Increased sensitivity of CLL‐derived lymphocytes to apoptotic death activation by the proteasome‐specific inhibitor lactacystin

Abstract

Summary. Ubiquitin-proteasome-dependent protein processing appears to be an essential component in the control of radiation-induced apoptosis in human lymphocytes. This control is altered in chronic lymphocytic leukaemia (CLL), compared to that of normal human lymphocytes which mainly showed high apoptotic values after irradiation, but in some cases no sensitivity was observed. Interestingly, lactacystin activated the apoptotic pathway in both radioresistant and sensitive CLL cells, at doses which had no effect in normal cells where significantly higher concentrations were required. Therefore the resistance of some CLL cells to apoptosis initiation by radiation does not correlate to observed increased sensitivity to lactacystin. The nuclear level of the transcription factor NF-kB or the cytoplasmic level of IkBa remained unaltered upon irradiation or lactacystin CLL cells treatment, suggesting that the activity of the other factors involved in apoptotic death control were altered through proteasomal inhibition. These results strongly suggest an essential role of the ubiquitin system in apoptotic cell death control in CLL lymphocytes. The inhibition of proteasome-ubiquitin-dependent processing could be a discriminatory apoptotic stimulus between normal versus malignant lymphocytes and therefore might potentially be of use in this specific human pathology.

The proteasome: a novel target for cancer chemotherapy

“…Although initially proposed to involve an inhibition of NF-B, this was subsequently shown not to be involved. 36 More recently, this group have also demonstrated that only specific proteasomal inhibitors, lactacystin and its active metabolite clasto-lactacystin ␤-lactone, induce selective apoptotic death in B-CLL cells and not in normal lymphocytes whereas non-specific inhibitors such as MG132 and LLnL (which also inhibits calpains) induce death equally in both cell types. 37 B-CLL cells have a constitutively higher chymotrypsin-like proteasomal activity compared to normal human lymphocytes and an altered proteolytic regulation of p53.…”

“…33 However, there are notable exceptions to this generalization as proteasome inhibitors are potent inducers of apoptosis in malignant B cell chronic lymphocytic leukaemia (B-CLL) cells, which are predominantly quiescent. [34][35][36][37][38][39] Proteasome inhibitors induce apoptosis and overcome drug resistance in many different types of cancer cell lines, and in primary leukaemic cells from patients with B-CLL ( Table 2). In the National Cancer Institute (NCI) pre-clinical screen, the proteasome inhibitor PS-341 (Table 1, Figure 2) is cytotoxic to a broad range of the 60 human tumour cell lines used, and interestingly, its mechanism of cytotoxicity differs from the other 60 000 compounds in the NCI database.…”

Mitochondrial DNA mutations in primary leukemia cells after chemotherapy: clinical significance and therapeutic implications

“…32,33 Such drug-resistance mechanisms might be a factor in the chemotherapy failure observed in CLL patients. [34][35][36] The fact that the five refractory patients all had high frequencies of mtDNA mutations with amino-acid alterations in the encoded proteins (Table 4) seems to suggest a relationship between mtDNA mutations and drug resistance, although the small number of patients did not allow a formal statistical analysis. Furthermore, clinical drug resistance is likely caused by multiple factors.…”

Proteasome inhibitor-induced apoptosis of B-chronic lymphocytic leukaemia cells involves cytochrome c release and caspase activation, accompanied by formation of an ∼700 kDa Apaf-1 containing apoptosome complex

“…MG132 induced apoptosis in glucocorticoid-resistant B-CLL cells, 4 and lactacystin activated apoptotic cell death and sensitised resistant B-CLL cells to TNF␣-induced apoptosis. 5,6 Caspases, a family of aspartate-specific cysteine proteases, are responsible for many of the characteristic biochemical and morphological changes associated with the execution phase of apoptosis. 7,8 'Initiator' caspases with long prodomains such as caspases-8, -9 and -10 either directly or indirectly activate 'effector' caspases, such as caspases-3, -6 and -7, which have short prodomains.…”