Sphingosine kinase and sphingosine-1-phosphate regulate epithelial cell architecture by the modulation of de novo sphingolipid synthesis

The functional antagonist of sphingosine-1-phosphate, FTY720, impairs gut barrier function

FTY720 or fingolimod is a known functional antagonist of sphingosine-1-phosphate (S1P), and it is effective in treating multiple sclerosis and preventing inflammatory bowel disease (IBD). Evidence shows that its use in mice can increase the susceptibility to mucosal infections. Despite the significant contribution of S1P to barrier function, the effect of the administration of FTY720 on the mucosal barrier has never been investigated. In this study, we looked into how FTY720 therapy affected the function of the gut barrier susceptibility. Administration of FTY720 to C57BL/6 mice enhances the claudin-2 expression and reduces the expression of claudin-4 and occludin, as studied by qPCR, Western blot, and immunofluorescence. FTY720 inhibits the Akt-mTOR pathway to decrease occludin and claudin-4 expression and increase claudin-2 expression. FTY720 treatment induced increased colonic inflammation, with notably greater immune cell infiltration, colon histopathology, and increased production of TNF-α, IFN-γ, CXCL-1, and CXCL-2 than that in control mice. Taking into account the close association of "the leaky gut" and gut dysbiosis among the major diseases, we therefore can infer that the vigilance of gut pathology should be maintained, where FTY720 is used as a treatment option.

Sphingosine-1-phosphate receptor 2 plays a dual role depending on the stage of cell differentiation in renal epithelial cells

Epithelial renal cells have the ability to adopt different cellular phenotypes through epithelial-mesenchymal transition (EMT) and mesenchymal-epithelial transition (MET). These processes are increasingly recognized as important repair factors following acute renal tubular injury. Sphingosine-1-phosphate (S1P) is a bioactive sphingolipid with impact on proliferation, growth, migration, and differentiation which has significant implication in various diseases including cancer and kidney fibrosis. Here we demonstrated that S1P can exert by activating S1P receptor 2 (S1PR2) different functions depending on the stage of cell differentiation. We observed that the differences in the migratory profile of Madin-Darby canine kidney (MDCK) cells depend both on their stage of cell differentiation and the activity of S1PR2, a receptor that can either promote or inhibit the migratory process. Meanwhile in non-differentiated cells S1PR2 activation avoids migration, it is essential on fully differentiated cells. This is the first time that an antagonist effect of S1PR2 was reported for the same cell type. Moreover, in fully differentiated cells, S1PR2 activation is crucial for the progression of EMT - characterized by adherent junctions disassembly, β-catenin and SNAI2 nuclear translocation and vimentin expression- and depends on ERK 1/2 activation and nuclear translocation. These findings provide a new perspective about the different S1PR2 functions depending on the stage of cell differentiation that can be critical to the modulation of renal epithelial cell plasticity, potentially paving the way for innovative research with pathophysiologic relevance.

A Rheostat of Ceramide and Sphingosine-1-Phosphate as a Determinant of Oxidative Stress-Mediated Kidney Injury

Reactive oxygen species (ROS) modulate sphingolipid metabolism, including enzymes that generate ceramide and sphingosine-1-phosphate (S1P), and a ROS-antioxidant rheostat determines the metabolism of ceramide-S1P. ROS induce ceramide production by activating ceramide-producing enzymes, leading to apoptosis, while they inhibit S1P production, which promotes survival by suppressing sphingosine kinases (SphKs). A ceramide-S1P rheostat regulates ROS-induced mitochondrial dysfunction, apoptotic/anti-apoptotic Bcl-2 family proteins and signaling pathways, leading to apoptosis, survival, cell proliferation, inflammation and fibrosis in the kidney. Ceramide inhibits the mitochondrial respiration chain and induces ceramide channel formation and the closure of voltage-dependent anion channels, leading to mitochondrial dysfunction, altered Bcl-2 family protein expression, ROS generation and disturbed calcium homeostasis. This activates ceramide-induced signaling pathways, leading to apoptosis. These events are mitigated by S1P/S1P receptors (S1PRs) that restore mitochondrial function and activate signaling pathways. SphK1 promotes survival and cell proliferation and inhibits inflammation, while SphK2 has the opposite effect. However, both SphK1 and SphK2 promote fibrosis. Thus, a ceramide-SphKs/S1P rheostat modulates oxidant-induced kidney injury by affecting mitochondrial function, ROS production, Bcl-2 family proteins, calcium homeostasis and their downstream signaling pathways. This review will summarize the current evidence for a role of interaction between ROS-antioxidants and ceramide-SphKs/S1P and of a ceramide-SphKs/S1P rheostat in the regulation of oxidative stress-mediated kidney diseases.

Apoptotic cell extrusion depends on single-cell synthesis of sphingosine-1-phosphate by sphingosine kinase 2

Collecting duct cells are physiologically subject to the hypertonic environment of the kidney. This condition is necessary for kidney maturation and function but represents a stress condition that requires active strategies to ensure epithelial integrity. Madin-Darby Canine Kidney (MDCK) cells develop the differentiated phenotype of collecting duct cells when subject to hypertonicity, serving as a model to study epithelial preservation and homeostasis in this particular environment. The integrity of epithelia is essential to achieve the required functional barrier. One of the mechanisms that ensure integrity is cell extrusion, a process initiated by sphingosine-1-phosphate (S1P) to remove dying or surplus cells while maintaining the epithelium barrier. Both types start with the activation of S1P receptor type 2, located in neighboring cells. In this work, we studied the effect of cell differentiation induced by hypertonicity on cell extrusion in MDCK cells, and we provide new insights into the associated molecular mechanism. We found that the different stages of differentiation influence the rate of apoptotic cell extrusion. Besides, we used a novel methodology to demonstrate that S1P increase in extruding cells of differentiated monolayers. These results show for first time that cell extrusion is triggered by the single-cell synthesis of S1P by sphingosine kinase 2 (SphK2), but not SphK1, of the extruding cell itself. Moreover, the inhibition or knockdown of SphK2 prevents cell extrusion and cell-cell junction protein degradation, but not apoptotic nuclear fragmentation. Thus, we propose SphK2 as the biochemical key to ensure the preservation of the epithelial barrier under hypertonic stress.

Bioavailable wine pomace attenuates oxalate-induced type II epithelial mesenchymal transition and preserve the differentiated phenotype of renal MDCK cells

The functional renal epithelium is composed of differentiated and polarized tubular cells with a strong actin cortex and specialized cell-cell junctions. If, under pathological conditions, these cells have to resist higher kidney osmolarity, they need to activate diverse mechanisms to survive external nephrotoxic agents such as inflammation and oxidative stress. Wine pomace polyphenols exert protective effects on renal cells. In this study, two wine-pomace products and their protective effects upon promotion and preservation of normal cell differentiation and attenuation of oxalate-induced type II epithelial mesenchymal transition (EMT) are evaluated. Treatment with gastrointestinal and colonic bioavailable fractions from red (rWPP) and white (wWPP) wine pomaces, both in the presence and the absence of oxalate, showed similar cell numbers and nuclear size than the non-treated differentiated MDCK cells. Immunofluorescence analysis showed the reduction of morphological changes and the preservation of cellular junctions for the rWPP and wWPP pre-treatment of cells exposed to oxalate injury. Hence, both rWPP and wWPP attenuated oxalate type II EMT in MDCK cells that conserved their epithelial morphology and cellular junctions through the antioxidant activities of grape pomace polyphenols.

Erythrocyte Metabolic Reprogramming by Sphingosine 1-Phosphate in Chronic Kidney Disease and Therapies

RATIONALE

Hypoxia promotes renal damage and progression of chronic kidney disease (CKD). The erythrocyte is the only cell type for oxygen (O₂) delivery. Sphingosine 1-phosphate (S1P)-a highly enriched biolipid in erythrocytes-is recently reported to be induced under high altitude in normal humans to enhance O₂ delivery. However, nothing is known about erythrocyte S1P in CKD.

OBJECTIVE

To investigate the function and metabolic basis of erythrocyte S1P in CKD with a goal to explore potential therapeutics.

METHODS AND RESULTS

Using erythrocyte-specific SphK1 (sphingosine kinase 1; the only enzyme to produce S1P in erythrocytes) knockout mice (eSphK1^-/-) in an experimental model of hypertensive CKD with Ang II (angiotensin II) infusion, we found severe renal hypoxia, hypertension, proteinuria, and fibrosis in Ang II-infused eSphk1^-/- mice compared with controls. Untargeted metabolomics profiling and in vivo U-¹³C₆ isotopically labeled glucose flux analysis revealed that SphK1 is required for channeling glucose metabolism toward glycolysis versus pentose phosphate pathway, resulting in enhanced erythroid-specific Rapoport-Luebering shunt in Ang II-infused mice. Mechanistically, increased erythrocyte S1P functioning intracellularly activates AMPK (AMP-activated protein kinase) 1α and BPGM (bisphosphoglycerate mutase) by reducing ceramide/S1P ratio and inhibiting PP2A (protein phosphatase 2A), leading to increased 2,3-bisphosphoglycerate (an erythrocyte-specific metabolite negatively regulating Hb [hemoglobin]-O₂-binding affinity) production and thus more O₂ delivery to counteract kidney hypoxia and progression to CKD. Preclinical studies revealed that an AMPK agonist or a PP2A inhibitor rescued the severe CKD phenotype in Ang II-infused eSphK1^-/- mice and prevented development of CKD in the control mice by inducing 2,3-bisphosphoglycerate production and thus enhancing renal oxygenation. Translational research validated mouse findings in erythrocytes of hypertensive CKD patients and cultured human erythrocytes.

CONCLUSIONS

Our study elucidates the beneficial role of eSphk1-S1P in hypertensive CKD by channeling glucose metabolism toward Rapoport-Luebering shunt and inducing 2,3-bisphosphoglycerate production and O₂ delivery via a PP2A-AMPK1α signaling pathway. These findings reveal the metabolic and molecular basis of erythrocyte S1P in CKD and new therapeutic avenues.