Human retinoic acid receptor‐related orphan receptor α1 overexpression protects neurones against oxidative stress‐induced apoptosis

Abstract

Retinoic acid receptor-related orphan receptor a (RORa) is a transcription factor belonging to the superfamily of nuclear receptors. Disruption of the Rora gene in the mouse results in a defect in the development of Purkinje cells leading to a cerebellar atrophy, which suggests a neuroprotective role for RORa. To test this hypothesis, the survival rate of lentiviralmediated human RORa1-overexpressing neurones has been evaluated in response to different stressors disturbing the redox homeostasis, such as b-amyloid peptide, c 2 -ceramide and H 2 O 2 . We show that overexpression of human RORa1 provides neuroprotection by increasing the expression of the antioxidant proteins glutathione peroxidase 1 and peroxiredoxin 6, leading to a reduction in the accumulation of stressinduced reactive oxygen species. We further demonstrate that the neuroprotective effect of RORa is predominantly mediated by glutathione peroxidase 1 and peroxiredoxin 6. These results suggest a new role for RORa in the control of the neuronal oxidative stress and thus represents a new transcription factor of interest in the regulation of reactive oxygen species-induced neurodegenerative processes during ageing. Keywords: apoptosis, lentivirus, neuroprotection, oxidative stress, retinoic acid receptor-related orphan receptor a. J. Neurochem. 96, 1778-1789.There is accumulating evidence that a proper balance between oxidants and antioxidants is of systemic importance for health and longevity. Homeostasis of redox status is normally maintained by intracellular regulatory molecules and enzymes such as glutathione, thioredoxin, catalase, superoxide dismutase and peroxidases. Exposure to oxidants can lead to oxidative stress wherein the antioxidant status of the cell is thrown out of balance and may lead to a variety of symptoms including apoptosis. How the expression of these enzymes, implicated in homeostasis of redox status, is regulated and which factors are involved is still poorly understood.Retinoic acid receptor-related orphan receptor a (RORa) is a member of the superfamily of the nuclear receptors (Giguere 1999). Initially described as an orphan receptor, RORa had long been considered a constitutive activator of transcription in the absence of exogenous ligand (Atkins et al. 1999;Harris et al. 2002). Recently, however, cholesterol and 7-dehydrocholesterol were shown to be natural ligands of the transcription factor RORa (Kallen et al. 2002;Bitsch et al. 2003).Mutant mice lacking functional RORa protein (Hamilton et al. 1996;Steinmayr et al. 1998) develop a severe cerebellar ataxia due to a massive cerebellar neurodegeneration (Sidman et al. 1962 Abbreviations used: Ab, b-amyloid peptide; GFP, green fluorescent protein; Gpx, glutathione peroxidase; hRORa, human retinoic acid receptor-related orphan receptor a; Prx, peroxiredoxin; RORa, retinoic acid receptor-related orphan receptor a; ROS, reactive oxygen species; si-RNA, small interfering RNA.

Cell-Autonomous and Non-Cell-Autonomous Neuroprotective Functions of RORα in Neurons and Astrocytes during Hypoxia

“…The increase in susceptibility to hypoxia was therefore not due to changes in neuronal development in vitro. Moreover, this cell-autonomous function of ROR␣ in neurons was not associated with a deregulation of antioxidant gene expression, contrary to what was previously shown with other stress (Boukhtouche et al, 2006). The target genes of ROR␣ responsible for modulating neuronal survival after hypoxia have yet to be identified.…”

“…The principal CNS phenotype of this mouse is extensive cerebellar neurodegeneration resulting in severe ataxia (Gold et al, 2007). In addition to its function in the cerebellum, ROR␣ has other important functions in the CNS including a neuroprotective role in cortical neurons (Boukhtouche et al, 2006). The upregulation of ROR␣ expression during hypoxia and its neuroprotective function raise questions about the possible role of neuronal and/or astrocyte ROR␣ in neuroprotective mechanisms during hypoxia in the cortex.…”

“…Oxidative stress is one of the major events leading to neuronal death after hypoxia. ROR␣ has been shown to protect cortical neurons against oxidative stress-induced apoptosis by increasing the expression of antioxidant proteins, thereby reducing the accumulation of reactive oxygen species (Boukhtouche et al, 2006). We thus compared the expression of various antioxidant genes in Rora ϩ/ϩ and Rora sg/sg neurons and astrocytes after hypoxia.…”

Rorα, a pivotal nuclear receptor for Purkinje neuron survival and differentiation: From development to ageing

“…To test whether an increased RORα expression could be neuroprotective, we have recently developed a recombinant lentiviral vector to perform RORα overexpression in cultured neurons. The survival rate of RORα-overexpressing cortical neurons was evaluated in response to different stressors disturbing redox homeostasis, such as Aß peptide, c 2 -ceramide and H 2 O 2 (51). In this study, we have shown that lentiviral-mediated hRORα1-overexpression provides neuroprotection against reactive oxygen species (ROS)-induced apoptosis.…”

“…Down-regulation of Gpx1 or Prx6 by si-RNA experiments partially suppressed the RORα-mediated neuroprotection, further demonstrating that this protection is, at least in part, mediated by an up-regulation of the anti-oxidant enzymes glutathione peroxidase 1 and peroxiredoxin 6, which leads to a decrease of the oxidative stress in neurons. RORα appears thus as a factor controlling the oxidative stress in neurons (51).…”