From sequence to function: using RNAi to elucidate mechanisms of human disease.

RNA interference (RNAi) has emerged as one of the most powerful tools for functionally characterizing large sets of genomic data. Capabilities of RNAi place it at the forefront of high-throughput screens, which are able to span the human genome in search of novel targets. Although RNAi screens have been used to elucidate pathway components and discover potential drug targets in lower organisms, including Caenorhabditis elegans and Drosophila, only recently has the technology been advanced to a state in which large-scale screens can be performed in mammalian cells. In this review, we will evaluate the major advancements in the field of mammalian RNAi, specifically in terms of high-throughput assays. Crucial points of experimental design will be highlighted, as well as suggestions as to how to interpret and follow-up on potential cell death targets. Finally, we assess the prospective applications of high-throughput screens, the data they are capable of generating, and the potential for this technique to further our understanding of human disease.

MEK inhibition enhances paclitaxel-induced tumor apoptosis.

The anti-cancer drug paclitaxel (Taxol) alters microtubule assembly and activates pro-apoptotic signaling pathways. Previously, we and others found that paclitaxel activates endogenous JNK in tumor cells, and the activation of JNK contributes to tumor cell apoptosis. Here we find that paclitaxel activates the prosurvival MEK/ERK pathway, which conversely may compromise the efficacy of paclitaxel. Hence, a combination treatment of paclitaxel and MEK inhibitors was pursued to determine whether this treatment could lead to enhanced apoptosis. The inhibition of MEK/ERK with a pharmacologic inhibitor, U0126, together with paclitaxel resulted in a dramatic enhancement of apoptosis that is four times more than the additive value of the two drugs alone. Enhanced apoptosis was verified by the terminal transferase-mediated dUTP nick end labeling assay, by an enzyme-linked immunosorbent assay for histone-associated DNA fragments, and by flow cytometric analysis for DNA content. Specificity of the pharmacologic inhibitor was confirmed by the use of (a) a second MEK/ERK inhibitor and (b) a transdominant-negative MEK. Enhanced apoptosis was verified in breast, ovarian, and lung tumor cell lines, suggesting this effect is not cell type-specific. This is the first report of enhanced apoptosis detected in the presence of paclitaxel and MEK inhibition and suggests a new anticancer strategy.