Dysregulated expression but redundant function of the long non-coding RNA HOTAIR in diabetic kidney disease.

AIMS/HYPOTHESIS: Long non-coding RNAs (lncRNAs) are garnering increasing attention for their putative roles in the pathogenesis of chronic diseases, including diabetic kidney disease (DKD). However, much about in vivo lncRNA functionality in the adult organism remains unclear. To better understand lncRNA regulation and function in DKD, we explored the effects of the modular scaffold lncRNA HOTAIR (HOX antisense intergenic RNA), which approximates chromatin modifying complexes to their target sites on the genome. METHODS: Experiments were performed in human kidney tissue, in mice with streptozotocin-induced diabetes, the db/db mouse model of type 2 diabetes, podocyte-specific Hotair knockout mice and conditionally immortalised mouse podocytes. RESULTS: HOTAIR was observed to be expressed by several kidney cell-types, including glomerular podocytes, in both human and mouse kidneys. However, knockout of Hotair from podocytes had almost no effect on kidney structure, function or ultrastructure. Glomerular HOTAIR expression was found to be increased in human DKD, in the kidneys of mice with streptozotocin-induced diabetes and in the kidneys of db/db mice. Likewise, exposure of cultured mouse podocytes to high glucose caused upregulation of Hotair expression, which occurred in a p65-dependent manner. Although HOTAIR expression was upregulated in DKD and in high glucose-exposed podocytes, its knockout did not alter the development of kidney damage in diabetic mice. Rather, in a bioinformatic analysis of human kidney tissue, HOTAIR expression closely paralleled the expression of its genic neighbour, HOXC11, which is important to developmental patterning but which has an uncertain role in the adult kidney. CONCLUSIONS/INTERPRETATION: Many lncRNAs have been found to bind to the same chromatin modifying complexes. Thus, there is likely to exist sufficient redundancy in the system that the biological effects of dysregulated lncRNAs in kidney disease may often be inconsequential. The example of the archetypal scaffold lncRNA, HOTAIR, illustrates how lncRNA dysregulation may be a bystander in DKD without necessarily contributing to the pathogenesis of the condition. In the absence of in vivo validation, caution should be taken before ascribing major functional roles to single lncRNAs in the pathogenesis of chronic diseases.

Histone H3 Serine 10 Phosphorylation Facilitates Endothelial Activation in Diabetic Kidney Disease.

The posttranslational histone modifications that epigenetically affect gene transcription extend beyond conventionally studied methylation and acetylation patterns. By examining the means by which podocytes influence the glomerular endothelial phenotype, we identified a role for phosphorylation of histone H3 on serine residue 10 (phospho-histone H3Ser10) in mediating endothelial activation in diabetes. Culture media conditioned by podocytes exposed to high glucose caused glomerular endothelial vascular cell adhesion protein 1 (VCAM-1) upregulation and was enriched for the chemokine CCL2. A neutralizing anti-CCL2 antibody prevented VCAM-1 upregulation in cultured glomerular endothelial cells, and knockout of the CCL2 receptor CCR2 diminished glomerular VCAM-1 upregulation in diabetic mice. CCL2/CCR2 signaling induced glomerular endothelial VCAM-1 upregulation through a pathway regulated by p38 mitogen-activated protein kinase, mitogen- and stress-activated protein kinases 1/2 (MSK1/2), and phosphorylation of H3Ser10, whereas MSK1/2 inhibition decreased H3Ser10 phosphorylation at the VCAM1 promoter. Finally, increased phospho-histone H3Ser10 levels were observed in the kidneys of diabetic endothelial nitric oxide synthase knockout mice and in the glomeruli of humans with diabetic kidney disease. These findings demonstrate the influence that histone protein phosphorylation may have on gene activation in diabetic kidney disease. Histone protein phosphorylation should be borne in mind when considering epigenetic targets amenable to therapeutic manipulation in diabetes.

Histone Deacetylase Inhibitors and Diabetic Kidney Disease.

Despite recent clinical trial advances and improvements in clinical care, kidney disease due to diabetes remains the most common cause of chronic kidney failure worldwide. In the search for new treatments, recent attentions have turned to drug repurposing opportunities, including study of the histone deacetylase (HDAC) inhibitor class of agents. HDACs are a group of enzymes that remove functional acetyl groups from histone and non-histone proteins and they can affect cellular function through both epigenetic and non-epigenetic means. Over the past decade, several HDAC inhibitors have been adopted into clinical practice, primarily for the treatment of hematological malignancy, whereas other existing therapies (for instance valproate) have been found to have HDAC inhibitory effects. Here we review the current HDAC inhibitors in the clinic and under development; the literature evidence supporting the renoprotective effects of HDAC inhibitors in experimental diabetic kidney disease; and the adverse effect profiles that may prevent existing therapies from entering the clinic for this indication. Whereas recent research efforts have shed light on the fundamental actions of HDACs in the diabetic kidney, whether these efforts will translate into novel therapies for patients will require more specific and better-tolerated therapies.

The Dipeptidyl Peptidase 4 Substrate CXCL12 Has Opposing Cardiac Effects in Young Mice and Aged Diabetic Mice Mediated by Ca2+ Flux and Phosphoinositide 3-Kinase γ.

Blood glucose-lowering therapies can positively or negatively affect heart function in type 2 diabetes, or they can have neutral effects. Dipeptidyl peptidase 4 (DPP-4) inhibitors lower blood glucose by preventing the proteolytic inactivation of glucagon-like peptide 1 (GLP-1). However, GLP-1 is not the only peptide substrate of DPP-4. Here, we investigated the GLP-1-independent cardiac effects of DPP-4 substrates. Pointing to GLP-1 receptor (GLP-1R)-independent actions, DPP-4 inhibition prevented systolic dysfunction equally in pressure-overloaded wild-type and GLP-1R knockout mice. Likewise, DPP-4 inhibition or the DPP-4 substrates substance P or C-X-C motif chemokine ligand 12 (CXCL12) improved contractile recovery after no-flow ischemia in the hearts of otherwise healthy young adult mice. Either DPP-4 inhibition or CXCL12 increased phosphorylation of the Ca2+ regulatory protein phospholamban (PLN), and CXCL12 directly enhanced cardiomyocyte Ca2+ flux. In contrast, hearts of aged obese diabetic mice (which may better mimic the comorbid patient population) had diminished levels of PLN phosphorylation. In this setting, CXCL12 paradoxically impaired cardiac contractility in a phosphoinositide 3-kinase γ-dependent manner. These findings indicate that the cardiac effects of DPP-4 inhibition primarily occur through GLP-1R-independent processes and that ostensibly beneficial DPP-4 substrates can paradoxically worsen heart function in the presence of comorbid diabetes.