α-Tocopherol transfer protein mediates protective hypercapnia in murine ventilator-induced lung injury

Hypercapnia is common in mechanically ventilated patients. Experimentally, 'therapeutic hypercapnia' can protect, but it can also cause harm, depending on the mechanism of injury. Hypercapnia suppresses multiple signalling pathways. Previous investigations have examined mechanisms that were known a priori, but only a limited number of pathways, each suppressed by CO₂, have been reported.

Because of the complexity and interdependence of processes in acute lung injury, this study sought to fill in knowledge gaps using an unbiased screen, aiming to identify a specifically upregulated pathway.

Using genome-wide gene expression analysis in a mouse model of ventilator-induced lung injury, we discovered a previously unsuspected mechanism by which CO₂ can protect against injury: induction of the transporter protein for α-tocopherol, α-tocopherol transfer protein (αTTP). Pulmonary αTTP was induced by inspired CO₂ in two in vivo murine models of ventilator-induced lung injury; the level of αTTP expression correlated with degree of lung protection; and, absence of the αTTP gene significantly reduced the protective effects of CO₂. α-Tocopherol is a potent antioxidant and hypercapnia increased lung α-tocopherol in wild-type mice, but this did not alter superoxide generation or expression of NRF2-dependent antioxidant response genes in wild-type or in αTTP^-/- mice. In concordance with a regulatory role for α-tocopherol in lipid mediator synthesis, hypercapnia attenuated 5-lipoxygenase activity and this was dependent on the presence of αTTP.

Inspired CO₂ upregulates αTTP which increases lung α-tocopherol levels and inhibits synthesis of a pathogenic chemoattractant.