Nuclear Receptor DAF-12 Is Required For Starvation Resistance Through Octopamine Signaling And Antioxidant Responses

Starvation is probably the most common stressful situation for animals in nature.However,animals can undergo physiological and behavioral changes in order to cope with nutrient deprivation.For instance,the free-living nematode Caenorhabditis eleganscan developmentally arrest at multiple stages,such as L1 diapause,dauer diapause,and adult reproductive diapause,in response to nutrient limitation.Meanwhile,octopamine,one of the most abundant amines,is elevated in worms during starvation,and mediates astarvation-induced response,such as suppression of egg laying and promotion of locomotion.In addition,worms maintain energy balance through increases in fat granule hydrolysis by a variety of lipases.In C.elegans,the nuclear receptor DAF-12 acts as a dietary and environmental sensor.By binding with its steroidal ligands dafachronic acids(DA),DAF-12 promotes reproductive development under favorable conditions such as an abundant food supply.When nematodes detect environmental stresses,such as food limitation,the production of the ligand DA is inhibited.DAF-12 binds to its co-repressor DIN-1 to form a complex.However,role of DAF-12/DIN-1 in starvation resistance remains unknown.In this study,we found that starvation up-regulated the expression of tbh-1 that encodes tyramine ?-hydroxylase,a key enzyme for octopamine biosynthesis,in the RIC neurons,thereby promoting the octopamine levels.Octopamine induced the expression of the lipase gene lips-6 via its receptor SER-3 in the intestine.LIPS-6,in turn,elicited lipid mobilization.Interestingly,in the tbh-1 promoter,there are three AGTACA hexamer elements(-4139,-3193,and-2711 upstream of ATG),which are putative DAF-12 binding sites.Further study revealed that DAF-12/DIN-1 complex induced the expression of tbh-1 during starvation.5'-deletion analysis of promoter and ChIP-qPCR demonstrated that the three AGTACA hexamer elements were crucial for DAF-12/DIN-1-medaited expression of tbh-1 Our results also demonstrated that the DAF-12/DIN-1/TBH-1/SER-3/LIP-6 signaling plays an important role in starvation tolerance in worms.We noted that the survival rates of daf-12(rh61rh411)and din-l(dh127)mutants were significantly lower than that of tbh-1(n3247)mutants,implicating that there is another pathway through which DAF-12/DIN-1 complex acts to regulate starvation resistance.We found that loss of function mutations in daf-12(rh61rh411)or din-1(dh127)resulted in an increase in reactive oxygen species(ROS)formation,which mediated systemic necrosis,during starvation.By screening the genes involved in necrosis,we identified that tra-3 and asp-4 were required for starvation-induced worm death in daf-12(rh61rh411)and din-1(dh127)mutants.In addition,the DAF-12/DIN-1 complex up-regulated the expression of antioxidant genes,such as gst-4 encoding a glutathione S-transferase.GST-4 in turn inhibited the formation of ROS,promoting the survival of worms.Taken together,our current study has revealed that the DAF-12/DIN-1 complex induces lipolysis tomaintain energy homeostasis by activating the octopamine signaling.On the other hand,the DAF-12/DIN-1 complex suppresses ROS-mediated necrosis by up-regulating expression of antioxidant genes.Thus,our study reveals a novel mechanism underlying the DAF-12/DIN-1 signaling-mediated starvation tolerance in C.eleagns.