Proteasome pathway operates for the degradation of ornithine decarboxylase in intact cells

Ornithine decarboxylase (ODC) is degraded in an ATP-dependent manner in vitro by the 26 S proteasome in the presence of antizyme, an ODC destabilizing protein induced by polyamines. In the present study we examined whether the proteasome catalyses ODC degradation in living mammalian cells. Lactacystin, the most selective proteasome inhibitor, strongly inhibited the degradation of ODC that had been induced in hepatoma tissue-culture (HTC) cells by refeeding with fresh medium. Furthermore the inhibitor inhibited the rapid degradation of ODC that had been induced by hypotonic shock. Interestingly, hypertonic shock was found to increase the proportion of ODC present as a complex with antizyme (the ratio of ODC–antizyme complex to total ODC). Cycloheximide, which partly inhibits rapid ODC degradation caused by hypertonic shock, also partly inhibited the increase in the ratio of ODC–antizyme complex to total ODC. These results suggest that a common ODC degradation pathway, namely the antizyme-dependent and 26 S proteasome-catalysed ODC degradation pathway, is also operating in intact cells for osmoregulated ODC degradation.