Bortezomib activity and in vitro interactions with anthracyclines and cytarabine in acute myeloid leukemia cells are independent of multidrug resistance mechanisms and p53 status

The therapeutic potential of the proteasome in leukaemia

Many cellular processes converge on the proteasome, and its key regulatory role is increasingly being recognized. Proteasome inhibition allows the manipulation of many cellular pathways including apoptotic and cell cycle mechanisms. The proteasome inhibitor bortezomib has enhanced responses in newly diagnosed patients with myeloma and provides a new line of therapy in relapsed and refractory patients. Malignant cells are more sensitive to proteasome inhibition than normal haematopoietic cells. Proteasome inhibition enhances many conventional therapies and its role in leukaemia is promising.

Differential bortezomib sensitivity in head and neck cancer lines corresponds to proteasome, nuclear factor-kappaB and activator protein-1 related mechanisms

Head and neck squamous cell carcinomas (HNSCC) exhibit constitutive activation of transcription factors nuclear factor-kappaB (NF-kappaB) and activator protein-1 (AP-1), which are modulated by the proteasome and promote resistance to cell death. HNSCC show variable sensitivity to the proteasome inhibitor bortezomib in vitro as well as in murine xenografts and patient tumors in vivo, and the mechanisms are not well understood. To address this question, the sensitivities of nine HNSCC cell lines to bortezomib were determined using 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assays, and the potential relationship between the sensitivity and bortezomib effects on biological processes was examined in HNSCC lines of differential bortezomib sensitivity. The most sensitive cell line (UM-SCC-11B) underwent cell death at 10(-9) mol/L in vitro and tumor regression at a maximally tolerated dose of bortezomib in a murine xenograft model. The differential sensitivity between UM-SCC-11A and UM-SCC-11B cells corresponded to differences in the extent of suppression of proteasome activity, ubiquitinated protein degradation, and NF-kappaB and AP-1 activation. Lower concentrations of bortezomib transiently increased NF-kappaB and sustained AP-1 activation in UM-SCC-11A cells. AP-1 reporter activity and cell density of UM-SCC-11A were suppressed when bortezomib was combined with c-Jun NH(2)-terminal kinase and p38 kinase pathways inhibitors. Thus, the differential sensitivities to bortezomib corresponded to dissimilar effects on the proteasome, NF-kappaB and AP-1 activities. Inhibition of c-Jun NH(2)-terminal kinase and p38 pathways blocked AP-1 activity and enhanced the antitumor effects. These findings revealed molecular mechanisms of bortezomib sensitivity and resistance, which are under development as biomarkers for clinical trials in patients with HNSCC.

Molecular basis of bortezomib resistance: proteasome subunit beta5 (PSMB5) gene mutation and overexpression of PSMB5 protein

The proteasome inhibitor bortezomib is a novel anticancer drug that has shown promise in the treatment of refractory multiple myeloma. However, its clinical efficacy has been hampered by the emergence of drug-resistance phenomena, the molecular basis of which remains elusive. Toward this end, we here developed high levels (45- to 129-fold) of acquired resistance to bortezomib in human myelomonocytic THP1 cells by exposure to stepwise increasing (2.5-200 nM) concentrations of bortezomib. Study of the molecular mechanism of bortezomib resistance in these cells revealed (1) an Ala49Thr mutation residing in a highly conserved bortezomib-binding pocket in the proteasome beta5-subunit (PSMB5) protein, (2) a dramatic overexpression (up to 60-fold) of PSMB5 protein but not of other proteasome subunits including PSMB6, PSMB7, and PSMA7, (3) high levels of cross-resistance to beta5 subunit-targeted cytotoxic peptides 4A6, MG132, MG262, and ALLN, but not to a broad spectrum of chemotherapeutic drugs, (4) no marked changes in chymotrypsin-like proteasome activity, and (5) restoration of bortezomib sensitivity in bortezomib-resistant cells by siRNA-mediated silencing of PSMB5 gene expression. Collectively, these findings establish a novel mechanism of bortezomib resistance associated with the selective overexpression of a mutant PSMB5 protein.

Phase I and pharmacokinetic study of bortezomib in combination with idarubicin and cytarabine in patients with acute myelogenous leukemia

PURPOSE

Proteasome inhibition results in cytotoxicity to the leukemia stem cell in vitro. We conducted this phase I study to determine if the proteasome inhibitor bortezomib could be safely added to induction chemotherapy in patients with acute myelogenous leukemia (AML).

EXPERIMENTAL DESIGN

Bortezomib was given on days 1, 4, 8, and 11 at doses of 0.7, 1.0, 1.3, or 1.5 mg/m(2) with idarubicin 12 mg/m(2) on days 1 to 3 and cytarabine 100 mg/m(2)/day on days 1 to 7.

RESULTS

A total of 31 patients were enrolled. The median age was 62 years, and 16 patients were male. Nine patients had relapsed AML (ages, 18-59 years, n = 4 and > or = 60 years, n = 5). There were 22 patients of > or = 60 years with previously untreated AML (eight with prior myelodysplasia/myeloproliferative disorder or cytotoxic therapy). All doses of bortezomib, up to and including 1.5 mg/m(2), were tolerable. Nonhematologic grade 3 or greater toxicities included 12 hypoxia (38%; 11 were grade 3), 4 hyperbilirubinemia (13%), and 6 elevated aspartate aminotransferase (19%). Overall, 19 patients (61%) achieved complete remission (CR) and three had CR with incomplete platelet recovery. Pharmacokinetic studies revealed that the total body clearance of bortezomib decreased significantly (P < 0.01, N = 26) between the first (mean +/- SD, 41.9 +/- 17.1 L/h/m(2)) and third (18.4 +/- 7.0 L/h/m(2)) doses. Increased bone marrow expression of CD74 was associated with CR.

CONCLUSIONS

The combination of bortezomib, idarubicin, and cytarabine showed a good safety profile. The recommended dose of bortezomib for phase II studies with idarubicin and cytarabine is 1.5 mg/m(2).

Analysis of changes in the proteome of HL-60 promyeloid leukemia cells induced by the proteasome inhibitor PSI

Proteasome inhibitors display potent anti-neoplastic and anti-angiogenic properties both in vitro and in vivo. The mechanisms, however, by which proteasome inhibitors kill tumor cells are still fairly elusive as is the molecular basis of resistance to treatment. To address these questions, we employed a high-throughput Western blotting procedure to analyze changes in a subproteome of approximately 800 proteins in the promyelocytic leukemia cell line HL-60 upon treatment with the proteasome inhibitor PSI (Z-Ile-Glu(OtBu)-Ala-Leu-aldehyde) and correlated the changes of selected target proteins with the changes in two multidrug-resistant HL-60 variants. In total, 105 proteins were upregulated more than 1.5-fold after PSI treatment, while 79 proteins were downregulated. Activation of caspases-3 and -8, modulation of members of the Bcl-2 family as well as stimulation of stress signaling pathways was prominent during HL-60 apoptosis. We also identified changes in the abundance of proteins previously not known to be affected by proteasome inhibitors. In contrast, two multidrug-resistant HL-60 cell lines, overexpressing either MRP1 or P-glycoprotein were largely resistant to PSI-induced apoptosis and could not be resensitized by the pharmacological inhibitors of the drug efflux pumps MK571 or PSC833. Drug resistance was also independent of the upregulation of Bad. Overexpression of multidrug resistance proteins, P-glycoprotein and MRP-1 is thus not sufficient to explain resistance of HL-60 cells to treatment with proteasome inhibitor PSI, which remains more closely related to a low level of Bax expression and to the inability to activate JNK. Alternative routes to the acquisition of resistance to PSI have therefore to be considered.

M4 and M5 acute myeloid leukaemias display a high sensitivity to Bortezomib-mediated apoptosis

The present study explored the sensitivity of leukaemic blasts derived from 30 acute myeloid leukaemia (AML) patients to Bortezomib. Bortezomib induced apoptosis of primary AML blasts: 18/30 AMLs were clearly sensitive to the proapoptotic effects of Bortezomib, while the remaining cases were moderately sensitive to this molecule. The addition of tumour necrosis factor-related-apoptosis-inducing ligand, when used alone, did not induce apoptosis of AML blasts and further potentiated the cytotoxic effects of Bortezomib. The majority of AMLs sensitive to Bortezomib showed immunophenotypic features of the M4 and M5 French-American-British classification subtypes and displayed myelomonocytic features. All AMLs with mutated FLT3 were in the Bortezomib-sensitive group. Biochemical studies showed that: (i) Bortezomib activated caspase-8 and caspase-3 and decreased cellular FLICE [Fas-associated death domain (FADD)-like interleukin-1beta-converting enzyme]-inhibitory protein (c-FLIP) levels in AML blasts; (ii) high c-FLIP levels in AML blasts were associated with low Bortezomib sensitivity. Finally, analysis of the effects of Bortezomib on leukaemic cells displaying high aldehyde dehydrogenase activity suggested that this drug induced in vitro killing of leukaemic stem cells. The findings of the present study, further support the development of Bortezomib as an anti-leukaemic drug and provide simple tools to predict the sensitivity of AML cells to this drug.

Knockdown of PgP resensitizes leukemic cells to proteasome inhibitors

Overexpression of MDR-1 represents a critical mechanism of drug resistance in cancer. Proteasome inhibitors recently entered the clinic for treatment of multiple myeloma. We provide evidence that the proteasome-inhibitors Bortezomib and MLN273 are both substrates of MDR-1 by using knockdown of MDR-1 via a transposon-based vector system stably expressing siRNA against MDR-1 in MDR-1-overexpressing K562/Dox cells. Notably, the efficacy of MLN273 (EC(50) from 253 ng/ml in MDR-1(+) to 9.7 ng/ml in MDR-1(-) cells) was much more dependent on MDR-1 expression than Bortezomib (EC(50) from 24.9 ng/ml in MDR-1(+) to 4.5 ng/ml in MDR-1(-) cells). Growth inhibition in MDR-1 negative cells was in part due to increased rate of apoptosis. The enhanced inhibitory effect on the proteasome by loss of MDR-1 was corroborated by a reduced proteasomal activity. Our report provides evidence that MLN273 and, to a lesser degree, Bortezomib are both MDR-1-substrates, which might be relevant for drug-resistance in cancer.