Gene expression profiling of HepG2 cell response to bortezomib in combination with nelfinavir or ISRIB, compounds targeting the eIF2a-ATF4 pathway

We investigate how modulation of the eIF2alpha-ATF4 stress pathway affects hepatoma cell response to bortezomib. Transcriptome profiling revealed that many ATF4 transcriptional target genes are among the highest upregulated genes in bortezomib-treated HepG2 human hepatoma cells. While pharmacological enhancement of the eIF2alpha-ATF4 pathway activity results in the elevation of the activities of all branches of the unfolded protein response (UPR) and sensitizes cells to bortezomib toxicity, the suppression of ATF4 induction delays bortezomib-induced cell death. The pseudokinase TRIB3, an inhibitor of ATF4, is expressed at a high basal level in hepatoma cells and is strongly upregulated in response to bortezomib. Bortezomib treatment leads to a robust enrichment of TRIB3 binding near genes induced by bortezomib and involved in the ER stress response and cell death. Disruption of TRIB3 increases C/EBP-ATF-driven transcription, augments ER stress and cell death in cells exposed to bortezomib, while TRIB3 overexpression enhances the cell survival. Thus, TRIB3, colocalizing with ATF4 and limiting its transcriptional activity, functions as a factor increasing resistance to bortezomib, while pharmacological over-activation of eIF2alpha-ATF4 can overcome the endogenous restraint mechanisms and sensitize cells to bortezomib. Overall design: mRNA expression profiling of single-compound and combination treatments in HepG2 cells