SRC is a proto-oncogene encoding a non-receptor tyrosine kinase, similar to the v-Src gene of the Rous sarcoma virus (14), which was initially discovered by Bishop and Varmus (15). The Src protein is formed of seven functional regions: i) N-terminal Src homology domain 4 (SH4) containing a myristic acid moiety, essential for its localization to the inner surface of the cell membrane; ii) a unique domain providing functional specificity to each member of the Src family; iii) SH3 domain, which binds proline-rich sequences to mediate intra- and intermolecular interactions; iv) SH2 domain, which binds phosphorylated tyrosine residues on Src and other proteins; v) a catalytic domain (SH1); and vi) C-terminal tail containing negative-regulatory Tyr530 (in humans) (16–18) (Fig. 1).

The activity of Src is regulated by the structural changes that occur following phosphorylation and dephosphorylation of its tyrosine residues, which is determined by the relative activities of protein kinases and phosphatases (19). The enzymatic activity of the 60 kDa human c-Src tyrosine kinase is predominantly regulated at two phosphorylation sites: Tyr527 and Tyr416. Phosphorylation at Tyr527 reduces the activity of Src, while dephosphorylation of phosphotyrosine 527 increases activity; autophosphorylation of Tyr416 also enhances activity (20,21). Phosphatases that may interact with phosphotyrosine 527 include cytoplasmic protein tyrosine phosphatase (PTP) 1B, Shp1 and Shp2, and transmembrane enzymes including CD45, PTPα, PTPε, and PTPκ (22,23). Furthermore, PTP-BL and PTP-BAS have been shown to dephosphorylate phosphotyrosine 416 to decrease Src kinase activity (24) (Fig. 2).

Src has been identified as an important factor in several human malignancies, and in the promotion of tumor progression during the multistep process of cancer pathogenesis (25). Src deregulation primarily involves protein overexpression and abnormalities in Src kinase activity. Differences in Src expression have been observed in lung, breast, pancreatic, colon and prostate cancer cells, compared with normal adjacent tissue, fibroblasts or normal mucosal cells (26–31). In the tumor microenvironment, Src activation has been observed in cancer and inflammatory cells, and may serve as a critical mechanistic link between inflammation and cancer. Src propagates a cycle between immune and tissue cells, ultimately leading to the development and progression of cancer (32,33). The abnormal activation of Src may result in the promotion of survival, angiogenesis, proliferation and invasion pathways observed in tumors cells (34,35). However, despite the evidence indicating a major role for Src in the development and progression of cancers, its mechanism of action is not fully understood.

A number experimental studies have proposed that Src may be involved in the transmission of signals from extra and intracellular stimuli. Interactions between the Src pathway and Signal Transducer and Activator of Transcription (STAT) 5, STAT3, N-cadherin and basic fibroblast growth factor receptors and β-catenin have been reported in melanoma cells (36,37). It may also be of value to understand the effect of Src inhibition on a number of the environment-sensing and growth-promoting pathways known to be aberrant in cancer cells, including the phosphoinositide 3-kinase/protein kinase B/mammalian target of rapamycin (PI3K/Akt/mTOR), Ras/mitogen-activated protein kinase (MAPK), platelet-derived growth factor (PDGF), Erb1/Erb2 and vascular endothelial growth factor (VEGF) pathways (38–40). Currently, the complex interactions between Src and other pathways remain to be established. The crosstalk signaling mechanisms that link inflammatory cells with cancer cells, including SDF-1-CXCR4-Src and Src-IL-6 signaling axes, result in a cycle leading to cancer development and progression (41–43). In leukemia, SDF-1 has been found to induce ‘inside-out’ signaling, which involves CXCR4 and Lyn, leading to aberrant adhesive responses. Furthermore, previous studies have shown that Src and Hck, the Src family members, are involved in the production of IL-6 in osteoblasts and inflammatory macrophages (42,43).

Src was the first transforming protein and the first gene product with protein tyrosine kinase activity to be discovered and isolated (44). With the use of immunohistochemistry and Western blotting, the total Src and phosphorylated Src (Y419) were found to be activated in human sarcoma tissues (leiomyosarcoma, high-grade osteosarcoma and liposarcoma) and sarcoma cell lines (osteosarcoma, Ewing's sarcoma, leiomyosarcoma and rhabdomyosarcoma) (45). Furthermore, Src was identified as one of the most strongly phosphorylated kinases in synovial sarcoma cells (46). Src activity was demonstrated to be upregulated in anoikis-resistant human osteosarcoma cells, SAOS-2, compared with their parental population (47).

With regard to different subtypes of sarcoma, Src is thought to be the most reliable discriminator to distinguish high-grade leiomyosarcoma from undifferentiated pleomorphic sarcoma, based on gene expression profiling and meta-analysis (48). Due to its aberrant expression in sarcoma, Src has been proposed to be important in signal transduction in human sarcomas, including osteosarcoma, rhabdomyosarcoma, leiomyosarcoma, fibrosarcoma and Ewing's sarcoma (49).

A fundamental trait of cancer cells is their ability to sustain chronic proliferation. The overexpression of Src in U2OS and MG63 osteosarcoma cells significantly enhances proliferation and reduces apoptosis of these cells (45,50). In human osteosarcoma cells SAOS-2, Src was revealed to be activated in anoikis resistance (47). Furthermore, Src was identified in 0–20% chondrosarcoma specimens. However, its expression had no prognostic significance, particularly in serving as an indicator of cell proliferation (51). Src and its downstream signaling via the p38 MAPK-AKT pathway may be activated by the signaling adaptor protein, Crk, to promote proliferation of human synovial sarcoma cells (52,53). Inhibition of Src signaling in Ewing's sarcoma cells was observed to induce apoptosis (45).

These findings indicate that Src may increase sarcoma proliferation and reduce apoptosis. However, in some subtypes of sarcoma, there is conflicting evidence with regard to the expression of Src. For example, high Src expression has been identified in high-grade leiomyosarcoma, while Src expression has been found to be variable in chondrosarcoma (48). Additionally, the mechanisms of proliferation and apoptosis require further investigation.

Despite continual research and increasing knowledge of the biology of sarcoma, invasion and metastasis remain poorly understood, and are the predominant cause of sarcoma-related mortality. The ability of cancer cells to leave their primary site of growth, move into different tissue compartments, and survive and proliferate in these foreign environments, defines the biological program known as ‘invasive growth’ (54). Invasive growth is important for cancer progression and thus, presents a target for the treatment of sarcoma. In mouse models of osteosarcoma, depletion of Src phosphorylation in SaOS-2 cells has been shown to decrease tumor mass (55). However, other reports indicate that inhibition of Src phosphorylation in HOS and SaOS-2 cells may only decrease the metastatic potential of osteosarcoma cells in vitro, and not in vivo (56). The effect of Src on the metastasis of osteosarcoma cells is therefore controversial. A number of studies reported that inhibition of c-Src signaling was able to reduce metastasis of chondrosarcoma (57,58). Other studies found that Src inhibition could overcome chemoresistance to induce apoptosis and to inhibit migration (59). In Ewing's sarcoma cells, inhibition of c-Src was also observed to reduce migration and metastasis (45).

It has been established that epithelial cells may acquire migratory capability, a feature typical of the mesenchymal cells, and gain invasive ability, resistance to apoptosis and the ability to disseminate (60), in a process known as the epithelial-mesenchymal transition (EMT). EMT is a complicated process, whereby cancer cells acquire migratory and invasive abilities, which are influenced by the tumor microenvironment and intercellular communication. Src activity affects metastatic progression, suggesting that Src-induced EMT may be associated with enhanced metastatic potential (61). However, the effect of Src-related EMT has yet to be investigated in sarcoma.