Oncogenic Ras and its role in tumor cell invasion and metastasis
Introduction
As the initiation and progression of solid tumors is in part dependent on a variety of signaling pathways, so too are the processes that allows for invasion and metastasis. In metastasis, cells migrate from the primary tumor, cross the tumor marginal border gaining access to the vascular and/or lymphatic system, and enter distal tissue. At this point, these “rogue” cells may lie dormant for an indefinite period of time, but upon stimulation by signals that at the present are not entirely known, these micrometastases can subsequently proliferate into solid tumors themselves. Because the process of metastasis can lead to potentially many secondary tumor sites, and because evidence indicates a more aggressive and lethal phenotype for the metastatic tumor versus the primary lesion, it is imperative to gain further knowledge of the biology of metastasis and invasion. Understanding the signaling mechanisms that lead to such migration will enable discovery of additional targets for therapeutic design.
Ras proteins (of which H-, N-, and K-Ras4A/4B are prototypical) are associated with the inner face of the plasma membrane where they facilitate signaling initiated by diverse extracellular stimuli [1]. Ras activity is regulated by cycling between inactive GDP-bound and active GTP-bound forms [2] (Fig. 1). Increase in GDP/GTP nucleotide exchange involves interaction between Ras and guanine exchange factors (GEFs), for example Sos1/2, RasGRP and RasGRF1/2 proteins [3]. When GTP-bound, Ras binds to and activates a plethora of effector molecules [1], [4], [5]. Hydrolysis of GTP by Ras is facilitated by GTPase-activating proteins (GAPs) such as p120GAP and NF1. Mutated variants of Ras (mutations at residues 12, 13 or 61) are found in 30% of all human cancers, are insensitive to GAP stimulation, and are consequently rendered constitutively activated [6], [7].
In addition to mutational activation, Ras GTPase signaling can be upregulated due to increased coupling to cell surface receptors. In particular, members of the epidermal growth factor (EGF) family of receptor tyrosine kinases (RTKs; including EGFR/ErbB/HER1 and ErbB2/Her2/Neu) [8], [9], [10] or other tyrosine kinases (e.g. Bcr-Abl) are commonly overexpressed in many cancers, causing persistent activation of Ras in the absence of mutations in Ras genes. Thus, Ras activation has been shown to be an important mediator of tumor cell invasion and metastasis caused by these and other tyrosine kinases. However, for the purpose of this review, we will limit our focus to the effects of mutationally activated Ras proteins on invasive and metastatic phenotypes.
The aberrant activation of Ras proteins been implicated in facilitating virtually all aspects of the malignant phenotype of the cancer cell, including cellular proliferation, transformation, invasion and metastasis (reviewed in [11]). Additionally, the functions of other Ras-related proteins are also regulated by Ras signaling and also contribute to oncogenesis. While much is known regarding the mechanisms by which aberrant Ras promotes uncontrolled proliferation by deregulation of cell cycle progression and promotion of cell survival, less is known regarding how Ras promotes tumor cell invasion and metastasis. In this review, we summarize the current understanding of the mechanisms by which oncogenic Ras promotes the malignant phenotype of cancer cells.
Section snippets
Mouse and in vitro experimental models
A variety of experimental approaches have been undertaken to ascertain the degree to which Ras GTPases are involved in and/or causative for metastasis and invasion. In both in vitro and in vivo experimental models, transfection of mutated, constitutively active forms of Ras into previously noncancerous cells can lead to invasive and metastatic phenotypes [12], [13]. One of the most commonly studied models of Ras activation is the murine NIH 3T3 fibroblast cell line. Ectopic expression of
Contribution of specific effectors downstream of oncogenic Ras
Ras interacts with and regulates multiple downstream effectors that stimulate diverse cytoplasmic signaling activities [1], [4], [5]. As new effectors continue to be identified, one of the critical issues concerns the specific role of each effector in Ras-mediated oncogenesis. While some are clearly important positive mediators of the oncogenic properties of Ras (e.g. Raf, PI3K, RalGEF, Tiam1), others may serve negative regulatory roles in oncogenesis (e.g. Nore1, RASSF).
One important area of
Met
The protooncogene Met is a RTK that is activated by its ligand hepatocyte growth factor/scatter factor (HGF/SF) [70]. Met and HGF/SF are overexpressed in metastases, and aberrant Met–HGF/SF signaling increased motility and invasion of cells in vitro and in vivo in part by augmenting the activity of urokinase plasminogen activator (uPa) [71]. uPa is known to be involved in the destruction of ECM/basement membrane, a necessary event in the migration of cells from the solid tumor.
Vande Woude and
Actin cytoskeleton
Gelsolin is a protein able to disrupt the actin cytoskeleton by cleaving F-actin subunits. It has been proposed that the upregulation of gelsolin may account for the transition from benign to invasive cell growth in some but not all tumors [87], [88]. Kwiatkowski and coworkers demonstrated a connection between gelsolin activity and Rac in fibroblast motility [89], and Gettemans and coworkers demonstrate that gelsolin activity is affected by the oncogenic Ras pathway [90], and that invasion of
Conclusions
Much work has been accomplished in discovering the many facets of oncogenic Ras signaling evident in the growth transformation of cells. These data have revealed that the mechanisms downstream of Ras are much more complex than originally thought. Similar conclusions are being formed about mutant Ras and its contribution to increased motility, invasiveness, and metastatic potential. Clearly, crosstalk and feedback with a multiplicity of signaling networks are in evidence, and the pathways
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