Background
Non-muscle invasive bladder cancer (NMIBC) is the most common histological subtype of bladder cancer, accounting for approximately 80% of all cases. Approximately 20% of these patients experience disease progression to muscle invasive bladder cancer (MIBC) after treatment, a development that is associated with a very poor prognosis for survival. Conventional histopathological parameters, such as tumor stage or grade, are generally considered to be prognostic factors, and numerous biomarkers have been investigated as prognostic indicators of the likelihood that NMIBC will develop into MIBC [
1‐
6].
Members of the
S100 family of calcium-binding proteins play essential roles in epithelial tissues and participate in a wide range of cellular processes, including transcription, proliferation, and differentiation [
7‐
11]. At least 16 genes that encode members of the
S100 family, including the gene for
S100A8, are clustered on human chromosome 1q21 [
12,
13], in a region that frequently experiences chromosomal rearrangement during tumor development [
14,
15].
S100A8 is reportedly up-regulated in many cancers, including bladder cancer [
16‐
23], and has been implicated in the regulation of tumor cell proliferation and metastasis [
16,
24‐
26].
Although numerous diagnostic markers have been investigated as indicators of the risk of disease progression [
1‐
6], none are able to sufficiently predict the behavior of NMIBC [
1‐
6].
S100A8 has been suggested to be a predictive biomarker of bladder cancer outcome in several studies [
21‐
23], however, the regulation of
S100A8 gene expression and whether the genes associated with its expression provides additional insight into the mechanisms of disease progression or tumor invasion have not been studied. Therefore, we analyzed the expression pattern of
S100A8 and its correlated genes to assess whether their molecular signature could identify patients with a higher likelihood of disease progression.
Discussion
The data presented in this study demonstrates that the expression signature of S100A8-correlated genes is able to predict the likelihood of bladder cancer progression. The validity of this signature as a prognostic indicator was confirmed by analysis of 302 cancers from an independent European cohort. In addition, based on the results of gene network analysis, we have identified a putative mechanism that may be responsible for disease progression.
Although considerable effort has been devoted to the establishment of a prognostic model of NMIBC that can provide information concerning survival and treatment options at diagnosis [
1‐
6], the ability to predict the course of disease progression for patients with bladder cancer remains a major clinical challenge. A number of the molecular markers that have been identified to date have been explored as potential predictors of disease progression. Some of these, such as
p53, have been suggested to be independent markers, while others do not appear to be effective as prognostic indicators [
4‐
6]. Thus, there is a crucial need for methods capable of identifying patients with NMIBC that is likely to develop into MIBC. In the present study, we developed a method to predict the progression of primary NMIBC based on a gene expression signature. We showed that our method has strong predictive value through multivariate regression analysis and a validation study in an independent cohort. These data underscore the effectiveness of this molecular signature as a prognostic indicator in NMIBC, and suggest that this signature could be clinically useful.
The accumulation of recent reports that document the deregulated expression of calcium-binding proteins of the
S100 family in a variety of human cancers implies that
S100 proteins are required for neoplastic or metastatic transformation of tumor cells [
26]. Previous observations that
S100A8 expression is enhanced in bladder cancer suggest a role for this protein in the progression of bladder cancer [
21‐
23]. The data obtained from the current study demonstrated that upregulation of
S100A8 expression was strongly associated with disease progression. This result substantiates the involvement of
S100A8 during the progression of NMIBC.
Expression of
S100A8 is not the only indicator of
S100A8 activity, because it is regulated by many different mechanisms. The identification of stable and reliable human gene-to-gene relationships is an essential step towards unraveling the interactions and functional correlations between human genes [
38]. Therefore, we performed gene network analysis to identify the association of
S100A8-correlated genes with progression of NMIBC. Strikingly, expression of both
S100A8 and its correlated genes were strong predictors of the progression of bladder cancer (Figures
1 and
2). This finding was further supported by multivariate analysis, which showed that the molecular signature of
S100A8-correlated genes was a strong predictor of cancer progression-free survival (Table
2). This result suggests that the
S100A8-correlated gene signature retains its prognostic relevance even after additional pathological prognostic features have been taken into account.
The standard strategy for evaluating the accuracy of classification methods is application of a training-validation approach, in which a training set is utilized to identify the molecular signature and a validation set is used to estimate the degree of reliability. For the validation assay, a large sample size is needed before expression profiling can be utilized in a clinical setting [
39]. In the current study, the accuracy of a predictive
S100A8-correlated gene signature was tested using five independent algorithms with gene expression data from a large independent European cohort (Figure
3). This result demonstrated not only a strong association between gene expression patterns and progression, but also provided strong evidence of the reliability of the prediction.
Based on an analysis of the
S100A8-correlated genes in the context of gene networks, we identified a putative role for
S100A8 in disease progression of bladder cancer. Because
S100A8 is the downstream target of
IL1B [
37], our results strongly support the potential involvement of
IL1B in tumor progression. Recent studies indicate that
IL1B is associated with tumor invasiveness and metastasis [
40,
41]. Many of the satellite genes associated with
IL1B (i.e.,
CXCL2,
PLA2G7,
CCL11,
S100A9,
CD163,
RNASE7, and
OSM) (Figure
5) participate in inflammatory and immune responses, which are the best known activities of
IL1B. Previous studies have demonstrated that
IL1B activates inflammation that promotes tumor invasiveness [
40]. It was reported previously that
S100A9 is up-regulated in conjunction with
S100A8 in many cancers, including gastric cancer [
20], prostate cancer [
16,
17], and colorectal cancer [
18,
19]. Both
S100A8 and
S100A9 have been implicated in the regulation of cell proliferation [
16,
24] and metastatic processes [
25]. Thus,
S100A9 may prove a valuable target for prevention of the migration of tumor cells to pre-metastatic sites [
26].
Our results demonstrate that an expression signature consisting of S100A8 and its correlated genes can be a reliable prognostic indicator of progression in NMIBC, independent of traditional pathologic prognostic parameters. The use of this signature as a predictive indicator could potentially enable the prognosis of heterogeneous NMIBC patients to be determined at diagnosis, which would allow for individualized treatment and evaluation.
Competing interests
The authors declare that they have no competing interests.
Authors' contributions
SKK conceived of the study, carried out the statistical analyses, and drafted the manuscript. EJK and SHL performed microarray preparation, sample selection, and RNA isolation. YSH investigated the clinical records of the considered patients and contributed to the interpretation of the results. YJK performed the statistical analyses, contributed to the interpretation of the results, and helped to draft the manuscript. WJK designed the study concept, interpreted the results and approved the final manuscript. All authors have read and approved the final manuscript.