Ras caught in another affair: the exchange factors for Ral
Introduction
Activation of the guanine-nucleotide-binding Ras proteins is a crucial component in the transduction of extracellular signals that stimulate proliferation and differentiation [1]. Wild-type Ras is transiently activated by the exchange of bound GDP for GTP, a process which is catalysed by guanine nucleotide exchange factors (GEFs), whereas oncogenic Ras proteins are stalled in their active, GTP-bound form [2]. Active Ras associates with a number of downstream targets, or effectors, to exert its biological effects 3, 4. Although there are indications that the GTP-bound form of Ras has many binding partners [5], three classes of Ras effectors are now established as being functional in vivo: the protein kinases of the Raf family, the catalytic subunit of a phosphatidylinositol-3 lipid kinase (PI3K), and the Ral guanine nucleotide exchange factors (RalGEFs).
Raf-kinases are the best-described Ras effectors and control the activation of the MAP kinases Erk1 and Erk2, which are involved in protein phosphorylation events in mitogenesis and differentiation (e.g. see [6]). PI3K protects Ras-transformed cells from going into apoptosis and links Ras activation to rearrangements of the actin cytoskeleton 7, 8. RalGEFs promote the activation of the ubiquitously expressed Ras family member Ral [9]. The roles of the RalGEFs in transmitting signals from Ras and the putative cellular functions of the RalGEF and Ral proteins have started to emerge more recently and will be discussed in our review.
Section snippets
Activation of RalGEFs by Ras
RalGDS, identified on the basis of its homology with yeast RasGEFs, was shown to exhibit specific in vitro guanine nucleotide exchange activity for the highly related RalA and RalB [10]. To date, three additional types of Ral-specific GEFs have been cloned: Rgl, Rlf and Rgr [9]. Rgr was identified as the oncogenic region of a fusion protein designated Rsc ([11]; Figure 1). The concept that RalGEFs are Ras effectors originates from the identification of RalGDS, Rgl and Rlf as binding partners of
Extracellular signal-induced activation of Ral
As expected if RalGEFs function as Ras effectors, stimulation of a variety of receptors including G-protein-coupled serpentine receptors and tyrosine-kinase-associated receptors induces rapid activation of endogenous Ral 27•, 28•, 29•. In serum-starved fibroblasts, most mitogens activate Ral at least five fold, converting up to an estimated 15–50% of Ral to its GTP-bound form 27•, 28•. Insulin and EGF-induced Ral activation in A14-NIH3T3 cells is blocked by dominant negative Ras, providing
A role for RalGEFs in cellular transformation by Ras
Rodent fibroblast cell lines that express oncogenic Ras display profound alterations in morphology and attachment, as well as enhanced proliferation that is no longer inhibited by cell–cell contacts or low concentrations of serum [34].
Important work from White et al. [35] established that concurrent activation of distinct effector pathways is necessary for oncogenic Ras to induce cellular transformation [35]. Effector domain mutants of oncogenic Ras, which interact with either Raf, PI3K or
RalGEF-mediated signal transduction
RalGEFs may exert their biological effects by stimulating the promoter activity of growth regulatory genes, such as fos 17, 18••, 44. This may involve activation of the serum response element (SRE) by a pathway that is independent of Erk activation, as was shown for active Rlf [18••]. As phosphorylation of the ternary complex factors Elk1, Sap1 and Sap2 plays a role in Ras-induced activation of the SRE 45, 46, RalGEFs might signal to a kinase that activates these and perhaps other transcription
Conclusions
In the past year, the RalGEFs have been established as a third class of Ras effectors, demonstrating directly that signal transduction in mammalian cells also involves cascades of Ras-like GTPases. Activation of RalGEFs is sufficient to induce low-serum growth and tumorigenicity of NIH3T3 cells, providing a novel signaling pathway by which activated Ras exerts its oncogenic effects.
It is likely that enhanced transcription of genes involved in growth regulation contributes to the cellular
Acknowledgements
The authors thank Boudewijn Burgering, Kris Reedquist and Fried Zwartkruis for critical reading the manuscript and all our colleagues for stimulating discussions. We thank the Netherlands organisation for Scientific Research (NWO) and the Dutch Cancer Society (KWF) for the support of our work on Ras signaling.
References and recommended reading
Papers of particular interest, published within the annual period of review, have been highlighted as:
• of special interest
•• of outstanding interest
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