Epigenetic Changes Induced by Hypoxia-Inducible Factor: a Long Way Still To Go as a Target for Therapy?

MicroRNA (miR) are small noncoding RNA molecules that regulate gene expression and play important roles during kidney development, homeostasis, and disease. Novel contributions to thefieldofcelldeath, cell cycle, andmiRregulation in the kidney have come from the Laboratory of Dong, and in the present issue of JASN, a study from this laboratory clarifies a role of miR-687 as a key regulator and therapeutic target in acute ischemic injury to the kidney.1 AKI is associated with high morbidity and mortality and is closely intertwined with CKD, with each disease serving as a risk factor for developing the other and sharing causes and other risk factors in common. Furthermore, ischemia is a major cause of AKI, and hypoxia serves as a final common pathway to end stage kidney failure. Therefore, their choice of a model is appropriate. Theauthorsusedmicroarrays andprofiledmiRexpression in kidney tissues rendered to ischemia reperfusion. Among themiR,miR-687 showed a highest up-regulation at 12 hours of reperfusion. Their beautiful in situ hybridization study localized miR-687 induction predominantly in the cells of renal cortical tubules. Then the authors studied the upstream of miR-687 induction and found that miR-687 induction was mediated by a transcriptional factor, hypoxia-inducible factor-1 (HIF-1), that plays an integral role in the body’s response to low oxygen concentrations.2,3 The amount of HIF-1 is regulated mainly at the degradation of HIF-1a via hydroxylation of the proline residues and subsequent polyubiquitylation by the von Hippel–Lindau protein that acts as its E3 ligase. This proline hydroxylation is regulated by oxygen-dependent activity prolyl hydroxylases (PHD) that belong to the family of ironand 2-oxoglutamate-dependent dioxygenase enzyme, and hypoxia inactivates PHD, allowingHIF-1a to escape degradation. Then the a subunit translocates to the nucleus where it forms a heterodimer with a constitutively expressed b-subunit and transactivates 100–200 target genes. The representative HIF target genes include those involved in erythropoiesis (e.g., erythropoietin) and those involved in angiogenesis (e.g., vascular endothelial growth factor) to increase oxygen delivery to tissues. The authors used HIF-1a-null mouse embryonic fibroblasts and proximal tubule-specific HIF-1a knockout mice, confirming HIF-1-dependent miR-687 induction. This demonstration of specificity is important because HIF has two active isoforms, HIF-1 and HIF-2. HIF-1a and HIF-2a have 48% amino acid sequence identity, but they are often nonredundant and have distinct target genes. This isoform specificity can explain tubular specific localization of miR-687 because HIF-1a is expressed in tubular cells and HIF-2a is expressed in endothelial cells and interstitial cells in the kidney.4 To investigate the downstream pathway of miR-687, the authors predicted targets of miR-687 using several databases and confirmed phosphatase and tensin homolog (PTEN), a modulatorof cell cycle and cell death, as the downstreamgene target. As previously described, HIF-1 is amaster regulator of adaptive responses against hypoxia, and their functional studies using cultured tubular cells suggested that miR-687mediated downregulation of PTEN facilitated cell cycle progression for tubular cell proliferation and kidney repair during hypoxia. Previous studies of experimental animals demonstrated protective effects of pharmacologic HIF activation against ischemia reperfusion injuryof the kidney, andmiR-687-mediated downregulation of PTEN may be one of the mechanisms of renoprotection by HIF activation.5,6 However, the authors’ functional studies of mice showed paradoxical results. The authors used locked-nucleic-acid oligonucleotides to neutralize the effects of miR-687 in vivo. Mice treated with anti-miR-687 developed significantly less kidney injury as evidenced by the examination of physiologic parameters, histology, and renal apoptosis. Therefore, blockade of miR-687 and preservation of PTEN expression resulted in protection against kidney injury in vivo. Although HIF-1 is the most potent defensive mechanism against hypoxia, it regulates a number of pathways. It is possible that miR-687 induced by HIF-1 may provoke proapoptotic responses in a PTEN-independent manner. The presence of other types of cells beside tubular cells may explain the discrepancy from the in vitro experimental system composed of exclusively tubular cells. In addition, various stresses, such as oxidative stress and endoplasmic reticulum stress interact Published online ahead of print. Publication date available at www.jasn.org.