ERN1-dependent regulation of BAG cochaperone 1 expression and the sensitivity to glutamine deprivation in U87MG glioblastoma cells

Abstract Objective. The BAG cochaperone 1 (BAG1) binds to oncogene BCL2 and markedly enhances its anti-apoptotic effects. This cochaperone represents a link between growth factor receptors and anti-apoptotic mechanisms mediated by endoplasmic reticulum stress. BAG1 interacts with the glucocorticoid receptor and modulates its transcription activity. As a cochaperone for several HSP70 proteins, it participates in control of protein folding. The present study aims to investigate the regulation of the BAG1 mRNA expression in U87MG glioblastoma cells by hypoxia and glucose or glutamine deprivation, depending on the inhibition of ERN1 (endoplasmic reticulum to nucleus signaling 1) with the intent to reveal the role of ERN1 signaling in the regulation of this gene expression and function in oncogenesis. Methods. The U87MG glioblastoma cells (transfected by an empty vector; control) and cells with inhibited ERN1 endoribonuclease and protein kinase (dnERN1) or only ERN1 endoribonuclease (dnrERN1) were used. Silencing of ERN1 and XBP1 mRNAs for suppression of ERN1 function was also used. A hypoxic condition was created by dimethyloxalylglycine (4 h). DMEM medium without glucose or glutamine was used for glucose and glutamine deprivation (16 h). The expression level of the BAG1 mRNA was studied by real-time qPCR and normalized to the beta-actin mRNA. Results. Inhibition of the endoribonuclease activity of ERN1 significantly decreased BAG1 mRNA expression. However, a lesser suppression of this mRNA expression was observed in dnERN1 cells (with inhibited ERN1 endoribonuclease and protein kinase) indicating the involvement of protein kinase in controlling BAG1 expression. The silencing of ERN1 and XBP1 mRNAs also reduced the expression of BAG1 mRNA demonstrating the involvement of XBP1s in this regulation. The expression of the BAG1 gene was resistant to glutamine deprivation and upregulated in response to glucose deprivation in control glioblastoma cells. However, the inhibition of ERN1 increased the sensitivity of BAG1 gene expression to both glucose and glutamine deprivation. Furthermore, the expression of the BAG1 gene was increased under hypoxia in control U87MG cells; however, a greater induction was observed in dnERN1 cells. Conclusion. The results of this study demonstrated that ERN1 inhibition reduces BAG1 mRNA expression through the endoribonuclease activity of ERN1 and that protein kinase activity counteracts endoribonuclease in regulating the expression of BAG1 mRNA. Moreover, ERN1 inhibition also enhances the sensitivity of BAG1 mRNA expression to nutrient supply and hypoxia resulting in reduced resistance of glioblastoma cells.