By Megan Knight, BSc
In 1911, Francis Peyton Rous made a groundbreaking discovery that certain types of viruses could trigger sarcoma formation in chickens. At this time, when filtration methods that purified particles smaller than both eukaryotic and bacterial cells had just been developed, Rous injected a filtrate containing sarcoma cells of a chicken into another chicken. In the following weeks, the sarcoma formation was observed in the injected chicken. This experiment showed that viruses could induce sarcoma formation, and therefore cancer.
The virus particle that Rous isolated in his experiments was therefore named ‘Rous Sarcoma Virus’ (RSV). This is a simple retrovirus, which encodes genes such as glycoproteins and proteins needed for genome coat formation, and a reverse transcriptase that is essential for viral replication in host cells.
In the 1960s, RSV was introduced into cultured cells. Introduction of RSV induced abnormal morphology and cell behavior causing ‘foci’ formations in cell cultures. This confirmed that RSV could transform cell phenotypes triggering cells to become cancerous. A subsequent experiment showed that transformation of cells to the cancerous phenotype relies on active viral proteins. At 37oC, a temperature sensitive RSV mutant has stable polypeptides, and shows its ability to transform cell phenotypes. However, when the temperature is increased to 40oC, the cell phenotype reverses to a normal morphology due to de-stabilization and de-naturation of polypeptides encoded by the virus.
Identification of Src
RSV possesses a gene encoding a protein called ‘Src’, which is a protein tyrosine kinase. It was shown that Src is an oncogene, which is a gene that possesses the potential to transform normal cells into cancer cells if a mutation causes aberrant activation. Homology studies found that the Src gene is of avian origin; this suggests that an avian virus strain infected a chicken cell and integrated into the host cell’s chromosomal DNA, next to the c-Src gene. When packaging capsids, c-Src was included thereby forming the v-Src gene present in RSV virions.
Src is a 60kDa protein and is a one of nine Src family kinase members. Its three major domains are Src homology (SH) SH2 and SH3, and the catalytic domain that contains the kinase active site. SH2 and SH3 are both needed for protein-protein interactions. C-Src is activated either by protein-protein interactions or by phosphorylation events. Tyr416 and Tyr 527 are the two main phosphorylation sites in Src. Tyr416 can be intrinsically phosphorylated in the presence of mitogenic signals that remove the activation loop from the catalytic cleft. This allows c-Src to phosphorylate downstream targets involved in the promotion of cell proliferation. Tyr527 phosphorylation by other proteins negatively regulates c-Src activity, causing inactivation of its kinase activity by blocking the catalytic cleft.
When becoming incorporated into RSV, c-Src loses an important tyrosine residue needed for its inactivation that forms v-Src. As a result, v-Src loses its ability to be inactivated and is constitutively active as an oncogene in infected host cells.
Src’s Role in Cancer Formation
In humans, Src is ubiquitously expressed. In mammalian cells, Src has pleiotropic effects on cell morphology, adhesion, invasion, proliferation, and differentiation. Normally, Src is predominantly inactive in cells and only activated in the presence of high concentrations of mitogens. If this balance is disrupted, Src activity becomes aberrant.
Over activation of Src kinase is common in many types of cancers, including colon and breast cancer. Src can become mutated and lose the domain containing the residue Tyr527, needed for its inactivation, or proteins which regulate Src can become mutated and result in its increased activation in the absence of a mitogen. Proteins activating Src are often over expressed in cancer cells, and proteins negatively regulating Src are down regulated in cancer cells.
Src over activation in cancer cells promotes processes such as the endothelial-mesenchymal transition, which is required for metastases, cell survival, mitogenesis, invasion, and angiogenesis. It also reduces cell adhesion, reducing contact inhibition of cancerous cells.
Src as a Therapeutic Target
Due to Src over activation and its oncogenic actions in a wide spectrum of cancers, it has been identified as a key molecule in tumor progression. This makes inhibition of Src a promising target for anti-cancer therapies.
Clinical trials have shown that Src inhibition reduces cancer progression in several types of cancer, such as breast, pancreatic and ovarian cancer. Src inhibitors are small molecules that are designed to be used in combination with other chemotherapeutic agents. Several such molecules are currently under clinical trial and have shown promising preliminary results.