Proto-oncogene tyrosine-protein kinase Src, also known as proto-oncogene c-Src, or simply c-Src (cellular Src; pronounced "sarc", as it is short for sarcoma), is a non-receptor tyrosine kinase protein that in humans is encoded by the SRC gene. It belongs to a family of Src family kinases and is similar to the v-Src (viral Src) gene of Rous sarcoma virus. It includes an SH2 domain, an SH3 domain and a tyrosine kinase domain. Two transcript variants encoding the same protein have been found for this gene. [5]
c-Src phosphorylates specific tyrosine residues in other tyrosine kinases. It plays a role in the regulation of embryonic development and cell growth. An elevated level of activity of c-Src is suggested to be linked to cancer progression by promoting other signals. [6] Mutations in c-Src could be involved in the malignant progression of colon cancer. c-Src should not be confused with CSK (C-terminal Src kinase), an enzyme that phosphorylates c-Src at its C-terminus and provides negative regulation of Src's enzymatic activity.
c-Src was originally discovered by American scientists J. Michael Bishop and Harold E. Varmus, for which they were awarded the 1989 Nobel Prize in Physiology or Medicine. [7]
In 1979, J. Michael Bishop and Harold E. Varmus discovered that normal chickens possess a gene that is structurally closely related to v-Src . [8] The normal cellular gene was called c-src (cellular-src). [9] 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.
There are 9 members of the Src family kinases: c-Src, Yes, Fyn, Fgr, Yrk, Lyn, Blk, Hck, and Lck. [10] 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. [11] [12] [13] [14]
c-Src is made up of 6 functional regions: Src homology 4 domain (SH4 domain), unique region, SH3 domain, SH2 domain, catalytic domain and short regulatory tail. [15] 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. [16] [17] [18]
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).
Src contains at least three flexible protein domains, which, in conjunction with myristoylation, can mediate attachment to membranes and determine subcellular localization. [19]
This proto-oncogene may play a role in the regulation of embryonic development and cell growth.
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, [20] [21] and regulates matrix metalloproteinase-9 activity after intracerebral hemorrhage. [22]
The activation of the c-Src pathway has been observed in about 50% of tumors from colon, liver, lung, breast and the pancreas. [23] 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.
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. [24] [25] [26] 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. [27] [28]
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. [29] [30] [31]
Overexpression of Human Epidermal Growth Factor Receptor 2 (HER2), also known as erbB2, is correlated with a worse prognosis for breast cancer. [32] [33] Thus, c-Src plays a key role in the tumor progression of breast cancers.
Members of the Src family kinases Src, Lyn and Fgr are highly expressed in malignant prostate cells compared to normal prostate cells. [34] 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. [35] So the use of a tyrosine kinase inhibitor is a possible way of reducing the progression of prostate cancers.
A number of tyrosine kinase inhibitors that target c-Src tyrosine kinase (as well as related tyrosine kinases) have been developed for therapeutic use. [36] 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). [37] 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, [38] bafetinib, Saracatinib(AZD-0530), XLl-999, KX01 and XL228. [6] HSP90 inhibitor NVP-BEP800 has been described to affect stability of Src tyrosine kinase and growth of T-cell and B-cell acute lymphoblastic leukemias. [39]
Src (gene) has been shown to interact with the following signaling pathways:
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An oncogene is a gene that has the potential to cause cancer. In tumor cells, these genes are often mutated, or expressed at high levels.
A tyrosine kinase is an enzyme that can transfer a phosphate group from ATP to the tyrosine residues of specific proteins inside a cell. It functions as an "on" or "off" switch in many cellular functions.
The epidermal growth factor receptor is a transmembrane protein that is a receptor for members of the epidermal growth factor family of extracellular protein ligands.
The insulin-like growth factor 1 (IGF-1) receptor is a protein found on the surface of human cells. It is a transmembrane receptor that is activated by a hormone called insulin-like growth factor 1 (IGF-1) and by a related hormone called IGF-2. It belongs to the large class of tyrosine kinase receptors. This receptor mediates the effects of IGF-1, which is a polypeptide protein hormone similar in molecular structure to insulin. IGF-1 plays an important role in growth and continues to have anabolic effects in adults – meaning that it can induce hypertrophy of skeletal muscle and other target tissues. Mice lacking the IGF-1 receptor die late in development, and show a dramatic reduction in body mass. This testifies to the strong growth-promoting effect of this receptor.
Receptor tyrosine kinases (RTKs) are the high-affinity cell surface receptors for many polypeptide growth factors, cytokines, and hormones. Of the 90 unique tyrosine kinase genes identified in the human genome, 58 encode receptor tyrosine kinase proteins. Receptor tyrosine kinases have been shown not only to be key regulators of normal cellular processes but also to have a critical role in the development and progression of many types of cancer. Mutations in receptor tyrosine kinases lead to activation of a series of signalling cascades which have numerous effects on protein expression. The receptors are generally activated by dimerization and substrate presentation. Receptor tyrosine kinases are part of the larger family of protein tyrosine kinases, encompassing the receptor tyrosine kinase proteins which contain a transmembrane domain, as well as the non-receptor tyrosine kinases which do not possess transmembrane domains.
Proto-oncogene c-KIT is the gene encoding the receptor tyrosine kinase protein known as tyrosine-protein kinase KIT, CD117 or mast/stem cell growth factor receptor (SCFR). Multiple transcript variants encoding different isoforms have been found for this gene. KIT was first described by the German biochemist Axel Ullrich in 1987 as the cellular homolog of the feline sarcoma viral oncogene v-kit.
v-Src is a gene found in Rous sarcoma virus (RSV) that encodes a tyrosine kinase that causes a type of cancer in chickens.
Fibroblast growth factor receptor 1 (FGFR-1), also known as basic fibroblast growth factor receptor 1, fms-related tyrosine kinase-2 / Pfeiffer syndrome, and CD331, is a receptor tyrosine kinase whose ligands are specific members of the fibroblast growth factor family. FGFR-1 has been shown to be associated with Pfeiffer syndrome, and clonal eosinophilias.
Proto-oncogene tyrosine-protein kinase Fyn is an enzyme that in humans is encoded by the FYN gene.
SHC-transforming protein 1 is a protein that in humans is encoded by the SHC1 gene. SHC has been found to be important in the regulation of apoptosis and drug resistance in mammalian cells.
Receptor tyrosine-protein kinase erbB-3, also known as HER3, is a membrane bound protein that in humans is encoded by the ERBB3 gene.
Signal transducer and activator of transcription 5A is a protein that in humans is encoded by the STAT5A gene. STAT5A orthologs have been identified in several placentals for which complete genome data are available.
Breast cancer anti-estrogen resistance protein 1 is a protein that in humans is encoded by the BCAR1 gene.
EPH receptor A2 is a protein that in humans is encoded by the EPHA2 gene.
Tyrosine-protein kinase Yes is a non-receptor tyrosine kinase that in humans is encoded by the YES1 gene.
Tyrosine-protein kinase receptor UFO is a protein that in human is encoded by the AXL gene. The gene was initially designated as UFO, in allusion to the unidentified function of this protein. However, in the years since its discovery, research into AXL's expression profile and mechanism has made it an increasingly attractive target, especially for cancer therapeutics. In recent years, AXL has emerged as a key facilitator of immune escape and drug-resistance by cancer cells, leading to aggressive and metastatic cancers.
Cytoplasmic tyrosine-protein kinase BMX is an enzyme that in humans is encoded by the BMX gene.
Proto-oncogene tyrosine-protein kinase ROS is an enzyme that in humans is encoded by the ROS1 gene.
A non-receptor tyrosine kinase (nRTK) is a cytosolic enzyme that is responsible for catalysing the transfer of a phosphate group from a nucleoside triphosphate donor, such as ATP, to tyrosine residues in proteins. Non-receptor tyrosine kinases are a subgroup of protein family tyrosine kinases, enzymes that can transfer the phosphate group from ATP to a tyrosine residue of a protein (phosphorylation). These enzymes regulate many cellular functions by switching on or switching off other enzymes in a cell.
Src kinase family is a family of non-receptor tyrosine kinases that includes nine members: Src, Yes, Fyn, and Fgr, forming the SrcA subfamily, Lck, Hck, Blk, and Lyn in the SrcB subfamily, and Frk in its own subfamily. Frk has homologs in invertebrates such as flies and worms, and Src homologs exist in organisms as diverse as unicellular choanoflagellates, but the SrcA and SrcB subfamilies are specific to vertebrates. Src family kinases contain six conserved domains: a N-terminal myristoylated segment, a SH2 domain, a SH3 domain, a linker region, a tyrosine kinase domain, and C-terminal tail.
for their discovery of 'the cellular origin of retroviral oncogenes'