Diabetic Nephropathy: Novel Molecular Mechanisms and Therapeutic Targets

Abstract

Diabetic nephropathy (DN) is one of the major microvascular complications of diabetes mellitus and the leading cause of end-stage kidney disease. The standard treatments for diabetic patients are glucose and blood pressure control, lipid lowering, and renin-angiotensin system blockade; however, these therapeutic approaches can provide only partial renoprotection if started late in the course of the disease. One major limitation in developing efficient therapies for DN is the complex pathobiology of the diabetic kidney, which undergoes a set of profound structural, metabolic and functional changes. Despite these difficulties, experimental models of diabetes have revealed promising therapeutic targets by identifying pathways that modulate key functions of podocytes and glomerular endothelial cells. In this review we will describe recent advances in the field, analyze key molecular pathways that contribute to the pathogenesis of the disease, and discuss how they could be modulated to prevent or reverse DN.

Keywords: angiotensin 1–7; diabetic nephropathy; hypoxia inducible factor; notch signaling; renin-angiotensin system; sirtuins; sodium-glucose cotransporter 2; thyroid hormone signaling.

FIGURE 1

Schematic representation of the renin-angiotensin system (RAS) showing that the angiotensin converting enzyme 2 (ACE2)/Angiotensin-(1–7)/Mas receptor axis exerts opposite effects to those of ACE/Angiotensin II/AT1 receptor axis. Renin cleaves hepatic angiotensinogen into Angiotensin I which is then cleaved via ACE into Angiotensin II. The effects of Angiotensin II are exerted mainly through the activation of the Angiotensin II type 1 receptor (AT1R) and includes vasoconstriction, inflammation, fibrosis, oxidative stress and cell growth. Angiotensin II also binds to Angiotensin II type 2 receptor (AT2R) which usually opposes the actions of AT1R. Angiotensin-(1–7), which is a specific Mas receptor (MasR) agonist, can be formed directly from Angiotensin II via ACE2, or it can be generated through the ACE2-catalyzed hydrolysis of Angiotensin I to the inactive Angiotensin-(1–9) which is then converted to Angiotensin-(1–7) by ACE or neprilysin (NEP). However, Angiotensin-(1–7) is mainly formed through the action of ACE2 on Angiotensin II which has more affinity to ACE2 than Angiotensin I. When levels of Ang II are not sufficiently elevated, Ang-(1–7) can also be formed directly from Ang I via NEP. Interaction of Angiotensin-(1–7) with MasR triggers intracellular signaling pathways leading to beneficial actions such as vasodilation, anti-inflammatory, anti-fibrotic and anti-oxidative effects, and inhibition of cell proliferation.

FIGURE 2

Role of sirtuin-3 dysregulation in kidney disease progression in diabetes. Sirtuin-3 is downregulated in the diabetic kidney. Reduced expression in podocytes and glomerular endothelial cells impairs SOD2 antioxidant activity as a consequence of enzyme acetylation, resulting in increased ROS generation, which promotes mitochondrial dysfunction and cell loss. These changes contribute to the development of albuminuria associated with inflammation and mesangial matrix expansion. Renal tubules with reduced sirtuin-3 undergo a metabolic reprogramming with a shift toward abnormal glycolysis, display EMT, and acquire a profibrotic phenotype. Rescuing sirtuin-3 by the specific activator honokiol prevents glomerular and tubule dysfunction and ameliorates diabetic nephropathy. acSOD2, acetylated SOD2; PKM2, pyruvate kinase isozyme M2; EMT, epithelial to mesenchymal transition; ECM, extracellular matrix.

FIGURE 3

Modulation of podocyte Notch1 signaling in diabetic nephropathy (DN). (A) Pharmacological inhibition of active Notch1 inducers. Ang II and TGF-β drive podocyte injury in diabetes through the activation of Notch1 signaling. Following Jag1 engagement Notch1 is cleaved by γ-secretase and the released Notch1 intracellular domain (NICD1), via Hes1, induces sustained Snail expression, which represses nephrin, leading to podocyte dedifferentiation. On the other hand, NICD1 downregulates Notch2 and NICD2 and activates proapoptotic pathways, promoting podocyte apoptosis. Targeting Ang II by RAS inhibition and TGF-β-induced upregulation of Jag1 with the Rho kinase inhibitor fasudil prevents Notch1-mediated podocyte phenotypic changes and loss, ameliorating DN (B,C). Epigenetic regulation of Notch1 signaling through miRNAs (B) or posttranslational histone modification (C). (B) In normal, mature, differentiated podocytes miR-146a and miRNA 34a/c prevent Notch1 signaling activation by targeting the 3′UTR of Notch1 or both Notch1 and Jag1, thus decreasing their mRNA and protein expression. Conversely, diabetes and hyperglycemia-induced downregulation of miR-146a and miRNA 34a/c result in Notch1 signaling activation in podocytes (C) In healthy podocytes the trimethylation of lysine residue 27 on histone protein H3 (H3K27me3) in the Jag1 promoter and the Sirt6-mediated deacetylation of lysine residue nine on histone protein H3 (H3K9) in the Notch1 promoter keep the Notch1 signaling pathway silent. In diabetes, reduced H3K27me3 – dependent on the overexpression of the demethylase UTX–and increased H3K9ac due to Sirt6 downregulation relieve the repression of Jag1 and Notch1, respectively, switching on Notch1 signaling in podocytes. ACEi, angiotensin converting enzyme inhibitor; ARB, angiotensin receptor blocker; UTX, Jumonji C domain-containing family: ubiquitously transcribed tetratricopeptide repeat on chromosome X.

FIGURE 4

In the fetus, when the levels of the active form of TH L-triiodothyronine (T3) are low, TRα1 mainly acts as an apo-receptor (unliganded state) to repress adult genes (thus protecting the embryo from premature differentiation) and enhances cell proliferation and organ growth. In contrast, after birth when T3 levels increase, TRα1 switches to the holo-receptor (liganded state) to induce the expression of adult genes, thus promoting cell differentiation, physiological organ maturation and function. In adult life, local T₃ availability is controlled by the T₃-inactivating enzyme DIO3, which converts excessive T₃ into rT₃ and T₂. In response to diabetic injury, systemic T₃ levels drop markedly, and TRα1 and DIO3 are overexpressed locally, resulting in the coordinated adoption of the fetal ligand/receptor relationship profile (i.e., low T₃ availability/high local TRα1). Apo-TRα1 binds DNA and represses the transcription of target adult genes (as happens in the fetus), leading to cell dedifferentiation, metabolic and structural remodeling, and cell cycle reactivation. The figure is modified from Benedetti et al., 2019.