Low-dose endothelial monocyte-activating polypeptide-II increases permeability of blood-tumor barrier via a PKC-ζ/PP2A-dependent signaling mechanism

Our previous study demonstrated that low-dose endothelial monocyte-activating polypeptide-II (EMAP-II) induces blood tumor barrier (BTB) opening via the RhoA/Rho kinase/protein kinase C (PKC)-alpha/beta signaling pathway and that PKC-zeta is involved in this process via other mechanisms. In the present study, using an in vitro BTB model, we detected the exact signaling mechanisms by which PKC-zeta activation affects EMAP-II-induced BTB hyperpermeability. Our results showed that three types of serine/threonine (Ser/Thr) protein phosphatases (PPs), namely PP1, PP2A, and PP2B, were expressed by rat brain microvascular endothelial cells (RBMECs). There was an interaction between PKC-zeta and PP2A in RBMECs. In addition, EMAP-II induced a significant increase in both the expression and the activity of PP2A in RBMECs. Inhibition of PKC-zeta with PKC-zeta pseudosubstrate inhibitor (PKC-zeta-PI) completely blocked EMAP-II-induced PP2A activation. Conversely, inhibition of PP2A with okadaic acid (OA) had no effect on EMAP-II-induced PKC-zeta activation. Like PKC-zeta-PI, OA partially prevented EMAP-II-induced BTB hyperpermeability and occludin redistribution in RBMECs. Neither PKC-zeta-PI nor OA affected EMAP-II-induced phosphorylation of myosin light chain and redistribution of actin cytoskeleton in RBMECs. Taken together, our present study demonstrated that low-dose EMAP-II increases BTB permeability by activating the PKC-zeta/PP2A signaling pathway, which consequently leads to the disruption of TJs and impairment of endothelial barrier function. (C) 2015 Elsevier Inc. All rights reserved.