Effect of inflammation-mediated endothelial metabolic shift on endothelial barrier function
Authors: Aslam M , Idrees H , Hamm C W , Ladilov Y 
Published: October 2021

Abstract

Background
The integrity of the endothelial cell barrier of the microvasculature is compromised by inflammation. The increased vascular permeability leads to tissue injury and organ dysfunction. In recent years, considerable advances have been made in the understanding of signalling mechanisms regulating the endothelial barrier integrity. The role of endothelial metabolism as a modulator of endothelial barrier integrity is not yet well-studied. The aim of the present study was to investigate the effect of inflammation on endothelial metabolism and its role in the maintenance of endothelial barrier integrity.

Methods
The study was carried out on cultured human umbilical vein endothelial cells and rat coronary microvascular endothelial cells. Inflammatory condition was simulated by treating cells with low concentrations (1 ng/mL) of TNFα for 24h. Endothelial barrier function was analysed by measuring the flux of albumen through endothelial monolayers cultured on filter membranes. Gene expression was analysed by qPCR-based assays. The capacity of endothelial cells for maximal ATP synthesis rate was investigated by the real-time live-cell imaging using FRET-based ATP-biosensor (live cell FRET). Total cellular ATP concentration was measured using luminescence-based commercial kit (ATPLite, PerkinElmer). Mitochondrial mass was analysed by the ratio of mitochondrial DNA (mtDNA) to nuclear DNA (nDNA). The cellular glucose uptake was measured by fluorescent microscopy using a fluorescent analogue of glucose (2-NBDG).

Results
Treatment of human endothelial cells with TNFα resulted in significant suppression of mitochondrial and upregulation of glycolytic ATP synthesis rate, suggesting a metabolic switch. This was accompanied by a reduction in mitochondrial content (mtDNA/nDNA), reduction in total cellular ATP levels, an enhanced expression of glycolytic enzymes 6-phosphofructo-2-kinase/fructose-2,6-biphosphatase 3 (PFKFB3) and phosphofructokinase 1 (PFK1), and enhanced glucose uptake by endothelial cells (n=5; p&lt;0.05 for all parameters tested). Moreover, TNFα caused a 3-fold increase in endothelial permeability. Pharmacological inhibition of glycolysis either by partial replacement of glucose with 2-deoxy glucose (2DG) or an inhibition of PFKFB3 resulted in further worsening (a 5-fold increase in permeability) of TNFα-induced endothelial barrier failure. On the other hand pharmacological activation of AMPK, a potent inducer of mitochondrial biogenesis, could attenuate TNFα-induced but not 2DG-induced endothelial hyperpermeability.

Conclusion
The study demonstrates that TNFα induces metabolic switch towards glycolysis in endothelial cells. Moreover, the data suggest that upregulation of glycolysis may serve as an endogenous metabolic adaptation to the TNFα-induced suppression of mitochondrial ATP synthesis, which protects endothelial barrier integrity.

Funding Acknowledgement
Type of funding sources: Public grant(s) – National budget only. Main funding source(s): Justus-Liebig University GiessenDZHK (German Centre for Cardiovascular Research), partner site Rhein-Main, Bad Nauheim, Germany