Mini-reviewTargeting the RAF–MEK–ERK pathway in cancer therapy
Introduction
Mitogen-activated protein kinases (MAPKs) mediate intracellular signals transduced by a wide variety of cell surface receptors. Mammalian cells employ four well-characterized MAPK cascades and numerous associated proteins that constitute a complex signaling network. The basic functional “architecture” of each MAPK cascade comprises a MAPK kinase kinase (MAP3K), a MAPK kinase (MAP2K) and a MAPK. The terminal MAPKs are the Erk, JNK (also called SAPK), p38 and ERK5 kinases. Although MAPKs cascades are usually simplified as linear unidirectional pathways, it is important to note the greater complexity of such networks in the context of signal cross-talk with a large number of additional associated proteins [1], [2]. Therefore, further efforts to model the MAPK pathway are of great value in understanding how the pathway potentially responds and adapts when challenged with small molecule inhibitors.
Among the MAPK-mediated pathways, the RAF–MAPK/extracellular signal-related kinase (ERK) kinase (MEK)–ERK signaling cascade has been the most extensively characterized thus far. This largely reflects its established role in a variety of human cancers, particularly those associated with activation of RAS proteins, which bind to and activate the RAF kinase, triggering engagement of this pathway. Upon RAS activation, RAF is recruited to the cell membrane where subsequent changes in RAF phosphorylation status result in stimulation of its serine-threonine kinase activity [3], [4]. Activated RAF triggers sequential phosphorylation and activation of the MEK1/MEK2 dual-specificity protein kinases and ERK, which translocates to the nucleus where they regulate the activity of several transcription factors that induce the expression of multiple genes required for survival and proliferation [5], [6].
Since the first reports of the oncogenic transforming activity of the viral oncogenic form of c-RAF-1 more than twenty years ago, there has been accumulating evidence of the importance of RAF–MEK–ERK pathway in oncogenesis [7]. Aberrant activation of the pathway is frequently seen in human cancers (Fig. 1), and is clearly contributing to oncogenic properties such as independence from growth factors and enhanced proliferation and survival [8], [9]. Constitutive activation of the RAF–MEK–ERK pathway can be driven either by ligand-induced stimulation of membrane associated receptor tyrosine kinases or by ligand-independent mechanisms such as increased expression or mutational activation of proteins that constitute the pathway. The most frequently observed mutations are those that arise in one of the three human RAS genes, HRAS, NRAS, and KRAS, which occur in approximately 20% of human neoplasms [3]. Moreover, recent sequencing efforts have led to the identification of activating mutations in the BRAF gene in 7% of all cancers (discussed below) [10]. Interestingly, NRAS and BRAF mutations appear to be mutually exclusive, suggesting that both proteins function in the same signaling pathway and that activation of either one is sufficient to promote engagement of the MEK–ERK cascade [11], [12].
The fact that the RAF–MEK–ERK cascade plays a central role in the regulation of cell proliferation and survival, together with the high frequency with which this pathway is deregulated in human cancer makes it an attractive target for drug development. The therapeutic approach of selectively targeting activated kinases has proven to be of great value for the treatment of a variety of malignancies, with the best examples thus far being the success of the ABL, KIT, PDGFR inhibitor imatinib in chronic myelogenous leukemias and gastrointestinal stromal tumors (GIST), and the EGFR kinase inhibitor erlotinib in a subset of non-small cell lung cancers [13], [14].
It is difficult to predict which of the various RAS–RAF–MEK–ERK pathway components will constitute the most useful therapeutic target. Moreover, considering the well documented widespread function of each of these proteins in a variety of normal physiological processes, it would not be surprising to find that inhibition of these proteins leads to unacceptable toxicity. Therefore, it is hoped that, as has been seen in other kinase-driven cancers, the mutational activation of this pathway in tumors leads to a therapeutic window that reflects enhanced dependency on tumor cells relative to normal cells [14]. While substantial efforts made over the years to target the activated RAS protein have been unsuccessful, leading many to conclude that RAS is a pharmacologically intractable target, the downstream kinases in the RAS cascade remain attractive as therapeutic targets [15]. Among these, BRAF has emerged as a particularly appealing target due to its high mutation prevalence in tumors. Consequently, intensive preclinical and clinical research has led to the development of small molecule kinase inhibitors of RAF, as well as its downstream effector MEK (Fig. 2). While clinical evaluation of these inhibitors is still in its early days, some promising findings have begun to emerge. In the following sections, we will focus on the various inhibitors of the RAF–MEK–ERK pathway that are currently being tested.
Section snippets
RAF kinase inhibitors
The RAF serine/threonine family is comprised of three isoforms: ARAF, BRAF and CRAF (also referred as c-RAF-1). While these three RAF isoforms share three common conserved regions, they are expressed in a tissue- and cell-specific manner and have been associated with distinct normal biological functions [16]. Recent data show that BRAF exhibits higher basal kinase activity than ARAF and CRAF which require a greater number of phosphorylations within the N-terminal region of the kinase domain to
MEK kinase inhibitors
The two MEK homologues – MEK1 and MEK2- are dual specificity kinases that share the two consensus kinase motifs, one involved in phosphorylation of serine-threonine residues and another in the phosphorylation of tyrosine residues [55]. The only known catalytic substrates for both MEK isoforms are the MAP kinases ERK1 and ERK2 [56]. Such signaling selectivity confers upon MEK a central role in transducing MAPK-mediated proliferation signals and makes it a particularly attractive target for
Future directions
Although clear progress has been made in the development of drugs that target RAF and its downstream effector MEK, new challenges and drug-related issues have emerged from the analysis thus far. Whereas RAF inhibitors are still in a relatively early phase of their clinical development, lessons learned from the evaluation from MEK inhibitors should serve to guide us in the future rational and successful clinical development of RAF/MEK inhibitors. As with other targeted therapies, a current
Conflicts of interest statement
The authors declare that they have no financial interest that could bias their presentation of findings in this manuscript.
Acknowledgements
We apologize to the authors of many excellent and relevant publications that we were unable to include in this review due to limitations of space.
C.M. has been supported by RTICC 06/0020/0109 and Fundació Cellex (Barcelona).
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