Chemical genetics inC. elegansidentifies anticancer mycotoxins chaetocin and chetomin as potent inducers of a nuclear metal homeostasis response

Abstract

C. elegans numr-1/2(nuclear-localizedmetal-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 fornumr-1p::GFPactivation. The most potent activator was chaetocin, a fungal 3,6-epidithiodiketopiperazine (ETP) with promising anticancer activity. Chaetocin activatesnumr-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 inC. elegansas genetic disruption and RNAi screening of homologs did not inducenumr-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 inducenumr-1/2.ETP monomer gliotoxin, despite possessing a disulfide linkage, had almost no effect onnumr-1/2, suggesting a dimer requirement. Chetomin inhibitsC. elegansgrowth at low micromolar levels and loss ofnumr-1/2increases sensitivity;C. elegansand Chaetomiaceae fungi inhabit similar environments raising the possibility thatnumr-1/2functions as a defense mechanism. There is no direct ortholog ofnumr-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.