Hypoxia controls the expression of genes responsible for serine synthesis in U87MG cells on ERN1-dependent manner

Abstract

Objective. Serine synthesis as well as endoplasmic reticulum stress and hypoxia are important factors of malignant tumor growth including glioblastoma. Previous studies have shown that the knockdown of ERN1 (endoplasmic reticulum to nucleus signaling) significantly suppressed the glioblastoma cell proliferation and modified the hypoxia regulation. The present study is aimed to investigate the impact of hypoxia on the expression of PHGDH (phosphoglycerate dehydrogenase), PSAT1 (phosphoserine aminotransferase 1), PSPH (phosphoserine phosphatase), ATF4 (activating transcription factor 4), and SHMT1 (serine hydroxymethyltransferase 1) in U87MG glioblastoma cells in relation to knockdown of ERN1 with the intent to reveal the role of ERN1 signaling pathway on the endoplasmic reticulum stress-dependent regulation of expression of these genes.

Methods. The control U87MG glioblastoma cells (transfected by empty vector) and ERN1 knockdown cells (transfected by dominant-negative ERN1) were exposed to hypoxia introduced by dimethyloxalylglycine for 4 h. RNA was extracted from cells and reverse transcribed. The expression level of PHGDH, PSAT1, PDPH, SHMT1, and ATF4 genes was studied by real-time qPCR and normalized to ACTB.

Results. It was found that hypoxia up-regulated the expression level of PHGDH, PSAT1, and ATF4 genes in control U87MG cells, but PSPH and SHMT1 genes expression was down-regulated. The expression of PHGDH, PSAT1, and ATF4 genes in glioblastoma cells with knockdown of ERN1 signaling protein was more sensitive to hypoxia, especially PSAT1 gene. At the same time, the expression of PSPH gene in ERN1 knockdown cells was resistant to hypoxia. The expression of SHMT1 gene, encoding the enzyme responsible for conversion of serine to glycine, showed similar negative sensitivity to hypoxia in both control and ERN1 knockdown glioblastoma cells.

Conclusion. The results of the present study demonstrate that the expression of genes responsible for serine synthesis is sensitive to hypoxia in gene-specific manner and that ERN1 knockdown significantly modifies the impact of hypoxia on the expression of PHGDH, PSAT1, PSPH, and ATF4 genes in glioblastoma cells and reflects the ERN1-mediated reprograming of hypoxic regulation at gene expression level.