Sphingosine Kinases as Druggable Targets

Current advancements and future perspectives of long noncoding RNAs in lipid metabolism and signaling

BACKGROUND

The investigation of lncRNAs has provided a novel perspective for elucidating mechanisms underlying diverse physiological and pathological processes. Compelling evidence has revealed an intrinsic link between lncRNAs and lipid metabolism, demonstrating that lncRNAs-induced disruption of lipid metabolism and signaling contribute to the development of multiple cancers and some other diseases, including obesity, fatty liver disease, and cardiovascular disease.

AIMOF REVIEW

The current review summarizes the recent advances in basic research about lipid metabolism and lipid signaling-related lncRNAs. Meanwhile, the potential and challenges of targeting lncRNA for the therapy of cancers and other lipid metabolism-related diseases are also discussed.

KEY SCIENTIFIC CONCEPT OF REVIEW

Compared with the substantial number of lncRNA loci, we still know little about the role of lncRNAs in metabolism. A more comprehensive understanding of the function and mechanism of lncRNAs may provide a new standpoint for the study of lipid metabolism and signaling. Developing lncRNA-based therapeutic approaches is an effective strategy for lipid metabolism-related diseases.

Sphingosine 1-phosphate receptor-targeted therapeutics in rheumatic diseases

Sphingosine 1-phosphate (S1P), which acts via G protein-coupled S1P receptors (S1PRs), is a bioactive lipid essential for vascular integrity and lymphocyte trafficking. The S1P-S1PR signalling axis is a key component of the inflammatory response in autoimmune rheumatic diseases. Several drugs that target S1PRs have been approved for the treatment of multiple sclerosis and inflammatory bowel disease and are under clinical testing for patients with systemic lupus erythematosus (SLE). Preclinical studies support the hypothesis that targeting the S1P-S1PR axis would be beneficial to patients with SLE, rheumatoid arthritis (RA) and systemic sclerosis (SSc) by reducing pathological inflammation. Whereas most preclinical research and development efforts are focused on reducing lymphocyte trafficking, protective effects of circulating S1P on endothelial S1PRs, which maintain the vascular barrier and enable blood circulation while dampening leukocyte extravasation, have been largely overlooked. In this Review, we take a holistic view of S1P-S1PR signalling in lymphocyte and vascular pathobiology. We focus on the potential of S1PR modulators for the treatment of SLE, RA and SSc and summarize the rationale, pathobiology and evidence from preclinical models and clinical studies. Improved understanding of S1P pathobiology in autoimmune rheumatic diseases and S1PR therapeutic modulation is anticipated to lead to efficacious and safer management of these diseases.

Ceramide synthases: Reflections on the impact of Dr. Lina M. Obeid

Sphingolipids are a family of lipids that are critical to cell function and survival. Much of the recent work done on sphingolipids has been performed by a closely-knit family of sphingolipid researchers, which including our colleague, Dr. Lina Obeid, who recently passed away. We now briefly review where the sphingolipid field stands today, focusing in particular on areas of sphingolipid research to which Dr. Obeid made valued contributions. These include the 'many-worlds' view of ceramides and the role of a key enzyme in the sphingolipid biosynthetic pathway, namely the ceramide synthases (CerS). The CerS contain a number of functional domains and also interact with a number of other proteins in lipid metabolic pathways, fulfilling Dr. Obeid's prophecy that ceramides, and the enzymes that generate ceramides, form the critical hub of the sphingolipid metabolic pathway.

Targeting the Sphingosine-1-Phosphate Axis for Developing Non-narcotic Pain Therapeutics

Chronic pain is a life-altering condition affecting millions of people. Current treatments are inadequate and prolonged therapies come with severe side effects, especially dependence and addiction to opiates. Identification of non-narcotic analgesics is of paramount importance. Preclinical and clinical studies suggest that sphingolipid metabolism alterations contribute to neuropathic pain development. Functional sphingosine-1-phosphate (S1P) receptor 1 (S1PR1) antagonists, such as FTY720/fingolimod, used clinically for non-pain conditions, are emerging as non-narcotic analgesics, supporting the repurposing of fingolimod for chronic pain treatment and energizing drug discovery focused on S1P signaling. Here, we summarize the role of S1P in pain to highlight the potential of targeting the S1P axis towards development of non-narcotic therapeutics, which, in turn, will hopefully help lessen misuse of opioid pain medications and address the ongoing opioid epidemic.

Dissecting Gq/11-Mediated Plasma Membrane Translocation of Sphingosine Kinase-1

Diverse extracellular signals induce plasma membrane translocation of sphingosine kinase-1 (SphK1), thereby enabling inside-out signaling of sphingosine-1-phosphate. We have shown before that G_q-coupled receptors and constitutively active Gα_q/11 specifically induced a rapid and long-lasting SphK1 translocation, independently of canonical G_q/phospholipase C (PLC) signaling. Here, we further characterized G_q/11 regulation of SphK1. SphK1 translocation by the M₃ receptor in HEK-293 cells was delayed by expression of catalytically inactive G-protein-coupled receptor kinase-2, p63Rho guanine nucleotide exchange factor (p63RhoGEF), and catalytically inactive PLCβ₃, but accelerated by wild-type PLCβ₃ and the PLCδ PH domain. Both wild-type SphK1 and catalytically inactive SphK1-G82D reduced M₃ receptor-stimulated inositol phosphate production, suggesting competition at Gα_q. Embryonic fibroblasts from Gα_q/11 double-deficient mice were used to show that amino acids W263 and T257 of Gα_q, which interact directly with PLCβ₃ and p63RhoGEF, were important for bradykinin B₂ receptor-induced SphK1 translocation. Finally, an AIXXPL motif was identified in vertebrate SphK1 (positions 100–105 in human SphK1a), which resembles the Gα_q binding motif, ALXXPI, in PLCβ and p63RhoGEF. After M₃ receptor stimulation, SphK1-A100E-I101E and SphK1-P104A-L105A translocated in only 25% and 56% of cells, respectively, and translocation efficiency was significantly reduced. The data suggest that both the AIXXPL motif and currently unknown consequences of PLCβ/PLCδ(PH) expression are important for regulation of SphK1 by G_q/11.

Murine platelet production is suppressed by S1P release in the hematopoietic niche, not facilitated by blood S1P sensing

The bioactive lipid mediator sphingosine 1-phosphate (S1P) was recently assigned critical roles in platelet biology: whereas S1P₁ receptor-mediated S1P gradient sensing was reported to be essential for directing proplatelet extensions from megakaryocytes (MKs) toward bone marrow sinusoids, MK sphingosine kinase 2 (Sphk2)-derived S1P was reported to further promote platelet shedding through receptor-independent intracellular actions, and platelet aggregation through S1P₁ Yet clinical use of S1P pathway modulators including fingolimod has not been associated with risk of bleeding or thrombosis. We therefore revisited the role of S1P in platelet biology in mice. Surprisingly, no reduction in platelet counts was observed when the vascular S1P gradient was ablated by impairing S1P provision to plasma or S1P degradation in interstitial fluids, nor when gradient sensing was impaired by S1pr1 deletion selectively in MKs. Moreover, S1P₁ expression and signaling were both undetectable in mature MKs in situ, and MK S1pr1 deletion did not affect platelet aggregation or spreading. When S1pr1 deletion was induced in hematopoietic progenitor cells, platelet counts were instead significantly elevated. Isolated global Sphk2 deficiency was associated with thrombocytopenia, but this was not replicated by MK-restricted Sphk2 deletion and was reversed by compound deletion of either Sphk1 or S1pr2, suggesting that this phenotype arises from increased S1P export and S1P₂ activation secondary to redistribution of sphingosine to Sphk1. Consistent with clinical observations, we thus observe no essential role for S1P₁ in facilitating platelet production or activation. Instead, S1P restricts megakaryopoiesis through S1P₁, and can further suppress thrombopoiesis through S1P₂ when aberrantly secreted in the hematopoietic niche.

Sphingosine 1-phosphate: Lipid signaling in pathology and therapy

Sphingosine 1-phosphate (S1P), a metabolic product of cell membrane sphingolipids, is bound to extracellular chaperones, is enriched in circulatory fluids, and binds to G protein-coupled S1P receptors (S1PRs) to regulate embryonic development, postnatal organ function, and disease. S1PRs regulate essential processes such as adaptive immune cell trafficking, vascular development, and homeostasis. Moreover, S1PR signaling is a driver of multiple diseases. The past decade has witnessed an exponential growth in this field, in part because of multidisciplinary research focused on this lipid mediator and the application of S1PR-targeted drugs in clinical medicine. This has revealed fundamental principles of lysophospholipid mediator signaling that not only clarify the complex and wide ranging actions of S1P but also guide the development of therapeutics and translational directions in immunological, cardiovascular, neurological, inflammatory, and fibrotic diseases.

The WD40 repeat protein, WDR36, orchestrates sphingosine kinase-1 recruitment and phospholipase C-β activation by Gq-coupled receptors

Sphingosine kinases (SphK) catalyse the formation of sphingosine-1-phosphate (S1P) and play important roles in the cardiovascular, nervous and immune systems. We have shown before that G_q-coupled receptors induce a rapid and long-lasting translocation of SphK1 to the plasma membrane and cross-activation of S1P receptors. Here, we further addressed G_q regulation of SphK1 by analysing the influence of the WD40 repeat protein, WDR36. WDR36 has been described as a scaffold tethering Gα_q to phospholipase C (PLC)-β and the thromboxane A₂ receptor-β (TPβ receptor). Overexpression of WDR36 in HEK-293 cells enhanced TPβ receptor-induced inositol phosphate production, as reported (Cartier et al. 2011), but significantly attenuated inositol phosphate production induced by muscarinic M₃ and bradykinin B₂ receptors. In agreement with its effect on PLCβ, WDR36 augmented TPβ receptor-induced [Ca²⁺]_i increases. Surprisingly, WDR36 also augmented M₃ receptor-induced [Ca²⁺]_i increases, which was due to increased Ca²⁺ mobilization while the Ca²⁺ content of thapsigargin-sensitive stores remained unaltered. Interestingly, overexpression of WDR36 significantly delayed SphK1 translocation by G_q-coupled M₃, B₂ and H₁ receptors in HEK-293 cells, while TPβ receptor-induced SphK1 translocation was generally slow and not altered by WDR36 in these cells. Finally, in C2C12 myoblasts, overexpression of WDR36 delayed SphK1 translocation induced by B₂ receptors. It is concluded that WDR36 reduces signalling of G_q-coupled receptors other than TPβ towards PLC and SphK1, most likely by scavenging Gα_q and PLCβ. Our results support a role of WDR36 in orchestration of G_q signalling complexes, and might help to functionally unravel its genetic association with asthma and allergy.

The regulation of p53, p38 MAPK, JNK and XBP-1s by sphingosine kinases in human embryonic kidney cells

Since inhibitors of sphingosine kinases (SK1, SK2) have been shown to induce p53-mediated cell death, we have further investigated their role in regulating p53, stress activated protein kinases and XBP-1s in HEK293T cells. Treatment of these cells with the sphingosine kinase inhibitor, SKi, which fails to induce apoptosis, promoted the conversion of p53 into two proteins with molecular masses of 63 and 90 kDa, and which was enhanced by over-expression of ubiquitin. The SKi induced conversion of p53 to p63/p90 was also enhanced by siRNA knockdown of SK1, but not SK2 or dihydroceramide desaturase (Degs1), suggesting that SK1 is a negative regulator of this process. In contrast, another sphingosine kinase inhibitor, ABC294640 only very weakly stimulated formation of p63/p90 and induced apoptosis of HEK293T cells. We have previously shown that SKi promotes the polyubiquitination of Degs1, and these forms positively regulate p38 MAPK/JNK pathways to promote HEK293T cell survival/growth. siRNA knockdown of SK1 enhanced the activation of p38 MAPK/JNK pathways in response to SKi, suggesting that SK1 functions to oppose these pro-survival pathways in HEK293T cells. SKi also enhanced the stimulatory effect of the proteasome inhibitor, MG132 on the expression of the pro-survival protein XBP-1s and this was reduced by siRNA knockdown of SK2 and increased by knockdown of p53. These findings suggest that SK1 and SK2 have opposing roles in regulating p53-dependent function in HEK293T cells.

Sphingosine 1-Phosphate Receptors and Metabolic Enzymes as Druggable Targets for Brain Diseases

The central nervous system is characterized by a high content of sphingolipids and by a high diversity in terms of different structures. Stage- and cell-specific sphingolipid metabolism and expression are crucial for brain development and maintenance toward adult age. On the other hand, deep dysregulation of sphingolipid metabolism, leading to altered sphingolipid pattern, is associated with the majority of neurological and neurodegenerative diseases, even those totally lacking a common etiological background. Thus, sphingolipid metabolism has always been regarded as a promising pharmacological target for the treatment of brain disorders. However, any therapeutic hypothesis applied to complex amphipathic sphingolipids, components of cellular membranes, has so far failed probably because of the high regional complexity and specificity of the different biological roles of these structures. Simpler sphingosine-based lipids, including ceramide and sphingosine 1-phosphate, are important regulators of brain homeostasis, and, thanks to the relative simplicity of their metabolic network, they seem a feasible druggable target for the treatment of brain diseases. The enzymes involved in the control of the levels of bioactive sphingoids, as well as the receptors engaged by these molecules, have increasingly allured pharmacologists and clinicians, and eventually fingolimod, a functional antagonist of sphingosine 1-phosphate receptors with immunomodulatory properties, was approved for the therapy of relapsing-remitting multiple sclerosis. Considering the importance of neuroinflammation in many other brain diseases, we would expect an extension of the use of such analogs for the treatment of other ailments in the future. Nevertheless, many aspects other than neuroinflammation are regulated by bioactive sphingoids in healthy brain and dysregulated in brain disease. In this review, we are addressing the multifaceted possibility to address the metabolism and biology of bioactive sphingosine 1-phosphate as novel targets for the development of therapeutic paradigms and the discovery of new drugs.

Sphingolipids in neurodegeneration (with focus on ceramide and S1P)

For many decades, research on sphingolipids associated with neurodegenerative disease focused on alterations in glycosphingolipids, particularly glycosylceramides (cerebrosides), sulfatides, and gangliosides. This seemed quite natural since many of these glycolipids are constituents of myelin and accumulated in lipid storage diseases (sphingolipidoses) resulting from enzyme deficiencies in glycolipid metabolism. With the advent of recognizing ceramide and its derivative, sphingosine-1-phosphate (S1P), as key players in lipid cell signaling and regulation of cell death and survival, research focus shifted toward these two sphingolipids. Ceramide and S1P are invoked in a plethora of cell biological processes participating in neurodegeneration such as ER stress, autophagy, dysregulation of protein and lipid transport, exosome secretion and neurotoxic protein spreading, neuroinflammation, and mitochondrial dysfunction. Hence, it is timely to discuss various functions of ceramide and S1P in neurodegenerative disease and to define sphingolipid metabolism and cell signaling pathways as potential targets for therapy.