Semi-high throughput method of measuring proteasome inhibition in vitro and in cultured cells

Abstract

The ubiquitin proteasome-proteolytic pathway has emerged as one of the most significant pathways in modulating protein homeostasis under both normal and disease states. The use of proteasome inhibitors (PI) has played a pivotal role in understanding protein turn over. The main objective of this work was to develop a comprehensive, fast, and reliable, yet simple in vitro assay that would allow for the identification and characterization of a wide range of PIs. The assays consist of a 96-well plate high throughput (HTP) method to assess proteasome activity in Hs578T breast cancer cell extracts, purified 20S proteasome, using a fluorogenic substrate, Suc-leu-leu-val-tyr-7-AMC, specific to the chymotrypsin-like enzymatic activity of the proteasome. We showed that the chymotrypsin-like activity of the proteasome was inhibited in the two in vitro systems, albeit to different degrees. The assay system also includes two cell-based assays consisting of a vector expressing a fusion protein of green fluorescent protein (gfp) and Mouse Ornithine Decarboxylase (MODC) in Zs578T (parental Hs578T carrying the vector that expresses the fusion protein). In the cell-based assay analyses (qualitatively by microscopy and quantitatively by flow cytometry), treatment of Zs578T with PIs prevented the degradation of MODC, accumulated gfp, indicative of increased proteasome inhibition. Because no single assay represents a definitive proof of proteasome inhibitory activity, combined, these assays should serve as a comprehensive benchmark for the identification and partial characterization of novel inhibitors. In summary, the four-step assay protocol can easily be adapted into a high throughput format to rapidly screen unknown inhibitors.

The effects of proteasomal inhibition on synaptic proteostasis

“…Thus, lactacystin is considered to be a highly selective inhibitor, and when used at a concentration of 10 lM, it has been shown to robustly block proteasome activity (e.g. Zhao et al, 2003;Willeumier et al, 2006;Machiya et al, 2010;Keyomarsi et al, 2011;Bajic et al, 2012).…”

Live-Cell Imaging of Ubiquitin–Proteasome System Function

“…For these studies, it is optimal to select clonal stable cell lines that exhibit a narrow distribution of basal intensities yielding uniform histograms that are easy to quantify and permit the detection of small changes in mean fluorescence (see Note 1). Such cell lines have been successfully used in experiments that quantify proteasome inhibition by pharmacological agents (10, 14). …”

“…While clonal cell lines stably expressing UPS reporters are excellent tools for assessing the effects of pharmacological inhibition on proteasome function or upstream steps in the degradation pathway of UPS substrates (9, 10), using these lines to analyze the effects of transient protein-overexpression or knockdown can be confounded by cell-to-cell variation in expression level and transfection efficiency. This problem can be circumvented by use of fluorescent proteins co-expressed with the protein of interest as transfection markers (11).…”