In this study, a total of 41 up-regulated genes and 79 down-regulated genes were identified from three microarray data sets and thoroughly analyzed. Pathway analysis showed that these genes are mainly involved in extracellular matrix organization and proteoglycans in cancer. Several hub genes, CXCL8, THBS1, KIT, and DCN, were found in the PPI network, and interestingly, also found in the predicted drug-gene interactions. However, according to the independent test results of the TCGA database, the difference of CXCL8, THBS1 and KIT in mRNA expression changes was significant. In addition, it was verified in clinical samples that the expression level of the three genes was consistent with the mRNA expression pattern.
The extracellular matrix (ECM) performs many functions in addition to its structural role; as a major component of the cellular microenvironment it influences cell behaviors such as proliferation, adhesion and migration, and regulates cell differentiation and death[18]. Abnormal ECM dynamics can lead to deregulated cell proliferation and invasion, failure of cell death, and loss of cell differentiation, resulting in congenital defects and pathological processes including tissue fibrosis and cancer. Proteoglycans, as ECM constituents, is lost in aged fibroblasts, resulting in a more aligned ECM that promoted metastasis of melanoma cells[19].
CXCL8(interleukin-8) is considered to be a typical chemokine belonging to the CXC family, responsible for the recruitment and activation of neutrophils and granulocytes at the site of inflammation. Its role in the progression of melanoma mainly depends on its interaction with specific cell surface G protein coupled receptor (GPCR), C-X-C chemokine receptor type 1 (CXCR1) and C-X-C chemokine receptor type 2 (CXCR2)[20-22]. Varney et al. examined the expression of CXCL8, its receptors, CXCR1 and CXCR2, and vessel density in human melanoma by immunohistochemical analysis of tumors from different Clark levels, depths and thicknesses, and found that the expression of CXCL8 and CXCR2 was lower in Clark level I and II specimens than in level III through V specimens and metastases[23]. It indicates that the expression of CXCL8 and CXCR2 contributes to the aggressive growth and metastasis of human malignant melanoma. Three years later, a live mouse study demonstrated that CXCR2 plays a key role in melanoma lung metastasis through a gene knockout model[24]. In addition, Wu et al. evaluated the role of CXCL8 in the growth and progression of melanoma by regulating its expression in melanoma cell lines expressing different levels of CXCL8, and found that the expression of CXCL8 is a key in regulating multiple cell phenotypes associated with melanoma growth and metastasis[25]. It shows that CXCL8 is an important biomarker in the process of melanoma metastasis.
As a matricellular glycoprotein, THBS1 regulates cellular phenotype and extracellular structure during tissue genesis and remodeling, and has been shown to regulate tumor progression and metastasis[26, 27]. There is increasing evidence that the acquisition of invasive and metastatic features of melanoma cells involves the reactivation of a developmental EMT-like program[28-30]. More importantly, the results of the biological pathway enrichment of DEGs in the study also confirmed this conclusion. As the main physiological activator of transforming growth factor-β (TGF-β), THBS1 may activate the latent TGF-β1 in the progress of melanoma to promote EMT of melanoma[31-33]. Another study also validated that increased expression of THBS1 is associated with an invasive and metastatic phenotype of melanoma, as part of a Slug-independent motility program that includes the melanoma-related VEGF/VEGFR-1 and FGF-2 pathways[34]. In addition, THBS1 has been shown to promote cell invasion of breast cancer, thyroid cancer, colon cancer and prostate cancer. Therefore, we can draw a clear conclusion that THBS1 promotes the invasion and metastasis of melanoma, which is expected to become a target for future treatment.
KIT, a tyrosine kinase receptor encoding stem cell factor, plays an important role in the development, migration and proliferation of melanocytes[35, 36]. Although KIT is expressed in some melanomas, as the disease progresses from the superficial stage to infiltration and then to the metastasis stage, the loss of KIT expression indicates that KIT has tumor suppressive function[37-39]. A recent study also found that in patients without lymph node metastasis at the initial diagnosis, the expression of KIT was significantly higher than that of patients with lymph node metastasis, indicating that melanoma with missing KIT expression is more likely to progress and metastasize[40]. In addition, KIT is the target of several small molecule inhibitors such as imatinib and nilotinib. These drugs have been used clinically and can significantly extend the lifespan of patients with metastatic melanoma carrying KIT mutations[41, 42]. Therefore, we believe that it mediates the metastasis of melanoma and can be used as a target for the treatment of metastatic melanoma[43].
In this study, we highlighted the potential role of CXCL8, THBS1 and KIT in melanoma metastasis. However, we acknowledged that the study has some certain limitations. Although we have verified the differences in mRNA and protein expression levels of these genes in TCGA databases and clinical specimens, in our future studies, the biological function of these genes in melanoma needs further study.