Fluorescent labeling of specific cysteine residues using CyMPL

Comparison of cell-based assays to quantify treatment effects of anticancer drugs identifies a new application for Bodipy-L-cystine to measure apoptosis

Cell-based assays that measure anticancer drug effects are essential for evaluating chemotherapeutic agents. Many assays targeting various cellular mechanisms are available, leading to inconsistent results when using different techniques. We critically compared six common assays, as well as a new assay using Bodipy.FL.L-cystine (BFC), to identify the most accurate and reproducible in measuring anticancer drug effects. We tested three common chemotherapies (methotrexate, paclitaxel, and etoposide) in two cell lines (Ln229 and MDA-MB231). Spectroscopic assays such as Cell Titer Blue, and 2′,7′-dichlorofluorescin diacetate (DCFDA) yielded a strong drug dose response, especially for paclitaxel and etoposide (R² = 0.9). MTT and Calcein-AM fluorescent dye-based assays were less consistent in that regard. Among three flow cytometry assays, Propidium Iodide (PI)-based DNA content analysis and a new BFC-based glutathione-redox (GSH) assay produced drug dose dependent results. Compared to PI, BFC showed a better correlation (R² = 0.7–0.9) in depicting live and apoptotic cells. We found that the combination of Cell Titer Blue spectroscopy and BFC flow cytometry assays were most accurate in assessing anticancer drug effects by clear distinction between live and apoptotic cells, independent of drug mechanism of action. We present a new application of BFC as an agent for measuring cellular apoptosis.

Evidence for a Negative Cooperativity between eIF5A and eEF2 on Binding to the Ribosome

eIF5A is the only protein known to contain the essential and unique amino acid residue hypusine. eIF5A functions in both translation initiation due to its stimulation of methionyl-puromycin synthesis and translation elongation, being highly required for peptide-bound formation of specific ribosome stalling sequences such as poly-proline. The functional interaction between eIF5A, tRNA, and eEF2 on the surface of the ribosome is further clarified herein. Fluorescence anisotropy assays were performed to determine the affinity of eIF5A to different ribosomal complexes and reveal its interaction exclusively and directly with the 60S ribosomal subunit in a hypusine-dependent manner (Ki60S-eIF5A-Hyp = 16 nM, Ki60S-eIF5A-Lys = 385 nM). A 3-fold increase in eIF5A affinity to the 80S is observed upon charged-tRNAiMet binding, indicating positive cooperativity between P-site tRNA binding and eIF5A binding to the ribosome. Previously identified conditional mutants of yeast eIF5A, eIF5AQ22H/L93F and eIF5AK56A, display a significant decrease in ribosome binding affinity. Binding affinity between ribosome and eIF5A-wild type or mutants eIF5AK56A, but not eIF5AQ22H/L93F, is impaired in the presence of eEF2 by 4-fold, consistent with negative cooperativity between eEF2 and eIF5A binding to the ribosome. Interestingly, high-copy eEF2 is toxic only to eIF5AQ22H/L93F and causes translation elongation defects in this mutant. These results suggest that binding of eEF2 to the ribosome alters its conformation, resulting in a weakened affinity of eIF5A and impairment of this interplay compromises cell growth due to translation elongation defects.