2.1. Glomerular Hemodynamic Perturbations
Hemodynamic effects are vital to maintain the glomerular homeostasis and surround the renin-angiotensin-aldosterone system (RAAS). SGLT2 is principal for increasing reabsorption of proximal tubular Na and glucose, which suppress the tubule-glomerular feedback due to reduced delivery of sodium chloride (NaCl) to macula densa[
19] and deteriorate hyperglycemia[
20]. Reduced tubule-glomerular feedback cause dilated afferent arteriole and increased angiotensin II in efferent arteriole, leading to vasoconstriction[
19]. Vasodilatation of afferent arteriole and vasoconstriction of efferent arteriole could lead to hyperfiltration, which is recognized to initiate the pathogenesis of DKD[
2]. Endothelin (ET) could modulate renal flow blood and glomerular filtration[
21], implicating a potential vasoconstriction effect in renal blood. Dyslipidemia, hyperglycemia, endothelial dysfunction and oxidative stress elevate plasma ET[
22]. Endothelin receptor (ER) blockade is demonstrated to reverse the progression of CKD[
23]. Cyclo-oxygenase 2(COX-2) derived prostanoids that are expressed in endothelial cells in renal tissue have been considered to regulate renal auto-regulatory functions at the macula densa and mediate dilated function of afferent arteriole[
24], resulting in hyperfiltration. Glomerular hyperfiltration results in progressive albuminuria, gradual decreased eGFR, and finally ESRD[
25]. Hyperglycemia, glomerular hypertension and high levels of amino acids could exacerbate glomerular hyperfiltration[
19].
Renin-Angiotensin-Aldosterone System (RAAS)
RAAS participates in the progression of DKD[
26]. The renin is produced by the juxtaglomerular cells of the nephron and found contiguous to the afferent arterioles. The renin is key to trigger RAS, which generates greater vasoconstriction in efferent arteriole than that in afferent arteriole[
27]. Angiotensin converting enzyme 2 (ACE2) play important roles in dilating glomerular afferent arteriole through degrading angiotensin II into angiotensin 1-7. Produced by activation of RAS, angiotensin II binds to specific receptors, namely AT1 and AT2. Activation of AT1 modulates the elevated resistance of efferent arteriole[
28], contributing to hyperfiltration, and activation of AT2 exerts protective counterregulatory role in renal flow, including prostaglandin release and regulating renal vasodilation[
29]. High angiotensin II accelerates renal damage through modulating calcium influx into podocyte[
30], stimulating the expression of proinflammatory (tumor necrosis factor α [ TNF-α], interleukin [IL-1,IL-6, IL-18], monocyte chemoattractant protein-1[MCP-1]), matrix metalloproteinase-9 [MMP-9] and profibrotic mediators (transforming growth factor [TGF- β])[
6,
31], macrophage activation[
32] and increased adrenal aldosterone secretion. Adrenal aldosterone could upregulate profibrotic factors such as TGF- β, which boost macrophage infiltration and fibrosis of kidney[
33].
2.2. Inflammatory and Fibrotic Factors
Inflammation and fibrosis are dominant interrelated promotors of the progression of DKD. Growth factors, inflammatory cytokines and chemokines are substantiated to elevate in renal biopsy samples from DKD patients[
34]. Substantial components of immune system including circulating leukocytes, chemokines and cytokines are activated in diabetes[
35]. Pathological variations of DKD are characterized by nodular and diffuse mesangial expansion, thickening of the glomerular and tubular basement membranes, podocyte damage and detachment, which are attributed to sustained glomerular hypertension and hyperfiltration, subsequent to tubular atrophy and glomerular sclerosis, and eventually apparent decline in renal function[
36].
TNF-α is produced by activated macrophages and resident kidney cells in glomerular and tubular, which plays vital roles in evoking chemokines, cytokines, cyto-toxic effects and apoptosis[
6]. The activation of NF- κB could lead to the production of inflammatory factors such as TNF-α which prompts the progression of DKD[
37]. Diabetic cohorts revealed that TNF-α receptor superfamily members were related to high risk of ESRD in diabetes[
38]. The cytokines, such as IL-1, IL-6, IL-16 and IL-18 have been incriminated to involve in pathogenesis of DKD. IL-1 could cause hyperpermeability of endothelial cells and glomerular hyper-perfusion through promoting the release of phospholipase A2 and prostaglandin E[
39]. Infiltrating macrophages and hyperglycemia contribute to releasing of IL-1β, which is the superfamily of IL-1, intimately involving in the pathogenesis of DKD[
40]. IL-6 recruits neutrophil infiltration in the tubulointerstitium, which is correlated to podocyte hypertrophy, and GBM thickening[
6], eventually resulting in albuminuria and decrease in renal function. The injection of IL-6 neutralizing antibody into diabetic mice leaded to a prominent reduction of collagen and fibrosis by ameliorating mesenchymal transition[
41]. IL-18 instigates unleashing of interferon-γ, expression of adhesion molecules and apoptosis[
39]. The expression of IL-18 in renal tissues is intimately associated with increased albuminuria in DKD[
39].
MCP-1, also referred to CC chemokine ligand 2, have been confirmed to elevate in biopsied kidneys from patients with DKD, which may elicit inflammatory cell recruitment, migration and interplay, and finally contributing to kidney injury[
42,
43]. MMP-9, which is expressed in the proximal renal tubular epithelial cells, is validated to modulate the degradation of extracellular matrix during renal fibrosis[
44]. Downregulation of MMP-9 leads to slowing the progression of DKD by improving creatinine and proteinuria[
45].
Kidney damage in diabetes is pronouncedly featured by monocytes and macrophages. Amassment of macrophages exhibits close relationship with histological severity of kidney disease in diabetes[
42,
46]. Macrophages exacerbates kidney injury by modulating tissue repair and fibrosis[
47,
48]. Hyperglycemia, angiotensin II, endothelial cell dysfunction, oxidized low density lipoprotein (LDL) and advanced glycation end products (AGEs) promote the accumulation of macrophages[
32]. M1 macrophages could switch to anti-inflammatory M2 macrophage[
47]. M1 macrophages could secret substantial inflammatory factors IL-1, IL-6, MMP-9 and TNF-α after kidney injury[
47]. The balance between M1 and M2 macrophage is the major challenging to develop macrophage-based therapy for DKD.
Tubulointerstitial fibrosis is inevitable outcome and the final convergent pathway of progressing kidney disease, and is correlated to extracellular matrix accumulation and tubular atrophy[
49]. TGF- β, which is expressed by nearly all cell types of kidney, infiltrates macrophages and leukocytes, and plays pleiotropic effects including immunomodulation, angiogenesis and extracellular matrix formation in the progression of kidney diseases. TGF- β acts as master mediator of DKD via regulating inflammation and fibrosis[
50].They summarized the pathogenetic roles of TGF- β and its downstream Smad signaling molecules in the progression of DKD. Smad3 fosters autophagy dysregulation by provoking lysosome depletion in tubular epithelial cells of DKD[
51]. Recent study demonstrated leucine-rich -2-glycoprotein 1 (LRG1) could exacerbate kidney fibrosis by augmenting TGF- β/ Smad3 signal transduction[
52]. Klotho, which was mainly expressed in kidney cells, was reported to be a potential therapy for DKD through regulating calcium and phosphate metabolism, downregulating apoptosis, guarding against oxidative stress, and playing anti-inflammatory and antifibrotic roles[
53].
Angiotensin II – mediated reactive oxygen species (ROS) or protein kinase C (PKC) and p38 mitogen-activated protein kinase could trigger CTGF, and plasminogen activator inhibitor (PAI-1) could be activated by TGF- β[
54,
55]. PAI-1accelerates kidney fibrosis by restraining the production of plasmin from plasminogen, which maintains extracellular matrix accumulation.
Hyperglycemia, AGEs and glomerular hypertension could upregulate the expression of TGF- β[
56]. Fibronectin is validated to result in mesangial expansion and deterioration of albuminuria, contributing to exacerbated kidney function[
57]. Treatment of DKD with mesenchymal stem cell therapy could diminish fibronectin and mitigate renal function and albuminuria[
58]. Metformin is reported to reduce collagen-1 together with fibronectin[
59]. Studies reveal collagen-1 propels the progression of renal fibrosis and overabundant accumulation of extracellular matrix in DKD[
60]. The precise mechanism of collagen-1 in the pathogenesis of DKD remains unclear and need further to be explored. The serine/threonine kinase, which is an apoptosis signal -regulating kinase 1(ASK1) induced by oxidative stress, evokes apoptosis, inflammation and fibrosis[
61]. ASK1 has been incriminated to participate in the pathogenesis of DKD through phosphorylating and activating c-Jun N-terminal kinase (JNK) and p38 mitogen-activated protein kinase[
62]. Glucose dysmetabolism could activate protein kinase C (PKC) and the Janus kinase (JAK)-signal transducer and activator of transcription (STAT) pathways[
63]. The JAK-STAT pathway prompts the expression proinflammatory factors and multiple chemokines, enhancing inflammatory response in DKD[
42]. JAK-STAT are highly expressed in glomeruli and tubulointerstitial cells in population with T2D and exhibits inverse relationship with eGFR[
64].
CTGF is found to be associated with tubulointerstitial fibrosis and glomerulosclerosis in various renal disease[
54]. Urinary CTGF concentrations is related to high risk of albuminuria and decreased eGFR[
54]. The synthesis of fibronectin and type 1 collagen elevated when mesangial cells were exposed to CTGF[
54]. Phosphatase and tensin homolog (PTEN) increases the risk of decreased eGFR of DKD patients[
65]. PTEN potentiates the expression of IL-6 and CTGF[
66].
2.3. Metabolic Factors
Hyperglycemia, increased adiposity and dyslipidemia could enhance the production of vasoactive mediators, including AGEs and ROS[
67,
68]. Upon interacting with the receptors for AGEs (RAGEs), AGEs lead to tissue damage in DKD through enhancing variation of extracellular matrix architecture and regulating cellular functions[
69]. RAGE are detected throughout the kidney. The accumulation of AGEs in renal basement could upregulate the expression of RAGE on podocytes in DKD, inducing NF- κB mediated inflammation, fibrosis and oxidative stress[
70]. Targeting on AGEs or AGE/RAGE induced oxidative stress could hold promising therapy for DKD. AGEs also could contribute to impaired vasodilatation in diabetes through suppressing the bioavailability of endothelium-derived nitric oxide (NO) and elevating the production of ROS[
71]. ROS accelerate the progression of DKD through podocytes apoptosis and epithelial to mesenchymal transition (EMT)[
68,
72]. ROS are generated by nucleotide leukin rich polypeptide 3 (NLRP3) inflammasomes, and also foster the activation of NLRP3[
72]. Knocking down of NLRP3 has been reported to impede podocytes injury through reducing hyperglycemia induced production of mitochondrial ROS in renal mesangial cells and preventing lipid accumulation[
73]. The overexpression of pro-oxidant enzyme NADPH oxidase 5 (NOX 5) is demonstrated to promote albuminuria, inflammation and renal fibrosis in diabetes by increasing ROS formation [
73]. Hyperglycemia also could modulate poyol and protein kinase C (PKC) pathways to diminish endothelial nitric oxide synthase and amplifying oxidative stress, respectively, resulting in higher vascular endothelial growth factor and endothelin levels[
32]. Hyperglycemia, dyslipidemia and insulin resistance are the common features of diabetes and could potentiate vicious cycle of inflammatory and oxidative process[
42,
74].
2.4. Dietary AGEs and Gut Microbiome Variation
AGEs exposure could partly result from diet as well as hyperglycemia[
75]. AGEs contribute to glomerular pathological alterations including glomerular hypertrophy, glomerular basement membrane widening, mesangial expansion and glomerular sclerosis[
76]. Dietary AGEs could interact with gut microbiota, evoking local inflammation and the release of inflammatory factors[
77]. AGEs-rich foods interrupt intestinal mucosal barrier and translocation of inflammatory mediators into systemic circulation, causing local kidney inflammation[
78]. As the progression of DKD, uremic toxins result in a relocation towards Gram-negative (G
-) bacteria in the gut. Gut microbiota derived phenyl sulfate has been reported to lead to podocyte injury and albuminuria. Lipopolysaccharides (LPS) from the cell wall of G
- bacteria bind to Toll-like receptor (TLR)-4 to elevate local cytokine production, recruitment of inflammatory cells and the release of LPS[
79]. Exposure to TLR-4 on podocytes or other kidney cells, LPS contribute to inflammation and fibrosis, ultimately resulting in podocyte damage, tubular injury, glomerular hypertrophy and hypercellularity as well as albuminuria in STZ induced diabetic mice[
80]. Alteration in gut microbiota has been incriminated in the pathogenesis of DKD. Reduction in dietary associated short chain fatty acids (SCFAs) from gut microbiota could worsen podocyte damage, interstitial fibrosis and albuminuria by promoting epithelial cell dysfunction and gut inflammation[
81].