Nucleoside transporters, bcl-2 and apoptosis in CLL cells exposed to nucleoside analogues in vitro

Purine analog-like properties of bendamustine underlie rapid activation of DNA damage response and synergistic effects with pyrimidine analogues in lymphoid malignancies

Bendamustine has shown considerable clinical activity against indolent lymphoid malignancies as a single agent or in combination with rituximab, but combination with additional anti-cancer drugs may be required for refractory and/or relapsed cases as well as other intractable tumors. In this study, we attempted to determine suitable anti-cancer drugs to be combined with bendamustine for the treatment of mantle cell lymphoma, diffuse large B-cell lymphoma, aggressive lymphomas and multiple myeloma, all of which are relatively resistant to this drug, and investigated the mechanisms underlying synergism. Isobologram analysis revealed that bendamustine had synergistic effects with alkylating agents (4-hydroperoxy-cyclophosphamide, chlorambucil and melphalan) and pyrimidine analogues (cytosine arabinoside, gemcitabine and decitabine) in HBL-2, B104, Namalwa and U266 cell lines, which represent the above entities respectively. In cell cycle analysis, bendamustine induced late S-phase arrest, which was enhanced by 4-hydroperoxy-cyclophosphamide, and potentiated early S-phase arrest by cytosine arabinoside (Ara-C), followed by a robust increase in the size of sub-G1 fractions. Bendamustine was able to elicit DNA damage response and subsequent apoptosis faster and with shorter exposure than other alkylating agents due to rapid intracellular incorporation via equilibrative nucleoside transporters (ENTs). Furthermore, bendamustine increased the expression of ENT1 at both mRNA and protein levels and enhanced the uptake of Ara-C and subsequent increase in Ara-C triphosphate (Ara-CTP) in HBL-2 cells to an extent comparable with the purine analog fludarabine. These purine analog-like properties of bendamustine may underlie favorable combinations with other alkylators and pyrimidine analogues. Our findings may provide a theoretical basis for the development of more effective bendamustine-based combination therapies.

Association of a novel regulatory polymorphism (−938C>A) in the BCL2 gene promoter with disease progression and survival in chronic lymphocytic leukemia

Bcl-2 plays a key role in the regulation of apoptosis. We investigated the role of a novel regulatory single-nucleotide polymorphism (−938C>A) in the inhibitory P2 BCL2 promoter in B-cell chronic lymphocytic leukemia (B-CLL). The −938C allele displayed significantly increased BCL2 promoter activity and binding of nuclear proteins compared with the A allele. Concomitantly, Bcl-2 protein expression in B cells from CLL patients carrying the −938 AA genotype was significantly increased compared with CC genotypes. Genotype distribution between 123 CLL patients (42 AA, 55 AC, 26 CC) and 120 genotyped healthy controls (36 AA, 63 AC, 21 CC) was not significantly different, suggesting that genotypes of this polymorphism do not increase the susceptibility for B-CLL. However, median time from first diagnosis to initiation of chemotherapy and median overall survival were significantly shorter in patients with −938AA genotype (38 and 199 months, respectively) compared with AC/CC genotypes (120 and 321 months, respectively; P = .008 and P = .003, respectively). Multivariable Cox regression identified the BCL2−938AA genotype as an independent prognostic factor for the time to first treatment (hazard ratio [HR] 1.9; P = .034) together with disease stage at diagnosis (HR 2.5; P = .004) and ZAP-70 status (HR 3.0; P = .001). The BCL2−938AA genotype is associated with increased Bcl-2 expression and a novel unfavorable genetic marker in patients with B-CLL.

Analysis of human equilibrative nucleoside transporter 1 (hENT1) protein in non-Hodgkin's lymphoma by immunohistochemistry

The human equilibrative nucleoside transporter 1 (hENT1) is a member of the equilibrative nucleoside transporter family that mediates cellular entry of gemcitabine, cytarabine, and fludarabine. Deficiency in hENT1 confers resistance to toxicity of these drugs in a variety of model systems. Since some nucleoside analogs have a role in treating patients with non-Hodgkin's lymphoma (NHL), this study was undertaken to assess hENT1 abundance in NHL. A total of 115 cases of NHL of various subtypes and 15 reactive lymph nodes were evaluated for the presence of hENT1 protein using immunohistochemistry applied to frozen tissues. Samples were considered positive when >or=50% of neoplastic cells showed immunostaining. In reactive lymph nodes, hENT1 was confined to the germinal centers, whereas mantle zone B-cells and interfollicular T-cells were negative. In NHL, a relatively high frequency of hENT1 positivity was found in Burkitt lymphoma/leukemia (63%), diffuse large B-cell lymphoma (DLCL; 45%), and follicular lymphoma (40%). In DLCL, 26% of cases were positive for CD10, and CD10-positive DLCL cases were more likely to be hENT1 positive than CD10-negative cases (P=0.025). A lower frequency of hENT1 positivity was found in mantle cell lymphoma (13%) and peripheral T-cell lymphomas (37%). All marginal zone lymphomas (n=5), chronic lymphocytic leukemia small lymphocytic lymphomas (n=10), plasmacytoma (n=3), acute lymphoblastic lymphoma/leukemia, and anaplastic large-cell lymphomas (n=5) were negative. In conclusion, hENT1 was most frequently found in benign and malignant follicular center cells. Prospective studies to assess the value of hENT1 immunostaining in predicting resistance to nucleoside chemotherapy for NHL are warranted.

Nucleoside transporters in chronic lymphocytic leukaemia

Nucleoside derivatives have important therapeutic activity in chronic lymphocytic leukaemia (CLL). Experimental evidence indicates that in CLL cells most of these drugs induce apoptosis ex vivo, suggesting that programmed cell death is the mechanism of their therapeutic action, relying upon previous uptake and metabolic activation. Although defective apoptosis and poor metabolism often cause resistance to treatment, differential uptake and/or export of nucleosides and nucleotides may significantly modulate intracellular drug bioavailability and, consequently, responsiveness to therapy. Two gene families, SLC28 and SLC29, encode transporter proteins responsible for concentrative and equilibrative nucleoside uptake (CNT and ENT, respectively). Furthermore, selected members of the expanding ATP-binding cassette (ABC) protein family have recently been identified as putative efflux pumps for the phosphorylated forms of these nucleoside-derived drugs, ABCC11 (MRP8) being a good candidate to modulate cell sensitivity to fluoropyrimidines. Sensitivity of CLL cells to fludarabine has also been recently correlated with ENT-type transport function, suggesting that, besides the integrity of apoptotic pathways and appropriate intracellular metabolism, transport across the plasma membrane is also a relevant event during CLL treatment. As long as nucleoside transporter expression in leukaemia cells is not constitutive, the possibility of regulating nucleoside transporter function by pharmacological means may also contribute to improve therapy.

Acadesine activates AMPK and induces apoptosis in B-cell chronic lymphocytic leukemia cells but not in T lymphocytes

Acadesine, 5-aminoimidazole-4-carboxamide (AICA) riboside, induced apoptosis in B-cell chronic lymphocytic leukemia (B-CLL) cells in all samples tested (n = 70). The half-maximal effective concentration (EC(50)) for B-CLL cells was 380 +/- 60 microM (n = 5). The caspase inhibitor Z-VAD.fmk completely blocked acadesine-induced apoptosis, which involved the activation of caspase-3, -8, and -9 and cytochrome c release. Incubation of B-CLL cells with acadesine induced the phosphorylation of adenosine monophosphate-activated protein kinase (AMPK), indicating that it is activated by acadesine. Nitrobenzylthioinosine (NBTI), a nucleoside transport inhibitor, 5-iodotubercidin, an inhibitor of adenosine kinase, and adenosine completely inhibited acadesine-induced apoptosis and AMPK phosphorylation, demonstrating that incorporation of acadesine into the cell and its subsequent phosphorylation to AICA ribotide (ZMP) are necessary to induce apoptosis. Inhibitors of protein kinase A and mitogen-activated protein kinases did not protect from acadesine-induced apoptosis in B-CLL cells. Moreover, acadesine had no effect on p53 levels or phosphorylation, suggesting a p53-independent mechanism in apoptosis triggering. Normal B lymphocytes were as sensitive as B-CLL cells to acadesine-induced apoptosis. However, T cells from patients with B-CLL were only slightly affected by acadesine at doses up to 4 mM. AMPK phosphorylation did not occur in T cells treated with acadesine. Intracellular levels of ZMP were higher in B-CLL cells than in T cells when both were treated with 0.5 mM acadesine, suggesting that ZMP accumulation is necessary to activate AMPK and induce apoptosis. These results suggest a new pathway involving AMPK in the control of apoptosis in B-CLL cells and raise the possibility of using acadesine in B-CLL treatment.

All- trans-retinoic acid increases cytotoxicity of 1-beta-D-arabinofuranosylcytosine in NB4 cells

PURPOSE

Clinically, the benefits of combining all- trans-retinoic acid (ATRA) with chemotherapy have been well documented in the treatment of acute promyelocytic leukemia (APL). Changes in nucleoside transporter expression and activity have been shown to occur in NB4 cells in vitro following treatment with ATRA. In this study we investigated whether ATRA treatment increases sensitivity to ara-C in NB4 cells. Specifically, we examined the role of ATRA-associated changes in nucleoside transporter expression and activity in eliciting ara-C cytotoxicity.

METHODS

Cellular uptake of [(3)H]-ara-C and nucleoside transporter abundance were determined in untreated cells and cells treated with 1 microM ATRA for 12-72 h using an inhibitor and oil stop procedure, and an equilibrium [(3)H]-NBMPR binding assay, respectively. Cytotoxicity of ara-C and the apoptotic response prior to and following ATRA treatment were determined using the MTT viability assay and the TUNEL assay, respectively.

RESULTS

ATRA treatment increased ara-C cytotoxicity and potency, ara-C transport, and augmented ara-C-induced apoptosis. The combination effect was supraadditive under some conditions and sequence-dependent whereby the maximum effect was seen when the addition of ATRA preceded the addition of ara-C, and when ara-C administration closely followed ATRA administration.

CONCLUSIONS

The ATRA-induced increase in cytotoxicity of ara-C was, in part, the result of an increase in the functional expression of nucleoside transporters, and a role for bcl-2 was also indicated. Our results would suggest that timing of ara-C therapy should be tied to maximal es transporter expression, which is likely to be 24 h after ATRA treatment begins. It remains to be seen whether the response in the clinic can be further enhanced in APL by taking advantage of ara-C transporter regulation by ATRA.

Resistance to fludarabine-induced apoptosis in Epstein-Barr virus infected B cells

The Epstein-Barr virus (EBV) transforms B cells in part by inhibiting the cellular apoptotic programme. This is also observed when Burkitt lymphoma cell lines are infected with EBV. Induction of apoptosis is one of the mechanisms by which fludarabine inhibits the growth of cells with low proliferative capacity. This compound can also inhibit several other mechanisms in the cell, including inhibition of the synthesis of factors such as STAT1. To analyse the relationship between EBV status, fludarabine-induced apoptosis, and transcription factors we studied the EBV-negative Burkitt lymphoma cell line BL2, its EBV-infected counterpart BL2.B95.8 and the EBV-transformed cell line PRI. The BL2 cell line was found to be very sensitive to fludarabine. The BL2.B95.8 and PRI cells were both resistant but the latter to a lesser extent. In the PRI cells fludarabine activated p53, but not in the BL2.B95.8 cells in which the p53 pathway is inactivated. We observed that this inactivation results in part from the lack of expression of the MDM2 inhibitor p14ARF. Conversely, there was a substantial constitutive activation of STAT1, and not of the other STATs, in the BL2.B95.8 cells and a modest one in the PRI cells. Furthermore, expression of STAT1 was significantly reduced by fludarabine treatment in the PRI cells, but not in the BL2.BL95.8 cells. Finally, the expression of p21WAF1/CIP1 was detected only in the BL2.B95.8 and PRI cells. This protein, known to play a role in cell survival, may therefore be involved in the resistance of the BL2.B95.8 cells to fludarabine.

Treatment-induced expression of anti-apoptotic proteins in WSU-CLL, a human chronic lymphocytic leukemia cell line

Bryostatin 1 (bryo 1) has been shown to potentiate the anti-tumor activity of 2-chloro-2-deoxyadenosine (2-CdA) in chronic lymphocytic leukemia (CLL) and in the WSU-CLL cell line. However, like resistant CLL, WSU-CLL cells lose their sensitivity to bryo 1/2-CdA treatment. We report that 2-CdA-induced IAP expression may be a possible mechanism whereby resistance to apoptosis is acquired in these cells. In WSU-CLL cells, three members of the Inhibitors of Apoptosis (IAP) family were identified. Bryo 1 treatment of WSU-CLL cells leads to initiation of the apoptotic cascade and induced a marginal increase in XIAP protein expression. In contrast, 2-CdA treatment, alone or in combination with bryo 1, induced a substantial increase in survivin and XIAP proteins and phosphorylation of BAD. Bryo 1 alone induced caspase-7 and -9 dependent [poly ADP-ribose] polymerase (PARP) cleavage, while sequential treatment with bryo 1 (72 h) followed by 2-CdA (24 h) induced caspase-3,-7, and -9 dependent PARP cleavage and increased apoptosis. Although exposure to bryo 1 initiated apoptotic events, apoptosis was first enhanced by 2-CdA, and then reversed in a time-dependent manner by 2-CdA-induced expression of survival proteins. Taken together, resistance to bryo 1/2-CdA treatment may be the result of 2-CdA-induced IAP inhibition of the intrinsic apoptotic pathway caspases.

Early induction of apoptosis in B-chronic lymphocytic leukaemia cells by hydroxychloroquine: activation of caspase-3 and no protection by survival factors

We have investigated the effect of hydroxychloroquine (HCQ), an anti-rheumatic drug, on malignant B cells from 20 patients with B-chronic lymphocytic leukaemia (B-CLL). HCQ induced a decrease in cell viability in a dose- and time-dependent manner. The mean IC50 was 32 +/- 7 microg/ml (range, 10-75 microg/ml) for 24 h of exposure. This cytotoxic effect was owing to apoptosis, as demonstrated by morphological changes, annexin V binding capacity and DNA fragmentation (28 +/- 4% of apoptotic cells as early as 5 h post incubation, increasing to 82 +/- 4% at 18 h post treatment). The apoptosis was associated with caspase-3 activation because the cleavage and activity of caspase-3 were increased by HCQ. The amount of bcl-2 protein was reduced during apoptosis, evidenced using quantitative flow cytometry. As early as 1 h post-HCQ treatment, a reduction of the mitochondrial transmembrane potential was measured by 3,3'-dihexyloxacarbocyanine iodide. Interestingly, the HCQ effect was not affected by exposure to interleukin-4 or co-culture with bone marrow stromal cells. Our observations suggest that HCQ may offer a new therapeutic tool in the treatment of B-CLL patients.

Induction of apoptosis by 2-chloro-2'deoxyadenosine (2-CdA) alone and in combination with other cytotoxic drugs: synergistic effects on normal and neoplastic lymphocytes by addition of doxorubicin and mitoxantrone

2-CdA is active as a single agent in the treatment of low-grade lymphomas. We analyzed the induction of apoptosis by 2-CdA alone (n=5) and in combination with other drugs in peripheral lymphocytes from 25 patients with leukemic low-grade lymphomas and from 25 healthy volunteers. 2-CdA was tested in 4 escalating concentrations (0.05 microg/ml to 0.4 microg/ml). Linear regressions showed a dose dependent apoptosis rate of 0.29 x microg 2-CdA/ml + 0.11 (r2=0.88, p=0.006) in normal cells and 0.41 x microg 2-CdA/ml + 0.15 (r2=0.88, p=0.005) in leukemic cells. Intracellular metabolization of 2-CdA into 2-CdA-5'mono-, -di- and the active metabolite -triphosphate was analyzed by HPLC and paralleled the dose dependent increase of apoptosis. The combination of 2-CdA with doxorubicin or mitoxantrone had a synergistic effect on the induction of apoptosis (p<0.001) in both normal and neoplastic lymphocytes, whereas 2-CdA plus etoposide or cytosine arabinoside were only additive. Due to the flat slope of the dose response of 2-CdA concentration on apoptosis we assume that higher in vivo dosages of 2-CdA in the treatment of low-grade lymphomas may not result in a higher clinical efficacy. The synergistic lymphocytotoxic effect of 2-CdA combined with doxorubicin or mitoxantrone may be relevant for new treatment approaches.

Apoptosis in B-chronic lymphocytic leukaemia

B-chronic lymphocytic leukaemia (B-CLL) is characterised by the progressive accumulation of monoclonal B cells, which may be the result of several factors leading to extended B-CLL cell lifespan, increased proliferative capacity and diminished cell death. Here we review the implications of several signals mediated by receptors, such as surface IgM, CD6 and CD40, for the B-CLL cell survival, together with data on gene modulation in relation to the apoptosis process in B-CLL cells. We also describe some features of the Fas/FasL system in B-CLL that hypothetically might contribute to the accumulation of leukaemic cells and the progression of the disease, by downregulating the apoptotic response or avoiding the autologous immune response.

Regulation of B-CLL apoptosis through membrane receptors and Bcl-2 family proteins

The accumulation of monoclonal chronic lymphocytic leukemia B (B-CLL) cells may be due to excessive proliferation and longevity. Clinical progression may thus come from a constitutive but altered expression of a number of genes that results in extended B-CLL cells life span, increased proliferative capacity and diminished cell death. B-CLL cells express a number of surface markers that characterise the normal B-cells phenotype. However, B-CLL cells are CD5 positive and most of them also express CD6, surface receptors that are present in just a small subset of normal B-cells. When exploring CD6 function, we found out that cross-linking of CD6 protected B-CLL from anti-IgM-induced apoptosis. CD6 activation blocked anti-IgM- induced Bax(alpha) up-regulation and, by doing so, corrected an imbalance in the Bcl-2/Bax ratio that accompanies apoptosis. Here, we review all surface receptors and cytokines that have been described as participating in the induction or protection of B-CLL apoptosis together with data on chemosensitivity and gene modulation, data on the Fas receptor/Fas ligand system, and the implications of all the latter for B-CLL cell survival.

Controversies and new approaches to treatment of Langerhans cell histiocytosis

There continues to be genuine ambivalence as to whether Langerhans cell histiocytosis (LCH) is a primary neoplastic or immuno-dysregulatory disorder. Treatment strategies have moved from one camp to the other depending upon the most current alleged successes or failures. This has been particularly true for patients who fall outside of the sphere where treatment is minimal or where known treatment approaches are clearly beneficial. However, there is growing evidence that LCH is both the result of clonal proliferation of Langerhans cells and the immunologic consequence of increased cellular activation. This new knowledge should be the basis for the development of new therapeutic approaches for patients with LCH and its complications.

The Novel Cyclin-Dependent Kinase Inhibitor Flavopiridol Downregulates Bcl-2 and Induces Growth Arrest and Apoptosis in Chronic B-Cell Leukemia Lines

Flavopiridol is a novel, potent inhibitor of cyclin-dependent kinases (CDK). This synthetic flavone has been reported to exhibit antitumor activity in murine and human tumor cell lines in vitro and in vivo and is currently undergoing clinical phase I evaluation. In the present study, 1 Epstein-Barr virus (EBV)-transformed B-prolymphocytic cell line (JVM-2), 1 EBV-transformed B-CLL cell line (I83CLL), and 1 non-EBV transformed B-CLL cell line (WSU-CLL) were used as targets. Treatment of the cells with flavopiridol (100 nmol/L to 400 nmol/L) led to a marked dose- and time-dependent inhibition of cell growth and survival as determined using trypan blue exclusion. Morphologic analysis showed characteristic apoptotic changes such as chromatin condensation and fragmentation, membrane blebbing, and formation of apoptotic bodies. Furthermore, quantitative assessment of apoptosis-associated DNA strand breaks by in situ TdT labeling showed that a significant number of flavopiridol-treated cells underwent apoptosis. These cellular effects were associated with a significant decrease in bcl-2 expression as observed by Northern and Western blotting. The results showed that flavopiridol downregulates bcl-2 mRNA and bcl-2 protein expression within 24 hours. Genistein and quercetin, two flavonoids that do not inhibit CDKs, did not affect bcl-2 expression. These data suggest an additional mechanism of action of this new flavone which might be useful as an agent in the treatment of chronic lymphoid malignancies.

The Novel Cyclin-Dependent Kinase Inhibitor Flavopiridol Downregulates Bcl-2 and Induces Growth Arrest and Apoptosis in Chronic B-Cell Leukemia Lines

Flavopiridol is a novel, potent inhibitor of cyclin-dependent kinases (CDK). This synthetic flavone has been reported to exhibit antitumor activity in murine and human tumor cell lines in vitro and in vivo and is currently undergoing clinical phase I evaluation. In the present study, 1 Epstein-Barr virus (EBV)-transformed B-prolymphocytic cell line (JVM-2), 1 EBV-transformed B-CLL cell line (I83CLL), and 1 non-EBV transformed B-CLL cell line (WSU-CLL) were used as targets. Treatment of the cells with flavopiridol (100 nmol/L to 400 nmol/L) led to a marked dose- and time-dependent inhibition of cell growth and survival as determined using trypan blue exclusion. Morphologic analysis showed characteristic apoptotic changes such as chromatin condensation and fragmentation, membrane blebbing, and formation of apoptotic bodies. Furthermore, quantitative assessment of apoptosis-associated DNA strand breaks by in situ TdT labeling showed that a significant number of flavopiridol-treated cells underwent apoptosis. These cellular effects were associated with a significant decrease in bcl-2 expression as observed by Northern and Western blotting. The results showed that flavopiridol downregulates bcl-2 mRNA and bcl-2 protein expression within 24 hours. Genistein and quercetin, two flavonoids that do not inhibit CDKs, did not affect bcl-2 expression. These data suggest an additional mechanism of action of this new flavone which might be useful as an agent in the treatment of chronic lymphoid malignancies.