PROTEASOME ACTIVITIES DECREASE DURING DEXAMETHASONE-INDUCED APOPTOSISOF THYMOCYTES

The induction of apoptosis in thymocytes by the glucocorticoid dexamet hasone was used as a model system to investigate whether there are cha nges in 20 S and 26 S proteasome activities during apoptosis, We obser ved that thymocytes contain high concentrations of proteasomes and tha t following treatment with dexamethasone, cell extracts showed a decre ase in proteasome chymotrypsin-like activity which correlated with the degree of apoptosis observed. The decrease in chymotrypsin-like activ ity of 20 S and 26 S proteasomes was still apparent after these comple xes had been partially purified from apoptotic thymocyte extracts and was therefore not due to competition resulting from a general increase in protein turnover. The trypsin-like and peptidylglutamylpeptide hyd rolase activities of proteasome complexes were also observed to decrea se during apoptosis, but these decreases were reversed by the inhibiti on of apoptosis by the caspase inhibitor benzyloxycarbonyl-Val-Ala-Asp (OMe)-fluor omethylketone. However, the chymotrypsin-like activity of proteasomes decreased further in the presence of the apoptosis inhibit or. Val-Ala-Asp-fluoromethylketone was found to inhibit the chymotryps in- and trypsin-like activity of 26 S proteasomes in vitro. The decrea se in proteasome activities in apoptosis did not appear to be due to a decrease in the concentration of total cellular proteasomes. Thus, th e early decreases in 20 S and 26 S proteasome activities during apopto sis appear to be due to a down-regulation of their proteolytic activit ies and not to a decrease in their protein concentration. These data s uggest that proteasomes may be responsible, in thymocytes, for the tur nover of a protein that functions as a positive regulator of apoptosis .