Orphan Nuclear Receptor Nur77 Promotes Acute Kidney Injury and Renal Epithelial Apoptosis

Nur77 and its family members Nurr1 and Nor-1 are inducible orphan nuclear receptors that orchestrate cellular responses to diverse extracellular signals. In epithelia, Nur77 can act as a potent proapoptotic molecule in response to cellular stress, suggesting a possible role for this nuclear receptor in the tissue response to injury. Here, we found that Nur77 promotes epithelial cell apoptosis after AKI. Injury of proximal tubular epithelial cells rapidly and strongly induced Nur77, Nor-1, and Nurr1 both in vitro and in vivo. After renal ischemia-reperfusion, Nurr77-deficient mice exhibited less apoptosis of tubular epithelial cells and better renal function than wild-type mice. Nur77-mediated renal injury involved a conformational change of Bcl2 and an increase in the protein levels of proapoptotic Bcl-xS. Ligand-activated retinoic acid receptors repressed Nur77 induction and function. Pretreatment of wild-type mice with retinoic acid before renal ischemia-reperfusion blunted the induction of Nur77, conferred protection of renal function, attenuated renal histologic injury, and reduced the expression of epithelial-derived proinflammatory cytokines. Retinoic acid also inhibited hypoxia-mediated induction of proinflammatory cytokines in cultured renal epithelial cells. Results obtained from proximal tubule cultures derived from Nur77-deficient mice suggested that the inhibition of Nur77 expression mediated the renoprotective effects of retinoic acid. In summary, Nur77 promotes epithelial apoptosis after ischemia-reperfusion injury, and retinoic acid-mediated inhibition of Nur77 expression is a promising therapeutic strategy for the prevention of AKI.