Podocyte injury due to either drug, toxin, infection, or metabolic abnormality is a great concern as it increases the risk of developing focal segmental glomerulosclerosis (FSGS) and proteinuric kidney diseases. The direct podocyte injury due to doxorubicin is associated with an increase in proinflammatory cytokines and induction of cathepsin L. The increased activity of cathepsin L in turn may degrade the glomerular slit diaphragm resulting in proteinuric kidney injury. The angiotensin-II type 2 receptor (AT2R) has earlier been reported to be associated with the preservation of slit diaphragm proteins and prevention of proteinuria. Recent in vivo findings by Zhang and colleagues further support the anti-proteinuric role of AT2R in preventing podocyte injury via down-regulating cytokines ccl2, and hence, cathepsin L, thereby, limiting the progression of FSGS.
The glomerular filtration barrier (GFB) is composed of fenestrated capillary endothelial cells (GECs) containing glycocalyx (a charged luminal network of proteoglycans, glycoproteins, and glycolipids), the glomerular basement membrane (GBM), and the terminally differentiated epithelial cells (podocytes) connected by interposed slit diaphragms between filamentous actin (F-actin)-rich foot processes . A cytoskeletal integrity of podocytes and slit diaphragms is critically important in providing mechanical stability to the GFB. The foot processes are attached to the GBM via the matrix receptors (dystroglycan and ανβ3 integrin) that in turn regulate the actin–myosin contractile apparatus to maintain the projections (i.e. foot processes) and motility . At the molecular level, the slit diaphragm is composed of protein complexes made of nephrin, synaptopodin, and cadherin, among other cytoskeletal proteins that prevent the passing of macromolecules . The connections of protein complexes of the slit diaphragm with the actin–myosin apparatus via CD2-associated protein (CD2AP) and podocin at focal adhesions are critically important for the formation and maintenance of podocyte processes. For instance, chronic activation of integrins (by their ligands, e.g. fragments of soluble urokinase plasminogen activator surface receptor, fibronectin, and fibrinogen) at the focal adhesions on the podocytes activates focal adhesion kinase (FAK). Activated FAK leads to actin remodeling, formation of stress fibers via RhoA, Rac1, and Cdc42, disrupts actin–myosin apparatus and its connection with CD2AP and other junctional protein, and causes retraction of podocyte foot processes that eventually damages the slit diaphragm . Such structural changes are associated with podocyte dysfunction, deposition of extracellular matrix, capillary dilation due to reduced compressive forces that oppose filtration pressure, breakdown of GFB, detachment, and loss of podocytes in the urine, and persistent albuminuria [1,2]. Over time, the focal segmental solidification of glomerular tuft may occur resulting in occlusion of capillaries and impaired blood flow eventually leading to glomerular obsolescence . This phenomenon is known as focal segmental glomerulosclerosis (FSGS), a common histopathologic lesion. This is also a common cause of primary glomerular disease, which is frequently associated with nephrotic syndrome (Figure 1). If untreated, the proteins that leak into glomerular filtrate will activate the tubular epithelium resulting in loss of tubular function and tubulointerstitial fibrosis . The incident rate of FSGS was around 3.2 cases per 100,000 person-years between 2004 and 2013. The risk is 1.5- to 2-fold higher in men compared with women. The risk is 4-fold higher in Black patients compared with White patients and Asian patients . Among, six distinct etiologies involved in the FSGS, the drug- or toxin-induced FSGS relates to the direct injury to podocyte as a causal mechanism and perhaps involves the soluble factors (e.g. complement proteins, immune complexes, autoantibodies, etc.) as well leading to FSGS; however, the mechanisms are not understood [2,4,5].
The role of AT2R in preventing the progression of albuminuria in mice models of doxorubicin-induced FSGS
Albuminuria is an independent risk factor for kidney disease progression. There is an unmet need for therapeutic targets to treat kidney diseases, such as FSGS [2,5]. The FSGS is a commonly identified histopathologic lesion during podocyte injury. The detailed pathogenesis-based classification, pathological and clinical manifestations, and treatment approaches are well described by Ahn and Bomback . For instance, despite sodium-glucose co-transporter 2 inhibitors showing improvement in hyperglycemia and cardiovascular and renal outcomes, they have had minimal effect on protein leakage . Liao and colleagues earlier have shown that the expression of nephrin and podocyte integrity is dependent on the expression of angiotensin (ang)-II type 2 (AT2R) [6,7]. The findings by Liao et al. [6–8] reaffirm the anti-inflammatory, anti-fibrotic, and anti-albuminuric effects of AT2R [9,10], and strengthen the position of AT2R as a pharmacological target to rescue podocyte loss and limit FSGS independent of ang-II/ang-II type 1 receptor (AT1R) signaling .
Podocytes express the ang-AT1R and AT2R receptors. AT2R is considered a protective arm of the renin–angiotensin–aldosterone system (RAAS) and an AT1R functional antagonist. Hence, considering the potential use of RAAS blockers (e.g. ang-II-converting enzyme inhibitor or antagonists of AT1R or mineralocorticoid receptor) in kidney disease patients, findings of Liao et al.  suggest that the combination of AT2R agonist C21 with RAAS blocker may offer enhanced protection.
Doxorubicin is a mitochondrial toxin and has been widely employed as a nephrotoxic and cardiotoxic agent to model tissue injury in experimental animals . Although, the doxorubicin-induced inflammation and induction of cathepsin L, a lysosomal cysteine proteinase, in the podocytes have been reported , the mechanisms are far from understood. The cathepsin L is involved in the breakdown of CD2-associated protein, synaptopodin, and dynamin contributing to the degradation of the slit diaphragm, GBM, and the actin cytoskeleton. The findings suggest that doxorubicin treatment of mice caused induction of cathepsin L via an increase in inflammatory cytokines, disrupted the slit diaphragm, and resulted in a loss of podocytes, albuminuria, and FSGS. However, the changes in cytokines determined by Liao and colleagues  appear secondary and heterogeneous, i.e. neither primarily involved in the pathogenesis of FSGS nor in AT2R-mediated protection of podocytes and glomerulus. If any cytokine is involved, it may be ccl2 because the expression of ccl2 was increased in FSGS which was further increased upon deletion of AT2R and the treatment with AT2R agonist C21 prevented the increase in ccl2. However, such a notion requires further studies, nonetheless, it is the translated cytokine protein that is important for the function.
The AT2R activation by its selective agonist C21 is associated with the activation of endothelial nitric oxide synthase (eNOS). The findings suggest the central role of healthy mitochondria and the activation of eNOS in AT2R-mediated protection. Podocyte dysfunction and loss in FSGS have been associated with mitochondrial dysfunction, reduced cellular ATP content, and the formation of oxidative stress . Podocytes require a significant amount of ATP to maintain the larger surface area of foot processes . And, AT2R has long been implicated in the inhibition of NADPH oxidase, oxidative stress, preservation of mitochondrial function, and preventing the protein leak. Literature suggests that the loss of eNOS accelerated podocyte injury, albuminuria, and nephropathy in mice .
The G1/G2 coding region on the APOL1 gene is an established risk factor for the development of not only FSGS, but also non-genetic diseases of vascular pathologies such as hypertensive nephrosclerosis because intracellular APOL1 gene product is toxic to mitochondria in kidney cells including podocytes, and even to in GECs and vasculature. However, in healthy individuals carrying G1 or G2 genotypes, secondary insults such as inflammation, consumption of a diet rich in sodium, or infection are required to trigger kidney diseases . Hence, considering the anti-inflammatory effects of AT2R it is reasonable to believe that AT2R activation can reduce inflammation and APOL1 product toxicity, and may hold the therapeutic premise even in genetic forms of FSGS. Moreover, in hypertensive kidney diseases, the enhanced expression of transient receptor potential channel (TRPC)-6 in podocytes and overactivity of TRPC6 in response to AT1R activation, stretch, oxidative stress, the local milieu of ang-II leading to activation of Ca2+/calmodulin/calcineurin pathway, and activation of nuclear factor of activated T cells (NFAT) has been implicated [2,13]. Although, opposite to findings of Liao et al. , considering AT2R to antagonize ang-II/AT1R signaling, the inhibition of ang-II/AT1R/TRPC6/calcineurin/NFAT in AT2R agonist C21-mediated renoprotection should be investigated as findings related to renal content of ang-II and protein levels of TRPC6 or AT1R in the podocytes are not available . Moreover, AT2R is located in close proximity to F-actin in proximal tubules  and has been implicated in the regulation of renal RhoA and Cdc42, and actin polymerization during ischemic insult ; hence, the additional AT2R-mediated renoprotective mechanism(s) in podocyte are probably deserves further investigation.
Interdependence among various cell types exists in the organ system and so in the glomerulus of the kidney. Hence, much remains to be identified about the role of AT2R in preventing the dysfunction of GECs and preserving the glycocalyx in FSGS. For instance, the mitochondrial dysfunction in GECs rather than podocytes could also contribute to GFB loss in FSGS because the mitochondrial dysfunction in GECs is associated with the loss of fenestrations and glycocalyx. Moreover, the role of glomerular filtration rate and increased blood pressure (BP) in the pathogenesis of FSGS as well as the glomerular filtration rate- or BP-independent role of AT2R activation in lessening FSGS is difficult to delineate [10,16,17].
The activation of AT2R has been investigated for its anti-fibrotic benefits in patients suffering from idiopathic pulmonary fibrosis and for its anti-inflammatory benefits in SARS-CoV-2 patients. It is safe and, we believe that AT2R activation should be tested comprehensively in human studies to limit myriad kidney diseases.
The authors declare that there are no competing interests associated with the manuscript.
National Institutes of Health [grant numbers R01 DK117495 and R01 DK0615787].
CRediT Author Contribution
Sanket Patel: Conceptualization, Formal analysis, Writing—original draft, Writing—review & editing. Kalyani Kulkarni: Visualization, Writing—review & editing. Tahir Hussain: Resources, Formal analysis, Writing—review & editing.
ang-II/ang-II type 1 receptor
endothelial nitric oxide synthase
focal adhesion kinase
focal segmental glomerulosclerosis
glomerular basement membrane
glomerular filtration barrier
nuclear factor of activated T cells
transient receptor potential channel-6