1. ¹Department of Pathology, Herbert Irving Comprehensive Cancer Center, Columbia University Medical Center, College of Physicians and Surgeons, New York, NY 10032, USA
2. ²Department of Microbiology and Immunology, Sylvester Comprehensive Cancer Center, University of Miami School of Medicine, Miami, FL 33136, USA
3. ³Department of Radiation Oncology, Virginia Commonwealth University, Richmond, VA 23298, USA
4. ⁴Department of Neurosurgery, Herbert Irving Comprehensive Cancer Center, Columbia University Medical Center, College of Physicians and Surgeons, New York, NY 10032, USA
5. ⁵Department of Urology, Herbert Irving Comprehensive Cancer Center, Columbia University Medical Center, College of Physicians and Surgeons, New York, NY 10032, USA

Correspondence: PB Fisher, Department of Pathology, Columbia University Medical Center, College of Physicians and Surgeons, BB-1501, 630 West 168th Street, New York, NY 10032, USA. Tel: +1 212 305 3966; Fax: +1 212 305 8177; E-mail: pbf1@columbia.edu

⁶These authors contributed equally to this work

Received 24 August 2005; Revised 14 February 2006; Accepted 15 February 2006; Published online 31 March 2006.

Abstract

Melanoma differentiation-associated gene-5 (mda-5) was the first molecule identified in nature whose encoded protein embodied the unique structural combination of an N-terminal caspase recruitment domain and a C-terminal DExD/H RNA helicase domain. As suggested by its structure, cumulative evidences documented that ectopic expression of mda-5 leads to growth inhibition and/or apoptosis in various cell lines. However, the signaling pathways involved in mda-5-mediated killing have not been elucidated. In this study, we utilized either genetically modified cloned rat embryo fibroblast cells overexpressing different functionally and structurally distinct oncogenes or human pancreatic and colorectal carcinoma cells containing mutant active ras to resolve the role of the Ras/Raf signaling pathway in mda-5-mediated growth inhibition/apoptosis induction. Rodent and human tumor cells containing constitutively activated Raf/Raf/MEK/ERK pathways were resistant to mda-5-induced killing and this protection was antagonized by intervening in this signal transduction cascade either by directly inhibiting ras activity using an antisense strategy or by targeting ras-downstream factors, such as MEK1/2, with the pharmacological inhibitor PD98059. The present findings provide a further example of potential cross-talk between growth-inhibitory and growth-promoting pathways in which the ultimate balance of these factors defines cellular homeostasis, leading to survival or induction of programmed cell death.