The invasive growth of GB makes complete tumor resection challenging and induces the tumor to be prone to recurrence [
27,
28]. Elucidating the molecular mechanisms underlying GB cellular migration and invasion, as well as screening for potential therapeutic targets, have received significant attention in the GB research community. The present study showed that SWAP-70 was highly expressed in human high-grade glioma tissues, and high-grade glioma patients with high expression of SWAP-70 in tumor tissues had a poorer prognosis and shorter median survival time compared to those with low SWAP-70 expression, suggesting that SWAP-70 plays an important role in the degree of malignancy of GBs. To verify these findings, we clarified that SWAP-70 downregulation inhibited GB cellular migration and invasion. In contrast, SWAP-70 overexpression promoted GB cellular migration and invasion. Importantly, our results suggest that SWAP-70 in GB may regulate CD44s protein expression to affect GB cellular migration and invasion.
As SWAP-70 is a GEF, it participates in cytoskeletal rearrangement through guanosine triphosphate-guanosine diphosphate conversion and promotes the formation of membrane folds and pseudopods, thereby regulating cellular movement [
29,
30]. Recently, many studies have shown that SWAP-70 is involved in the activation of B-cell signaling and may have potential carcinogenic functions in tumorigenesis [
18,
19,
31]. Heerema et al. conducted immunohistochemistry on 86 cases of human B-cell neoplasms, and all six cases of nodular lymphocyte-predominant Hodgkin lymphoma had positive SWAP-70 staining [
32]. Fukui et al. studied mouse embryo fibroblasts (MEFs) and showed that cells lacking SWAP-70 expression failed to grow on soft agarose, whereas cells overexpressing SWAP-70 were susceptible to cloning. Inoculation of MEFs into nude mice showed that tumor volumes from MEFs lacking SWAP-70 were significantly smaller than those from MEFs expressing exogenous SWAP-70 [
33]. These results indicate that the carcinogenesis of MEFs requires the involvement of SWAP-70, suggesting that SWAP-70 may be an oncogene. SWAP-70 is involved in tumor cell migration and invasion in GB [
17]; however, the underlying mechanism has not been fully clarified. In the present study, we established GB cell lines with stable SWAP-70 downregulation and SWAP-70 overexpression and used in vitro scratch assays and transwell invasion assays to show that SWAP-70 promoted GB cellular migration and invasion. Interestingly, SWAP-70 expression was positively correlated with the expression of CD44s in GB tissues. Downregulation of SWAP-70 also reduced CD44s protein expression. In contrast, SWAP-70 overexpression enhanced CD44s protein expression. We suspect that there may be a regulatory relationship between the two proteins in GB invasion and metastasis, and that both SWAP-70 and CD44s may be synergistically involved in GB cellular migration and invasion.
CD44 is a transmembrane glycoprotein that is expressed in the plasma membrane. The human
CD44 gene is located on the short arm of chromosome 11 and has 20 highly conserved exons [
34]. CD44 molecules are classified into the following two categories based on the types of exons contained in the
CD44 gene: CD44s and the variant isoform of CD44 (CD44v) [
35]. The structural difference between CD44s and CD44v is mainly in terms of the extracellular segment. Alternative splicing during transcription of the structural variation region in the extracellular segment results in different protein expression levels [
35]. CD44 is involved in intercellular adhesion. CD44 binds to cytoskeletal proteins and participates in cellular movement, as well as intracellular and extracellular signal transduction [
36,
37]. Abnormal expression of CD44 significantly enhances tumor cell migration, which is closely related to tumor metastasis [
35,
38,
39]. Research has shown that CD44s is the predominant form in human GB, accounting for approximately 83.3% of CD44 types [
40]. CD44s plays an important role in GB cellular migration and invasion and is involved in a variety of signaling pathways to promote GB cellular migration and invasion [
24,
26]. In addition, the binding of the GEF T-cell lymphoma invasion and metastasis 1 (Tiam 1) to the intracellular domain of CD44v3 promotes breast cancer cellular invasion and migration [
41]. The intracellular domain of CD44 is conserved between CD44s and CD44v. SWAP-70, another GEF, may also have a similar mechanism to regulate GB invasion and metastasis. In the present study, our results showed that changes in SWAP-70 expression affected CD44s expression, while CD44s did not affect SWAP-70 expression, suggesting that SWAP-70 may be upstream of CD44s and therefore regulate CD44s protein expression. Further experiments showed that SWAP-70 did not affect the mRNA expression of
CD44s but regulated the expression of CD44s protein. The dissociation of CD44 is regulated by protein kinase C, calcium ion flux, and small G-proteins of the Ras homologous (Rho) family. The Rho family molecules recruit metalloproteinases to surround CD44, promote CD44 dissociation, and alter the CD44 distribution on the plasma membrane [
42]. SWAP-70 regulates cytoskeletal changes through Rac activation. Further studies will be necessary to determine if SWAP-70 affects CD44s expression through Rho family molecules. In addition, we overexpressed CD44s in GB cell lines with downregulated SWAP-70 and showed that CD44s overexpression partially reversed the inhibitory effect of the downregulated SWAP-70 on GB cellular migration and invasion. The above-mentioned results suggest that SWAP-70 is involved in GB cellular migration and invasion via regulation of CD44s expression.