Introduction
Chronic kidney disease (CKD) has become a major global public health concern. A nationally representative cross-sectional study of 176,874 individuals from the Sixth Chinese Chronic Kidney Disease Surveillance Center revealed that the prevalence of CKD among Chinese adults was 8.2%. According to the sixth Chinese census, approximately 82 million adults are estimated to suffer from CKD [1]. Renal fibrosis (RF) serves as a prevalent mechanism in the advancement of many renal disorders to end-stage renal disease (ESRD), often necessitating dialysis and renal transplantation for life-sustaining measures, thereby diminishing patients’ quality of life. Current medicines mostly focus on symptom management rather than addressing the underlying disease process, exhibit little efficacy in preventing or reversing fibrosis, and lack effective treatments that possess high specificity and minimal adverse effects. Despite several tries, no singular method has successfully eradicated RF yet. Therefore, investigating the mechanism underlying RF in CKD and identifying effective treatment targets aimed at decelerating its progression are imperative.
Mechanisms underlying renal fibrosis
Signaling pathways and renal fibrosis
Transforming growth factor-beta/Smad signaling pathway
Transforming growth factor-beta (TGF-β) is a member of the TGF-β superfamily and consists of three isoforms: TGF-β1, TGF-β2, and TGF-β3. TGF-β1 is widely expressed in renal cells and promotes RF by stimulating extracellular matrix (ECM) production, inhibiting degradation, mediating epithelial-mesenchymal transition (EMT), and promoting apoptosis [2]. Upon binding as a homodimer to TGF-βRII, TGF-β1 induces the transphosphorylation of TGF-βII to activate downstream Smad signaling molecules to regulate RF through the phosphorylation of specific Ser residues in its C-terminal region. Smad3 enhances RF, whereas Smad2 and Smad7 inhibit it; meanwhile, Smad4 plays a dual role by augmenting Smad3-mediated RF and inhibiting nuclear factor kappa-B (NF-κB)–induced inflammation through the activation of Smad7 transcription [3].
Wnt/β-catenin signaling pathway
The Wnt/β-catenin signaling pathway is an extensively conserved and intricate developmental mechanism that governs cellular fate, organogenesis, tissue homeostasis, and injury repair. Temporary stimulation of Wnt/β-catenin signaling is essential in the repair and regeneration of acute kidney injuries, whereas persistent dysregulation of this pathway results in CKD-associated RF and podocyte damage. The stimulation of Wnt/β-catenin in renal cells increases the expression of fibrosis-related genes, including Snail1, plasminogen activator inhibitor-1 (PAI-1), and matrix metalloproteinase-7 (MMP-7) [4].
The Wnt/β-catenin signaling pathway is essential for the regulation of cellular EMT, which is an essential process in RF development. Activating the Wnt signaling pathway increases downstream β-catenin expression and attenuates its degradation, thus enabling the transformation of renal tubular epithelial cells into a mesenchymal or senescent phenotype. The activation of downstream target genes that produce various ECM components is responsible for this process [5]. Research has shown an elevated expression of Wnt receptors in the unilateral ureteral obstruction (UUO) mice model of RF, resulting in the accumulation of β-catenin and subsequent development of RF. However, when the Wnt antagonist Dickkopf-1 (DKK1) was used, it resulted in reduced β-catenin accumulation and attenuated RF in the UUO mouse model [6]. These data suggest that increased Wnt/β-catenin signaling is crucial to the pathophysiology of RF.
Notch signaling pathway
The Notch family contains numerous receptors and ligands. Four transmembrane receptors (Notch1, Notch2, Notch3, and Notch4) have been identified, along with five ligands (Jagged [JAG] 1, JAG2, delta-like [Dll] 1, Dll3, and Dll4) [7]. The Notch signaling pathway is crucial in numerous biological processes, such as embryonic development and tissue repair. The activation of the Notch signaling pathway has been demonstrated to be involved in numerous chronic renal disorders in humans, contributing to renal regeneration, podocyte apoptosis, proliferation, fibroblast activation, and the promotion of EMT in renal tubular epithelial cells. The expression levels of cleaved Notch1, Notch2, and JAG1 are markedly elevated in podocytes during proteinuric nephropathy and exhibit a strong correlation with the degree of proteinuria, as well as the severity of glomerulosclerosis and tubulointerstitial fibrosis. Among these factors, the expression of Notch1/JAG1 in renal tubules is strongly correlated with the severity of RF [8].
Hedgehog signaling pathway
The Hedgehog (Hh) signaling pathway is essential in RF and encompasses three types of ligands: sonic Hedgehog (Shh), Indian Hedgehog, and desert Hedgehog. Hh is mainly composed of a Hh ligand, a patched receptor (Pcth receptor), coreceptors (Cdon/Ihog, Boc/Boi, and Gas1), a transducer smoothened (Smo), and the downstream transcription factor Gli. Among these components, Shh is thought to play a role in kidney growth and tissue regeneration following injury. Overactive Shh signaling leads to fibroblast activation, proliferation, and excessive matrix synthesis. Studies have demonstrated that downregulating Shh expression in renal tubules and inhibiting the Shh signaling pathway through inhibitors such as cyclobenzaprine targeting Smo, which is a downstream transducer of Hh signaling, effectively attenuate RF [9].
Other signaling pathways
The Janus kinase (JAK)/signal transducer and activator of transcription (STAT) signaling pathway plays a critical function in transferring signals from various cytokines and growth factors in vivo, and its activation significantly contributes to inflammatory infiltration, ECM deposition, and mesenchymal fibrosis. Patients with diabetic kidney disease and animal models exhibit elevated expression of JAK/STAT isoforms, particularly JAK2/STAT3. The activation of these isoforms leads to tubulointerstitial fibrosis and glomerulosclerosis. In animal models of diabetic nephropathy (DN), the inhibition of JAK2 via S3I-201 or a STAT3 inhibitor attenuates RF and the inflammatory response [10]. This evidence implies that the suppression of the JAK/STAT pathway could alleviate fibrotic changes in the kidneys and safeguard their functional integrity.
Interleukin-6 (IL-6) is also involved in RF development. Research has demonstrated that as fibrosis advances in mice treated with UUO, the levels of soluble IL-6 receptor in the kidneys rise. Additionally, treatment with the recombinant IL-6 trans-signaling–specific antagonist Fc-gp130 reduces tubular atrophy, ECM production, inflammation, immune cell infiltration, profibroblastic cytokine expression, and fibroblast accumulation in renal tissues [11].
Cross-talks among signaling pathways
As RF initiates and progresses, the aforementioned pathways individually facilitate its development and participate in a network of intricate interactions. The TGF-β signaling pathway, in particular, has a predominant role in the development of fibrosis. 1) TGF-β and Wnt: By downregulating the Wnt antagonist DKK1 and upregulating Wnt1-associated β-catenin, the TGF-β pathway promotes the activation of the Wnt pathway. β-catenin binds to Smad 3 and promotes the transcription of Smad proteins. 2) TGF-β and JAK/STAT: TGF-β activates JAK 1, STAT 1, STAT 3, and STAT 5 to regulate fibrogenesis, and STAT 3 enhances fibrosis by stimulating TGF-β expression. 3) TGF-β and Notch: The TGF-β pathway promotes the expression of Notch ligands, including JAG, which activates Notch to Notch intracellular domain, subsequently interacting with Smad 3 and augmenting the activity of the TGF-β signaling. 4) Wnt and Notch: JAG1 functions as a target gene of β-catenin and is activated by Wnt/β-catenin signaling. Wnt10b facilitates the activation of both Wnt and Notch signaling pathways, with Wnt/β-catenin signaling serving as an upstream mediator of Notch signaling [12] (Fig. 1).
MicroRNAs and renal fibrosis
MicroRNAs (miRNAs) are endogenous tiny noncoding RNAs, ranging from 19 to 25 nucleotides, that interact with the 3’ untranslated region of messenger RNAs (mRNAs) to modulate gene expression. Many miRNAs, including miR-21, miR-34a, miR-130a-3p, miR-192, miR-214, miR-433, miR-23a, miR-26, miR-27a, miR-132, miR-135a, miR-142-3p, and miR-146a, promote RF. In addition, several miRNA inhibitory RFs, including the miR29 family (miR-29a, -29b, and -29c), the miR30 family (miR-30a, -30b, -30c, -30d, and -30e), the miR200 family (miR-200a, -200b, -200c, -429, and -141), miR23b, miR26a, miR-129-5p, miR130b, and miR152, have been identified [13]. Table 1 [14-22] shows the function of the above miRNAs in renal fibrosis.
Aging and renal fibrosis
The kidneys suffer more from aging than any other organ does. Hypertension, diabetes mellitus, and dyslipidemia can accelerate aging and cause kidney damage during CKD. During the progression of CKD, senescent renal cells secrete senescence-associated secretory phenotypes (SASPs), including pro-inflammatory factors such as IL-1 and IL-6 and profibrotic mediators such as TGF-β1 and matrix metalloproteinases (MMPs), which promote the senescence of the cells themselves as well as the senescence of neighboring renal cells by paracrine secretion, wherein pro-inflammatory factors can contribute to the onset of fibrosis by promoting phenotypic transformation of the cells and ECM protein deposition. Renal tubular epithelial cells undergo senescence due to oxidative stress, telomere depletion, and DNA damage [23]. Therefore, the search for regulators and pathways involved in renal cellular senescence may yield valuable targets for the treatment of RF.
Autophagy and renal fibrosis
Autophagy is the mechanism via which eukaryotic cells preserve cellular homeostasis and integrity by lysosomal degradation of cytoplasmic proteins, damaged organelles, or invading pathogens, regulated by conditional autophagy-related genes. Under stress settings, autophagy is meticulously regulated by signaling pathways that modulate cellular autophagic flux, including mammalian target of rapamycin (mTOR), adenosine monophosphate-activated protein kinase (AMPK), and sirtuins, which are essential regulators of autophagy. Dysregulated autophagy results in acute kidney injury and inadequate renal repair post-injury, along with several CKDs [24]. Autophagy is involved in the RF process, and its effects on RF vary slightly among resident renal cells. Autophagy activation during stress protects renal tubular epithelial cells; however, prolonged autophagy activation causes renal cell senescence and increases RF. In contrast, the basal autophagic activity of podocytes and glomerular endothelial cells (GECs) is extremely high and plays an essential function in preserving the equilibrium of the glomerular filtration barrier, to which podocytes contribute, as well as the integrity of the glomerular capillaries based on GECs, which can inhibit RF [25].
Fibrotic ecological niche of renal fibrosis
RF lesions are not evenly distributed throughout the renal parenchyma but rather begin at specific focal sites, forming a focal and fibrotic ecological niche microenvironment that activates fibroblasts and induces fibrotic lesions. This microenvironment is composed of resident renal cells, infiltrating inflammatory cells, ECM networks, extracellular vesicles, soluble factors, and metabolites. ECM proteins in the fibrous ecological niche recruit soluble factors from the extracellular environment, including Wnts, TGF-β, and Hh, resulting in a unique profibrotic microenvironment that independently stimulates fibroblast proliferation, tubular EMT, macrophage activation, and endothelial apoptosis [26]. Immune cells are also central players in the fibrotic ecological niche.
Myofibroblasts
Myofibroblast activation and subsequent ECM buildup are the primary events in RF. The genesis of myofibroblasts in RF is contentious, with potential progenitors including local fibroblasts, pericytes, mesenchymal stem cell-like cells, epithelial cells, and endothelial cells [27]. Kuppe et al. [28] reported three major myofibroblast sources in the kidney via genetic analysis: PDGFRα+ PDGFRβ+ MEG 3+ fibroblasts; PDGFRβ+ COLEC 11+ CXCL 12+ fibroblasts; and PDGFRα- PDGFRβ+ RGS 5+ NOTCH 3+ pericytes. Damaged renal tubular cells and infiltrating immunoinflammatory cells can secrete a variety of profibrotic mediators (including TGF-β, Wnt, Hh, and tissue inhibitor of MMP) [27], which target myofibroblast precursors via autocrine or paracrine mechanisms, resulting in an intricate series of signaling events associated with myofibroblast activation, proliferation, and ECM formation.
Neutrophils
Neutrophils are crucial constituents of innate immune cells participating in RF. The prompt recruitment of neutrophils to inflamed areas generates pro-inflammatory cytokines and releases a network of DNA and granule proteins referred to as neutrophil extracellular traps (NETs). NETs have the potential to be toxic, causing glomerular injury, activating autoimmune processes, inducing vascular injury, and promoting RF [29]. Ryu et al. [30] determined that neutrophils were the predominant immune cell population in advanced fibrotic kidneys, as revealed by a flow cytometry examination of UUO mice. TGF-β1 and granulocyte-macrophage colony-stimulating factor facilitate the transformation of neutrophils into Siglec-F+ neutrophils, which generate profibrotic mediators and secrete collagen 1 to enhance RF [30].
Lymphocytes
• T lymphocytes: CD4+ T lymphocytes directly facilitate RF independently of macrophages. Among the T-cell subsets, Th2 cells assume a more pivotal role in RF. Compared with Th 1 cell adoptive transfer, the establishment of Th 2 cells in CD4+ T-cell–deficient mice led to exacerbated RF. The Th 17/IL-17 axis has a profibrotic effect. In contrast, CD8+ T lymphocytes may cause apoptosis in fibroblasts, hence restricting RF [31].
• B lymphocytes: In the senescent kidney, T and B lymphocytes, along with resident fibroblasts, constitute tertiary lymphoid tissue, which causes uncontrollable inflammation and delays tissue repair [27].
• Natural killer (NK) lymphocytes: Human NK cells are CD 3-/CD 56+/CD 335 (NKp 46+) monocytes, which are further classified into low-density (CD 56dim) and high-density (CD 56bright) subpopulations on the basis of the level of CD 56 (NCAM) expression. Law et al. [32] reported that in fibrotic kidneys, the number of CD 56 bright NK cells is increased and promotes tissue scarring through the release of pro-inflammatory cytokines, including interferon gamma (IFN-γ) and tumor necrosis factor alpha (TNF-α).
Macrophages
Macrophages are categorized into two phenotypes, M1 and M2; M1 macrophages generate pro-inflammatory mediators, while M2 macrophages exhibit anti-inflammatory properties and contribute to fibrosis. Activated M1 macrophages exacerbate inflammation and tissue injury, whereas Th2 cells and regulatory T cells facilitate a transition to the M2 phenotype, aiding in the alleviation of inflammation and tissue healing. However, as a result of progressive injury and persistent inflammation, M2 macrophages secrete pro-fibrotic factors, such as TGF-β1, and also produce fibronectin and collagen or transdifferentiate into collagen-producing fibroblasts, which promotes myofibroblast proliferation and ECM accumulation, leading to RF [33].
Other immune cells
Dendritic cells and mast cells participate in RF. Kitamoto et al. [34] documented a four-fold increase in dendritic cell populations within the fibrotic kidneys of UUO mice, and the targeted elimination of F4/80+ dendritic cells using liposomal clodronate markedly reduced renal tubular cell death and RF. Summers et al. [35] indicated that collagen deposition and the production of TGF-β, alpha smooth muscle actin (α-SMA), and chemokines were markedly diminished in the kidneys of mast cell-deficient UUO model mice, while RF was aggravated by the reconstitution of the mast cell population.
Treatment strategies for renal fibrosis
Preclinical investigations have found many treatment strategies to mitigate RF but also do not necessarily target fibrosis directly, as reducing fibrosis indirectly by directly preventing acute renal cell injury or reducing inflammation may be possible. Currently, the U.S. Food and Drug Administration has sanctioned only pirfenidone (PFD) and nintedanib for the treatment of pulmonary fibrosis disease; nonetheless, these medications are ineffective in correcting the degenerative progression of pulmonary fibrosis and exhibit adverse consequences. In addition, no antifibrotic medications are available for hepatic, cardiac, or RF [36].
Current clinical use of renoprotective drugs and renal fibrosis
Renin-angiotensin system inhibitors
All renin-angiotensin system (RAS) components are expressed in renal tissue and have direct renal injury and profibrotic effects, while also elevating ultrafiltration, blood pressure, and inflammation [37], and decreasing Klotho expression [38]. The blockade of the RAS by an angiotensin-converting enzyme inhibitor (ACEI) or angiotensin receptor blocker (ARB) mitigates RF in many animal models of renal disease. Nonetheless, the direct targeting of fibrosis is not the principal mechanism of renal protection [39]. In addition, persistent administration of ACEI or ARB can lead to high levels of angiotensin II and aldosterone, a phenomenon referred to as angiotensin II and aldosterone escape. Elevated renin levels may result in the progression of renal disease [37].
Mineralocorticoid receptor antagonists
Hyperactivation of the mineralocorticoid receptor (MR) plays a pro-inflammatory and fibrotic role in kidney disease. The binding of mineralocorticoid receptor antagonist (MRA) to MR reduces the transcription of pro-inflammatory and profibrotic genes, hence lowering inflammation and fibrosis, which mitigates renal injury [40]. Eplerenone markedly decreased oxidative stress, prevented renal interstitial fibrosis, and mitigated the inflammatory response caused by macrophage and monocyte infiltration. Eplerenone therapy markedly diminished 24-hour urine protein, microalbuminuria, and urine albumin-to-creatinine ratio levels, while also lowering laminin levels in individuals with DN, demonstrating the antifibrotic effect of eplerenone [41]. Finerenone is a novel nonsteroidal MRA that exhibits a reduced occurrence of treatment-related hyperkalemia and acute kidney injury compared to steroidal MRAs such as spironolactone and eplerenone. Improvement in renal outcomes in DN with finerenone was observed in the phase III FIDELIO-DKD trial, but the study did not incorporate RF as an endpoint [42]. Additional preclinical and clinical research is required to validate the direct effects of finerenone on RF.
Sodium/glucose cotransporter protein 2 inhibitors
Sodium/glucose cotransporter protein 2 (SGLT-2) inhibitors are utilized in diabetes mellitus management and have demonstrated direct renoprotective properties. In a mouse model of DN, Canagliflozin demonstrates anti-inflammatory and antifibrotic properties by diminishing intrarenal angiotensinogen synthesis, monocyte/macrophage infiltration, and oxidative stress, while lowering plasma concentrations of TNF receptor 1 (TNFR 1), IL-6, MMP7, and fibronectin. Empagliflozin enhanced mitochondrial activity and autophagy via AMPK activation, and ameliorated kidney morphology in diabetic mice. Dapagliflozin markedly decreased urine metabolites linked to mitochondrial metabolism [43]. Clinical investigations indicate that SGLT-2 inhibitor treatment correlates with decreased levels of circulating inflammatory and fibrotic markers, specifically NF-κB, IL-6, monocyte chemotactic protein 1 (MCP-1), TNFR 1, MMP7, and fibrinogen 1 [44]. These findings further support the antifibrotic and anti-inflammatory properties of SGLT-2i.
Pentoxifylline
Pentoxifylline (PTF) is a nonspecific phosphodiesterase (PDE) inhibitor exhibiting robust antiproliferative and anti-inflammatory effects through a mechanism that entails the inhibition of cyclic-3-Mobi, 5′-PDE, resulting in elevated intracellular cyclic adenosine monophosphate levels and subsequent activation of protein kinase A [45]. Research with animals has demonstrated that PTF can impede the advancement of RF by suppressing cell proliferation, diminishing renal inflammation, and decreasing the deposition of the ECM [46]. A randomized trial showed that PTF treatment decreased serum high sensitivity C-reactive protein, TNF-α, and fibrinogen levels while increasing estimated glomerular filtration rate (eGFR) [47]. These studies suggest a potential renoprotective and antifibrotic effect of PTF.
Ongoing trials of new drugs for renal fibrosis
Pirfenidone
PFD is extensively utilized in idiopathic pulmonary fibrosis and exhibits antifibrotic, anti-inflammatory, antioxidant, and antiapoptotic properties. The effects of PFD in attenuating RF and protecting renal function have been demonstrated in various animal experiments [48]. In a model of RF produced by cyclosporin A, PFD therapy reduced the mRNA expression levels of TGF-β1. In an animal model of DN, PFD therapy reduced the expression levels of renal type I collagen, type IV collagen, and fibronectin genes [49]. In a clinical study of DN, PTF treatment improved the decrease in eGFR without reducing urinary protein or urinary TGF-β levels [50]. Whether PTF has definitive efficacy in improving nephropathy outcomes and attenuating RF in the clinic needs to be confirmed by more clinical studies.
Nintedanib
Nintedanib is utilized for the treatment of idiopathic pulmonary fibrosis and has demonstrated the capacity to block EMT by obstructing TGF-β/Smad signaling. Preclinical trials have demonstrated antifibrotic properties in renal disease [51]. Nintedanib inhibited the course of kidney disease in two mice models of autosomal dominant polycystic kidney disease (ADPKD), but reduced fibrosis in just one model [52]. In a controlled model of spontaneous human RF utilizing precision-cut kidney sections, nintedanib suppressed cellular proliferation and diminished type I collagen buildup as well as the expression of fibrosis-associated genes [53], suggesting a strong antifibrotic effect. These trials indicate that nintedanib is a viable choice for the treatment of RF.
Atrasentan
Atrasentan is a specific inhibitor of endothelin receptor A, and endothelin induces harmful consequences including ultrafiltration, podocyte injury, proteinuria, and eventually a reduction in glomerular filtration rate via endothelin receptor type A activation [54]. In patients with CKD, such as DN, atrasentan has reduced proteinuria, RF, and inflammation. Other endothelin receptor antagonists (ERAs), including avosentan and zibotentan, have also been proposed as therapeutic agents for renal illness. Nonetheless, the application of ERAs in clinical practice for kidney disease prevention is constrained, as some ERAs have not shown efficacy or have resulted in side effects, including edema and an elevated risk of congestive heart failure in phase III randomized clinical studies [55].
Bardoxolone methyl
Bardoxolone methyl interacts with Kelch-like ECH-related protein 1, inducing a conformational alteration that facilitates the release of Nrf2, a transcription factor that translocates to the nucleus to exhibit antioxidant and anti-inflammatory actions in response to oxidative stress. In a rat model, methylbadoxolone alleviated kidney fibrosis via inhibiting TGF-β [56]. Early increases in eGFR with bardoxolone methyl treatment have been demonstrated in clinical trials in DN; however, these increases cause serious adverse events, including increased albuminuria and blood pressure, and cause hypomagnesemia, weight loss, edema, and worsening of heart failure [57]. According to the results of recent clinical trials, treatment with bardoxolone methyl did not change the time to onset of ESRD [58].
Other potential drugs for the treatment of renal fibrosis
The TGF-β/Smad, Wnt/β-catenin, Notch, Hedgehog, and other important signaling pathways are inappropriately activated throughout the RF process. Modulating these signaling pathways may serve as a potential therapeutic strategy for RF.
Research with animals has demonstrated that the inhibition of the TGF-β1 signaling pathway can significantly reduce RF in multiple renal models. Examples include PFD, a small-molecule drug that blocks the TGF-β1 promoter; fresolimumab and LY2382770, high-affinity neutralizing antibodies targeting TGF-β isoforms; and SIS3, a selective inhibitor of Smad3 phosphorylation that improves RF in DN models. Clonidine phosphate has anti-RF effects by inhibiting Smad3 binding to the promoter of the homologous structural domain-interacting protein kinase 2 (HIPK2) gene, reducing Smad pathway activation [2].
Blocking Wnt/β-catenin signaling helps with CKD RF. Endogenous Wnt inhibitors, including DKK1, secreted frizzled protein 1 (SFRP1), Wnt inhibitor 1 (WIF-1), and Klotho, can obstruct the Wnt signaling by competitively binding to Wnt ligands at Wnt receptors or coreceptors [59]. Vitamin D receptor agonists, including paricalcitol, can bind and chelate β-catenin, blocking the Wnt/β-catenin signaling cascade and decreasing proteinuria and fibrosis [60].
Blocking Notch with pharmacological inhibitors or soluble ligands improved CKD RF. Treatment with the γ-secretase inhibitor dibenzazepine, which hydrolyzes Notch, reduced the transcript levels of fibrosis indicators, including collagen 1a1, collagen 3a1, and fibronectin, as well as the expression of α-SMA and vimentin [7]. RF can be ameliorated by blocking the components of the Hedgehog pathway; for example, arsenic trioxide has shown efficacy in inhibiting the synthesis of renal Gli1 and Gli2 proteins in animal models, along with the genes that regulate Ptch and Smo, thereby blocking the Hedgehog pathway and exerting an anti-RF effect [61].
To date, there are no authorized targeted antifibrotic medicines specifically for RF. Potential factors contributing to this include: challenges in thoroughly addressing intricate pathological pathways and difficulties in the design of clinical trials.
Antiaging to treat renal fibrosis
Current treatments and therapeutic options for cellular senescence encompass calorie restriction (CR), exercise regimens, Klotho, senolytics, senostatics, and other associated pharmacological agents [62].
Calorie restriction
CR, characterized by a decrease in daily caloric consumption below discretionary levels without inducing malnutrition, diminishes the expression of aging markers in the kidneys, postpones renal aging, and mitigates age-related functional and structural alterations, including tubulointerstitial fibrosis, glomerulosclerosis, reduced renal blood flow, and the impairment of various tubular transport functions [62]. The potential mechanisms encompass the activation of AMPK phosphorylation, suppression of inflammation, restoration of autophagy, diminution of oxidative stress, and improvement of mitochondrial dysfunction [63]. CR reduces cyst size, RF, inflammation, and injury in an ADPKD mouse model [64]. In a randomized clinical trial involving individuals with type 2 diabetes and obesity, CR enhanced glomerular hyperfiltration and mitigated the long-term decline in GFR [65]. However, protein-energy depletion and hypoalbuminemia are prevalent in individuals with CKD, correlating with increased morbidity and mortality, and CR conditions may further aggravate protein-energy wasting [66], the safety of any future randomized controlled trial of CR in patients with CKD should be carefully evaluated.
Exercise regimens
Exercise regimens are directly beneficial for improving renal function and renal senescence by significantly reducing biomarkers of cellular senescence. Regular exercise was demonstrated to reduce reactive oxygen species and serum advanced glycation end-product levels and attenuate aging-induced oxidative stress in a mouse model of aging [67]. A meta-analysis of human and rodent trials demonstrated that exercise training can decelerate cellular senescence by increasing telomerase reverse transcriptase gene expression and telomerase activity, as well as by mitigating telomere shortening [68]. Prolonged exercise is believed to indirectly mitigate the risk of renal dysfunction by lowering the likelihood of hyperglycemia, which triggers pro-inflammatory and fibrotic signaling, and hypertension, which exacerbates renal deterioration via the RAS, oxidative stress, and endothelial dysfunction [69].
Regulates Klotho expression
Klotho, an antiaging protein encoded by the Klotho gene, was discovered by Kuro-o et al. [70] in transgenic mice and is predominantly expressed in the kidney (distal tubules) and brain (choroid plexus) [71]. Klotho can downregulate several cytokines and growth factors, including Wnt/β-catenin and TGF-β1, to modulate cellular senescence and suppress apoptosis, inflammation, and oxidative stress, thus significantly contributing to the delay of renal aging, RF, and CKD progression [23]. Doi et al. [72] indicated that RF correlated with a marked decrease in renal Klotho expression, and that Klotho replacement therapy mitigated RF in a UUO mice model by blocking TGF-β1, Wnt, and insulin-like growth factor 1 (IGF-1) signaling pathways. Yuan et al. [73] reported that Klotho-derived peptide 1 inhibits renal fibroblast activation, attenuates fibrotic lesions, and restores endogenous Klotho expression by targeting TGF-β1 signaling in vitro and in vivo in UUO murine models. These studies suggest that Klotho may inhibit RF by simultaneously inhibiting multiple signaling pathways.
Klotho expression is decreased by oxidative stress, inflammation, RAS system activation, proteinuria associated with renal aging, and a variety of CKD pathologies [23]. Therefore, by increasing Klotho expression, renal senescence can be inhibited, delaying CKD progression. Klotho’s advancement as a fundamental pharmacological agent for CKD and RF exhibits significant promise, with KLOTHO Therapeutics, Inc. (www.klotho.com) intending to produce recombinant Klotho specifically for CKD treatment [71]. In preclinical studies, diabetes mellitus medications (metformin, glucagon-like peptide-1 [GLP-1], and peroxisome proliferator-activated receptor gamma [PPAR-γ] agonists) were shown to increase the level of Klotho. Exercise and physical activity also increased the level of Klotho [74]. Clinical investigations have shown that some presently utilized medications, including angiotensin II receptor antagonists (losartan, valsartan), vitamin D receptor agonists (osteotriol, paricalcitol), and statins (fluvastatin), may enhance endogenous Klotho expression [75].
Senolytics, senostatics, and other related agents
Senolytics, including dasatinib, quercetin (D + Q group), ABT-263, and FOXO4-DRI, can postpone senescence and RF in tubular cells by diminishing the expression of senescence marker proteins in epithelial cells and promoting apoptosis in senescent cells. Senostatics, including rapamycin, metformin, and resveratrol, are pharmacological agents that suppress the senescent phenotype and diminish SASP release while preserving cell viability. Additional treatments comprise nicotinamide riboside and nicotinamide mononucleotide, which serve as NAD+ precursor supplements, as well as nonsteroidal anti-inflammatory drugs [23].
Regulating autophagy to treat renal fibrosis
Autophagy regulates RF in a bidirectional manner. mTOR inhibitors, such as rapamycin (also known as sirolimus) and everolimus, have been shown to decrease the course of RF and CKD by inhibiting inflammatory responses, EMT, collagen deposition, and podocyte damage through autophagy [76]. Curcumin [77], Cordyceps sinensis, and similar compounds [78] are examples of herbal ingredients that can reduce RF via autophagy modulation.
Animal and basic research indicates that sirolimus and its analogs may provide advantages for humans with ADPKD and metabolic or immune-mediated nephropathy by diminishing glomerular hypertrophy, parenchymal inflammation, and fibrosis [79]. Wang et al. [80] revealed that sirolimus alleviated renal tubulointerstitial inflammation and fibrosis in a murine model of RF by blocking the mTOR signaling pathway both in vivo and in vitro. Nonetheless, the translation of mTOR inhibitors to therapeutic applications has been problematic due to their lack of cell-type specificity, which may result in adverse consequences in patients with renal disease. In podocytosis, mTOR is crucial for preserving glomerular podocyte form and function; thus, mTOR inhibitors (sirolimus and everolimus) may compromise the integrity of the actin cytoskeleton, reduce cell adhesion, and disrupt podocyte functionality [81]. Studies have reported that sirolimus treatment may cause proteinuria [82] and a decline in renal function [83,84]. Consequently, mTOR inhibitors represent attractive pharmacological agents for RF treatment; nevertheless, their clinical efficacy and safety in this context have yet to be established.
Summary and outlook
RF is a dynamic process that occurs when practically all CKD progresses to ESRD, and it involves almost all renal intrinsic cells. Numerous studies have revealed that the development of RF is linked to multiple signaling pathways, including senescence, autophagy, and that there are interactions between the pathways, implying that it is a very complex process. Although research has shown that various therapeutic techniques can slow or stop the course of RF and CKD, they cannot reverse the process, and novel anti-fibrotic strategies cannot be transmitted from the laboratory to the clinic due to the intricacy of RF. Therefore, it is critical to investigate RF-related mechanisms and anti-RF therapeutic techniques for the prevention, early detection, and treatment of this disease, as well as to improve patients’ quality of life and extend their lifespan (Fig. 2).
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