Action Mechanisms and Therapeutic Targets of Renal Fibrosis

Renal fibrosis was a chronic and progressive process affecting kidneys in chronic kidney disease (CKD), regardless of cause. Although no effective targeted therapy yet existed to retard renal fibrosis, a number of important recent advances have highlighted the cellular and molecular mechanisms underlying the renal fibrosis. The advances including TGF-β/Smad pathway, oxidative stress and inflammation, hypoxia and gut microbiota-derived from uremic solutes were highlighted that could provide therapeutic targets. New therapeutic targets and strategies that are particularly promising for development of new treatments for patients with CKD were also highlighted. DOI : 10.14302/issn.2574-4488.jna-18-2443 Corresponding author: Wei Su, MD, Department of Nephrology, Baoji Central Hospital, No. 8 Jiangtan Road, Baoji, Shaanxi 721008, China, E-mail: suwei831@foxmail.com


Introduction
Chronic kidney disease (CKD) had a high prevalence all over the world and was closely associated with high mortality [1][2][3]. The prevalence of CKD was estimated to be 8-16% worldwide. In patients over 64 year old, the prevalence elevated to 23 Stage 5: End-stage renal failure; glomerular filtration rate <15 mL per min per 1.73 m 2 [1,4,5].
Renal fibrosis was characterized as a common endpoint of diverse CKD which resulted in functional damage ultimately leading to terminal renal failure [6][7][8][9].
Renal fibrosis is generally regarded as the dark side of tissue repair mechanisms. Fibrogenesis might be involved in the tubulointerstitium resulting in tubulointerstitial fibrosis, glomeruli resulting in glomerulosclerosis or the arterial vasculature resulting in atherosclerotic lesions [5,10]. Various action mechanisms were implicated in renal diseases and renal fibrosis [11][12][13][14][15][16][17][18]. Knowledge of the complex pathophysiological mechanisms contributed to CKD remains limited. In this review, we verify the critical roles of transforming growth factor-β (TGF-β)/Smad pathway, oxidative stress and inflammation, hypoxia and gut microbiota-derived from uremic solutes in the pathophysiology of CKD and renal fibrosis, summarize the action mechanisms of renal fibrosis, and discuss the effects of these mediators in the context of renal fibrosis.

TGF-β/Smad in Renal Fibrosis
TGF-β was essential for normal tissue development, repair and maintenance for organ functions. TGF-β1 was known as an antiinflammation cytokine [18]. It produced anti-inflammatory effects through inhibition of mitogenesis and cytokine responses in glomerular cells and inhibiting infiltrating cells [18].
There is extensive evidence pointing to TGF-β1 upregulation and its role in the pathogenesis of renal fibrosis in both animal models and patients with CKD [18,26]. TGF-β1 mediated progressive renal fibrosis by stimulating production and suppressing degradation of extracellular matrix (ECM). Moreover, TGF-β1 caused renal fibrosis by the transformation of tubular epithelial cells to myofibroblasts through epithelial-to-mesenchymal transition (EMT) [23]. The central role of TGF-β1 on EMT and renal fibrosis has been confirmed by many experiments which indicated the ability of TGF-β1 blockade with decorin, neutralizing TGF-β antibody or anti-sense oligonucleotides to attenuate renal fibrosis [18]. Direct evidence for the causal role of TGF-β1 in renal fibrosis is confirmed in mice over-expressing an active TGF-β1 form [27]. TGF and diabetic nephropathy. The upregulation of the three TGF-β isoforms and TGFβRI and TGFβRII has been uncovered in the glomeruli and tubulointerstitium in kidney diseases [28]. Upregulation of TGF-β1 caused excessive ECM productions, reduced ECM-degrading proteinase activity and upregulated proteinase inhibitor, that resulted in excessive ECM deposition. In progressive podocyte-associated glomerular diseases, excessive TGF -β1 expression in the podocytes has been indicated the role of TGF-β1 in podocyte injury in patients with IgA nephropathy, focal and segmental glomerulosclerosis (FSGS) and diabetic nephropathy [29]. Tubular and glomerular TGF-β expression was increased in early and late stages of diabetic nephropathy and inversely correlates with glycemic control in diabetic patients [30].
TGF-β1 expression was stimulated by glomerular stretch and hyperglycemia in early stage, and by angiotensin II, advanced glycation end-product and platelet-derived growth factor [30]. Angiotensin II has been demonstrated to raise expression of TGF-β1 and its receptors [31,32].

Fibrosis
Oxidative stress and inflammation played a central part in the pathogenesis and progression of CKD [77][78][79][80][81][82]. Renal fibrosis was a relatively common cause of CKD in humans. Rats or mice fed an adenine-containing diet exhibited severe renal fibrosis resembling that seen in humans [83]. and growth factors that contribute to renal fibrosis [87,88].

Hypoxia and Renal Fibrosis
The kidney was physiologically hypoxic despite its plentiful blood supply, because an oxygen shunt is transgenic mice compared to control mice, they did not display renal injury or dysfunction [92]. These results were consistent with study indicating that conditional knockout of HIF-1α in the proximal tubules lessened fibrosis in mouse UUO [89]. Given that deposition of ECM was a part of repair processes unless it is uncontrolled, hypoxia-inducible factor activation by hypoxia in tubular cells mitigated renal injury by the upregulation of angiogenic and fibrogenic factors. At least five uremic toxins showed a direct link to EMT and renal fibrosis [96][97][98][99].

Uremic Solutes and Renal Fibrosis
Uremic toxin IS was a small organic aromatic polycyclic anion derived from dietary tryptophan by gut microbiota that has widely been investigated in linking with CKD-associated cardiovascular disease [96,[100][101][102], and IS can induce vascular calcification and correlates with coronary artery disease and mortality [103]. IS also contributed to a plethora of pathologies observed in dialysis patients, including tubulointerstitial inflammation and kidney damage [96]. IS overload augmented the gene expression of tissue inhibitor of metalloproteinases-1, intercellular adhesion molecule-1, alpha-1 type I collagen, and TGF-β in the renal cortex of 5/6 nephrectomized rats [104]. Moreover, IS stimulated the production of TGF-β in renal proximal tubular cells.
Other study indicated that stimulation of HK-2 cells to IS resulted in a reactive oxygen species-mediated upregulation of plasminogen activator inhibitor-1, a downstream signaling mediator of the TGF-β signaling related to most aggressive kidney diseases [105].
Furthermore, another study demonstrated that IS can increase α-SMA and TGF-β expression in HK-2 cells by activation of the (pro)renin receptor through reactive oxygen species-Stat3-NF-κB pathways [106]. IS also activated the TGF-β signaling, as showed by an increased Smad2/3 phosphorylation [97,107].
Although EMT contribution to fibrosis was controversial, phenotypic alterations reminiscent of EMT, also presented as epithelial phenotypic changes, might play an important role in the fibrogenesis and disease progression [108].  [97]. Similar effects of IS have also been observed in human renal cell models [109].
In addition, genetic or microRNA-based mechanisms are also reported to inhibit renal fibrosis through modulating signaling pathways to prevent the progression of renal fibrosis during CKD. Knockdown of profibrotic factor Smad4 alleviated renal fibrosis in mice [110].