Inhibition of ERN1/IRE1α (endoplasmic reticulum to nucleus signaling 1/inositol requiring enzyme-1α), the major signaling pathway of endoplasmic reticulum stress, significantly decreases tumor growth. We have studied the expression of transcription factors such as E2F8 (E2F transcription factor 8), EPAS1 (endothelial PAS domain protein 1), TBX3 (T-box 3), ATF3 (activating transcription factor 3), FOXF1 (forkhead box F1), and HOXC6 (homeobox C6) in U87 glioma cells overexpressing dominant-negative ERN1/IRE1α defective in endoribonuclease (dnr-ERN1) as well as defective in both kinase and endonuclease (dn-ERN1) activity of ERN1/IRE1α. We have demonstrated that the expression of all studied genes is decreased at the mRNA level in cells with modified ERN1/IRE1α; TBX3, however, is increased in these cells as compared to control glioma cells. Changes in protein levels of E2F8, HOXC6, ATF3, and TBX3 corresponded to changes in mRNAs levels. We also found that two mutated ERN1/IRE1α have differential effects on the expression of studied transcripts. The presence of kinase and endonuclease deficient ERN1/IRE1α in glioma cells had a less profound effect on the expression of E2F8, HOXC6, and TBX3 genes than the blockade of the endoribonuclease activity of ERN1/IRE1α alone. Kinase and endonuclease deficient ERN1/IRE1α suppresses ATF3 and FOXF1 gene expressions, while inhibition of only endoribonuclease of ERN1/IRE1α leads to the up-regulation of these gene transcripts. The present study demonstrates that fine-tuning of the expression of proliferation related genes is regulated by ERN1/IRE1α an effector of endoplasmic reticulum stress. Inhibition of ERN1/IRE1α, especially its endoribonuclease activity, correlates with deregulation of proliferation related genes and thus slower tumor growth.
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