Cold Inducible RNA Binding Protein Is Involved in Chronic Hypoxia Induced Neuron Apoptosis by Down-Regulating HIF-1α Expression and Regulated By microRNA-23a

Abstract

Background: Neuron apoptosis mediated by hypoxia inducible factor 1α (HIF-1α) in hippocampus is one of the most important factors accounting for the chronic hypobaric hypoxia induced cognitive impairment. As a neuroprotective molecule that is up-regulated in response to various environmental stress, CIRBP was reported to crosstalk with HIF-1α under cellular stress. However, its function under chronic hypobaric hypoxia remains unknown.Objective: In this study, we tried to identify the role of CIRBP in HIF-1α mediated neuron apoptosis under chronic hypobaric hypoxia and find a possible method to maintain its potential neuroprotective in long-term high altitude environmental exposure.Methods: We established a chronic hypobaric hypoxia rat model as well as a tissue culture model where SH-SY5Y cells were exposed to 1% hypoxia. Based on these models, we measured the expressions of HIF-1α and CIRBP under hypoxia exposure and examined the apoptosis of neurons by TUNEL immunofluorescence staining and western blot analysis of apoptosis related proteins. In addition, by establishing HIF-1α shRNA and pEGFP-CIRBP plasmid transfected cells, we confirmed the role of HIF-1α in chronic hypoxia induced neuron apoptosis and identified the influence of CIRBP over-expression upon HIF-1α and neuron apoptosis in the process of exposure. Furthermore, we measured the expression of the reported hypoxia related miRNAs in both models and the influence of miRNAs' over-expression/knock-down upon CIRBP in the process of HIF-1α mediated neuron apoptosis.Results: HIF-1α expression as well as neuron apoptosis was significantly elevated by chronic hypobaric hypoxia both in vivo and in vitro. CIRBP was induced in the early stage of exposure (3d/7d); however as the exposure was prolonged (21d), CIRBP level of the hypoxia group became significantly lower than that of control. In addition, HIF-1α knockdown significantly decreased neuron apoptosis under hypoxia, suggesting HIF-1α may be pro-apoptotic in the process of exposure. CIRBP over-expression significantly suppressed HIF-1α up-regulation in hypoxia and inhibited HIF-1α mediated neuron apoptosis. Interestingly, miR-23a was also induced by hypoxia exposure and showed the same changing tendency with CIRBP (increasing in 3d/7d, decreasing in 21d). In addition, over-expressing miR-23a up-regulated CIRBP, down-regulated HIF-1α and attenuated neuron apoptosis.Conclusion: Cold inducible RNA binding protein is involved in chronic hypoxia induced neuron apoptosis by down-regulating HIF-1α expression, and MiR-23a may be an important tool to maintain CIRBP level and function.

Recent progress in the research of cold-inducible RNA-binding protein

“…In addition, CIRP is also upregulated upon UV radiation, mild hypoxia and glucose deprivation, but is decreased in response to heat stress and inflammatory cytokines including TNF-α and TGF-β [ 9 ], suggesting that CIRP is a more general stress responsive protein ( Figure 2 ). Although, CIRP is upregulated upon mild hypoxia, chronic hypoxia has been reported to depress CIRP expression [ 10 ]. The influence of H 2 O 2 on CIRP expression is controversial with some studies showing downregulation and others showing no change [ 3 , 11 , 6 ].…”

miR-21-5p Under-Expression in Patients with Obstructive Sleep Apnea Modulates Intermittent Hypoxia with Re-Oxygenation-Induced-Cell Apoptosis and Cytotoxicity by Targeting Pro-Inflammatory TNF-α-TLR4 Signaling

“…MiR-23a can protect retinal pigment epithelial cells against oxidative injury through regulation of Fas, alleviate hypoxia-induced neuronal apoptosis through suppressing Apaf-1, and inhibit vascular permeability of endothelial cells through regulation of JAM-C/ZO-2, whereas it may contribute to myocardial dysfunction due to ischemia-reperfusion injury through regulation of glutamine metabolism, facilitate cell proliferation and migration by targeting BMPR2/Smad1 signaling in hypoxia-induced human pulmonary artery smooth muscles, and potentiate hypoxia-induced injury of cardimyocytes [37][38][39][40][41]. On the other hand, miR-23a has been shown to be downregulated in mice under short-term IHR exposure, and inhibition of miR-23a in stressed cells represents a general mechanism for inducing apoptosis through targeting pro-apoptotic genes [42,43].…”

Extracellular CIRP (eCIRP) and inflammation

“…The genes induced by iCIRP are thought to be a function of the intensity (mild, moderate, or severe) and timing (early or late exposure) of the stressors. In a recent study, for example, the neuronal expression of iCIRP was increased in rats subjected to chronic hypoxia, which in turn inhibited HIF‐α and, thus, protected neuronal cells from apoptosis . Besides, its role in regulation of mRNA stability and translation in the cytoplasm, nuclear iCIRP has also been shown to play a role in posttranscriptional regulation of its targets by controlling alternative polyadenylation that leads to generating alternative mRNA transcripts .…”

“…The transcription factor specificity protein 1 (sp1) is induced by hypoxia and has been found to bind to the CIRP's cold responsive element, suggesting that it may mediate hypoxia‐induced CIRP transcription . Recently, the overexpression of miRNA‐23a has also been shown to induce the expression of CIRP during prolonged hypoxia condition, possibly through its binding to CIRP's transcriptional enhancer . A recent study also revealed that hypothermia increases intracellular calcium levels via TRPV4, which regulates CIRP's expression in response to cold stress .…”