Ras caught in another affair: the exchange factors for Ral

https://doi.org/10.1016/S0959-437X(99)80016-1Get rights and content

Abstract

The Ral guanine nucleotide exchange factors are direct targets of Ras, providing a mechanism for Ral activation by extracellular signals. In addition, Ral can be activated by a Ras-independent pathway. Ral guanine nucleotide exchange factors contribute to cellular transformation induced by oncogenic Ras through an Erk-independent mechanism which may involve activation of transcription.

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

References (60)

  • A Kikuchi et al.

    Regulation of interaction of ras p21 with RalGDS and Raf-1 by cyclic AMP-dependent protein kinase

    J Biol Chem

    (1996)
  • RMF Wolthuis et al.

    Ras-dependent activation of the small GTPase Ral

    Curr Biol

    (1998)
  • KL Wang et al.

    Identification and characterization of a calmodulin-binding domain in Ral-A, a Ras-related GTP-binding protein purified from human erythrocyte membrane

    J Biol Chem

    (1997)
  • MA White et al.

    Multiple Ras functions can contribute to mammalian cell transformation

    Cell

    (1995)
  • P Rodriguez-Viciana et al.

    Role of phosphoinositide 3-OH kinase in cell transformation and control of the actin cytoskeleton by Ras

    Cell

    (1997)
  • MJ Miller et al.

    RalGDS functions in Ras- and cAMP-mediated growth stimulation

    J Biol Chem

    (1997)
  • C Rommel et al.

    Ras — a versatile cellular switch

    Curr Opin Genet Dev

    (1998)
  • J Downward

    Cell cycle control: routine role for Ras

    Curr Biol

    (1997)
  • R Treisman

    Regulation of transcription by MAP kinase cascades

    Curr Opin Cell Biol

    (1996)
  • M Schmidt et al.

    Specific inhibition of phorbol ester-stimulated phospholipase D by Clostridium sordellii lethal toxin and Clostridium difficile toxin B-1470 in HEK-293 cells

    J Biol Chem

    (1998)
  • JH Exton

    New developments in phospholipase D

    J Biol Chem

    (1997)
  • M Ikeda et al.

    Identification and characterization of a novel protein interacting with Ral-binding protein 1, a putative effector protein of Ral

    J Biol Chem

    (1998)
  • A Yamaguchi et al.

    An Eps homology (EH) domain protein that binds to the Ral-GTPase target, RalBP1

    J Biol Chem

    (1997)
  • MA White et al.

    A role for the Ral guanine nucleotide dissociation stimulator in mediating Ras-induced transformation

    J Biol Chem

    (1996)
  • JA Herberg et al.

    TAPASIN, DAXX, RGL2, HKE2 and four new genes (BING 1, 3 to 5) form a dense cluster at the centromeric end of the MHC

    J Mol Biol

    (1998)
  • C Herrmann et al.

    Differential interaction of the ras family GTP-binding proteins H-Ras, Rap1A, and R-Ras with the putative effector molecules Raf kinase and Ral-guanine nucleotide exchange factor

    J Biol Chem

    (1996)
  • C Herrmann et al.

    Quantitative analysis of the complex between p21ras and the Ras-binding domain of the human Raf-1 kinase

    J Biol Chem

    (1995)
  • S Cowley et al.

    Activation of MAP kinase kinase is necessary and sufficient for PC12 differentiation and for transformation of NIH 3T3 cells

    Cell

    (1994)
  • JL Bos

    ras oncogenes in human cancer: a review

    Cancer Res

    (1989)
  • JL Bos

    Ras-like GTPases

    Biochim Biophys Acta

    (1997)
  • Cited by (0)

    View full text