Long-term exposure to ethanol downregulates tight junction proteins through the protein kinase Cα signaling pathway in human cerebral microvascular endothelial cells

Abstract

Brain microvascular endothelial cells (BMECs) are the primary component of the blood-brain barrier (BBB). Tight junction (TJ) proteins, including claudin, occludin and zonula occludens (ZO)-1, ZO-2 and ZO-3, maintain the structural integrity of BMECs. Ethanol activates the assembly and disassembly of TJs, which is a process that is regulated by protein kinase C (PKC). In addition, ethanol treatment leads to the loss of structural integrity, which damages the permeability of the BBB and subsequently affects central nervous system homeostasis, thus allowing additional substances to enter the brain. However, the mechanisms underlying ethanol-induced loss of BBB structure remain unknown. It has been hypothesized that long-term exposure to ethanol reduces the expression of claudin-5, occludin and ZO-1 via the PKC signaling pathway, thereby affecting BBB structural integrity. In the current study, the human cerebral microvascular endothelial cell line, HCMEC/D3, was treated with 50, 100, 200 and 400 mM ethanol for 24, 48 and 72 h. Cell viability was determined using an MTS assay. The expression of claudin-5, occludin and ZO-1 protein and mRNA was measured using western blot analysis and reverse transcription-quantitative polymerase chain reaction, respectively. Following the pretreatment of HCMEC/D3 cells with the PKCα-specific inhibitor, safingol (10 µmol/l), the expression of claudin-5, occludin, ZO-1 and phosphorylated (p)-PKCα was measured using western blot analysis, and PKCα localization was determined by immunofluorescence. With increasing concentrations of ethanol, the expression of claudin-5, occludin and ZO-1 protein decreased, while the expression of claudin-5, occludin and ZO-1 mRNA increased. Exposure to ethanol significantly increased the expression of p-PKCα, whereas no significant effect on the expression of PKCα was observed. Following 48 h treatment with 200 mM ethanol, the expression of claudin-5, occludin and ZO-1 protein was significantly decreased when compared with the control. By contrast, the expression of p-PKCα was increased, and increased translocation of PKCα from the cytoplasm to the nuclear membrane and nucleus was observed. In addition, the results demonstrated that safingol significantly reversed these effects of ethanol. In conclusion, long-term exposure to ethanol downregulates the expression of claudin-5, occludin and ZO-1 protein in HCMEC/D3 s, and this effect may be mediated via activation of PKCα.