ABSTRACT
Hyperactive Ras signaling has strong oncogenic effects causing several different forms of cancer. Hyperactivity is frequently induced by mutations within Ras itself, which account for up to 30% of all human cancers. In addition, hyperactive Ras signaling can also be triggered independent of Ras by either mutation or by misexpression of various upstream regulators and immediate downstream effectors. We have previously reported that C-kinase potentiated protein phosphatase-1 inhibitor of 17 kDa (CPI-17) can drive Ras activity and promote tumorigenic transformation by inhibition of the tumor suppressor Merlin. We now describe an additional element of this oncogenic mechanism in the form of the ezrin-radixin-moesin (ERM) protein family, which exhibits opposing roles in Ras activity control. Thus, CPI-17 drives Ras activity and tumorigenesis in a two-fold way; inactivation of the tumor suppressor merlin and activation of the growth promoting ERM family. The in vivo significance of this oncogenic switch is highlighted by demonstrating CPI-17's involvement in human melanoma pathogenesis.
Subject(s)
Cytoskeletal Proteins/metabolism , Melanoma/enzymology , Membrane Proteins/metabolism , Microfilament Proteins/metabolism , Phosphoprotein Phosphatases/metabolism , Skin Neoplasms/enzymology , ras Proteins/metabolism , Animals , Cell Proliferation , Cell Transformation, Neoplastic/genetics , Cell Transformation, Neoplastic/metabolism , Cell Transformation, Neoplastic/pathology , Cytoskeletal Proteins/genetics , Gene Expression Regulation, Neoplastic , HeLa Cells , Humans , Intracellular Signaling Peptides and Proteins , Melanoma/genetics , Melanoma/pathology , Membrane Proteins/genetics , Mice , Microfilament Proteins/genetics , Muscle Proteins , Myosin-Light-Chain Phosphatase/genetics , Myosin-Light-Chain Phosphatase/metabolism , NIH 3T3 Cells , Neurofibromin 2/genetics , Neurofibromin 2/metabolism , Phosphoprotein Phosphatases/genetics , Phosphorylation , RNA Interference , Signal Transduction , Skin Neoplasms/genetics , Skin Neoplasms/pathology , Time Factors , Transfection , ras Proteins/geneticsABSTRACT
BACKGROUND: Receptor tyrosine kinases (RTKs) participate in a multitude of signaling pathways, some of them via the small G-protein Ras. An important component in the activation of Ras is Son of sevenless (SOS), which catalyzes the nucleotide exchange on Ras. PRINCIPAL FINDINGS: We can now demonstrate that the activation of Ras requires, in addition, the essential participation of ezrin, radixin and/or moesin (ERM), a family of actin-binding proteins, and of actin. Disrupting either the interaction of the ERM proteins with co-receptors, down-regulation of ERM proteins by siRNA, expression of dominant-negative mutants of the ERM proteins or disruption of F-actin, abolishes growth factor-induced Ras activation. Ezrin/actin catalyzes the formation of a multiprotein complex consisting of RTK, co-receptor, Grb2, SOS and Ras. We also identify binding sites for both Ras and SOS on ezrin; mutations of these binding sites destroy the interactions and inhibit Ras activation. Finally, we show that the formation of the ezrin-dependent complex is necessary to enhance the catalytic activity of SOS and thereby Ras activation. CONCLUSIONS: Taking these findings together, we propose that the ERM proteins are novel scaffolds at the level of SOS activity control, which is relevant for both normal Ras function and dysfunction known to occur in several human cancers.
