Abstract Objective. Phosphoenolpyruvate carboxykinase (PCK) catalyzes the conversion of oxaloacetate to phosphoenolpyruvate and regulates pyruvate metabolism and gluconeogenesis in response to glucocorticoid and insulin stimuli. Mitochondrial isoform of this enzyme (PCK2) is overexpressed in glioblastoma cells and participates in metabolic reprogramming and cell proliferation. This study aims to examine the impact of ERN1 (endoplasmic reticulum to nucleus signaling 1) inhibition on PCK2 expression and sensitivity to glucose and glutamine deprivation to determine the role of ERN1 signaling in the regulating its expression in glioblastoma cells. Methods. The glioblastoma cell line U87MG and two genetically modified variants of these cells were used. These were glioblastoma cell sublines with suppressed endoribonuclease and protein kinase activities of ERN1 (dnERN1) or only ERN1 endoribonuclease (dnrERN1), and control cells transfected with an empty vector. The suppression of ERN1 function by silencing of ERN1 and XBP1 mRNAs was also used. Hypoxia was generated using the HIF1A prolyl hydroxylase inhibitor dimethyloxalylglycine. For glucose and glutamine deprivation, DMEM medium without glucose or glutamine was used. The expression level of the PCK2 mRNA was analyzed by real-time qPCR and normalized to the beta-actin mRNA. Results. It has been demonstrated that PCK2 mRNA expression is significantly decreased in dnERN1 glioblastoma cells. Similar suppression of this mRNA expression was also observed in cells with only the endoribonuclease activity of ERN1 inhibited, indicating that this enzymatic activity is involved in the regulation of PCK2 expression. The silencing of ERN1 and XBP1 mRNAs also induced similar changes in PCK2 mRNA expression, possibly mediated by XBP1s. The expression of PCK2 was enhanced under glutamine deprivation in control glioblastoma cells, but inhibition of ERN1 activity strongly increased this effect. Upregulated PCK2 expression was also observed in control glioblastoma cells under glucose deprivation. However, the inhibition of ERN1 activity strongly increased the sensitivity of this gene expression to glucose deprivation. Furthermore, PCK2 mRNA expression was resistant to hypoxic conditions in cells with native ERN1. At the same time, in glioblastoma cells with inhibited ERN1 activity, a strong induction of PCK2 expression was observed. Conclusion. The results of this study demonstrated that ERN1 inhibition reduces PCK2 mRNA expression through the ERN1 endoribonuclease activity. This mRNA expression is upregulated under glutamine and glucose deprivation. Moreover, ERN1 inhibition strongly enhanced the sensitivity of PCK2 mRNA expression to glucose and glutamine deprivation as well as to hypoxia.
Viletska et al. (2026) studied this question.