ERN1 inhibition modifies hypoxic regulation of EGLN1 expression in U87MG glioblastoma cells

Objective. Glioblastomas are the most common and highly aggressive malignant brain tumors that are difficult to treat. Hypoxia is a significant factor in glioblastoma growth. Previous studies have shown that the inhibition of ERN1 (endoplasmic reticulum to nucleus signaling 1) significantly suppresses the glioblastoma cell proliferation and modifies the hypoxic regulation. The present study aimed to investigate the impact of hypoxia on the expression of endoplasmic reticulum stress-dependent genes including EGLN1 in U87MG glioblastoma cells in relation to ERN1 inhibition to reveal a possible role of the ERN1 signaling pathway in the hypoxic regulation of these genes' expression. Methods. The control U87MG glioblastoma cells (transfected by an empty vector) and ERN1 knockdown cells with inhibited ERN1 endoribonuclease and protein kinase (dnERN1) were used. Hypoxia was induced at normoxic conditions with dimethyloxalylglycine (0.5 mM for 4 h). RNA was extracted and reverse transcribed. The expression levels of EGLN1, HSPA5, DDIT4, PA2G4, XIAP, and MTOR genes were studied by real-time qPCR and normalized to ACTB mRNA. Results. It was established that hypoxia increases the expression level of EGLN1, HSPA5, DDIT4, and XIAP in glioblastoma cells with native ERN1. However, the expression levels of two other stress-dependent genes (PA2G4 and MTOR) were reduced in these glioblastoma cells under hypoxia. Furthermore, inhibition of ERN1 enzymatic activity attenuated the effects of hypoxia on the expression of most of the genes studied in glioblastoma cells, while XIAP gene expression was dramatically increased. Conclusion. The results of this study showed that hypoxia differentially affects the expression of genes related to endoplasmic reticulum stress in glioblastoma cells and is dependent on ERN1 activity. This reflected the ERN1-mediated reprogramming of hypoxic regulation of gene expression.