ERN1 signaling in the unfolded protein response regulates Ras homolog family member B expression in glioblastoma cells: Effect of hypoxia

Endoplasmic reticulum (ER) stress and hypoxia are essential factors for the effective growth of malignant tumors, including glioblastomas—the most malignant brain tumors, which are difficult to treat. Inhibition of the signaling protein ER to nucleus signaling 1 (ERN1) reduces the proliferation rate of glioblastoma cells by altering the expression of numerous genes. The present study aims to investigate the regulation of tumor suppressor Ras homolog family member B (RHOB) expression by ERN1 knockdown and hypoxia using genetically modified glioblastoma cells (dnERN1 and dnrERN1) and mRNA silencing. The expression level of the RHOB gene was assessed using quantitative polymerase chain reaction. It was found that inhibition of the endoribonuclease activity of ERN1 increased RHOB expression levels. However, a greater induction of RHOB expression was observed in dnERN1 cells, with inhibited ERN1 endoribonuclease and protein kinase activities, indicating their involvement in controlling RHOB expression. ERN1 and X-box binding protein 1 mRNA silencing also increased RHOB expression levels. The expression of microRNA miR-21-5p and miR-223-3p, which have binding sites in RHOB mRNA, was decreased in dnERN1 glioblastoma cells. RHOB expression was also increased under hypoxia; however, in dnERN1 glioblastoma cells, the effect of hypoxia was reduced, indicating that ERN1 is involved in this regulation. Furthermore, this gene expression was unchanged by glucose deprivation in control cells; however, it was increased in dnERN1 glioblastoma cells. These findings highlight the significant role of ERN1 protein kinase and endoribonuclease activities in the transcriptional and possibly post-transcriptional regulation of RHOB expression.