Manipulation of the Sphingolipid Rheostat Influences the Mediator of Flow-Induced Dilation in the Human Microvasculature

Background Elevated levels of ceramide, a sphingolipid known to cause a transition from nitric oxide (NO)- to hydrogen peroxide-dependent flow-induced dilation (FID) in human arterioles, correlate with adverse cardiac events. However, elevations of ceramide are associated with changed concentrations of other sphingolipid metabolites. The effects of sphingolipid metabolites generated through manipulation of this lipid pathway on microvascular function are unknown. We examined the hypothesis that inhibition or activation of the ceramide pathway would determine the mediator of FID. Methods and Results Using videomicroscopy, internal diameter changes were measured in human arterioles collected from discarded adipose tissue during surgery. Inhibition of neutral ceramidase, an enzyme responsible for the hydrolysis of ceramide, favored hydrogen peroxide-dependent FID in arterioles from healthy patients. Using adenoviral technology, overexpression of neutral ceramidase in microvessels from diseased patients resulted in restoration of NO-dependent FID. Exogenous sphingosine-1-phosphate, a sphingolipid with opposing effects of ceramide, also restored NO as the mediator of FID in diseased arterioles. Likewise, exogenous adiponectin, a known activator of neutral ceramidase, or, activation of adiponectin receptors, favored NO-dependent dilation in arterioles collected from patients with coronary artery disease. Conclusions Sphingolipid metabolites play a critical role in determining the mediator of FID in human resistance arterioles. Manipulating the sphingolipid balance towards ceramide versus sphingosine-1-phosphate favors microvascular dysfunction versus restoration of NO-mediated FID, respectively. Multiple targets exist within this biolipid pathway to treat microvascular dysfunction and potentially improve patient outcomes.

Sphingosine-1-Phosphate Receptor 1 Regulates Cardiac Function by Modulating Ca2+ Sensitivity and Na+/H+ Exchange and Mediates Protection by Ischemic Preconditioning

Background

Sphingosine‐1‐phosphate plays vital roles in cardiomyocyte physiology, myocardial ischemia–reperfusion injury, and ischemic preconditioning. The function of the cardiomyocyte sphingosine‐1‐phosphate receptor 1 (S1P₁) in vivo is unknown.

Methods and Results

Cardiomyocyte‐restricted deletion of S1P₁ in mice (S1P₁^αMHCCre) resulted in progressive cardiomyopathy, compromised response to dobutamine, and premature death. Isolated cardiomyocytes from S1P₁^αMHCCre mice revealed reduced diastolic and systolic Ca²⁺ concentrations that were secondary to reduced intracellular Na⁺ and caused by suppressed activity of the sarcolemmal Na⁺/H⁺ exchanger NHE‐1 in the absence of S1P₁. This scenario was successfully reproduced in wild‐type cardiomyocytes by pharmacological inhibition of S1P₁ or sphingosine kinases. Furthermore, Sarcomere shortening of S1P₁^αMHCCre cardiomyocytes was intact, but sarcomere relaxation was attenuated and Ca²⁺ sensitivity increased, respectively. This went along with reduced phosphorylation of regulatory myofilament proteins such as myosin light chain 2, myosin‐binding protein C, and troponin I. In addition, S1P₁ mediated the inhibitory effect of exogenous sphingosine‐1‐phosphate on β‐adrenergic–induced cardiomyocyte contractility by inhibiting the adenylate cyclase. Furthermore, ischemic precondtioning was abolished in S1P₁^αMHCCre mice and was accompanied by defective Akt activation during preconditioning.

Conclusions

Tonic S1P₁ signaling by endogenous sphingosine‐1‐phosphate contributes to intracellular Ca²⁺ homeostasis by maintaining basal NHE‐1 activity and controls simultaneously myofibril Ca²⁺ sensitivity through its inhibitory effect on adenylate cyclase. Cardioprotection by ischemic precondtioning depends on intact S1P₁ signaling. These key findings on S1P₁ functions in cardiac physiology may offer novel therapeutic approaches to cardiac diseases.

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.