Arsenic trioxide, a therapeutic agent for APL

The molecular pathogenesis of acute promyelocytic leukaemia: implications for the clinical management of the disease

Acute promyelocytic leukaemia (APL) is characterised by chromosomal rearrangements of 17q21, leading to fusion of the gene encoding retinoic acid receptor alpha (RARalpha) to a number of alternative partner genes (X), the most frequent of which are PML (>95%), PLZF (0.8%) and NPM (0.5%). Over the last few years, it has been established that the X-RARalpha fusion proteins play a key role in the pathogenesis of APL through recruitment of co-repressors and the histone deacetylase (HDAC)-complex to repress genes implicated in myeloid differentiation. Paradoxically, the X-RARalpha fusion protein has the potential to mediate myeloid differentiation at pharmacological doses of its ligand (all trans-retinoic acid (ATRA)), which is dependent on the dissociation of the HDAC/co-repressor complex. Arsenic compounds have also been shown to be promising therapeutic agents, leading to differentiation and apoptosis of APL blasts. It is now apparent that the nature of the RARalpha-fusion partner is a critical determinant of response to ATRA and arsenic, underlining the importance of cytogenetic and molecular characterisation of patients with suspected APL to determine the most appropriate treatment approach. Standard protocols involving ATRA combined with anthracycline-based chemotherapy, lead to cure of approximately 70% patients with PML-RARalpha-associated APL. Patients at high risk of relapse can be identified by minimal residual disease monitoring. The challenge for future studies is to improve complete remission rates through reduction of induction deaths, particularly due to haemorrhage, identification of patients at high risk of relapse who would benefit from additional therapy, and identification of a favourable-risk group, for which treatment intensity could be reduced, thereby reducing risks of treatment toxicity and development of secondary leukaemia/myelodysplasia. With the advent of ATRA and arsenic, APL has already provided the first example of successful molecularly targeted therapy; it is hoped that with further understanding of the pathogenesis of the disease, the next decade will yield further improvements in the outlook for these patients.

Caspase activation is accelerated by the inhibition of arsenite-induced, membrane rafts-dependent Akt activation

Renewed interest in arsenic has been shown recently due to its dual nature of being a potent toxin and a drug for treatment of acute promyelocytic leukemia (APL) because of its ability to trigger caspase activation. Here, we found that sodium arsenite (NaAsO(2)) also triggers the signal for activation of Akt and downstream glycogen synthase 3beta (GSK3beta). Such Akt/GSK3beta activation was abrogated completely by wortmannin, an inhibitor of PI-3 kinase, and greatly by pertussis toxin, a G-protein inhibitor. Arsenite-induced Akt phosphorylation also was inhibited by sequestrating membrane cholesterol with beta cyclodextrin. Reducing reagents/reactive oxygen species (ROS) scavengers reduced arsenite-induced Akt phosphorylation and beta cyclodextrin reduced arsenite-mediated ROS production, suggesting that arsenite-induced G-protein/Akt/GSK3beta pathway is membrane raft dependent and redox linked. We also found that a combination of a low concentration (1 microM) of arsenite and wortmannin triggers the signal for caspase activation, whereas neither of these elements alone did so. These results suggested that selective blockade of the arsenite-provoked PI-3 kinase/Akt pathway can promote the arsenite-triggered pathway for caspase activation, and this may open a new study area for wider applications of arsenic as a drug for treating various kinds of leukemia.

Arsenic trioxide promotes histone H3 phosphoacetylation at the chromatin of CASPASE-10 in acute promyelocytic leukemia cells

Arsenic trioxide (As(2)O(3)) is highly effective for the treatment of acute promyelocytic leukemia, even in patients who are unresponsive to all-trans-retinoic acid therapy. As(2)O(3) is believed to function primarily by promoting apoptosis, but the underlying molecular mechanisms remain largely unknown. In this report, using cDNA arrays, we have examined the changes in gene expression profiles triggered by clinically achievable doses of As(2)O(3) in acute promyelocytic leukemia NB4 cells. CASPASE-10 expression was found to be potently induced by As(2)O(3). Accordingly, caspase-10 activity also substantially increased in response to As(2)O(3) treatment. A selective inhibitor of caspase-10, Z-AEVD-FMK, effectively blocked caspase-3 activation and significantly attenuated As(2)O(3)-triggered apoptosis. Interestingly, the treatment of NB4 cells with As(2)O(3) markedly increased histone H3 phosphorylation at serine 10, an event that is associated with acetylation of the lysine 14 residue. Chromatin immunoprecipitation assays revealed that As(2)O(3) potently enhances histone H3 phosphoacetylation at the CASPASE-10 locus. These results suggest that the effect of As(2)O(3) on histone H3 phosphoacetylation at the CASPASE-10 gene may play an important role in the induction of apoptosis and thus contribute to its therapeutic effects on acute promyelocytic leukemia.

Pink-eyed dilution protein modulates arsenic sensitivity and intracellular glutathione metabolism

Mutations in the mouse p (pink-eyed dilution) and human P genes lead to melanosomal defects and ocular developmental abnormalities. Despite the critical role played by the p gene product in controlling tyrosinase processing and melanosome biogenesis, its precise biological function is still not defined. We have expressed p heterologously in the yeast Saccharomyces cerevisiae to study its function in greater detail. Immunofluorescence studies revealed that p reaches the yeast vacuolar membrane via the prevacuolar compartment. Yeast cells expressing p exhibited increased sensitivity to a number of toxic compounds, including arsenicals. Similarly, cultured murine melanocytes expressing a functional p gene were also found to be more sensitive to arsenical compounds compared with p-null cell lines. Intracellular glutathione, known to play a role in detoxification of arsenicals, was diminished by 50% in p-expressing yeast. By using the glutathione-conjugating dye monochlorobimane, in combination with acivicin, an inhibitor of vacuolar gamma-glutamyl cysteine transpeptidase, involved in the breakdown of glutathione, we found that p facilitates the vacuolar accumulation of glutathione. Our data demonstrate that the pink-eyed dilution protein increases cellular sensitivity to arsenicals and other metalloids and can modulate intracellular glutathione metabolism.

The cytotoxicity of arsenic trioxide to normal hematopoietic progenitors and leukemic cells is dependent on their cell-cycle status

Arsenic trioxide (ATO) is a novel agent to treat acute promyelocytic leukemia (APL). ATO can degrade chimeric PML-RAR proteins and induce apoptosis in various cancer cells. However, its effects on primary hematopoietic CD34+ have not been examined. In this study, we compared the effects of ATO on HL60 leukemic cells and primary umbilical cord blood (UCB) CD34+ cells. HL60 cells and UCB CD34+ cells were cultured with different concentrations of ATO for up to three weeks and examined for changes of cell cycle. We found that ATO (< or = 5 microM) caused prolongation of G1/S and G2/M phase in a dose-dependent manner. The percentage of cells in G2/M increased significantly (from 8.6 to 53.8%). High-dose ATO (> or = 25 microM) caused non-specific cell death in HL60 cells without any changes in cell cycle. In contrast to HL60 cells, UCB CD34+ cells were more resistant to high-dose ATO and most ATO-resistant CD34+ cells remained in G0/G1 phase. Primary cells that were resistant to ATO were rich in CD34+ cells. We further show that the ATO resistance was not related to the expression of P-glycoprotein (MDR-1). Our results suggest that the resistance to ATO in primitive UCB CD34+ cells is most likely related to its cell-cycle status. These results could be useful to design treatments for non-APL malignancies and to enrich hematopoietic stem cells in clinically applicable settings.

Variations on a theme: the alternate translocations in APL

The t(15;17)(q22;q21) translocation is tightly linked to the APL phenotype, and the resultant PML-RAR fusion can be demonstrated in 98% of APL cases. Rare variant translocations have been reported, the majority of which on detailed analysis represent cryptic PML-RAR fusions. However, a handful of APL cases have been described with different genotypes. These include the t(11;17)(q23;q21) that produces the PLZF-RAR fusion, t(5;17)(q35;q21) that forms NPM-RAR, t(11;17)(q13;q21) that generates NUMA-RAR, and der(17) that creates STAT5b-RAR. In this review we will discuss these variant translocations, and discuss the insights that we have gained from their study.

Epigenetics wins over genetics: induction of differentiation in tumor cells

Malignant cells are genetically abnormal, but can the malignant phenotype revert to a non-malignant phenotype without correcting these genetic abnormalities? It has been found that this reversion can be achieved by reprogramming tumor cells by epigenetic changes induced by differentiation. The epigenetic suppression of malignancy by inducing differentiation bypasses the genetic abnormalities in tumor cells. Studies with myeloid leukemic cells have shown that some leukemic cells can be induced to differentiate by cytokines that control normal hematopoiesis, and that myeloid leukemic cells resistant to normal cytokines can be induced to differentiate by compounds that use alternative differentiation pathways. The epigenetic reprogramming of tumor cells by inducing differentiation has also been found with other types of tumors and can be used for tumor therapy. By this reversion of the malignant to non-malignant phenotype, epigenetics wins over genetics.