Unique structure and regulation of the nematode detoxification gene regulator, SKN-1: implications to understanding and controlling drug resistance

Abstract

Nematodes parasitize an alarming number of people and agricultural animals globally and cause debilitating morbidity and mortality. Anthelmintics have been the primary tools used to control parasitic nematodes for the past several decades, but drug resistance is becoming a major obstacle. Xenobiotic detoxification pathways defend against drugs and other foreign chemicals in diverse organisms, and evidence is accumulating that they play a role in mediating resistance to anthelmintics in nematodes. Related anti-oxidation pathways may also provide filarial parasites protection against host free radical-mediated immune responses. Upstream regulatory pathways have received almost no attention in nematode parasites despite their potential to co-regulate multiple detoxification and anti-oxidation genes. The NRF2 transcription factor mediates inducible detoxification and anti-oxidation defenses in mammals and recent studies have demonstrated that it promotes multidrug resistance in some human tumors. Recent studies in the free-living model nematode Caenorhabditis elegans have defined the homologous transcription factor SKN-1 as a master regulator of detoxification and anti-oxidation genes. Despite similar functions, SKN-1 and NRF2 have important differences in structure and regulatory pathways. Protein alignment and phylogenetic analyses indicate that these differences are shared among many nematodes making SKN-1 a candidate for specifically targeting nematode detoxification and anti-oxidation.

Direct Interaction between the WD40 Repeat Protein WDR-23 and SKN-1/Nrf Inhibits Binding to Target DNA

“…We measured in vivo gst-4::GFP fluorescence in all 10 sequenced wdr-23 mutants and the single skn-1 mutant to confirm pathway activation; gst-4 is a core SKN-1 target gene that is strongly activated in multiple tissues under numerous stress conditions (8,16,18,37). As shown in Fig.…”

“…By promoting redox homeostasis and small-molecule detoxification, SKN-1 is also a potential pharmacological target for host defense and drug resistance in parasitic nematodes (16,17). The skn-1 locus encodes three splice variants-SKN-1a, SKN1b, and SKN-1c-with distinct expression patterns and functions.…”

“…We previously determined that Keap1 is absent from C. elegans and other nematodes (16) and identified WDR-23, a conserved WD40 repeat protein, as a direct repressor of SKN-1 (8,23,24). Transcriptional and translational green fluorescent protein (GFP) reporters suggest that WDR-23 is expressed in many tissues, including the intestine, and regulates SKN-1 activity cell autonomously (8,23).…”

Inhibition of the oxidative stress response by heat stress inCaenorhabditis elegans

“…HSF-1 is itself negatively regulated by Hsp70 and Hsp90 chaperone complexes that bind HSF-1 during non-stress conditions and reduce its transcriptional activity (Morton and Lamitina, 2013;Schöffl et al, 1998;Voellmy and Boellmann, 2007). The OxSR in C. elegans is largely mediated by a single CNC transcription factor, SKN-1, that translocates into nuclei during oxidative stress to activate the transcription of over 2000 genes, including more than 100 antioxidant and detoxification genes (An and Blackwell, 2003;Choe et al, 2012;Oliveira et al, 2009;Park et al, 2009). Important OxSR genes upregulated by SKN-1 activation include those encoding glutathione S-transferases (e.g.…”

A Negative-Feedback Loop between the Detoxification/Antioxidant Response Factor SKN-1 and Its Repressor WDR-23 Matches Organism Needs with Environmental Conditions

“…We recently determined that C. elegans and other nematodes lack an ortholog of KEAP1 (20) and identified a WD40 repeat protein named WDR-23 as the direct repressor of SKN-1 (21). Biochemical and genetic evidence suggests that WDR-23 functions to constitutively recruit SKN-1 to a DDB1-CUL4 ubiquitin ligase.…”