The cell adhesion process plays a key role in tumor progression. Several adhesion molecules, including ICAM-1, have been implicated in the tumor transformation and metastasis via the mediating adhesion of cancer cells to the vascular endothelium. This is an important procedure for executing the subsequent steps of invasion and survival of cancer cells . It has been shown that CRC metastases are related to the ability of cancer cells to adhere to the microvascular endothelium of the lung that expresses the ligands for ICAM-1 on the tumor cell surface . Previous studies demonstrated that ICAM-1 is expressed in CRC cells, and patients with higher ICAM-1 levels in plasma reveal poor prognosis compared to healthy controls [14, 17, 33]. In addition, up-regulation of ICAM-1 expression is correlated with the cancer stage and distant metastasis, suggesting that ICAM-1 plays a central role in tumor progression and metastasis . The findings of the present study suggest that the expression of ICAM-1 in DLD-1 cells may enhance their adhesiveness to HUVECs: 1) ICAM-1 was up-regulated by SDF-1 stimulation in DLD-1 cells; 2) functional blockade of ICAM-1 by siRNA and neutralizing antibody resulted in markedly decreased adherence of DLD-1 cells to HUVECs.
The expression of SDF-1 has been detected in several cancer cells, indicating that these pathways may be important in the metastasis of tumor cells [35, 36]. SDF-1 is a constitutively expressed and inducible chemokine that modulates biological and physiological processes, including embryonic development and organ homeostasis . Abnormal expression of SDF-1 has been detected in various solid tumor tissues such as ovarian cancer , breast cancer  and colorectal cancer . SDF-1 has been shown to promote the growth, invasion and metastasis of CRC cells , implying that SDF-1 plays a role in the progression of CRC; however, the mechanisms of how SDF-1 stimulates metastasis of CRC cells are not totally understood. In this study, we evaluated the molecular mechanisms underlying the roles of SDF-1 in modulating ICAM-1 expression and cell adhesion to HUVECs in colon cancer cells. Our results have demonstrated that SDF-1-induced expression of ICAM-1 expression is mediated by the MAPKs, and downstream Sp1, C/EBP-β and NF-κB signaling pathways.
The promoter region of the ICAM-1 gene has different binding sites for various transcriptional factors . Previous studies have shown that the transcription factors NF-κB, Sp1 and C/EBP-β can be activated through the MAPK pathways in colorectal cancer cells [20, 43, 44]. Regulation of gene expression through the use of combinations of different transcription factors has been observed [45, 46]. It has been reported that NF-κB and Sp1 can be activated by MAPK family members, depending on the target gene in different cells . Patel et al. reported that MAPKs mediate NF-κB and C/EBP-β activation, which leads to the transcription of specific genes . To evaluate the signaling factors upstream of the activation of NF-κB, Sp1 and C/EBP-β that lead to ICAM-1 transcriptional activation in DLD-1 cells, we investigated the role of MAPKs using specific inhibitors. In addition, we performed TF ELISA assays to demonstrate that the regulation of ICAM-1 gene expression in DLD-1 cells was mediated by increased NF-κB p65, Sp1- and C/EBP-β-DNA binding activities. Based on our results, we propose a possible signal transduction pathway in DLD-1 cells in which SDF-1 induces ERK, JNK and p38 phosphorylation, which activates NF-κB, Sp1 and C/EBP-β to lead to their binding to respective sites in the ICAM-1 promoter, thus resulting in ICAM-1 transcriptional activation.
Several studies have indicated that CRC cells express CXCR4 and CXCR7, the receptors for SDF-1, and that their survival and migration to distant tissues are promoted by SDF-1 [41, 49]. The binding of SDF-1 to CXCR4 may affect several signaling pathways, which can lead to multiple responses. Our previous study revealed that the expression of urokinase plasminogen activator in CRC DLD-1 cells by SDF-1 stimulation is mediated by CXCR4 . In order to analyze the role of CXCR4 in ICAM-1 expression in human CRC cells, we evaluated the effect of CXCR4 function-blocking antibody on SDF-1–induced ICAM-1 expression. Our results suggest that when DLD-1 cells encounter an external stimulus of SDF-1, SDF-1 induces ICAM-1 expression via CXCR4 up-regulation. Interestingly, DLD-1 cells did not express surface CXCR4 , but they had abundant CXCR4 after SDF-1 stimulation, as determined by flow cytometry. Up-regulation of CXCR4 cells is functionally important, as SDF-1-stimulated DLD-1 cells in contrast to control cells respond to SDF-1 by inducing the ICAM-1 expression and adhering to HUVECs in response to SDF-1. Previously it had been reported that CXCR4 surface expression is hardly detectable on neutrophils, but expression became apparent gradually after incubation . An alternative possibility described recently postulates that CXCR7 may serve as a co-receptor for CXCR4 and mediate SDF-1–induced intracellular signaling . Several studies have indicated that CRC cells also express CXCR7, the receptors for SDF-1, and that their survival and migration to distant tissues is promoted by SDF-1 . However, the mechanisms of CXCR4 expression in DLD-1 cells and the interaction between CXCR7 remains unknown, and further study is required to identify whether SDF-1 affects CXCR7 function and CXCR4 expression in CRC cells.
ICAM-1 expression modulates several cell functions and processes that regulate the progression of malignancies, including apoptosis, cell motility, invasion and angiogenesis. It has been shown that ICAM-1 may support mesothelial adhesion of CRC cells . Interaction of ICAM-1 on vascular or lymphatic endothelial cells has been found to promote the adhesion of lung cancer and breast cancer cells [54, 55]. In addition, ICAM-1 may be expressed by cancer cells themselves and contribute to their invasiveness .