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. 2009 Jun 9;100(12):1879–1888. doi: 10.1038/sj.bjc.6605034

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Copyright © 2009 Cancer Research UK

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GSI1 inhibits proteasome activity both in vivo and in vitro but has less cytotoxic effect on breast cancer cells than MG132. (A) Inhibition of proteasome activity in vitro. The 20S proteasomal component was pre-incubated with GSI1, MG132, or vehicle for 15 min at room temperature, and then the proteolysis of suc-LLVY-AMC was monitored by fluorimetry. One unit of activity was defined as the amount of protein producing one μmol of AMC per minute at 40°C. Data represent the average ±s.d. of two experiments. (B) Effect of both GSI1 and MG132 on proteasome activity in cell extracts from breast cancer cells. Both drugs where added to the culture medium of breast cancer cells, and after 4 h, proteasome activity was determined in cell extracts by proteolysis of suc-LLVY-AMC. Data represent the average ±s.d. of three experiments. (C) Cytotoxic effect of GSI1 and MG132 on breast cancer cells. The effect of increasing drug concentrations on breast cancer cells was determined by staining with SRB after 48 h. Data represent the average ±s.d. of two independent experiments with six replicates at each time point.