Relevance of TNF-α in the context of other inflammatory cytokines in the progression of diabetic nephropathy

An ancillary paradigm that has evolved recently in the pathogenesis of diabetic nephropathy includes subclinical microinflammation with influx of macrophages and consequent generation of myriad proinflammatory cytokines and ensuing kidney damage. Among various proinflammatory cytokines, tumor necrosis factor-α (TNF-α) has attracted the most attention, since it amplifies the inflammatory network of cytokines, leading to worsening of the progression of diabetic nephropathy. The article by Awad et al. examines the role of TNF-α in the pathogenesis of experimental diabetic nephropathy. An ancillary paradigm that has evolved recently in the pathogenesis of diabetic nephropathy includes subclinical microinflammation with influx of macrophages and consequent generation of myriad proinflammatory cytokines and ensuing kidney damage. Among various proinflammatory cytokines, tumor necrosis factor-α (TNF-α) has attracted the most attention, since it amplifies the inflammatory network of cytokines, leading to worsening of the progression of diabetic nephropathy. The article by Awad et al. examines the role of TNF-α in the pathogenesis of experimental diabetic nephropathy. Diabetic nephropathy is the major prototypic example of renovascular complications of diabetes, and it is the leading cause of end-stage renal disease in the Western population and in developing countries. It has reached epidemic proportions worldwide and is an important cause of morbidity and mortality in 30% of diabetic patients, and thus there is an urgent need for the development of effective therapies. Although many therapeutic interventions, including reducing hyperglycemia and intraglomerular pressure, have been shown to slow down the progression of diabetic nephropathy, many patients still develop end-stage renal disease. This may result from the fact that damage to various renal compartments due to diabetes may be multifactorial and the processes involved may be intricately interwoven and difficult tease out to treat them individually in an effective manner. Recently, the attention of several investigators has been drawn to control subclinical chronic inflammation (microinflammation) in the kidney, which probably plays a critical role in promoting progression of diabetic nephropathy.1.Lim A.K.H. Tesch G.H. Inflammation in diabetic nephropathy.Mediators Inflamm [online]. 2012; 2012: 1461254Google Scholar, 2.Donate-Correa J. Martin-Nunez E. Muros-de-Fuentes M. et al.Inflammatory cytokines in diabetic nephropathy.J Diabetes Res [online]. 2015; 2015: 948417Google Scholar In this regard, Awad et al.3.Awad A.S. You H. Gao T. et al.Macrophage-derived tumor necrosis factor-α mediates diabetic renal injury.Kidney Int. 2015; 88Abstract Full Text Full Text PDF Scopus (121) Google Scholar (this issue) report effectiveness of anti-tumor necrosis factor-α (anti-TNF-α) antibody in the amelioration of experimental diabetic nephropathy in Ins2Akita diabetic mice. Likewise, they observed protection from streptozotocin-induced hyperglycemic renal injury in another mouse model deficient in macrophage-specific TNF-α (CD11bCre/TNF-αFlox/Flox). In both models, besides reduced levels of TNF-α, the authors noted improvement in renal function and morphology, and reduced macrophage influx associated with downregulation of inflammatory cytokines, including monocyte chemoattractant protein-1 (MCP-1)/chemokine (C-C motif) ligand 2 (CCL2), which is involved in recruitment of monocytes. These novel findings reinforce the importance of previous studies that have elucidated the roles of inflammatory cytokines in orchestrating a complex series of signaling events resulting in renal injury in diabetes. We will therefore briefly discuss the importance of the current study in the context of previous research. Hyperglycemia-induced metabolic perturbations and hemodynamic disturbances have been considered the main factors promoting diabetic renal injury for the past two to three decades. The hyperglycemia-induced metabolic derangements coupled with channeling of glucose intermediaries into polyol and hexosamine pathways lead to the activation of protein kinase C and the generation of advanced glycation endproducts (AGEs) and reactive oxygen species (ROS). These factors lead to activation of the renin–angiotensin–aldosterone system that dramatically worsens the progression of diabetic nephropathy.4.Kanwar Y.S. Sun L. Xie P. et al.A glimpse of various pathogenetic mechanisms of diabetic nephropathy.Annu Rev Pathol. 2011; 6: 395-423Crossref PubMed Scopus (509) Google Scholar During the past decade the roles of innate immunity and microinflammation have emerged as important factors in causing diabetic renal injury.1.Lim A.K.H. Tesch G.H. Inflammation in diabetic nephropathy.Mediators Inflamm [online]. 2012; 2012: 1461254Google Scholar The evolution of this concept stemmed from studies in the late 1980s and early 1990s indicating that the AGEs and AGE-modified glomerular basement membranes are capable of stimulating and inducing secretion of TNF-α and interleukin-1β (IL-1β) by macrophages, which express the receptor for AGEs.5.Hasegawa G. Nakano K. Sawada M. et al.Possible role of tumor necrosis factor and interleukin-1 in the development of diabetic nephropathy.Kidney Int. 1991; 40: v1007-v1012Abstract Full Text PDF PubMed Scopus (224) Google Scholar This set the stage for investigators to explore the role of inflammatory cytokines in progression of diabetic nephropathy in humans and in various animal model systems. More than two decades ago, Furuta et al. demonstrated an important role for macrophages in diabetic nephropathy, observing a transient influx of macrophages predominantly into glomeruli, which declined in advanced stages of diabetic glomerulosclerosis.6.Furuta T. Saito T. Ootaka T. et al.The role of macrophages in diabetic glomerulosclerosis.Am J Kidney Dis. 1993; 21: 480-485Abstract Full Text PDF PubMed Scopus (273) Google Scholar This matter was later revisited, and now there is growing evidence that the macrophages are indeed the major inflammatory cells infiltrating the diabetic kidney in the setting of subclinical chronic inflammation associated with diabetic renal injury. After accumulating in glomeruli and/or the interstitium, macrophages initiate a series of events involving complex interactions with resident kidney cells inducing them to secrete proinflammatory cytokines that ultimately result in diabetic kidney injury. In the diabetic milieu, factors including hyperglycemia, AGEs, oxidized low-density lipoprotein, and ROS as well as angiotensin II can activate resident kidney cells by inducing the stress-activated protein kinases p38 MAPK and JNK, which increases secretion of chemokines including MCP-1 and colony-stimulating factor-1 (CSF-1) and expression of adhesion molecules, such as intercellular adhesion molecule (ICAM) and vascular cell adhesion molecule (VCAM).7.Hickey F.B. Martin F. Diabetic kidney disease and immune modulation.Curr Opin Pharmacol. 2013; 13: 602-613Crossref PubMed Scopus (38) Google Scholar These signaling cascades facilitate the recruitment of monocytes and T lymphocytes that, in conjunction with renal parenchymal cells, elaborate proinflammatory cytokines and ROS in an autocrine and paracrine manner, thereby establishing a vicious cycle of inflammatory injury in the glomerular and tubulointerstitial compartments of the kidney. The most important inflammatory cytokines that are induced in this setting include TNF-α, IL-1, IL-6, and IL-18. There is also increased activation of the transcription factor nuclear factor-κB, a master regulator of innate immunity, which contributes to further progression of diabetic nephropathy.8.Lampropoulou I.T. Stangou M. Papagianni A. et al.TNF-α and microalbuminuria in patients with type 2 diabetes mellitus.J Diabetes Res [online]. 2014; 2014: 394206Google Scholar The evidence for the M1-type macrophage influx and cytokine-induced inflammatory damage in the setting of hyperglycemia is derived from both clinical and experimental studies. In humans, accumulation and activation of macrophages in kidneys of diabetic patients positively correlates with severity of hyperglycemia, plasma creatinine, and hemoglobin A1c, and the degree of albuminuria and glomerular and tubular damage.1.Lim A.K.H. Tesch G.H. Inflammation in diabetic nephropathy.Mediators Inflamm [online]. 2012; 2012: 1461254Google Scholar Likewise, the serum levels of inflammatory cytokines, for instance TNF-α, were positively correlated with macro- and microalbuminuria; however, a recent study refutes these findings.8.Lampropoulou I.T. Stangou M. Papagianni A. et al.TNF-α and microalbuminuria in patients with type 2 diabetes mellitus.J Diabetes Res [online]. 2014; 2014: 394206Google Scholar Nevertheless, experimental animal data convincingly support the notion that macrophage-induced signaling correlates with severity of renal injury in hyperglycemia. For instance, db/db mice deficient in the adhesion molecule ICAM have reduced renal monocyte influx, albuminuria, glomerular hypertrophy, and tubular damage.1.Lim A.K.H. Tesch G.H. Inflammation in diabetic nephropathy.Mediators Inflamm [online]. 2012; 2012: 1461254Google Scholar, 2.Donate-Correa J. Martin-Nunez E. Muros-de-Fuentes M. et al.Inflammatory cytokines in diabetic nephropathy.J Diabetes Res [online]. 2015; 2015: 948417Google Scholar These investigators also reported decreased accumulation of glomerular and interstitial macrophages and attenuated renal fibrosis in CCL2/MCP−/− mice with streptozotocin-induced diabetes. Similar results have been reported with the ablation of CCR2, the receptor for MCP-1/CCL2. CCR2–/– mice were noted to have attenuated albuminuria, glomerular macrophage influx, and extracellular matrix expression.3.Awad A.S. You H. Gao T. et al.Macrophage-derived tumor necrosis factor-α mediates diabetic renal injury.Kidney Int. 2015; 88Abstract Full Text Full Text PDF Scopus (121) Google Scholar Since MCP-1 pathobiology in the context of diabetic nephropathy is well established and MCP-1 and TNF-α are central to the pathogenesis of inflammatory damage in hyperglycemia, Awad et al. further explored the relevance of TNF-α in diabetic injury in the current study.3.Awad A.S. You H. Gao T. et al.Macrophage-derived tumor necrosis factor-α mediates diabetic renal injury.Kidney Int. 2015; 88Abstract Full Text Full Text PDF Scopus (121) Google Scholar TNF-α is a transmembrane homotrimeric protein with a molecular mass of 34 kDa and, after cleavage by TNF-α-converting enzyme 17 (ADAM-17), is released into the circulation and binds to TNF receptor 1 (TNFR1) or TNFR2 and modulates a complex series of immune and inflammatory responses.8.Lampropoulou I.T. Stangou M. Papagianni A. et al.TNF-α and microalbuminuria in patients with type 2 diabetes mellitus.J Diabetes Res [online]. 2014; 2014: 394206Google Scholar, 9.Al-Lamki R.S. Mayadas T.N. TNF receptors: signaling pathways and contribution to renal dysfunction.Kidney Int. 2015; 87: 281-296Abstract Full Text Full Text PDF PubMed Scopus (115) Google Scholar Almost all resident kidney cells, including mesangial cells, podocytes, endothelial and tubular cells, are capable of producing TNF-α. However, TNF-α is predominantly synthesized by infiltrating T cells and monocytes/macrophages, where hyperglycemia and AGEs serve as potent inducers of TNF-α in the resident kidney cells. TNF-α can amplify the renal inflammatory response by increasing the expression of adhesion molecules and by inducing the cells to release other cytokines, growth factors, and proinflammatory chemokines, IL-8, MCP-1, and macrophage colony-stimulating factor (M-CSF) in an autocrine and paracrine manner. Since the infiltrating and intrinsic glomerular cells can synthesize TNF-α, both can be considered equally important in the causation and perpetuation of glomerular injury. In addition, TNF-α can directly activate NADPH oxidase, leading to local generation of ROS via phosphodiesterase-dependent mechanisms. Overall, it seems that TNF-α is a prime inducer and driver of renal microinflammation and thus is believed to play a central role in the network of proinflammatory molecules during the progression of diabetic nephropathy (Figure 1). TNF-α was hypothesized to play a pathogenetic role in the progression of diabetic nephropathy by Hasegawa et al. more than two decades ago.5.Hasegawa G. Nakano K. Sawada M. et al.Possible role of tumor necrosis factor and interleukin-1 in the development of diabetic nephropathy.Kidney Int. 1991; 40: v1007-v1012Abstract Full Text PDF PubMed Scopus (224) Google Scholar Since then many investigators have attempted to delineate the mechanism(s) by which macrophage-derived TNF-α causes renal injury in the diabetic milieu.8.Lampropoulou I.T. Stangou M. Papagianni A. et al.TNF-α and microalbuminuria in patients with type 2 diabetes mellitus.J Diabetes Res [online]. 2014; 2014: 394206Google Scholar, 9.Al-Lamki R.S. Mayadas T.N. TNF receptors: signaling pathways and contribution to renal dysfunction.Kidney Int. 2015; 87: 281-296Abstract Full Text Full Text PDF PubMed Scopus (115) Google Scholar In this regard, a number of mechanisms have been proposed by which TNF-α may induce renal injury directly or indirectly via the elaboration of other inflammatory cytokine. TNF-α may stimulate the production of endothelin-1, leading to dysregulation of vascular tone resulting in reduced intrarenal blood flow and glomerular filtration rate. It may also disrupt endothelial intercellular junctions and thus adversely affect the integrity of the glomerular filtration barrier, thereby causing an increase in its permeability and albuminuria. In addition, TNF-α may also have direct cytotoxic effects on glomerular podocytes and tubular cells and thus may induce apoptosis and cell death. Alternatively, it may cause activation of the protein kinase/phosphatidylinositol-3-kinase (PI3K) pathway or activation of NADPH oxidase, culminating in the generation of ROS and consequential cellular damage. With respect to glomerular podocytes, it may reduce the expression of nephrin via activation of the PI3K/Akt pathway, and decrease Akt activity in the setting of diabetes, leading to reduced cell survival. Lastly, it may synergize with the actions of another profibrogenic cytokine, transforming growth factor-β, in promoting ECM accumulation by increasing the expression of fibronectin and TIMP-1.8.Lampropoulou I.T. Stangou M. Papagianni A. et al.TNF-α and microalbuminuria in patients with type 2 diabetes mellitus.J Diabetes Res [online]. 2014; 2014: 394206Google Scholar These myriad effects of TNF-α support its role as an inducer as well as a driver of renal injury by modulation of the expression of other downstream cytokines. In line with this notion are the findings of the current study by Awad et al., in which inhibition of TNF-α led to decreased expression of other cytokines, that is, MCP-1, keratinocyte-derived cytokine (KC), and granulocyte–macrophage colony-stimulating factor (GM-CSF), along with improvement in renal functions and reduction of urinary albumin excretion.3.Awad A.S. You H. Gao T. et al.Macrophage-derived tumor necrosis factor-α mediates diabetic renal injury.Kidney Int. 2015; 88Abstract Full Text Full Text PDF Scopus (121) Google Scholar Despite the large literature on the roles of inflammatory cytokines in the pathogenesis of diabetic nephropathy, studies need to determine whether TNF-α is a suitable biomarker or target for therapeutic intervention to ameliorate the progression of diabetic nephropathy. Many studies have reported a relationship between elevated serum and urinary levels of TNF-α and abnormal urinary protein excretion and reduced glomerular filtration rate. Increased expression of TNF-α in both glomerular and tubulointerstitial compartments has also been reported in patients with diabetic nephropathy and in murine models of experimental diabetes. Furthermore, elevated concentrations of circulating TNF-α, TNFR1, and TNRF2 are associated with loss of renal function.9.Al-Lamki R.S. Mayadas T.N. TNF receptors: signaling pathways and contribution to renal dysfunction.Kidney Int. 2015; 87: 281-296Abstract Full Text Full Text PDF PubMed Scopus (115) Google Scholar However, a recent report indicates a positive correlation only of urinary but not of serum TNF-α with severity of microalbuminuria in type 2 diabetes.8.Lampropoulou I.T. Stangou M. Papagianni A. et al.TNF-α and microalbuminuria in patients with type 2 diabetes mellitus.J Diabetes Res [online]. 2014; 2014: 394206Google Scholar The issue of whether the TNF-α/TNF-α receptor system could serve as a therapeutic target for the deceleration of diabetic nephropathy in diabetic mice or patients has been addressed in several investigations.2.Donate-Correa J. Martin-Nunez E. Muros-de-Fuentes M. et al.Inflammatory cytokines in diabetic nephropathy.J Diabetes Res [online]. 2015; 2015: 948417Google Scholar, 3.Awad A.S. You H. Gao T. et al.Macrophage-derived tumor necrosis factor-α mediates diabetic renal injury.Kidney Int. 2015; 88Abstract Full Text Full Text PDF Scopus (121) Google Scholar, 8.Lampropoulou I.T. Stangou M. Papagianni A. et al.TNF-α and microalbuminuria in patients with type 2 diabetes mellitus.J Diabetes Res [online]. 2014; 2014: 394206Google Scholar, 9.Al-Lamki R.S. Mayadas T.N. TNF receptors: signaling pathways and contribution to renal dysfunction.Kidney Int. 2015; 87: 281-296Abstract Full Text Full Text PDF PubMed Scopus (115) Google Scholar Like the study by Awad et al.,3.Awad A.S. You H. Gao T. et al.Macrophage-derived tumor necrosis factor-α mediates diabetic renal injury.Kidney Int. 2015; 88Abstract Full Text Full Text PDF Scopus (121) Google Scholar other studies have also documented a reduction of urinary albumin excretion and improvement in renal functions following the administration of monoclonal antibody directed against TNF-α (infliximab), or circulating receptor fusion protein (etanercept) or antagonist of TNF-α receptor (progranulin). Another anti-inflammatory small-molecule inhibitor, pentoxifylline, has been shown to slow the progression of diabetic nephropathy.2.Donate-Correa J. Martin-Nunez E. Muros-de-Fuentes M. et al.Inflammatory cytokines in diabetic nephropathy.J Diabetes Res [online]. 2015; 2015: 948417Google Scholar Pentoxifylline is a methylxanthine-derived phosphodiesterase inhibitor that reduces TNF-α gene transcription. Administration of pentoxifylline was shown to reduce albuminuria and estimated glomerular filtration rate, most likely by improving the local renal microcirculatory fluid dynamics. In addition to above-mentioned pharmacological interventions, the new information provided by Awa et al.3.Awad A.S. You H. Gao T. et al.Macrophage-derived tumor necrosis factor-α mediates diabetic renal injury.Kidney Int. 2015; 88Abstract Full Text Full Text PDF Scopus (121) Google Scholar indicates that genetic ablation of TNF-α also leads to amelioration of diabetic injury. Finally, in view of the above discussion of the vast amount of scientific and clinical work encompassing the past two to three decades, one can make a timely comment at this juncture that the TNF-α/TNF-α receptor system certainly has some added value in serving as a potential therapeutic target besides controlling hyperglycemia and hypertension to improve outcomes in the progression of diabetic nephropathy toward end-stage renal disease. Supported in part by US National Institutes of Health grant DK60635 and the National Science Foundation of China.