New insights into the role of sphingosine 1-phosphate and lysophosphatidic acid in the regulation of skeletal muscle cell biology

Adipose tissue adipokines and lipokines: Functions and regulatory mechanism in skeletal muscle development and homeostasis

Skeletal muscle plays important roles in normal biological activities and whole-body energy homeostasis in humans. The growth and development of skeletal muscle also directly influence meat production and meat quality in animal production. Therefore, regulating the development and homeostasis of skeletal muscle is crucial for human health and animal production. Adipose tissue, which includes white adipose tissue (WAT) and brown adipose tissue (BAT), not only functions as an energy reserve but also has attracted substantial attention because of its role as an endocrine organ. The novel signalling molecules known as "adipokines" and "lipokines" that are secreted by adipose tissue were identified through the secretomic technique, which broadened our understanding of the previously unknown crosstalk between adipose tissue and skeletal muscle. In this review, we summarize and discuss the secretory role of adipose tissues, both WAT and BAT, as well as the regulatory roles of various adipokines and lipokines in skeletal muscle development and homeostasis. We suggest that adipokines and lipokines have potential as drug candidates for the treatment of skeletal muscle dysfunction and related metabolic diseases and as promising nutrients for improving animal production.

Pathogenic Role of the Sphingosine 1-Phosphate (S1P) Pathway in Common Gynecologic Disorders (GDs): A Possible Novel Therapeutic Target

Sphingosine 1-phosphate (S1P) is a bioactive sphingolipid, noteworthy for its involvement both in the modulation of various biological processes and in the development of many diseases. S1P signaling can be either pro or anti-inflammatory, and the sphingosine kinase (SphK)–S1P–S1P receptor (S1PR) axis is a factor in accelerating the growth of several cells, including endometriotic cells and fibrosis. Gynecologic disorders, including endometriosis, adenomyosis, and uterine fibroids are characterized by inflammation and fibrosis. S1P signaling and metabolism have been shown to be dysregulated in those disorders and they are likely implicated in their pathogenesis and pathophysiology. Enzymes responsible for inactivating S1P are the most affected by the dysregulation of S1P balanced levels, thus causing accumulation of sphingolipids within these cells and tissues. The present review highlights the past and latest evidence on the role played by the S1P pathways in common gynecologic disorders (GDs). Furthermore, it discusses potential future approaches in the regulation of this signaling pathway that could represent an innovative and promising therapeutical target, also for ovarian cancer treatment.

S1P Signalling Axis Is Necessary for Adiponectin-Directed Regulation of Electrophysiological Properties and Oxidative Metabolism in C2C12 Myotubes

Background: Adiponectin (Adn), released by adipocytes and other cell types such as skeletal muscle, has insulin-sensitizing and anti-inflammatory properties. Sphingosine 1-phosphate (S1P) is reported to act as effector of diverse biological actions of Adn in different tissues. S1P is a bioactive sphingolipid synthesized by the phosphorylation of sphingosine catalyzed by sphingosine kinase (SK) 1 and 2. Consolidated findings support the key role of S1P in the biology of skeletal muscle. Methods and Results: Here we provide experimental evidence that S1P signalling is modulated by globular Adn treatment being able to increase the phosphorylation of SK1/2 as well as the mRNA expression levels of S1P₄ in C2C12 myotubes. These findings were confirmed by LC-MS/MS that showed an increase of S1P levels after Adn treatment. Notably, the involvement of S1P axis in Adn action was highlighted since, when SK1 and 2 were inhibited by PF543 and ABC294640 inhibitors, respectively, not only the electrophysiological changes but also the increase of oxygen consumption and of aminoacid levels induced by the hormone, were significantly inhibited. Conclusion: Altogether, these findings show that S1P biosynthesis is necessary for the electrophysiological properties and oxidative metabolism of Adn in skeletal muscle cells.

A2 B Adenosine Receptors and Sphingosine 1-Phosphate Signaling Cross-Talk in Oligodendrogliogenesis

Oligodendrocyte-formed myelin sheaths allow fast synaptic transmission in the brain. Impairments in the process of myelination, or demyelinating insults, might cause chronic diseases such as multiple sclerosis (MS). Under physiological conditions, remyelination is an ongoing process throughout adult life consisting in the differentiation of oligodendrocyte progenitor cells (OPCs) into mature oligodendrocytes (OLs). During pathological events, this process fails due to unfavorable environment. Adenosine and sphingosine kinase/sphingosine 1-phosphate signaling axes (SphK/S1P) play important roles in remyelination processes. Remarkably, fingolimod (FTY720), a sphingosine analog recently approved for MS treatment, plays important roles in OPC maturation. We recently demonstrated that the selective stimulation of A_{2 B} adenosine receptors (A_{2 B} Rs) inhibit OPC differentiation in vitro and reduce voltage-dependent outward K⁺ currents (I _K ) necessary to OPC maturation, whereas specific SphK1 or SphK2 inhibition exerts the opposite effect. During OPC differentiation A_{2 B} R expression increases, this effect being prevented by SphK1/2 blockade. Furthermore, selective silencing of A_{2 B} R in OPC cultures prompts maturation and, intriguingly, enhances the expression of S1P lyase, the enzyme responsible for irreversible S1P catabolism. Finally, the existence of an interplay between SphK1/S1P pathway and A_{2 B} Rs in OPCs was confirmed since acute stimulation of A_{2 B} Rs activates SphK1 by increasing its phosphorylation. Here the role of A_{2 B} R and SphK/S1P signaling during oligodendrogenesis is reviewed in detail, with the purpose to shed new light on the interaction between A_{2 B} Rs and S1P signaling, as eventual innovative targets for the treatment of demyelinating disorders.

Sphingosine 1-phosphate signaling in uterine fibroids: implication in activin A pro-fibrotic effect

OBJECTIVE

To explore the link between sphingosine 1-phosphate (S1P) signaling and leiomyoma and the possible S1P cross-talk with the fibrotic effect of activin A.

DESIGN

Case-control laboratory study.

SETTING

University institute and university hospital.

PATIENT(S)

Patients with uterine fibroids (n = 26).

INTERVENTIONS(S)

Tissue specimens of leiomyoma and normal myometrium were obtained from patients undergoing myomectomy or total hysterectomy.

MAIN OUTCOME MEASURE(S)

Expression of mRNA levels of the enzyme involved in S1P metabolism, S1P receptors, and S1P transporter Spns2 was evaluated in matched leiomyoma/myometrium specimens and cell populations. The effects of inhibition of S1P metabolism and signaling was evaluated on activin A-induced fibrotic action in leiomyoma cell lines.

RESULT(S)

The expression of the enzymes responsible for S1P formation, sphingosine kinase (SK) 1 and 2, and S1P₂, S1P₃, and S1P₅ receptors was significantly augmented in leiomyomas compared with adjacent myometrium. In leiomyoma cells, but not in myometrial cells, activin A increased mRNA expression levels of SK1, SK2, and S1P₂. The profibrotic action of activin A was abolished when SK1/2 were inhibited or S1P_2/3 were blocked. Finally, S1P augmented by itself mRNA levels of fibrotic markers (fibronectin, collagen 1A1) and activin A in leiomyomas but not in myometrial cells.

CONCLUSION(S)

This study shows that S1P signaling is dysregulated in uterine fibroids and involved in activin A-induced fibrosis, opening new perspectives for uterine fibroid treatment.

Role of sphingosine 1-phosphate signalling in tissue fibrosis

Fibrosis is characterized by the excessive accumulation of extracellular matrix components, leading to loss of tissue function in affected organs. Although the majority of fibrotic diseases have different origins, they have in common a persistent inflammatory stimulus and lymphocyte-monocyte interactions that determine the production of numerous fibrogenic cytokines. Treatment to contrast fibrosis is urgently needed, since some fibrotic diseases lead to systemic fibrosis and represent a major cause of death. In this article, the role of the bioactive sphingolipid sphingosine 1-phosphate (S1P) and its signalling pathway in the fibrosis of different tissue contexts is extensively reviewed, highlighting that it may represent an innovative and promising pharmacological therapeutic target for treating this devastating multifaceted disease. In multiple tissues S1P influences different aspects of fibrosis modulating the recruitment of inflammatory cells, as well as cell proliferation, migration and transdifferentiation into myofibroblasts, the cell type mainly involved in fibrosis development. Moreover, at the level of fibrotic lesions, S1P metabolism is profoundly influenced by multiple cross-talk with profibrotic mediators, such as transforming growth factor β, thus finely regulating the development of fibrosis. This article is part of a Special Issue entitled "Physiological and pathological roles of bioactive sphingolipids".

Adenosine A2B receptors inhibit K+ currents and cell differentiation in cultured oligodendrocyte precursor cells and modulate sphingosine-1-phosphate signaling pathway

Oligodendrocytes are the only myelinating cells in the brain and differentiate from their progenitors (OPCs) throughout adult life. However, this process fails in demyelinating pathologies. Adenosine is emerging as an important player in OPC differentiation and we recently demonstrated that adenosine A_2A receptors inhibit cell maturation by reducing voltage-dependent K⁺ currents. No data are available to date about the A_2B receptor (A_2BR) subtype. The bioactive lipid mediator sphingosine-1-phosphate (S1P) and its receptors (S1P_1-5) are also crucial modulators of OPC development. An interaction between this pathway and the A_2BR is reported in peripheral cells. We studied the role of A_2BRs in modulating K⁺ currents and cell differentiation in OPC cultures and we investigated a possible interplay with S1P signaling. Our data indicate that the A_2BR agonist BAY60-6583 and its new analogue P453 inhibit K⁺ currents in cultured OPC and the effect was prevented by the A_2BR antagonist MRS1706, by K⁺ channel blockers and was differently modulated by the S1P analogue FTY720-P. An acute (10 min) exposure of OPCs to BAY60-6583 also increased the phosphorylated form of sphingosine kinase 1 (SphK1). A chronic (7 days) treatment with the same agonist decreased OPC differentiation whereas SphK1/2 inhibition exerted the opposite effect. Furthermore, A_2BR was overexpressed during OPC differentiation, an effect prevented by the pan SphK1/2 inhibitor VPC69047. Finally, A_2BR silenced cells showed increased cell maturation, decreased SphK1 expression and enhanced S1P lyase levels. We conclude that A_2BRs inhibit K⁺ currents and cell differentiation and positively modulate S1P synthesis in cultured OPCs.

Lysophosphatidic Acid Inhibits Simvastatin-Induced Myocytoxicity by Activating LPA Receptor/PKC Pathway

Statins such as simvastatin have many side effects, including muscle damage, which is known to be the most frequent undesirable side effect. Lysophosphatidic acid (LPA), a kind of biolipid, has diverse cellular activities, including cell proliferation, survival, and migration. However, whether LPA affects statin-linked muscle damage has not been reported yet. In the present study, to determine whether LPA might exert potential protective effect on statin-induced myocyotoxicity, the effect of LPA on cytotoxicity in rat L6 myoblasts exposed to simvastatin was explored. Viability and apoptosis of rat L6 myoblasts were detected via 2,3-bis(2-methoxy-4-nitro-5-sulfophenyl)-5- [(phenylamino)carbonyl]-2H-tetrazolium hydroxide (XTT) assay and terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling (TUNEL) assay, respectively. Protein expression levels were detected via Western blotting. Simvastatin decreased viability of L6 cells. Such decrease in viability was recovered in the presence of LPA. Treatment with LPA suppressed simvastatin-induced apoptosis in L6 cells. In addition, treatment with LPA receptor inhibitor Ki16425, protein kinase C (PKC) inhibitor GF109203X, or intracellular calcium chelator BAPTA-AM attenuated the recovery effect of LPA on simvastatin-induced L6 cell toxicity. These findings indicate that LPA may inhibit simvastatin-induced toxicity in L6 cells probably by activating the LPA receptor-PKC pathway. Therefore, LPA might have potential as a bioactive molecule to protect muscles against simvastatin-induced myotoxicity.

Roles of lysophosphatidic acid and sphingosine-1-phosphate in stem cell biology

Stem cells are unique in their ability to self-renew and differentiate into various cell types. Because of these features, stem cells are key to the formation of organisms and play fundamental roles in tissue regeneration and repair. Mechanisms controlling their fate are thus fundamental to the development and homeostasis of tissues and organs. Lysophosphatidic acid (LPA) and sphingosine-1-phosphate (S1P) are bioactive phospholipids that play a wide range of roles in multiple cell types, during developmental and pathophysiological events. Considerable evidence now demonstrates the potent roles of LPA and S1P in the biology of pluripotent and adult stem cells, from maintenance to repair. Here we review their roles for each main category of stem cells and explore how those effects impact development and physiopathology.

Severe murine limb-girdle muscular dystrophy type 2C pathology is diminished by FTY720 treatment

INTRODUCTION

Limb-girdle muscular dystrophy type 2C (LGMD-2C) is caused by mutations in γ-sarcoglycan and is a devastating, progressive, and fully lethal human muscle-wasting disease that has no effective treatment. This study examined the efficacy of the sphingosine-1-phosphate receptor modulator FTY720 in treating Sgcg^-/- DBA2/J, a severe mouse model of LGMD-2C. FTY720 treatment was expected to target LGMD-2C disease progression at 2 key positions by reducing chronic inflammation and fibrosis.

METHODS

The treatment protocol was initiated at age 3 weeks and was continued with alternate-day injections for 3 weeks.

RESULTS

The treatment produced significant functional benefit by plethysmography and significant reductions of membrane permeability and fibrosis. Furthermore, the protocol elevated protein levels of δ-sarcoglycan, a dystrophin-glycoprotein family member.

CONCLUSION

This study showed that FTY720 is an effective muscular dystrophy treatment when therapy is initiated early in the disease progression. Muscle Nerve 56: 486-494, 2017.

Sphingosine 1-phosphate signaling axis mediates fibroblast growth factor 2-induced proliferation and survival of murine auditory neuroblasts

Hearing loss affects millions of people in the world. In mammals the auditory system comprises diverse cell types which are terminally differentiated and with no regenerative potential. There is a tremendous research interest aimed at identifying cell therapy based solutions or pharmacological approaches that could be applied therapeutically alongside auditory devices to prevent hair cell and neuron loss. Sphingosine 1-phosphate (S1P) is a pleiotropic bioactive sphingolipid that plays key role in the regulation of many physiological and pathological functions. S1P is intracellularly produced by sphingosine kinase (SK) 1 and SK2 and exerts many of its action consequently to its ligation to S1P specific receptors (S1PR), S1P_1-5. In this study, murine auditory neuroblasts named US/VOT-N33 have been used as progenitors of neurons of the spiral ganglion. We demonstrated that the fibroblast growth factor 2 (FGF2)-induced proliferative action was dependent on SK1, SK2 as well as S1P₁ and S1P₂. Moreover, the pro-survival effect of FGF2 from apoptotic cell death induced by staurosporine treatment was dependent on SK but not on S1PR. Additionally, ERK1/2 and Akt signaling pathways were found to mediate the mitogenic and survival action of FGF2, respectively. Taken together, these findings demonstrate a crucial role for S1P signaling axis in the proliferation and the survival of otic vesicle neuroprogenitors, highlighting the identification of possible novel therapeutical approaches to prevent neuronal degeneration during hearing loss.

Sphingosine 1-phosphate signaling pathway in inner ear biology. New therapeutic strategies for hearing loss?

Hearing loss is one of the most prevalent conditions around the world, in particular among people over 60 years old. Thus, an increase of this affection is predicted as result of the aging process in our population. In this context, it is important to further explore the function of molecular targets involved in the biology of inner ear sensory cells to better individuate new candidates for therapeutic application. One of the main causes of deafness resides into the premature death of hair cells and auditory neurons. In this regard, neurotrophins and growth factors such as insulin like growth factor are known to be beneficial by favoring the survival of these cells. An elevated number of published data in the last 20 years have individuated sphingolipids not only as structural components of biological membranes but also as critical regulators of key biological processes, including cell survival. Ceramide, formed by catabolism of sphingomyelin (SM) and other complex sphingolipids, is a strong inducer of apoptotic pathway, whereas sphingosine 1-phosphate (S1P), generated by cleavage of ceramide to sphingosine and phosphorylation catalyzed by two distinct sphingosine kinase (SK) enzymes, stimulates cell survival. Interestingly S1P, by acting as intracellular mediator or as ligand of a family of five distinct S1P receptors (S1P₁–S1P₅), is a very powerful bioactive sphingolipid, capable of triggering also other diverse cellular responses such as cell migration, proliferation and differentiation, and is critically involved in the development and homeostasis of several organs and tissues. Although new interesting data have become available, the information on S1P pathway and other sphingolipids in the biology of the inner ear is limited. Nonetheless, there are several lines of evidence implicating these signaling molecules during neurogenesis in other cell populations. In this review, we discuss the role of S1P during inner ear development, also as guidance for future studies.

Defining the role of mesenchymal stromal cells on the regulation of matrix metalloproteinases in skeletal muscle cells

Recent studies indicate that mesenchymal stromal cell (MSC) transplantation improves healing of injured and diseased skeletal muscle, although the mechanisms of benefit are poorly understood. In the present study, we investigated whether MSCs and/or their trophic factors were able to regulate matrix metalloproteinase (MMP) expression and activity in different cells of the muscle tissue. MSCs in co-culture with C2C12 cells or their conditioned medium (MSC-CM) up-regulated MMP-2 and MMP-9 expression and function in the myoblastic cells; these effects were concomitant with the down-regulation of the tissue inhibitor of metalloproteinases (TIMP)-1 and -2 and with increased cell motility. In the single muscle fiber experiments, MSC-CM administration increased MMP-2/9 expression in Pax-7(+) satellite cells and stimulated their mobilization, differentiation and fusion. The anti-fibrotic properties of MSC-CM involved also the regulation of MMPs by skeletal fibroblasts and the inhibition of their differentiation into myofibroblasts. The treatment with SB-3CT, a potent MMP inhibitor, prevented in these cells, the decrease of α-smooth actin and type-I collagen expression induced by MSC-CM, suggesting that MSC-CM could attenuate the fibrogenic response through mechanisms mediated by MMPs. Our results indicate that growth factors and cytokines released by these cells may modulate the fibrotic response and improve the endogenous mechanisms of muscle repair/regeneration.

Sphingosine 1-phosphate axis: a new leader actor in skeletal muscle biology

Sphingosine 1-phosphate (S1P) is a bioactive lipid involved in the regulation of biological processes such as proliferation, differentiation, motility, and survival. Here we review the role of S1P in the biology and homeostasis of skeletal muscle. S1P derives from the catabolism of sphingomyelin and is produced by sphingosine phosphorylation catalyzed by sphingosine kinase (SK). S1P can act either intracellularly or extracellularly through specific ligation to its five G protein-coupled receptors (GPCR) named S1P receptors (S1PR). Many experimental findings obtained in the last 20 years demonstrate that S1P and its metabolism play a multifaceted role in the regulation of skeletal muscle regeneration. Indeed, this lipid is known to activate muscle-resident satellite cells, regulating their proliferation and differentiation, as well as mesenchymal progenitors such as mesoangioblasts that originate outside skeletal muscle, both involved in tissue repair following an injury or disease. The molecular mechanism of action of S1P in skeletal muscle cell precursors is highly complex, especially because S1P axis is under the control of a number of growth factors and cytokines, canonical regulators of skeletal muscle biology. Moreover, this lipid is crucially involved in the regulation of skeletal muscle contractile properties, responsiveness to insulin, fatigue resistance and tropism. Overall, on the basis of these findings S1P signaling appears to be an appealing pharmacological target for improving skeletal muscle repair. Nevertheless, further understanding is required on the regulation of S1P downstream signaling pathways and the expression of S1PR. This article will resume our current knowledge on S1P signaling in skeletal muscle, hopefully stimulating further investigation in the field, aimed at individuating novel molecular targets for ameliorating skeletal muscle regeneration and reducing fibrosis of the tissue after a trauma or due to skeletal muscle diseases.

Sphingosine-1-phosphate receptor 3 influences cell cycle progression in muscle satellite cells

Skeletal muscle retains a resident stem cell population called satellite cells, which are mitotically quiescent in mature muscle, but can be activated to produce myoblast progeny for muscle homeostasis, hypertrophy and repair. We have previously shown that satellite cell activation is partially controlled by the bioactive phospholipid, sphingosine-1-phosphate, and that S1P biosynthesis is required for muscle regeneration. Here we investigate the role of sphingosine-1-phosphate receptor 3 (S1PR3) in regulating murine satellite cell function. S1PR3 levels were high in quiescent myogenic cells before falling during entry into cell cycle. Retrovirally-mediated constitutive expression of S1PR3 led to suppressed cell cycle progression in satellite cells, but did not overtly affect the myogenic program. Conversely, satellite cells isolated from S1PR3-null mice exhibited enhanced proliferation ex-vivo. In vivo, acute cardiotoxin-induced muscle regeneration was enhanced in S1PR3-null mice, with bigger muscle fibres compared to control mice. Importantly, genetically deleting S1PR3 in the mdx mouse model of Duchenne muscular dystrophy produced a less severe muscle dystrophic phenotype, than when signalling though S1PR3 was operational. In conclusion, signalling though S1PR3 suppresses cell cycle progression to regulate function in muscle satellite cells.

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Expression of S1PR3 is associated with non-cycling myoblasts.

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Constitutive expression of S1PR3 leads to reduced cell proliferation.

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Satellite cells lacking S1PR3 have enhanced proliferation.

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Muscle regeneration is improved in the absence of S1PR3.

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The dystrophic phenotype in mdx mice is improved by the absence of S1PR3.

Role of sphingosine 1-phosphate in skeletal muscle cell biology

Studies performed in the last fifteen years have clearly established that the bioactive sphingolipid sphingosine 1-phosphate (S1P) affects various different biological properties of myogenic precursor cells as well as physiological features of adult skeletal muscle. Noticeably, in myogenic precursor cells multiple growth factors and cytokines cross-communicate with S1P axis and the engagement of distinct S1P receptor subtypes appears to be crucially implicated in transmitting specific biological effects. This paper summarizes current research findings and discloses the potential for new therapeutics designed to alter S1P signaling with the aim of improving skeletal muscle repair.