Sphingosine-1-phosphate receptor: a novel therapeutic target in ulcerative colitis

KEYWORDS:

• Sphingolipids
• inflammatory bowel disease
• gut damage
• lymphocyte trafficking
• S1P modulators

1. Introduction

Ulcerative colitis (UC) is a chronic inflammatory condition of the colon that affects several million people worldwide. UC is histologically characterized by mucosal infiltration with macrophages, lymphocytes, neutrophils and plasma cells, formation of crypt abscesses, and disruption of epithelial barrier. Patients with UC suffer from recurrent episodes of rectal bleeding and diarrhea and may have reduced quality of life and increased risk of developing colorectal cancer [1]. The underlying cause of UC is unknown, but there is evidence that the pathological process is triggered, in genetically predisposed individuals, by environmental insults, which promote an excessive mucosal immune response. Among environmental factors, diet seems to play a decisive role in the pathogenesis of UC [2].

Patients with mild-to-moderate UC are mainly treated with oral and rectal mesalamine and/or steroids, while patients with more severe forms need systemic steroids and/or biologics (i.e. antibodies against tumor necrosis factor [TNF] or integrins). Immunosuppressive drugs (i.e. azathioprine and 6-mercaptopurine) are also used to prevent flare-ups in steroid-dependent patients and (i.e. cyclosporine A) to control the active phases. However, not all the patients respond to these pharmacological approaches, and the use of such drugs (e.g. steroids, immunosuppressors, and biologics) can associate with enhanced risk of side effects [1]. This indicates the necessity of safer and more effective compounds.

2. Sphingosin-1-phosphate: synthesis and function

Sphingosin-1-phosphate (S1P) is a bioactive lipid generated through degradation of endogenous and dietary mammalian sphingolipids containing the long chain base sphingosine. S1P is formed by phosphorylation of sphingosine, a process that is catalyzed by two distinct sphingosine kinases (SPHKs) 1 and 2. S1P is synthesized by SPHK1 at the plasma membrane level and by SPHK2 in several intracellular compartments such as endoplasmic reticulum (ER), nucleus, and mitochondria [3]. Depending on the site of production, S1P can exert divergent functions [4]. For example, S1P produced by SPHK2 located in the ER is associated with inhibition of cell growth and induction of apoptosis, while that induced by SPHK1 promotes cell growth and inhibits apoptosis [5]. Once formed, S1P can be either dephosphorylated back to sphingosine by two specific S1P phosphatases (SPP1 and SPP2) [5] or irreversibly degraded to hexadecenal and phosphoethanolamine by S1P lyase. S1P controls many cellular functions, including cell adhesion, migration, proliferation and differentiation, the development, homeostasis, and functions of many organs and tissues. Although, as pointed out earlier, there are proposed intracellular roles for S1P, most of its effects are mediated by the activation of a family of five G-protein-coupled cell membrane receptors, termed S1P receptors (S1PR) [6] through an autocrine or paracrine manner [7]. Cellular and temporal expression of the S1PRs determines their specific roles in various organs. While S1PR1, 2 and 3 are ubiquitously expressed, S1PR4 is primarily expressed in lymphoid tissues, and S1PR5 shows restricted tissue distribution to brain and spleen. Binding of S1P to each of these receptors activates distinctive intracellular signals and cellular responses that are, in some cases, antagonistic. S1PR expressed in immune cells enable their egress from lymph nodes to lymph and plasma [8,9].

3. Role of S1P in the amplification of intestinal inflammatory processes

Accumulating evidence supports the hypothesis that S1P plays important roles in the pathogenesis of colitis. For instance, SPHK1 is overexpressed in inflamed colon of patients with UC as well as in colon of mice with dextran sodium sulfate (DSS)-induced colitis [10], a murine model that shows some clinical and immunomorphological similarities with UC. Moreover, increased levels of S1P have been documented in the systemic circulation of mice with DSS-colitis. Mice lacking SPHK1 are less susceptible to DSS-colitis [10], and colitic mice given pharmacologic inhibitors of SPHK exhibit attenuation of the ongoing intestinal inflammation [11]. These data fit with the demonstration that SPHK1 is activated by TNF-α, a major mediator of the UC-associated inflammation, and is required for mediating TNF-α inflammatory responses in cells [12]. On the other hand, gene targeting of S1P lyase in gut epithelium, which promotes S1P accumulation, or of other molecules involved in S1P metabolism (e.g. neutral ceramidase, intestinal alkaline sphingomyelinase) enhances the development of experimental colitis and colitis-associated colon cancer [13,14]. More recent studies have documented a divergent role of SPP1 and SPP2 in the control of gut inflammation. In particular, mice lacking SPP2, an isoenzyme that is mainly expressed in the gastrointestinal tract, develop reduced DSS-colitis, whereas deletion of ubiquitously expressed SPP1 associates with enhanced expression of interleukin (IL)-6, TNF-α, and IL-1β, activation of the transcription factor signal transducer and activator of transcription 3, immune cell infiltration into the colons, and exacerbation of colitis [15]. Analysis of SPP2 expression in human samples revealed enhanced expression of such enzyme in inflamed colon of UC patients [16]. S1P1 promotes blood vessel barrier function, and the increased levels of S1P in UC reflect increased vascular density in the inflamed mucosa of UC patients [15]. Genetic targeting of S1PR1 in mice increases colonic vascular permeability under basal conditions and rectal bleeding during experimental colitis, thus suggesting a role for S1P1 in maintaining colonic vascular integrity [15].

A proof-of-concept study showed that W-061, a S1PR1/5 modulator, is effective in reducing inflammation and epithelial damage in mice with DSS-colitis [17]. Along the same line is the demonstration that FTY720 (fingolimod) attenuates intestinal inflammation in mice with DSS-colitis, as well as in mice with UC-like oxazolone-mediated colitis model [18,19].

4. Agonists of S1PRs in UC

Various inhibitors of S1P synthesis and function have been developed for the treatment of human disorders. Among these, S1PR1 agonists induce internalization and degradation of the receptor, thus rendering lymphocytes incapable of migrating from secondary lymphoid organs with the downstream effect of reducing the number of cells circulating in the blood. One such a compound, fingolimod, has already been approved for the treatment of relapsing multiple sclerosis, and it is currently under investigation in UC. Treatment of patients with fingolimod may associate with adverse events, such as bradycardia, second-degree atrioventricular blocks, elevation of liver aminotransferase levels, and macular edema, perhaps due to the fact that this drug binds not only S1PR1 but also S1PR3, S1PR4, and S1PR5.

Another modulator of S1PR1 and S1PR5 is ozanimod (RPC-1063). Ozanimod has no activity on S1PR2, S1PR3, and S1PR4, and its oral administration was associated with clinical benefit in patients with relapsing multiple sclerosis. A randomized, double-blind, placebo-controlled, phase 2 of induction and maintenance was recently performed in patients with moderate-to-severe UC [20]. One hundred ninety-seven patients were randomly assigned to receive ozanimod at a dose of 0.5 or 1 mg or placebo daily; 186 out of the 197 patients completed the induction period (8 weeks). The primary end point was clinical remission at week 8; secondary end points at week 8 were clinical response and mucosal healing. At week 8, 103 patients who were considered by the investigators to have clinical improvement continued in the maintenance phase, and 91 of these 103 patients completed the trial (week 32). Exploratory outcomes included clinical remission, clinical response, and mucosal healing at week 32 and histologic remission at week 8 and week 32. The safety profile of the drug was also evaluated. At week 8, more patients treated with the highest dose of ozanimod achieved clinical remission as compared to those treated with placebo (16% vs. 9%). Similarly, the rate of clinical response was significantly greater in patients treated with 1 mg ozanimod (54%) than in those receiving placebo (37%). In contrast, differences in the primary outcome and clinical response between the group receiving 0.5 mg ozanimod and the placebo group were not significant. The anti-inflammatory effect of ozanimod was supported by the greater percentages of patients achieving mucosal healing and histologic remission. Ozanimod showed a good safety profile, even though four patients (one treated with 0.5 mg and three treated with 1 mg) had increased hepatic aminotransferase levels.

5. Conclusions

The data described in this article highlight the inflammatory role of S1P/S1PR signaling in the gut and support previous studies showing that compounds interfering with lymphocyte trafficking and/or blocking/attenuating mucosal inflammatory pathways are an effective therapeutic approach for UC patients. Further studies on larger populations are, however, needed to confirm the clinical benefit seen in patients treated with ozanimod, to examine whether other inhibitors of S1P are useful in the management of active phases of the disease and to identify which subsets of patients could benefit from such treatments. In this context, it would be useful to assess whether ozanimod and other inhibitors of S1P can be effective when given rectally to patients with distal colitis. So far, treatment with S1P modulators has been associated with some hepatic and cardiac effects, but adverse events were minimal following treatment with ozanimod. If this is due to the selective controls of S1PR1 and S1PR5 by ozanimod or reflects the short period of treatment of UC patients with ozanimod remains to be verified. In this context, it is also noteworthy that ozanimod treatment associates with marked decrease of the absolute lymphocyte count in the blood, raising the possibility that this approach can alter tissue immunosurveillance, thereby increasing the risk of infections and malignancies. This must be verified in future studies.

Declaration of interest

The authors have no relevant affiliations or financial involvement with any organization or entity with a financial interest in or financial conflict with the subject matter or materials discussed in the manuscript. This includes employment, consultancies, honoraria, stock ownership or options, expert testimony, grants or patents received or pending, or royalties.

Additional information

Funding

This paper was not funded.