SKN-1 in Caenorhabditis elegans is functionally orthologous to mammalian NRF2, 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. We used a SKN-1Colon, two colonsGFP translational fusion to record changes in both
skn-1 expression and SKN-1 nuclear localization, and a gst-4Colon, two colonsGFP transcriptional fusion to measure SKN-1 transcriptional activity. GST-4 is involved in the Phase II oxidative stress response and its expression is lost in a
skn-1(
zu67) mutant. In this study, we show that the regulation of
skn-1 is tied to the protein-degradation machinery of the cell. RNAi-targeted removal of most proteasome subunits in C. elegans caused nuclear localization of SKN-1 and in some cases induced transcription of
gst-4. Most intriguingly, RNAi knockdown of proteasome core subunits caused nuclear localization of SKN-1 and induced
gst-4, while RNAi knockdown of proteasome regulatory subunits resulted in nuclear localization of SKN-1 but did not induce
gst-4. RNAi knockdown of ubiquitin-specific hydrolases and chaperonin components also caused nuclear localization of SKN-1 and in some cases also induced
gst-4 transcription.
skn-1 activation by proteasome dysfunction could be occurring by one or several mechanisms: 1) the reduced processivity of dysfunctional proteasomes may allow oxidatively damaged by-products to build-up which, in turn, activate the
skn-1 stress response; 2) dysfunctional proteasomes may activate the
skn-1 stress-response by blocking the constitutive turnover of SKN-1; 3) dysfunctional proteasomes may activate an unidentified signaling pathway that feeds back to control the
skn-1 stress-response.