Chemical Genetics in C. elegansIdentifies Anticancer Mycotoxins Chaetocin and Chetomin as Potent Inducers of a Nuclear Metal Homeostasis Response

C. elegansnumr-1/2(nuclear-localized metal-responsive) is an identical gene pair encoding a nuclear protein previously shown to be activated by cadmium and disruption of the integrator RNA metabolism complex. We took a chemical genetic approach to further characterize regulation of this novel metal response by screening 41,716 compounds and extracts for numr-1p::GFPactivation. The most potent activator was chaetocin, a fungal 3,6-epidithiodiketopiperazine (ETP) with promising anticancer activity. Chaetocin activates numr-1/2strongly in the alimentary canal but is distinct from metal exposure, because it represses canonical cadmium-responsive metallothionine genes. Chaetocin has diverse targets in cancer cells including thioredoxin reductase, histone lysine methyltransferase, and acetyltransferase p300/CBP; further work is needed to identify the mechanism in C. elegansas genetic disruption and RNAi screening of homologues did not induce numr-1/2in the alimentary canal and chaetocin did not affect markers of integrator dysfunction. We demonstrate that disulfides in chaetocin and chetomin, a dimeric ETP analog, are required to induce numr-1/2.ETP monomer gliotoxin, despite possessing a disulfide linkage, had almost no effect on numr-1/2, suggesting a dimer requirement. Chetomin inhibits C. elegansgrowth at low micromolar levels, and loss of numr-1/2increases sensitivity; C. elegansand Chaetomiaceaefungi inhabit similar environments raising the possibility that numr-1/2functions as a defense mechanism. There is no direct orthologue of numr-1/2in humans, but RNaseq suggests that chaetocin affects expression of cellular processes linked to stress response and metal homeostasis in colorectal cancer cells. Our results reveal interactions between metal response gene regulation and ETPs and identify a potential mechanism of resistance to this versatile class of preclinical compounds.