Two pathways of apoptosis induced with all-trans retinoic acid and etoposide in the myeloid cell line P39.

P39/Tsugane is a myelomonocytoid cell line derived from a patient with myelodysplastic syndrome (MDS). The cells readily undergo apoptosis in response to various agents, and the cell line has been suggested as a useful model to study apoptosis in MDS. The aims of the present study were to assess differentiation and apoptosis induced with all-trans retinoic acid (ATRA) and etoposide, to characterize the mode of apoptosis in these two model systems, and to assess the influence of granulocyte colony-stimulating factor (G-CSF), which in combination with erythropoietin has been shown to inhibit apoptosis in MDS. ATRA induced differentiation and apoptosis in a concentration- and time-dependent manner. Differentiated cells were partially rescued (by 50%) from apoptosis with G-CSF. Etoposide induced apoptosis in a concentration- and time-dependent manner, but no signs of preceding maturation or G-CSF rescue were detected. ATRA- and etoposide-induced apoptosis were both mediated through the caspase pathway and were partially blocked with the general caspase inhibitor zVAD-fmk. Simultaneous treatment with G-CSF and zVAD-fmk additively blocked ATRA-induced apoptosis. However, the two pathways differed in terms of substrate cleavage during apoptosis. ATRA-induced apoptosis caused actin cleavage, which was not affected by G-CSF, and Bcl-2 downregulation. Etoposide induced a caspase-dependent cleavage of Bcl-2, while actin remained intact. The Fas system did not seem to play a major role in any of these apoptotic pathways. Our results may provide new tools to study the mechanisms of apoptosis in MDS.     

Ellagic acid,a natural polyphenolic compound,induces apoptosis and potentiates retinoic acid-induced differentiation of human leukemia HL-60 cells

All-trans retinoic acid (ATRA) is a standard drug used for differentiation therapy in acute promyelocytic leukemia. To potentiate this therapy, we examined the effect of ellagic acid (EA), a natural polyphenolic compound with antiproliferative and antioxidant properties, on the growth and differentiation of HL-60 acute myeloid leukemia cells. EA was found to induce apoptosis, which was blocked by pan-caspase inhibitor, Z-VAD-FMK. EA activated the caspase-3 pathway and enhanced the expressions of myeloid differentiation markers (CD11b, MRP-14 protein, granulocytic morphology) induced by ATRA treatment. These results indicate that EA is a potent apoptosis inducer and also effectively potentiates ATRA-induced myeloid differentiation of HL-60 cells.     

Differentiation-independent retinoid induction of folate receptor type beta, a potential tumor target in myeloid leukemia

Folate receptor (FR) type beta is expressed in the myelomonocytic lineage, predominantly during neutrophil maturation and in myeloid leukemias. FR-beta expression was elevated up to 20-fold by all-trans retinoic acid (ATRA) in KG-1 myeloid leukemia cells in a dose-dependent and reversible manner in the absence of terminal differentiation or cell growth inhibition. ATRA also increased FR-beta expression in vitro in myeloid leukemia cells from patient marrow. FR-beta was not up-regulated in KG-1 cells treated with phorbol ester, dexamethasone, 1,25-dihydroxy vitamin D(3), or transforming growth factor beta. ATRA did not induce FR-beta expression in receptor negative cells of diverse origin. The ATRA-induced increase in FR-beta expression in KG-1 cells occurred at the level of messenger RNA synthesis, and in 293 cells containing a stably integrated FR-beta promoter-luciferase reporter construct, ATRA induced expression of the reporter. From experiments using retinoid agonists and antagonists and from cotransfection studies using the FR-beta promoter and expression plasmids for the nuclear receptors retinoic acid receptor (RAR)alpha, RARbeta, or RARgamma, it appears that the retinoid effect on FR-beta expression could be mediated by ligand binding to RARs alpha, beta, or gamma, but not to retinoid X receptors. Furthermore, there was apparent cross-talk between RARalpha and RARgamma selective agonists or antagonists, suggesting a common downstream target for RAR isoforms in inducing FR-beta expression. Thus, blocks in the RARalpha-specific pathway of retinoid-induced differentiation may be bypassed during retinoid induction of FR-beta expression. The results suggest that to facilitate FR-targeted therapies, retinoids may be used to modulate FR-beta expression in myeloid leukemia cells refractory to retinoid differentiation therapy.     

The vitamin D3 analog EB1089 induces apoptosis via a p53-independent mechanism involving p38 MAP kinase activation and suppression of ERK activity in B-cell chronic lymphocytic leukemia cells in vitro

EB1089, a novel vitamin D3 analog, has been shown to have cytotoxic and antiproliferative properties in a variety of malignant cells. However, its potential as a treatment for B-cell chronic lymphocytic leukemia (B-CLL) has not been evaluated. EB1089 induced apoptosis in all of the 102 B-CLL samples tested with a mean LD(50) (the concentration of EB1089 required to kill 50% of cells) value (+/- SD) of 2.1 x 10(-8) M (+/- 1.4 x 10(-8) M). Furthermore, no significant difference was found in the cytotoxicity of EB1089 in B-CLL samples from previously treated and untreated patients (P =.1637). Induction of apoptosis was associated with a reduction in Bcl-2 and Mcl-1 protein expression, but this was evident only in the apoptotic cells. In contrast, the expression of Bax, p21, and p53 was not altered in the viable or apoptotic cells from either B- or T-lymphocyte lineages. EB1089-induced apoptosis was preceded by activation of p38 mitogen-activated protein (MAP) kinase and suppression of extracellular signal-regulated kinase (ERK) activity, and this was associated with downstream activation of caspase-3. The pancaspase inhibitor (Z-VAD-FMK) and the caspase-9 inhibitor (Z-LEHD-FMK) were able to partially abrogate the apoptotic effects of EB1089 but did not affect the phosphorylation of p38 MAP kinase or the suppression of ERK. The B-CLL cells in the study were shown to highly express vitamin D receptor, but an additional receptor-independent mechanism of cell killing cannot be ruled out at this stage. These findings show that EB1089 is a potent apoptosis-inducing agent in B-CLL cells and may be useful in the treatment of B-CLL patients, particularly those with p53 mutations or drug-resistant disease.     

全反式维甲酸诱导胰腺癌细胞凋亡机制研究

目的 维甲酸类药物调节肿瘤细胞生长、分化和凋亡是其防治肿瘤的基础。本研究观察全反式维甲酸(ATRA)诱导体外胰腺癌细胞凋亡的可能机制。方法 胰腺癌细胞Patu8988与ATRA共同孵育。通过DNA断裂分析、TUNEL标记证实凋亡存在，并用流式细胞仪检测凋亡细胞比例。用RT-PCR和Western blot方法检测凋亡诱导过程中p53、bcl-2和bax基因的水平及其表达变化。结果 ATRA处理组细胞凋亡比例明显增加。TUNEL标记和DNA断裂分析发现典型凋亡特征。ATRA处理组bax和phospho-p53(Ser 46)表达上调，但bcl-2表达下调。结论 ATRA能诱导胰腺癌细胞凋亡，其分子机制可能与bcl-2/bax和p53的表达相关。     

p53 pathway in apoptosis induced by all-trans-retinoic acid in acute myeloblastic leukaemia cells

The role of the p53 pathway in apoptosis induced by all-trans-retinoic acid (ATRA) was studied in 5 human acute myeloid leukaemia (AML) cell lines, OU-AML-3, -4, -5, -7 and -8, previously established and characterized by the authors. Although all the cell lines have a wild-type (wt) p53 gene, the protein is in a mutant conformation detectable by the anti-p53 antibody PAb 240. Exposure of the cell lines to 1.0 microM ATRA for 72 h caused induction of apoptosis detectable by morphology and the annexin V assay. The number of apoptotic cells according to the annexin V assay varied from 16 +/- 8% (OU-AML-7) to 61 +/- 4% (OU-AML-3) in ATRA-treated cells, while it was 7 +/- 6% in control cells. Western blotting and flow cytometry showed down-regulation of the p53 protein by ATRA. The conformation of p53 remained unchanged, being detectable in flow cytometry by PAb 240, but not by PAb 1620 (an antibody which only detects p53 in wt conformation). At the same time bcl-2 was down-regulated as shown by Western blotting and flow cytometry, while no induction of bax was observed by ATRA. On the basis of these results, ATRA-induced apoptosis in these AML cell lines is independent of the p53 pathway, although it is associated with the down-regulation of bcl-2.     

Chlorambucil resistance in B-cell chronic lymphocytic leukaemia is mediated through failed Bax induction and selection of high Bcl-2-expressing subclones

Our previous data have shown that high Bct-2/ Bax ratios in chronic lymphocytic leukaemia (B-CLL) correlate with in vitro apoptosis and clinical resistance. We have now monitored the in vitro viability of B-CLL cells in relation to Bcl-2 and Bax expression over a 48 h time course following exposure to chlorambucil. The results showed that Bax up-regulation was essential for chlorambucil-induced apoptosis in B-CLL cells and a 3-fold increase in expression within 4 h of exposure to drug was typically observed in sensitive cells; resistant cells failed to up-regulate Bax at all. In contrast, the constitutively high levels of Bcl-2 found in B-CLL cells were found to be down-regulated in apoptotic cells but the mean Bcl-2 expression in viable cells was increased, probably as a result of the loss of lower Bcl-2-expressing cells into the apoptotic compartment. Taken together, these data add further weight to the suggestion that Bcl-2/Bax ratios may be pivotal in determining the fate of B-CLL cells. Furthermore, the Bcl-2/Bax ratios found in apoptotic B lymphocytes were remarkably similar in the treated, untreated and normal control cells, which suggests that there is a universal Bcl-2/Bax ratio threshold for cell survival and cell death.     

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 leukemia (B-CLL) lymphocytes to apoptotic death activation by the proteasome-specific inhibitor lactacystin. Here, we show that only specific-not nonspecific-proteasomal inhibitors can discriminate between malignant and normal lymphocytes in inducing the apoptotic death response. Indeed, lactacystin and its active metabolite clasto-lactacystin beta-lactone induced apoptotic death in CLL but not in normal lymphocytes. This difference was completely abolished when tripeptide aldehydes such as MG132 or LLnL (which can also inhibit calpains) were used as less specific proteasomal inhibitors. Moreover, B-CLL cells exhibited a constitutive altered ubiquitin-proteasome system, including a threefold higher chymotrypsin-like proteasomal activity and high levels of nuclear ubiquitin-conjugated proteins compared with normal lymphocytes. Interestingly, B-CLL cells also displayed altered proteolytic regulation of wild-type p53, an apoptotic factor reported to be a substrate for the ubiquitin-proteasome system. Nuclear wild-type p53 accumulated after lactacystin 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 in normal lymphocytes p53 is regulated mainly by calpains and not by the ubiquitin-proteasome system. This work raises the possibility that two different proteolytic pathways controlling p53 stability may be pathologically imbalanced. This could result in modification of apoptosis control, since in CLL-lymphocytes a highly upregulated ubiquitin-proteasome system, which controls p53 stability among other apoptotic factors, was correlated with an increased propensity of these cells to apoptosis triggered by lactacystin.     

Functional integrity of the p53-mediated apoptotic pathway induced by the nongenotoxic agent nutlin-3 in B-cell chronic lymphocytic leukemia (B-CLL)

Deletions and/or mutations of p53 are relatively rare and late events in the natural history of B-cell chronic lymphocytic leukemia (B-CLL). However, it is unknown whether p53 signaling is functional in B-CLL and if targeted nongenotoxic activation of the p53 pathway by using nutlin-3, a small molecule inhibitor of the p53/MDM2 interaction, is sufficient to kill B-CLL cells. In vitro treatment with nutlin-3 induced a significant cytotoxicity on primary CD19(+) B-CLL cells, but not on normal CD19(+) B lymphocytes, peripheral-blood mononuclear cells, or bone marrow hematopoietic progenitors. Among 29 B-CLL samples examined, only one was resistant to nutlin-3-mediated cytotoxicity. The induction of p53 by nutlin-3 in B-CLL samples was accompanied by alterations of the mitochondrial potential and activation of the caspase-dependent apoptotic pathway. Among several genes related to the p53 pathway, nutlin-3 up-regulated the steady-state mRNA levels of PCNA, CDKN1A/p21, GDF15, TNFRSF10B/TRAIL-R2, TP53I3/PIG3, and GADD45. This profile of gene activation showed a partial overlapping with that induced by the genotoxic drug fludarabine. Moreover, nutlin-3 synergized with both fludarabine and chlorambucil in inducing B-CLL apoptosis. Our data strongly suggest that nutlin-3 should be further investigated for clinical applications in the treatment of B-CLL.     

AM580, a stable benzoic derivative of retinoic acid, has powerful and selective cyto-differentiating effects on acute promyelocytic leukemia cells

All-trans retinoic acid (ATRA) is successfully used in the cyto- differentiating treatment of acute promyelocytic leukemia (APL). Paradoxically, APL cells express PML-RAR, an aberrant form of the retinoic acid receptor type alpha (RAR alpha) derived from the leukemia- specific t(15;17) chromosomal translocation. We show here that AM580, a stable retinobenzoic derivative originally synthesized as a RAR alpha agonist, is a powerful inducer of granulocytic maturation in NB4, an APL-derived cell line, and in freshly isolated APL blasts. After treatment of APL cells with AM580 either alone or in combination with granulocyte colony-stimulating factor (G-CSF), the compound induces granulocytic maturation, as assessed by determination of the levels of leukocyte alkaline phosphatase, CD11b, CD33, and G-CSF receptor mRNA, at concentrations that are 10- to 100-fold lower than those of ATRA necessary to produce similar effects. By contrast, AM580 is not effective as ATRA in modulating the expression of these differentiation markers in the HL-60 cell line and in freshly isolated granulocytes obtained from the peripheral blood of chronic myelogenous leukemia patients during the stable phase of the disease. In NB4 cells, two other synthetic nonselective RAR ligands are capable of inducing LAP as much as AM580, whereas RAR beta- or RAR gamma-specific ligands are totally ineffective. These results show that AM580 is more powerful than ATRA in modulating the expression of differentiation antigens only in cells in which PML-RAR is present. Binding experiments, using COS-7 cells transiently transfected with PML-RAR and the normal RAR alpha, show that AM580 has a lower affinity than ATRA for both receptors. However, in the presence of PML-RAR, the synthetic retinoid is a much better transactivator of retinoic acid-responsive element-containing promoters than the natural retinoid, whereas, in the presence of RAR alpha, AM580 and ATRA have similar activity. This may explain the strong cyto-differentiating potential of AM580 in PML-RAR-containing leukemic cells.     

Novel triterpenoid CDDO-Me is a potent inducer of apoptosis and differentiation in acute myelogenous leukemia.

It has been shown that the novel synthetic triterpenoid CDDO inhibits proliferation and induces differentiation and apoptosis in myeloid leukemia cells. In the current study the effects of the C-28 methyl ester of CDDO, CDDO-Me, were analyzed on cell growth and apoptosis of leukemic cell lines and primary acute myelogenous leukemia (AML). CDDO-Me decreased the viability of leukemic cell lines, including multidrug resistant (MDR)-1-overexpressing, p53(null) HL-60-Dox and of primary AML cells, and it was 3- to 5-fold more active than CDDO. CDDO-Me induced a loss of mitochondrial membrane potential, induction of caspase-3 cleavage, increase in annexin V binding and DNA fragmentation, suggesting the induction of apoptosis. CDDO-Me induced pro-apoptotic Bax protein that preceded caspase activation. Furthermore, CDDO-Me inhibited the activation of ERK1/2, as determined by the inhibition of mitochondrial ERK1/2 phosphorylation, and it blocked Bcl-2 phosphorylation, rendering Bcl-2 less anti-apoptotic. CDDO-Me induced granulo-monocytic differentiation in HL-60 cells and monocytic differentiation in primary cells. Of significance, colony formation of AML progenitors was significantly inhibited in a dose-dependent fashion, whereas normal CD34(+) progenitor cells were less affected. Combinations with ATRA or the RXR-specific ligand LG100268 enhanced the effects of CDDO-Me on cell viability and terminal differentiation of myeloid leukemic cell lines. In conclusion, CDDO-Me is an MDR-1- and a p53-independent compound that exerts strong antiproliferative, apoptotic, and differentiating effects in myeloid leukemic cell lines and in primary AML samples when given in submicromolar concentrations. Differential effects of CDDO-Me on leukemic and normal progenitor cells suggest that CDDO-Me has potential as a novel compound in the treatment of hematologic malignancies.