Background
Colorectal cancer (CRC) is one of the most common forms of cancer worldwide with approximately 1 million new cases detected every year [
1]. Early detection, adequate surgical excision and optimal adjuvant treatment are of critical importance for outcome. Although several prognostic and predictive CRC biomarkers have been proposed [
2], serum carcino-embryonic antigen (s-CEA) is currently the only accepted marker incorporated into clinical practice. S-CEA is used for early detection of metastasis during follow-up of patients with stage II and III disease and for monitoring response to adjuvant treatment in stage IV disease.
In a previous study by our group, we found that a high expression of tumour-associated trypsin inhibitor (TATI; synonymous to pancreatic secretory trypsin inhibitor, PSTI, and serine protease inhibitor Kazal type 1, SPINK1) in tumour tissue (t-TATI) was associated with an increased risk of metachronous liver metastasis and an impaired prognosis in CRC patients [
3]. These findings are supported by
in vitro data, demonstrating that TATI promotes invasiveness of CRC cells [
4].
Several studies have found s-TATI to be of potential prognostic value in ovarian cancer [
5,
6], a good serum marker for monitoring [
7] and prognosis [
8] of bladder cancer, prognosis of renal cancer [
9] and more accurate than CEA, carbohydrate antigen (CA) 15-3, CA 125 and CA 19-9 in post-operative follow up of renal cancer patients [
10]. Previous studies on s-TATI in various cancer forms have been performed on rather small cohorts with diverging conclusions regarding its prognostic value. In a study from 1991, s-TATI was found to be a good predictor of liver metastasis in CRC and breast cancer [
11]. Satake
et al found elevated s-TATI concentrations in CRC patients, but the results were not considered to be of sufficient diagnostic value for clinical use [
12]. In a study on 62 CRC patients, Pasanen
et al found s-TATI to be a useful biomarker for staging of CRC, however less useful than s-CEA [
13]. Similar results were obtained in another study comprising 53 CRC patients [
14]. Solakidi
et al found s-TATI to be a useful complementary biomarker for diagnosing and monitoring of gastrointestinal malignancies, having a higher sensitivity than s-CEA [
15].
Three main mechanisms have been proposed to cause increased levels of TATI in serum; leakage from tumour-derived TATI into the circulation and as a response to tissue destruction and inflammation [
16]. A transitory elevation of s-TATI levels have been found after surgery, suggesting that TATI may behave as an acute phase protein [
14,
17]. Elevated levels of s-TATI can also be detected temporarily in some non-malignant conditions, especially in pancreatitis [
18], and in severe inflammatory diseases, injuries, and sepsis [
12,
19].
The purpose of the present study was to examine the prognostic value of s-TATI in a cohort of 324 prospectively collected CRC patients, including 308 cases previously analysed for t-TATI [
3]. Furthermore, the prognostic value of s-CEA was assessed, as well as the association between s-TATI and t-TATI.
Discussion
In this study, we have shown that elevated levels of s-TATI independently predict an adverse outcome in CRC patients and this was also the case in separate analysis of patients with Stage II and III disease.
We have previously shown that high expression of TATI in tumour tissue correlates with an adverse prognosis in two independent cohorts of CRC patients, including the cohort studied here, in which t-TATI was also significantly associated with a reduced time to metachronous liver metastasis [
3]. The lack of any association between s-TATI levels and t-TATI is therefore somewhat surprising, given the significant relationship between s-TATI and an adverse prognosis demonstrated in this study. Indeed, s-TATI and t-TATI remained independent prognostic factors when both were included in multivariate analysis. In addition, while s-TATI levels were significantly higher in patients presenting with distant metastasis at diagnosis, we could not confirm any relationship between s-TATI and time to liver or lung metastases.
To our knowledge, the relationship between s-TATI and t-TATI has only been investigated in one previous study on renal cell carcinoma, which also failed to show a significant association between these variables [
21]. However, in that study, it was also shown that an impaired renal function could contribute to elevated s-TATI levels in patients with renal cancer. A limitation to this study is the lack of information on renal function for the patients included. The relationship between s-TATI and renal function in CRC patients is an important issue that should be addressed in future studies. In this study, age at diagnosis was significantly associated with high s-TATI levels, which is in line with the study by Solakidi et al. [
15]. As high age is associated with a poor DFS and OS in CRC, and also with an impaired renal function, this relationship could to some extent contribute to the adverse prognostic impact of high s-TATI levels. Notably, the independent prognostic value of s-TATI remained significant for both DFS and OS when adjusted for age in the multivariate Cox regression analysis.
Elevation of s-TATI may also be caused by several non-malignant conditions such as severe inflammatory disease [
22], pancreatitis and hepatobiliary disease [
23], and tissue destruction [
24,
25]. TATI is produced at high concentration in the pancreas, from which it leaks into serum in pancreatic disease [
24]. However, normal s-TATI levels have been found after total pancreatectomy showing that TATI is produced in several tissues [
26,
27]. Solakidi
et al suggested that leakage from the tumour cells into the circulation might be the mechanism behind elevated s-TATI levels [
15], but the lack of a correlation between s-TATI and t-TATI in this study implies that leakage from the tumour into the circulation is not a major cause of elevated s-TATI levels. Apart from its trypsin inhibiting function, TATI has also been ascribed properties promoting a more malignant phenotype in CRC [
4] and in prostate cancer cells [
28]. Interestingly, TATI has been shown to stimulate the EGF receptor and thus increase tumour aggressiveness [
29]. Therefore, it could well be that t-TATI contributes to the biological aggressiveness of the tumour per se, whereas s-TATI levels reflect a local and/or systemic response to the disease.
Notably, while there was no association between t-TATI and tumour location, s-TATI levels were significantly lower in left-sided tumours compared to right-sided tumours and in rectal cancers compared to colonic cancers, the majority of which had received preoperative RT. None of these associations were seen for s-CEA. The significant association between lower s-TATI levels and RT indicates that treatment might directly affect s-TATI levels. This would be of interest to investigate in future studies, comparing levels of s-TATI before, during and after RT. In this study, s-TATI was an independent prognostic factor in patients not receiving RT, whereas it was only prognostic in univariate analysis in patients treated with RT. As the number of patients that had received RT was relatively small (n = 85) and the number of patients that had not received RT was even smaller (n = 23), no conclusions can be drawn regarding the value of s-TATI as a predictor of response to preoperative RT in rectal cancer patients. This should be investigated in a prospective setting, preferably a randomised trial.
Notably, the significantly lower s-TATI levels in left-sided compared to right-sided colonic tumours indicate that differences in s-TATI levels according to tumour location might also be due to differences in biological characteristics of the tumours, e.g., microsatellite instability (MSI), which is more frequently present in tumours arising in the right colon [
30,
31]. Information on MSI status was not available for the tumours in this study, but should be considered in future studies.
Earlier studies on s-TATI in colorectal cancer have only concerned its diagnostic value, and for this purpose s-TATI has not been found to provide information additional to that obtained with s-CEA [
13,
14]. The prognostic value of s-TATI in CRC has not been studied before. In the present study s-TATI was found to be a prognostic biomarker that was independent of conventional clinicopathological factors, and also independent of s-CEA. Furthermore, in most settings, the value of s-TATI was stronger than that of s-CEA.
Individualised treatment is still not available for patients with CRC in the adjuvant setting; currently the TNM stage is used for selection of patients for adjuvant treatment. In stage II patients about 20% will develop cancer recurrence, and there is modest evidence that these patients benefit from adjuvant chemotherapy [
32]. Several high risk factors have been suggested for selecting stage II patients for adjuvant chemotherapy, i.e few lymph nodes examined, T4 disease, obstruction and tumour perforation [
33]. Stage III CRC patients, on the other hand, have a well documented benefit from adjuvant chemotherapy [
34] although it is estimated that up to 80% will relapse despite adjuvant treatment. It is therefore obvious that more accurate prognostic factors are needed when selecting patients for adjuvant treatment in CRC. The data presented here show that s-TATI is an independent prognostic biomarker in the whole cohort and in separate analysis of patients with Stage II and III disease.
Competing interests
The authors declare that they have no competing interests.
Authors' contributions
AG participated in the collection of data, performed statistical analyses and drafted the manuscript. BN assisted with the serum analyses and revision of the manuscript. EN assisted with the serum analyses. KH performed the serum analyses and revised the manuscript. UHS participated in the design of the study and helped with revision of the manuscript. AB participated in the design of the study and helped with the revision of the manuscript. HB included the patients into the study, collected data, drafted the manuscript and participated in the statistical analysis. KJ participated in the conception and design of the study, statistical analysis, drafted and revised the manuscript. All authors read and approved the final manuscript.