Tumour-associated processes such as invasion and metastasis are critically dependent on dynamic alterations in the organization of the actin cytoskeleton. Dysregulation of actin-based motility is a prominent factor in cell transformation and probably is associated with carcinogenesis [
22,
23]. To date, a large number of actin-binding proteins have been cloned, many of which are involved in the malignant transformation process; and distinctive protein expression patterns of some of these genes in cancer and progressive carcinogenesis processes have been observed [
24‐
28]. Previously, we reported significant overexpression of the actin-binding protein CapG expression at the protein level in OSCC-derived cell lines using a 2-D-DIGE system and MALDI-TOF/MS [
7].
CapG is a member of the gelsolin superfamily of the actin-binding proteins [
29]. In addition to respective roles in actin filament remodelling, the proteins of the gelsolin superfamily have specific roles in several cellular processes, including cell motility, signal control, and apoptosis and regulation of phagocytosis [
30,
31]. Variations in expression of the gelsolin superfamily proteins are thought to affect major cytoskeletal changes during differentiation and carcinogenesis, and considerable evidence has shown a significant association between the proteins and a wide range of human malignancies including OSCCs [
32‐
35]. Experimental evidence has shown that CapG also is crucial for regulating cell motility [
36,
37]; however, its exact function in the development and progression of malignant tumours remains controversial. Watari et al. reported an apparent decrease in CapG protein levels in some human tumor cell lines compared with their corresponding benign counterparts. In addition, those investigators showed that CapG protein expression reduces the ability of a transformed cell to induce tumour formation, suggesting that CapG is a tumour suppressor gene [
38]. However, accumulating evidence has indicated a number of theories on the possible function of CapG as a tumor activator. A recent study identified CapG as a target of the AP-1 transcription factor complex, which has emerged as a critical regulator of gene expression in response to the activation of a variety of oncogenic signal transduction cascades, including c-Fos and c-Jun [
39]. De Corte et al. reported that overexpression of CapG promotes cancer cells to invade collagen through the Ras-phosphoinositide 3-kinase signaling pathway [
40]. More recently, they reported that targeting human cancer cells, including breast and prostate cancer cells with an RNAi procedure against CapG, significantly reduced the invasive and motile properties of both cells examined as well as cell aggregation [
41]. Similar results were obtained by others in pancreatic cancer cells [
42]. These findings support the hypothesis that CapG also is a tumour activator. Furthermore, significant overexpression of CapG was reported at both the mRNA and protein levels in several types of human primary tumours, i.e., ocular melanomas [
43], glioblastomas [
44], and pancreatic ductal adenocarcinomas [
42], all of which are aggressive cancers that kill patients by metastasis and local invasion. Because OSCC is also a solid neoplasm exhibiting aggressive tumour phenotypes, we hypothesized that CapG is a potential emerging therapeutic target of interest for the treatment of oral cancer. However, the status of CapG in OSCC remains unclear and therefore we selected it for further investigation.
To confirm our hypothesis, we determined the protein/mRNA expression in a series of human primary OSCCs using IHC. By evaluating the CapG-IHC scores, significant up-regulation was evident in the primary OSCCs compared with normal tissues (P < 0.01). The CapG protein expression levels in primary OSCCs were significantly associated with tumor size (P = 0.014). In pancreatic ductal adenocarcinomas, a clinical study also showed that high CapG expression was correlated with increased tumour size [
42], which was consistent with results obtained in the current study. Moreover, the state of CapG protein expression differed significantly between the early stages (I and II) and the advanced stages (III and IV) (Mann-Whitney U-test, P < 0.001). Given the known roles of CapG in cytoskeletal organization and cell migration, these findings indicated the potential clinical significance of CapG as a marker or prospective therapeutic target for the most aggressive forms of OSCCs. Furthermore, high levels of CapG protein expression were detected even in the OPLs examined, suggesting that CapG may play an important role in early-stage OSCC development. However, the development of OSCC is generally predicated on the development of multiple clonal genetic alterations [
45,
46], and additional research is needed to establish whether and how CapG-stained OPLs give rise to strongly CapG-stained OSCCs.