Orexin-A regulates cell apoptosis in human H295R adrenocortical cells via orexin receptor type 1 through the AKT signaling pathway

Interaction of Orexin and Bone Morphogenetic Proteins in Steroidogenesis by Human Adrenocortical Cells

Orexins are neuropeptides that play important roles in sleep-wake regulation and food intake in the central nervous system, but their receptors are also expressed in peripheral tissues, including the endocrine system. In the present study, we investigated the functions of orexin in adrenal steroidogenesis using human adrenocortical H295R cells by focusing on its interaction with adrenocortical bone morphogenetic proteins (BMPs) that induce adrenocortical steroidogenesis. Treatment with orexin A increased the mRNA levels of steroidogenic enzymes including StAR, CYP11B2, CYP17, and HSD3B1, and these effects of orexin A were further enhanced in the presence of forskolin. Interestingly, orexin A treatment suppressed the BMP-receptor signaling detected by Smad1/5/9 phosphorylation and Id-1 expression through upregulation of inhibitory Smad7. Orexin A also suppressed endogenous BMP-6 expression but increased the expression of the type-II receptor of ActRII in H295R cells. Moreover, treatment with BMP-6 downregulated the mRNA level of OX1R, but not that of OX2R, expressed in H295R cells. In conclusion, the results indicate that both orexin and BMP-6 accelerate adrenocortical steroidogenesis in human adrenocortical cells; both pathways mutually inhibit each other, thereby leading to a fine-tuning of adrenocortical steroidogenesis.

α-Synuclein Induced the Occurrence of RBD via Interaction with OX1R and Modulated Its Degradation

Rapid eye movement (REM) sleep behavior disorder (RBD) is a powerful early sign of Parkinson's disease (PD), but the pathogenetic mechanism involved in RBD remains largely unexplored. α-Synuclein has been verified to form Lewy bodies in the orexin neurons, whose activity and function rely on the orexin 1 receptor (OX1R). Dysfunction of the OX1R may induce the occurrence of RBD. Here, we determined the role of the interaction between α-Synuclein and OX1R in the pathogenesis of RBD, in vitro and in vivo. We found that injection of α-Synuclein into the lateral hypothalamus area (LHA) damaged orexin neurons and induced the RBD-like sleep pattern, to further damage dopaminergic neurons and result in locomotor dysfunction in mice. α-Synuclein interacted with OX1R, promoting the degradation of OX1R through proteasomal and lysosomal pathways. In addition, overexpression of α-Synuclein downregulated OX1R-mediated signaling, subsequently leading to orexin neuron damage. We conclude that α-Synuclein induced the occurrence of RBD via interaction with OX1R and modulated its degradation. These findings provide evidence for a novel mechanism by which the association of α-Synuclein with OX1R was attributed to α-Synuclein-induced orexin neuron damage, which may be a new molecular target for an effective therapeutic strategy for RBD pathology.

Orexin-A protects SH-SY5Y cells against H2O2-induced oxidative damage via the PI3K/MEK1/2/ERK1/2 signaling pathway

Orexin-A elicits multiple potent effects on a variety of tumor cells via different signaling pathways. However, it is unknown whether it has a neuroprotective effect on SH-SY5Y human neuroblastoma cells. This study investigated the neuroprotective effect of Orexin-A against hydrogen peroxide (H₂O₂)-induced oxidative damage in SH-SY5Y cells and the underlying mechanism. H₂O₂ treatment decreased the viability of SH-SY5Y cells, induced apoptosis, and decreased superoxide dismutase activity. Orexin-A attenuated these effects, indicating that it protects SH-SY5Y cells against H₂O₂-induced oxidative damage. Pre-treatment with Orexin-A also attenuated H₂O₂-induced increases in phosphorylation of MEK_1/2 and ERK_1/2. Moreover, these effects of Orexin-A were reduced in the presence of the PI3K inhibitor LY294002. Finally, pre-treatment with LY294002 abrogated attenuation of the H₂O₂-induced decrease in cell viability and increase in caspase-3/7 activity by Orexin-A. These results show that the PI3K/MEK_1/2/ERK_1/2 signaling pathway is involved in the neuroprotective effects of Orexin-A against H₂O₂-induced oxidative damage in SH-SY5Y cells. Our findings provide insight into the neuroprotective effects of Orexin-A and the underlying mechanism, which will be useful for the treatment of nervous system diseases.

Global analysis of gene expression mediated by OX1 orexin receptor signaling in a hypothalamic cell line

The orexins and their cognate G-protein coupled receptors have been widely studied due to their associations with various behaviors and cellular processes. However, the detailed downstream signaling cascades that mediate these effects are not completely understood. We report the generation of a neuronal model cell line that stably expresses the OX₁ orexin receptor (OX₁) and an RNA-Seq analysis of changes in gene expression seen upon receptor activation. Upon treatment with orexin, several families of related transcription factors are transcriptionally regulated, including the early growth response genes (Egr), the Kruppel-like factors (Klf), and the Nr4a subgroup of nuclear hormone receptors. Furthermore, some of the transcriptional effects observed have also been seen in data from in vivo sleep deprivation microarray studies, supporting the physiological relevance of the data set. Additionally, inhibition of one of the most highly regulated genes, serum and glucocorticoid-regulated kinase 1 (Sgk1), resulted in the diminished orexin-dependent induction of a subset of genes. These results provide new insight into the molecular signaling events that occur during OX₁ signaling and support a role for orexin signaling in the stimulation of wakefulness during sleep deprivation studies.