The role of TGF-? and epithelial-to mesenchymal transition in diabetic nephropathy

Transforming Growth Factor-beta (TGF-?) is a pro-sclerotic cytokine widely associated with the development of fibrosis in diabetic nephropathy. Central to the underlying pathology of tubulointerstitial fibrosis is epithelial-to-mesenchymal transition (EMT), or the trans-differentiation of tubular epithelial cells into myofibroblasts. This process is accompanied by a number of key morphological and phenotypic changes culminating in detachment of cells from the tubular basement membrane and migration into the interstitium. Ultimately these cells reside as activated myofibroblasts and further exacerbate the state of fibrosis. A large body of evidence supports a role for TGF-? and downstream Smad signalling in the development and progression of renal fibrosis. Here we discuss a role for TGF-? as the principle effector in the development of renal fibrosis in diabetic nephropathy, focusing on the role of the TGF-?1 isoform and its downstream signalling intermediates, the Smad proteins. Specifically we review evidence for TGF-?1 induced EMT in both the proximal and distal regions of the nephron and describe potential therapeutic strategies that may target TGF-?1 activity.</p

The role of TGF-? and epithelial-to mesenchymal transition in diabetic nephropathy

Transforming Growth Factor-beta (TGF-?) is a pro-sclerotic cytokine widely associated with the development of fibrosis in diabetic nephropathy. Central to the underlying pathology of tubulointerstitial fibrosis is epithelial-to-mesenchymal transition (EMT), or the trans-differentiation of tubular epithelial cells into myofibroblasts. This process is accompanied by a number of key morphological and phenotypic changes culminating in detachment of cells from the tubular basement membrane and migration into the interstitium. Ultimately these cells reside as activated myofibroblasts and further exacerbate the state of fibrosis. A large body of evidence supports a role for TGF-? and downstream Smad signalling in the development and progression of renal fibrosis. Here we discuss a role for TGF-? as the principle effector in the development of renal fibrosis in diabetic nephropathy, focusing on the role of the TGF-?1 isoform and its downstream signalling intermediates, the Smad proteins. Specifically we review evidence for TGF-?1 induced EMT in both the proximal and distal regions of the nephron and describe potential therapeutic strategies that may target TGF-?1 activity.</p

TGF-?1-induced epithelial-to-mesenchymal transition and therapeutic intervention in diabetic nephropathy

Background/Aims: Epithelial-to-mesenchymal cell transformation (EMT) is the trans-differentiation of tubular epithelial cells into myofibroblasts, an event underlying progressive chronic kidney disease in diabetes, resulting in fibrosis. Mainly reported in proximal regions of the kidney, EMT is now recognized as a key contributor to the loss of renal function throughout the nephron in diabetic nephropathy (DN). Concomitant upregulation of TGF-? in diabetes makes this pro-fibrotic cytokine an obvious candidate in the development of these fibrotic complications. This article reviews recent findings clarifying our understanding of the role of TGF-? and associated sub-cellular proteins in EMT. Methods: To understand the pathology of EMT and the role of TGF-?, we reviewed the literature using PubMed for English language articles that contained key words related to EMT, TGF-? and DN. Results: EMT and phenotypic plasticity of epithelial cells throughout the nephron involves cytoskeletal reorganization and de novo acquisition of classic mesenchymal markers. Concurrent downregulation of epithelial adhesion molecules results in a loss of function and decreased cell coupling, contributing to a loss of epithelial integrity. TGF-?1 is pivotal in mediating these phenotypic changes. Conclusion: TGF-?-induced EMT is a key contributor to fibrotic scar formation as seen in DN, and novel routes for future therapeutic intervention are discussed.</p

Glucose-evoked changes in Transforming Growth Factor Beta1 modulate cell-substrate binding in human proximal tubule derived epithelial cells.

Aims: The tubular-basement membrane is a highly regulated microenvironment that facilitates numerous cell-matrix interactions critical in maintaining epithelial phenotype. Currently, we know little of how cell substrate and cell-cell interactions are modulated in diabetic nephropathy. This study identifies a role for glucose-evoked changes in TGF-?1, in modulation of interactions between proximal tubule derived epithelial cells and key components of the extracellular matrix.Methods: HK2 cells were cultured in either 5mM-glucose +/- TGF-?1 (2-10ng/mL) or 25mM-glucose. Glucose evoked increases in TGF-?1 secretion were determined by ELISA. HK2-ECM interactions were assessed via ECM arrays.Results: Cell culture supernatant of HK2-cells cultured in 25mM-glucose exhibited increased TGF-?1 secretion from 334pg/mL±4.1% to 994pg/mL ±4.3% as compared to 5mM-control (n=3 PCollagenIV>Collagen I>Laminin as determined by an ECM assay. TGF-?1 treated HK2 cells (48hrs) evoked increased binding to Collagen I, Collagen IV and Laminin to 340±26%, 228±38% and 289±42% respectively, whilst binding to fibronectin was unaltered as compared to control (n=3 P<0.01). HK2 cells cultured in 25mM glucose exhibited increased binding to Collagen I, Collagen IV and Laminin to 183±4%, 157±3% and 175±20% respectively, whilst binding to fibronectin was reduced to 80±5% as compared to control (n=3 P<0.001).Conclusions: The current study suggests that glucose-evoked changes in TGF-?1 are instrumental in reorganizing the extracellular-matrix and cell-substrate interactions in proximal tubule epithelial cells, changes that alter cell architecture, integrity and function, which can ultimately result in kidney damage ahead of overt renal failure in Diabetic-Nephropathy.This work was supported wholly or in part by the generous support of Diabetes UK (BDA: 11/0004215), and The Warwickshire Private Hospital (WPH) Charitable Trust.</p

Editorial: A special issue on from bench to bedside: An integrated and multidisciplinary approach to tackling diabetic kidney disease

An editorial for the special issue entitled 'A special issue on from bench to bedside: An integratedand multidisciplinary approach to tackling diabetic kidney disease'</p

TGF-?1-induced epithelial-to-mesenchymal transition and therapeutic intervention in diabetic nephropathy

Background/Aims: Epithelial-to-mesenchymal cell transformation (EMT) is the trans-differentiation of tubular epithelial cells into myofibroblasts, an event underlying progressive chronic kidney disease in diabetes, resulting in fibrosis. Mainly reported in proximal regions of the kidney, EMT is now recognized as a key contributor to the loss of renal function throughout the nephron in diabetic nephropathy (DN). Concomitant upregulation of TGF-? in diabetes makes this pro-fibrotic cytokine an obvious candidate in the development of these fibrotic complications. This article reviews recent findings clarifying our understanding of the role of TGF-? and associated sub-cellular proteins in EMT. Methods: To understand the pathology of EMT and the role of TGF-?, we reviewed the literature using PubMed for English language articles that contained key words related to EMT, TGF-? and DN. Results: EMT and phenotypic plasticity of epithelial cells throughout the nephron involves cytoskeletal reorganization and de novo acquisition of classic mesenchymal markers. Concurrent downregulation of epithelial adhesion molecules results in a loss of function and decreased cell coupling, contributing to a loss of epithelial integrity. TGF-?1 is pivotal in mediating these phenotypic changes. Conclusion: TGF-?-induced EMT is a key contributor to fibrotic scar formation as seen in DN, and novel routes for future therapeutic intervention are discussed.</p

C-Peptide and its intracellular signaling

Although long believed to be inert, C-peptide has now been shown to have definite biological effects both in vitro and in vivo in diabetic animals and in patients with type 1 diabetes. These effects point to a protective action of C-peptide against the development of diabetic microvascular complications. Underpinning these observations is undisputed evidence of C-peptide binding to a variety of cell types at physiologically relevant concentrations, and the downstream stimulation of multiple cell signaling pathways and gene transcription via the activation of numerous transcription factors. These pathways affect such fundamental cellular processes as re-absorptive and/or secretory phenotype, migration, growth, and survival. Whilst the receptor remains to be identified, experimental data points strongly to the existence of a specific G-protein-coupled receptor for C-peptide. Of the cell types studied so far, kidney tubular cells express the highest number of C-peptide binding sites. Accordingly, C-peptide exerts major effects on the function of these cells, and in the context of diabetic nephropathy appears to antagonise the pathophysiological effects of major disease mediators such as TGF?1 and TNF?. Therefore, based on its cellular activity profile C-peptide appears well positioned for development as a therapeutic tool to treat microvascular complications in type 1 diabetes.</p

Intracellular signalling by C-peptide

C-peptide, a cleavage product of the proinsulin molecule, has long been regarded as biologically inert, serving merely as a surrogate marker for insulin release. Recent findings demonstrate both a physiological and protective role of C-peptide when administered to individuals with type I diabetes. Data indicate that C-peptide appears to bind in nanomolar concentrations to a cell surface receptor which is most likely to be G-protein coupled. Binding of C-peptide initiates multiple cellular effects, evoking a rise in intracellular calcium, increased PI-3-kinase activity, stimulation of the Na+/K+ ATPase, increased eNOS transcription, and activation of the MAPK signalling pathway. These cell signalling effects have been studied in multiple cell types from multiple tissues. Overall these observations raise the possibility that C-peptide may serve as a potential therapeutic agent for the treatment or prevention of long-term complications associated with diabetes.</p

Cellular and physiological effects of C-peptide

In recent years, accumulating evidence indicates a biological function for proinsulin C-peptide. These results challenge the traditional view that C-peptide is essentially inert and only useful as a surrogate marker of insulin release. Accordingly, it is now clear that C-peptide binds with high affinity to cell membranes, probably to a pertussis-toxin-sensitive G-protein-coupled receptor. Subsequently, multiple signalling pathways are potently and dose-dependently activated in multiple cell types by C-peptide with the resulting activation of gene transcription and altered cell phenotype. In diabetic animals and Type 1 diabetic patients, short-term studies indicate that C-peptide also enhances glucose disposal and metabolic control. Furthermore, results derived from animal models and clinical studies in Type 1 diabetic patients suggest a salutary effect of C-peptide in the prevention and amelioration of diabetic nephropathy and neuropathy. Therefore a picture of Type 1 diabetes as a dual-hormone-deficiency disease is developing, suggesting that the replacement of C-peptide alongside insulin should be considered in its management.</p

Serum and glucocorticoid regulated kinase and disturbed renal sodium transport in diabetes

Diabetes is associated with a number of side effects including retinopathy, neuropathy, nephropathy and hypertension. Recent evidence has shown that serum and glucocorticoid regulated kinase-1 (SGK1) is increased in models of diabetic nephropathy. While clearly identified as glucocorticoid responsive, SGK1 has also been shown to be acutely regulated by a variety of other factors. These include insulin, hypertonicity, glucose, increased intracellular calcium and transforming growth factor-beta, all of which have been shown to be increased in type II diabetes. The principal role of SGK1 is to mediate sodium reabsorption via its actions on the epithelial sodium channel (now known is sodium channel, nonvoltage-gated 1). Small alterations in the sodium resorptive capacity of the renal epithelia may have dramatic consequences for fluid volume regulation, and SGK1 maybe responsible for the development of hypertension associated with diabetes. This short commentary considers the evidence that Supports the involvement of SGK1 in diabetic hypertension, but also discusses how aberrant sodium reabsorption may account for the cellular changes seen in the nephron.</p