Abstract LB-232: MEK2 signaling pathway alone is sufficient but not necessary for melanoma cell proliferation

Abstract

Abstract The Raf-MEK-ERK signaling pathway plays a critical role in survival of many types of cancers, including melanoma. Targeting this pathway by using MEK inhibitors is a rational therapeutic strategy for melanoma treatment. Despite their success in pre-clinical studies, MEK inhibitors have failed to obtain satisfactory responses in clinical cancer trials. An improved understanding of MEK signaling in melanoma survival will facilitate the design of more effective therapeutics. There are two isoforms of MEK; MEK1 and MEK2. Although they are thought to be functionally redundant, recent studies have suggested that these two signaling molecules have non-overlapping cellular functions. To test this we developed a novel experimental system to allow signaling through individual MEK pathway in human melanoma SK-MEL-28 cells. In this system, multiple MEK/MKK pathways are inhibited by anthrax lethal toxin (LeTx), a Bacillus anthracis-secreted toxin which specifically and proteolytically inhibits members of MEK/MKK proteins. MEK1 or MEK2 signaling is rescued by expressing a cleavage-resistant MEK mutant (MEK1cr or MEK2cr) able to actively signal in the presence of LeTx. Using this approach, we determined that MEK2 signaling, and not MEK1, is sufficient for melanoma cell proliferation and anchorage-independent growth. To explain this we hypothesized that MEK1 and MEK2 signaling differentially regulate downstream effectors to control cell survival. To test this and to identify the non-overlapping effectors downstream to MEK1 and MEK2, we compared gene expression profiles using Agilent 60-mer Whole Human Genome Microarrays. We found that global suppression of MKK signaling by LeTx treatment significantly altered expression of 20% of represented genes. These mainly include genes involved in survival pathways such as the pathways of DNA replication, E2F transcription factors, rRNA processing, and cell proliferation, as well as oncogenic pathways such as Myc and Ras. Of these genes, approximately half were rescued by expression of one of MEK1cr or MEK2cr. Whereas MEK2cr had a more pronounced effect on expression and predominantly rescued signaling associated with cell proliferation, MEK1cr rescued signaling normally associated with epithelial to mesenchymal transition. Our findings demonstrate that MEK1 and MEK2 have non-redundant cellular functions and differentially regulate downstream effectors in SK-MEL-28 cells. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr LB-232.