Acute promyelocytic leukemia: A model of molecular target based therapy

NLS-RARα is a novel transcriptional factor

Acute promyelocytic leukemia (APL) is characterized by the presence of the promyelocytic leukemia (PML)-retinoic acid receptor-α (RAR-α) fusion protein. PML-RARα can be cleaved by neutrophil elastase (NE) in several positions in cells in the promyelocytic stage, nuclear location signal (NLS)-negative PML and NLS-RARα may be the products of PML-RARα by NE. The function of NLS-RARα may be affected by the addition of NLS, which would alter its localization in cells, as the role of NLS is to identify proteins for transport to the nucleus. Preliminary experiments demonstrated that the overexpression of NLS-RARα in HL-60 cells could promote cellular proliferation and inhibit cellular differentiation. Following treatment with all-trans retinoic acid (ATRA), the degree of cellular differentiation was enhanced. In the present study, the localization of NLS-RARα was identified and its activity as a novel transcriptional factor was assessed, which may be critical in the development of APL. The location of NLS-RARα was detected in the nucleus and cytoplasm by indirect immunofluorescence and western blot analysis, with expression in the nucleus revealed to be increased compared with that in the cytoplasm. Next, native-PAGE was performed and NLS-RARα and RXRα were revealed to form heterodimers in the nucleus. In addition, co-immunoprecipitation revealed an interaction between NLS-RARα and retinoid X receptor-α (RXRα). An electrophoresis mobility shift assay (EMSA) indicated that NLS-RARα could bind retinoic acid response elements (RAREs) in the presence of ATRA. Indeed, NLS-RARα could bind RAREs just as WTRARα could, including the RAREs direct repeat-2 (DR-2) and DR-5. In addition, results from a luciferase reporter gene assay demonstrated that NLS-RARα could mediate the activity of RAREs that it bound. Together, these results indicated that NLS-RARα may be a novel transcription factor that contributes to leukemogenesis by competitively binding RAREs as heterodimers with RXRα, just as PML-RARα does, thus repressing the gene transcription essential for myeloid differentiation. These findings indicate the potential role of NLS-RARα targeted therapy in APL.

Enhanced proliferation inhibition of HL60 cells treated by synergistic all-trans retinoic acid/blue light/nanodiamonds

This work explores a strategy using drug all-transretinoic (ATRA) combined with nanodiamond (ND) and blue light (BL) irradiation on the typical HL60 cell line, to establish a approach for improving the treatment efficacy of human leukemia cells.

TRIM32 promotes retinoic acid receptor α-mediated differentiation in human promyelogenous leukemic cell line HL60

Ubiquitination, one of the posttranslational modifications, appears to be involved in the transcriptional activity of nuclear receptors including retinoic acid receptor α (RARα). We previously reported that an E3 ubiquitin ligase, TRIM32, interacts with several important proteins including RARα and enhances transcriptional activity of RARα in mouse neuroblastoma cells and embryonal carcinoma cells. Retinoic acid (RA), which acts as a ligand to nuclear receptors including RARα, plays crucial roles in development, differentiation, cell cycles and apoptosis. In this study, we found that TRIM32 enhances RARα-mediated transcriptional activity even in the absence of RA and stabilizes RARα in the human promyelogenous leukemic cell line HL60. Moreover, we found that overexpression of TRIM32 in HL60 cells suppresses cellular proliferation and induces granulocytic differentiation even in the absence of RA. These findings suggest that TRIM32 functions as one of the coactivators for RARα-mediated transcription in acute promyelogenous leukemia (APL) cells, and thus TRIM32 may become a potentially therapeutic target for APL.

Arsenic trioxide

BACKGROUND

The ancient drug, arsenic, has remarkable efficacy in the treatment of relapsed acute promyelocytic leukemia (APL) and this success has led to exploration of its use in other malignancies.

OBJECTIVE

To provide an overview of the mechanism of action of arsenic and summarize its development in the treatment of APL and other malignant disorders.

METHODS

A 20-year search of MEDLINE, EMBASE and Web of Science was conducted.

RESULTS/CONCLUSIONS

A series of clinical trials with arsenic trioxide has confirmed its benefit in the therapy of APL. Its role in the treatment of other malignancies remains to be determined. Careful attention to the clinical management of patients on arsenic trioxide therapy can significantly lessen the risk of major side effects.

Molecular and chromosomal alterations: new therapies for relapsed acute myeloid leukemia

Acute myeloid leukemia (AML) remains the most common form of leukemia and the most common cause of leukemia death. Although conventional chemotherapy can cure between 25 and 45% of AML patients, the majority of patients die after relapse or of complications associated with treatment. Thus, more specific and less toxic treatments for AML patients are needed, especially for elderly patients. An indispensable prerequisite to investigate tailored approaches for AML is the recent progress in the understanding the molecular features that distinguish leukemia progenitors from normal hematopoietic counterparts and the identification of a variety of dysregulated molecular pathways. This in turn would allow the identification of tumor-specific characteristics that provide a rational basis for the development of more tailored, and hence potentially more effective and less toxic, therapeutic approaches. In this review, we describe some of the signaling pathways that are aberrantly regulated in AML, with a specific focus on their pathogenetic and therapeutic significance, and we examine some recent therapies directed against these targets, used in clinical trial for relapsed patients or unfit for conventional chemotherapy.

Drug metabolism by tumours: its nature, relevance and therapeutic implications

Drug-metabolising enzymes (DMEs) are present in tumours and are capable of biotransforming a variety of antineoplastics. Tumoural drug metabolism is both a potential mechanism of resistance and a means of achieving optimal therapy. This review addresses the classes of DMEs, their cytotoxic substrates and distribution in specific malignancies. The limitations of preclinical and clinical studies are highlighted. Their role in predicting therapeutic response, the activation of prodrugs and the potential for their modulation for gain is also addressed. The contribution of tumoural DMEs to cancer therapy can only be ascertained through large prospective studies and supported by new technologies. Only then can efforts be concentrated in the design of better prodrugs or combination therapy to optimise individual therapy.

Role of Myc in differentiation and apoptosis in HL60 cells after exposure to arsenic trioxide or all-trans retinoic acid

Acute promyelocytic leukemia (APL) is highly malignant and frequently expresses the PML-RARalpha (promyelocytic leukemia-retinoic acid receptor-alpha) fusion protein. This fusion protein is targeted by all-trans retinoic acid (ATRA) and arsenic trioxide (As2O3), presently used in APL therapy. We have evaluated effects of ATRA and As2O3 treatment in PML-RARalpha-negative HL60 promyelocytic leukemia cells, harboring amplified c-myc. Characterization of expression and activity of c-Myc and its target genes hTERT (human telomerase reverse transcriptase) and CAD (carbamoyltransferase-dihydroorotase) revealed marked down-regulation in response to ATRA, but not As2O3. We suggest that blockage of terminal differentiation upon As2O3 treatment may be mediated through c-Myc.

From dissection of disease pathogenesis to elucidation of mechanisms of targeted therapies: leukemia research in the genomic era

Leukemia is a group of heterozygous diseases of hematopoietic stem/progenitor cells that involves dynamic change in the genome. Dissection of genetic abnormalities critical to leukemia initiation provides insights into the elusive leukemogenesis, identifies distinct subsets of leukemia and predicts prognosis individually, and can also provide rational therapeutic targets for curative approaches. The past three decades have seen tremendous advances in the analysis of genotype-phenotype connection of leukemia, and in the identification of molecular biomarkers for leukemia subtypes. Intriguingly, differentiation therapy, targeted therapy and chemotherapy have turned several subtypes of leukemia from highly fatal to highly curable. The use of all-trans retinoic acid and arsenic trioxide, which trigger degradation of PML-RARalpha, the causative fusion protein generated by t (15;17) translocation in acute promyelocytic leukemia (APL), has led to a dramatic improvement of APL clinical outcome. Imatinib mesylate/ Gleevec/STI571, which inhibits the tyrosine kinase activity of BCR-ABL oncoprotein, has now become the new gold standard for the treatment of chronic myeloid leukemia. Optimal use of chemotherapeutic agents together with a stringent application of prognostic factors for risk-directed therapy in clinical trials has resulted in a steady improvement in the treatment outcome of acute lymphoblastic leukemia. Hence, the pace of progress extrapolates to a prediction of leukemia control in the twenty-first century.

Developmental and genetic modulation of arsenic biotransformation: a gene by environment interaction?

The complexity of arsenic toxicology has confounded the identification of specific pathways of disease causation. One focal point of arsenic research is aimed at fully characterizing arsenic biotransformation in humans, a process that appears to be quite variable, producing a mixture of several arsenic species with greatly differing toxic potencies. In an effort to characterize genetic determinants of variability in arsenic biotransformation, a genetic association study of 135 subjects in western Sonora, Mexico was performed by testing 23 polymorphic sites in three arsenic biotransformation candidate genes. One gene, arsenic 3 methyltransferase (AS3MT), was strongly associated with the ratio of urinary dimethylarsinic acid to monomethylarsonic acid (D/M) in children (7-11 years) but not in adults (18-79 years). Subsequent analyses revealed that the high D/M values associated with variant AS3MT alleles were primarily due to lower levels of monomethylarsonic acid as percent of total urinary arsenic (%MMA5). In light of several reports of arsenic-induced disease being associated with relatively high %MMA5 levels, these findings raise the possibility that variant AS3MT individuals may suffer less risk from arsenic exposure than non-variant individuals. These analyses also provide evidence that, in this population, regardless of AS3MT variant status, children tend to have lower %MMA5 values than adults, suggesting that the global developmental regulation of arsenic biotransformation may interact with genetic variants in metabolic genes to result in novel genetic effects such as those in this report.