CXCL14, a novel non-ELR chemokine with yet uncharacterised receptor, has been reported to be associated with tumor progression and metastasis [15, 16, 30]. In this study, we found that CXCL14 was frequently up-regulated in colorectal tumors compared with the adjacent normal colorectal mucosa. This finding is consistent with previous studies that CXCL14 expression was elevated in tumor tissues compared with adjacent normal tissues [19, 20, 30]. To the best of our knowledge, this is the first study that analyzes the expression of CXCL14 protein in colorectal carcinoma by immunohistochemistry using surgically resected neoplastic tissues. Furthermore, in early stage I colorectal cancer, CXCL14 expression was up-regulated compared with adjacent normal mucosa, suggesting that CXCL14 over-expression is an early event in the development of CRC. It is intriguing to speculate on possible reasons for these high expression levels of CXCL14 during tumorigenesis. Immunohistochemical analysis showed that CXCL14 expression was mainly located in malignant colorectal epithelial cells, and partially in stroma cells. Hence, it could be postulated that CXCL14, similar to other chemokines, is likely to signal through a G-coupled receptor and contributes to progression of CRC via both autocrine and paracrine pathways. Additionally, the levels of CXCL14 expression were significantly associated with some clinicopathologic factors including TNM stage, histodifferentiation, and tumor size. Previous studies have reported that these factors are possible predictors of early recurrence and cancer-death [31–33]. Among them, TNM stage is the strongest predictor of survival for patients with colorectal cancer . The correlation between CXCL14 expression and these factors implies that elevated expression of CXCL14 may be associated with poor patient outcome.
Multivariate analysis showed that elevated CXCL14 level in patients with stage I/II colorectal carcinoma was an independent risk factor for developing recurrence, and that the level of CXCL14 expression in patients with stage III/IV colorectal carcinomas was inversely correlated with duration of survival. CXCL14 is therefore a useful prognostic factor for predicting the outcome of patients with CRC who have had a surgical resection of their tumor. Therefore, patients with colorectal carcinoma showing elevated CXCL14 expression should be carefully followed-up, while those with no or low CXCL14 expression should avoid being overtreated. In addition, tumors with high CXCL14 expression had a worse prognosis, even within the same clinicopathologic stage. The staging system, which is based on the extent of the tumor spread at the time of primary surgical treatment, does not always account for the aggressiveness of the tumor itself. From a clinical perspective, the ability to predict outcome within the same clinicopathologic stage is the key to individualizing treatment options. CXCL14, to some degree, helps makes up for the fact that TNM stage per se cannot predict the outcome of patients with the same clinical stage. Although clinicopathologic data dealing with CXCL14 expression at the protein level are currently scarce, increasing attention has been paid to the prognostic significance of CXCL14 in other diseases. Using antibody arrays, CXCL14 has been identified as a potential diagnostic marker of hepatocellular carcinoma . Additionally, CXCL14 have been suggested to be a poor prognostic marker in papillary thyroid carcinoma by qPCR analysis .
The functional role(s) of CXCL14 in tumor biology has been addressed in a few previous reports [15, 17]. In the present study, immunohistochemical analyses and a number of in vitro studies were performed to validate the aggressive role of CXCL14 in colorectal cancer progression. The observation that there was good correlation between CXCL14 levels and Ki67 in human colorectal cancer suggested that CXCL14 was involved in human colorectal cancer cell proliferation. Indeed, we found that CXCL14 enhanced malignant colorectal cell proliferation in vitro. Consistent with these findings, CAF-derived CXCL14 was reported to exert paracrine stimulatory effects on the proliferation of the prostate cancer cell line LNCaP . Meanwhile, rhCXCL14 was shown to enhance the proliferation of MCF-7 breast cancer cell accompanied by a robust increase in ERK phosphorylation in an autocrine manner . Intriguingly, CXCL14 expression was observed to be restricted to the myoepithelial cells in ductal carcinoma in situ and the tumor epithelial cells in invasive breast carcinoma, suggesting CXCL14 might be converted into an autocrine factor from a paracrine factor .
In almost all the specimens studied, the deeply invaded cancer cells were more intensely stained than those in mucosal regions. Additionally, the number of CXCL14-positive cancer cells increased as cancer cells invaded deeply, with more than 90% of the cells in the regions of the muscularis propria and subserosa immunopositive. Furthermore, almost all the cancer cells in the lymph node metastases were also CXCL14-immunopositive. These results suggest CXCL14 may be involved in the process of invasion and metastasis, especially lymphatic permeation and nodal metastasis of colorectal cancer cells. In vitro studies further validated the role of CXCL14 in colorectal cancer cell migration and invasion. The underlying mechanisms of CXCL14-enhanced colorectal cancer cell migration and invasion are not clear. Recent reports showed that activation of chemokine signaling could promote tumor cell metastasis by regulating polymerization of intracellular actins, inducing formation of pseudopodia, and facilitating extracellular matrix degradation and remodeling [36, 37]. These findings are consistent with previous studies that implicated altered chemokine expression levels as an indicator of progression to tumorigenesis and metastasis capacity [38, 39]. However, few studies have reported the potential of CXCL14 in colorectal carcinoma initiation and progression. In contrast to previous studies in which CXCL14 was reported to be deficient in tumors and acted as a tumor suppressor [17, 18], CXCL14 is recently identified as a novel potential cancer-stimulatory protein . Results obtained from the present study together with others indicate a need for further analysis of possible tumor type- and stage-specific effects of CXCL14 in other tumors [15, 17, 30]. The mechanisms through which CXCL14 promotes or inhibits tumor progression need to be further analyzed.
A series of topics worthy of further studies are suggested by the present findings. Firstly, identification of the receptor for CXCL14 is pressingly needed. The “seed and soil” theory that target organs releasing specific chemokines attract tumor cells bearing corresponding receptors has provided an acceptable explanation for many important roles of chemokine receptors in cancer metastasis [37, 40]. Analyses of the effects of GPCR inhibitors with known target profiles and GPCR profiling of CXCL14-responsive and -nonresponsive cells, will possibly be useful for receptor identification. Identification of the factor(s) that cause up-regulation of CXCL14 in tumor epithelial cells is another relevant issue warranting further analyses. Understanding the mechanisms through which the chemokine CXCL14 promotes cell transformation and tumor progression is key to effective therapy. We examined the expression pattern of CXCL14 in CRC and its clinical value based on a small number of cases. Thus, there is a requirement for further investigations to clarify the correlation of CXCL14 expression with the clinical outcome of CRC with a larger patient cohort.