Transforming growth factor-β inhibits alveolar epithelial protein transport by glycogen synthase kinase 3-β-mediated downregulation of megalin

A hallmark of acute lung injury/acute respiratory distress syndrome (ALI/ARDS) is the accumulation of protein-rich edema fluid in the distal airspaces removal of which is critical for patient survival. We have previously shown that albumin transport across the alveolar epithelial barrier is an active process mediated by megalin. Here we set out to identify the mechanisms that may impair megalin function upon ALI/ARDS. Particularly, we investigated the role of glycogen synthase kinase 3-β (GSK3-β), a serine-threonine kinase that is involved in various inflammatory disorders and that has been proposed to phosphorylate megalin at the PPPSP motif thereby causing its downregulation.

We found that in the intact rabbit lung model TGFβ1 decreases clearance of excess albumin. In A549 and primary rat alveolar epithelial cells treatment with TGF-β1 led to inhibition of FITC-albumin uptake by the cells and activation of GSK3-β. Importantly, maximal activation of GSK3-β was observed within minutes, which was in line with the rapid impairment of albumin transport detected in intact rabbit lungs after TGF-β1 treatment. When GSK3 activity was inhibited by siRNA or administration of the specific inhibitor SB 216763, the TGF-β1-induced downregulation of megalin membrane expression was prevented and albumin transport capacity returned to baseline. Collectively those findings suggest that TGF-β1 induced activation of GSK3-β, which results in phosphorylation and subsequent endocytosis of megalin, thereby disrupting albumin transport.

These findings may suggest a potential role for GSK3-β inhibition as a novel therapeutic approach in ALI/ARDS.