Inflammation-Mediated Dysregulation of the NFκB /Sox18 Axis during VILI

Acute respiratory distress syndrome (ARDS) is a refractory lung disease characterized by severe hypoxemia and an unacceptable high mortality that ranges from 30 to 50 percent. And while it can be a life-saving intervention in critically ill patients with respiratory failure, mechanical ventilation also creates excessive mechanical stress that directly worsens lung injury, a syndrome known as ventilator-induced lung injury (VILI). VILI may also ensue in mechanically-ventilated patients even when ARDS is not initially present. It shares many pathobiology features with ARDS including increased inflammatory cytokine expression and marked pulmonary capillary endothelial cell (EC) leakage, however, specific mechanisms involved in the development of VILI remain elusive.

These findings mandate a more thorough understanding of VILI pathobiology and highlight the need to develop novel targets and strategies. The labs of Drs. Stephen Black and Joe G. N. Garcia both examine mechanisms involved in the initial inflammatory phase of VILI focusing on the inflammatory signaling evoked by the transcription factor, nuclear factor (NF)-κB. Whereas Dr. Garcia’s lab targets nicotinamide phosphoribosyltransferase/toll-like receptor 4 (NAMPT/TLR4) induced NF-κB signaling, Dr. Black’s lab extends long standing interest in lung vascular EC responses to mechanical stress and examines how excessive mechanical stress regulates NF-κB signaling proteins via increased reactive oxygen species- or ROS-mediated post-translational modifications (PTMs) including nitration. These novel data indicate that lipopolysaccharide (LPS) and VILI mediate nitration and degradation of the NF-κB inhibitory protein, IκBα. These events, in concert with recruitment of histone deacetylases, are important in suppressing promoter activity of SOX18, a transcription factor intimately involved in preservation of lung vascular EC barrier integrity via transcriptional regulation of the tight junction protein, claudin 5 (CLDN5). Thus, NF-κB appears to be critically involved in the suppression of SOX18 and CLDN5 expression in response to mechanical stress.

The work from Dr. Black’s lab has shown that SOX18 expression is increased by fluid shear stress and a decrease in lung vascular EC exposed to 18 percent cyclic stretch. SOX18 transcriptionally increases expression of proteins intimately involved in EC barrier maintenance and protects against shear stress- and LPS-induced loss of EC barrier integrity. In addition, enhancing SOX18 expression in the mouse lung attenuates EC barrier disruption associated with high tidal volume mechanical ventilation. The mechanism by which LPS and excessive mechanical stress reduce SOX18 expression in both human lung EC and the mouse lung is unknown.

Currently, Dr. Black and his colleagues are pursuing a project focusing on four specific aims:

  • Aim 1 is proposed to elucidate the genetic and epigenetic influence of SOX18-promoter single nucleotide polymorphisms (SNPs) and DNA methylation on mechanical stress-mediated SOX18 and CLDN5 expression.
  • Aim 2 is to define the mechanisms by which mechanical stress-mediated PTMs, including nitration, activate NF-κB and alter EC junctional integrity.
  • Aim 3 determines how mitochondrial dysfunction, the ubiquitin-mediated degradation of GTP cyclohydrolase I (GCH1), and eNOS uncoupling interconnect to increase the mechanical stress-induced nitration of protein targets including IκBα.
  • Aim 4 translates the data obtained from Aims 1-3 into actionable information to attenuate VILI/ARDS by defining the therapeutic efficacy of directly increasing SOX18 expression (SOX18 over-expression, Nrf2 activators, simvastatin), preventing the epigenetic down-regulation of SOX18 (HDAC inhibitors), preventing nitration-mediated NF-κB activation (shielding peptides) and reducing mitochondrial-derived ROS (mitochondrial targeted peptides). Completion of the proposed four specific aims (see Figure) will advance our understanding of the interplay between genetics, epigenetics and post-translational modifications in the dysregulation of lung vascular EC tight junctions during VILI pathobiology.

Figure: Effects of Inflammatory Signaling on the SOX18/CLDN5 Axis. SA #1 will assess the genetic and epigenetic regulation of SOX18 and CLDN5 promoter activities by mechanical stress and define how promoter SNPs alter promoter activities. SA #2 will define mechanical stress-evoked PTMs (nitration) involved in NFκ-B mediated activation and tight junction disruption. SA #3 will define the signaling pathway responsible for generating the peroxynitrite in response to mechanical stress and the PTMs involved (phosphorylation, ubiquitination). SA #4 will evaluate SOX18 over-expression, novel mitochondrial targeted ROS-scavenging peptides, IκBα targeted nitration-shielding peptides, and HDAC inhibitors as potential VILI therapeutic strategies to reducing the severity of VILI/ARDS.

Related lines of research include: