Lyn, a Src kinase family member, is a promising therapeutic target for sepsis-associated acute kidney injury

Sepsis is a life-threatening medical emergency occurring as a dysregulated and overwhelming host response to infection and results in multi-organ dysfunction. Sepsis remains a health and economic concern worldwide despite recent improvements in the standard of care. Furthermore, approximately half of the patients develop sepsis-associated acute kidney injury (SA-AKI), which brings even higher mortality and morbidity (1). Some patients may develop an infection-related glomerulonephritis, which presents clinically with overt urinary abnormalities and pathologically with glomerular endocapillary proliferation and exudative lesions with distinct complement C3 deposition (2). Of course, this disease entity should not be overlooked; however, in most SA-AKI patients, urinary findings are scarce, and acute tubular necrosis is the main pathological finding.

Another problem that has recently been raised is that although AKI was previously considered a completely reversible condition, epidemiological studies have shown that some patients with AKI go on to develop chronic kidney disease (CKD), end-stage kidney disease, or death (3). The phenomenon is referred to as AKI-to-CKD transition and is associated with a further social burden (4).

There has been an urgent need to find promising therapeutic approaches to SA-AKI, but no effective strategy is available for preventing or treating SA-AKI yet. Accumulating evidence has shown that various factors, such as hemodynamic changes, innate and adaptive immune systems, cytokines, reactive oxygen species, and cell apoptosis, are intricately intertwined in the SA-AKI disease process. However, our understanding of its pathogenesis is still limited.

Src family kinases (SFKs) a non-receptor tyrosine kinase that regulates intracellular signal transduction through phosphorylation of specific tyrosine residues in the substrate proteins and contributes to various aspects of cellular function, such as proliferation, migration, invasion, and metabolism. Previous studies have shown that Lyn, a member of SFKs, regulates inflammation and apoptosis processes in various conditions (5). Under the circumstances, Li et al. have recently provided evidence that Lyn alleviated the condition of SA-AKI in a mouse model through inhibiting signaling of signal transducer and activator of transcription 3 (STAT3), which is a cell-signal transcription factor and a substrate for Lyn (6). STAT3 has recently attracted much interest in various research areas, including SA-AKI, because its vital roles in directing immune responses and sustaining inflammatory pathways have been shown.

What did the study show?

The authors demonstrated several important findings using the SA-AKI model induced by the cecal ligation and puncture (CLP) technique in mice (6). First, Lyn expression, but not Src and Fyn, which are two other SFK members, was decreased in the renal tissues of SA-AKI mice both in total and phosphorylated forms. SA-AKI mice with CLP exhibited a significant increase in the levels of serum blood urea nitrogen (BUN) and serum creatinine (sCr), severe injury of renal tubular epithelial cells, prominent renal inflammation that was demonstrated by macrophage infiltration as well as the increased levels of inflammatory cytokines in the renal tissues, and aggravated cell apoptosis that was shown by increased pro-apoptotic Bax levels and decreased anti-apoptotic Bcl-2 levels in the tissues, compared with controls. Compared with Lyn^+/+ wild-type mice with CLP, all these findings in Lyn-deficient mice with SA-AKI were significantly aggravated, which was another novel finding. The CLP procedure increased the expression of p-STAT3 in the renal tissues, which was more pronounced in Lyn^-/- mice, but STAT3 inhibitor administration to these mice ameliorated the above parameters. Furthermore, Lyn knockdown augmented both inflammatory and pro-apoptotic signaling, accompanied by increased STAT3 phosphorylation in LPS-treated mouse proximal tubular epithelial cells, whereas Lyn overexpression suppressed these changes. Finally, administration of a Lyn agonist to Lyn^+/+ mice, but not to Lyn^-/- mice, reversed STAT3 phosphorylation and mitigated renal inflammation and apoptosis, thereby protecting from CLP-induced SA-AKI.

Thus, using both in vivo and in vitro investigations, the authors showed Lyn’s protective roles in the SA-AKI disease process, at least partly through inhibition of STAT3 phosphorylation and resultant mitigation of both inflammation and cell apoptosis.

Why is the present study important?

The protective effects of Lyn in SA-AKI, which Li et al. elegantly demonstrated this time, were supposedly through the inhibition of STAT3 signaling. Besides that, various reports have shown the anti-inflammatory functions of Lyn; it inhibits the activation of the NLR family pyrin domain containing 3 (NLRP3) inflammasome by phosphorylating NLRP3, which promotes its ubiquitination and degradation (5). Lyn also inhibits IL-13-induced endoplasmic reticulum (ER) stress, which is associated with NF-κB activity (5). In line with these findings, the protective roles of Lyn through the downregulation of NF-κB activity and inflammasome signaling in a murine model of endotoxin-induced lung inflammation are reported (7).

Bim protein triggers subsequent Bax-mediated apoptosis, and it is reported that Lyn negatively regulates the mitochondrial apoptotic pathway by phosphorylating Bim and increasing its interaction with anti-apoptotic molecules (8). This function seems highly important; renal tissue consumes a lot of energy and is rich in mitochondria, and when SA-AKI develops, mitochondrial dysfunction/destruction is almost certainly observed. Mitochondrial dysfunction enhances mitochondrial reactive oxygen species production, but Lyn is also shown to reduce oxidative stress (5). ER stress is known to cause cell apoptosis, so inhibition of ER stress can be another candidate for the anti-apoptotic mechanisms of Lyn.

It is reported that Src and Fyn, other SFK members, generally play pathogenic roles in the mechanisms of AKI (5). Lyn and Fyn are shown to exert opposing effects through phosphorylation of the same substrates at different residues (9). Whether or not they play different roles in the same cells/tissues in the process of SA-AKI remains unclear. There has been an increasing number of studies that target SFKs for the treatment of AKI. For example, nintedanib, a well-known multi-kinase inhibitor approved for clinical use, reportedly suppresses renal inflammation and fibrosis (10). Nintedanib inhibits the phosphorylation of Lyn, which could raise doubt about the renoprotective roles of Lyn; however, it also inhibits other SFKs, such as Src. Modulating a single member of the kinase family using specific agonists and/or small molecule inhibitors is therefore required to determine the function of the kinase, as such a strategy has already proven to be available both in in vitro investigations and animal models. Data regarding SFKs other than Lyn, Src, and Fyn is very limited to date, and a broad research area is left in the field of SA-AKI.

Although the authors focused on the macrophage infiltration in the kidney tissues, various types of immune cells supposedly play certain roles in developing SA-AKI. A recent study suggested the critical involvement of innate immune cells (especially CD56⁺ T cells, a functional counterpart of murine natural killer NKT cells) in the disease process of SA-AKI through pro-apoptotic molecules, such as fas ligand (FasL) or perforin (11). The tumor necrosis factor (TNF)/FasL system is shown to be involved in the injury of tubular epithelial cells, and the perforin-mediated pathway is presumably involved in vascular endothelial cell injury. Furthermore, the interaction between macrophages and NKT/NK cells after bacterial infections is suggested (12). The effects of Lyn on these pro-apoptotic molecules and the functions of immune cells other than macrophages are issues for future study.

It should be kept in mind that, as the authors discussed, some studies have shown that STAT3 is not inhibited but rather activated by Lyn, which is a contrary finding to the authors (6). It has also been reported very recently that Lyn expression is increased not only in the diabetic mouse model but also in patients with diabetic kidney disease (13). The pathogenesis of diabetic kidney disease differs considerably from that of SA-AKI, and different experimental models might have affected these discrepant findings. The roles of Lyn in specific cell types, such as immune cells or vascular endothelial cells, during the disease process of SA-AKI should be investigated in future studies using cell-specific Lyn knockdown and/or overexpression techniques, as demonstrated by the authors using mouse proximal tubular epithelial cells (6); Lyn is expressed in diverse cell types, and pan Lyn deletion cannot determine its function in a specific cell type. Finally, the authors investigated the roles of Lyn only in the “pre-treatment” models; administration of STAT3 inhibitor or Lyn agonist was performed only before the SA-AKI model was established. Although the critical roles of Lyn in the prevention of SA-AKI were elegantly demonstrated in this study, its potential as a therapeutic option for already existing SA-AKI should be further investigated in the future.

Despite these limitations, Li et al. demonstrated new possibilities that Lyn is a therapeutic target for SA-AKI, which remains a substantial worldwide burden. Putative protective mechanisms that Lyn exerts in the pathogenesis of SA-SKI are summarized in Figure 1. Knowledge regarding SFKs, including Lyn, has been accumulating, and future studies will further unravel their roles in the development of SA-AKI, for which no effective approaches have been established yet.

Figure 1 Putative roles of Lyn in the pathogenesis of sepsis-associated acute kidney injury [based on the study from Li et al. (6)]. SA-AKI, sepsis-associated acute kidney injury; STAT3, signal transducer and activator of transcription 3.

Acknowledgments

We thank Editage (www.editage.jp) for English editing of the manuscript.

Footnote

Provenance and Peer Review: This article was commissioned by the editorial office, Annals of Translational Medicine. The article has undergone external peer review.

Peer Review File: Available at https://atm.amegroups.com/article/view/10.21037/atm-23-1932/prf

Funding: None.

Conflicts of Interest: Both authors have completed the ICMJE uniform disclosure form (available at https://atm.amegroups.com/article/view/10.21037/atm-23-1932/coif). The authors have no conflicts of interest to declare.

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