The ICAM‐1 expression level determines the susceptibility of human endothelial cells to simulated microgravity

Microgravity is a principal risk factor hampering human cardiovascular regulation during space flights. Endothelial dysfunction associated with the impaired integrity and uniformity of the monolayer represents a potential trigger for vascular damage. We characterized the expression profile of the multi-step cascade of adhesion molecules (ICAM-1, VCAM-1, E-selectin, VE-cadherin) in umbilical cord endothelial cells (ECs) after 24 hrs of exposure to simulated microgravity (SMG), pro-inflammatory cytokine TNF-α, and the combination of the two. Random Positioning Machine (RPM)-mediated SMG was used to mimic microgravity effects. SMG stimulated the expression of E-selectin, which is known to be involved in slowing leukocyte rolling. Primary ECs displayed heterogeneity with respect to the proportion of ICAM-1-positive cells. ECs were divided into two groups: pre-activated ECs displaying high proportion of ICAM-1+-cells (ECs-1) (greater than 50%) and non-activated ECs with low proportion of ICAM-1+-cells (ECs-2) (less than 25%). Only non-activated ECs-2 responded to SMG by elevating gene transcription and increasing ICAM-1 and VE-cadherin expression. This effect was enhanced after cumulative SMG-TNF-α exposure. ECs-1 displayed an unexpected decrease in number of E-selectin- and ICAM-1-positive ECs and pronounced up-regulation of VCAM1 upon activation of inflammation, which was partially abolished by SMG. Thus, non-activated ECs-2 are quite resistant to the impacts of microgravity and even exhibited an elevation of the VE-cadherin gene and protein expression, thus improving the integrity of the endothelial monolayer. Pre-activation of ECs with inflammatory stimuli may disturb the EC adhesion profile, attenuating its barrier function. These alterations may be among the mechanisms underlying cardiovascular dysregulation in real microgravity conditions. This article is protected by copyright. All rights reserved