Role of cAMP-dependent protein kinase A activity in low-dose endothelial monocyte-activating polypeptide-II-induced opening of blood-tumor barrier

Endothelial tight junctions and their regulatory signaling pathways in vascular homeostasis and disease

Endothelial tight junctions (TJs) regulate the transport of water, ions, and molecules through the paracellular pathway, serving as an important barrier in blood vessels and maintaining vascular homeostasis. In endothelial cells (ECs), TJs are highly dynamic structures that respond to multiple external stimuli and pathological conditions. Alterations in the expression, distribution, and structure of endothelial TJs may lead to many related vascular diseases and pathologies. In this review, we provide an overview of the assessment methods used to evaluate endothelial TJ barrier function both in vitro and in vivo and describe the composition of endothelial TJs in diverse vascular systems and ECs. More importantly, the direct phosphorylation and dephosphorylation of TJ proteins by intracellular kinases and phosphatases, as well as the signaling pathways involved in the regulation of TJs, including and the protein kinase C (PKC), PKA, PKG, Ras homolog gene family member A (RhoA), mitogen-activated protein kinase (MAPK), phosphatidylinositol 3-kinase (PI3K)/Akt, and Wnt/β-catenin pathways, are discussed. With great advances in this area, targeting endothelial TJs may provide novel treatment for TJ-related vascular pathologies.

The Role of miR-330-3p/PKC-α Signaling Pathway in Low-Dose Endothelial-Monocyte Activating Polypeptide-II Increasing the Permeability of Blood-Tumor Barrier

This study was performed to determine whether EMAP II increases the permeability of the blood-tumor barrier (BTB) by affecting the expression of miR-330-3p as well as its possible mechanisms. We determined the over-expression of miR-330-3p in glioma microvascular endothelial cells (GECs) by Real-time PCR. Endothelial monocyte-activating polypeptide-II (EMAP-II) significantly decreased the expression of miR-330-3p in GECs. Pre-miR-330-3p markedly decreased the permeability of BTB and increased the expression of tight junction (TJ) related proteins ZO-1, occludin and claudin-5, however, anti-miR-330-3p had the opposite effects. Anti-miR-330-3p could enhance the effect of EMAP-II on increasing the permeability of BTB, however, pre-miR-330-3p partly reversed the effect of EMAP-II on that. Similarly, anti-miR-330-3p improved the effects of EMAP-II on increasing the expression levels of PKC-α and p-PKC-α in GECs and pre-miR-330-3p partly reversed the effects. MiR-330-3p could target bind to the 3′UTR of PKC-α. The results of in vivo experiments were similar to those of in vitro experiments. These suggested that EMAP-II could increase the permeability of BTB through inhibiting miR-330-3p which target negative regulation of PKC-α. Pre-miR-330-3p and PKC-α inhibitor decreased the BTB permeability and up-regulated the expression levels of ZO-1, occludin and claudin-5 while anti-miR-330-3p and PKC-α activator brought the reverse effects. Compared with EMAP-II, anti-miR-330-3p and PKC-α activator alone, the combination of the three combinations significantly increased the BTB permeability. EMAP-II combined with anti-miR-330-3p and PKCα activator could enhance the DOX’s effects on inhibiting the cell viabilities and increasing the apoptosis of U87 glioma cells. Our studies suggest that low-dose EMAP-II up-regulates the expression of PKC-α and increases the activity of PKC-α by inhibiting the expression of miR-330-3p, reduces the expression of ZO-1, occludin and claudin-5, and thereby increasing the permeability of BTB. The results can provide a new strategy for the comprehensive treatment of glioma.

Low-Dose Endothelial Monocyte-Activating Polypeptide-II Induces Blood-Tumor Barrier Opening Via the cAMP/PKA/Rac1 Pathway

Previous studies have demonstrated that low-dose endothelial monocyte-activating polypeptide-II (EMAP-II) induces blood-tumor barrier (BTB) hyperpermeability via both paracellular and transcellular pathways. In a recent study, we revealed that cyclic adenosine monophosphate (cAMP)/protein kinase A (PKA)-dependent signaling pathway is involved in EMAP-II-induced BTB hyperpermeability. This study further investigated the exact mechanisms through which the cAMP/PKA-dependent signaling pathway affects EMAP-II-induced BTB hyperpermeability. In an in vitro BTB model, low-dose EMAP-II (0.05 nM) induced a significant decrease in Rac1 activity in rat brain microvascular endothelial cells (RBMECs). Pretreatment with forskolin to elevate intracellular cAMP concentration completely blocked EMAP-II-induced inactivation of Rac1. Besides, pretreatment with 6Bnz-cAMP to activate PKA partially attenuated EMAP-II-induced Rac1 inactivation. Moreover, 6Bnz-cAMP pretreatment significantly diminished EMAP-II-induced changes in BTB permeability, myosin light chain (MLC) phosphorylation, expression and distribution of ZO-1, and actin cytoskeleton arrangement in RBMECs. These effects of 6Bnz-cAMP were completely blocked in the presence of NSC-23766 (the specific inhibitor of Rac1). In conclusion, this study demonstrates that low-dose EMAP-II induces BTB hyperpermeability via the cAMP/PKA/Rac1 signaling pathway.

Overexpression of miR-18a negatively regulates myocyte enhancer factor 2D to increase the permeability of the blood-tumor barrier via Krüppel-like factor 4-mediated downregulation of zonula occluden-1, claudin-5, and occludin

miR-18a represses angiogenesis and tumor evasion by weakening vascular endothelial growth factor and transforming growth factor-β signaling to prolong the survival of glioma patients, although it is thought to be an oncogene. This study investigates the potential effects of miR-18a on the permeability of the blood-tumor barrier (BTB) and its possible molecular mechanisms. An in vitro BTB model was successfully established. The endogenous expression of miR-18a in glioma vascular endothelial cells (GECs) was significantly lower than that in normal vascular ECs, and the overexpression of miR-18a significantly increased the permeability of the BTB as well as downregulating the mRNA and protein expressions of tight junction-related proteins zonula occluden-1 (ZO-1), claudin-5, and occludin in GECs. Dual luciferase reporter assays revealed that miR-18a bound to the 3'-untranslated region (3'UTR) of myocyte enhancer factor 2D (MEF2D). The overexpression of both miR-18a and MEF2D with the 3'UTR significantly weakened the effect caused by miR-18a of decreasing the mRNA and protein expressions of ZO-1, claudin-5 and occludin and of increasing the permeability of the BTB. Chromatin immunoprecipitation showed that MEF2D could directly bind to KLF4 promoter. This study shows that miR-18a targets and negatively regulates MEF2D, which further regulates tight junction-related proteins ZO-1, claudin-5, and occludin through transactivation of KLF4 and, finally, changes the permeability of the BTB. MiR-18a should garner growing attention because it might serve as a potential target in opening the BTB and providing a new strategy for the treatment of gliomas.

Functions for the cAMP/Epac/Rap1 Signaling Pathway in Low-Dose Endothelial Monocyte-Activating Polypeptide-II-Induced Opening of Blood-Tumor Barrier

Previous studies have demonstrated that low-dose endothelial monocyte-activating polypeptide-II (EMAP-II) induces blood-tumor barrier (BTB) hyperpermeability via both paracellular and transcellular pathways. In a recent study, we revealed that cAMP/PKA-dependent and cAMP/PKA-independent signaling pathways are both involved in EMAP-II-induced BTB hyperpermeability. The present study further investigated the exact mechanisms through which the cAMP/PKA-independent signaling pathway affects EMAP-II-induced BTB hyperpermeability. In an in vitro BTB model, low-dose EMAP-II (0.05 nM) induced a significant decrease in Rap1 activity in RBMECs. Pretreatment with forskolin to elevate intracellular cAMP concentration completely blocked EMAP-II-induced Rap1 inactivation. Epac/Rap1 activation by 8-pCPT-2'-O-Me-cAMP significantly prevented EMAP-II-induced activation of RhoA/ROCK. Furthermore, 8-pCPT-2'-O-Me-cAMP pretreatment significantly inhibited EMAP-II-induced decreases in TEER and increases in HRP flux. Pretreatment also significantly prevented EMAP-II-induced changes in MLC phosphorylation, actin cytoskeleton arrangement, and expression and distribution of ZO-1 in RBMECs. This study demonstrates that the cAMP/Epac/Rap1 signaling cascade is a crucial pathway in EMAP-II-induced BTB hyperpermeability.