Immediate early gene-X1 interferes with 26S proteasome activity by attenuating expression of the 19S proteasomal components Rpn10 and Rpn2

The stress response gene IEX-1 is involved in the regulation of cell growth and cellular viability. To some extent, these effects include an interference with the proteasomal turnover of certain regulatory proteins. Here, we show that IEX-1 directly attenuates the activity and formation of the 26S-proteasome in 293-cells. We further demonstrate that IEX-1 reduces the overall expression levels of certain protein components of the 19S-proteasomal subunit such as S5a/Rpn10 and S1/Rpn2, whereas the expression of other proteasomal proteins was less or not affected. In contrast to direct apoptotic stimuli, such as the anti-cancer drug etoposide, leading to caspase dependent degradation of Rpn10 and Rpn2, the effect of IEX-1 is independent of proteolytic cleavage of these proteins. Furthermore, the decreasing effect of IEX-1 on Rpn10 and Rpn2 expression is still seen in the presence of cycloheximide, but not in the presence of actinomycinD, and quantitative real time PCR revealed lower mRNA levels of Rpn10 and Rpn2 in IEX-1 overexpressing cells, suggesting an interference of IEX-1 with the gene transcription of Rpn10 and Rpn2. Additionally, luciferase assays confirmed an interference of IEX-1 with the activity of the Rpn10 promoter. These findings indicate a role of IEX-1 in the maintenance and assembly of the 26S-proteasome, obviously involving an altered gene expression of certain proteasomal proteins. Thereby, IEX-1 may essentially modulate signaling pathways related to 26S-proteasome activity and involved in cellular growth control and apoptosis.