Ras acts as a switch that turns on and off a cascade of protein kinases known as the mitogen-activated protein kinase (MAP kinase) cascade. Ras accomplishes this by binding to GTP and then cleaving bound GTP to GDP. When bound to GTP Ras is in its active state, while it is made inactive by binding to GDP (Janeway et al., 1999). Activation of Ras is mediated by Guanine-nucleotide exchange factors (GEF's), such as SOS, that coordinate the transfer of GTP to deactivated Ras molecules and exchange GTP for GDP. There are two types of GEF's: a stimulatory type, GDS, and an inhibitory type, GDI. However, the effector capabilities of the two molecules are not equal; the inhibitory effect of GDI is much stronger than the stimulatory effect of GDS. It was discovered that only the stimulatory type of GEF is active in the Ras mediated signal transduction pathway (Kikuchi et al., 1992). Ras is inactivated by the innate GTPase activity of Ras itself, which is catalyzed by GTPase activating protein (rasGAP) (Gunzberg 1997).
The Ras mediated MAP kinase cascade encompasses the following steps. Activation of Ras by SOS causes a serine/threonine kinase, called Raf protein (specifically, c-Raf, as there are a number of different types of Raf protein, c-Raf being the most highly characterized of them) to translocate from the cytosol to the plasma membrane, where its N-terminal region binds to Ras. This in turn causes the carboxy terminal of the c-Raf to bind to another protein, called MEK. Upon binding, all tyrosine and serine aa residues in MEK are phosphorylated. Phosphorylated MEK then activates a MAP kinase (ERK1 or 2), again by phosphorylation. Finally, MAP kinase migrates to the nucleus where it phosphorylates other proteins and triggers many transcription factors, including epidermal growth factor (EGF), platelet-derived growth factor (PDGF), insulin, an interleukin (IL), granulocyte colony-stimulating factor (G-CSF), and granulocyte/macrophage colony-stimulating factor (GM-CSF) (Gunzberg 1997). The end result of the cascade is an extracellular signal being transduced and propogated intracellulary, until it finally reaches the nucleus, where it is interpreted and cell growth and differentiation quickly ensue.
A mutated form of the Ras protein has been implicated in certain forms of Cancer, specifically colo-rectal cancers (King 1999). This oncogenic form of the gene contains a point mutation that locks Ras in its active state, causing the cell to which Ras is bound to proliferate uncontrollably, as Ras is unable to switch itself off. There are two types of Oncogenic Ras proteins: the first results from a point mutation in residues 12, 13, 59, or 61 that causes Ras to be unresponsive towards GAP, and not express any hydrolytic activity, or to lose its GTPase activity altogether. In either case, Ras cannot cleave GTP and it remains in its active state indefinitely, resulting in tumor growth (Trahey et al., 1987). The second type of Oncogenic Ras protein results from point mutations in residues 116 or 119 that alter the binding affinity of GTP to the Ras binding pocket. As a result, Ras is no longer able to hold the GTP long enough to cleave it into GDP, [GTP] dominates [GDP] intracellularly, and the rate of transfer becomes so accelerated that Ras is constantly in its active state (Walter et al., 1986). It has been estimated that mutated forms of Ras have been dcan be found in 10-30% of all human tumors (Cancer Drugs 1999).
Drugs that are effective in preventing the activity of mutated Ras proteins are antisense inhibitors of Ras gene expression. They work by binding to the mRNA of a specific Ras gene, and preventing the translation of that specific Ras protein. These types of drugs are very effective because they only bind to the mRNA of the Ras protein for which they are specific, and not any of the other mRNAs of the Ras family. An example of an antisense drug that binds to Ras and is designed to prevent tumor growth is ISIS 2503. It's particular specificity is for human Ha-Ras mRNA, though it has been shown to be effective in binding to mutant Ki-ras and tumor types that express the normal Ras protein. Unfortunately, ISIS 2503 is still in the preclinical testing stages, though it has shown anti-bladder, breast and colon cancer capability (Cancer Drugs 1999).