Current Pharmaceutical Design, 2005, 11, 1119-1130 1119 1381-6128/05 $50.00+.00 © 2005 Bentham Science Publishers Ltd. Novel Insights Into c-Src Ö. Alper 1, * and E. T. Bowden 2 1 National Institute of Health, National Institute of Neurological Disorders, Surgical Neurology, Bldg 10, 5D37, Bethesda, MD, 2 Department of Oncology, Lombardi Cancer Center, Research Bldg W303, Georgetown University Medical School, Washington, D.C. Abstract: Since identifying a transmissible agent responsible for tumorigenesis in chickens, the v-Src oncogene, significant progress has been made in determining the functions of its cellular homologue. c-Src is the product of the SRC gene and has been found both over-expressed and highly activated in a number of human cancers. In fact the relationship between c-Src activation and cancer progression is significant. Furthermore c-Src may play a role in the acquisition of the invasive and metastatic phenotype. In this review we will summarize some of the latest evidence for the role of c-Src in tumorigenesis and particularly in human tumor progression. In this review, specifically, we will address growth signals, adhesion, migration, invasion, angiogenesis and functional genomics. Key Words: c-Src, v-Src, tumorigenesis, angiogenesis, genomics. INTRODUCTION Almost a century ago, in 1911, Peyton Rous described the transmissible agent, Rous sarcoma virus (RSV) that is responsible for the induction of tumors in chickens [1]. The first cellular homologue, c-Src, of the retroviral v-Src protein was identified in the late 1970s. This protein is expressed in normal cells [2, 3]. c-Src and v-Src are protein tyrosine kinases. Although c-Src is non-transforming, when expressed at high levels, it can be converted to a transforming protein by various amino acid substitutions including replacement or truncation of the carboxy-terminus and by dephosphorylation of a tyrosine specifically residue 527 (numbering is based on chicken c-Src) [4-11]. c-Src is widely expressed in most avian and mammalian cells, and at particularly high concen- trations in brain, platelets and bone-resorbing osteoclasts. All Src family members are composed of several well- characterized protein domains. The amino-terminus of Src is myristylated and most members of the Src family are also palmitoylated. These lipid modifications are essential for the targeting of Src kinases to the inner leaflet of cell mem- branes. Two Src homology domains (SH2 and SH3) are adjacent to the N-terminus and participate in protein-protein interactions. The catalytic domain has tyrosine kinase acti- vity that contains an Src trans-phosphorylation site, tyrosine residue 416, that must be autophosphorylated for maximal tyrosine kinase activity. Tyrosine-416 is highly conserved in tyrosine kinases and is the major phosphorylation site in v- Src. Detailed biochemical and x-ray crystallography studies have provided a model for the regulation of Src activation. The protein tyrosine kinase domain of c-Src is main- tained in an inactive state by intramolecular interactions. The SH3 domain of Src binds to a short proline type II helix region between the tyrosine kinase and the SH2 domains. *Address correspondence to this author at the National Institute of Health, National Institute of Neurological Disorders, Bldg. 10, Rm. 5D37, Bethesda, MD, 20892-1414; Tel: 301-496-1155; Fax: 301-402-2286; E-mail: alpero@mail.nih.gov The SH2 domain is bound to tyrosine residue 527 in the carboxy terminus of c-Src. These interactions are the basis for the “closed” and inactive conformation of c-Src. In this state, c-Src is tethered to the inner membrane leaflet via its lipid modifications and can be activated via a variety of signals. Src tyrosine kinase activity can result from a number of protein-protein interactions. For example, a PXXP motif can function as a binding site for the SH3 domain of Src [12]. The interaction of Src with a protein containing this motif will result in tyrosine kinase activation. This mechan- ism of activation is thereby mediated via molecular dis- placement of either SH2 or SH3 mediated intramolecular interactions by high affinity ligands. Further-more, many tyrosine-phosphorylated proteins can bind to the SH2 domain of Src and therefore will compete with the SH2 domain binding to tyrosine residue 527; this competition can also result in Src activation. In support of this model for activation, it has been demonstrated that binding of the SH2 domain of Src to either the activated platelet-derived growth factor receptor (PDGFR) or a phosphorylated peptide derived from the PDGFR can activate c-Src. However, there is no direct evidence that these mechanisms for activation have particular significance in tumorigenesis. The C-terminal Src tyrosine kinase (Csk) and its struc- turally related homolog, Csk homologous kinase (Chk), are both able to phosphorylate c-Src in vitro at the inhibitory carboxyl-terminal tyrosine residue 527 [13-16]. This interaction results in the change of c-Src molecule to closed or locked conformation [17-20]. Although the mechanism of Csk regulation is still poorly understood, downregulation has been observed in hepatocellular carcinoma, as compared with levels in normal liver and these reduced levels of expression correlate with enhanced Src activation [21]. Finally, Src can also be activated by protein tyrosine phosphatases that specifically dephosphorylate the tyrosine 527-autoinhibitory residue. This forces Src into an “open” conformation resulting in activation of the tyrosine kinase domain via autophosphorylation of a key tyrosine residue 416, required for full activation as mentioned above.