Aging Cells in Kidney Scarring: Their Role in Disease and Possible Treatments

Cellular senescence is defined as a permanent cease of cell cycle progression even in the presence of growth factor stimulation. However, it is challenging to identify senescent cells with certainty in vivo due to the lack of a sensitive and specific marker. The characteristic features of senescent cells include their resistance to apoptosis and phenotypic changes such as altered morphology with large flattened cell bodies (Knoppert et al., 2019). As summarized in Table 1, there are several characteristic markers associated with cellular senescence and they are frequently used in the field (Gorgoulis et al., 2019; Docherty et al., 2020). Although a single marker may not be specific and sufficient, combination of several of these markers is considered adequate for identifying senescent cells in vitro and in vivo.

One widely used marker for senescent cells is the increased senescence-associated β-galactosidase (SA-β-Gal), which reflects the enhanced lysosomal content of senescent cells. However, it should be noted that SA-β-Gal activity itself may not be necessary for cellular senescence to occur (Hernandez-Sequra et al., 2018) and it is also increased in non-senescent confluent cells with high density in cell cultures, which limits its use as a standalone marker for detecting senescent cells. Furthermore, SA-β-Gal activity cannot be used for paraffin-embedded tissue sections or in live cells.

Senescent cells can also be detected based on the signal pathways leading to their irreversible exit from cell cycle. In this regard, several markers are used, including increased phosphorylation of the Ser-139 residue of the histone variant H2AX (γ-H2AX), accumulation of tumor suppressor p53 and its downstream cyclin-dependent kinase (CDK) inhibitors (CKI) such as p21^Cip1 (p21) and p16^Ink4a (p16), and increase of the senescence-associated heterochromatin foci, which manifests altered DNA packing within senescent cells. Senescent cells are well known to produce a host of proinflammatory and profibrotic cytokines and factors termed as the senescence-associated secretory phenotype (SASP). However, it remains to be determined what percentage of the cells positive for these markers are true senescent cells that never enter cell cycle again.

Aside from these positive markers, senescent cells cease to express proliferation-related markers such as Ki-67, a nuclear protein associated with cell proliferation. Another feature of senescent cells is the loss of the structural nuclear lamina protein lamin B1 (LMNB1). Of note, decrease of lamin B1 is also observed in apoptotic cells but not in quiescent cells. However, lamin B1 is degraded by caspase in apoptotic cells, whereas its reduction in senescent cells results from a decreased mRNA stability.

Senescence most likely occurs in a dynamic, step-wise fashion. It has been proposed that senescence proceeds with two stages: early senescence and full senescence (Childs et al., 2015). The early senescence is characterized by permanent cell cycle arrest and SA-β-GAL positivity, whereas full senescence is irreversible cell cycle arrest, SA-β-GAL positivity and SASP production and secretion.

Given the difficulty in identifying senescent cells, a three-step multi-marker workflow has been proposed recently for detecting senescent cells (Gorgoulis et al., 2019). The first step involves assessing SA-β-Gal activity, while the second step is to co-stain for additional senescent markers, such as p16, p21, γ-H2AX and LMNB1. In the final step, one needs to examine senescence markers that are expected to change in the biological context studied. Together, combinations of these methods provide a robust and sufficient approach for detection and monitoring of senescent cells in different settings.

Cellular senescence can be triggered by a wide variety of stressors, and senescent cells may exhibit immense diversity with different cell origins and varying degree of genomic and epigenomic instability. Depending on the developmental stage, different timing and distinct function, cellular senescence can be divided into three categories: developmental senescence, acute senescence and chronic senescence. Developmentally programmed senescence and acute cellular senescence are usually beneficial, as these senescent cells are generated by defined triggers, regulated by transient signaling and controlled by eventual clearance. However, chronic cellular senescence triggered by aging and sustained injury leads to permanent cell cycle arrest, resists to apoptosis and is unable to proliferate, ultimately causing tissue fibrosis and organ dysfunction (Schmitt and Melk, 2012; van Deursen, 2014; Childs et al., 2015).

Developmental senescence is believed to play a critical role in the fine-tuning of embryogenesis. Senescent cells are transiently present in multiple embryonic structures including the mesonephros, the neural tube, the developing limb, and the endolymphatic sac of the inner ear, as detected by SA-β-Gal staining, p21 expression and SASP activation. In developing kidney, senescent cell accumulation precedes macrophages infiltration, suggesting its role in attracting immune cells to facilitate mesonephros regression via clearance of the senescent cells. Loss of developmental senescence in p21^−/− knockout mice leads to compensatory increase in apoptosis. Although p21^−/− mice remain viable, they exhibit detectable abnormalities in their kidney, limbs and vagina (Munoz-Espin et al., 2013). These studies suggest a critical role of developmental senescence, as a mechanism that complements apoptosis, in eliminating specific groups of cells during embryogenesis.

Acute cellular senescence is induced as an integral part of the healing and regenerative response after an initial insult. Studies show an early induction of senescence in endothelial cells and fibroblasts in response to cutaneous injury (Demaria et al., 2014). This acute senescence is transient and promotes wound healing by secreting platelet-derived growth factor (PDGF). Senescent cell ablation delays, but does not prevent, wound healing. Senescent fibroblast accumulation is observed after skin wound or post myocardial infarction, which functionally limits tissue fibrosis. Similarly, senescence of hepatic stellate cells also appears to attenuate liver fibrosis following injury. Therefore, acute senescence, if it is transient and under tight control of immune clearance, is beneficial in reducing fibrosis and promoting tissue regeneration and recovery.

Chronic senescence, however, leads to progressive accumulation of senescent cells when clearance is insufficient, contributing to kidney aging and CKD. Evidence is mounting that increased numbers of senescent cells are not merely a biomarker of aging or previous injury, they contribute to maladaptive repair and promote tissue fibrosis and organ dysfunction, and therefore represent a novel therapeutic target. There is a close association between senescent cell accumulation and CKD severity (Melk et al., 2005; Schafer et al., 2018). In the kidney, tubular epithelial cells are the major source of senescent cells in CKD, although other cells may also be involved, depends on the location and type of injury (Sis et al., 2007; Valentijn et al., 2017). Senescent cells after chronic injury are resistant to apoptosis, persistently reside within the injured kidney and secrete proinflammatory and profibrotic SASP components, thereby promoting progressive damage.

A diverse array of stressors can induce cellular senescence, such as telomere shortening, DNA damage, oncogenic stimuli, nephrotoxins, ischemia, mitochondrial dysfunction, ROS generation and mechanical stress. To ensure cell integrity after these stresses, DNA is checked during cell cycle progression in the G1/S or G2/M phases (Figure 2). Ataxia telangiectasia mutated (ATM) and ATM-Rad3-related (ATR) kinases play a central role in mediating DNA damage response (DDR). Both kinases rapidly phosphorylase the tumor suppressor p53 upon DNA damage, leading to the induction of p21^Cip1. Furthermore, ATM also phosphorylates the checkpoint kinases Chk1 and Chk2. Phosphorylation of p21 inhibits CDK, leading to the dephosphorylation of retinoblastoma protein (Rb). This enables Rb to bind to transcription factor E2F, thereby leading to G1/S cell cycle arrest. Likewise, p38 MAPK activation induced by ROS and other stimuli also increases p53 activity. In addition to p53, accumulation of tumor suppressor p16^Ink4a also leads to cell cycle arrest via the inhibition of CDK4/CDK6 (Figure 2). Senescent cells produce and secrete SASP, leading to increased inflammation, oxidative stress, hyperactive RAAS, Wnt/β-catenin and TGF-β signaling, and decreased Klotho, Sirtuin 1 (SIRT1), peroxisome proliferator-activated receptors (PPARs) and nuclear factor erythroid 2-related factor 2 (Nrf2) signaling. These senescence-associated signaling such as RAAS, Wnt and TGF-β further promote secondary cellular senescence, creating a vicious cycle.

Senescence-provoking signaling can be different and involve distinct pathways between senescence programs. For instance, developmental senescence is mainly mediated by p21^cip1, but does not involve a DDR and p53. Developmental senescence is also regulated by TGF-β/Smad and phosphatidylinositol 3-kinases (PI3K)/forkhead box protein O (FOXO) pathways, which can also be independent of p53 (Munoz-Espin et al., 2013). However, acute and chronic senescence triggered by various stressors often involve induction of p16^INK4A and activation of DDR and its downstream key effectors p53 and p21, which converge to inhibit CDKs (Figure 2). These studies suggest that developmental senescence is differentially regulated and is unlikely to be the consequence of cumulative cellular stress.

The p53 protein is strategically positioned and plays a vital role in cell fate determination after injury by an array of stressors. In the setting of DNA damage, the level of p53 expression may determine whether a cell cycle-arrested cell continues to replicate after the damage is repaired, or undergo cellular senescence or apoptosis. When DNA is irreparable, the fate of the cells is either apoptosis or senescence by permanently inhibiting cell cycle progression. Although the exact mechanisms under cell fate decision are still unclear (Valentijn et al., 2017), it is believed that the level of intracellular p53 may be the key. High level of p53 leads to apoptosis while lower level of p53 induces cellular senescence. In this regard, chronic senescence may represent a pro-survival choice of the cells after a sublethal injury.

Senescent cells drive kidney fibrosis mainly by producing and releasing SASP, a collection of proinflammatory and profibrotic factors and cues. SASP forms a detrimental microenvironment that regulates the behavior of neighboring cells. SASP components include interleukin-1β (IL-1β), IL-6, IL-8, C-X-C motif chemokine ligand 1 (CXCL1), transforming growth factor-β1 (TGF-β1). They act in a paracrine and autocrine fashion and induce a series of downstream effects (Docherty et al., 2019). A study shows that treatment of human bronchial epithelial cells with serum from patients of chronic obstructive pulmonary disease induces senescent phenotype, including increased SA-β-Gal, γ-H2AX, and p21 (Kuznar-Kaminska et al., 2018). These results suggest that the extracellular factors produced by senescent cells may mediate non-senescent cells to cell cycle arrest, leading to secondary senescence. Of note, SASP produced by senescent cells exhibits some unique feature, which distinguishes it from that secreted by quiescent and other types of non-proliferating cells (van Deursen, 2014). As shown in Table 2, SASP components can be roughly divided into three major categories: soluble signaling factors such as interleukins, chemokines and growth factors, secreted proteases, and secreted insoluble proteins or extracellular matrix (ECM) ingredients (Coppe et al., 2010; Liu et al., 2020b).

The regulation of SASP expression is controlled at multiple levels including transcription, mRNA stability, protein translation and secretion. In addition, SASP forms a stable positive loop via autocrine and paracrine mechanism, which leads to signal amplification (Herranz and Gil, 2018). The expression of the majority of SASP is regulated by nuclear factor κ-B (NF-κB), CCAT/enhancer binding protein-β (C/EBP-β), p38 mitogen-activated protein kinase (MAPK) (Flanagan et al., 2018). DNA damage and oxidative stress activate NF-κB signaling, leading to overexpression of downstream SASP (Taniguchi and Karin, 2018; Wei and Ji, 2018). In kidney proximal tubular cells, indoxyl sulfate induces cellular senescence through reactive oxygen species (ROS)/NF-κB/p53 pathway, and NF-κB also suppresses cellular proliferation and upregulates TGF-β1 and α-SMA to induce fibrosis (Shimizu et al., 2010; Shimizu et al., 2011). P38 MAPK is another regulator of cell senescence. By activating p38 MAPK pathway, SASP can be induced by a mechanism independent of DDR (Alimbetov et al., 2016; Wei and Ji, 2018). C/EBP-β is upregulated in oncogene-induced senescence and it stimulates SASP expression (Lopes-Paciencia et al., 2019).

Although SASP components are relatively conserved and ubiquitous, senescent cells induced by different stressors may have some unique features with heterogeneous SASP, thereby inciting distinct responses in regulating cell proliferation, migration, inflammation and oxidative stress. Another characteristic of SASP is related to its temporal secretion, and not all factors are induced in the same time (Coppe et al., 2010). It is also unclear whether all SASP components are secreted as soluble factors or packaged in the extracellular vesicles including exosomes for efficient intercellular communication (Liu et al., 2020a). Further characterization of SASP composition, dynamics of its secretion, mode of its transmission and functional classification warrants more investigation.

In response to oxidative, toxic and metabolic insults, kidney cells, particularly the proximal tubular epithelial cells, respond in different ways and undergo a spectrum of changes, such as partial epithelial-mesenchymal transition (pEMT), metabolic reprogramming, cell cycle arrest and cellular senescence, and various forms of cell death including apoptosis and necrosis (Figure 3) (Zhou and Liu, 2016). The majorities of these responses to sublethal injury are believed to be evolutionarily conserved programs in an effort of promoting survival and preventing cell death. However, if these responses are not under control or resolved promptly after chronic or repeated injury, they induce maladaptive changes and promote the initiation and progression of CKD. One shared and convergent consequence of these responses including pEMT, metabolic reprogramming and cellular senescence is converting the affected cells into secretory phenotype, thereby producing substantial amount of inflammatory and profibrotic factors and releasing into extracellular space to incite secondary responses (Figure 3).

As one of the injury responses, the exact relationship of cellular senescence to other responses after kidney injury remains elusive. Earlier studies suggest that pEMT precedes cell cycle arrest/senescence after kidney injury (Lovisa et al., 2015), suggesting a temporal relationship between these two events. At present, it is unclear how kidney cells decide to proceed through a special route in response to an insult. As aforementioned, the level of p53 may be an arbitrator in determining the cell fate such as apoptosis or senescence. More studies are needed in this area.

Senescent cells can be detected in different tissue compartments during aging and CKD. Generally, proximal tubular epithelium is the major site of senescent cells after kidney injury, although other sites such as glomeruli and endothelium also harbor senescent cells. It is commonly believed that different stressors that target varying cells may determine the localization and type of senescent cells. Table 3 lists the detection of senescent cells in various kidney diseases.

Aging kidneys have poor outcome when transplanted, and cellular senescence is a crucial prognosis for poorer graft survival (Melk, 2003; Sosa Pena et al., 2018). In a cohort of 75 cases with preimplantation renal allograft biopsies, the levels of cellular senescence marker p16^INK4a predicates postoperative renal function in humans (McGlynn et al., 2009). A retrospective clinical study also indicates that cellular senescence in kidney tubular epithelia of IgA nephropathy (IgAN) patients corelates with its progression (Liu et al., 2012). P16^INK4a positive cells can be found in the glomeruli and tubulointerstitium of renal biopsies from patients with minimal change disease (MCD), membranous nephropathy (MN), focal and segmental glomerulosclerosis (FSGS) (Sturmlechner et al., 2017). In type 2 DN, senescent cells are found in the glomerulus and renal tubules, which positively correlates with the severity of DN (Verzola et al., 2008).

Cell cycle arrest/senescence plays a critical role in mediating AKI-CKD progression (Clements et al., 2013). Using multiple animal models, investigators show that G2/M arrest in proximal tubular cells produces profibrotic factors through JNK pathway (Yang et al., 2010). Genetic ablation of p16^INK4a improves kidney regeneration and reduces capillary rarefaction after ischemic injury in p16^INK4a null mice (Lee et al., 2012). Furthermore, using transgene INK-ATTAC technology to selectively remove p16^INK4a-positive senescent cells upon drug treatment attenuates age-related phenotypes and extends healthy lifespan (Baker et al., 2011; Baker et al., 2016). Epithelial cell-specific deletion of Myd88, an adapter protein in NF-κB signaling, reduces expression of proinflammatory cytokines such as IL-1α, IL-1β, IL-6, TNF-α, and MCP-1 after folic acid injury and attenuates renal fibrosis. This suggests that innate immune signaling also participates in cellular senescence after kidney injury (Jin et al., 2019).

Given the importance of cellular senescence in determining organismal longevity, it is not surprising that this program is delicately controlled by both positive and negative regulators. There are several naturally occurring, endogenous anti-aging factors that negatively regulate cellular senescence and aging. Among them, Klotho is the most well-studied anti-aging protein. It is a transmembrane glycosidase with extracellular KL1and KL2 domains. Klotho is mainly expressed in kidney, brain and parathyroid glands (Ide et al., 2016). Although all tubular segments in the kidney express Klotho, its level in the distal convoluted tubules is the highest. Global or kidney-specific deletion of Klotho in mice exhibits a premature aging phenotype with short lifespan, growth retardation, decreased physical activity, ectopic calcification, atrophy of skin and internal organs (Kuro-o et al., 1997; Lindberg et al., 2014). In contrast, transgenic mice with overexpression of Klotho resist to pathogenesis and have longer lifespan than wild type counterparts (Kurosu et al., 2005).

The full-length, membranous form of Klotho (mKlotho) acts as a co-receptor for the fibroblast growth factor 23 (FGF23). Klotho and FGF receptor (FGFR) work together to mediate FGF23 action and play a crucial role in maintaining phosphate homeostasis. Besides mKlotho, there is a secreted, soluble form of Klotho (sKlotho), which is generated from either alternative mRNA splicing or proteolytic shedding of the extracellular domain of mKlotho. The sKlotho can be excreted into blood circulation, body fluid and urine, and therefore can act on distant organs as a hormone.

Loss of Klotho is a common finding after kidney injury, which plays a pathogenic role in the onset and progression of kidney disorders. In virtually all kidney disease models including UUO, DN, adriamycin nephropathy, remnant kidney after 5/6 nephrectomy (5/6NX), ischemia/reperfusion injury (IRI) and cisplatin nephropathy, the expression of Klotho is markedly decreased in serum and the kidneys (Zhou et al., 2013; Zhou et al., 2015b). In aging and age-related diseases such as CKD, diabetes, atherosclerosis, Alzheimer’s disease and cancer, Klotho is significantly lower than normal and healthy subjects (Koh et al., 2001; Bian et al., 2015). A clinical study on health aging and body composition (Health ABC) also reveals that higher sKlotho level in serum independently associates with a lower risk of decline in kidney function (Drew et al., 2017).

Klotho is known to promote potassium secretion into urine by upregulating the renal outer medullary potassium channel 1 (ROMK1) on the plasma membrane. It also regulates the transient receptor potential cation channel, subfamily V, member 5 (TRPV5), a calcium channel in renal tubular cells, causing Ca²⁺ reabsorption. Klotho inhibits insulin/insulin-like growth factor 1 (IGF-1) activity, which is related to its ability to decrease oxidative stress and prolong lifespan.

Mounting evidence suggests a special role of Klotho in preventing kidney fibrosis by targeting multiple key profibrotic signaling. Earlier studies show that Klotho directly binds to the type II TGF-β receptor (TβR2), which prevents its engagement with TGF-β1, thereby blocking TGF-β1 signaling (Doi et al., 2011). Studies from our laboratory show that Klotho intercepts Wnt/β-catenin signaling by binding to various Wnt ligands, thereby inhibiting Wnt signal transduction (Zhou et al., 2013). As Wnt/β-catenin plays a fundamental role in kidney fibrogenesis and cellular senescence (Zuo and Liu, 2018), inhibition of its signaling by Klotho results in alleviation of fibrotic lesions in various models of CKD (Zhou et al., 2013; Zhou et al., 2015b). Furthermore, Klotho has been shown to suppress renal tubulo-interstitial fibrosis by constraining FGF-2 signaling (Guan et al., 2014). These findings establish that Klotho explicitly targets TGF-β, Wnt and FGF-2, the three important signal pathways implicated in kidney fibrosis.

As an intrinsic anti-aging protein, Klotho may have a unique role in halting cellular senescence. The connection between loss of Klotho and increased cellular senescence is validated in many studies. Klotho reduces endothelial cell senescence induced by uremia through blocking NF-κB signaling (Zhao et al., 2011; Buendia et al., 2015). As Wnt/β-catenin signaling promotes kidney tubular epithelial cell senescence (Luo et al., 2018), Klotho can play an anti-aging effect by downregulating Wnt signaling. Along this line, loss of Klotho in CKD de-represses Wnt signaling, leading to an exaggerated tubular cell senescence, kidney fibrosis and dysfunction. A recent study indicates that high phosphate induces kidney tubular epithelial cell senescence, which is blocked by Klotho (Maique et al., 2020). Furthermore, hyperphosphatemia induced by Klotho depletion in CKD is a trigger to renal osteopathy and vascular calcification, which may indirectly accelerate aging and increase mortality (Kuro-o, 2011).

Sirtuins are an evolutionarily conserved family of NAD⁺-dependent deacetylases and regulate the stress response programs by modulating many crucial regulators, such as NF-κB, p53, hypoxia-inducible factor 1α (HIF-1α), FOXOs, PGC-1α (Hekmatimoghaddam et al., 2017; Ryu et al., 2019). There are seven sirtuins (SIRT1-7) localizing in different tissues and subcellular compartments. Through controlling key transcription factors via epigenetic regulation, they play a critical role in the development and progression of a wide variety of diseases, such as premature aging, CKD, metabolic syndrome and cancer (Huang et al., 2017).

SIRT1 is the prototypic member of the sirtuins family and one of the most well-characterized sirtuins. In the kidney, SIRT1 is mainly expressed in tubular epithelial cells and glomerular podocytes. SIRT1 is renal protective and prevents renal lesions after various injuries. The beneficial effects of SIRT1 are mediated by its ability to inhibit renal fibrosis, reduce tubular and glomerular cell apoptosis and inflammation, promote autophagy and control blood pressure (Kitada et al., 2013). In AKI and DN models, SIRT1 stabilizes mitochondrial function, activates PGC-1α/PPARγ and FOXO3/FOXO4 signaling in tubular cells, and reduces podocytes damages through acetylating of the p65 NF-κB and STAT3 (Yacoub et al., 2014). In CKD induced by UUO, SIRT1-deficient mice exhibit more severe renal fibrosis and apoptosis compared with wild-type controls (He et al., 2010). In a study with podocyte-specific SIRT1 knockdown mice, aged mice display more accumulation of senescent cells and severer sclerosis in glomerulus compared to controls, and this effect is mediated by an increased acetylation of FOXO3, FOXO4, NF-κB and PGC-1α (Chuang et al., 2017). Therefore, loss or dysregulation of SIRT1, an intrinsic inhibitor of aging, promotes cellular senescence and CKD progression.

As senescence plays a crucial role in aging and disease, senescent cells have emerged as promising new target for therapeutic intervention known as senotherapy. The senotherapy includes depleting senescent cells (senolytic), suppressing SASP (senostatic) and boost/restoration of senescence inhibitors (Figure 4). The goal of senotherapy is to develop agents or strategies to prevent, alleviate, or reverse age-related diseases.

The senolytic approach to remove senescent cells for preventing aging and age-related disease is initially conceptualized based on the studies on transgenic mice. To deplete senescent cells selectively, the promoter of the p16^INK4a gene is used to drive the expression of a suicide gene which can be activated by administration of a drug or dietary compound. Using this approach, a landmark paper demonstrated successful elimination of senescent cells in vivo as shown by SA-β-Gal staining (Baker et al., 2016). The kidney of mice with senescent cell depletion show significant protection with less glomerulosclerosis and lower blood urea levels. A similar approach using the P16:3MR mouse model, where p16^INK4a promoter drives the expression of a viral thymidine kinase, conferring selective sensitivity to the effects of the anti-viral drug ganciclovir (Baar et al., 2017). Mice with senescent cell depletion had lower levels of serum creatinine and blood urea nitrogen. These studies provide a proof-of-principle for the feasibility of senescent cell depletion in preventing age-related disease (Baker et al., 2011).

To translate these studies on mouse genetic models to clinical setting, pharmacological approaches capable of targeting senescent cells in vivo are needed. Senescent cells typically resist to apoptosis and enter the permanent cell cycle arrest by upregulating pro-survival proteins such as Bcl-2, Bcl-xL and Bcl-w. ABT-263, also known as navitoclax, and related compounds ABT-737 and ABT-199 are Bcl-2/xL/w inhibitors, which act as BH3 mimetics and occupy the BH3-binding groove of these pro-survival proteins, thereby preventing their binding to pro-apoptotic proteins BAX and BAK (Knoppert et al., 2019). This enables BAX and BAK to release and form oligomers and initiate the caspase cascade leading to apoptosis. Studies show that BH3 mimetics leads senescent cell to apoptosis, promotes tissue repair and rejuvenates aged stem cells. This observation is confirmed in two mouse models including ionizing radiation-induced lung damage and senescence and a transgenic model of senescence in the skin (Yosef et al., 2016). Whether this approach is efficacious in CKD models remains to be tested. One concern is the specificity of this approach. Whether the BH3 mimetics merely work in the senescent cells, but not in other non-senescent cells, remains to be carefully validated.

The p53 is an arbitrator dictating cell apoptosis or senescence, and its activity is subjected to regulation by FOXO4 protein. By binding to p53, FOXO4 prevents senescent cells from apoptosis. FOXO4-DRI (FOXO4-D-Retro-Inverso) is an interfering peptide that competes with endogenous FOXO4 for p53 binding, thereby enabling senescent cells to undergo p53-mediated apoptosis (Baar et al., 2017). Treatment with FOXO4-DRI reduces senescent cells in the kidney as assessed by SA-β-Gal staining and loss of LMNB1 in a genetic model of fast-aging mice or CKD model induced by adriamycin. FOXO4-DRI also preserves kidney function and inhibits IL-6 expression (Baar et al., 2017). These studies indicate that an increase of p53 by disrupting its binding to FOXO4 results in selective clearance of senescent cells in diseased kidneys, leading to renal protection. It remains to be seen whether this senolytic approach also works in other age-related disease models.

Senescent cells drive tissue fibrosis and CKD progression mainly by secreting proinflammatory and profibrotic SASP components. As such, a potential strategy for senotherapy is to modulate or impede senescent cells to produce and release deleterious SASP, instead of killing senescent cells themselves. This kind of “senostatic” approach may be less specific, in view of that the SASP is context-dependent and dynamic. However, one advantage is the fact that there are many drugs or strategies currently available to modulate SASP production and secretion. Therefore, repurposing these existing drugs as senostatics would greatly speed up their clinical translation.