The Ras/Raf/MAPK Pathway

Abstract

The Ras/Raf/MAPK pathway is probably the best characterized signal transduction pathway in cell biology. The function of this pathway is to transduce signals from the extracellular milieu to the cell nucleus where specific genes are activated for cell growth, division and differentiation. The Ras/Raf/MAPK pathway is also involved in cell cycle regulation, wound healing and tissue repair, integrin signaling and cell migration.1–3 Finally, the Ras/Raf/MAPK pathway is able to stimulate angiogenesis through changes in expression of genes directly involved in the formation of new blood vessels.4 Thus, signaling through the Ras/Raf/MAPK regulates a variety of cellular functions that are important for tumorigenesis. Dysregulation of this pathway is a common event in cancer as Ras is the most frequently mutated oncogene in human cancer. Mutations in the K-ras oncogene have been localized in codons 12, 13, 59 and 61 with those at codons 12 and 61 occurring most frequently. K-ras mutations are present in 15-50% of lung cancers and in 72–90% of pancreatic cancers.5,6 Even in the absence of activating mutations, K-ras still plays a role in oncogenesis via Ras gene amplification, overexpression or upstream activation of the pathway. Each of these potential cellular alterations will produce increased activation of Ras effectors, thereby promoting development of tumors. For example, 40% of esophageal cancers have amplification of the K-ras gene7 and in approximately 50% of breast cancers, Ras is highly active in association with expression of HER-2/Neu receptors.8 Mutations in other members of the pathway are also common in other forms of cancers. Somatic B-raf mutations have been found in 60-70% of malignant melanomas and are also seen in papillary thyroid cancer, colon and ovarian cancers.9 Furthermore, activation of MAPK –not associated with either K-ras or BRAF activating mutations are seen in 41% of low-grade ovarian serous carcinomas.10 The ras superfamily of genes encodes small GTPbinding proteins that are responsible for regulation of many cellular processes, including differentiation, cytoskeletal organization, and protein trafficking.11 Oncogenic ras genes in human cells include H-ras, N-ras and K-ras.12 The 21-kd transforming proteins Hand Kras genes were first identified as the counterparts of the oncogenes of the Harvey and Kirsten rat sarcoma viruses, whereas the N-ras oncogene was isolated from a neuroblastoma and has not been found in any retroviruses.12 Since mutations in K-ras and not in H-ras or N-ras are common in lung cancer, this review will focus on K-ras. K-ras is initially synthesized as an inactive cytosolic pro-peptide. Then the protein undergoes a series of posttranslational modifications at its carboxyl terminus that increase its hydrophobicity allowing its localization to the lipid-rich cell membrane.13 An important post-translational modification is farnesylation in the hydrophobic tail of the carboxyl terminal group. This reaction is catalyzed by the enzyme farnesyltransferase which adds a 15-carbon hydrobobic farnesyl isoprenyl to the carboxyl terminus of Ras. Once in the cell membrane, K-ras cycles between inactive guanosine diphosphate–bound and active guanosine triphosphate (GTP) –bound states, thereby activating a series of effector kinases –such as Raf and MAPKthat phosphorylate a cascade of signaling proteins.14 The principal consequence of the mutated proteins is a marked decrease in interactions between Ras and its GTPase activator protein.15 Instead of reverting to its inactive guanosine diphosphate–bound state, the modified conformation of mutant Ras favors its active GTP-bound state, which has a higher propensity to activate downstream effectors even in the absence of growth factor stimulation, conferring a proliferative advantage to tumors. Activation of the pathway begins when a signal binds to a protein tyrosine kinase receptor. The epidermic growth factor receptor (EGFR) and the platelet-derived growth factor receptor (PDGFR) are the best-known receptors in the pathway. However, multiple upstream receptors including other receptor tyrosine kinases, integrins, serpentine receptors, heterotrimeric G-proteins and cytokine receptors are able to activate K-ras.16 Binding of a ligand to EGF receptor induces oligomerization of the receptor, a process that results in juxtaposition of the cytoplasmic, catalytic domains in a manner that allows activation of the kinase activity and transphosphorylation.17 Adaptor proteins such as Grb2 are now able to recognize sequence homology 2 (SH2) domains such as Shc, which in turn, recruit guanine nucleotide exchange factors (GEFs) like SOS-1 or CDC25 to the cell membrane17 (figure 1). The GEF becomes capable of interacting with Ras proteins at the cell membrane to promote a conformational change and the exchange of GDP for GTP. Following Ras activation, Raf is recruited to the cell membrane through Division of Medical Oncology Mayo Clinic, Rochester, MN Submitted for publication Accepted for publication Correspondence to: Julian R. Molina, MD., Ph.D Division of Medical Oncology Mayo Clinic Rochester, MN 55905 Phone: 507284 2511 FAX: 507 -284 1803 Molina.julian@mayo.edu Copyright © 2006 by the International Association for the Study of Lung Cancer ISSN: 1556-0864/06/0101-0007