Active Endothelial Inactivation of Hyperpermeability: The Role of Nitric Oxide-Driven cAMP/Epac1 Signaling.

Endothelial hyperpermeability is a hallmark of diverse inflammatory and vascular pathologies, including sepsis, acute respiratory distress syndrome (ARDS), ischemia-reperfusion injury, and atherosclerosis. Traditionally considered a passive return to baseline following stimulus withdrawal, barrier recovery is now recognized as an active, endothelial-driven process. Earlier work identified individual components of this restorative phase, such as cyclic adenosine monophosphate (cAMP)/exchange protein directly activated by cAMP 1 (Epac1) signaling, Rap1/Rac1 activation, vasodilator-stimulated phosphoprotein (VASP) phosphorylation, and targeted cytoskeletal remodeling, as well as kinase pathways involving PKA, PKG, and Src. However, these were often regarded as discrete events lacking a unifying framework. Recent integrative analyses, combining mechanistic insights from multiple groups, reveal that nitric oxide (NO) generated early during hyperpermeability can initiate a delayed cAMP/Epac1 cascade. This axis coordinates Rap1/Rac1-mediated cortical actin polymerization, VASP-driven junctional anchoring, retro-translocation of endothelial nitric oxide synthase (eNOS) to caveolar domains, PP2A-dependent suppression of actomyosin tension, and Krüppel-like factor 2 (KLF2)-driven transcriptional programs that sustain endothelial quiescence. Together, these pathways form a temporally orchestrated, multi-tiered "inactivation" program capable of restoring barrier integrity even in the continued presence of inflammatory stimuli. This conceptual shift reframes NO from solely a barrier-disruptive mediator to the initiating trigger of a coordinated, pro-resolution mechanism. The unified framework integrates cytoskeletal dynamics, junctional reassembly, focal adhesion turnover, and redox/transcriptional control, providing multiple potential intervention points. Therapeutically, Epac1 activation, Rap1/Rac1 enhancement, RhoA/ROCK inhibition, PP2A activation, and KLF2 induction represent strategies to accelerate endothelial sealing in acute microvascular syndromes. Moreover, applying these mechanisms to arterial endothelium could limit low-density lipoprotein (LDL) entry and foam cell formation, offering a novel adjunctive approach for atherosclerosis prevention. In this review, we will discuss both the current understanding of endothelial hyperpermeability mechanisms and the emerging pathways of its active inactivation, integrating molecular, structural, and translational perspectives.