1. Introduction
S100B is a protein belonging to the S100 protein family that was first reported by B.W. Moor in 1965 [
1]. The S100 proteins, a family of calcium (Ca
2+)-binding cytosolic proteins, are composed of 25 known members, namely, S100A1-18, hair hyaluronan, keratin fibrin, reptin, S100B, S100P, S100Z and S100G [
2]. These proteins have diverse intracellular and/or extracellular functions, including the regulation of cell proliferation, differentiation, apoptosis, migration, energy metabolism, calcium balance, protein phosphorylation, and inflammation [
3,
4,
5,
6]; therefore, altered expression of S100 proteins may be associated with disease development. S100B has been reported to be expressed mainly in astrocytes and other glial cells, such as oligodendrocytes and Schwann cells. It has been reported to be associated with various neurological disorders [
7]. However, S100B has also been detected in definite nonneural cell types, such as dendritic cells, certain lymphocyte subpopulations, chondrocytes, Langerhans cells, melanocytes, adrenal medulla satellite cells, skeletal muscle satellite cells, and adipocytes [
8,
9,
10,
11]. Additionally, we have previously reported that S100B is expressed in the liver of mice and that the interaction with the receptor for advanced glycation end products (RAGE) is observed in cholestatic liver injury by bile duct ligation [
12]. Another experimental study has reported that serum S100B levels increase in the setting of local ischemia and perfusion of the rat liver without intracranial injury, suggesting that the liver may be a possible source of S100B [
13].
To date, serum levels of S100B protein have been extensively studied in certain conditions, such as neural tissue injury, but rarely in relatively stable cirrhotic patients without any types of hepatic encephalopathy (HE). LC is the end stage of various liver diseases and a major cause of morbidity and mortality worldwide. HE is one of the main complications of LC and develops in 30–45% of patients with cirrhosis [
14]. HE is defined as brain dysfunction caused by liver insufficiency and/or portosystemic shunting. Its manifestations include a wide spectrum of neurological or psychiatric abnormalities, such as inappropriate behavior, disorientation, confusion, slurred speech, stupor, and coma [
15]. Depending on the severity of symptoms, HE can be classified into minimal hepatic encephalopathy (MHE) and overt hepatic encephalopathy (overt HE). Several studies have suggested that elevated serum S100B levels may be a diagnostic marker of overt HE in fulminant hepatic failure or cirrhosis [
16,
17]. However, little is known about the roles of serum S100B levels in diagnosing MHE or predicting 1-year mortality in cirrhotic patients.
In the present study, we aimed to investigate the diagnostic ability of serum S100B to discriminate MHE and the parameters correlated with serum S100B levels in cirrhotic patients, depending on whether HE is present. Second, we investigated whether serum S100B levels can be used to predict 1-year mortality in cirrhotic patients.
4. Discussion
In the present study, serum S100B level was not useful for differentiating the severity of HE. However, serum S100B levels were associated with age and the main parameters of the MELD score, such as serum bilirubin and creatinine levels in cirrhotic patients. Additionally, serum S100B levels showed the potential in the prediction of 1-year mortality in cirrhotic patients.
LC is a life-threatening disorder leading to 1.03 million deaths per year worldwide [
18]. Patients with decompensated LC characterized by variceal bleeding or ascites have a 10-fold higher risk of death than the general population [
19]. The CTP and MELD scores have been widely used as noninvasive tools for predicting the prognosis of LC. Because the CTP scoring system includes rather subjective variables, such as the degree of evaluation of ascites and HE, the MELD score is considered a more reproducible and accurate tool than the CTP score. In the present study, serum S100B levels showed a positive correlation with MELD scores, and the relationship was more distinct than that with CTP scores.
The biological half-life of S100B has been reported to be in the range of 30–132 min, depending on the clinical situation [
20,
21,
22]. Similar to other small proteins, S100B is presumed to be eliminated by degradation in the proximal tubules in the kidney. Therefore, impaired renal function may affect the serum levels of S100B. Although a significant difference in renal function was not observed among our study groups, the serum S100B level was correlated well with the serum creatinine level, which is one of the main parameters of the MELD scoring system. Regarding the elimination of S100B, Jönsson et al. reported that S100B levels were not influenced by a moderate decrease in glomerular filtration rate (GFR) [
22]. At the same time, Gross et al. demonstrated that serum S100B was negatively related to creatinine clearance [
23]. In cirrhotic patients, the serum creatinine level is not an accurate marker for estimating renal function due to various factors, such as reduced muscle mass, increased tubular secretion of creatinine, and impaired production of creatine, which is the precursor of serum creatinine. Therefore, to establish the relationship between S100B levels and renal dysfunction in cirrhotic patients, further study using other renal biomarkers, such as cystatin C, is necessary.
In the present study, total serum bilirubin level was associated with serum S100B levels. Similar to our result, Okumus et al. found a significant correlation between serum bilirubin and S100B levels [
24]. However, it is unclear how serum bilirubin levels affect serum S100B levels. Serum bilirubin levels have been considered an important factor in the diagnosis and prognosis of patients with liver disorders. Almost all prognostic scores for liver diseases include bilirubin in their calculations. These findings suggest that liver dysfunction can affect serum S100B levels. Additionally, the present study showed that age was associated with serum S100B levels. Similar to our data, Gross et al. found that serum S100B was positively related to age [
23]. On the other hand, Portela et al. demonstrated that a correlation between age and S100B was not evident in individuals >20 years of age but was present in individuals <20 years of age [
25]. Therefore, further study is needed to clarify the effect of age on serum S100B levels. To date, most of the research on the clinical use of S100B has been conducted in patients with neurologic disorders or some malignancies [
26,
27,
28,
29]. Regarding liver diseases, there have been a few studies reporting serum S100B as a neuromarker of HE in patients with fulminant hepatitis [
16,
17,
30]. Especially, Toney et al. investigated that individual neuromarkers derived from specialized cell types within the brain, such as neuron-specific enolase (NSE) present in neurons, S100B present in astrocytes, and myelin basic protein (MBP) in oligodendrocytes, would be associated with the development of HE in pediatric acute liver failure [
16]. They found that only S100B was associated with HE. Additionally, they showed no relationship between S100B and the severity of HE, similar to the present study. There is another study using samples from 54 subjects in the US Acute Liver Failure Study Group. In that study, Vaquero et al. found that S100B was increased in subjects with (i) stage 1–2 HE who did not progress, (ii) stage 1–2 HE who progressed to a severe encephalopathy, (iii) stage 3–4 HE who survived and (iv) stage 3–4 HE who died or required transplantation [
31]. However, a significant difference in S100B level according to the severity of HE was not observed. Hepatic stellate cells, which share many functional and morphologic characteristics with glial cells, were suggested as another relevant source of serum S100B in the setting of liver failure.
Taken together, obvious astrocyte swelling and damage developed in acute liver failure may release S100B protein; thus, elevated serum S100B may be a marker suggesting severe HE. However, based on the results of the present study, the elevated serum S100B levels observed in fulminant hepatic failure may be the result of not only astrocyte injury but also liver damage itself.
HE is a typical cirrhosis-related complication and is associated with repeated readmission and, more importantly, high rates of mortality [
32,
33]. Although MHE is the mildest form in the HE spectrum, it is associated with low health-related quality of life (QOL) and worse survival [
34,
35]. Because patients with MHE have no clinically detectable neurological-psychiatric abnormalities, neuropsychological or electrophysiological tests are required to diagnose MHE [
36]. Under real-world conditions, these diagnostic procedures are cumbersome and time-consuming for clinicians. Therefore, the identification of serologic markers for diagnosing MHE will benefit the current clinical practice for cirrhotic patients. Unfortunately, in the present study, serum S100B levels did not show diagnostic value for MHE or overt HE. Ammonia levels also failed to show a direct correlation with the severity of HE.
However, in the present study, serum S100B levels had a significant relationship with the MELD score regardless of the presence of HE. It is uncertain how serum S100B levels are elevated in cirrhotic patients without brain injury. Cell damage followed by apoptosis or necrosis, which occurs in the liver, is presumed to be the possible source of blood S100B protein. S100B is known to play various roles through intracellular and extracellular functions [
37]. In our previous experimental study, we found that hepatic stellate cells, the key players in liver fibrosis, are activated by recombinant S100B protein treatment [
12]. Recently, another study has elucidated the role of S100B in hepatocellular carcinoma (HCC) [
38]. Yan et al. showed that S100B expression is correlated with hypoxia and the immune response in a human HCC cell line. Therefore, S100B may be involved in various liver diseases. To clarify the mechanism of S100B in liver diseases, further research is needed.
The present study indicated that the combination of a high MELD score and S100B levels, high ammonia levels, and low cholesterol levels were associated with 1-year mortality in cirrhotic patients. Recently, Tranah et al. reported that plasma ammonia is an independent predictor of hospitalization with liver-related complications and mortality in clinically stable outpatients with cirrhosis through a prospective cohort study [
39]. As a gut-derived neurotoxin, hyperammonemia resulting from impaired metabolism in chronic liver diseases leads to cerebral edema and intracranial pressure elevation [
40,
41]. Additionally, ammonia is known to be related to the pathogenesis of liver-related complications, including liver cell damage, immune dysfunction, sarcopenia, and portal hypertension [
42]; thus, together with S100B, ammonia may be an interesting indicator of LC. Similar to our findings, Feng et al. reported that decreased total cholesterol is significantly associated with reduced survival in cirrhotic patients [
43].
The results of the present study should be interpreted considering its limitations. First, the sample size was relatively small, but this study was conducted prospectively and used proper neuropsychological tests to discriminate MHE and overt HE. In the present data, the prevalence of MHE and mortality-related factors in cirrhotic patients were similar to those of recently published studies, reflecting the adequacy of the present study. The sample size of the non-HE group was also too small to perform survival analysis. Nevertheless, our results showed that S100B is worthy of further study in cirrhotic patients without any types of HE. Second, the value of serum S100B in the age-matched healthy control group was not investigated. Furthermore, seasonal variation (summer/winter) or genetic factors (S100B single nucleotide polymorphism rs9722 and S100B haplotype T-G-G-A) could affect the levels of serum S100B [
44,
45]. Therefore, in liver disease, subsequent research on the clinical value of S100B should include a normal control group and be designed considering the source of variation affecting the levels of serum S100B concentrations. Lastly, in the present study, MHE was diagnosed using MMSE and PHES. Recently, two abnormal tests among neuropsychological assessments (i.e., PHES, critical flicker frequency test, inhibitory control test, and Stroop test et al.) have been required to diagnose MHE [
46]. Unfortunately, the present study was designed in early 2013. At that time, no agreement existed as to what combination of tests should be carried out to confirm MHE. We followed a general approach to the diagnosis of MHE based on Ferenci et al. [
47]. Therefore, subsequent research using more various neuropsychological assessment tools is necessary.
In summary, this is a negative study showing that serum S100B is not useful for MHE diagnosis; however, the present study found that serum S100B levels can be affected by age, serum bilirubin, and creatinine in cirrhotic patients, and associated with MELD score. Additionally, serum S100B levels showed the possibility of predicting 1-year mortality in cirrhotic patients. In the future, basic and clinical studies are necessary to identify the effect of liver dysfunction on serum S100B levels in various liver diseases.