In vitro activity: Treatment with 50 μM LDN-57444 for 24 h leads to 70% inhibition of the proteasome activity. LDN-57444 causes a significant and concentration-dependent decrease in cell viability at concentrations above 25 μM and the cell viability reduced to 61.81% at 50 μM. LDN-57444 is able to cause cell death through the apoptosis pathway by decreasing the activity of ubiquitin proteasome system and increasing the levels of highly ubiquitinated proteins, both of which can activate unfolded protein response. The apoptosis induced by LDN-57444 may be triggered by the activation of endoplasmic reticulum stress (ERS).

Kinase Assay: To start an assay, 0.5 μL of 5 mg/mL test compound (about 50 μM final reaction concentration) or DMSO control is aliquoted into each well. Both enzyme and substrate are prepared in UCH reaction buffer (50 mM Tris-HCl [pH 7.6], 0.5 mM EDTA, 5 mM DTT, and 0.5 mg/mL ovalbumin). 25 μL of 0.6 nM UCH-L1 is then added to each well except substrate control wells, followed by plate shaking for 45–60 s on an automatic shaker. The enzyme/compound mixture is incubated at room temperature for 30 min before 25 μL of 200 nM Ub-AMC is added to initiate the enzyme reaction. The reaction mixture (300 pM UCH-L1, 100 nM Ubiquitin-AMC with 2.5 μg test compound) is incubated at room temperature for 30 additional minutes prior to quenching the reaction by the addition of 10 μL 500 mM acetic acid per well. The fluorescence emission intensity is measured on a LJL Analyst using a coumarin filter set (ex = 365 nm, em = 450 nm) and is subtracted by the intrinsic compound fluorescence to reveal the enzyme activity. A DMSO control (0.5 μL of DMSO, 25 μL of UCH-L1, 25 μL of ubiquitin-AMC, 10 μL of acetic acid), enzyme control (25 μL of UCH-L1, 25 μL of buffer, 10 μL of acetic acid), substrate control (25 μL of buffer, 25 μL of ubiquitin-AMC, 10 μL of acetic acid), and inhibitor control (0.5 μL of ubiquitin aldehyde [100 nM stock], 25 μL of UCH-L1, 25 μL of ubiquitin-AMC, 10 μL of acetic acid) are also performed in each assay plate to ensure quality and reproducibility. Potential UCH-L1 inhibitors are selected if the compounds demonstrated greater than 60% inhibition compared to the controls. The UCH-L1 enzymatic reactions are manually repeated twice using the same protocol to confirm the results for the hit compounds from the primary robot-assisted screen.

Cell Assay: MTT assay

Cell. 2006 Aug 25;126(4):775-88; Mol Cell Biochem. 2008 Nov;318(1-2):109-15.