Proteasomal dysfunction activates the transcription factor SKN-1 and produces a selective oxidative-stress response in Caenorhabditis elegans

摘要

pSKN-1 in the nematode worm iCaenorhabditis elegans/i is functionally orthologous to mammalian NRF2 [NF-E2 (nuclear factor-E2)-related factor 2], a protein regulating response to oxidative stress. We have examined both the expression and activity of SKN-1 in response to a variety of oxidative stressors and to down-regulation of specific gene targets by RNAi (RNA interference). We used an SKN-1–GFP (green fluorescent protein) translational fusion to record changes in both iskn/i-i1/i expression and SKN-1 nuclear localization, and a igst/i-i4/i–GFP transcriptional fusion to measure SKN-1 transcriptional activity. GST-4 (glutathione transferase-4) is involved in the Phase II oxidative stress response and its expression is lost in an iskn/i-i1/i(izu67/i) mutant. In the present study, we show that the regulation of iskn/i-i1/i is tied to the protein-degradation machinery of the cell. RNAi-targeted removal of most proteasome subunits in iC. elegans/i caused nuclear localization of SKN-1 and, in some cases, induced transcription of igst/i-i4/i. Most intriguingly, RNAi knockdown of proteasome core subunits caused nuclear localization of SKN-1 and induced igst/i-i4/i, whereas RNAi knockdown of proteasome regulatory subunits resulted in nuclear localization of SKN-1 but did not induce igst/i-i4/i. RNAi knockdown of ubiquitin-specific hydrolases and chaperonin components also caused nuclear localization of SKN-1 and, in some cases, also induced igst/i-i4/i transcription. iskn/i-i1/i activation by proteasome dysfunction could be occurring by one or several mechanisms: (i) the reduced processivity of dysfunctional proteasomes may allow oxidatively damaged by-products to build up, which, in turn, activate the iskn/i-i1/i stress response; (ii) dysfunctional proteasomes may activate the iskn/i-i1/i stress response by blocking the constitutive turnover of SKN-1; and (iii) dysfunctional proteasomes may activate an unidentified signalling pathway that feeds back to control the iskn/i-i1/i stress response./p