Insights into the Mechanisms Involved in the Expression and Regulation of Extracellular Matrix Proteins in Diabetic Nephropathy

Abstract

Diabetic Nephropathy (DN) is believed to be a major microvascular complication of diabetes. The hallmark of DN includes deposition of Extracellular Matrix (ECM) proteins, such as, collagen, laminin and fibronectin in the mesangium and renal tubulo-interstitium of the glomerulus and basement membranes. Such an increased expression of ECM leads to glomerular and tubular basement membranes thickening and increase of mesangial matrix, ultimately resulting in glomerulosclerosis and tubulointerstitial fibrosis. The characteristic morphologic glomerular mesangial lesion has been described as Kimmelstiel-Wilson nodule, and the process at times is referred to as diabetic nodular glomerulosclerosis. Thus, the accumulation of ECM proteins plays a critical role in the development of DN. The relevant mechanism(s) involved in the increased ECM expression and their regulation in the kidney in diabetic state has been extensively investigated and documented in the literature. Nevertheless, there are certain other mechanisms that may yet be conclusively defined. Recent studies demonstrated that some of the new signaling pathways or molecules including, Notch, Wnt, mTOR, TLRs and small GTPase may play a pivotal role in the modulation of ECM regulation and expression in DN. Such modulation could be operational for instance Notch through Notch1/Jagged1 signaling, Wnt by Wnt/β- catenin pathway and mTOR via PI3-K/Akt/mTOR signaling pathways. All these pathways may be critical in the modulation of ECM expression and tubulo-interstitial fibrosis. In addition, TLRs, mainly the TLR2 and TLR4, by TLR2- dependent and TGF-β-dependent conduits, may modulate ECM expression and generate a fibrogenic response. Small GTPase like Rho, Ras and Rab family by targeting relevant genes may also influence the accumulation of ECM proteins and renal fibrosis in hyperglycemic states. This review summarizes the recent information about the role and mechanisms by which these molecules and signaling pathways regulate ECM synthesis and its expression in high glucose ambience in vitro and in vivo states. The understanding of such signaling pathways and the molecules that influence expression, secretion and amassing of ECM may aid in developing strategies for the amelioration of diabetic nephropathy.

Figure 1

Schematic drawing depicting events related to Notch signaling that are relevant to ECM accumulation in diabetic nephropathy. High glucose increases TGF-β levels and up-regulates JAG-I expression. This induces a conformational change in Notch1 by proteolytic cleavage resulting in the release and translocation of Notch intracellular domain (NICD) into nucleus, where NICD interacts with Rbp1, p300 and CSL, and forms CBF1/Su(H)/Lag-1 transcription factor complex. Conceivably, the complex triggers transcription of target genes, such as, of ECM, EMT and VEGF, and ultimately leading to renal fibrosis in diabetic nephropathy.

Figure 2

An overview of different signaling pathways activated by high glucose ambience, which leads to an altered expression of various ECM glycoproteins, and of genes relevant to EMT and apoptosis processes. WNT/β-CATENIN signaling: Under high-glucose ambience, Wnt proteins are secreted into the extracellular space and bind with LRP. These interactions send a signal to the phosphoprotein Dishevelled (Dsh) in the cytoplasm, while simultaneously inhibit the activity of glycogen synthase kinase 3 (GSK-3β) followed by its falling off from the Axin complex. Increased cytoplasmic β-catenin is translocated into the nucleus, and it serves as a transcriptional co-activator of various transcription factors which stimulate the expression of the target EMT genes. TLRs signaling: It can be activated by the ligand TLR2, and subsequents events are channeled via MyD88-dependent signaling pathway. Whereas, TLR4 activation leads to channeling of events via MyD88-dependent as well as MyD88-independent pathways. Both pathways lead to the activation of nuclear factor Kappa-B (NF-κB), which in turn increases the expression and secretion of pro-fibrotic and pro-inflammatory cytokines with over-expression of ECM proteins like fibronectin. PI3-K/Akt/mTOR signaling: High glucose activates PI3-K/Akt/mTOR signaling with increase of phospho-p70S6 and phospho-eEF2 kinase activities while reducing phospho-eEF2 (factor) expression. In addition, mTOR pathway activation causes over-activation of PP2Ac. In all these three pathways there is an increased expression of ECM glycoproteins and initiation of events related to the process of EMT with consequential apparent thickening of GBMs and expansion of glomerular mesangium.

Figure 3

Schematics depicting potential mechanism(s) by which high glucose induces ECM glycoprotein synthesis via the activation of small GTPases, Ras or Rho, in diabetic nephropathy. Under high glucose conditions, PKC is activated which induces Rap1-GDP transition to Rap1-GTP, the activated form of small GTPase. This up-regulates the expression of ECM related genes in various cells of the kidney via B-Raf /MEK pathway. On the other hand, high glucose by activation of Rho/Rock signaling induces over-expression of transcription factor(s), such as NF-κB, AP-1 and p21, which are associated with increased fibronectin matrix protein synthesis and genes relevant to the process of EMT, ultimately leading to the progression of DN.