Introduction
Gastrointestinal stromal tumors (GISTs) are the most common type of gastrointestinal mesenchymal tumor (GIMT), and can be classified as being of spindle, epithelioid or mixed cell type based on morphology (1). Recent studies have identified that a gain-of-function mutation in the c-kit gene can be found in 95% of GIST cases. This mutation facilitates ligand-independent activation of the tyrosine kinase function of KIT [also known as cluster of differentiation (CD)117] and can be used to aid the diagnosis of GISTs (2,3). GISTs are commonly diagnosed on the basis of the expression levels of a range of proteins, which include CD117, CD34, smooth muscle actin (SMA), desmin and S-100 (4,5). However, a small percentage of GIST cases are negative for CD117 expression, which can complicate the diagnosis (6).
The primary treatment modality for GISTs is surgical removal, with traditional radiation and chemotherapy being ineffective (7). Furthermore, the recurrence and metastasis rate following surgical treatment is as high as 80%, and the five-year survival rate is only 50% (8). Treatment with imatinib offers hope to GIST sufferers, but this also has limitations (9). Effective decision-making as to which treatment option to pursue may therefore be aided by accurate evaluation of the biological behavior of individual GIST cases.
Since its proposal, the precise definition of GIST has been controversial. Currently, it is universally acknowledged that GISTs possess malignant potential; however, the classification of the degree of this malignancy does not have a common standard (10). It is typically believed that the tumor size and mitotic activity are the most important criteria to distinguish benign from malignant tumors (11). The study by Fletcher et al provides a reasonable and detailed classification that divides GISTs into four grades, with an extremely low, low, moderate or high risk of invasion based on tumor size and mitotic activity (12). However, studies have shown that certain small tumors that are <2 cm in size and without detectable mitotic activity possess an invasive capacity (7,10,13). Therefore, this field remains an active research area, with current efforts focused on seeking novel molecular markers that have a higher sensitivity and specificity for the prediction of the biological behavior of tumors (14).
CD171, also known as L1, is an adhesion molecule that regulates attachments between neighboring cells, or between cells and their surrounding extracellular matrix (ECM) through interactions with additional L1 molecules or with other ligands, such as integrin β1 and CD24 (15). L1 is important in the development of the nervous system, and high expression levels of L1 protein have been identified in nerve-derived tumors and other solid tumors, including pancreatic cancer, ovarian cancer, melanoma, small cell lung cancer, colorectal cancer and breast cancer, where it is associated with metastasis (16,17). Similar to L1, CD24 is also an adhesion molecule, and is expressed on the surface of human B cells and cells of the nervous system. Recently, high levels of CD24 have been identified on cancerous cells of the hematopoietic system and certain solid tumors. In such cases, CD24 functions as a ligand of P-selectin and has been indicated to be involved in the invasion and metastasis of tumors (18). Integrin β1 is an important receptor for cellular adhesive molecules, and it plays a central role in the processes of tumor formation, growth, differentiation and metastasis, by mediating signal transduction and regulating adhesion between cells or between a cell and the ECM (19). The present study examined the expression levels of L1, CD24 and integrin β1 in human GIST samples, and assessed the association between these molecules and the invasion, local recurrence and distant metastasis of cancer cells, and patient prognosis.
Discussion
In 1983, Mazur and Clark discovered that the gastric stromal tumors once diagnosed as leiomyoma (smooth muscle tumors and leiomyosarcoma) or schwannoma lacked immunohistochemical staining for SMA or S-100 protein, and that no particular cell features could be observed under the electron microscope (1). Therefore, the term ‘gastric stromal tumor’ with a neutral tissue origin was suggested (1). Recently, a gain-of-function mutation of c-kit has been found in the majority of GISTs, which results in the ligand-independent activation of tyrosine kinase and the expression of CD117 (KIT), providing the basis for the clinical diagnosis of GISTs (2,20). However, a small proportion of GIST cases with no CD117 expression remain, which introduces difficulties for the diagnosis and differential diagnosis of GISTs. The accurate evaluation of the biological behavior of GISTs is associated with the treatment and outcome of patients. The dispute has existed since the concept of GIST was proposed. Although it is well-established that all the GISTs have malignant potential, its malignant degree classification does not have a uniform standard. The tumor size and mitotic activity are typically considered as the most important criteria to differentiate between benign and malignant tumors, which is largely consistent with the classification standard proposed by Fletcher et al (12). However, studies have suggested that certain tumors, with a size of <2 cm and without mitotic activity, may also metastasize (21,22). Therefore, a search for markers of the biological behavior of GIST still has important research value for clinical application.
L1 is a transmembrane adhesion molecule originally believed to be expressed only in the nervous system, where it is involved in mediating adhesion between neurons, axonal spontaneous contraction, synapse formation, axon growth and neuronal migration (23). Expression of L1 has since been identified in various tumors of the intestine, urogenital tract, lung, rectum and kidney, and in rhabdomyosarcoma (24). High expression levels of L1 were first identified in GISTs in 2006 using immunohistochemical staining, with 73.6% (53/72) of sampled tumors expressing L1 at a high level (25). However, L1 is not expressed in fibromatosis or in gastrointestinal smooth muscle tumors.
CD24 is a highly glycosylated adhesion molecule of low molecular weight, which has been found to be expressed at high levels on the cell membrane within human hematopoietic system tumors and solid tumors, including primary liver, gastric, colorectal and breast cancers (26–28). As an adhesion molecule receptor, integrin β1 plays a role in the developmental processes of tumors, mainly by mediating the adhesion between cells and between the cell and the ECM (29,30). The present data found that L1, CD24 and integrin β1 were expressed at a significantly higher level in GISTs than in normal control tissues. Smooth muscle tumors had barely detectable L1 expression, while there was no significant difference in L1 expression between GISTs and schwannomas. No differences in CD24 and integrin β1 expression were identified across the three types of tumors. These results suggested that L1, CD24 and integrin β1 could be useful markers in the differential diagnosis of GISTs.
L1 is an adhesion molecule with a role in regulating cell attachments and an important signaling molecule (31). CD24 is an adhesion molecule that mainly localizes to the membrane surface (32), and integrin β1 is also an adhesion molecule receptor (33). The interaction of these three molecules has played a positive role in the migration of tumor cells (34). Fogel et al (35) found that the overexpression of L1 in melanoma promoted tumor progression and metastasis, and was closely associated with the prognosis of the patients. Gavert et al (36) determined that L1 could promote the progression and metastasis of colon cancer with the use of cell culture and animal models, and further elucidated the responsible mechanism. Firstly, L1 itself has a weak adhesion capacity, but this can be enhanced through interaction with homotypic or heterotypic molecules (37). The combination of L1 and CD24 molecules alters their conformation, facilitating the activation of endothelial cells and platelets, and enhancing the attachment of L1 with ECM ligands (38). Furthermore, the interaction of CD24 and L1 can increase the intracellular calcium concentration and strengthen signal transduction, promoting adhesion (39). Secondly, the integrin family contains multiple L1 receptors, and L1 mainly combines with integrin β1 in the presence of Mn2+. This combination not only mediates the transfer of L1 into cells to reduce attachments between the tumor cells, but also induces the increased expression of adhesion molecule receptors in vascular endothelial cells and the ECM, promoting the adhesion and migration of tumor cells in these environments (40). Lastly, integrin β1 is the receptor for CD24, and its interaction with L1 is regulated by CD24, likely through a change in the conformation of an arginine-glycine-aspartate sequence following the binding of CD24 and L1 (41). The present data revealed that the expression levels of L1, CD24 and integrin β1 were significantly correlated in the GIST cases, and that they were also associated with the expression of Ki-67. Moreover, L1 and integrin β1 levels were significantly increased with increased invasion risk in the GISTs. However, this was not the case for CD24 expression, indicating the synergistic effects of L1, CD24 and integrin β1 in the progression of GISTs. Therefore, the detection of L1 and integrin β1 in combination with Ki-67 favors an accurate prediction of the biological behavior of GISTs, while elucidation of the full function of CD24 in GIST requires further investigation.