In eukaryotes, the ubiquitin-proteasome system (UPS) is responsible for the degradation of most proteins. Proteasome inhibition, which has been associated with various diseases, can cause alterations in various intracellular processes including the expression of heat shock protein (hsp) genes. In this study, we show that celastrol, a quinone methide triterpene and anti-inflammatory agent, inhibited proteasome activity and enhanced HSP accumulation in Xenopus laevis A6 kidney epithelial cells. Treatment of cells with celastrol induced the accumulation of ubiquitinated protein and inhibited chymotrypsin-like activity. This was accompanied by a dose- and time-dependent accumulation of HSP30 and HSP70. Celastrol-induced HSP accumulation was mediated by HSF1-DNA binding activity since this response was inhibited by the HSF1 activation inhibitor, KNK437. Simultaneous exposure of cells with celastrol plus either mild heat shock or the proteasome inhibitor, MG132, produced an enhanced accumulation of HSP30 that was greater than the sum of the individual stressors alone. Immunocytochemical analysis revealed that celastrol-induced HSP30 accumulation occurred in the cytoplasm in a granular pattern supplemented with larger circular HSP30 staining structures. HSP30 was also noted in the nucleus with less staining in the nucleolus. In some cells, celastrol induced the collapse of the actin cytoskeleton and conversion to a rounder morphology. In conclusion, this study has shown that celastrol inhibited proteasome activity and induced HSF1-mediated expression of hsp genes in amphibian cells.
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