Mechanisms Underlying Nonsteroidal Anti-Inflammatory Drug-Induced p27 Expression

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Abstract
We demonstrated previously that nonsteroidal anti-inflammatory drugs (NSAIDs) increased p27Kip1 by inhibiting protein degradation to suppress the proliferation of human lung cancer cells. In this study, we elucidate the molecular mechanism by which NSAIDs modulate p27Kip1 proteolysis. Immunoblotting and in vitro ubiquitination assays indicated that the expression of Cul1 and Skp2 and ubiquitination activity toward p27Kip1 were not regulated by NSAIDs. On the contrary, we found that NSAIDs inhibited proteasome activity to increase p27Kip1 protein levels. NSAIDs suppressed the expression of chymotrypsin-like catalytic subunits (β5, LMP7, and LMP2), but did not directly block enzymatic activity, to inhibit proteasome activity. Reverse transcriptase-competitive polymerase chain reaction and promoter activity assays showed that this inhibition occurred at the transcriptional level. In vitro degradation experiments showed that p27Kip1 degradation was inhibited by NS398, and the addition of purified 26S proteasome reversed this inhibitory effect. Collectively, our results revealed the mechanism by which NSAIDs modulate p27Kip1protein degradation and suggest that NSAIDs are a novel class of proteasome inhibitors.