Sphingosine-1-phosphate (S1P) in ovarian physiology and disease

Effects and action mechanism of gonadotropins on ovarian follicular cells: A novel role of Sphingosine-1-Phosphate (S1P). A review

Follicle-stimulating hormone (FSH) and luteinizing hormone (LH) control antral follicular growth by regulating several processes, such as the synthesis of hormones and signaling molecules, proliferation, survival, apoptosis, luteinization, and ovulation. To exert these effects, gonadotropins bind to their respective Gs protein-coupled receptors, activating the protein kinase A (PKA) pathway or recruiting Gq proteins to activate protein kinase C (PKC) signaling. Although the action mechanism of FSH and LH is clear, recently, it has been shown that both gonadotropins promote the synthesis of sphingosine-1-phosphate (S1P) in granulosa and theca cells through the activation of sphingosine kinase 1. Moreover, the inhibition of SPHKs reduces S1P synthesis, cell viability, and the proliferation of follicular cells in response to gonadotropins, and the addition of S1P to the culture medium increases the proliferation of granulosa and theca cells without apparent effects on sexual steroid synthesis. Therefore, we consider that S1P is a crucial signaling molecule that complements the canonical gonadotropin pathway to promote the proliferation and viability of granulosa and theca cells.

Sphingosine-1-phosphate mediates FSH-induced cell viability but not steroidogenesis in bovine granulosa cells

Follicle-stimulating hormone (FSH) stimulates the proliferation, survival, and estradiol synthesis of granulosa cells by binding to their G protein-coupled receptors. Although FSH activates sphingosine kinase-1 (SPHK1) to induce sphingosine-1-phosphate (S1P) synthesis, which is required to mediate the proliferative and survival effect of this gonadotrophin, the mechanisms, and the role of S1P in estradiol synthesis have not been reported. This study aimed to evaluate the importance of FSH-induced S1P synthesis as a mediator of the effects of this gonadotrophin on granulosa cell viability and steroidogenesis and to determine if FSH-induced S1P synthesis depends on estradiol, cAMP, PKA, or PKC. To achieve these objectives, we tested the effects of FSH, a sphingosine kinase-1 inhibitor (SKI-178), estradiol and inhibitors of aromatase, cAMP, PKA, and PKC (Formestane, MDL-12330A, H-89 dihydrochloride hydrate and Calphostin C respectively), on granulosa cell viability, S1P and estradiol production, and the mRNA expression of CYP19A1 and STAR in four in vitro culture experiments. The addition of FSH (1 ng/mL) increased (P < 0.05) granulosa cells number and S1P concentration in the culture media. Conversely, the addition of SKI-178 (10 μM) reduced (P < 0.05) S1P concentration negating the effect of FSH on cell viability. Inhibition of PKC and PKA, but not cAMP, reduced (P < 0.05) S1P secretion of FSH treated granulosa cells. It is important to note that the reduction in S1P secretion was strong (49 %) with the use of the PKC inhibitor. The use of formestane (10 μg) did not modify (P > 0.05) S1P secretion in FSH-treated cells; however, the addition of 5 or 10 ng/mL of estradiol increased (P < 0.05) S1P secretion. Finally, FSH increased (P < 0.05) estradiol concentration in the culture media, but this effect was not blocked by the inhibition of S1P synthesis. Similarly, FSH, SKI-178 or their combination did not modify the mRNA expression of CYP19A1 and STAR. In conclusion, S1P synthesis is stimulated FSH in granulosa cells and mediated mainly by PKC. S1P in turn promotes the granulosa cell viability, however, this does not influence estradiol synthesis. Additionally, estradiol synthesis induced by FSH is not essential for S1P synthesis, however high estradiol concentration may stimulate S1P production by granulosa cells.

The vascular endothelial growth factor (VEGF) system as a key regulator of ovarian follicle angiogenesis and growth

The vascular endothelial growth factor-A (VEGFA) system is a complex set of proteins, with multiple isoforms and receptors, including both angiogenic (VEGFxxx, VEGFR2) and antiangiogenic members (VEGFxxxb, VEGFR1 and soluble forms of VEGFR). The members of the VEGF system affect the proliferation, survival, and migration of endothelial and nonendothelial cells and are involved in the regulation of follicular angiogenesis and development. The production of VEGF by secondary follicles stimulates preantral follicular development by directly affecting follicular cells and promoting the acquisition of the follicular vasculature and downstream antrum formation. Additionally, the pattern of expression of the components of the VEGF system may provide a proangiogenic milieu capable of triggering angiogenesis and stimulating follicular cells to promote antral follicle growth, whereas, during atresia, this milieu becomes antiangiogenic and blocks follicular development.

Sphingosine-1-Phosphate Recruits Macrophages and Microglia and Induces a Pro-Tumorigenic Phenotype That Favors Glioma Progression

Glioblastoma is the most aggressive brain tumor in adults. Treatment failure is predominantly caused by its high invasiveness and its ability to induce a supportive microenvironment. As part of this, a major role for tumor-associated macrophages/microglia (TAMs) in glioblastoma development was recognized. Phospholipids are important players in various fundamental biological processes, including tumor-stroma crosstalk, and the bioactive lipid sphingosine-1-phosphate (S1P) has been linked to glioblastoma cell proliferation, invasion, and survival. Despite the urgent need for better therapeutic approaches, novel strategies targeting sphingolipids in glioblastoma are still poorly explored. Here, we showed that higher amounts of S1P secreted by glioma cells are responsible for an active recruitment of TAMs, mediated by S1P receptor (S1PR) signaling through the modulation of Rac1/RhoA. This resulted in increased infiltration of TAMs in the tumor, which, in turn, triggered their pro-tumorigenic phenotype through the inhibition of NFkB-mediated inflammation. Gene set enrichment analyses showed that such an anti-inflammatory microenvironment correlated with shorter survival of glioblastoma patients. Inhibition of S1P restored a pro-inflammatory phenotype in TAMs and resulted in increased survival of tumor-bearing mice. Taken together, our results establish a crucial role for S1P in fine-tuning the crosstalk between glioma and infiltrating TAMs, thus pointing to the S1P-S1PR axis as an attractive target for glioma treatment.

Role of sphingolipid metabolites in the homeostasis of steroid hormones and the maintenance of testicular functions

With the acceleration of life pace and the increase of work pressure, the problem of male infertility has become a social problem of general concern. Sphingolipids are important regulators of many cellular processes like cell differentiation and apoptosis, which are ubiquitously expressed in all mammalian cells. Various sphingolipid catabolic enzymes can generate multiple sphingolipids like sphingosine-1-phosphate and sphingomyelin. Present studies have already demonstrated the role of steroid hormones in the physiological processes of reproduction and development through hypothalamus-pituitary-gonad axis, while recent researches also found not only sphingolipids can modulate steroid hormone secretion, but also steroid hormones can control sphingolipid metabolites, indicating the role of sphingolipid metabolites in the homeostasis of steroid hormones. Furthermore, sphingolipid metabolites not only contribute to the regulation of gametogenesis, but also mediate damage-induced germ apoptosis, implying the role of sphingolipid metabolites in the maintenance of testicular functions. Together, sphingolipid metabolites are involved in impaired gonadal function and infertility in males, and further understanding of these bioactive sphingolipids will help us develop new therapeutics for male infertility in the future.

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.

The Use of Ceramides to Predict Metabolic Response to Metformin in Women With PCOS

Context

Polycystic ovarian syndrome (PCOS) is a complex disorder in which metabolic abnormalities are associated with reproductive dysfunction. Women with PCOS have increased ceramide concentrations. Previous studies demonstrated that treating metabolic abnormalities of PCOS with metformin improved glucose effectiveness after 12 weeks.

Objective

We evaluated whether, in women with PCOS, lower baseline ceramide, diacylglycerol (DAG), and triacylglycerol (TAG) concentrations were associated with improved metabolic response to metformin.

Methods

Women (n = 29), aged 29 ± 5 years and diagnosed with PCOS by the NIH criteria underwent an intravenous glucose tolerance test (IVGTT) before and after 12-week treatment with metformin (1500 mg per day). Metabolic responders were defined by improved glucose effectiveness, specifically, the ability of glucose to stimulate uptake and suppress production, after metformin treatment.

Results

Twelve weeks of metformin resulted in weight loss (-1.7 ± 2.6 kg, P < 0.01) and a reduction in BMI (-0.6 ± 0.9 kg/m², P < 0.01) with no change in HbA1c. The concentrations of Cer(d18:1/22:0), Cer(d18:1/24:0), total ceramides, total Cer(d16:0), total Cer(d18:2), DAG, dihydrosphingomyelin (DHSM), and TAG decreased after metformin treatment (P < 0.05). Baseline total Cer(d16:0) concentration <204.1 pmol/mL was 82% sensitive (AUC 0.72, P = 0.03) and total DHSM concentration <32237 pmol/mL was 100% specific (AUC 0.73, P = 0.03) in predicting improved metabolic response to metformin, as measured by IVGTT.

Conclusion

Lower total Cer(16:0) and DHSM concentrations are associated with a beneficial metabolic response to metformin in women with PCOS. Based on the known association between higher ceramide levels and type 2 diabetes, the data suggest that metformin improves metabolic parameters in women with mild metabolic derangements.

Novel functions of S1P in chronic itchy and inflammatory skin diseases

S1P is a pleotropic sphingolipid signalling molecule that acts through binding to five high-affinity G-protein coupled receptors. S1P-signaling affects cell fate in a multitude of ways, e.g. influencing cell differentiation, proliferation, and apoptosis, as well as playing an important role in immune cell trafficking. Though many effects of S1P-signaling in the human body have been discovered, the full range of functions is yet to be understood. For inflammatory skin diseases such as atopic dermatitis and psoriasis, evidence is emerging that dysfunction and imbalance of the S1P-axis is a contributing factor. Multiple studies investigating the efficacy of S1PR modulators in alleviating the severity and symptoms of skin conditions in various animal models and human clinical trials have shown promising results and validated the interest in the S1P-axis as a potential therapeutic target. Even though the involvement of S1P-signalling in inflammatory skin diseases still requires further clarification, the implications of the recent findings may prompt expansion of research to additional skin conditions and more S1P-axis modulatory pharmaceuticals.

Sphingolipids as multifaceted mediators in ovarian cancer

Ovarian cancer is the most lethal gynaecological malignancy. It is commonly diagnosed at advanced stage when it has metastasised to the abdominal cavity and treatment becomes very challenging. While current standard therapy involving debulking surgery and platinum + taxane-based chemotherapy is associated with high response rates initially, the large majority of patients relapse and ultimately succumb to chemotherapy-resistant disease. In order to improve survival novel strategies for early detection and therapeutics against treatment-refractory disease are urgently needed. A promising new target against ovarian cancer is the sphingolipid pathway which is commonly hijacked in cancer to support cell proliferation and survival and has been shown to promote chemoresistance and metastasis in a wide range of malignant neoplasms. In particular, the sphingosine kinase 1-sphingosine 1-phosphate receptor 1 axis has been shown to be altered in ovarian cancer in multiple ways and therefore represents an attractive therapeutic target. Here we review the roles of sphingolipids in ovarian cancer progression, metastasis and chemoresistance, highlighting novel strategies to target this pathway that represent potential avenues to improve patient survival.

Sphingosine-1-phosphate and its mimetic FTY720 do not protect against radiation-induced ovarian fibrosis in the nonhuman primate†

Oocytes are highly radiosensitive, so agents that prevent radiation-induced ovarian follicle destruction are important fertility preservation strategies. A previous study in rhesus macaques demonstrated that ovarian treatment with antiapoptotic agents, sphingosine-1-phosphate (S1P) and FTY720, its long-acting mimetic, preserved follicles following a single dose of 15 Gy X-ray radiation, and live offspring were obtained from FTY720-treated animals. However, it is unknown whether these antiapoptotic agents also protected the ovarian stroma from late effects of radiation, including vascular damage and fibrosis. Using ovarian histological sections from this study, we evaluated the vasculature and extracellular matrix in the following cohorts: vehicle + sham irradiation, vehicle + irradiation (OXI), S1P + irradiation (S1P), and FTY720 + irradiation (FTY720). One ovary from each animal was harvested prior to radiation whereas the contralateral ovary was harvested 10 months post-treatment. We assessed vasculature by immunohistochemistry with a PECAM1 antibody, hyaluronan by a hyaluronan binding protein assay, and collagen by picrosirius red and Masson's trichrome staining. Disorganized vessels were observed in the medulla in the OXI and S1P cohorts relative to the sham, but the vasculature in the FTY720 cohort appeared intact, which may partially explain fertoprotection. There were no differences in the hyaluronan matrix among the cohorts, but there was thickening of the tunica albuginea and fibrosis in the OXI cohort relative to the sham, which was not mitigated by either S1P or FTY720 treatment. Thus, the fertoprotective properties of S1P and FTY720 may be limited given their inability to protect the ovarian stroma against the late effects of radiation-induced fibrosis.

Sphingosine-1 phosphate induces cAMP/PKA-independent phosphorylation of the cAMP response element-binding protein (CREB) in granulosa cells

BACKGROUND AND AIMS

Sphingosine-1 phosphate (S1P) is a lysosphingolipid present in the ovarian follicular fluid. The role of the lysosphingolipid in gonads of the female is widely unclear. At nanomolar concentrations, S1P binds and activates five specific G protein-coupled receptors (GPCRs), known as S1P_1-5, modulating different signaling pathways. S1P₁ and S1P₃ are highly expressed in human primary granulosa lutein cells (hGLC), as well as in the immortalized human primary granulosa cell line hGL5. In this study, we evaluated the signaling cascade activated by S1P and its synthetic analogues in hGLC and hGL5 cells, exploring the biological relevance of S1PR-stimulation in this context.

METHODS AND RESULTS

hGLC and hGL5 cells were treated with a fixed dose (0.1 μM) of S1P, or by S1P₁- and S1P₃-specific agonists SEW2871 and CYM5541. In granulosa cells, S1P and, at a lesser extent, SEW2871 and CYM5541, potently induced CREB phosphorylation. No cAMP production was detected and pCREB activation occurred even in the presence of the PKA inhibitor H-89. Moreover, S1P-dependent CREB phosphorylation was dampened by the mitogen-activate protein kinase (MEK) inhibitor U0126 and by the L-type Ca²⁺ channel blocker verapamil. The complete inhibition of CREB phosphorylation occurred by blocking either S1P₂ or S1P₃ with the specific receptor antagonists JTE-013 and TY52156, or under PLC/PI3K depletion. S1P-dependent CREB phosphorylation induced FOXO1 and the EGF-like epiregulin-encoding gene (EREG), confirming the exclusive role of gonadotropins and interleukins in this process, but did not affect steroidogenesis. However, S1P or agonists did not modulate granulosa cell viability and proliferation in our conditions.

CONCLUSIONS

This study demonstrates for the first time that S1P may induce a cAMP-independent activation of pCREB in granulosa cells, although this is not sufficient to induce intracellular steroidogenic signals and progesterone synthesis. S1P-induced FOXO1 and EREG gene expression suggests that the activation of S1P-S1PR axis may cooperate with gonadotropins in modulating follicle development.

Sphingosine 1-phosphate protects against radiation-induced ovarian injury in female rats-impact on mitochondrial-related genes

The toxic effects of ionizing radiation on the gonads have been widely recognized. Sphingosine 1-phosphate (S1P) has a protective effect on ovarian injury, and although it is known that mitochondria are involved in this process, the specific mechanism is not fully understood. The present study analysed the changes in the serum AMH and ovarian histology in Sprague-Dawley female rats exposed to X-ray radiation only or co-administered with S1P. The mRNA expression profile of ovarian tissue was further analysed via next-generation sequencing and bioinformatics approaches to screen out candidate mitochondria-related genes. Finally, differentially expressed target genes were verified by real-time PCR. The results showed that ionizing radiation could reduce the serum AMH level, destroy ovarian structure and decrease the number of follicles in rats, while S1P administration significantly attenuated the impairment of ovarian function. Gene ontology (GO) and KEGG pathway analysis revealed that a variety of genes related to mitochondrial function were differentially expressed, and the protective effect of S1P on mitochondria was more obvious in the acute phase 24 h after radiation. The differentially expressed mitochondrial function-related genes associated with the protective effect of S1P were UQCRH, MICU2 and GPX4, which were subsequently verified by RT-PCR. Therefore, ionizing radiation has a significant effect on ovarian function, and S1P has a protective effect on radiation-induced ovarian injury, in which mitochondria may play an important role. This study sheds new light on the mechanism of radiation-induced ovarian injury and helps develop a novel potential strategy to control it.

Structure-function analysis of lipid substrates and inhibitors of sphingosine kinases

The sphingosine kinases, SK1 and SK2, catalyse the formation of the bioactive signalling lipid, sphingosine 1-phosphate (S1P), from sphingosine. SK1 and SK2 differ in their subcellular localisation, trafficking and regulation, but the isoforms are also distinct in their selectivity toward naturally occurring and synthetic ligands as substrates and inhibitors. To date, only the structure of SK1 has been determined, and a structural basis for selectivity differences in substrate handling by SK2 has yet to be established. Here we present a structural rationale, based on homology modelling and ligand docking, to account for the capacity of SK2, but not SK1, to efficiently process the pharmacologically active substances, fingolimod (FTY720) and safingol, as substrates. We propose that two key residue differences in hSK2 (Ser305/Thr584 in place of Ala175/Ala339 in hSK1) facilitate conformational switching in the lipid head group anchor residue, Asp308 (corresponding to Asp178 in hSK1), to accommodate substrate diversity for SK2. Our analysis accounts for the contrasting behaviour of fingolimod and safingol as non-turnover inhibitors of SK1, but substrates for SK2, and the observed stereoselectivity for phosphorylation of the pro-S hydroxymethyl group of fingolimod to generate (S)-FTY720-P in vivo. We also rationalise why methylation of the pro-R hydroxymethyl of FTY720 switches the behaviour of the resulting compound, (R)-FTY720 methyl ether (ROMe), to SK2-selective inhibition. Whilst the pharmacological significance of (S)-FTY720-P is firmly established, as the active principle of fingolimod in treating relapsing-remitting multiple sclerosis, the potential importance of SK-mediated phosphorylation of other substrates, such as safingol and non-canonical naturally occuring substrates such as (4E,nZ)-sphingadienes, is less widely appreciated. Thus, the contribution of SK2-derived safingol 1-phosphate to the anti-cancer activity of safingol should be considered. Similarly, the biological role of sphingadiene 1-phosphates derived from plant-based dietary sphingadienes, which we also show here are substrates for both SK1 and SK2, merits investigation.

Recent advances in the role of sphingosine 1-phosphate in cancer

Sphingosine 1-phosphate (S1P) is a bioactive lipid that binds to a family of G protein-coupled receptors (S1P_1-5 ) and intracellular targets, such as HDAC1/2, that are functional in normal and pathophysiologic cell biology. There is a significant role for sphingosine 1-phosphate in cancer underpinning the so-called hallmarks, such as transformation and replicative immortality. In this review, we survey the most recent developments concerning the role of sphingosine 1-phosphate receptors, sphingosine kinase and S1P lyase in cancer and the prognostic indications of these receptors and enzymes in terms of disease-specific survival and recurrence. We also provide evidence for identification of new therapeutic approaches targeting sphingosine 1-phosphate to prevent neovascularisation, to revert aggressive and drug-resistant cancers to more amenable forms sensitive to chemotherapy, and to induce cytotoxicity in cancer cells. Finally, we briefly describe current advances in the development of isoform-specific inhibitors of sphingosine kinases for potential use in the treatment of various cancers, where these enzymes have a predominant role. This review will therefore highlight sphingosine 1-phosphate signalling as a promising translational target for precision medicine in stratified cancer patients.