Treatment for advanced tumors: SRC reclaims center stage.

The non–receptor protein tyrosine, Src, is a 60-kDa protein that is the archetypal member of a nine-gene family, including Src, Yes, Fyn, Lyn, Lck, Hck, Fgr, Blk, and Yrk, that plays a critical role in regulation of proliferation, differentiation, migration, adhesion, invasion, angiogenesis, and immune function (for detailed reviews, see refs. 1–4). All Src family kinases are myristoylated at the NH2 terminus, and several are additionally palmitoylated downstream of the myristoylation site. These fatty acid moieties are required for localization to cellular membranes. Each Src family kinase contains a poorly conserved (‘‘unique’’) domain and three conserved Src homology domains: SH2, SH3, and SH1 or protein tyrosine kinase domain. Critical to the regulation of Src is a COOHterminal tyrosine (Y530) that, when phosphorylated by Cterminal Src kinase (Csk), leads to a more inactive Src conformation, as has been shown by numerous molecular studies and confirmed by crystal structures (5). An autophosphorylation site (Y418) is indicative of, but not entirely diagnostic for, Src activation. Biochemical and structural studies have discerned the general mechanism by which Src activity is regulated (6). A myriad of cellular stimuli, including polypeptide growth factors, hormones, integrin aggregation, stress, cell cycle progression, and undoubtedly others, lead to the phosphorylation of signaling proteins for which the Src SH2 domain has higher affinity than for its own COOH-terminal phosphorylated site. Src thus associates with numerous proteins (depending on the input signal) and assumes an active conformation through phosphatase-mediated dephosphorylation of Y530 and autophosphorylation of Y418. Through its SH2 and SH3 domains, Src further associates with structural and signaling proteins, and the resulting complexes are critical to Src’s role in diverse cellular processes, as illustrated in Fig. 1. Specificity of Src signaling stems from the composition of these complexes as well as their duration and subcellular localization and may be additionally influenced by phosphorylation sites on Src mediated by protein kinase C, protein kinase A, cdc2 kinase, and growth factor receptors (7). In turn, Src substrates include signaling enzymes, cytoskeletal proteins, mitotic regulators, etc. (8). Aberrantly high Src activity is seen in diverse cancers (9). Although a number of mechanisms may account for these increases, including increased transcription (10) and perhaps very rare activating mutations (4), activation is likely primarily a consequence of genetic and epigenetic alterations in tumor cells. As examples, overexpression and mutation of growth factor receptors and alterations in integrin regulation and downstream mediators, such as overexpression of focal adhesion kinase, frequently lead to activation of Src and deregulation of one or more of the pathways depicted in Fig. 1. As these epigenetic changes accumulate, Src activity increases during cancer progression, with the highest expression and/or activity of Src observed in metastases (11, 12). Src activity can be predictive of poor prognosis (13). Thus, current expectation is that Src inhibitors will hold the most promise for later stage neoplasms, and clinical trials are focusing on this potential. Src activation is not always confined to late-stage tumors, however, as it occurs in colon polyps of high malignant potential (14) as well as in ulcerative colitis (15) and can promote hyperproliferation in mouse models of skin carcinogenesis (16). Thus, in some malignancies, Src inhibitors may have efficacy in earlier-stage disease.