IRF-1 Links Cytoskeletal Contraction With Inflammatory Response in mTOR-Inhibited Endothelial Cells

Clinical therapies targeting mammalian target of rapamycin (mTOR) are associated with high rates of pneumonitis. Recent studies independently revealed the upregulation of the proinflammatory transcription factor interferon regulatory factor-1 (IRF-1) by mTOR inhibition (mTORi) of endothelial cells (EC) and further highlighted a mechanism converging on myosin light chain (MLC) phosphorylation-dependent cytoskeletal dynamics in promoting the endothelial hyperpermeability and pulmonary inflammation caused by mTORi. This study investigated a role for this mechanism in linking the regulation of IRF-1 expression with downstream responses in mTOR-inhibited EC. IRF-1 was transcriptionally upregulated in cultured EC by treatment with mTOR inhibitor rapamycin or torin 1, or by silencing either Raptor or Rictor expression to disrupt mTOR complex 1 (mTORC1) or 2 (mTORC2). Inhibition of MLC kinase (MLCK) activity or activation of MLC phosphatase (MLCP) to suppress MLC phosphorylation, or direct inhibition of actin polymerization, attenuated IRF-1 expression as well as transcription of an array of proinflammatory cytokines. Moreover, IRF-1 in turn upregulated MLCK expression to enhance MLC phosphorylation and promote endothelial hyperpermeability in mTOR-inhibited EC. Consistent with these observations in culture, targeted endothelial deficiency of IRF-1 in mice significantly reduced lung edema and inflammation elicited by separate or combined treatment of rapamycin and lipopolysaccharide. In conclusion, activation of actomyosin contractility by mTORi upregulated IRF-1, which promoted the development of lung injury by mediating inflammation and hyperpermeability responses in EC.