| The orphan nuclear receptor Nur77(also known as NGFI-B or TR3),encoded by the immediate early gene NR4A1,belongs to the nuclear receptor(NR)superfamily and plays an important regulatory role in a variety of cancers.Our previous study found that glucose deprivation can activate ERK2 to phosphorylate Nur77,which further facilitates the mitochondrial targeting of Nur77.Nur77 then binds to TPβ,a thiolase in fatty acid oxidation,thereby impeding ROS-induced oxidative inactivation of TPp,which in turn promotes fatty acid oxidation to produce NADPH for the elimination of ROS and cell survival under glucose deprivation conditions.However,it is not clear that how Nur77 enters mitochondria and how it protects TPβ.In the present study,we found that the mitochondrial outer membrane protein Nix interacts with phosphorylated Nur77,which promotes the translocation of Nur77 into mitochondria to bind to TPβ.The mitochondrial localization of Nur77 and its binding to TPβ is a prerequisite for Nur77 to protect TPβ from oxidative inactivation and is also necessary for Nur77 to promote cell survival under glucose deprivation conditions.Further studies reveals that under glucose deprivation,Cys505,Cys551 and Cys566 on Nur77 LBD were oxidized.If these three oxidation sites are mutated,Nur77 loses the ability to protect TPβ from oxidative inactivation.We further found that Nur77 is more sensitive than TPβ to ROS-induced oxidation under glucose deprivation.Nur77 binding to TPβ protects TPβ from oxidation by self-sacrifice of Nur77,which protects the enzymatic activity of TPβ and thus promotes fatty acid oxidation in melanoma cells.It was demonstrated that the key phosphorylation sites and oxidation sites of Nur77 are important for reducing ROS and improving cell viability in melanoma cells under glucose deprivation conditions.To study the protective effect of Nur77 under physiologic conditions,we tested the survival of circulation tumor cells,which undergo loss of attachment to the matrix and glucose starvation.Mutations of the key phosphorylation sites and oxidation sites on Nur77 inhibits not only the survival of mouse circulating tumor cells but also the lung metastasis of melanoma.In summary,this study not only uncovers new functions of Nur77 and Nix under metabolic stress,but also provides new ideas for the therapy of melanoma. |
