT I independently of its interaction with mTOR, leading to suppression of MLCP (MLC phosphatase) activity. EC-specific deficiency in mTOR, Raptor or Rictor aggravated lung inflammation in LPS-treated mice. These findings reveal that mTORi induces PKC-dependent endothelial MLC phosphorylation, contraction, and hyperpermeability that promote pneumonitis.