Oxidative stress is one of the main determinants involved in the onset and progression of DN pathogenesis ^[1][112]. This condition is promoted by hyperglycemia through the increased production of ROS, which alters the metabolism of carbohydrates and causes complications including DN ^[2][3][113,114]. Oxidative stress-related renal damage in DN is characterized by significant structural and functional alterations in glomerular and renal tubular cells. The three main sources of ROS in DM are NOX, AGE, and polyol chain. In particular, the NOX4 (family of NOX) enzyme is critically necessary for the kidneys to produce ROS. Furthermore, ROS is produced via NOX, uncoupled nitric xanthine oxidase, oxide synthase, lipoxygenase, and mitochondrial respiratory chain dysfunction ^[4][5][20,115]. In general, renal cells have a self-defense system that protects them from ROS. This system is known as an antioxidant, which can be enzymatic (superoxide dismutase (SOD), catalase (CAT), and glutathione peroxidase (GPx)) or non-enzymatic (GSH). In hyperglycemic conditions, the amount of produced ROS may exceed the amount that renal antioxidants can control ^[6][7][116,117]. The imbalance in the ROS: antioxidant ratio causes changes in the redox signaling of the cell, which in turn leads to an impairment in the cell metabolism ^[8][118]. In addition, multiple pathways involved in DN pathogenesis are induced by this imbalance. As a result, antioxidative-stress treatment strategies may efficaciously maintain normal renal function while halting or delaying DN progression ^[9][119].

Natural antioxidants can attenuate oxidative stress by inhibiting the formation of ROS, scavenging and inactivating ROS, inducing the activity of antioxidant enzymes, and forming other proteins involved in the antioxidant pathway ^[10][120]. Plant extracts exhibit antioxidant activity in experimental animal models of DN. The water extract of the seeds of Trigonella foenum graecum counteracts the free radicals and reduces the renal damage in high-sucrose diet and STZ (25 mg/kg) induced diabetic Sprague–Dawley rats ^[11][121]. In diabetic Wistar rats induced by STZ (30 mg/kg), the water extract of the fruit pulp of Passiflora ligularis Juss decreases the oxidative stress ^[12][122]. TIn this research, the therapy was conducted for 30 days at doses of 200 mg/kg, 400 mg/kg, and 600 mg/kg orally per day. The same result was also obtained by Atawodi et al. ^[13][123], who administered the methanol extract of the leaves of Tetrapleura tetraptera to diabetic Wistar rats induced by alloxan (120 mg/kg) and boosted 4 days later (120 mg/kg). Furthermore, the methanol extract of the pods of Acaciella angustissima (25 mg/kg, 50 mg/kg, and 100 mg/kg) showed antioxidant activity by reducing renal TBARS in diabetic Wistar rats induced by STZ (45 mg/kg) ^[14][124]. The extract was given at a dose of 50 mg/kg orally per day for 7 days.

Medicinal plant extracts exhibit antioxidant activity by restoring the enzymatic antioxidative defense system. Dogan et al. ^[15][125] administered the water extract of the leaves of Quercus brantii with doses 100 mg/kg, 250 mg/kg, and 500 mg/kg to diabetic Wistar rats induced by 50 mg/kg of STZ. After 21 days, a decrease in renal GSH, GST, CAT, GPx, and SOD was observed in the treatment group compared with those in the diabetic control. The water extract of the leaves of Cyclocarya paliurus (47 and 94 mg/kg orally per day for 56 days) improved the renal CAT, GPx, and SOD in diabetic Wistar rats induced by HFD and 35 mg/kg of STZ ^[16][126]. The ethanolic extract of the stem barks of Ficus recemosa (200 mg/kg and 400 mg/kg orally per day for 56 days) restored the renal GSH and SOD in diabetic Wistar rats induced by 45 mg/kg of STZ ^[17][127]. The ethanolic extract of the flowers of Diplotaxis simplex (100 mg/kg and 200 mg/kg orally per day for 30 days) improved renal SOD, CAT, and GPx in diabetic Wistar rats induced by 150 mg/kg of alloxan ^[18][128]. The methanolic extract of the barks of Syzygium mundagam (100 mg/kg and 200 mg/kg orally per day for 28 days) increased the renal SOD, CAT, GSH, and GST in diabetic Wistar rats induced by 60 mg/kg of STZ and 120 mg/kg of nicotinamide ^[19][129].

Other findings were also reported in the restoration of the enzymatic antioxidative defense system. These treatments include the methanolic extract of the leaves of Anogeissus acuminata (100 mg/kg and 300 mg/kg for 56 days) administered to diabetic Wistar rats induced by 50 mg/kg of STZ ^[20][130]; the ethanolic extract of the buds and flowers of Cassia auriculata (250 mg/kg and 500 mg/kg orally per day for 21 days) administered to diabetic Wistar rats induced by HFD and 35 mg/kg of STZ ^[21][131]; the ethanolic extract of the stems and roots of Nerium oleander (200 mg/kg orally per day for 20 days) administered to diabetic Swiss albino mice induced by 150 mg/kg of alloxan ^[22][132]; the water extract of the leaves of Nelumbo nucifera (0.5% and 1% (w/w) orally per day for 42 days) administered to diabetic Sprague–Dawley rats induced by HFD and 35 mg/kg of STZ ^[9][119]; the hydroethanolic extract of the aerial parts of Ficus religiosa (50 mg/kg, 100 mg/kg, and 200 mg/kg orally per day for 45 days) administered to diabetic Wistar rats induced by 65 mg/kg of STZ and 230 mg/kg of nicotinamide ^[23][133]; the methanolic extract of the aerial parts of Phyllanthus fraternus (200 mg/kg and 400 mg/kg orally per day for 14 days) administered to diabetic Wistar rats induced by 130 mg/kg of alloxan ^[24][134]; the aqueous extract of the flower of Etlingera elatior (1000 mg/kg orally per day for 42 days) administered to diabetic Sprague–Dawley rats induced by HFD and 35 mg/kg of STZ ^[25][135]; the methanolic extract of the aerial part Centaurium erythraea (100 mg/kg orally per day for 28 days) administered to diabetic Wistar rats induced by STZ (40 mg/kg) for 5 consecutive days ^[26][136]; and the hydroalcoholic extracts of the aerial part of Thuja occidentalis (50 mg/kg, 100 mg/kg, and 200 mg/kg orally per day for 30 days) administered to diabetic Wistar rats induced by 65 mg/kg of STZ and 230 mg/kg of nicotinamide ^[27][137]. In general, these findings indicate the ability of plant extracts to reduce malondialdehyde (MDA) levels in the kidneys of DN models. MDA is a marker compound that shows cellular damage due to oxidative stress ^[28][138]. Animal models with DN have higher MDA levels than normal animal models ^[29][139].

Hyperglycemic-induced oxidative stress increases proinflammatory protein levels by invading macrophages. The inflammatory cytokines are then released, leading to local and systemic inflammation ^[30][165]. Excessive ROS production in pancreatic β-cells can activate stress signaling pathways, which in turn activate inflammatory and apoptotic transcription factors, including NF-κB. The result is the death of β-cells and a reduction in insulin. Most kidney cells, such as renal tubular, mesangial, glomerular endothelial, and dendritic cells, exhibit increased NF-κB expression in response to oxidative stress production ^[31][32][33][166,167,168]. Once NF-kB is activated, it triggers the transcription of proinflammatory genes encoding cytokines (TNFα, IL-1β, IL-2, IL-6, IL-12, and IL-18) and chemokines (MCP-1). The transcription of profibrotic genes involved in the production of growth factors (TGF-β) and leukocyte adhesion molecules (E-selectin, VCAM1, and ICAM-1) is also promoted by NF-κB. The production of these proinflammatory and profibrotic proteins results in inflammation, atherosclerosis, and vascular dysfunction. Thus, the approach targeting the inflammatory response may be effective for DN therapy ^[34][169]. An increased peroxisome proliferator-activated receptor-γ (PPARγ) expression has been found in diabetic animals. PPARγ is a ligand-activated transcription factor that belongs to the nuclear hormone receptor superfamily and is crucial for the control of cell cycle, insulin sensitivity, glucose, and lipid homeostasis. It is also involved in the development of diabetic kidney injury. Gao et al. ^[35][144] reported increased renal PPARγ expression and reduced renal damage after the administration of the ethanolic (70%) extract of the fruits of Cornus officinalis SIEB. et Zucc (100 mg/kg, 200 mg/kg or 400 mg/kg orally per day for 40 days) to diabetic Wistar rats induced by 60 mg/kg of STZ.

Medicinal plant extracts have been used to reduce the levels of proinflammatory mediators in animal models. Honoré et al. ^[36][143] found that the administration of the water extract of the leaves of Smallanthus sonchifolius (70 mg/kg orally per day for 28 days) on diabetic Wistar rats induced by STZ (45 mg/kg) decreased the renal TGF-β1 expression. Lee et al. ^[37][145] showed that the administration of water extract of the of the aerial parts of Portulaca oleracea (300 mg/kg orally per day for 70 days) on C57BL/KsJ-db/db mice decreased the renal TGF-β1 level. The expression of NF-κB p65 and ICAM-1 in renal tissues was also observed. Lu et al. ^[38][146] revealed that the administration of aqueous–ethanol extract of the tuberous roots of Liriope spicata var. prolifera (100 mg/kg and 200 mg/kg orally per day for 56 days) to diabetic Wistar rats induced by STZ (60 mg/kg) decreased the renal expression of ICAM-1, MCP-1, TNF-α, IL-1β, and NF-κB. Studies on the effect of plant extracts on reducing the levels and expression of inflammatory mediators used the following treatments: methanolic extract of the leaves of Paederia foetida (250 mg/kg and 500 mg/kg orally per day for 28 days) administered to diabetic Wistar rats induced by 150 mg/kg of alloxan ^[39][152]; ethanolic extract of the leaves of Lycium chinense (100 mg/kg, 200 mg/kg, and 400 mg/kg orally per day for 40 days) administered to diabetic Sprague–Dawley rats induced by 65 mg/kg of STZ ^[40][153]; hydroethanolic extract of the seeds of Cassia obtusifolia (27 mg/kg, 54 mg/kg, and 81 mg/kg orally per day for 60 days) administered to diabetic Wistar rats induced by 40 mg/kg of STZ ^[41][156]; aqueous extract of the leaves of Gongronema latifolium (6.36 mg/kg, 12.72 mg/kg, and 25.44 mg/kg orally per day for 13 days) administered to diabetic Wistar rats induced by 150 mg/kg of alloxan ^[42][159]; methanolic extract of the leaves of Croton hookeri (200 mg/kg orally per day for 14 days) administered to diabetic Sprague–Dawley rats induced by 45 mg/kg of STZ ^[43][160]; and methanolic extract of the aerial part of Trifolium alexandrinum (200 mg/kg orally per day for 35 days) administered to diabetic Wistar rats induced by HFD and 35 mg/kg of STZ ^[44][164].

AGEs are lipids or proteins that have been glycated as a consequence of their interaction with glucose or related metabolites ^[45][170]. AGEs are generated through the Maillard process via a non-enzymatic reaction between ketones or aldehydes of reducing sugars and the terminal α-amino groups or ε-amino groups of protein lysine residues. The accumulation of AGEs at the site of microvascular injury plays a significant role in renal complication ^[46][47][171,172]. Cytotoxic AGEs may modify lipids and proteins, leading to renal inflammation and oxidative stress, both of which are hallmarks of diabetic kidney disease ^[48][173]. Diabetic rats having high AGE levels may be three times more likely to develop nephropathy compared with those having normal AGE levels ^[49][10]. AGE receptors are found in a variety of renal cells, including proximal tubular cells, mesangial cells, and podocytes. RAGE, LOX-1, galactin-3, CD-36, and SR-B1 are types of AGE receptors. AGEs increase inflammatory responses by activating and expressing various inflammatory mediators, such as IL-6, TGFβ1, and NF-κB ^[50][174].

Medicinal plant extracts show a potential therapeutic effect on the DN animal model via AGE suppression. AGE production is inhibited in the kidney. Gutierrez and Ortiz ^[51][148] confirmed that the administration of Azadirachta indica chloroform extract (200 mg/kg orally per day for 30 days) to diabetic Wistar rats induced by STZ (50 mg/kg) inhibited the formation of AGEs. Other studies on the inhibition of AGE formation in diabetic rat models administered the following treatments: methanol extract of Punica granatum leaves (100 mg/kg, 200 mg/kg, and 400 mg/kg orally per day for 56 days) to diabetic Sprague–Dawley rats induced by 45 mg/kg of STZ ^[52][151]; hydroethanolic (80%) extract of Allium cepa bulbs (150 mg/kg and 300 mg/kg orally per day for 28 days) to diabetic Wistar rats induced by 50 mg/kg of STZ ^[53][155]; petroleum ether extract of Coriandrum sativum seeds (100 mg/kg, 200 mg/kg, and 400 mg/kg orally per day for 45 days) to diabetic Wistar rats induced by 65 mg/kg of STZ and 230 mg/kg of nicotinamide ^[54][16]; and Lagerstroemia speciosa leaf extract (400 mg/kg orally per day for 40 days) to diabetic Wistar rats induced by HFD and 35 mg/kg of STZ ^[55][163].

The interactions between AGEs and RAGEs trigger oxidative stress and promote the creation and release of cytokines, both of which amplify tissue damage ^[56][175]. Given the importance of the AGE–RAGEs axis in DN pathogenesis, inhibiting the formation of AGEs may be a promising treatment for DN. Lee et al. ^[57][141] found that administering the water extract of the roots of Salvia miltiorrhiza (501 mg/kg orally per day for 56 days) to Sprague–Dawley rats induced by STZ (45 mg/kg) decreased the level of AGEs. The expression of glomerular RAGE (the receptor for AGEs) also decreased. The treatment of Wistar rats induced by STZ (60 mg/kg) with the hydroalcoholic extract of the roots of Angelica Acutiloba (50 mg/kg, 100 mg/kg, and 200 mg/kg orally per day for 56 days) reduced the overexpression of renal AGEs and RAGE ^[58][142]. Furthermore, the expression of renal RAGE increased in Wistar rats induced by STZ (55 mg/kg). However, this expression decreased after the administration of the ethanolic (70%) extract of the aerial parts Scrophularia striata at doses of 100 mg/kg and 200 mg/kg orally per day for 60 days ^[59][158]. High AGE levels in kidney tissues during diabetes can induce the secretion of CTGF, which has important roles in many biological processes. Diabetic rats with DN have high CTGF expression. Extracts that can suppress CTGF expression in the kidney may be useful to decrease the progression of DN. An increase in renal CTGF and RAGE expression levels was observed in diabetic Wistar rats induced by STZ (55 mg/kg) ^[60][162]. The treatment using the ethanolic extract of Allium jesdianum rhizomes at doses of 250 mg/kg and 500 mg/kg orally per day for 42 days decreased the renal CTGF and RAGE expression levels.

Cell death is crucial for the development of DN. When renal tissues are subjected to oxidative stress over time, a variety of pathophysiological events may occur, ultimately resulting in cell death. Undesired cells are eliminated by apoptosis in normal tissues to maintain tissue homeostasis. When cells are damaged due to oxidative stress, apoptosis occurs and activates cell death receptors such as TNFRs ^[61][62][176,177]. Some pro- and anti-apoptotic proteins and cysteinyl aspartic acid-specific proteases (caspases) are the primary executors of apoptotic pathways ^[63][178]. Bcl-2 (anti-apoptotic) proteins act on mitochondria to regulate cytochrome c release and initiate the caspases-dependent apoptotic pathway ^[64][179]. In normal animal model, the expression of Bcl2, an anti-apoptotic protein, is mostly found in proximal and distal tubules and capsular parietal cells ^[65][180]. Bax is a pro-apoptotic protein that can modulate pro-apoptotic processes by inhibiting the expression of Bcl-2 members. In diabetic animals, caspase-3 and caspase-9 are upregulated in renal tissues. The activation of Bax inhibits Bcl-2, indicating the occurrence of apoptotic cell damage in renal tissues with the progression of DN ^[66][181].

Khanra et al. ^[67][149] administered the methanol extract of the leaves of Abroma augusta at doses of 100 and 200 mg/kg orally per day for 28 days to diabetic rats induced by STZ (65 mg/kg) and nicotinamide (110 mg/kg) and found that the expression of renal Bcl-2 increased and that of renal Bax, caspase 3, and caspase 9 decreased. In diabetic Wistar rats induced by 55 mg/kg of STZ, the administration of the ethanolic extract of the aerial parts of Artemisia absinthium (200 mg/kg and 400 mg/kg orally per day for 60 days) decreased the expression of renal Bax and eventually increased the expression of renal Bcl-2 ^[68][157]. Alabi et al. ^[69][15] observed an increase in renal Bcl2 and caspase-3 expression levels on diabetic rats induced by fructose 10% and STZ (40 mg/kg). The administration of the water extract of Anchomanes difformis at doses of 200 mg/kg and 400 mg/kg orally per day for 42 days also produced this result.

Podocytes secret vascular endothelial growth factor (VEGF), which is required for the survival of endothelial cells, podocytes, and mesangial cells. The expression of anti-apoptotic protein Bcl-2 is also increased by VEGF ^[70][182]. Ibrahim and Abd El-Maksoud ^[71][150] showed that the administration of the water extract of Fragaria x ananassa leaves (50 mg/kg, 100 mg/kg, and 200 mg/kg orally per day for 30 days) to diabetic Wistar rats induced by STZ (45 mg/kg) increased the level of renal VEGF-A and decreased the activity of caspase 3. The production of ROS in mitochondria can result in the release of cytochrome c into the cytosol, leading to caspase 3 activation and apoptotic cell death. The administration of the hydroethanolic extract of the rhizomes of Zingiber officinale (400 mg/kg and 800 mg/kg orally per day for 42 days) to diabetic Wistar rats induced by STZ (50 mg/kg) reduced the levels of renal cytochrome c and caspase-3 ^[72][154].

Nephrin is a crucial transmembrane protein located in the slit diaphragm complex. It serves as the framework for the podocyte slit diaphragm and is involved in podocyte survival ^[73][74][183,184]. Through podocin and CD2-associated protein (CD2AP), nephrin is connected to the actin cytoskeleton ^[75][76][185,186]. This compound is crucial for controlling the insulin sensitivity of podocytes because its cytoplasmic domain permits the docking of GLUT1 and GLUT4 with the vesicle-associated membrane protein-2, which facilitates insulin signaling. It can act as an early diabetic kidney-disease biomarker. Damage to podocytes has been connected to modifications in nephrin excretion ^[77][78][187,188]. Podoclin, a protein, helps form tight connections between the foot processes of podocytes. Its levels in urine can be measured to monitor the development of kidney damage in people with diabetes ^[79][189]. DN affects nephrin and podocin expression. Therefore, DN progression might be slowed down by upregulating these proteins. Tzeng et al. ^[80][147] reported that the administration of the ethanolic extract of Zingiber zerumbet (200 mg/kg and 300 mg/kg orally per day for 56 days) to diabetic Wistar rats induced by STZ (60 mg/kg) increased the expression levels of renal nephrin and podocin.

Extracellular matrix (ECM) protein builds up in the tubulointerstitial space and the glomerular mesangium of DN, which results in renal fibrosis and ultimately renal failure ^[81][82][190,191]. The ECM is a highly charged, dynamic structure that participates actively in cell signaling and serves as a support system for the cells. It is made up of molecules of elastin, glycoproteins, and collagen that interact with one another and the cells around them to form a complex network ^[83][192]. Renal fibrosis is a pathogenic intermediary for chronic kidney disease (CKD) progression and a histological marker of CKD ^[84][193]. Abid et al. ^[85][161] showed that the administration of the water extract of the aerial parts of Thymelaea hirsute (at a dose of 200 mg/kg orally per day for 28 days) to diabetic Wistar rats induced by STZ (50 mg/kg) decreased the levels of urinary creatinine and tubulointerstitial renal collagen. This result demonstrates that the extract could protect against renal fibrosis by inhibiting ECM protein accumulation.