miR‐21 contributes to renal protection by targeting prolyl hydroxylase domain protein 2 in delayed ischaemic preconditioning

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Acute kidney injury (AKI), associated with high morbidity and mortality, can be mainly caused by renal ischaemia/reperfusion injury (IRI). Hypoxia/reoxygenation is the crucial part in mechanism of IRI and evokes a series of cellular adaptive response.1
MicroRNAs (miRNAs) are short noncoding RNA that regulate gene expression by binding target mRNA, leading to mRNA degradation or translational inhibition.3 miRNAs participate in regulation of diverse cellular physiological process and even pathogenesis of various diseases.5 Aberrant miRNAs expression have been found differently in ischaemic insult of kidney.8miR‐21 is one of the upregulated miRNAs in renal IRI. Some studies revealed that protective role of miR‐21 was associated with targeting anti‐apoptosis genes.8 And upregulation of miR‐21 contributes to protection of renal ischaemic preconditioning (IPC).11 IPC is known as a short period of ischaemia followed by reperfusion to protect against a subsequent, sustained ischaemic insult.11The protection of delayed IPC appears later and maintains longer, compared with early IPC.
Hypoxia inducible factor (HIF), as a pivotal transcription factor, mediates a variety of genes responding to hypoxia.12 HIF is composed of two subunits: HIF‐α (HIF‐1α , HIF‐2α or HIF‐3α) and constitutive HIF‐β. Under normoxic condition, HIF‐α is hydroxylated by enzyme prolyl hydroxylase domain (PHD, also known as EGLN), then HIF‐α was degraded via the ubiquitin‐proteasome pathway.13 The PHD family consists of three different isoforms: PHD1, PHD2 and PHD3, all of which require iron and ascorbate as cofactors. In the absence of oxygen, activity of PHDs decreased, resulting in HIF‐α accumulation.12 Therefore, PHD/HIF system regulate adaptive response against hypoxia by promoting hypoxia‐target genes expression.14
Activated HIF‐1α through administration of PHD inhibitor, exerts protection against renal IRI.15 HIF‐1α also can be accumulated by IPC, then attenuate the followed prolonged ischaemia.11 So we hypothesized that miR‐21 might play an important role in the renal IRI by targeting PHD2 for accumulation of HIF‐1α. We used the mouse delayed renal IPC model and suppression of miR‐21 via intravenous delivery of locked nucleic acid (LNA) modified anti‐miR‐21. With this approach, we were able to determine the impact of miR‐21 on HIF‐1α expression and identify PHD2 as a novel target of miR‐21.
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