Deregulation of the ubiquitin system and p53 proteolysis modify the apoptotic response in B-CLL lymphocytes

We recently reported increased sensitivity of B-cell chronic lymphocytic le ukemia (B-CLL) lymphocytes to apoptotic death activation by the proteasome- specific inhibitor lactacystin. Here, we show that only specific-not nonspe cific-proteasomal inhibitors can discriminate between malignant and normal lymphocytes in inducing the apoptotic death response. Indeed, lactacystin e nd its active metabolite clasto-lactacystin p-lactone induced apoptotic dea th in CLL but not in normal lymphocytes. This difference was completely abo lished when tripeptide aldehydes such as MG132 or LLnL (which can also inhi bit calpains) were used as less specific proteasomal inhibitors, Moreover, B-CLL cells exhibited a constitutive altered ubiquitin-proteasome system, i ncluding a threefold higher chymotrypsin-like proteasomal activity and high levels of nuclear ubiquitin-conjugated proteins compared with normal lymph ocytes, Interestingly, B-CLL cells also displayed altered proteolytic regul ation of wild-type p53, an apoptotic factor reported to be a substrate for the ubiquitin-proteasome system. Nuclear wild-type p53 accumulated after la ctacystin treatment used at the discriminating concentration in malignant, but not in normal, lymphocytes. In contrast, p53 was stabilized by MG132 or LLnL in malignant and normal cells undergoing apoptosis, indicating that i n normal lymphocytes p53 is regulated mainly by calpains and not by the ubi quitin-proteasome system. This work raises the possibility that two differe nt proteolytic pathways controlling p53 stability may be pathologically imb alanced. This could result in modification of apoptosis control, since in C LL-lymphocytes a highly upregulated ubiquitin-proteasome system, which cont rols p53 stability among other apoptotic factors, was correlated with an in creased propensity of these cells to apoptosis triggered by lactacystin. (B lood. 2000;96:269-274) (C) 2000 by The American Society of Hematology.