Differential Modulation of S1PR(1–5) and Specific Activities of SphK and nSMase in Pulmonary and Cerebral Tissues of Rats Exposed to Hypobaric Hypoxia

Short Periods of Hypoxia Upregulate Sphingosine Kinase 1 and Increase Vasodilation of Arteries to Sphingosine 1-Phosphate (S1P) via S1P3

Sphingosine kinase [(SK), isoforms SK1 and SK2] catalyzes the formation of the bioactive lipid, sphingosine 1-phosphate (S1P). This can be exported from cells and bind to S1P receptors to modulate vascular function. We investigated the effect of short-term hypoxia on SK1 expression and the response of arteries to S1P. SK1 expression in rat aortic and coronary artery endothelial cells was studied using immunofluorescence and confocal microscopy. Responses of rat aortic rings were studied using wire myography and reversible hypoxia induced by bubbling myography chambers with 95% N₂:5% CO₂ Inhibitors were added 30 minutes before induction of hypoxia. S1P induced endothelium-dependent vasodilation via activation of S1P₃ receptors and generation of nitric oxide. Hypoxia significantly increased relaxation to S1P and this was attenuated by (2R)-1-[[(4-[[3-methyl-5-[(phenylsulfonyl)methyl] phenoxy]methyl]phenyl]methyl]-2-pyrrolidinemethanol [(PF-543), SK1 inhibitor] but not (R)-FTY720 methyl ether [(ROMe), SK2 inhibitor]. Hypoxia also increased vessel contractility to the thromboxane mimetic, 9,11-dideoxy-11α,9α-epoxymethanoprostaglandin F2α, which was further increased by PF-543 and ROMe. Hypoxia upregulated SK1 expression in aortic and coronary artery endothelial cells and this was blocked by PF-543 and 2-(p-hydroxyanilino)-4-(p-chlorophenyl)thiazole [(SKi), SK1/2 inhibitor]. The effects of PF-543 and SKi were associated with increased proteasomal/lysosomal degradation of SK1. A short period of hypoxia increases the expression of SK1, which may generate S1P to oppose vessel contraction. Under hypoxic conditions, upregulation of SK1 is likely to lead to increased export of S1P from the cell and vasodilation via activation of endothelial S1P₃ receptors. These data have significance for perfusion of tissue during episodes of ischemia.

Acute Hypobaric Hypoxia-Mediated Biochemical/Metabolic Shuffling and Differential Modulation of S1PR-SphK in Cardiac and Skeletal Muscles

AIM

High altitude exposure alters biochemical, metabolic, and physiological features of heart and skeletal muscles, and hence has pathological consequences in these tissues. Central to these hypoxia-associated biochemical/metabolic shuffling are energy deficit accumulation of free radicals and ensuing oxidative damage in the tissue. Recent preclinical/clinical studies indicate sphingosine-1-phosphate (S1P) axis, comprising S1P G protein coupled receptors (S1PR_1-5) and its synthesizing enzyme-sphingosine kinase (SphK) to have key regulatory roles in homeostatic cardiac and skeletal muscle biology. In view of this, the aim of the present study was to chart the initiation and progression of biochemical/metabolic shuffling and assess the coincident differential modulation of S1PR_(1-5) expression and total SphK activity in cardiac and skeletal muscles from rats exposed to progressive hypobaric hypoxia (HH; 21,000 feet for 12, 24, and 48 hours).

RESULTS

HH-associated responses were evident as raised damage markers in plasma, oxidative stress, decreased total tissue protein, imbalance of intermediate metabolites, and aerobic/anaerobic enzyme activities in cardiac and skeletal muscles (gastrocnemius and soleus) culminating as energy deficit.

CONCLUSION

Cardiac and gastrocnemius muscles were more susceptible to hypoxic environment than soleus muscle. These differential responses were directly and indirectly coincident with temporal expression of S1PR_(1-5) and SphK activity.

S1P promotes inflammation-induced tube formation by HLECs via the S1PR1/NF-κB pathway

Inflammation-induced lymphangiogenesis is a widely accepted concept. However, most of the inflammatory factors and their related mechanisms have not been clarified. It has been reported that sphingosine-1-phosphate (S1P) is not only closely related to the chronic inflammatory process but also affects angiogenesis. Therefore, we investigated the inflammatory effects of S1P on human lymphatic endothelial cells (HLECs). Our results showed that S1P promotes tumor necrosis factor-α (TNF-α) and interleukin-1β (IL-1β) secretion in HLECs. We also confirmed that S1P-stimulated TNF-α and IL-1β secretion is mediated through S1P receptor 1 (S1PR1). Using TNF-α siRNA and IL-1β siRNA, we found that TNF-α and IL-1β play essential roles in S1P-induced HLEC proliferation, migration, and tube formation. S1P induces phosphorylation of NF-κB p65 and activation of NF-κB nuclear translocation. A S1PR1 antagonist (W146) and NF-κB inhibitor (BAY11-7082) inhibited S1P-induced TNF-α and IL-1β secretion and prevented NF-κB nuclear translocation. Taken together, the results demonstrated for the first time that S1P promotes the secretion of TNF-α and IL-1β in HLECs via S1PR1-mediated NF-κB signaling pathways, thus affecting lymphangiogenesis. The study provides a new strategy for finding treatments for lymphangiogenesis-related diseases.