Sphingosine-1-phosphate inhibits high glucose-mediated ERK1/2 action in endothelium through induction of MAP kinase phosphatase-3

Cynarin attenuates LPS-induced endothelial inflammation via upregulation of the negative regulator MKP-3

Clinical observations have revealed that non-resolving low-grade inflammation is linked to the pathogenesis of chronic inflammatory diseases, for example arthritis, atherosclerosis, Alzheimer’s disease, diabetes, and chronic kidney disease. Interestingly, low levels of circulating lipopolysaccharides (LPS) derived from the outer membrane of gram-negative bacteria appear to be one of the primary causes of persistent low-grade inflammation. The inner surface of the blood vessels is lined with endothelial cells; therefore, even low levels of circulating LPS can directly activate these cells and elicit specific cellular responses, such as an increase in the expression levels of cell adhesion molecules and proinflammatory mediators. In endothelial cells, LPS exposure results in an inflammatory response through activation of nuclear factor-kappa B (NF-κB) and mitogen-activated protein kinases. Cynarin, a phytochemical found in artichokes, has several pharmacological properties against endothelial inflammation. In the present study, we discovered that cynarin suppressed the LPS-induced increase in the expression levels of vascular cell adhesion molecule-1 and proinflammatory mediators such as monocyte chemoattractant protein-1 (MCP-1), tumor necrosis factor-α (TNF-α), and interleukin-1β in EA.hy926 cells. Further, cynarin inhibited the activation of p38 and NF-κB pathways by inducing the negative regulator mitogen-activated protein kinase phosphatase 3 (MKP-3) in LPS-stimulated EA.hy926 cells. In conclusion, cynarin alleviates inflammation by upregulating MKP-3, a negative regulator of p38 and NF-κB, and it may be a therapeutic option for treating endothelial inflammation-related diseases.

MKP-3 suppresses LPS-induced inflammatory responses in HUVECs via inhibition of p38 MAPK/NF-κB pathway

Endothelial cell dysfunction and inflammatory responses play critical roles in the development of atherosclerosis. Recent data on the processes underlying atherogenesis indicate the substantial role of endotoxins (lipopolysaccharides; LPS) of the intestinal microflora in the initiation and progression of atherosclerosis. Mitogen-activated protein (MAP) kinase phosphatase-3 (MKP-3) is a cytoplasmic dual-specificity protein phosphatase that specifically binds to and inactivates MAP kinases in mammalian cells, but its biological function in endothelial cell dysfunction and inflammatory responses remains largely unknown. The aim of the present study was to investigate the role of MKP-3 in endotoxin-induced endothelial inflammation by western blotting, quantitative polymerase chain reaction, and immunofluorescence. The results of our study demonstrated that MKP-3 overexpression markedly inhibited the adhesion of human monocytic THP-1 cells to human umbilical vein endothelial cells (HUVECs) by downregulating the expression of vascular cell adhesion protein 1 (VCAM-1) and pro-inflammatory cytokines. In contrast, MKP-3-encoding gene knockdown by small interfering RNA (siRNA) exacerbated LPS-induced endothelial dysfunction. Additionally, we found that MKP-3 overexpression inhibited LPS-induced p38 MAPK phosphorylation and decreased the nuclear translocation of nuclear factor kappa B (NF-κB) after LPS treatment, suggesting its implication in the LPS/Toll-like receptor 4 (TLR4)/p38/NF-κB pathway. Overall, these observations suggest that MKP-3 plays a protective role in endothelial dysfunction and may be a therapeutic target.

Emerging role of protein kinases in diabetes mellitus: From mechanism to therapy

Diabetes mellitus has emerged as a severe burden on the medical health system across the globe. Presently, around 422 million people are suffering from diabetes which is speculated to be expanded to about 600 million by 2035. Patients with type 2 diabetes are at increased risk of developing detrimental metabolic and cardiovascular complications. The scientific understanding of this chronic disease and its underlying root cause is not yet fully unraveled. Protein kinases are well known to regulate almost every cellular process through phosphorylation of target protein in diverse signaling pathways. The important role of several protein kinases including AMP-activated protein kinase, IκB kinase and protein kinase C have been well demonstrated in various animal models. They modulate glucose tolerance, inflammation and insulin resistance in the cells via acting on diverse downstream targets and signaling pathways. Thus, modulating the activity of potential human kinases which are significantly involved in diabetes by targeting with small molecule inhibitors could be an attractive therapeutic strategy to tackle diabetes. In this chapter, we have discussed the potential role of protein kinases in glucose metabolism and insulin sensitivity, and in the pathogenesis of diabetes mellitus. Furthermore, the small molecules reported in the literature that can be potentially used for the treatment of diabetes have been discussed in detail.

Neuroprotective effect of dual specificity phosphatase 6 against glutamate-induced cytotoxicity in mouse hippocampal neurons

Dual specificity phosphatase 6 (DUSP6), a member of the dual specificity protein phosphatase subfamily, can inactivate ERK1/2. However, its possible role in glutamate-induced oxidative cytotoxicity effects is not clear.Here, we aimed to investigate whether DUSP6 was neuroprotective against glutamate-induced cytotoxicity in HT22 mouse hippocampal cells and primary cultured hippocampal neurons (pc-HNeu). HT22 and pc-HNeu cells were treated with varying concentrations of glutamate (from 0.05mM to 5.0mM) and DUSP6 protein expression were detected by western blotting. DUSP6-overexpressing HT22 and pc-HNeu cells were generated by transfection with DUSP6-overexpressing plasmid. The effects of DUSP6 overexpression on glutamate-induced cytotoxicity, cell death, cell apoptosis, and cell autophagy were determined by cell proliferation assays, flow cytometry, transmission electron microscopy, and western blotting. Glutamate treatment from 0.5mM to 5.0mM downregulated DUSP6 protein expression in both HT22 and pc-HNeu cells. DUSP6 overexpression ameliorated glutamate-induced cell death, apoptosis, and autophagy in both HT22 and pc-HNeu cells. Furthermore, ERK1/2 phosphorylation was decreased by DUSP6 overexpression. In conclusion, DUSP6 has neuroprotective effects against glutamate-induced cytotoxicity in HT22 and pc-HNeu cells. Targeting DUSP6 may be a useful strategy to prevent neuronal death in neurodegenerative diseases including AD.

The role of sphingolipid signalling in diabetes‑associated pathologies (Review)

Sphingosine kinase (SphK) is an important signalling enzyme that catalyses the phosphorylation of sphingosine (Sph) to form sphingosine‑1‑phosphate (S1P). The multifunctional lipid, S1P binds to a family of five G protein-coupled receptors (GPCRs). As an intracellular second messenger, S1P activates key signalling cascades responsible for the maintenance of sphingolipid metabolism, and has been implicated in the progression of cancer, and the development of other inflammatory and metabolic diseases. SphK and S1P are critical molecules involved in the regulation of various cellular metabolic processes, such as cell proliferation, survival, apoptosis, adhesion and migration. There is strong evidence supporting the critical roles of SphK and S1P in the progression of diabetes mellitus, including insulin sensitivity and insulin secretion, pancreatic β‑cell apoptosis, and the development of diabetic inflammatory state. In this review, we summarise the current state of knowledge for SphK/S1P signalling effects, associated with the development of insulin resistance, pancreatic β‑cell death and the vascular complications of diabetes mellitus.

Switching the sphingolipid rheostat in the treatment of diabetes and cancer comorbidity from a problem to an advantage

Cancer and diabetes are among the most common diseases in western societies. Epidemiological studies have shown that diabetic patients have a significantly higher risk of developing a number of different types of cancers and that individuals with comorbidity (cancer and diabetes/prediabetes) have a poorer prognosis relative to nondiabetic cancer patients. The increasing frequency of comorbidity of cancer and diabetes mellitus, mainly type 2 diabetes, has driven the development of therapeutic interventions that target both disease states. There is strong evidence to suggest that balancing the sphingolipid rheostat, ceramide--sphingosine--sphingosine-1-phosphate (S1P) is crucial in the prevention of diabetes and cancer and sphingosine kinase/S1P modulators are currently under development for the treatment of cancer and diabetes. This paper will highlight some of the complexities inherent in the use of the emerging sphingosine kinase/S1P modulators in the treatment of comorbidity of diabetes and cancer.

Role of sphingosine-1-phosphate receptor 1 and sphingosine-1-phosphate receptor 2 in hyperglycemia-induced endothelial cell dysfunction

The hyperglycemia-induced production of oxidative stress results in endothelial cell dysfunction. Previous studies have demonstrated that sphingosine-1-phosphate (S1P) regulates an array of biological activities in endothelial cells mediated by sphingosine-1-phosphate receptors (S1PRs). However, the role of S1PR-mediated signaling pathways in hyperglycemia-induced endothelial cell dysfunction is currently unknown. In the present study, we aimed to explore the role of S1PRs in endothelial cell dysfunction. For this purpose, hyperglycemia-induced oxidative stress was examined using human umbilical vein endothelial cells (HUVECs) cultured with either normal (5.6 mM) or high (25 mM) levels of glucose. The levels of reactive oxygen species (ROS) and nitric oxide (NO) were determined by flow cytometric (FCM) analysis and nitrate reductase, respectively. Endothelial morphogenesis assay was performed in three-dimensional Matrigel. The mRNA and protein expression levels of S1PRs in the HUVECs were determined by RT-qPCR and western blot analysis, respectively. In addition, ROS, NO and endothelial morphogenesis assays were conducted using the high glucose-treated endothelial cells transfected with adenoviral vector expressing exogenous S1PR1 gene (pAd-S1PR1) or with adenoviral vector expressing S1PR2-specific shRNA (pAd-shRNA-S1PR2). The expression levels of S1PR1 and S1PR2 in the endothelial cells treated with high levels of glucose decreased and increased, respectively. However, the effects of high levels of glucose on S1PR3 were minimal. In addition, high levels of glucose enhanced ROS generation and markedly reduced NO generation and morphogenetic responses. Nevertheless, all the aforementioned changes were completely reversed by transfection with pAd-S1PR1 or pAd-shRNA-S1PR2, which increased S1PR1 and decreased S1PR2 expression, respectively. It can thus be concluded that S1PR1 and S1PR2 play crucial roles in hyperglycemia-induced endothelial cell dysfunction.

Roles of Sphingolipid Metabolism in Pancreatic β Cell Dysfunction Induced by Lipotoxicity

Pancreatic β cells secrete insulin in order to maintain glucose homeostasis. However, various environmental stresses such as obesity have been shown to induce loss of secretory responsiveness in pancreatic β cells and pancreatic β cell apoptosis which can favor the development of type 2 diabetes (T2D). Indeed, elevated levels of free fatty acids (FFAs) have been shown to induce β cell apoptosis. Importantly, the chronic adverse effects of FFAs on β cell function and viability are potentiated in the presence of hyperglycaemia, a phenomenon that has been termed gluco-lipotoxicity. The molecular mechanisms underlying the pathogenesis of gluco-lipotoxicity in pancreatic β cells are not completely understood. Recent studies have shown that sphingolipid metabolism plays a key role in gluco-lipotoxicity induced apoptosis and loss of function of pancreatic β cells. The present review focuses on how the two main sphingolipid mediators, ceramides and sphingoid base-1-phosphates, regulate the deleterious effects of gluco-lipotoxicity on pancreatic β cells. The review highlights the role of a sphingolipid biostat on the dysregulation of β cell fate and function induced by gluco-lipotoxicity, offering the possibility of new therapeutic targets to prevent the onset of T2D.

Ceramide 1-phosphate stimulates glucose uptake in macrophages

It is well established that ceramide 1-phosphate (C1P) is mitogenic and antiapoptotic, and that it is implicated in the regulation of macrophage migration. These activities require high energy levels to be available in cells. Macrophages obtain most of their energy from glucose. In this work, we demonstrate that C1P enhances glucose uptake in RAW264.7 macrophages. The major glucose transporter involved in this action was found to be GLUT 3, as determined by measuring its translocation from the cytosol to the plasma membrane. C1P-stimulated glucose uptake was blocked by selective inhibitors of phosphatidylinositol 3-kinase (PI3K) or Akt, also known as protein kinase B (PKB), and by specific siRNAs to silence the genes encoding for these kinases. C1P-stimulated glucose uptake was also inhibited by pertussis toxin (PTX) and by the siRNA that inhibited GLUT 3 expression. C1P increased the affinity of the glucose transporter for its substrate, and enhanced glucose metabolism to produce ATP. The latter action was also inhibited by PI3K- and Akt-selective inhibitors, PTX, or by specific siRNAs to inhibit GLUT 3 expression.

Sphingosine 1-phosphate (S1P) carrier-dependent regulation of endothelial barrier: high density lipoprotein (HDL)-S1P prolongs endothelial barrier enhancement as compared with albumin-S1P via effects on levels, trafficking, and signaling of S1P1

Sphingosine 1-phosphate (S1P) is a blood-borne lysosphingolipid that acts to promote endothelial cell (EC) barrier function. In plasma, S1P is associated with both high density lipoproteins (HDL) and albumin, but it is not known whether the carriers impart different effects on S1P signaling. Here we establish that HDL-S1P sustains EC barrier longer than albumin-S1P. We showed that the sustained barrier effects of HDL-S1P are dependent on signaling by the S1P receptor, S1P1, and involve persistent activation of Akt and endothelial NOS (eNOS), as well as activity of the downstream NO target, soluble guanylate cyclase (sGC). Total S1P1 protein levels were found to be higher in response to HDL-S1P treatment as compared with albumin-S1P, and this effect was not associated with increased S1P1 mRNA or dependent on de novo protein synthesis. Several pieces of evidence indicate that long term EC barrier enhancement activity of HDL-S1P is due to specific effects on S1P1 trafficking. First, the rate of S1P1 degradation, which is proteasome-mediated, was slower in HDL-S1P-treated cells as compared with cells treated with albumin-S1P. Second, the long term barrier-promoting effects of HDL-S1P were abrogated by treatment with the recycling blocker, monensin. Finally, cell surface levels of S1P1 and levels of S1P1 in caveolin-enriched microdomains were higher after treatment with HDL-S1P as compared with albumin-S1P. Together, the findings reveal S1P carrier-specific effects on S1P1 and point to HDL as the physiological mediator of sustained S1P1-PI3K-Akt-eNOS-sGC-dependent EC barrier function.

Ischemia/reperfusion-induced MKP-3 impairs endothelial NO formation via inactivation of ERK1/2 pathway

Mitogen-activated protein kinase phosphatases (MKPs) are a family of dual-specificity phosphatases. Endothelial cells express multiple MKP family members, such as MKP-3. However, the effects of MKP-3 on endothelial biological processes have not yet been fully elucidated. Here, we address the association between MKP-3 and endothelial Nitric oxide (NO) formation under ischemia/reperfusion (IS/RP) condition. Human umbilical vein endothelial cells (HUVECs) were subjected to IS/RP treatment. The MKP-3 expression and NO formation were examined. IS/RP induced endothelial MKP-3 expression and inhibited eNOS expression and NO formation, accompanied by an increase of endothelial apoptosis. The siRNA experiments showed that MKP-3 was an important mediator in impairing eNOS expression and NO production in endothelial cells. Transfection of HUVECs with constitutively active ERK plasmids suggested that the above mentioned effect of MKP-3 was via inactivation of ERK1/2 pathway. Furthermore, impairment of eNOS expression was restored by treatment of histone deacetylase (HDAC) inhibitor and related to histone deacetylation and recruitment of HDAC1 to the eNOS promoter. Finally, Salvianolic acid A (SalA) markedly attenuated induction of MKP-3 and inhibition of eNOS expression and NO formation under endothelial IS/RP condition. Overall, these results for the first time demonstrated that IS/RP inhibited eNOS expression by inactivation of ERK1/2 and recruitment of HDAC1 to the gene promoter, leading to decreased NO formation through a MKP-3-dependent mechanism in endothelial cells, and SalA has therapeutic significance in protecting endothelial cells from impaired NO formation in response to IS/RP.

Sphingosine-1-phosphate prevents permeability increases via activation of endothelial sphingosine-1-phosphate receptor 1 in rat venules

Sphingosine-1-phosphate (S1P) has been demonstrated to enhance endothelial barrier function in vivo and in vitro. However, different S1P receptor subtypes have been indicated to play different or even opposing roles in the regulation of vascular barrier function. This study aims to differentiate the roles of endogenous endothelial S1P subtype receptors in the regulation of permeability in intact microvessels using specific receptor agonist and antagonists. Microvessel permeability was measured with hydraulic conductivity (L(p)) in individually perfused rat mesenteric venules. S1P-mediated changes in endothelial intracellular Ca(2+) concentration ([Ca(2+)](i)) was measured in fura-2-loaded venules. Confocal images of fluorescent immunostaining illustrated the spatial expressions of three S1P subtype receptors (S1P(R1-3)) in rat venules. The application of S1P (1 μM) in the presence of S1P(R1-3) inhibited platelet-activating factor- or bradykinin-induced permeability increase. This S1P effect was reversed only with a selective S1P(R1) antagonist, W-146, and was not affected by S1P(R2) or S1P(R3) antagonists JTE-013 and CAY-10444, respectively. S1P(R1) was also identified as the sole receptor responsible for S1P-mediated increases in endothelial [Ca(2+)](i). S1P(R2) or S1P(R3) antagonist alone affected neither basal L(p) nor platelet-activating factor-induced permeability increase. The selective S1P(R1) agonist, SEW-2871, showed similar [Ca(2+)](i) and permeability effect to that of S1P. These results indicate that, despite the presence of S1P(R1-3) in the intact venules, only the activation of endothelial S1P(R1) is responsible for the protective action of S1P on microvessel permeability and that endogenous S1P(R2) or S1P(R3) did not exhibit functional roles in the regulation of permeability under basal or acutely stimulated conditions.

Vascular PTPs: current developments and challenges for exploitation in Type 2 diabetes-associated vascular dysfunction

Protein Tyrosine Phosphatases (PTPs) are important contributors to vascular cells normal function, by balancing signaling proteins activation exerted by phosphorylating kinases. Type 2 diabetes related insults, such as hyperglycemia, oxidative stress, and insulin resistance disturb the phosphorylation/dephosphorylation equilibrium towards an abnormal augmented phosphorylation of signaling proteins associated with changes in PTPs expression, enzymatic activity and interaction with cellular substrates. We briefly review here: (i) the new findings on receptor and non-receptor PTPs and their role in vascular cells, (ii) several data on oxidation and phosphorylation of these molecules in endothelial and smooth muscle cells, (iii) vascular PTPs intrinsic activity and dysregulation under the insults of diabetic milieu, and (iv) the potential use of PTPs and their inhibitors as therapeutic targets in Type 2 diabetes-associated vascular dysfunction.