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

Abstract

Rationale: Hypoxia promotes renal damage and progression of chronic kidney disease (CKD). The erythrocyte is the only cell type for oxygen (O 2 ) 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 O 2 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- 13 C 6 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]-O 2 –binding affinity) production and thus more O 2 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 O 2 delivery via a PP2A-AMPK1α signaling pathway. These findings reveal the metabolic and molecular basis of erythrocyte S1P in CKD and new therapeutic avenues.

COVID-19 infection alters kynurenine and fatty acid metabolism, correlating with IL-6 levels and renal status

“…In addition, all patients with COVID-19 in this study exhibited some degree of renal dysfunction, which may affect serum metabolism by compromising metabolite filtration capacity. Nonetheless, it is worth noting that the present study did not identify an effect of the inflammatory state (e.g., IL-6 levels) on the characteristic increases in purine breakdown and deamination/oxidation products seen in the context of chronic kidney disease (70) or acute kidney ischemia (73,76). However, accumulation of metabolites in this pathway did correlate with BUN and creatinine, consistent with a role of renal dysfunction in regulating circulating levels of oxidized purines.…”

“…Interestingly, increased serum sphingosine 1-phosphate levels were observed in patients with COVID-19. Circulating levels of this metabolite are significantly influenced by red blood cell (RBC) sphingosine kinase 1 activity in response to hypoxia (69), whereas alterations to this pathway mediate responses to angiotensin II-induced stimulation in the hypoxic kidney in chronic kidney disease (70); the latter may be relevant in light of renal dysfunction in some patients with COVID-19 in our study. In contrast, one would have predicted significantly increased serum levels of lactate and carboxylic acid markers of hypoxia (e.g., succinate) in patients with COVID-19, similar to that seen in patients following ischemic (71) or hemorrhagic shock (72).…”

Blood donor exposome and impact of common drugs on red blood cell metabolism

“…We have previously shown that RBC S1P promotes glycolysis by stabilizing the tense deoxygenated state of hemoglobin, which in turn outcompetes glycolytic enzymes that are otherwise bound to and inhibited by the N-terminus of band 3 (model summarized in Figure 4.D ). 35,44,45 Consistently, RBCs from the ranitidine positive unit were characterzied by end of storage lactate levels in this subject that ranked 2 nd out of all 599 samples tested in this study( Figure 4.E ). Given the structural similarity between ranitidine and S-Adenosyl-Homocysteine (SAH – Figure 4.F ), we hypothesized and observed an association between the detection of ranitidine and the levels of SAH and related metabolites (SAM and SAM/SAH ratios – Figure 4.F ), metabolites critical in oxidant stress-induced isoaspartyl protein damage-repair in the stored erythrocyte 5 .…”

“…lactic acid (marker of glycolysis – the main energy pathway in RBCs), acyl-carnitines (markers of membrane lipid homeostasis 42 ), amino acids (marker of ion and protein homeostasis 10 ), carboxylic acids (2-oxoglutarate and succinate – markers of altered homeostasis of reducing equivalents 43 ) and sphingosine 1-phosphate (S1P), a major regulator of RBC glycolysis and function (i.e., oxygen off-loading) under physiological and pathological conditions. 35,44,45 Of note, a subset of drugs were found to significantly decrease (e.g., melatonin, carbazochrome, 4-aminosalicylic acid – PAS, methazolamide) or increase (ranitidine, tiopronin and ketorolac) RBC S1P levels by over 4 standard deviations from the mean (Z-score normalized values) ( Figure 3.F ). Interestingly, melatonin has been reported to inhibit sphingosine kinase 1 (Sphk1) 46 – the rate-limiting enzyme of S1P synthesis in RBCs.…”

COVID-19 infection results in alterations of the kynurenine pathway and fatty acid metabolism that correlate with IL-6 levels and renal status

“…Interestingly, increased serum sphingosine 1-phosphate levels were observed in COVID-19 patients. Circulating levels of this metabolite are significantly influenced by red blood cell (RBC) Sphingosine Kinase 1 activity in response to hypoxia (66), whereas alterations to this pathway mediate responses to angiotensin II-induced stimulation in the hypoxic kidney in chronic kidney disease (67); the latter may be relevant in light of renal dysfunction in some of the COVID-19 patients in the current study. In contrast, one would have predicted significant increases in serum levels of lactate and carboxylic acid markers of hypoxia (e.g., succinate) in COVID-19 patients, similar to what is seen in hypoxemic patients following ischemic (68) or hemorrhagic shock (69).…”