Targeting sphingosine 1 phosphate receptor type 1 receptors in acute kidney injury

Neutral ceramidase deficiency protects against cisplatin-induced acute kidney injury

Cisplatin is a commonly used chemotherapeutic for the treatment of many solid organ cancers; however, its effectiveness is limited by the development of acute kidney injury (AKI) in 30% of patients. AKI is driven by proximal tubule cell death, leading to rapid decline in renal function. It has previously been shown that sphingolipid metabolism plays a role in regulating many of the biological processes involved in cisplatin-induced AKI. For example, neutral ceramidase (nCDase) is an enzyme responsible for converting ceramide into sphingosine, which is then phosphorylated to become sphingosine-1-phosphate, and our lab previously demonstrated that nCDase knockout (nCDase-/-) in mouse embryonic fibroblasts led to resistance to nutrient and energy deprivation-induced cell death via upregulation of autophagic flux. In this study, we further characterized the role of nCDase in AKI by demonstrating that nCDase-/- mice are resistant to cisplatin-induced AKI. nCDase-/- mice display improved kidney function, reduced injury and structural damage, lower rates of apoptosis, and less ER stress compared to wild-type mice following cisplatin treatment. Although the mechanism of protection is still unknown, we propose that it could be mediated by increased autophagy, as chloroquine treatment resensitized nCDase-/- mice to AKI development. Taken together, we conclude that nCDase may represent a novel target to prevent cisplatin-induced nephrotoxicity.

The role of sphingolipids in acute kidney injury

Acute kidney injury (AKI) is most simply defined as the rapid loss of kidney function in a matter of hours to days. AKI can manifest in a number of ways including pre-renal, post-renal, or intrinsic AKI. During acute kidney injury, multiple pathogenic processes are activated including inflammation, cell death, and the generation of reactive oxygen species, just to name a few. Sphingolipids are known to play a role in a number of the pathogenic pathways involved in the pathogenesis of many types of AKI, which suggests a role for sphingolipids in AKI. This short review will discuss the evidence for a role for sphingolipids in AKI.

Cellular and Molecular Mechanisms of AKI

In this article, we review the current evidence for the cellular and molecular mechanisms of AKI, focusing on epithelial cell pathobiology and related cell-cell interactions, using ischemic AKI as a model. Highlighted are the clinical relevance of cellular and molecular targets that have been investigated in experimental models of ischemic AKI and how such models might be improved to optimize translation into successful clinical trials. In particular, development of more context-specific animal models with greater relevance to human AKI is urgently needed. Comorbidities that could alter patient susceptibility to AKI, such as underlying diabetes, aging, obesity, cancer, and CKD, should also be considered in developing these models. Finally, harmonization between academia and industry for more clinically relevant preclinical testing of potential therapeutic targets and better translational clinical trial design is also needed to achieve the goal of developing effective interventions for AKI.

Sphingosine-1-phosphate acts as a key molecule in the direct mediation of renal fibrosis

The major sphingolipid metabolite, sphingosine‐1‐phosphate (S1P), has important biological functions. S1P serves as a ligand for a family of five G‐protein‐coupled receptors with distinct signaling pathways regulating important biological pathways. S1P induces renal fibrosis through an inflammatory pathway. However, its direct fibrosis‐inducing effect on the kidney has not been shown. The role of S1P as a direct mediator of renal fibrosis was investigated in normal rat kidney interstitial fibroblast (NRK‐49F) cells (in vitro) and kidneys of a unilateral ureteral obstruction (UUO) mouse model (in vivo). To clarify the role of S1P in renal fibrosis, we adopted nude UUO mice with immune response deficits. NRK‐49F cells were stimulated with various concentrations of exogenous S1P and FTY720 (a S1P receptor agonist) or N,N‐dimethylsphingosine (DMS; a sphingosine kinase inhibitor). C57BL6 and nude UUO mice were pretreated with FTY720, DMS, or saline. Expression levels of alpha‐smooth muscle actin (a‐SMA), E‐cadherin, collagen type 1 (COL1), collagen type 4 (COL4), tissue inhibitor of matrix metalloproteinase‐1 (TIMP1), and plasminogen activator inhibitor‐1 (PAI1) were examined. S1P stimulated fibrosis in NRK‐49F cells and UUO mice. Increased a‐SMA, COL1, COL4, TIMP1, and PAI1 and decreased E‐cadherin expression levels were observed in both the S1P‐stimulated cells and UUO mice. Nude UUO mouse kidneys expressed fibrotic markers. Fibrotic changes were successfully induced in both UUO and nude UUO mice, evident through prominent fibronectin and COL1 staining. These S1P‐induced fibrotic changes were suppressed by FTY720 and DMS both in vitro and in vivo. Thus, S1P essentially and directly mediates renal fibrosis.

Sphingosine‐1‐phosphate (S1P) stimulated fibrosis both in vitro and in vivo. Fibrotic changes were successfully induced in both unilateral ureteral obstruction (UUO) and nude mice with UUO. These results suggest that S1P is a pivotal fibrotic mediator in renal fibrosis that acts, in part, through direct fibrotic effects.

Lysophosphatidic acid increases proximal tubule cell secretion of profibrotic cytokines PDGF-B and CTGF through LPA2- and Gαq-mediated Rho and αvβ6 integrin-dependent activation of TGF-β

After ischemia-reperfusion injury (IRI), kidney tubules show activated transforming growth factor β (TGF-β) signaling and increased expression of profibrotic peptides, platelet-derived growth factor-B (PDGF-B) and connective tissue growth factor (CTGF). If tubule repair after IRI is incomplete, sustained paracrine activity of these peptides can activate interstitial fibroblast progenitors and cause fibrosis. We show that lysophosphatidic acid (LPA), a ubiquitous phospholipid that is increased at sites of injury and inflammation, signals through LPA2 receptors and Gαq proteins of cultured proximal tubule cells to transactivate latent TGF-β in a Rho/Rho-kinase and αvβ6 integrin-dependent manner. Active TGF-β peptide then initiates signaling to increase the production and secretion of PDGF-B and CTGF. In a rat model of IRI, increased TGF-β signaling that was initiated early during reperfusion did not subside during recovery, but progressively increased, causing tubulointerstitial fibrosis. This was accompanied by correspondingly increased LPA2 and β6 integrin proteins and elevated tubule expression of TGF-β1, together with PDGF-B and CTGF. Treatment with a pharmacological TGF-β type I receptor antagonist suppressed TGF-β signaling, decreased the expression of β6 integrin, PDGF-B, and CTGF, and ameliorated fibrosis. We suggest that LPA-initiated autocrine signaling is a potentially important mechanism that gives rise to paracrine profibrotic signaling in injured kidney tubule cells.