Apoptotic effect of fludarabine is independent of expression of IAPs in B-cell chronic lymphocytic leukemia

Subcellular localization of X-linked inhibitor of apoptosis protein (XIAP) in cancer: does that matter?

X-linked inhibitor of apoptosis protein (XIAP) finely tunes the balance between survival and death to control homeostasis. XIAP is found aberrantly expressed in cancer, which has been shown to promote resistance to therapy-induced apoptosis and confer poor outcome. Despite its predominant cytoplasmic localization in human tissues, growing evidence implicates the expression of XIAP in other subcellular compartments in sustaining cancer hallmarks. Herein, we review our current knowledge on the prognostic role of XIAP localization and discuss molecular mechanisms underlying differential biological functions played in each compartment. The comprehension of XIAP subcellular shuttling and functional dynamics might provide the rationale for future anticancer therapeutics.

Reactivation of Smac-mediated apoptosis in chronic lymphocytic leukemia cells: mechanistic studies of Smac mimetic

Dysfunctional apoptotic machinery is a hallmark feature of chronic lymphocytic leukemia (CLL). Accordingly, targeting apoptosis regulators has been proven a rational approach for CLL treatment. We show that CLL lymphocytes express high levels of XIAP, cIAP1, and cIAP2 compared to normal lymphocytes. Smac mimetic, Smac066, designed to bind to BIR3-domain of IAPs, induce apoptosis in primary CLL cells (n=71; p<0.0001), irrespective of prognostic markers. Apoptosis was mediated by diminished levels of IAPs (XIAP-p=0.02; cIAP-p<0.0001) and increased activation of caspases-8,-9,-3. The caspase-cleavage was in direct association with the levels of apoptosis (r2=0.8 for caspases-8,-9,-3). Correlative analysis revealed a direct relationship between reduction in IAPs and degree of apoptosis (r2=0.6 (XIAP); 0.5 (cIAP2)). There was a strong association between apoptosis, IAP-degradation, and concurrent caspase-activation. Pan-caspase inhibitor Z-Vad-fmk reversed the degradation of Mcl-1, but not IAPs suggesting that smac066 is selective to IAPs, however, Mcl-1 degradation is through caspase-mediated cleavage. Immunoprecipitation experiments revealed physical interaction between caspase-3 and XIAP that was disrupted by smac066. Importantly, XIAP and cIAP2 were markedly induced in bone-marrow and lymph-node microenvironments, providing a basis for IAP antagonists as anti-tumor agents in CLL. Smac066 synergized with ABT-737, revealing a mechanistic rationale to jointly target BH3 and BIR3 domains.

Targeting inhibitor of apoptosis proteins by Smac mimetic elicits cell death in poor prognostic subgroups of chronic lymphocytic leukemia

Inhibitor of apoptosis (IAP) proteins are highly expressed in chronic lymphocytic leukemia (CLL) cells and contribute to evasion of cell death and poor therapeutic response. Here, we report that Smac mimetic BV6 dose-dependently induces cell death in 28 of 51 (54%) investigated CLL samples, while B-cells from healthy donors are largely unaffected. Importantly, BV6 is significantly more effective in prognostic unfavorable cases with, e.g., non-mutated VH status and TP53 mutation than samples with unknown or favorable prognosis. The majority of cases with 17p deletion (10/12) and Fludarabine refractory cases respond to BV6, indicating that BV6 acts independently of p53. BV6 also triggers cell death under survival conditions mimicking the microenvironment, e.g., by adding CD40 ligand or conditioned medium. Gene expression profiling identifies cell death, NF-κB and redox signaling among the top pathways regulated by BV6 not only in CLL but also in core-binding factor (CBF) acute myeloid leukemia (AML). Consistently, BV6 stimulates production of reactive oxygen species (ROS), which are contributing to BV6-induced cell death, since antioxidants reduce cell death. While BV6 causes degradation of cellular inhibitor of apoptosis (cIAP)1 and cIAP2 and nuclear factor-kappaB (NF-κB) pathway activation in primary CLL samples, BV6 induces cell death independently of caspase activity, receptor-interacting protein (RIP)1 activity or tumor necrosis factor (TNF)α, as zVAD.fmk, necrostatin-1 or TNFα-blocking antibody Enbrel fail to inhibit cell death. Together, these novel insights into BV6-regulated cell death in CLL have important implications for developing new therapeutic strategies to overcome cell death resistance especially in poor prognostic CLL subgroups.

Selective IAP inhibition results in sensitization of unstimulated but not CD40-stimulated chronic lymphocytic leukaemia cells to TRAIL-induced apoptosis

Despite recent advances in therapy, chronic lymphocytic leukaemia (CLL) remains incurable and new treatment strategies are therefore urgently required. Inhibitor of apoptosis proteins (IAPs) are over-expressed in CLL, suggesting both a role in disease pathogenesis and the potential for therapeutic targeting. To explore these questions, we evaluated the effects on primary CLL cells of AZD5582, a novel potent and selective inhibitor of IAPs. AZD5582 at nanomolar concentrations induced extensive degradation of cIAP-1 and cIAP-2, but minimally of X chromosome-linked IAP (XIAP). However, these effects of AZD5582 produced little or no direct cytotoxicity, nor did they sensitize CLL cells to p53-dependent killing by fludarabine or p53-independent killing by dexamethasone. In contrast, AZD5582 significantly enhanced apoptosis induced by the death receptor (DR) agonist tumour necrosis factor-related apoptosis-inducing ligand (TRAIL). Importantly, killing by TRAIL plus AZD5582 was independent of adverse prognostic features including TP53 deletion which is strongly associated with chemoresistance in CLL. Coculture experiments involving transfected mouse fibroblasts expressing human CD40L (CD154) to mimic the effect of T cells at sites of tissue involvement showed that CD40 stimulation almost completely prevented the killing of CLL cells by TRAIL plus AZD5582 despite up-regulating TRAIL receptors 1 and 2. In conclusion, our findings confirm the rate-limiting, upstream involvement of IAPs in the extrinsic but not intrinsic apoptotic pathway of CLL cells and suggest that drug combinations that simultaneously activate DRs and inhibit IAPs may have therapeutic potential in patients with CLL who have failed T-cell-depleting chemotherapy.

Inhibitor of Apoptosis (IAP) proteins in hematological malignancies: molecular mechanisms and therapeutic opportunities

Inhibitor of Apoptosis (IAP) proteins exert essential functions during tumorigenesis as well as treatment resistance by simultaneously blocking cell death pathways and promoting cell survival. As IAP proteins are typically aberrantly expressed in human cancers including hematological malignancies, they represent in principle promising targets for therapeutic interventions. There are currently exciting opportunities to rationally exploit the therapeutic targeting of IAP proteins for the treatment of leukemia and lymphoma. Further insights into the signaling pathways that are under the control of IAP proteins and into the specific IAP protein-dependent vulnerabilities of hematological neoplasms are expected to pave the avenue to novel treatment strategies.

IAP proteins as targets for drug development in oncology

The inhibitors of apoptosis (IAPs) constitute a family of proteins involved in the regulation of various cellular processes, including cell death, immune and inflammatory responses, cell proliferation, cell differentiation, and cell motility. There is accumulating evidence supporting IAP-targeting in tumors: IAPs regulate various cellular processes that contribute to tumor development, such as cell death, cell proliferation, and cell migration; their expression is increased in a number of human tumor samples, and IAP overexpression has been correlated with tumor growth, and poor prognosis or low response to treatment; and IAP expression can be rapidly induced in response to chemotherapy or radiotherapy because of the presence of an internal ribosome entry site (IRES)-dependent mechanism of translation initiation, which could contribute to resistance to antitumor therapy. The development of IAP antagonists is an important challenge and was subject to intense research over the past decade. Six molecules are currently in clinical trials. This review focuses on the role of IAPs in tumors and the development of IAP-targeting molecules for anticancer therapy.

XIAP and P-glycoprotein co-expression is related to imatinib resistance in chronic myeloid leukemia cells

P-glycoprotein (Pgp) and XIAP co-expression has been discussed in the process of the acquisition of multidrug resistance (MDR) in cancer. Here, we evaluated XIAP and Pgp expression in chronic myeloid leukemia (CML) samples, showing a positive correlation between them. Furthermore, we evaluated the effects of imatinib in XIAP and Pgp expression using CML cell lines K562 (Pgp(-)) and K562-Lucena (Pgp(+)). Imatinib increased XIAP and Pgp expression in K562-Lucena cells, while in K562 cells a downregulation of these proteins was observed, suggesting that imatinib induces an increment of MDR phenotype of CML cells that previously exhibit high levels of Pgp/XIAP co-expression.

Single-agent Smac-mimetic compounds induce apoptosis in B chronic lymphocytic leukaemia (B-CLL)

Defective apoptosis is a hallmark of the progression of B chronic lymphocytic leukaemia (B-CLL). Smac-mimetics have been shown to induce apoptosis in several tumours. We describe the in vitro pro-apoptotic activity and regulation of the molecular pathway induced by new Smac-mimetics in B-CLL. The cytotoxic effect was significantly higher in B-CLL samples than in healthy controls. No significant synergistic effect was observed in combined treatment. In conclusion one of our compounds (Smac66), used as monotherapy and not in combination, is highly active against B-CLL cells thus suggesting a promising therapeutic potential as a new class of antileukemic drugs in haematology.

Imatinib increases apoptosis index through modulation of survivin subcellular localization in the blast phase of CML cells

Using MTT, Annexin V/flow cytometry, immunocytochemistry, subcellular fractionation, and Western blotting assays we analyzed the effect of imatinib in two blast phase of chronic myeloid leukemia (CML) cell lines: K562 P-glycoprotein (Pgp)-negative, and Lucena, Pgp-positive. In K562 cell line, the high apoptosis index induced by imatinib was associated with the survivin predominantly in the nucleus. In the Lucena cell line, the low apoptosis index induced by imatinib was associated with a cytoplasmatic survivin localization. Pgp and survivin might be subject to the same molecular regulation, and therefore represent a therapeutic target in the blast phase of CML.

Targeting IAP proteins for therapeutic intervention in cancer

Evasion of apoptosis is one of the crucial acquired capabilities used by cancer cells to fend off anticancer therapies. Inhibitor of apoptosis (IAP) proteins exert a range of biological activities that promote cancer cell survival and proliferation. X chromosome-linked IAP is a direct inhibitor of caspases - pro-apoptotic executioner proteases - whereas cellular IAP proteins block the assembly of pro-apoptotic protein signalling complexes and mediate the expression of anti-apoptotic molecules. Furthermore, mutations, amplifications and chromosomal translocations of IAP genes are associated with various malignancies. Among the therapeutic strategies that have been designed to target IAP proteins, the most widely used approach is based on mimicking the IAP-binding motif of second mitochondria-derived activator of caspase (SMAC), which functions as an endogenous IAP antagonist. Alternative strategies include transcriptional repression and the use of antisense oligonucleotides. This Review provides an update on IAP protein biology as well as current and future perspectives on targeting IAP proteins for therapeutic intervention in human malignancies.

Exploiting inhibitor of apoptosis proteins as therapeutic targets in hematological malignancies

Resistance to apoptosis is one of the hallmarks of human cancers and contributes to the insensitivity of many cancers to commonly used treatment approaches. Inhibitor of apoptosis (IAP) proteins, a family of anti-apoptotic proteins, have an important role in evasion of apoptosis, as they can both block apoptosis-signaling pathways and promote survival. High expression of IAP proteins is observed in multiple cancers, including hematological malignancies, and has been associated with unfavorable prognosis and poor patients' outcome. Therefore, IAP proteins are currently considered as promising molecular targets for therapy. Indeed, drug-discovery approaches over the last decade aiming at neutralizing IAP proteins have resulted in the generation of small-molecule inhibitors or antisense oligonucleotides that demonstrated in vitro and in vivo antitumor activities in preclinical studies. As some of these strategies have already entered the stage of clinical evaluation, for example, in leukemia, an update on this promising molecular-targeted strategy to interfere with apoptotic pathways is of broad interest.

Aggregation of spectrin and PKCtheta is an early hallmark of fludarabine/mitoxantrone/dexamethasone-induced apoptosis in Jurkat T and HL60 cells

It has been shown that changes in spectrin distribution in early apoptosis preceded changes in membrane asymmetry and phosphatidylserine (PS) exposure. PKCtheta was associated with spectrin during these changes, suggesting a possible role of spectrin/PKCtheta aggregation in regulation of early apoptotic events. Here we dissect this hypothesis using Jurkat T and HL60 cell lines as model systems. Immunofluorescent analysis of alphaIIbetaII spectrin arrangement in Jurkat T and HL60 cell lines revealed the redistribution of spectrin and PKCtheta into a polar aggregate in early apoptosis induced by fludarabine/mitoxantrone/dexamethasone (FND). The appearance of an alphaIIbetaII spectrin fraction that was insoluble in a non-ionic detergent (1% Triton X-100) was observed concomitantly with spectrin aggregation. The changes were observed within 2 h after cell exposure to FND, and preceded PS exposure. The changes seem to be restricted to spectrin and not to other cytoskeletal proteins such as actin or vimentin. In studies of the mechanism of these changes, we found that (i) neither changes in apoptosis regulatory genes (e.g., Bcl-2 family proteins) nor changes in cytoskeleton-associated proteins were detected in gene expression profiling of HL60 cells after the first hour of FND treatment, (ii) caspase-3, -7, -8, and -10 had minor involvement in the early apoptotic rearrangement of spectrin/PKCtheta, and (iii) spectrin aggregation was shown to be partially dependent on PKCtheta activity. Our results indicate that spectrin/PKCtheta aggregate formation is related to an early stage in drug-induced apoptosis and possibly may be regulated by PKCtheta activity. These findings indicate that spectrin/PKCtheta aggregation could be considered as a hallmark of early apoptosis and presents the potential to become a useful diagnostic tool for monitoring efficiency of chemotherapy as early as 24 h after treatment.

Expression and prognostic significance of the inhibitor of apoptosis protein (IAP) family and its antagonists in chronic lymphocytic leukaemia

Impaired apoptosis is still considered to be an important event in the development and progression of chronic lymphocytic leukaemia (CLL). However, mechanisms of this defect have not been fully elucidated. In this study, expression of inhibitor of apoptosis proteins, IAPs (cIAP1, cIAP2, XIAP and survivin), and their antagonists (Smac/DIABLO and HtrA2/Omi) was comprehensively analysed in 100 untreated CLL patients, using flow cytometry and Western blot techniques. Expression of anti-apoptotic cIAP1 and cIAP2 in leukaemic cells was significantly higher than in non-tumour lymphocytes (p=0.000001 and p=0.014, respectively), whereas the IAP-antagonist, Smac/DIABLO, was decreased in CLL (p=0.010). Higher expression of all analysed IAPs (cIAP1, p=0.002; cIAP2, p=0.026; XIAP, p=0.002; survivin, p=0.00006) and lower levels of Smac/DIABLO (p=0.006) were found in patients with progressive disease, compared to those with stable CLL. High baseline expression of cIAP1 and survivin correlated with worse response to treatment. Co-expression of these proteins was associated with shorter overall survival of CLL patients (p=0.005). In conclusion, CLL cells show the apoptosis-resistant profile of IAPs/IAP-antagonist expression. Upregulation of IAPs is associated with a progressive course of the disease. Co-expression of cIAP1 and survivin seems to be an unfavourable prognostic factor in CLL patients. Further studies with longer follow up are warranted to confirm and expand these findings.

A novel paradigm to trigger apoptosis in chronic lymphocytic leukemia

Evasion of apoptosis is a hallmark of chronic lymphocytic leukemia (CLL), calling for new strategies to bypass resistance. Here, we provide first evidence that small-molecule X-linked inhibitor of apoptosis (XIAP) inhibitors in combination with the death receptor ligand tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) present a novel approach to trigger apoptosis in CLL, including subgroups with resistant disease or unfavorable prognosis. XIAP, cellular IAP (cIAP) 1, and cIAP2 are expressed at high levels in primary CLL samples. Proof-of-concept studies in CLL cell lines show that subtoxic concentrations of XIAP inhibitors significantly enhance TRAIL-induced apoptosis and also sensitize for CD95-mediated apoptosis. Importantly also in primary CLL samples, XIAP inhibitor acts in concert with TRAIL to trigger apoptosis in 18 of 27 (67%) cases. This XIAP inhibitor-induced and TRAIL-induced apoptosis involves caspase-3 activation and is blocked by the caspase inhibitor zVAD.fmk. The cooperative interaction of XIAP inhibitor and TRAIL is even evident in distinct subgroups of patients with poor prognostic features (i.e., with 17p deletion, TP53 mutation, chemotherapy-refractory disease, or unmutated V(H) genes). Interestingly, cases with unmutated V(H) genes were significantly more sensitive to XIAP inhibitor-induced and TRAIL-induced apoptosis compared with V(H) gene-mutated samples, pointing to a role of B-cell receptor signaling in apoptosis regulation. By showing that XIAP inhibitors in combination with TRAIL present a new strategy to trigger apoptosis even in resistant forms and poor prognostic subgroups of CLL, our findings have important implications for the development of apoptosis-based therapies in CLL.

Modulation of reactive oxygen species by antioxidants in chronic myeloid leukemia cells enhances imatinib sensitivity through survivin downregulation

Survivin, a member of the inhibitor of apoptosis protein family and a target for new drugs, is modulated by reactive oxygen species in several types of neoplasms including leukemias. The aim of this study is to find mechanisms to enhance sensitivity to imatinib in imatinib-responsive cells. In this study, we demonstrated through fluorescein isothiocyanate-labeled annexin V for apoptotic cells detection and western blotting that by inhibiting catalase activity, imatinib apoptosis induction was significantly enhanced (P<0.05) through diminishing survivin expression in K562 cells. These findings might be of clinical relevance and might help improve the chemotherapeutic use of imatinib mesylate.

Inhibitor of apoptosis proteins in hematological malignancies

Apoptosis or programmed cell death is a key mechanism to control tissue homeostasis, for example, in the hematopoietic system. Thus, resistance to apoptosis can contribute to the development of leukemia or lymphoma. Inhibitors of apoptosis (IAP) proteins block cell death pathways at a central node by interfering with the activation of effector caspases. As increased expression levels of IAPs are found in hematological malignancies and have been correlated with poor prognosis, IAPs could be exploited as therapeutic targets and prognostic markers. Various strategies have been developed to target IAPs for therapeutic purposes in leukemia and lymphoma cells, including small-molecule inhibitors and antisense oligonucleotides. These agents could directly induce apoptosis in malignant cells or sensitize these cells to other cytotoxic agents. Thus, IAPs present promising targets for the development of new biomarkers and cancer therapeutics in hematological malignancies.

Bisphosphonates induce apoptosis in CLL cells independently of MDR phenotype

PURPOSE

The anti-tumoral activity of bisphosphonates (BPs) has been reported in leukemia. In this study, we attempted to evaluate the apoptotic effects of the BPs pamidronate (PAM) and zoledronic acid (ZOL) in chronic lymphocytic leukemia (CLL) samples, and to correlate it with clinical parameters and multidrug resistance (MDR) phenotype (P-glycoprotein and multidrug resistance-related protein expression and/or their functional activity).

RESULTS

Both BPs were able to induce apoptosis significantly. No correlation was observed between BP-induced apoptosis and clinical parameters or MDR phenotype.

CONCLUSION

Our data suggest that concurrent or sequential administration of BPs with conventional chemotherapeutic drugs may have significant therapeutic potential for CLL patients.

Pomolic acid-induced apoptosis in cells from patients with chronic myeloid leukemia exhibiting different drug resistance profile

Pomolic acid (PA) is a pentacyclic triterpene which has been previously described as active in inhibiting the growth of K562 cell line-originated from chronic myeloid leukemia (CML) in blast crisis-and its vincristine-resistant derivative K562-Lucena1. In this work, cells from CML patients were treated with PA and the apoptotic index was compared with the multidrug resistance (MDR) profile and clinical status of the patients. Our findings show that PA 12.5 microg/ml at 24 h (p = 0.000), at 48 h (p = 0.012) and at 72 h (p = 0.005) has a potent apoptotic index in CML cells as compared to mononuclear cells from healthy donors. PA was capable to induce apoptosis in cells from CML patients exhibiting functional MDR phenotype but not in P-glycoprotein expression. In addition, PA was effective in chronic as well as in blast phase of CML. Moreover, similar apoptotic index induced by PA was observed in low, intermediate and high-risk Sokal score as well as in samples from the group of patients with clinical resistance to interferon and/or imatinib and non-treated patients. These results suggest that PA may be an effective agent for the treatment of CML.