Sandbox Reserved 707: Difference between revisions

This cascade participates in the regulation of several cellular mechanisms like: apoptosis, cell cycle progression, differentiation, proliferation and transformation to the cancerous state in response to growth factors, cytokines and hormones. First discovered in 1938<ref>PMID:16649144</ref> as a retroviral oncogene B-RAF plays a key role on all studies concerning cancer therapies and other biomedical applications.  

'''1.'''    The open form allows access of ATP and release of ADP from the active site.<br />

'''2.''' The closed form brings the residues of the substrate into the active site.<br />

'''1.''' In the small lobe, this segment is a α-helix, which is called αC-helix. The αC-helix rotates and translates with respect to the rest of the lobe, making or breaking part of the active site.<br />

'''2.''' In the large lobe, the activation segment can make or break part of the ATP binding site<ref>PMID:3291115</ref>.<br />

The activation segment of each protein kinase has a specific domain that begins with a DFG amino acid sequence that can easily change its conformation (active or inactive conformation). In the inactive conformation<ref>PMID:19276351</ref> the phenylalanine side chain occupies the ATP binding pocket and the aspartate side chain faces away from the active site. This is the DFG Aspartate '''Out''' Conformation. In the active conformation, the phenylalanine side chain is rotated out of the ATP binding pocket and the Aspartate side chain can now face the ATP binding pocket and form coordinated links with the Mg²⁺. This is called the DFG Aspartate '''In''' Conformation. The activation segment can be phosphorylated by members of the same protein kinase family or by other protein kinases. <br />

A <scene name='Sandbox_Reserved_707/Gatekeeper_residue/1'>gatekeeper residue</scene> separates the adenine binding site from the hydrophobic pocket. Mutation on this residue can prevent the binding of kinase inhibitory drugs (the replacement of a threonine by a methionine for example).<br />

[[Image:The RAS RAF MEK ERK pathway.jpg|400px|thumb| Diagram of the RAS-RAF-MEK-ERK pathway<ref>PMID:17208430</ref>.]]

The regulation of RAF Kinases involves protein-protein interactions, phosphorylations, dephosphorylations and conformational changes<ref>PMID:15520807</ref>. <br />

=== Structural implication on the MAP Kinase Pathway ===

RAF kinase inhibitors effectively block MEK and ERK phosphorylation.

However the B-RAF specific inhibitor 885-A produces an unexpected increase in ERK phosphorylation in human melanoma cell lines. How can a RAF kinase inhibitor lead to the paradoxical increase in RAF kinase activity and ERK phosphorylation? Two additional studies noted below address this issue, and the common finding is that the binding of inhibitors to RAF kinases promotes RAS-dependent C-RAF homo- or heterodimerization and C-RAF activation. <br />

=== First Study ===

HCT116 (mutant K-RAS) cells reverses the phospho-MEK induction observed after RAF inhibitor treatment, indicating that C-RAF has a major role in signaling to MEK, a result that is in agreement with the above two studies. <br />

RAS mutations occur in 15–30% of all human cancers, and B-RAF mutations occur in 30–60% of melanomas, 30–50% of thyroid cancers, and 5–20% of colorectal cancers. B-RAF mutants occur in a variety of cancers while mutants of the other two RAF enzymes in cancers are very rare. The majority of B-RAF mutations occur in the activation segment or in the glycine-rich loop. These mutations disrupt the inactive state to favor the active state. A Val600Glu mutation is one of the more known B-RAF mutations. This mutation occurs within the activation segment where the introduction of negative charges favors the formation of an active conformation. The introduction of glutamate into the activation segment of C-RAF fails to produce an activated enzyme most likely owing to the need for a negatively charged N-region for activity. <br />

RAF kinases are attractive cancer drug targets. Pre-clinical studies with cell lines and tumor xenographs bearing B-RAF mutations indicate that RAF kinase inhibitors are effective in decreasing cell proliferation. PLX4032, which has higher affinity for B-RAF than wild-type B-RAF, inhibits cancer progression in several animal models. The compound has demonstrated efficacy in Phase I clinical<ref>PMID:20472680</ref> trials in the treatment of melanoma patients. Sorafenib has a lower affinity for B-RAF than C-RAF and is ineffective as monotherapy in the treatment of melanoma. RAF kinase inhibitor treatment of cancers with wild-type or activated mutant RAS co-expressed with wild-type B-RAF may be deleterious owing to up-regulation of RAF kinase signaling. Deciphering the mechanisms of RAS-RAF-MEK-ERK signaling continues to be an important and challenging task.

== External Ressources ==

== References ==

<references />

== Protreopedia Page Contributors and Editors ==

Kostika Sofroni and Gilles Dupouy