Involvement of ATF6 in Octreotide-Induced Endothelial Barrier Enhancement

<b>Background/Objectives</b>: Endothelial hyperpermeability is the hallmark of severe disease, including sepsis and acute respiratory syndrome (ARDS). The development of medical countermeasures to treat the corresponding illness is of utmost importance. Synthetic somatostatin analogs (SSA) are FDA-approved drugs prescribed in patients with neuroendocrine tumors, and they act via growth hormone (GH) suppression. Preclinical investigations suggest that Octreotide (OCT) alleviates Lipopolysaccharide (LPS)-induced injury. The aim of the study is to investigate the involvement of activating transcription factor 6 (ATF6) in the protective effects of OCT in endothelial dysfunction. To the best of our knowledge, the available information on that topic is limited. <b>Methods</b>: Human lung microvascular endothelial cells (HULEC-5a) and bovine pulmonary artery endothelial cells (BPAEC) which expressed elevated levels of ATF6 due to AA147 were exposed to OCT or vehicle. Protein expression, endothelial permeability, and reactive oxygen species (ROS) generation were assessed utilizing Western blot analysis, Fluorescein isothiocyanate (FITC)-Dextran assay, and Dichlorofluorescein diacetate measurements, respectively. <b>Results</b>: Our observations suggest that ATF6 activation significantly improves OCT-induced endothelial barrier enhancement. This combination led to increased expression of binding immunoglobulin protein (BiP) and glucose-regulated protein 94 (Grp94), which are downstream unfolded protein response (UPR) targets. Moreover, ATF6 activation prior to OCT treatment resulted in decreased activation of myosin light chain 2 (MLC2) and cofilin; and reduced reactive oxygen species (ROS) generation. ATF6 activation enhanced the anti-inflammatory effects of OCT, as reflected in the suppression of transducer and activator of transcription (STAT) 1, STAT3, and P38 phosphorylation. <b>Conclusions</b>: Our findings suggest that ATF6 activation prior to OCT treatment enhances the beneficial effects of OCT in the endothelium.