Epigenetic modifications in the pathogenesis of diabetic nephropathy

Abstract

Diabetic nephropathy (DN) is a leading cause of end-stage renal disease. Diabetic vascular complications such as DN can progress despite subsequent glycemic control, suggesting a metabolic memory of previous exposure to hyperglycemia. Diabetes profoundly impacts transcription programs in target cells through activation of multiple signaling pathways and key transcription factors leading to aberrant expression of pathologic genes. Emerging evidence suggests that these factors associated with the pathophysiology of diabetic complications and metabolic memory also might be influenced by epigenetic mechanisms in chromatin such as DNA methylation, histone lysine acetylation, and methylation. Key histone modifications and the related histone methyltransferases and acetyltransferases have been implicated in the regulation of inflammatory and profibrotic genes in renal and vascular cells under diabetic conditions. Advances in epigenome profiling approaches have provided novel insights into the chromatin states and functional outcomes in target cells affected by diabetes. Because epigenetic changes are potentially reversible, they can provide a window of opportunity for the development of much-needed new therapies for DN in the future. In this review, we discuss recent developments in the field of epigenetics and their relevance to diabetic vascular complications and DN pathogenesis.

Keywords: Chromatin; diabetic nephropathy; epigenomics; histone modifications; metabolic memory.

Figure 1. Major pathways involved in the pathophysiology of diabetic nephropathy

Complex interactions between metabolic and hemodynamic factors regulate the pathogenesis of diabetic nephropathy. Persistence of HG mediated damage including epigenetic modifications even after return to normoglycemia can lead to metabolic memory and increased risk for long term complications. TGF-β, transforming growth factor-β; AGEs, advanced glycation end product; RAAS, rennin angiotensin aldosterone system; PKC, protein kinase C; NF-κB, nuclear factor kappa-B; PTMs, posttranslational modifications; ECM, extracellular matrix.

Figure 2. Schematic diagram showing enrichment of histone modifications at various regulatory elements in chromatin

Transcriptionally active chromatin is characterized by open chromatin states with nucleosome depleted regions providing increased access to transcription factors (TF), RNA polymerase II (Pol II) and other components of the transcription machinery (T). Whereas repressed chromatin has a compact structure with higher density of nucleosomes and restricted accessibility. In general, active gene promoters are enriched with H3K4me3, H3K4me2 and H3/H4Kac, transcribed Exons (Ex) and Introns (Int) are enriched with H3K36me3 and H3K79me3. Enhancers (Enh) are enriched with H3K4me1 and the histone acetyl transferase p300 and active enhancers are marked by H3K4me2 and H3K27ac. Repressed promoters are enriched with H3K9me2/3, H3K27me3, H4K20me3 and DNA methylation (DNAme). Insulators (Ins) are enriched with the CCCTC-binding factor (CTCF) and demarcate active and inactive chromatin regions. TB-TF binding sites; Kme-lysine methylation; Kac-lysine acetylation.

Figure 3. Epigenetic mechanisms involved in Diabetic Nephropathy

High glucose (HG), and TGF-β-transforming growth factor-beta1 (TGF-β) regulate Extracellular matrix (ECM) genes and cell cycle genes by increasing active modifications H3K9/14ac and H3K4me1/2/3, and inhibiting repressive marks H3K9me2/3 at these gene promoters. TGF-β promotes the recruitment of CBP/p300 which increases H3K9/14ac and chromatin access to Smads and SP1 transcription factors. CBP/p300 also regulates Smad activity by direct acetylation. TGF-β mediated inhibition of HDAC1 and HDAC5 may also play a role in increased H3K9/14ac. TGF-β induces SET7 expression and promotes its recruitment to gene promoters, which increases H3K4me1/2. Similar epigenetic mechanisms are induced by HG and they are blocked by TGF-β antibodies (TGF-β Ab) implicating TGF-β as a major mediator of epigenetic events in Diabetic Nephropathy. R-Corepressors. HMT-histone methyl transferases; HDAC-histone deacetylases; SP1B-SP1 binding sites; SBE-Smad binding elements; Pr-promoter; Pol II-RNA Polymerase II; T- components of transcription machinery.

Figure 4. Histone Modifications and gene regulation in vascular complications

Diabetes enhances expression of NF-κB induced inflammatory genes (TNF-α, IL-6 and MCP-1) and inhibits antioxidant stress genes superoxide dismutase (SOD) in monocytes and vascular cells via multiple epigenetic mechanisms: A. in vascular smooth muscle cells, diabetes increases miR-125b, which blocks SUV39H1 expression, leading to inhibition of H3K9me3 (repressive mark) and recruitment of co-repressor HP1 at inflammatory gene promoters resulting in increased inflammatory gene expression; B. In retinal endothelial cells, expression of anti-oxidant genes such as SOD is repressed by HG induced SUV420H2, which mediates a repressive modification H4K20me3. 3. In monocytes and endothelial cells, diabetes conditions promote recruitment of co-activator Histone acetyl transferase (p300) and H3K4methyl transferase SET7, which increase activation marks H3/H4Kac and H3K4me respectively leading to enhanced NF-κB mediated inflammatory gene expression. Persistence of these epigenetic modifications even after removal of the diabetic stimuli might be an underlying mechanism involved in metabolic memory of diabetic complications. Inf genes-inflammatory genes; SOD-Super oxide dismutase; Pr-promoter; TSS-transcription start site; Pol II-RNA Polymerase II; T- components of transcription machinery.