Background
S100B is a calcium-binding protein of 92 residues, with its gene located on 21q22.3 [
1‐
3]. Within the central nervous system (CNS), S100B is thought to be a marker for astroglial activation, linking astrocyte dysfunction to schizophrenia [
1‐
4]. In addition to astrocytes, S100B is released from many cell types, such as, adipocytes, chondrocytes, cardiomyocytes and lymphocytes [
5‐
13]. S100B can act in a paracrine and autocrine manner, where low concentrations regulate proliferation and differentiation of neurons and glia. S100B also acts as a neurotrophic factor, regulating dopaminergic and glutamatergic synaptic function, in addition to synaptogenesis [
1,
2]. In contrast, excessive levels of S100B promote the expression of inducible nitric oxide synthase or pro-inflammatory cytokines causing neuronal dysfunction and apoptosis. In S100B knockout mice, increased brain-derived neurotrophic factor and decreased noradrenaline further suggest a role for S100B in regulating the levels of neurotrophic factors and neurotransmitters [
4]. Studies have also shown that copy number variations in S100B as well as polymorphisms in its gene and promoter region are associated with altered S100B protein levels in schizophrenia [
14‐
21]. In addition, polymorphisms in the receptor for S100B, RAGE (receptor for advanced glycation end products) have been linked with schizophrenia, as have increased levels of the soluble version of RAGE [
22,
23].
At the protein level, findings suggest that S100B is increased in schizophrenia, and that these protein levels are correlated with medication, gender, age and illness severity [
16,
23‐
44]. Importantly, astrocytes express dopamine receptors (DA2Rs) and antipsychotic medications regulate the cellular release of S100B [
45‐
48]. Of interest, S100B is also expressed in immune cells (including T cells and natural killer cells) and in adipocytes suggesting it may have a role in altered immune response and metabolic activity in schizophrenia [
2,
5‐
13]. Indeed, findings have suggested that altered S100B levels in schizophrenia may be due to metabolic disorder, visceral obesity, diabetes and/or immune dysfunction in this illness [
6,
8,
9,
34,
49,
50]. Here, the serum levels of S100B in patients diagnosed with schizophrenia was investigated and relationships between age, gender, illness severity, type of medication, treatment time and various metabolic factors, was examined. In this study, focus was given toward patients treated with clozapine, which has been previously shown to increase the incidence of hyperglycaemia, diabetes and metabolic syndrome [
51]. Moreover, stratification of patients and control groups into male and female subpopulations were performed to investigate the impact of body mass index (BMI) on levels of S100B.
Discussion
In the current study 97 subjects diagnosed with schizophrenia, treated with clozapine, 34 patients treated with depot antipsychotic medications and 27 healthy control subjects were recruited. Fasting glucose levels, triglyceride and HDL were raised in the patients compared to healthy controls. As expected the patient group showed an increased percentage of subjects who smoked and increased numbers with a known diagnoses of type 2 diabetes compared to control. Patients diagnosed with schizophrenia also showed higher BMI levels compared to healthy controls. When comparing S100B levels, no statistical difference was found between patients and controls. The serum levels of S100B were also not correlated with age, treatment time or illness severity. Analysis of S100B serum concentrations in patients treated with the depot antipsychotic medications including zuclopentixol, flupenthixol, risperidone, flupehazine and haloperidol also showed no observable differences in S100B levels compared to control. Importantly, the data here showed that S100B levels were elevated in female patients with schizophrenia compared to male patients and to those female and male healthy control subjects. Of interest, BMI levels were also elevated in female patients compared to male patients and healthy control groups. A correlation analysis showed that levels of S100B increased with BMI in female patients with schizophrenia. Taken together, the data suggested that levels of S100B are altered in schizophrenia and these levels are also likely related to patients BMI, in addition to astrocyte dysfunction.
The question has emerged if S100B is a specific marker for astrocyte dysfunction in schizophrenia? The widespread cellular expression of S100B and the lack of disease specificity for this protein [
34,
49], has questioned if S100B is a sole marker for astrocytes [
50]. Astrocytes have diverse roles including (i) direct communication of astrocytic end-feet with endothelial cells that allows astrocytes to control the blood–brain-barrier; (ii) uptake of neurotransmitters, such as glutamate at the synaptic cleft, that allow astrocytes to regulate synaptic transmission and excitoxicity; and (iii) release of transmitters, growth factors, and cytokine/chemokines, allowing astrocytes to regulate cellular communication, migration and survival, for example of neurons and lymphocytes [
65]. It is significant that astrocytes also play a role in scar formation after CNS injury, thus having apparent opposing roles in physiology and pathophysiology. In disease, astrocytes have been suggested to play roles in a range of psychiatric, neurological and neurodegenerative disorders. Previous studies have focused on the role of S100B in astrocytes, and suggested that astrocyte dysfunction may increase release of S100B in schizophrenia. However, S100B is also released from many other cell types including adipocytes and a number of
in vitro studies have shown that levels of S100B from these fat cells can be regulated by, for example, glucagon, adrenaline and insulin [
10,
66,
67]. Moreover, chronic fasting, weight gain and diet have also been shown to influence serum levels of S100B in patients and animal models [
67,
68]. The findings that S100B and BMI levels are elevated in female patients compared to male patients and to controls, and that S100B and BMI levels correlated in female patients (but not male patients), are in line with the hypothesis that visceral fat and altered adipocyte function could be a mechanism explaining elevated levels of S100B in schizophrenia [
9,
69]. On a more cautionary note, these results equally suggest that S100B can no longer be considered as a sole marker of astrocyte dysfunction in brain disease given its widespread distribution.
Previous studies have suggested that S100B levels in patients treated with clozapine are increased [
33], which in part is in agreement with the data in the current study showing raised levels of S100B in female patients treated with clozapine. The effects of clozapine are thought to be mediated primarily via antagonism at 5HT2AR and DA4R, with weak DA2R blocking activity [
70]. Astrocytes express DA2Rs and activation of these receptors using apomorphine decreases the levels of S100B in these cells, via a signalling pathway that involves inhibition of adenyl cyclase [
46]. Moreover, antipsychotic medications (such as clozapine, haloperidol and risperidone), which block DA2Rs, also decrease S100B levels [
46,
47]. Treatment of astrocytic C6 cells and oligodendrocytic OLN-93 cells with haloperidol and clozapine also decreases the levels of S100B [
48]. This data is in contrast with elevated levels of S100B observed in patients diagnosed with schizophrenia [
46]. However, contradictory to the hypothesis that astrocyte derived S100B levels are raised in schizophrenia, is the finding that chronic antipsychotic medication, such as haloperidol or olanzapine, reduce astrocyte numbers in Macaque Monkeys [
71]. Recent data also demonstrated that insulin downregulates levels of S100B in adipocytes (in addition to astrocytes) suggesting that other cell types may determine the levels of S100B [
9]. Interestingly, molecular links between S100B, DA2Rs and schizophrenia have been suggested where S100B has been shown to interact with the third cytoplasmic loop of the DA2R, and to enhance receptor signalling to ERK and inhibition of adenylate cyclase [
45,
72‐
74]. In S100B transgenic animals, there is a decrease in the expression of DA2R suggesting crosstalk between S100B and DA2R function [
75]. Taken together, these studies support the S100B/DA2R protein complex as a molecular target for antipsychotic medications and possible aberrant S100B/DA2R-mediated signalling in schizophrenia.
Conclusions
This current study showed that levels of S100B are raised in female patients diagnosed with schizophrenia and correlate with BMI, which is possibly linked to higher levels of release from adipocytes. A limitation of this study was the unequal distribution of genders among the two groups (controls
vs. patients). While ANOVA tests were performed, these assumed the sample size were equal and thus may have been too liberal. Another limitation of this study is the direct comparison of S100B levels between patients on medication and drug naive patients with schizophrenia. Previous studies have however reported that levels of S100B in drug naive patients with schizophrenia are raised [
32,
42]. Notably, however the levels of S100B were not correlated with BMI in these reports. In this current study we also did not investigate if the levels of S100B were associated with genetic mutations reported in schizophrenia. While it may be possible that our findings are explained, in part, by pathogenetic mechanisms, further studies would be required to determine this possibility, for example by investigating the levels of S100B in siblings of patients with schizophrenia. Moreover, further analysis of molecules that are more specific to adipocytes (rather than S100B) would be worthy of investigation. In addition, cellular studies demonstrating the effects of antipsychotic medications on the release of S100B in adipocytes would be warranted. In closing, while a number of studies have demonstrated that levels of S100B are altered in schizophrenia, many of these reports are contradictory when suggesting that age, gender, medication and illness severity all have an impact (or not) on S100B levels. The study here suggested two important factors that may help unify these apparent contradictory findings. Firstly, that the serum S100B concentrations are likely influenced by metabolic activity. Secondly, that the levels of S100B are not solely dependent on astrocyte dysfunction and may involve altered fat cell (adipocyte) function. Thus, the data presented here supports previous studies that have suggested an association between metabolic syndrome, diabetes and immune response dysfunction in schizophrenia. The clinical implications of this study are two-fold, firstly that S100B levels may no longer be considered as a biomarker that is dependent on neurological function alone, and secondly, that regulating metabolic dysfunction in schizophrenia may represent a novel drug target.
Competing interests
The authors declare that they have no competing interests.
Authors’ contributions
KKD and JT conceived the idea. KKD and KOC designed the methodology of this study. KOC was the major contributor in subject recruitment, in clinical and diagnostic assessments, in gathering experimental data and in conducted all the statistical analyses. KKD and KOC were the major contributors in the writing of the manuscript. All authors have approved the final manuscript.