Serum sphingosine-1-phosphate levels and Sphingosine-1-Phosphate gene polymorphisms in acute respiratory distress syndrome: a multicenter prospective study.

BACKGROUND: Sphingosine-1-phosphate (S1P) is a signaling phospholipid involved in pathophysiologic progression of acute respiratory distress syndrome (ARDS) through its roles in endothelial barrier function and immune modulation. We hypothesized that decreased serum S1P level is associated with the clinical outcomes of ARDS and polymorphisms in the S1P gene are associated with serum S1P levels.
METHODS: This multicenter prospective study includes ARDS patients and healthy blood donors as controls. Serum S1P levels were quantified using enzyme-linked immunosorbent assays. Eight tag single nucleotide polymorphisms (SNPs) in the S1P gene were detected, and their associations with S1P levels were evaluated.
RESULTS: A total of 121 ARDS patients and 100 healthy individuals were enrolled. Serum S1P levels were lower in ARDS patients than in controls (P < 0.001). Decreased S1P levels correlated with more organ dysfunction and higher Acute Physiology and Chronic Health Evaluation II scores. Changes in S1P levels in ARDS patients were associated with the clinical outcomes. The recessive model for SNP rs3743631 suggests that GG homozygote is associate with a higher risk for ARDS. The dominant model for SNP rs907045 suggests that AA or TA genotype might increase the risk for ARDS. In ARDS patients, the rs3743631 GG genotype showed lower S1P levels than those harboring AG and AA genotypes. The serum S1P levels of rs907045 AA or TA genotype patients were lower than those of TT genotype.
CONCLUSIONS: Serum S1P levels are dramatically decreased in ARDS patients. Reduced S1P levels are associated with worse clinical outcomes. There is a significant association between S1P rs3743631, rs907045 polymorphisms and susceptibility of ARDS.

Background

Acute respiratory distress syndrome (ARDS) is an acute life-threatening inflammatory lung injury characterized by severe hypoxemia and stiff lungs due to diffuse alveolar injury and immune cell infiltration [1, 2]. Although vast improvements have been made in ARDS treatment in the past decade, the mortality rate of patients with severe ARDS remains unacceptable at 45% [1]. Therefore, early identification of patients at risk is crucial to allow them to benefit from timely treatment. The pathophysiologic causes underlying ARDS include endothelial barrier disruption, dysregulated cytokine secretion, and profound lung inflammatory response [3, 4], all of which are potentially regulated by sphingosine-1-phosphate (S1P).

S1P, a biologically active lipid, signals through specific G-protein-coupled receptors (receptors 1–5, S1PR1-5) and elicits multiple physiologic functions, especially in the vascular and immune systems [5-7]. S1P is a potent barrier-enhancing agent and the major endothelial chemotactic factor present in the serum [8, 9]. Previous evidence has demonstrated that the circulating S1P level is significantly lower in the presence of sepsis [10] and acute dengue infection [11], in which inflammation and endothelial damage play causal roles. In an acute lung injury (ALI) model, S1P delivery reduced vascular leak and attenuated oxygenation impairment [12]. In animal models of infection-induced micro-vascular endothelial dysfunction, administration of S1P analogues stabilized the micro-vascular endothelium, reduced excessive cytokine production and improved the outcome [13, 14]. Considering these observations, S1P could be a critical factor for preserving the endothelial barrier integrity and eliciting immunomodulatory effects against the progression of ARDS.

The mechanisms underlying ARDS are complex and may involve a variety of processes influenced by genetic factors. S1PR3 gene polymorphism has been reported to constitute risk factors for ARDS [15]. We thus speculated that the polymorphism of S1P was associated with the risk of ARDS. It is unknown whether differences in serum S1P levels among patients are associated with polymorphisms in the S1P gene. Therefore, we postulated that (1) serum S1P levels are substantially decreased in ARDS patients and could be a valuable predictor of disease severity, and (2) genetic differences in the S1P gene reflect the serum S1P levels.

Methods

Patients and controls

All participants or their surrogate care providers gave written informed consent. The protocol was approved by the ethics committee of Jinling Hospital and Nanjing First Hospital (Approval No.: JLYY: 2013021).

This was a prospective, multicenter, observational cohort study performed in four intensive care units (ICUs) (two respiratory ICUs, one medical ICU, and one emergency ICU) at two tertiary-care, university-affiliated hospitals in Nanjing, China (Jinling Hospital and Nanjing First Hospital). Patients aged ≥ 18 years who were admitted to one of the four participating ICUs with identified ARDS from January 2018 to August 2019 were considered eligible for inclusion in this study.

All admitted patients with invasive mechanical ventilation (IMV) were screened for eligibility by senior intensive care physicians. ARDS patients were identified by physicians blinded to S1P levels and S1P genotypes, based on the criteria from the Berlin definition [1]. Patients with diffuse alveolar hemorrhage, chronic lung disease other than chronic obstructive pulmonary disease or asthma, directive to withhold intubation, and immunosuppression not secondary to corticosteroids, and those treated with granulocyte colony-stimulating factor were excluded. Patients were also ruled out if they had experienced a cardiac arrest before enrollment, had died or were discharged within 48 h of ICU admission, were admitted for uncomplicated overdose, or had been admitted to the ICU for ≥ 3 days before enrollment.

The control cohort consisted of 100 healthy blood donors. Healthy controls were defined as individuals without any recent acute illness or any chronic illness requiring evaluation by physicians. The controls were kept anonymous, and only age, gender and ethnicity were collected.

Clinical evaluations and assays

The patients’ demographic and baseline clinical characteristics, including age, gender, ethnicity, medical history and vital signs were recorded at enrollment. Within 24 h after admission, blood samples were obtained from ARDS patients as soon as possible after confirming that they met the inclusion criteria for S1P measurement, as were baseline levels. The blood samples were also used to determine other clinical parameters. To stratify the distinct time points during the illness, blood samples were collected on day 7 if the patients were alive at this time point. The primary outcome was all-cause in-hospital mortality and the secondary outcome was ventilator-free days (VFDs). The number of VFDs was defined as the number of days from day 1 to day 28 during which a patient had been breathing without assistance for at least 48 consecutive hours. Patients with ≥ 28 ventilator days and non-survivors were considered to have 0 VFD.

Serum preparation and S1P measurements

After coagulation at 4 °C, blood samples were cleared by centrifugation and serum was immediately frozen and stored at − 80 °C until S1P measurement. The measurements were carried out blindly in duplicates using enzyme-linked immunosorbent assays kits (Echelon Biosciences, Inc., Salt Lake City, UT, USA).

SNPs selection and genotyping

The Single nucleotide polymorphisms (SNPs) were selected using information from the GenBank and HapMap databases. We selected tag SNPs in the genomic region including the S1P gene and 2000 bp upstream and downstream, with the minimum allele frequency set at 5% and r2 at 0.8. The selected SNPs were located within the coding region, 5′ untranslated region (UTR) and 3′ UTR of the S1P gene. A total of eight tag SNPs with representativeness were genotyped.

Genomic DNA was extracted from the whole blood with the QIAamp DNA Blood Mini Kit (Qiagen, Berlin, Germany) using standard procedures. The selected tag SNPs were genotyped using the improved Multiple Ligase Detection Reaction assay technology on an ABI Prism 377 Sequence Detection System (Applied Biosystems, Foster City, CA, USA) with technical support from the Shanghai Genesky Biotechnology Company. To ensure the accuracy of genotyping, negative controls were included in each plate. Genotyping was performed by investigators blinded to clinical status.

Statistical analysis

Continuous variables were described as median (interquartile range [IQR]). For continuous variables, the Mann–Whitney U test or Kruskal–Wallis analysis were used to compare groups. Categorical variables summarized as proportions were compared using Pearson’s Chi square or Fisher’s exact test. The relationship between two variables was assessed using Pearson correlation analysis and Spearman rank analysis. Receiver operating characteristic (ROC) curves were constructed and the areas under the ROC curves (AUCs) were calculated. Diagnostic AUCs were compared using the Z-test. The genotypic data of each SNP was assessed in terms of Hardy–Weinberg equilibrium by using the Chi square goodness-of-fit test. The additive model, codominant model, dominant model and recessive model were used to compare the difference in genotype distribution between patients and controls. The strength of association between S1P polymorphism and the risk of ARDS was evaluated by odds ratio (OR) and 95% confidence interval (CI). Statistical analysis was performed using the SPSS 24.0 software and the GraphPad Prism 7 software. P < 0.05 was considered statistically significant.

Results

Serum S1P levels were decreased in ARDS patients

During the study period, 332 admitted patients with IMV were screened. According to the inclusion and exclusion criteria, 121 ARDS patients were enrolled for analysis. The detailed demographic and clinical characteristics of the study patients are provided in Table 1.

Table 1

Demographic and clinical characteristics of ARDS patients

Characteristic	Value (n = 121)
Age, years	61.0 (43.5–69.0)
Male sex	84 (69.4)
Current smokers	35 (28.9)
ARDS risk factor
Sepsis	71 (58.7)
Pneumonia	22 (18.2)
Aspiration	12 (11.6)
Acute pancreatitis	16 (15.7)
Vasopressors use at admission	36 (29.8)
Coexisting conditions
Hypertension	33 (27.3)
Diabetes	16 (13.2)
Coronary heart disease	11 (9.1)
Cerebrovascular disease	15 (12.4)
Chronic renal disease	20 (16.5)
COPD or asthma	11 (9.1)
Cancer	11 (9.1)
Laboratory values on diagnosis of ARDS
WBC count, × 109/L	12.7 (10.14–17.5)
Hematocrit, %	28.0 (23.5–34.1)
Platelet count, × 109/L	174 (92.5–259)
Serum bilirubin, μmol/L	26.3 (12.5–49.8)
Serum creatinine, μmol/L	120.6 (69.5–173.9)
Serum albumin, g/L	29.4 (27.3–31.95)
APACHE II score	25 (21–29)
No. of organ failures	2 (0–3)
Lowest PaO2/FiO2 ratio	170.4 (124.0–257.0)
Berlin categories
Mild	44 (36.4)
Moderate	56 (46.3)
Severe	21 (17.4)
Days in ICU	16.0 (10.0–29.5)
Ventilator-free days	12.0 (7.0–21.0)
Death in hospital	33 (27.3)

Continuous variables are presented as median (interquartile range); categorical variables are presented as No.(%)

ARDS Acute respiratory distress syndrome, WBC white blood cell, APACHE II Acute Physiology and Chronic Health Evaluation II, No. of organ failures includes only non-pulmonary organ failures; ICU intensive care unit

Serum S1P levels were measured in 121 ARDS patients and 100 healthy controls. Both groups showed a male predominance; however, the age and gender distribution did not differ between the groups (Table 2). Patients with ARDS had significantly lower serum S1P levels (median 303.0 nmol/L, IQR 221.3–418.5 nmol/L) than controls (median 930.5 nmol/L, IQR 733.2–1153.4 nmol/L, P < 0.001) (Fig. 1a). Gender bias was not detected in both controls and patients (Fig. 1b). ARDS patients also had significantly lower red blood cells (RBC) count, platelet count, albumin level, and high-density lipoprotein (HDL) level (Table 2).

Table 2

Characteristics of study groups

Variables	Control (n = 100)	ARDS (n = 121)	P value
Age, years	51.0 (39.75–65.75)	61.0 (43.5–69.0)	n.s.
Male/Female	66/34 (66.0/34.0)	84/37 (69.4/30.6)	n.s.
Ethnicity			n.s.
Hans	100 (100)	121 (100)
Serum-S1P, nmol/L	930.5 (733.2–1153.4)	303.0 (221.3–418.5)	< 0.001
RBC count, × 1012/L	4.5 (3.89–5.063)	2.8 (2.475–3.1)	< 0.001
Platelet count, × 109/L	199.5 (150.3–253.8)	174 (92.5–259)	0.025
Serum albumin, g/L	40 (36–44.28)	29.4 (27.3–31.95)	< 0.001
HDL, mmol/L	1.40 (1.04–2.13)	0.57 (0.36–0.89)	< 0.001

Continuous variables are presented as median (interquartile range); categorical variables are presented as No. (%)

ARDS Acute respiratory distress syndrome, n.s. non-significant, S1P sphingosine-1-phosphate, RBC red blood cell, HDL high-density lipoprotein

Fig. 1

a ARDS patients had significantly lower mean serum S1P levels than controls (P < 0.001). b The two study groups did not differ in terms of gender distribution. ARDS acute respiratory distress syndrome, S1P sphingosine-1-phosphate, n.s. non-significant

ARDS patients were then divided into three groups (mild, moderate, and severe) according to decreasing PaO2/FiO2 ratio. However, the S1P levels did not correlate with worsening Berlin oxygenation categories (P = 0.135) (Fig. 2a). The baseline levels of S1P differed with the underlying risk factor for the development of ARDS. Specifically, patients with sepsis as the primary cause of ARDS had lower S1P levels (Fig. 2b).

Fig. 2

a S1P levels did not correlate with worsening Berlin oxygenation categories (P = 0.135). b Patients with sepsis as the primary cause of ARDS had lower S1P levels. ARDS acute respiratory distress syndrome, S1P sphingosine-1-phosphate, PF PaO2/FiO2 ratio

S1P Levels were associated with disease severity

We, subsequently, analyzed the potential correlation between serum S1P levels and organ failures and Acute Physiology and Chronic Health Evaluation II (APACHE II) scores in ARDS patients. Interestingly, Spearman correlation analysis showed that the number of organ failures was negatively correlated with the baseline S1P levels (r = − 0.2477, P = 0.006). Decreasing S1P levels correlated with increasing number of organ failures (Fig. 3a). The APACHE II score was determined to evaluate the disease severity in the ICU [16, 17]. We also observed that the baseline serum S1P levels in ARDS patients demonstrated a high and significantly negative linear correlation with the APACHE II scores (r = − 0.286, P = 0.002), whereas the S1P levels on day 7 showed a weaker, albeit significant, negative linear correlation with disease severity (r = − 0.219, P = 0.034) (Figs. 3b, c).

Fig. 3

a S1P levels were associated with the number of non-pulmonary organ failures (P = 0.001). b APACHE II scores were highly negatively correlated with baseline serum S1P levels in ARDS patients. c APACHE II scores showed a weaker negative correlation with day 7 S1P levels in ARDS patients. ARDS acute respiratory distress syndrome, S1P sphingosine-1-phosphate, APACHE II acute physiology and chronic health evaluation II

Decreased serum S1P levels may reflect disease outcomes

To investigate whether changes in S1P levels in ARDS patients were associated with disease progression and outcome, we analyzed S1P levels in 94 patients for whom both baseline and day 7 serum S1P levels were available. These 94 ARDS patients were divided into three groups according to disease outcomes: patients who were discharged from the hospital within 28 days, patients who were discharged from the hospital after more than 28 days, and patients who died. Intriguingly, in the group of patients discharged from the hospital within 28 days, the serum S1P levels on day 7 significantly increased compared with the levels on day 1 (P = 0.008) (Fig. 4a). However, in the groups of patients hospitalized for a longer period or those who died, serum S1P levels remained low during the early stage of illness.

Fig. 4

a Baseline and day 7 S1P serum levels in ARDS patients who died (n = 18), in those who were hospitalized for>28 days (n = 27) and in those who were hospitalized for ≤ 28 days (n = 49). b S1P levels were lower in non-survivors than in survivors (P < 0.001). ARDS acute respiratory distress syndrome, S1P sphingosine-1-phosphate

We further analyzed whether S1P levels were related to fatal outcomes. The baseline serum S1P levels were lower in non-survivors (median 224.6 nmol/L, IQR 165.6-331.5 nmol/L) than in survivors (median 337.7 nmol/L, IQR 258.1–445.2 nmol/L, P < 0.001) (Fig. 4b). To evaluate the potential of S1P level for predicting mortality in ARDS patients, a ROC curve analysis was performed. S1P level had moderately good ability to indicate mortality with an identical AUC of 0.70 (95% CI 0.59–0.80, P = 0.001). On analysis of ROC, the AUCs for APACHE II score was 0.73 and PF ratio was 0.55, whereas the combination of S1P either with APACHE II score and PF ratio yielded the AUCs of 0.80 and 0.77, respectively (Fig. 5).

Fig. 5

The ROC curve depicted an improvement in the discriminatory ability of the severity categories based on the APACHE II score (a) and PF ratio (b) with the addition of S1P level to the model. The AUC increased from 0.73 to 0.80 (P > 0.05) and 0.55 to 0.77 (P < 0.01), respectively. ROC receiver operating characteristic, APACHE II Acute Physiology and Chronic Health Evaluation II, PF PaO2/FiO2, S1P sphingosine-1-phosphate, AUC areas under the curve

Association between S1P genetic variants and risk of ARDS

We analyzed the genotype and allele distribution of SNPs in the ARDS and control groups (Table 3). The genotype frequencies of the studied polymorphisms in control individuals were in Hardy–Weinberg equilibrium. The minor allele frequencies of all of the SNPs in this group were > 5%.

Table 3

Genotype distribution and allele frequency of the tested SNPs in patients and controls

SNP	Model	Genotype	Case	Control	OR (95% CI)	P value
rs1049884	Additive	T/T	2	1	1.173 (0.684–2.009)	0.5619
		A/T	37	28
		A/A	82	71
	Codominant	T/T	2	1	1.732 (0.1538–19.5)	0.6567
		A/T	37	28	1.144 (0.6376–2.053)	0.6517
		A/A	82	71	–	–
	Dominant	T/T,A/T	39	29	1.164 (0.6545–2.072)	0.6045
		A/A	82	71
	Recessive	T/T	2	1	1.664 (0.1487–18.62)	0.6795
		A/A,A/T	119	99
	Allele	T	41	30	1.156 (0.6918–1.931)	0.5801
		A	201	170
rs11550470	Additive	T/T	1	0	1.499 (0.713–3.151)	0.2857
		C/T	19	12
		C/C	101	88
	Codominant	T/T	1	0	NA (NA–NA)	NA
		C/T	19	12	NA (NA–NA)	NA
		C/C	101	88	–	–
	Dominant	T/T,C/T	20	12	1.452 (0.6719–3.138)	0.3427
		C/C	101	88
	Recessive	T/T	1	0	NA (NA–NA)	NA
		C/C,C/T	120	100
	Allele	T	21	12	1.489 (0.7135–3.106)	0.289
		C	221	188
rs11607	Additive	T/T	17	23	0.7049 (0.4887–1.017)	0.06143
		C/T	50	42
		C/C	54	35
	Codominant	T/T	17	23	0.4791 (0.2246–1.022)	0.05691
		C/T	50	42	0.7716 (0.4273–1.393)	0.3898
		C/C	54	35	–	–
	Dominant	T/T,C/T	67	65	0.6681 (0.3873–1.153)	0.1471
		C/C	54	35
	Recessive	T/T	17	23	0.5472(0.2737–1.094)	0.08804
		C/C,C/T	104	77
	Allele	T	84	88	0.6766 (0.4606–0.9941)	0.04658
		C	158	112
rs12933523	Additive	A/A	2	1	1.875(0.8935–3.934)	0.09646
		G/A	19	8
		G/G	100	91
	Codominant	A/A	2	1	1.82 (0.1623–20.41)	0.6273
		G/A	19	8	2.161 (0.9023–5.177)	0.08376
		G/G	100	91	–	–
	Dominant	A/A,G/A	21	9	2.123 (0.9251–4.874)	0.0757
		G/G	100	91
	Recessive	A/A	2	1	1.664 (0.1487–18.62)	0.6795
		G/G,G/A	119	99
	Allele	A	23	10	1.995(0.9263–4.299)	0.07766
		G	219	190
rs2280026	Additive	C/C	14	7	1.044 (0.6981–1.56)	0.8355
		T/C	42	42
		T/T	65	51
	Codominant	C/C	14	7	1.569 (0.5898–4.175)	0.3668
		T/C	42	42	0.7846 (0.4467–1.378)	0.3987
		T/T	65	51	–	–
	Dominant	C/C,T/C	56	49	0.8967 (0.5275–1.524)	0.6871
		T/T	65	51
	Recessive	C/C	14	7	1.738 (0.673–4.49)	0.2534
		T/T,T/C	107	93
	Allele	C	70	56	1.047 (0.6908–1.585)	0.8301
		T	172	144
rs3743631	Additive	G/G	16	4	1.638 (1.069–2.509)	0.02341
		A/G	55	44
		A/A	50	52
	Codominant	G/G	16	4	4.16 (1.301–13.3)	0.01623
		A/G	55	44	1.3 (0.7464–2.264)	0.354
		A/A	50	52	–	–
	Dominant	G/G,A/G	71	48	1.538(0.9021–2.623)	0.1137
		A/A	50	52
	Recessive	G/G	16	4	3.657 (1.181–11.32)	0.02452
		A/A,A/G	105	96
	Allele	G	87	52	1.598 (1.059–2.409)	0.02542
		A	155	148
rs907045	Additive	A/A	4	1	1.831 (1.055–3.177)	0.03147
		T/A	38	21
		T/T	79	78
	Codominant	A/A	4	1	3.949 (0.4317–36.13)	0.2239
		T/A	38	21	1.787 (0.963–3.314)	0.06569
		T/T	79	78	–	–
	Dominant	A/A,T/A	42	22	1.885 (1.031–3.446)	0.03945
		T/T	79	78
	Recessive	A/A	4	1	3.385 (0.3722–30.78)	0.279
		T/T,T/A	117	99
	Allele	A	46	23	1.806(1.052–3.1)	0.03196
		T	196	177
rs9922601	Additive	A/A	6	6	1.099 (0.6956–1.738)	0.685
		T/A	33	22
		T/T	82	72
	Codominant	A/A	6	6	0.878 (0.2712–2.843)	0.8283
		T/A	33	22	1.317 (0.7046–2.462)	0.3881
		T/T	82	72	–	–
	Dominant	A/A,T/A	39	28	1.223 (0.6851–2.183)	0.496
		T/T	82	72
	Recessive	A/A	6	6	0.8174 (0.2552–2.618)	0.7342
		T/T,T/A	115	94
	Allele	A	45	34	1.115 (0.6826–1.822)	0.6632
		T	197	166

SNP single nucleotide polymorphisms, OR odds ratio, CI confidence interval

Genotypic differences in rs3743631 between the case and control groups were statistically significant (P = 0.038). Calculation for odds ratios in accordance with a recessive model for rs3743631 suggests that individuals who were homozygous for GG homozygote had a higher risk for ARDS. The subjects of GG genotype were 3.657 times higher risk than those with AA and AG (Recessive model, OR = 3.657, 95% CI = 1.181–11.32, P = 0.024). The allele model for SNP rs3743631 suggests that G allele might increase the risk for ARDS compared to A allele.

We also found that individuals with AA or TA genotype of rs907045 had higher risk of ARDS compared with those with TT genotype (Additive model). The subjects of AA and TA genotype had 1.885 times higher risk of getting ARDS when compared with the subjects of TT (Dominant model, OR = 1.885, 95% CI = 1.031–3.446, P = 0.039). The A allele of rs907045 was significantly associated with increased risk of ARDS compared with T allele (P = 0.032).

For SNP rs11607, a statistical difference was found between ARDS group and control group in allele frequency of T and C (P = 0.046). No differences in genotypic or allelic frequencies were observed for other 5 SNPs.

Association of gene polymorphisms with S1P levels

Serum S1P levels of control individuals with rs3743631 GG genotype or AA + AG genotype did not differ from each other (Fig. 6a). The S1P levels between rs907045 AA or TA genotype and TT genotype in control group also showed no significant differences (Fig. 6b). However, ARDS patients with rs3743631 GG genotype (vs AA and AG genotypes) showed lower S1P levels (median 222.73 nmol/L, IQR 126.69–318.61 nmol/L vs median 314.81 nmol/L, IQR 240.10–436.90 nmol/L; P = 0.004). Similar results were obtained in the patients with rs907045 AA or TA genotype (AA and TA vs TT, median 281.31 nmol/L, IQR 201.69–342.93 nmol/L; vs median 335.73 nmol/L, IQR 235.20–450.46 nmol/L; P = 0.035).

Fig. 6

Serum S1P levels with different genotypes. a The S1P levels of ARDS patients with rs3743631 GG genotype were lower than those with AG + AA genotypes. b The S1P levels of ARDS patients with rs907045 AA + TA genotypes were lower than those with TT genotype. No difference was observed in controls. ARDS acute respiratory distress syndrome, S1P sphingosine-1-phosphate

Discussion

This is the first multi-center study to assess the role of S1P in ARDS patients. Detailed clinical data were prospectively collected, and multiple correlations were made with S1P. The major findings of this study were as follows: (1) serum S1P levels were significantly lower in ARDS patients than in healthy controls; (2) reduced serum S1P levels in ARDS patients were associated with more organ dysfunction and higher mortality; and (3) the S1P rs3743631, rs907045 polymorphisms are associated with susceptibility to ARDS.

The serum S1P levels in our healthy controls are consistent with those reported in two previous studies [10, 11]. Considering that blood samples from patients and controls were handled in the same way and there is no age dependency and gender bias in the S1P levels, we believe that the differences in serum S1P levels between controls and patients were due to ARDS. Platelets, RBCs and endothelial cells have been proven to be the main sources of S1P [18-20]. Thrombocytopenia and anemia in ARDS patients are probably related to the low serum S1P levels in these patients. Endothelial damage and barrier disruption play critical roles in ARDS [3, 4], which may cause decreased S1P production. Serum S1P levels are also dependent on the level of S1P carrier proteins. In the blood, S1P is predominantly associated with HDL-associated apolipoprotein M (apo M) and to a lesser extent with albumin [21, 22]. In ARDS, both HDL and albumin levels are decreased. Taken together, decreased S1P production, loss of S1P sources and reduced carrier proteins all contribute to lower S1P levels in ARDS patients.

Our study results demonstrate the prognostic value of serum S1P levels measured in the early course of ARDS. S1P, recognized as a potent endothelial cell agonist and angiogenic factor, directly contributes to the maintenance of the integrity of the vascular endothelium [23, 24]. Intravenous application of S1P to lung-injured animals was found to attenuate lung vascular dysfunction and to increase the endothelial barrier integrity [12, 25]. Mice with decreased circulating S1P levels showed increased vascular permeability, lung edema formation and decreased survival after inflammatory challenge [26]. In addition to maintaining the endothelial integrity, S1P seems to directly modulate the immune response. S1P can prevent neutrophil chemotaxis and the transmigration of neutrophils across an endothelial cell monolayer [27]. In ALI animal models, administration of S1PR agonists inhibited early pro-inflammatory cytokine production, inhibited innate immune cell recruitment and attenuated inflammatory lung injury [14, 28–31].

Serum S1P levels could stratify disease severity and predict the disease outcome in ARDS patients. We are not suggesting the use of S1P as a sole marker in risk stratification or for making decisions about treatment futility. Rather, we want to emphasize the association of S1P levels with mortality owing to its potential utility in combination with other biomarkers and clinical predictors. In our study, S1P levels correlated with the number of non-pulmonary organ failures, but they were not associated with ARDS severity according to the Berlin classification. Partially due to the limited sample size, it is possible that the relationship between S1P and mortality is mediated by multiple organ failure rather than by the severity of lung injury, measured according to oxygenation impairment. There is therefore a need to increase the sample size of ARDS patients to verify these results in future investigations.

S1P gene polymorphism in ARDS patients has never been examined. To gain insight into the role of S1P in clinic, we examined S1P gene polymorphism in the blood sample of ARDS patients. In this study, we tested eight candidate SNPs. The genotype and allele frequency of rs3743631, rs907045 and allele frequency of rs11607 had significant differences between ARDS and control. The frequency of the rs3743631 GG homozygote was significantly higher in ARDS patients compared with healthy controls, which indicates that individuals with GG genotype is susceptible to ARDS. The dominant model for SNP rs907045 suggests that AA or TA genotype might increase the risk for ARDS. Another finding indicates that the rs3743631 GG genotype is associated with lower plasma S1P levels compared with the AA and AG genotypes in ARDS patients. The serum S1P levels of rs907045 AA or TA genotype patients were lower than that of TT genotype patients. Thus, it is more likely that A to G variation of rs3743631 and T to A variation of rs907045 in S1P gene increase the onset of ARDS.

Our study also has limitations. First, the smaller sample size of patients may limit power to investigate the association between functional SNPs and S1P levels. All the subjects were from Chinese Han population. It is possible that ethnic and genetic differences may also influence the association between S1P polymorphism and serum S1P levels. Second, apoM was recently identified as the responsible binding protein of S1P in the blood [32]. We were not able to obtain information on apoM levels owing to limitations in our data. Third, because patients with trauma, drug overuse, and other less common risk factors for ARDS were not included, the present findings may not be generalizable. Considering the heterogeneity and various manifestations of ARDS, the study results should be examined in larger samples to evaluate our findings among specific subgroups.

Conclusions

Serum S1P levels are decreased in ARDS patients. Low serum S1P levels are associated with multiple organ dysfunction and adverse clinical outcomes in ARDS patients. Analysis of S1P gene polymorphism revealed that A to G variation of rs3743631 and T to A variation of rs907045 in S1P gene increased the risk of ARDS, as validated by actual measurements of serum S1P levels. A potential therapeutic strategy that would involve increasing the serum S1P levels during illness may be desirable. Further studies with larger populations are needed before our findings can be generalized to all ARDS patients.