Proto oncogene tyrosine protein kinase Src

Discovery

In 1979, J. Michael Bishop and Harold E. Varmus discovered that normal chickens contain a gene that is structurally closely related to v-Src. The normal cellular gene was called c-src (cellular-src). This discovery changed the current thinking about cancer from a model wherein cancer is caused by a foreign substance (a viral gene) to one where a gene that is normally present in the cell can cause cancer. It is believed that at one point an ancestral virus mistakenly incorporated the c-Src gene of its cellular host. Eventually this normal gene mutated into an abnormally functioning oncogene within the Rous sarcoma virus. Once the oncogene is transfected back into a chicken, it can lead to cancer.

Structure and function

There are 9 members part of the Src family kinases: c-Src , Yes, Fyn, Fgr, Yrk, Lyn, Blk, Hck, and Lck. The expression of these Src family members are not the same throughout all tissues and cell types. Src, Fyn and Yes are expressed ubiquitously in all cell types while the others are generally found in hematopoietic cells.

c-Src is made up of 6 functional regions: Src homology (SH) 4 domain (SH4 domain), unique region, SH3 domain, SH2 domain, catalytic domain and short regulatory tail. When Src is inactive, the phosphorylated tyrosine group at the 527 position interacts with the SH2 domain which helps the SH3 domain interact with the flexible linker domain and thereby keeps the inactive unit tightly bound. The activation of c-Src causes the dephosphorylation of the tyrosine 527. This induces long-range allostery via protein domain dynamics, causing the structure to be destabilized, resulting in the opening up of the SH3, SH2 and kinase domains and the autophosphorylation of the residue tyrosine 416.

c-Src can be activated by many transmembrane proteins that include: adhesion receptors, receptor tyrosine kinases, G-protein coupled receptors and cytokine receptors. Most studies have looked at the receptor tyrosine kinases and examples of these are platelet derived growth factor receptor (PDGFR) pathway and epidermal growth factor receptor (EGFR). When src is activated, it promotes survival, angiogenesis, proliferation and invasion pathways. It also regulates angiogenic factors and vascular permeability after focal cerebral ischemia-reperfusion, and regulates matrix metalloproteinase-9 activity after intracerebral hemorrhage.

Src contains at least three flexible protein domains, which, in conjunction with myristoylation, can mediate attachment to membranes and determine subcellular localization.

Role in cancer

The activation of the c-Src pathway has been observed in about 50% of tumors from colon, liver, lung, breast and the pancreas. Since the activation of c-Src leads to the promotion of survival, angiogenesis, proliferation and invasion pathways, the aberrant growth of tumors in cancers is observed. A common mechanism is that there are genetic mutations that result in the increased activity or the overexpression of the c-Src leading to the constant activation of the c-Src.

Colon

The activity of c-Src has been best characterized in colon cancer. Researchers have shown that Src expression is 5 to 8 fold higher in premalignant polyps than normal mucosa. The elevated c-Src levels have also been shown to have a correlation with advanced stages of the tumor, size of tumor, and metastatic potential of tumors.

Breast

EGFR activates c-Src while EGF also increases the activity of c-Src. In addition, overexpression of c-Src increases the response of EGFR-mediated processes. So both EGFR and c-Src enhance the effects of one another. Elevated expression levels of c-Src were found in human breast cancer tissues compared to normal tissues.

Overexpression of Human Epidermal Growth Factor Receptor 2 (HER2), also known as erbB2, is correlated with a worse prognosis for breast cancer. Thus, c-Src plays a key role in the tumor progression of breast cancers.

Prostate

Members of the Src family kinases Src, Lyn and Fgr are highly expressed in malignant prostate cells compared to normal prostate cells. When the primary prostate cells are treated with KRX-123, which is an inhibitor of Lyn, the cells in vitro were reduced in proliferation, migration and invasive potential. So the use of a tyrosine kinase inhibitor is a possible way of reducing the progression of prostate cancers.

As a drug target

A number of tyrosine kinase inhibitors that target c-Src tyrosine kinase (as well as related tyrosine kinases) have been developed for therapeutic use. One notable example is dasatinib which has been approved for the treatment of chronic myeloid leukemia (CML) and Philadelphia chromosome-positive (PH+) acute lymphocytic leukemia (ALL). Dasatinib is also in clinical trials for the use in non-Hodgkin’s lymphoma, metastatic breast cancer and prostate cancer. Other tyrosine kinase inhibitor drugs that are in clinical trials include bosutinib, bafetinib, AZD-530, XLl-999, KX01 and XL228.

Interactions

Src (gene) has been shown to interact with the following signaling pathways: