Raf Family Kinases: Old Dogs Have Learned New Tricks

Abstract

First identified in the early 1980s as retroviral oncogenes, the Raf proteins have been the objects of intense research. The discoveries 10 years later that the Raf family members (Raf-1, B-Raf, and A-Raf) are bona fide Ras effectors and upstream activators of the ubiquitous ERK pathway increased the interest in these proteins primarily because of the central role that this cascade plays in cancer development. The important role of Raf in cancer was corroborated in 2002 with the discovery of B-Raf genetic mutations in a large number of tumors. This led to intensified drug development efforts to target Raf signaling in cancer. This work yielded not only recent clinical successes but also surprising insights into the regulation of Raf proteins by homodimerization and heterodimerization. Surprising insights also came from the hunt for new Raf targets. Although MEK remains the only widely accepted Raf substrate, new kinase-independent roles for Raf proteins have emerged. These include the regulation of apoptosis by suppressing the activity of the proapoptotic kinases, ASK1 and MST2, and the regulation of cell motility and differentiation by controlling the activity of Rok-α. In this review, we discuss the regulation of Raf proteins and their role in cancer, with special focus on the interacting proteins that modulate Raf signaling. We also describe the new pathways controlled by Raf proteins and summarize the successes and failures in the development of efficient anticancer therapies targeting Raf. Finally, we also argue for the necessity of more systemic approaches to obtain a better understanding of how the Ras-Raf signaling network generates biological specificity.

Phosphorylation of RAF Kinase Dimers Drives Conformational Changes that Facilitate Transactivation

“…This pathway regulates many fundamental cellular functions, including cell proliferation, and is dysregulated in approximately 50 % of human cancers 1. This pathway has thus been a key focus in cancer drug development.…”

“…Owing to its important clinical implications, RAF dimerization has attracted enormous interest. RAF homo‐ and heterodimers show significantly higher kinase activity than monomers, and it has been shown that physiological RAF activation involves dimerization 1, 5. Dimer activity remains high even when one protomer (denoted as the activator) is kinase‐dead or inhibited, owing to allosteric transactivation of its binding partner (the receiver) 1, 5.…”

“…RAF homo‐ and heterodimers show significantly higher kinase activity than monomers, and it has been shown that physiological RAF activation involves dimerization 1, 5. Dimer activity remains high even when one protomer (denoted as the activator) is kinase‐dead or inhibited, owing to allosteric transactivation of its binding partner (the receiver) 1, 5. Recent data indicate that the N‐terminal acidic (NtA) motif6 is essential for the allosteric activation of RAF dimers 7.…”

“…This configuration of the NtA motif primes BRAF for activation, which may explain why single mutations of BRAF, such as V600E in the activation loop, can cause full activation and drive cancer, while RAF1 mutations are rare in cancer 9. There is no consensus on which kinase phosphorylates the NtA motif in vivo, since several kinases, including RAF1 itself, have been reported to be able to do this 1, 10. As shown by mutagenesis studies,7 the NtA motif in the activator is required for transactivation of the receiver in RAF dimers.…”

Differential localization of A-Raf regulates MST2-mediated apoptosis during epithelial differentiation

“…16 This anti-apoptotic role may explain why A-Raf levels are elevated in a variety of malignancies. 16,[19][20][21] Both A-Raf and MST2 were shown to localize to the mitochondria in tumor cell lines as well as primary tumors. 16 A-Raf was reported to associate with the mitochondrial transport system proteins hTOM and hTIM, 22 but the physiological significance of this interaction is still unknown.…”