Proteasome Inhibitors Reduce Luciferase and β-Galactosidase Activity in Tissue Culture Cells*

Reporter enzymes are commonly used in cell biology to study transcriptional activity of genes. Recently, reporter enzymes in combination with compounds that inhibit proteasome function have been used to study the effect of blocking transcription factor degradation on gene activation. While investigating the effect of proteasome inhibition on steroid receptor activation of the mouse mammary tumor virus (MMTV) promoter, we found that treatment with proteasome inhibitors enhanced glucocorticoid activation of the promoter attached to a chloramphenicol acetyltransferase (CAT) reporter, but inhibited activation of MMTV attached to a firefly luciferase or β-galactosidase reporter. MMTV RNA levels under these conditions correlated with the promoter activity observed using the CAT reporter, suggesting that proteasome inhibitor treatment interfered with luciferase or β-galactosidase reporter assays. Washout experiments demonstrated that the majority of luciferase activity was lost if the proteasome inhibitor was added at the same time luciferase was produced, not once the functional protein was made, suggesting that proteasome inhibition interferes with production of luciferase protein. Indeed, we found that proteasome inhibitor treatment dramatically reduced the levels of luciferase and β-galactosidase protein produced, as determined by Western blot. Thus, treatment with proteasome inhibitors interferes with luciferase and β-galactosidase reporter assays, possibly by inhibiting production of a functional reporter protein.