Inhibiting Ribosomal RNA Synthesis in C. elegans Protects Against Reductive Stress During de novo Fatty Acid Synthesis Deficiency.

Reductive stress has remained underappreciated as a significant disrupter of redox homeostasis. Recent studies have begun to link the accumulation of NADH and NADPH to the development and progression of metabolic diseases such as cancer, cardiac disease, and diabetes. In this study we use the nematode Caenorhabditis elegans to examine the phenomenon of catastrophic reductive-death caused by combined biguanide treatment and fasn-1 deficiency. This process of synergistic biguanide-induced reductive stress correlates with the activation of hypodermal stress response genes and aberrant alternations in the nucleolar morphology of hypodermal cells. Interestingly, we find that loss-of-function and RNAi-based knockdown of the catalytic RNA exosome subunit crn-3 significantly protects against reductive death. RNAi knockdown of multiple other genes involved in rRNA synthesis recapitulate this phenotype. We postulate that this reversal of reductive death can be attributed to impaired ribosomal RNA biogenesis that promotes tolerance of accumulated reducing equivalents NADPH and NADH while also preventing the accumulation of GSH by potentially activating downstream signaling pathways. Notably, we identify a downstream nuclear RNAi pathway that is activated by phenformin in a fasn-1 dependent manner and is also activated upon disruption of rRNA processing. Overall, we identify a novel mechanism by which pathologic states of reductive stress-related diseases could be ameliorated.